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TALAT Lecture 3205



              The Fluidity of Molten Metals
                                19 pages, 17 figures

                                  Advanced Level

     prepared by John Campbell and Richard A. Harding, IRC in Materials,
                       The University of Birmingham




Objectives:

− To introduce the concept of fluidity of molten metal and its influence on the
  production of castings
− The student will understand the relevance of fluidity, the means by which this is
  measured and the effect of alloy type.


Prerequisites:

− Basic understanding of foundry processes.
− Knowledge of phase diagrams.
− Basic physics and mathematics.




Date of Issue: 1994
 EAA - European Aluminium Association
3205 The Fluidity of Molten Metals

Table of Contents

  Introduction............................................................................................................... 2
  Measurements of Fluidity......................................................................................... 2
  Solidification Rates ................................................................................................... 4
    Short Freezing Range Alloys ...................................................................................5
    Long Freezing Range Alloys....................................................................................6
     Effect of Impurities ..................................................................................................7
  Map of Fluidity.......................................................................................................... 8
  Effect of Surface Tension ....................................................................................... 11
  Effect of Section Thickness .................................................................................... 13
  Concept of Continuous Fluidity............................................................................. 15
  Practical Feeding Problems.................................................................................... 16
  Literature................................................................................................................. 19
  List of Figures.......................................................................................................... 19




Introduction

Fluidity is, in casting terminology, the distance to which a metal, when cast at a given
temperature, will flow in a given test mould before it is stopped by solidification.
Fluidity is therefore a length, usually measured in millimetres or metres.

(It is not to be confused with the physicists' definition as the reciprocal of viscosity.)


Measurements of Fluidity

Traditionally fluidity has been measured in a spiral mould. The rationale behind this is
clearly the desire to compress the fluidity test into as small a mould as possible, and that
the flow distance is sensitive to levelling errors, and that these are minimised by the
spiral path of the liquid (Figure 3205.00.01).




TALAT 3205                                                     2
Measurement of Fluidity

             (a) Fluidity Spiral                                                            0    50       100
                                                                                                Scale


                                                                                9.1
                             Spiral section enlarged                      4.1         7.5




           (b) Laboratory Test
                            Maximum fluidity length

                                                                                                              To Vacuum


                                                 alu

       Training in Aluminium Application Technologies
                                                                            Measurement of Fluidity                                 3205.00.01



A large number of variations of the spiral test have been used over the last half century.
Although widely used, they are also widely criticised for a number of reasons, probably
the most important of which is that the test bears no clear relation to its application in
real castings. For instance, if it is found that a particular alloy at a reasonable casting
temperature gives a spiral fluidity length of 500 mm, how does this relate to a particular
casting which might be bottom gated and with a wall 350 mm high and 4 mm thick.
Will the casting fill or not? Will it be prevented from filling by premature freezing?
These are questions which have not been answered up till now. However, they are
capable of being answered even from the results of the spiral test as we shall see.


                                                                  New Design of Fluidity Testpiece

                                                                                Offset stepped
                                                                                pouring basin



                                           Tapered
                                           downsprue
                                                                                                                             2 mm
                                                                                                                         4
                                                                                                                      0.5
                                                                                                                  8

                                                                                                              6
                                        20 mm
                                                                                                          1
                                                                                                      5
                                                                                                 3

                                                                      500 mm



                                                        alu

                 Training in Aluminium Application Technologies
                                                                         New Design of Fluidity Testpiece                      3205.00.02




TALAT 3205                                                                              3
However, the questions are probably more easily and directly answered by more recent,
simpler designs of test which have been developed over recent years at The University
of Birmingham, UK. Figure 3205.00.02 shows the latest variant in which a series of
long strips of different thickness is filled simultaneously from a common runner bar.
One mould provides a plot of fluidity as a function of thickness and several moulds
poured at different temperatures completely characterises the fluidity of a given
material. Some results from this will be shown later.


Solidification Rates

                                                                    Solidification Rates
                               Mould                        Metal                                 Mould             Metal
                                                Solid           Liquid                                         Solid L Solid
                                                      x
                                                                                                                x           x


                                                                                                                     s

                                                          Metal die
                                                          t = ki x
                                                                                        s
                                                                                                     e
                Thickness, x




                                                                                                              d                 ts (sand) = km s² / 4
                                                                                                  Di




                                                                                                           San
                                                                                   Thickness, x




                                                                Sand mould
                                                                t =km x²
                                                                                                                                ts (die)   = ki s / 2


                                                      Time, t                                       ts (die)             ts (sand)     Time, t

                                                alu

         Training in Aluminium Application Technologies
                                                                             Solidification Rates                                      3205.00.03



Figure 3205.00.03: As a necessary piece of background, we need to know that the time t
for the solidified metal to reach a thickness x in a sand mould, where the rate of loss of
heat from the casting is controlled by the poor conductivity of the mould, is given by the
parabolic law of thickening:

                                                                    t = km x2                                                                           (1a)

In the case of a die, the heat flow away from the casting is fast through the casting and
die, since both are metals, but is now limited by heat flow across the metal/die interface,
especially as the air gap opens up as the casting contracts away from the die. The
equivalent relation in this case is a linear law for the rate of thickening:

                                                                    t = ki x                                                                            (1b)
where
− k is a constant,




TALAT 3205                                                                               4
− and the subscript m indicates heat transfer controlled by the mould (i.e. the mould is
  a poor conductor of heat)
− and the subscript i heat transfer controlled by the metal/mould interface (which is
  generally applied to most types of aluminium alloy die casting operations, including
  gravity, low pressure and high pressure die casting).

Thus the time for complete solidification, ts, of a sand casting of section thickness S
( = twice the maximum value of x)
                                              km ⋅ S 2                                  (2a)
                                       ts =
                                                  4
and an equivalent die casting
                                               ki ⋅ S
                                       ts =                                             (2b)
                                                  2


Short Freezing Range Alloys

We will now consider the fluidity of short freezing range metals and alloys
(Figure 3205.00.04). This shows the mode of solidification from the outside walls in
towards the centre as the metal proceeds along the mould. A point to notice is the
remelting of the part of the frozen solid nearest the source of hot metal. For this reason
the solidified zone migrates progressively along the mould, trailing behind the liquid tip.
However, the important point to be noted is that the flow of metal is stopped when the
two freezing fronts meet (although this point is some distance back from the flow tip).
This corresponds to 100 % solidification at this point.

We can therefore write down a formula for fluidity in terms of the solidification time tS,
the velocity of flow V, and the fluidity distance Lf. Assuming that V is approximately
constant, we have simply

                                L f = V · tS                                             (3)

and hence from equations 2a and 3 we have for sand moulds

                                       V ⋅ km ⋅ S 2
                                Lf =                                                    (4a)
                                              4
and for dies:
                                       V ⋅ ki ⋅ S
                                Lf =                                                    (4b)
                                              2
These formulae overestimate the fluidity length somewhat because V is not completely
constant; the melt does slow a little as the constriction due to freezing starts to take hold.




TALAT 3205                                        5
However, this is a relatively minor effect, and the formula is in fact quite a useful
approximation.


                                  Fluidity of Short Freezing Range Alloys
                                                               Solid
                                                                                       Liquid         Air

                                                              Mould

                                                                     Lf

                                        Fluidity Distance Lf = V ts       where        V = flow velocity
                                                                                       ts = solidification time


                         Sand Mould                   Metal Die           Predicted                               Difference due
                                                       ki s / 2               Actual                              to constriction
          ts              km s² / 4
                                                                                  Fluidity
          Lf                 V km s² / 4                V ki s / 2                length



                                                                                                 Time        ts

                                           alu
                                                     Fluidity of Short Freezing Range Alloys                       3205.00.04
    Training in Aluminium Application Technologies




Long Freezing Range Alloys

We will now move onto the fluidity of long freezing range alloys. The solidification of
such alloys in a rapidly flowing stream is somewhat different (Figure 3205.00.05).

The solidification front is now, of course, no longer planar but dendritic, and because
freezing is occurring in a moving liquid, the bulk turbulence in the liquid causes
turbulent eddies to sweep through the dendrites, carrying pockets of hot liquid into these
cooler regions, and thus remelting dendrite arms and other fragments, to build up a
slurry of dendrite debris. As heat is lost from the slurry, the slurry thickens, gradually
becoming so thick that it is too viscous to flow. This occurs at different fractions of
solid in different alloys, and also seems to be influenced by the metallostatic head
driving the flow. In general, however, the flow of liquid is arrested when the volume
fraction of solid is somewhere between 25 and 50 %.




TALAT 3205                                                                6
Fluidity of Long Freezing Range Alloys
                                                                                             Slurry of dendrites
                                     Liquid                                       Air
                                                 Mould

                                                     Lf                                              Sand Mould           Metal Die
                              Flow stops when 25 - 50%                                                                    t = ki x
                                                                                                     t = km x²
                              solid is present,
                                                                                     25% solid       t = km S² / 64       t = ki S / 8
                              i.e. when x = S/8 to S/4
                                                                                     50% solid       t = km S² / 16       t = ki S / 4

                                                                                     Therefore       Lf = V km S² / 64    V ki S / 8
                                                                                                             to              to
                                                                                                          V km S² / 16    V ki S² / 4


                                            Remember that for short freezing range alloys:           Lf = V km S² / 4     V ki S / 2


                                                     Therefore          Short freezing range     :        4 - 16             2-4
                                                                        long freezing range

                                           alu

    Training in Aluminium Application Technologies
                                                                 Fluidity of Long Freezing Range Alloys                  3205.00.05


If we now rewrite equation 1 allowing for the fact that only 25 to 50 % freezing is
required to arrest flow, then x = S/4 to S/8, and from equation 1 we obtain therefore
t = km S2 / 16 to t = km S2 / 64, so that

                                                                        V ⋅ km S 2           V ⋅ km ⋅ S 2
                                                                 Lf =                   to                                               (5)
                                                                            16                       64

By comparison with equation 4a, it is immediately clear that fluidity is expected to fall
by a factor of 4 to 16 in long freezing range alloys when cast in sand moulds. This is
often seen in practice.

For dies, a similar approach gives

                                                                        V ⋅ ki ⋅ S            V ⋅ ki ⋅ S
                                                                 Lf =                   to                                               (6)
                                                                             4                       8

By comparison with equation 4b, this shows that the fluidity falls by a factor of only
2 to 4 when long freezing range alloys are cast in dies.

Effect of Impurities

Sometimes, however, the large predicted fall for sand castings is not seen. In practice
the fall appears to be only perhaps a factor of 2. In such cases it is likely that the so
called pure metal components of the alloy are in fact not so pure; even slight impurities




TALAT 3205                                                                       7
will greatly reduce the fluidity of pure elements, thus easily accounting for the fact that
further alloying appears to reduce the fluidity further by only the factor of 2.
This effect is illustrated by the results (Figure 3205.00.06) which show the effect of
small concentrations of tin on the fluidity of pure aluminium.


                                                                            Fluidity of Al - Sn alloys
                                                                            700
                                                                                               100°C superheat
                                                                            600

                                                                            500
                                                            Fluidity (mm)




                                                                            400

                                                                            300

                                                                            200

                                                                            100

                                                                              0
                                                                                  0        5        10             15
                                                                                           Tin (mass %)
                                                  alu

           Training in Aluminium Application Technologies
                                                                                      Fluidity of Al - Sn Alloys        3205.00.06




Map of Fluidity

We can now create a useful map of the performance of a complete series of binary alloys
in terms of their ability to flow before being arrested by solidification
(Figure 3205.00.07).
The map can be created in stages as follows: firstly the pure metal components and the
eutectic are skin-freezing materials (all solidifying at a single temperature) and so have
high fluidities. These three points should lie on a single line, but the extreme sensitivity
of fluidity to even minor impurities often will seriously affect the height of these cusps.
Also, of course, the cusps are so high and narrow that it is easily possible in practice to
miss the peak when attempting to locate it experimentally.




TALAT 3205                                                                                      8
Mapping




                                                                             Fluidity distance
                              the Fluidity
                              of Binary Alloys

                                                                                                 A                  B




                                                                             Temperature
                                                                                                 A                 B
                                                                                                     Composition

                                                   alu

            Training in Aluminium Application Technologies
                                                             Mapping the Fluidity of Binary Alloys                 3205.00.07



The regions in between the three peaks have fluidities which are lower by a factor of
2 to 16, but more typically by a factor of 2 to 4. This factor can be different at the two
ends, of course. Furthermore, we may probably assume a method of mixtures type
argument between these extremes, which gives a straight line connection. The map is
then complete, as shown here.

Figure 3205.00.08: In practice the Al-Si system turns out to be a surprise when the peak
in fluidity is not at the equilibrium eutectic around 11% Si, but is nearer 15% Si. This
corresponds of course to the non-equilibrium eutectic composition. It is expected that
the presence of Na or Sr as promoters of the eutectic phase, and suppressers of the
primary Si, might influence the position and height of the fluidity peak somewhat. The
general increase in fluidity with increasing silicon content in this particular alloy is the
result of the powerful effect of Si. Its latent heat of solidification is among the highest of
all natural elements, and is nearly 5 times greater than that of Al. Thus tS is significantly
increased as Si levels are raised.




TALAT 3205                                                                   9
The Fluidity of Al - Si Alloys
                                                                              800
                                                                                        800°C



                                                                              600

                                                               Fluidity, mm

                                                                              400




                                                                              200




                                                                               0
                                                                                    0      5                      10               15            20               25              30
                                                                                                       Silicon, wt per cent
                                               alu

     Training in Aluminium Application Technologies
                                                                                          The Fluidity of Al - Si Alloys                                                                                  3205.00.08




Figure 3205.00.09 shows results for Al - Zn alloys and again confirms that fluidity is
highest for pure metals and at the eutectic composition. This is true whether a constant
pouring temperature (700° C) or constant superheat (liquidus temperature plus 50° C) is
used.


                              The Fluidity                                                                        1.0
                                                                                                                                                                                                 x
                                                                                                Fluidity, m




                              of Al - Zn Alloys                                                                                                                                                       x


                                                                                                                                                                                             xx
                                                                                                                                                                                                      x
                                                                                                                                                                                                     x+
                                                                                                                                                                                             x+
                                                                                                                  0.5                                         700°C               x    x x
                                                                                                                                                                            x x
                                                                                                                           x                                           x
                                                                                                                            x                                      x
                                                                                                                                x
                                                                                                                                x                 x x
                                                                                                                                                      x   x
                                                                                                                                                              x    Tliq + 50°C               +
                                                                                                                                                                                                 ++
                                                                                                                                 xxx        x x x                                                +
                                                                                                                                   x+ x x x
                                                                                                                                     x
                                                                                                                                                  + + +   +   +    + +      + +   + + +
                                                                                                                                       + + ++ + +
                                                                                                Temperature, °C




                                                                                                                       0
                                                                                                                                                                           Liquid
                                                                                                                  600
                                                                                                                  500
                                                                                                                                        Solid
                                                                                                                  400
                                                                                                                       0                                  50                                     100
                                                                                                                       Al                                                                        Zn
                                                                                                                                                 Zinc, wt per cent

                                                         alu

                  Training in Aluminium Application Technologies
                                                                                          The Fluidity of Al - Zn Alloys                                                                 3205.00.09


For more complicated systems, the fluidity map becomes even more complicated as seen
in this three dimensional plot for Al - Si - Cu alloys (Figure 3205.00.10). These



TALAT 3205                                                                                                             10
experimental data show that the maximum fluidity coincides with the ternary eutectic
'A'. When designing new foundry alloys it should be mandatory to create such a map.
The sensitivity or robustness of the fluidity of a new material would then be clear from
the start, or slight modifications might result in a much improved optimisation of the
castability of the material without significant loss of other properties. In the past the
importance of this critical parameter has been sadly overlooked.


                                                                                     The Fluidity of Al-Cu-Si Alloys
                                                                                                                                               The liquidus surface of the
                                                                                                                                                    Al-Cu-Si system

                                                                                                                                                    Cu      Si     T°C
                                                                                                                   40                          A 27.5       5.25   524   Ternary eutectic
                                                                           B
                                                                                                                                               B 51         4.5    571   Ternary eutectic
                                                                                               40
                                                                                                                   35                          C 33         -      548   Binary eutectic
                                                             A                                 35                                              D 51         -      591   CuAl2
                                                                                                    Fluidity, mm


                                                                                               30                  30                          E -          11.7   577   Binary eutectic
                                                                                          D
                                                                               C
                                                                                               25
                                                                                                                   25                              15
                                                                                               20
                                                                                                                   20
                                                                                                                                        %
                                                                                               15                                                       E
                                                                                                                                          t
                                                                                                                                      igh
                                                                                                                                         10                        Si
                   E                                                                                               15
                                                                                                                                   we


                                                                                          50                       10
                                                                                                                                n,




        14                                                                                                                                                   A
                                                                                                                             ico




                                                                                    40                                                                                       B
               12                                                                                                                 5
                                                                                          t%




                                                                                                                            Sil




                       10
                                                                                     gh




            Si
               lic
                                                                               30                                                             Al             CuAl2
                                                                                     ei




                     on 8
                                                                                    w




                       ,w            6                                20
                                                                             r,




                                                                                                                                                        C          D
                                                                           pe




                         eig                4                    10                                                     0
                                                                       op




                                  ht                 2
                                     %                                                                                      0                 20            40          50
                                                                      C




                                                         0   0
                                                                                                                                        Copper, weight %

                                           alu
                                                                               The Fluidity of Al - Cu - Si Alloys                                                           3205.00.10
    Training in Aluminium Application Technologies




Effect of Surface Tension

In addition to the effect of solidification on limiting the distance to which metals will
flow in moulds and dies, the effect of surface tension becomes important in thin sections
i.e. less than 5 mm or so (Figure 3205.00.11).




TALAT 3205                                                                                                     11
Influence of Surface Tension

                                                            Air
                                                                                               Liquid
                                                    r1
                                            r2
               Liquid
                                                                                               r

                                                          1    1                                            γ
            Back pressure ∆P = γ                             -                         Back pressure ∆P =
                                                          r1   r2                                           r



                                                                                         The limiting condition is when d = 2r
                                                                           d
                                        h                                                Metallostatic head = ρ g h
                                                                                         and back pressure = 2 γ / d

                                                                                         Forces balance when ρ g h = 2 γ / d,
                                                                             r
                                                                                         i.e. when d = 2 γ / ρ g h



                                              alu

       Training in Aluminium Application Technologies
                                                                    Influence of Surface Tension                         3205.00.11


The effect of surface tension can be likened to a back pressure, often known as capillary
repulsion - repulsion since moulds and dies are designed to avoid wetting by the liquid
metal; if the mould or die was wetted then the effect would be the familiar phenomenon
of capillary attraction, as water climbing a glass capillary tube. The formula which
allows the effect to be quantified is
                                                                              1 1 
                                                                        ∆P= γ  − 
                                                                               r1 r2 
where ∆ P is the pressure difference across the curved interface, γ is the surface tension
of the liquid metal, and r1 and r2 are the two radii at right angles which define the
curvature of the liquid meniscus. In the filling of a wide plate we may consider that one
of the radii becomes nearly infinity, so that if the remaining radius is now r, the
expression reduces to
                                                                                   γ
                                                                        ∆P=
                                                                                   r
Thus the pressure exerted by the meniscus to prevent a thin section from filling is
inversely proportional to the section thickness. If we equate this to the pressure available
due to the metallostatic head ρ g h, where
                        ρ is the density of the liquid,
                        g the acceleration due to gravity, and
                        h the metallostatic head,
then we can calculate the section thickness d = 2r which just will not fill. This is
                                                                                  2 ⋅γ
                                                                        d=                                                            (7)
                                                                                 ρ ⋅ g ⋅h



TALAT 3205                                                                        12
Effect of Section Thickness

Figure 3205.00.12 shows the effect of casting a series of strips of different thickness at
a variety of temperatures. Clearly they all extrapolate backwards to a critical thickness
0.63 mm which just will not fill (i.e. has zero fluidity distance under all conditions of
pouring temperature) for this height of mould. For the Zn - 27 % Al alloy we can work
out from Equation (7) the effective surface tension. When this exercise is carried out for
most aluminium alloys the result is approximately 1.5 - 2.0 N/m. Since γ for liquid
aluminium and its alloys is actually nearer 0.9 - 1 N/m, we can conclude that the
presence of the strong and tenacious aluminium oxide film approximately doubles the
effect of the natural surface tension of the liquid in preventing the liquid from entering
narrow sections.


              The Fluidity                                                                                     Pouring
                                                                                                               temperature
              of ZA 27 Alloy                                                        ZA 27 alloy cast
                                                                                                               °C
                                                                                    in greensand               600
                                                                        200                                !
                                                                                                           O
                                                                                                               570
                                                     Strip length, mm




                                                                                                       !
                                                                         150
                                                                                                       O       550
                                                                                                           #

                                                                         100                      !    #       545
                                                                                                  O        "
                                                                                                       "

                                                                                              !
                                                                                                  #
                                                                                                           $
                                                                                                               520
                                                                          50                      "
                                                                                              O
                                                                                              #   $    $
                                                                                              $
                                                                           0
                                                                               0        0.5     1.0    1.5  2.0
                                                                                        Strip thickness, mm
                                           alu

    Training in Aluminium Application Technologies
                                                                        The Fluidity of ZA 27 Alloy                  3205.00.12


The determination of fluidity as a function of thickness of section in this way is a
valuable method of determining the effective surface tension in filling problems. The
technique is recommended for more general use.

Figure 3205.00.13 shows more fluidity data for this alloy cast in sand moulds,
illustrating the considerably greater distance to which the metal will run in the wide
section of a fluidity spiral, as opposed to the narrow plate-like sections.




TALAT 3205                                                                         13
800
                       Comparison                                                                       ZA 27 alloy cast
                                                                                                                                          !
                                                                                                        in greensand
                       of                                                                      700
                                                                                                                     Fluidity
                                                                                                                                      !

                       fluidity                                                                600
                                                                                                                      spiral !
                                                                                                                          !
                       measurements                                                                                       !




                                                                         Fluidity length, mm
                                                                                               500                        !
                                                                                                                       !
                                                                                                                     !
                                                                                               400                    !
                                                                                                                                          Fluidity
                                                                                                                                           strips
                                                                                               300
                                                                                                                                              2.0 mm
                                                                                                                                                      o
                                                                                               200                                            o
                                                                                                                                      o       1.5 mm
                                                                                                                                                      x
                                                                                                                              o
                                                                                                                                      x
                                                                                               100                        o
                                                                                                                              x               1.0 mm
                                                                                                                o         x                           %
                                                                                                                                      %
                                                                                                                x
                                                                                                                %         %%
                                                                                                    0
                                                                                                          500         550                             600
                                                                                                                Temperature, °C
                                               alu

        Training in Aluminium Application Technologies
                                                          Comparison of Fluidity Measurements                                                         3205.00.13



However, Figure 3205.00.14 shows how all these results become equivalent when
allowance is made for the effect of surface tension and the effect of the different cooling
rates due to the difference in geometric modulus; all the results condense together onto a
single curve when reduced to the effective fluidity in a 2 mm plate section. This result
emphasises the fact that all the fluidity tests give equivalent information providing
allowance is made for the effects of the section on cooling rate and the effective loss of
pressure head due to surface tension.


             Rationalisation of                                                                                                                             x
                                                                                                                                                            %

             fluidity measurements                                                                                                                          o


                                                                                               200                                        x
                                                                                                                                          %
                                                                                                                                          o

                                                                                                                                                  !
                                                                       Fluidity, mm




                                                                                               150
                                                                                                                                          !
                                                                                                                           !% !
                                                                                                                                  x
                                                                                                                           o
                                                                                                                           !
                                                                                                                        !x!
                                                                                                                              %
                                                                                               100                   !        o
                                                                                                                      !
                  ! Spiral lengths / 4.44                                                                        x
                                                                                                                 %
                                                                                                                 o
                  o 2.0 effective strip thickness, mm                                          50
                  x 1.5 strip / 0.572                                                                   Tm
                  % 1.0 strip / 0.243


                                                                                                0
                                                                                                          500         550                                 600
                                                                                                                 Temperature, °C

                                              alu

       Training in Aluminium Application Technologies
                                                         Rationalisation of Fluidity Measurements                                                     3205.00.14




TALAT 3205                                                                                     14
Concept of Continuous Fluidity

I would now like to introduce the concept of continuous fluidity. So far we have
discussed only the maximum distance that an alloy will run in a mould. In some
instances it is useful to be able to define the distance to which a metal can flow in a
channel, in which the metal will continue to run for ever, at least in principle.
This can happen in short lengths of channel where the channel opens out subsequently
into a larger section. It occurs because the arrival of new hot metal in the channel
continuously remelts the originally solidified material, effectively redepositing this
further down the channel. The effect is seen in Figure 3205.00.15.


                                                         Continuous Fluidity




                                     Maximum fluidity length Lf



                                                                                        Continuous Fluidity
                                                                                        Length LC




                                               alu

        Training in Aluminium Application Technologies
                                                                  Continuous Fluidity                    3205.00.15



With the movement of the solidifying plug progressing down the channel, if the channel
opens out into a larger section before the plug finally closes, then the plug will
completely remelt, effectively being pushed out of the channel. Thus the subsequent
flow will be unchecked by solidification - the channel will remain completely open
while hot metal continues to flow.

This effect is valuable in running systems, where the section thickness is small, but the
system remains open because of the remelting effect. Interestingly, the continuous
fluidity concept can allow the casting engineer to predict how small a running system
section he can use, and what corresponding casting temperature will be required to avoid
premature freezing in the running system, resulting in a short-run casting.

Figure 3205.00.16 illustrates how continuous fluidity contrasts with normal (i.e.
maximum) fluidity. Maximum fluidity still exists at zero superheat because there is




TALAT 3205                                                             15
latent heat in the liquid which will allow the metal to continue to flow for a short time.
For continuous fluidity, some superheat is always required for the remelting action to
occur. The various regimes of continuous, partial, and impossible flow to make castings
are worth remembering.




                                                                      Fluidity
                     Regimes of                                                               No
                     continuous,                                                             flow




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                                                                                                      lim
                     partial                                                               regime




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                                                                                 Melting




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                     and




                                                                                               flu
                                                                                  point




                                                                                             um
                     impossible




                                                                                          im
                                                                                       ax
                     flow




                                                                                      M
                                                                                                Limited
                                                          Fluidity                                flow
                                                          at zero                               regime
                                                          superheat




                                                                                                                  ty
                                                                                                               idi
                                                                                                         s flu
                                                                                                      ou
                                                                                                          Continuous




                                                                                                    inu
                                                                                                             flow



                                                                                                  nt
                                                                                               Co
                                                                                                           regime
                                                                                                             Temperature
                                                                                           Some superheat required
                                                                                           for continuous fluidity

                                              alu

       Training in Aluminium Application Technologies
                                                        Flow Regimes of Molten Metals                                  3205.00.16



Very little experimental work has been performed on continuous fluidity. More deserves
to be carried out on this important and useful concept.


Practical Feeding Problems


Figure 3205.00.17: Finally, a significant practical difficulty needs to be emphasised for
the benefit of the unwary casting technologist.
It needs to be pointed out that although an alloy may be persuaded to flow a
considerable distance to fill a mould, this flow distance may greatly exceed the distance
to which the alloy may be fed. Thus although the mould may have been successfully
filled, so that the casting may appear to be reassuringly complete when taken out from
the mould, the casting may be scrap in any case as a result of internal shrinkage
problems, simply because the metal has been allowed to flow to a greater distance than
can easily be fed.
Unfortunately, it is not easy at this time to give any easy guidance on this important
issue. The reason is that despite a number of attempts to clarify this issue by research on
the feeding distance of aluminium casting alloys, the results have been unclear. This is
perhaps not surprising because the physical laws controlling feeding distance are
complicated (we shall briefly take a look at these in a moment).




TALAT 3205                                                        16
The imprecise situation for aluminium alloys contrasts with the precise and well known
condition in simple carbon-manganese steels of section thickness upwards of 15 mm
cast in green sand moulds. Here the feeding distance is easily demonstrated to be a
maximum of 2.5 times the section thickness away from the feeder.
However, as a useful rule of thumb, most common aluminium foundry alloys in a sand
mould will flow a distance of at least 50 times the section thickness in a complex
casting, and up to 100 times the section thickness in a simple, nicely gated casting.
(Beyond this are usually the headaches of regular scrap due to misrun castings. Beware!)
It is likely therefore that the feeding distance might be considerably exceeded in
aluminium alloys, and some castings are indeed observed to contain some centre-line
shrinkage porosity beyond a distance of a few section thicknesses from the feeder.



         Filling
            vs.                                                Pouring basin
                                                                                 Feeder
               feeding
                                                              Downsprue                   Feeding distance
                                                               Thin wall
                    Beware!!                                   bell-shaped
         Filling distance may exceed                           casting                             Shrinkage
                feeding distance
                                                               Runner



            ' In steels, feeding distance                 ' Thin wall aluminium castings often sound due to:
               = 2.5 x section thickness.                     ( Surface feeding [liquid drawn from surface]
            ' Al alloys: flow distance                        ( Solid feeding [solid skin sucked inwards]
               = 50 - 100 x section thickness.                ( Assistance from dispersed hydrogen porosity
            ' Al alloys: feeding distance not                 ( Assistance from low melting point eutectic
              well established.

                                             alu

      Training in Aluminium Application Technologies
                                                       Filling versus Feeding                    3205.00.17




However, this is not always the case. Many thin sections which have been run to the
maximum fluidity distance are still found to be sound when checked by X-ray
radiography. This appears to be the result of the beneficial actions of other feeding
processes such as:
•    surface feeding, in which liquid to feed internal shrinkage is withdrawn from the
     nearby casting surface. (In a thin section casting this feeding mechanism has a
     negligible effect on surface finish),
or




TALAT 3205                                                    17
•   solid feeding, in which the solid skin of the casting is sucked inwards. Once again,
    this is a negligible effect on casting dimensions when well-dispersed over an
    extensive thin wall.


Other effects which are occasionally present and which have an important influence on
whether the casting freezes sound or not are:
•   The occasional presence of some dispersed hydrogen porosity which will reduce the
    feeding difficulties.
•   Some alloys seem to enjoy the additional beneficial effects of minute traces of
    impurities in creating a low melting point eutectic phase which aids feeding.
For extensive thin-walled aerospace castings the fluidity and the soundness of the
casting are of paramount importnace.


However, in the manufacture of many thin walled aluminium alloys castings for normal
large commercial markets, the soundness requirements are in most cases easily met
because the castings often require no great mechanical properties, but are merely
required to be leak-tight. No X-ray requirements are therefore imposed, and a certain
amount of internal porosity (whether shrinkage or gas) as a result of exceeding the
feeding distance, is of little consequence to the serviceability of the component.


Clearly, the ultimate criterion has to be fitness for purpose at minimum cost.




TALAT 3205                                  18
Literature


Campbell, J.: Castings, Butterworth Heinemann, 1991




List of Figures


Figure No.   Figure Title (Overhead)
3205.00.01   Measurement of Fluidity
3205.00.02   New Design of Fluidity Testpiece
3205.00.03   Solidification Rates
3205.00.04   Fluidity of Short Freezing Range Alloys
3205.00.05   Fluidity of Long Freezing Range Alloys
3205.00.06   Fluidity of Al - Sn Alloys
3205.00.07   Mapping the Fluidity of Binary Alloys
3205.00.08   The Fluidity of Al - Si Alloys
3205.00.09   The Fluidity of Al - Zn Alloys
3205.00.10   The Fluidity of Al - Cu - Si Alloys
3205.00.11   Influence of Surface Tension
3205.00.12   The Fluidity of ZA 27 Alloy
3205.00.13   Comparison of Fluidity Measurements
3205.00.14   Rationalisation of Fluidity Measurements
3205.00.15   Continuous Fluidity
3205.00.16   Flow Regimes of Molten Metals
3205.00.17   Filling vs. Feeding




TALAT 3205                              19

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Fluidity of Molten Metals

  • 1. TALAT Lecture 3205 The Fluidity of Molten Metals 19 pages, 17 figures Advanced Level prepared by John Campbell and Richard A. Harding, IRC in Materials, The University of Birmingham Objectives: − To introduce the concept of fluidity of molten metal and its influence on the production of castings − The student will understand the relevance of fluidity, the means by which this is measured and the effect of alloy type. Prerequisites: − Basic understanding of foundry processes. − Knowledge of phase diagrams. − Basic physics and mathematics. Date of Issue: 1994  EAA - European Aluminium Association
  • 2. 3205 The Fluidity of Molten Metals Table of Contents Introduction............................................................................................................... 2 Measurements of Fluidity......................................................................................... 2 Solidification Rates ................................................................................................... 4 Short Freezing Range Alloys ...................................................................................5 Long Freezing Range Alloys....................................................................................6 Effect of Impurities ..................................................................................................7 Map of Fluidity.......................................................................................................... 8 Effect of Surface Tension ....................................................................................... 11 Effect of Section Thickness .................................................................................... 13 Concept of Continuous Fluidity............................................................................. 15 Practical Feeding Problems.................................................................................... 16 Literature................................................................................................................. 19 List of Figures.......................................................................................................... 19 Introduction Fluidity is, in casting terminology, the distance to which a metal, when cast at a given temperature, will flow in a given test mould before it is stopped by solidification. Fluidity is therefore a length, usually measured in millimetres or metres. (It is not to be confused with the physicists' definition as the reciprocal of viscosity.) Measurements of Fluidity Traditionally fluidity has been measured in a spiral mould. The rationale behind this is clearly the desire to compress the fluidity test into as small a mould as possible, and that the flow distance is sensitive to levelling errors, and that these are minimised by the spiral path of the liquid (Figure 3205.00.01). TALAT 3205 2
  • 3. Measurement of Fluidity (a) Fluidity Spiral 0 50 100 Scale 9.1 Spiral section enlarged 4.1 7.5 (b) Laboratory Test Maximum fluidity length To Vacuum alu Training in Aluminium Application Technologies Measurement of Fluidity 3205.00.01 A large number of variations of the spiral test have been used over the last half century. Although widely used, they are also widely criticised for a number of reasons, probably the most important of which is that the test bears no clear relation to its application in real castings. For instance, if it is found that a particular alloy at a reasonable casting temperature gives a spiral fluidity length of 500 mm, how does this relate to a particular casting which might be bottom gated and with a wall 350 mm high and 4 mm thick. Will the casting fill or not? Will it be prevented from filling by premature freezing? These are questions which have not been answered up till now. However, they are capable of being answered even from the results of the spiral test as we shall see. New Design of Fluidity Testpiece Offset stepped pouring basin Tapered downsprue 2 mm 4 0.5 8 6 20 mm 1 5 3 500 mm alu Training in Aluminium Application Technologies New Design of Fluidity Testpiece 3205.00.02 TALAT 3205 3
  • 4. However, the questions are probably more easily and directly answered by more recent, simpler designs of test which have been developed over recent years at The University of Birmingham, UK. Figure 3205.00.02 shows the latest variant in which a series of long strips of different thickness is filled simultaneously from a common runner bar. One mould provides a plot of fluidity as a function of thickness and several moulds poured at different temperatures completely characterises the fluidity of a given material. Some results from this will be shown later. Solidification Rates Solidification Rates Mould Metal Mould Metal Solid Liquid Solid L Solid x x x s Metal die t = ki x s e Thickness, x d ts (sand) = km s² / 4 Di San Thickness, x Sand mould t =km x² ts (die) = ki s / 2 Time, t ts (die) ts (sand) Time, t alu Training in Aluminium Application Technologies Solidification Rates 3205.00.03 Figure 3205.00.03: As a necessary piece of background, we need to know that the time t for the solidified metal to reach a thickness x in a sand mould, where the rate of loss of heat from the casting is controlled by the poor conductivity of the mould, is given by the parabolic law of thickening: t = km x2 (1a) In the case of a die, the heat flow away from the casting is fast through the casting and die, since both are metals, but is now limited by heat flow across the metal/die interface, especially as the air gap opens up as the casting contracts away from the die. The equivalent relation in this case is a linear law for the rate of thickening: t = ki x (1b) where − k is a constant, TALAT 3205 4
  • 5. − and the subscript m indicates heat transfer controlled by the mould (i.e. the mould is a poor conductor of heat) − and the subscript i heat transfer controlled by the metal/mould interface (which is generally applied to most types of aluminium alloy die casting operations, including gravity, low pressure and high pressure die casting). Thus the time for complete solidification, ts, of a sand casting of section thickness S ( = twice the maximum value of x) km ⋅ S 2 (2a) ts = 4 and an equivalent die casting ki ⋅ S ts = (2b) 2 Short Freezing Range Alloys We will now consider the fluidity of short freezing range metals and alloys (Figure 3205.00.04). This shows the mode of solidification from the outside walls in towards the centre as the metal proceeds along the mould. A point to notice is the remelting of the part of the frozen solid nearest the source of hot metal. For this reason the solidified zone migrates progressively along the mould, trailing behind the liquid tip. However, the important point to be noted is that the flow of metal is stopped when the two freezing fronts meet (although this point is some distance back from the flow tip). This corresponds to 100 % solidification at this point. We can therefore write down a formula for fluidity in terms of the solidification time tS, the velocity of flow V, and the fluidity distance Lf. Assuming that V is approximately constant, we have simply L f = V · tS (3) and hence from equations 2a and 3 we have for sand moulds V ⋅ km ⋅ S 2 Lf = (4a) 4 and for dies: V ⋅ ki ⋅ S Lf = (4b) 2 These formulae overestimate the fluidity length somewhat because V is not completely constant; the melt does slow a little as the constriction due to freezing starts to take hold. TALAT 3205 5
  • 6. However, this is a relatively minor effect, and the formula is in fact quite a useful approximation. Fluidity of Short Freezing Range Alloys Solid Liquid Air Mould Lf Fluidity Distance Lf = V ts where V = flow velocity ts = solidification time Sand Mould Metal Die Predicted Difference due ki s / 2 Actual to constriction ts km s² / 4 Fluidity Lf V km s² / 4 V ki s / 2 length Time ts alu Fluidity of Short Freezing Range Alloys 3205.00.04 Training in Aluminium Application Technologies Long Freezing Range Alloys We will now move onto the fluidity of long freezing range alloys. The solidification of such alloys in a rapidly flowing stream is somewhat different (Figure 3205.00.05). The solidification front is now, of course, no longer planar but dendritic, and because freezing is occurring in a moving liquid, the bulk turbulence in the liquid causes turbulent eddies to sweep through the dendrites, carrying pockets of hot liquid into these cooler regions, and thus remelting dendrite arms and other fragments, to build up a slurry of dendrite debris. As heat is lost from the slurry, the slurry thickens, gradually becoming so thick that it is too viscous to flow. This occurs at different fractions of solid in different alloys, and also seems to be influenced by the metallostatic head driving the flow. In general, however, the flow of liquid is arrested when the volume fraction of solid is somewhere between 25 and 50 %. TALAT 3205 6
  • 7. Fluidity of Long Freezing Range Alloys Slurry of dendrites Liquid Air Mould Lf Sand Mould Metal Die Flow stops when 25 - 50% t = ki x t = km x² solid is present, 25% solid t = km S² / 64 t = ki S / 8 i.e. when x = S/8 to S/4 50% solid t = km S² / 16 t = ki S / 4 Therefore Lf = V km S² / 64 V ki S / 8 to to V km S² / 16 V ki S² / 4 Remember that for short freezing range alloys: Lf = V km S² / 4 V ki S / 2 Therefore Short freezing range : 4 - 16 2-4 long freezing range alu Training in Aluminium Application Technologies Fluidity of Long Freezing Range Alloys 3205.00.05 If we now rewrite equation 1 allowing for the fact that only 25 to 50 % freezing is required to arrest flow, then x = S/4 to S/8, and from equation 1 we obtain therefore t = km S2 / 16 to t = km S2 / 64, so that V ⋅ km S 2 V ⋅ km ⋅ S 2 Lf = to (5) 16 64 By comparison with equation 4a, it is immediately clear that fluidity is expected to fall by a factor of 4 to 16 in long freezing range alloys when cast in sand moulds. This is often seen in practice. For dies, a similar approach gives V ⋅ ki ⋅ S V ⋅ ki ⋅ S Lf = to (6) 4 8 By comparison with equation 4b, this shows that the fluidity falls by a factor of only 2 to 4 when long freezing range alloys are cast in dies. Effect of Impurities Sometimes, however, the large predicted fall for sand castings is not seen. In practice the fall appears to be only perhaps a factor of 2. In such cases it is likely that the so called pure metal components of the alloy are in fact not so pure; even slight impurities TALAT 3205 7
  • 8. will greatly reduce the fluidity of pure elements, thus easily accounting for the fact that further alloying appears to reduce the fluidity further by only the factor of 2. This effect is illustrated by the results (Figure 3205.00.06) which show the effect of small concentrations of tin on the fluidity of pure aluminium. Fluidity of Al - Sn alloys 700 100°C superheat 600 500 Fluidity (mm) 400 300 200 100 0 0 5 10 15 Tin (mass %) alu Training in Aluminium Application Technologies Fluidity of Al - Sn Alloys 3205.00.06 Map of Fluidity We can now create a useful map of the performance of a complete series of binary alloys in terms of their ability to flow before being arrested by solidification (Figure 3205.00.07). The map can be created in stages as follows: firstly the pure metal components and the eutectic are skin-freezing materials (all solidifying at a single temperature) and so have high fluidities. These three points should lie on a single line, but the extreme sensitivity of fluidity to even minor impurities often will seriously affect the height of these cusps. Also, of course, the cusps are so high and narrow that it is easily possible in practice to miss the peak when attempting to locate it experimentally. TALAT 3205 8
  • 9. Mapping Fluidity distance the Fluidity of Binary Alloys A B Temperature A B Composition alu Training in Aluminium Application Technologies Mapping the Fluidity of Binary Alloys 3205.00.07 The regions in between the three peaks have fluidities which are lower by a factor of 2 to 16, but more typically by a factor of 2 to 4. This factor can be different at the two ends, of course. Furthermore, we may probably assume a method of mixtures type argument between these extremes, which gives a straight line connection. The map is then complete, as shown here. Figure 3205.00.08: In practice the Al-Si system turns out to be a surprise when the peak in fluidity is not at the equilibrium eutectic around 11% Si, but is nearer 15% Si. This corresponds of course to the non-equilibrium eutectic composition. It is expected that the presence of Na or Sr as promoters of the eutectic phase, and suppressers of the primary Si, might influence the position and height of the fluidity peak somewhat. The general increase in fluidity with increasing silicon content in this particular alloy is the result of the powerful effect of Si. Its latent heat of solidification is among the highest of all natural elements, and is nearly 5 times greater than that of Al. Thus tS is significantly increased as Si levels are raised. TALAT 3205 9
  • 10. The Fluidity of Al - Si Alloys 800 800°C 600 Fluidity, mm 400 200 0 0 5 10 15 20 25 30 Silicon, wt per cent alu Training in Aluminium Application Technologies The Fluidity of Al - Si Alloys 3205.00.08 Figure 3205.00.09 shows results for Al - Zn alloys and again confirms that fluidity is highest for pure metals and at the eutectic composition. This is true whether a constant pouring temperature (700° C) or constant superheat (liquidus temperature plus 50° C) is used. The Fluidity 1.0 x Fluidity, m of Al - Zn Alloys x xx x x+ x+ 0.5 700°C x x x x x x x x x x x x x x x x Tliq + 50°C + ++ xxx x x x + x+ x x x x + + + + + + + + + + + + + + ++ + + Temperature, °C 0 Liquid 600 500 Solid 400 0 50 100 Al Zn Zinc, wt per cent alu Training in Aluminium Application Technologies The Fluidity of Al - Zn Alloys 3205.00.09 For more complicated systems, the fluidity map becomes even more complicated as seen in this three dimensional plot for Al - Si - Cu alloys (Figure 3205.00.10). These TALAT 3205 10
  • 11. experimental data show that the maximum fluidity coincides with the ternary eutectic 'A'. When designing new foundry alloys it should be mandatory to create such a map. The sensitivity or robustness of the fluidity of a new material would then be clear from the start, or slight modifications might result in a much improved optimisation of the castability of the material without significant loss of other properties. In the past the importance of this critical parameter has been sadly overlooked. The Fluidity of Al-Cu-Si Alloys The liquidus surface of the Al-Cu-Si system Cu Si T°C 40 A 27.5 5.25 524 Ternary eutectic B B 51 4.5 571 Ternary eutectic 40 35 C 33 - 548 Binary eutectic A 35 D 51 - 591 CuAl2 Fluidity, mm 30 30 E - 11.7 577 Binary eutectic D C 25 25 15 20 20 % 15 E t igh 10 Si E 15 we 50 10 n, 14 A ico 40 B 12 5 t% Sil 10 gh Si lic 30 Al CuAl2 ei on 8 w ,w 6 20 r, C D pe eig 4 10 0 op ht 2 % 0 20 40 50 C 0 0 Copper, weight % alu The Fluidity of Al - Cu - Si Alloys 3205.00.10 Training in Aluminium Application Technologies Effect of Surface Tension In addition to the effect of solidification on limiting the distance to which metals will flow in moulds and dies, the effect of surface tension becomes important in thin sections i.e. less than 5 mm or so (Figure 3205.00.11). TALAT 3205 11
  • 12. Influence of Surface Tension Air Liquid r1 r2 Liquid r 1 1 γ Back pressure ∆P = γ - Back pressure ∆P = r1 r2 r The limiting condition is when d = 2r d h Metallostatic head = ρ g h and back pressure = 2 γ / d Forces balance when ρ g h = 2 γ / d, r i.e. when d = 2 γ / ρ g h alu Training in Aluminium Application Technologies Influence of Surface Tension 3205.00.11 The effect of surface tension can be likened to a back pressure, often known as capillary repulsion - repulsion since moulds and dies are designed to avoid wetting by the liquid metal; if the mould or die was wetted then the effect would be the familiar phenomenon of capillary attraction, as water climbing a glass capillary tube. The formula which allows the effect to be quantified is 1 1  ∆P= γ  −   r1 r2  where ∆ P is the pressure difference across the curved interface, γ is the surface tension of the liquid metal, and r1 and r2 are the two radii at right angles which define the curvature of the liquid meniscus. In the filling of a wide plate we may consider that one of the radii becomes nearly infinity, so that if the remaining radius is now r, the expression reduces to γ ∆P= r Thus the pressure exerted by the meniscus to prevent a thin section from filling is inversely proportional to the section thickness. If we equate this to the pressure available due to the metallostatic head ρ g h, where ρ is the density of the liquid, g the acceleration due to gravity, and h the metallostatic head, then we can calculate the section thickness d = 2r which just will not fill. This is 2 ⋅γ d= (7) ρ ⋅ g ⋅h TALAT 3205 12
  • 13. Effect of Section Thickness Figure 3205.00.12 shows the effect of casting a series of strips of different thickness at a variety of temperatures. Clearly they all extrapolate backwards to a critical thickness 0.63 mm which just will not fill (i.e. has zero fluidity distance under all conditions of pouring temperature) for this height of mould. For the Zn - 27 % Al alloy we can work out from Equation (7) the effective surface tension. When this exercise is carried out for most aluminium alloys the result is approximately 1.5 - 2.0 N/m. Since γ for liquid aluminium and its alloys is actually nearer 0.9 - 1 N/m, we can conclude that the presence of the strong and tenacious aluminium oxide film approximately doubles the effect of the natural surface tension of the liquid in preventing the liquid from entering narrow sections. The Fluidity Pouring temperature of ZA 27 Alloy ZA 27 alloy cast °C in greensand 600 200 ! O 570 Strip length, mm ! 150 O 550 # 100 ! # 545 O " " ! # $ 520 50 " O # $ $ $ 0 0 0.5 1.0 1.5 2.0 Strip thickness, mm alu Training in Aluminium Application Technologies The Fluidity of ZA 27 Alloy 3205.00.12 The determination of fluidity as a function of thickness of section in this way is a valuable method of determining the effective surface tension in filling problems. The technique is recommended for more general use. Figure 3205.00.13 shows more fluidity data for this alloy cast in sand moulds, illustrating the considerably greater distance to which the metal will run in the wide section of a fluidity spiral, as opposed to the narrow plate-like sections. TALAT 3205 13
  • 14. 800 Comparison ZA 27 alloy cast ! in greensand of 700 Fluidity ! fluidity 600 spiral ! ! measurements ! Fluidity length, mm 500 ! ! ! 400 ! Fluidity strips 300 2.0 mm o 200 o o 1.5 mm x o x 100 o x 1.0 mm o x % % x % %% 0 500 550 600 Temperature, °C alu Training in Aluminium Application Technologies Comparison of Fluidity Measurements 3205.00.13 However, Figure 3205.00.14 shows how all these results become equivalent when allowance is made for the effect of surface tension and the effect of the different cooling rates due to the difference in geometric modulus; all the results condense together onto a single curve when reduced to the effective fluidity in a 2 mm plate section. This result emphasises the fact that all the fluidity tests give equivalent information providing allowance is made for the effects of the section on cooling rate and the effective loss of pressure head due to surface tension. Rationalisation of x % fluidity measurements o 200 x % o ! Fluidity, mm 150 ! !% ! x o ! !x! % 100 ! o ! ! Spiral lengths / 4.44 x % o o 2.0 effective strip thickness, mm 50 x 1.5 strip / 0.572 Tm % 1.0 strip / 0.243 0 500 550 600 Temperature, °C alu Training in Aluminium Application Technologies Rationalisation of Fluidity Measurements 3205.00.14 TALAT 3205 14
  • 15. Concept of Continuous Fluidity I would now like to introduce the concept of continuous fluidity. So far we have discussed only the maximum distance that an alloy will run in a mould. In some instances it is useful to be able to define the distance to which a metal can flow in a channel, in which the metal will continue to run for ever, at least in principle. This can happen in short lengths of channel where the channel opens out subsequently into a larger section. It occurs because the arrival of new hot metal in the channel continuously remelts the originally solidified material, effectively redepositing this further down the channel. The effect is seen in Figure 3205.00.15. Continuous Fluidity Maximum fluidity length Lf Continuous Fluidity Length LC alu Training in Aluminium Application Technologies Continuous Fluidity 3205.00.15 With the movement of the solidifying plug progressing down the channel, if the channel opens out into a larger section before the plug finally closes, then the plug will completely remelt, effectively being pushed out of the channel. Thus the subsequent flow will be unchecked by solidification - the channel will remain completely open while hot metal continues to flow. This effect is valuable in running systems, where the section thickness is small, but the system remains open because of the remelting effect. Interestingly, the continuous fluidity concept can allow the casting engineer to predict how small a running system section he can use, and what corresponding casting temperature will be required to avoid premature freezing in the running system, resulting in a short-run casting. Figure 3205.00.16 illustrates how continuous fluidity contrasts with normal (i.e. maximum) fluidity. Maximum fluidity still exists at zero superheat because there is TALAT 3205 15
  • 16. latent heat in the liquid which will allow the metal to continue to flow for a short time. For continuous fluidity, some superheat is always required for the remelting action to occur. The various regimes of continuous, partial, and impossible flow to make castings are worth remembering. Fluidity Regimes of No continuous, flow it lim partial regime ty Melting idi and flu point um impossible im ax flow M Limited Fluidity flow at zero regime superheat ty idi s flu ou Continuous inu flow nt Co regime Temperature Some superheat required for continuous fluidity alu Training in Aluminium Application Technologies Flow Regimes of Molten Metals 3205.00.16 Very little experimental work has been performed on continuous fluidity. More deserves to be carried out on this important and useful concept. Practical Feeding Problems Figure 3205.00.17: Finally, a significant practical difficulty needs to be emphasised for the benefit of the unwary casting technologist. It needs to be pointed out that although an alloy may be persuaded to flow a considerable distance to fill a mould, this flow distance may greatly exceed the distance to which the alloy may be fed. Thus although the mould may have been successfully filled, so that the casting may appear to be reassuringly complete when taken out from the mould, the casting may be scrap in any case as a result of internal shrinkage problems, simply because the metal has been allowed to flow to a greater distance than can easily be fed. Unfortunately, it is not easy at this time to give any easy guidance on this important issue. The reason is that despite a number of attempts to clarify this issue by research on the feeding distance of aluminium casting alloys, the results have been unclear. This is perhaps not surprising because the physical laws controlling feeding distance are complicated (we shall briefly take a look at these in a moment). TALAT 3205 16
  • 17. The imprecise situation for aluminium alloys contrasts with the precise and well known condition in simple carbon-manganese steels of section thickness upwards of 15 mm cast in green sand moulds. Here the feeding distance is easily demonstrated to be a maximum of 2.5 times the section thickness away from the feeder. However, as a useful rule of thumb, most common aluminium foundry alloys in a sand mould will flow a distance of at least 50 times the section thickness in a complex casting, and up to 100 times the section thickness in a simple, nicely gated casting. (Beyond this are usually the headaches of regular scrap due to misrun castings. Beware!) It is likely therefore that the feeding distance might be considerably exceeded in aluminium alloys, and some castings are indeed observed to contain some centre-line shrinkage porosity beyond a distance of a few section thicknesses from the feeder. Filling vs. Pouring basin Feeder feeding Downsprue Feeding distance Thin wall Beware!! bell-shaped Filling distance may exceed casting Shrinkage feeding distance Runner ' In steels, feeding distance ' Thin wall aluminium castings often sound due to: = 2.5 x section thickness. ( Surface feeding [liquid drawn from surface] ' Al alloys: flow distance ( Solid feeding [solid skin sucked inwards] = 50 - 100 x section thickness. ( Assistance from dispersed hydrogen porosity ' Al alloys: feeding distance not ( Assistance from low melting point eutectic well established. alu Training in Aluminium Application Technologies Filling versus Feeding 3205.00.17 However, this is not always the case. Many thin sections which have been run to the maximum fluidity distance are still found to be sound when checked by X-ray radiography. This appears to be the result of the beneficial actions of other feeding processes such as: • surface feeding, in which liquid to feed internal shrinkage is withdrawn from the nearby casting surface. (In a thin section casting this feeding mechanism has a negligible effect on surface finish), or TALAT 3205 17
  • 18. solid feeding, in which the solid skin of the casting is sucked inwards. Once again, this is a negligible effect on casting dimensions when well-dispersed over an extensive thin wall. Other effects which are occasionally present and which have an important influence on whether the casting freezes sound or not are: • The occasional presence of some dispersed hydrogen porosity which will reduce the feeding difficulties. • Some alloys seem to enjoy the additional beneficial effects of minute traces of impurities in creating a low melting point eutectic phase which aids feeding. For extensive thin-walled aerospace castings the fluidity and the soundness of the casting are of paramount importnace. However, in the manufacture of many thin walled aluminium alloys castings for normal large commercial markets, the soundness requirements are in most cases easily met because the castings often require no great mechanical properties, but are merely required to be leak-tight. No X-ray requirements are therefore imposed, and a certain amount of internal porosity (whether shrinkage or gas) as a result of exceeding the feeding distance, is of little consequence to the serviceability of the component. Clearly, the ultimate criterion has to be fitness for purpose at minimum cost. TALAT 3205 18
  • 19. Literature Campbell, J.: Castings, Butterworth Heinemann, 1991 List of Figures Figure No. Figure Title (Overhead) 3205.00.01 Measurement of Fluidity 3205.00.02 New Design of Fluidity Testpiece 3205.00.03 Solidification Rates 3205.00.04 Fluidity of Short Freezing Range Alloys 3205.00.05 Fluidity of Long Freezing Range Alloys 3205.00.06 Fluidity of Al - Sn Alloys 3205.00.07 Mapping the Fluidity of Binary Alloys 3205.00.08 The Fluidity of Al - Si Alloys 3205.00.09 The Fluidity of Al - Zn Alloys 3205.00.10 The Fluidity of Al - Cu - Si Alloys 3205.00.11 Influence of Surface Tension 3205.00.12 The Fluidity of ZA 27 Alloy 3205.00.13 Comparison of Fluidity Measurements 3205.00.14 Rationalisation of Fluidity Measurements 3205.00.15 Continuous Fluidity 3205.00.16 Flow Regimes of Molten Metals 3205.00.17 Filling vs. Feeding TALAT 3205 19