Al 8mg

International Journal of of Mechanical Engineering Research and
International Journal Mechanical Engineering Research and Development (IJMERD), ISSN 2248 – 9347(Print)
Development (IJMERD), ISSN 2248 – 9347(Print)
ISSN 2228 – 9355(Online), Volume 1, Number 2, May-October (2011)
ISSN 2228 – 9355(Online), Volume 1, Number 2
IJMERD
May-October (2011), pp. 41-52
© PRJ PUBLICATION
© PRJ Publication, http://www.prjpublication.com/IJMERD.asp

MICROHARDNESS STUDIES ON AL-8MG ALLOYS TO ANALYZE THE
DEFORMATION BEHAVIOUR

Sagar Yanda, Swami Naidu Gurugubelli
Department of Mechanical Engineering
J N T U Kakinada
Vizianagaram Campus
Andhra Pradesh, India
ABSTRACT

Cylindrical samples of Al-8 wt pct Mg alloy were upset to 40% reduction followed by heat
treatment at 2000C, 3500C and 4500C, respectively. Inhomogeneity in deformation due to
friction at the platen interface and internal friction was studied by microhardness measurements
in the roll direction. Results were quantified and analyzed by generating bar charts.
Homogenized structures exhibit high and uniform hardness values compared to the as-cast ones.
Retention of higher Mg concentrations in the solidsolution (making it supersaturated) with the
cast-structures (chill-cast) is the reason for its greater in-homogeneity in deformation. Rejection
of Mg from the supersaturated solidsolution followed by formation of Al3Mg2 is the reason for
high hardness of homogenized structures. Upsetting followed by heat treatment not only
enhances the uniformity in structure but also decreases the average hardness, with increasing
heat treatment temperature. This could be attributed to the formation of new strain free grains
and grain growth. A mathematical model has been developed and verified at other temperatures
as well.

Key words: Microhardness, homogeneity, heat treatment, deformation

1. INTRODUCTION

Aluminum alloys are being considered for increased usage as structural materials for producing
lightweight automobiles. Al-Mg alloys have a good combination of strength and formability.
Commercial Al-Mg alloys of AA5000 series have excellent resistance to corrosion, mechanical
strength, weldability and machinability. The strength of Al-Mg alloys is mainly due to solid
solution strengthening and strain hardening. Increasing the Mg content in solid solution Al-Mg
alloys enhances strongly the multiplication of mobile dislocations and decreases the trapping of
mobile dislocations. This leads to the strong strengthening effect of Mg addition in Al-Mg alloy
(Gyozo Horvath et al, 2007). The microstructure evolution and strain hardening have been
investigated extensively in the case of Al-Mg alloys (Gubicza et al; 2004; Horvath et al, 2007;
Hughes, 1993; Lukac, 1996; Malygin, 1990; Waldron, 1965). It was shown that the addition of
Mg inhibits cell structure formation (Hughes, 1993; Waldron, 1965) . Increasing the Mg content
in solid solution Al-Mg alloys enhances strongly the multiplication of mobile dislocations and
decreases the trapping of mobile dislocations and the annihilation of immobile dislocations. This
41

International Journal of Mechanical Engineering Research and Development (IJMERD), ISSN 2248 – 9347(Print)

leads to the strong strengthening effect of Mg addition in Al-Mg alloys (Gyozo Harvath et at,
2007). Al-Mg alloys containing more than 3wt% Mg are susceptible to both intergranular
corrosion and stress corrosion cracking when exposed to elevated temperatures (≥500C) in
corrosive environments for sufficient length of time (Searls, Gouma and Bucheit, 2001). Hence
these alloys have wide industrial applications at room temperature, though the formability of
aluminum alloys at room temperature is generally lower than at both cryogenic and elevated
temperatures. Aluminum alloys with high magnesium content show more serrated flow due to
Luders banding (ØYvind Ryen, 2006). Hence the homogeneity of deformation is studied based
on the micro hardness measurements. Attempts have been made to observe the deformation
behavior of as-cast and homogenized structures based on the micro-hardness measurements and
the effect of heat treatment on upset structures in terms of homogeneity. A mathematical model
is developed to design the deformation and heat treatment cycles to get the required hardness.

2. EXPERIMENT
Pencil ingots of 100 mm x 10 mm ф of Al-8% Mg are produced by melting pure aluminum and
magnesium in an induction heating furnace and cast in cast iron moulds at 6900C. Chemical
composition of the alloy using optical emission spectrometer Q8 Magellan is given in table 1.
Element Al Mg Fe Si Mn Ni Cu Zn P
Weight% 92.23 7.573 0.124 0.050 0.018 0.0029 0.0012 <0.000010 <0.00010
Table 1. Chemical composition of Al-8Mg alloy.
Cylindrical specimens of 1.5:1 aspect ratio are prepared from the as-cast ingot and ingots
homogenized at 1000C for 24hours. Microstructures of the same are shown below, fig 1.
a b

10µ 10µ
Fig 1. Microstructures of Al-8Mg alloy, a. cast and b. homogenized
Specimens are given 40% deformation in a hydraulic press at a strain rate of 1mm/min and are
parted vertically (normal direction, ND) into two, fig 2. Microstructure of the parted surface in
the rolling direction, RD, is shown as fig 3. Parted samples are heat-treated at 200 and 4500C for
1 hour and the microstructures are shown as fig 4. Hardness measurements are made using
Vicker’s microhardness tester with a load of 100 g. applied for 15 seconds at an interval of 2 mm
on the parted surface along rolling direction.

42


ND

RD

a b c
Fig 2. Photographs of the specimen, a. before upsetting, b. after upsetting and c. after parting

3. RESULTS AND DISCUSSION

Figure 1 shows the cast and homogenized microstructures of Al-8Mg alloy. Cast structure
exhibit super saturated solid solution of Mg in Al. The Hazier appearance is due to the chilling
effect of the metal mold. Al-Mg system is characterized by the limited solid solubility of Mg
(upto 17.4%) in solid Aluminum (Lyakishev, 1996), decreases with decrease in temperature.
When the casting is homogenized at 1000C, Mg from the supersaturated solid solution comes out
with some Al as Al3Mg2 (Hatch, 1993; Mundalfo, 1976), resulting fine Al3Mg2 particles in the
microstructure of the homogenized sample. Formation of fine intermetallics enhances the
hardness of matrix.

10µ
Fig 3. Microstructure of deformed Al-8Mg alloy, parted surface in rolling direction, RD

Figure 3 shows the microstructure of the 40% cold deformed sample. Aluminum alloys with high Mg
content exhibit serrated flow due to Luders banding (ØYvind Ryen, 2006). Mazilkin et.al (2007), have
shown that the undeformed alloys with 10%Mg contain about 8%Mg in the solid solution. Due to
deformation, the system passes to a state that is closer to thermodynamic equilibrium than the initial state
was. The electron diffraction data (Mazilkin et al, 2007) has shown that the alloy structures in both the
initial and deformed states contain intermediate phases, namely, the β-phase (Al3Mg2). As the alloy is
subjected to deformation, the supersaturated solid solution decomposes. Elongated grains are observed in
the microstructure taken in the rolling direction for the deformed sample is shown in the figure. Since Mg
atoms are highly diffusive in nature, the increased dislocation density due to deformation has shown
thicker lines of diffused Mg along these dislocation populations.

43


Along X – direction
-10 -8 -6 -4 -2 2 4 6 8 10
-12 87.4 107.6 79.3 91.7 94 94 91.7 79.3 107.6 87.4
-10 87 101.7 78 93.2 83.4 83.4 93.2 78 101.7 87
-8 75.8 75.6 90.5 81.3 91.5 91.5 81.3 90.5 75.6 75.8
Along Y – direction

-6 83.9 95.1 94.7 95.3 97.5 97.5 95.3 94.7 95.1 83.9
-4 100.9 114.7 83.2 90.3 101.7 101.7 90.3 83.2 114.7 100.9
-2 80.9 78.3 89.9 109.2 112.2 112.2 109.2 89.9 78.3 80.9
2 80.9 78.3 89.9 109.2 112.2 112.2 109.2 89.9 78.3 80.9
4 100.9 114.7 83.2 90.3 101.7 101.7 90.3 83.2 114.7 100.9
6 83.9 95.1 94.7 95.3 97.5 97.5 95.3 94.7 95.1 83.9
8 75.8 75.6 90.5 81.3 91.5 91.5 81.3 90.5 75.6 75.8
10 87 101.7 78 93.2 83.4 83.4 93.2 78 101.7 87
12 87.4 107.6 79.3 91.7 94 94 91.7 79.3 107.6 87.4
Table 2. Microhardness values of cast-deformed Al-8Mg alloy
Tables 2 and 3 show the microhardness values along central cross-section in the rolling direction
of the cast-deformed and homogenized-deformed samples. The average microhardness value
obtained for the homogenized deformed sample is 95 VHN, which is higher than the average
micro hardness of cast-deformed sample of 92 VHN. This increase in hardness is due to the
formation of Al3Mg2 particles by the decomposition of Al-Mg supersaturated solid solution
(Nebti, amana and Cizeron, 1995). More uniform microhardness measurements are obtained for
the homogenized deformed sample than the cast-deformed one.

Contour maps are developed using MATLAB to identify the zones of different deformation
levels. Figures 4 and 5 show the contour maps developed for cast-deformed sample and
homogenized deformed sample respectively. Hardness is higher in the central region, due to
high plastic deformation achieved. Lower microhardness values are observed in the dead metal
zone where the deformation is minimum due to high friction at the interface.

Dead metal zone

Uniform deform
zone

Dead metal zone

Fig 4. Contour map showing variation in deformation in cast-deformed sample.

44


-10 -8 -6 -4 -2 2 4 6 8 10
-12 94.4 94.4 93.6 96 95.7 95.7 96 93.6 94.4 94.4
-10 84.8 84.8 78.8 92.8 89.7 89.7 92.8 78.8 84.8 84.8
-8 79.6 79.6 97.7 99.3 90.7 90.7 99.3 97.7 79.6 79.6

-6 77.7 77.7 117 114.7 103.6 103.6 114.7 117 77.7 77.7
-4 97.6 97.6 116.1 102.6 107.4 107.4 102.6 116.1 97.6 97.6
-2 97.9 97.9 104.6 100 83.5 83.5 100 104.6 97.9 9.9
2 97.9 97.9 104.6 100 83.5 83.5 100 104.6 97.9 97.9
4 97.6 97.6 116.1 102.6 107.4 107.4 102.6 116.1 97.6 97.6
6 77.7 77.7 117 114.7 103.6 103.6 114.7 117 77.7 77.7
8 79.6 79.6 97.7 99.3 90.7 90.7 99.3 97.7 79.6 79.6
10 84.8 84.8 78.8 92.8 89.7 89.7 92.8 78.8 84.8 84.8
12 94.4 94.4 93.6 96 95.7 95.7 96 93.6 94.4 94.4
Table 3 Microhardness values homogenized-deformed Al-8Mg alloy

Figure 6 shows is the graphical representation of the above, ie., variation in the degree of
homogeneity in deformation with cast and homogenized structures. The uniform deformation
zone is higher for the homogenized-deformed sample than the cast-deformed one. Higher Mg
concentrations with cast structures hinder the deformation as Mg atoms readily diffuses to the
dislocations assisting the deformation. This leads to sluggish movement of the material with a
variation from one region to the other. High hardness of the homogenized sample than the cast
one is a signature of the above discussion.

Dead metal zone

Uniform deform
zone

Dead metal zone

Fig 5: Contour map showing variation in deformation in homogenized-deformed sample.

45


Fig 6. Bar charts showing degree of homogeneity in deformation, a. cast and b. homogenized

Figure 7 shows the microstructure of upset samples heat treated at 2000C and 4500C. The
structure shows spherical grains with uniform distribution of β-phase. This is a signature of
recrystallization. An increase in grain size and uniformity in distribution of β-phase is observed
with increase in heat-treatment temperature, which indicates the grain growth.
Tables 4, 5 and 6 show the microhardness measurements taken along the central cross-section of
the deformed samples after heat-treating at 2000C, 3500C and 4500C respectively for 1 hour.
Figures 8, 9 and 10 show the corresponding contour maps developed for the above. With
increase in heat treatment temperature the average hardness of the uniform deformation zone and
the dead metal zone is decreased. The variation in the microhardness values between different
zones of deformation is also decreased which indicates the stress reliving effect. In other words,
the internal stresses developed in different regions of the component due to deformation are
related to the extent of deformation. The average microhardness values are 80 VHN, 67 VHN
and 65 VHN, respectively. With increase in the heat treatment temperature the average hardness
is decreased, which corroborate the earlier discussion.

a b

10µ 10µ
Fig 7. Microstructures of Al-8Mg alloy, after heat treatment, 1 hour, a. 200 C and b. 4500C
0

46


-10 -8 -6 -4 -2 2 4 6 8 10
-12 67.4 83.2 74 71.4 68.7 68.7 71.4 74 83.2 67.4
-10 60.3 75.5 68.7 77.5 78.5 78.5 77.5 68.7 75.5 60.3
-8 86.8 80.6 75.6 89.1 87.4 87.4 89.1 75.6 80.6 86.8
Along Y- direction

-6 74.7 89.7 77 67.1 90.1 90.1 67.1 77 89.7 74.7
-4 98.4 79.8 70.8 80.3 86.3 86.3 80.3 70.8 79.8 98.4
-2 88.2 94 96 85.3 91.7 91.7 85.3 96 94 88.2
2 88.2 94 96 85.3 91.7 91.7 85.3 96 94 88.2
4 98.4 79.8 70.8 80.3 86.3 86.3 80.3 70.8 79.8 98.4
6 74.7 89.7 77 67.1 90.1 90.1 67.1 77 89.7 74.7
8 86.8 80.6 75.6 89.1 87.4 87.4 89.1 75.6 80.6 86.8
10 60.3 75.5 68.7 77.5 78.5 78.5 77.5 68.7 75.5 60.3
12 67.4 83.2 74 71.4 68.7 68.7 71.4 74 83.2 67.4
Table 4 Microhardness values, heat treated, 2000C, 1 hour

Fig 8. Contour map showing variation in deformation, heat-treated at 2000C, 1 hour.

-10 -8 -6 -4 -2 2 4 6 8 10
-12 78.5 77.2 67.7 84.7 81.3 81.3 84.7 67.7 77.2 78.5
-10 56.4 58.8 44.1 60.5 77.2 77.2 60.5 44.1 58.8 56.4

-8 71.8 60.8 65.8 63.3 70.4 70.4 63.3 65.8 60.8 71.8
-6 75.3 54.6 60.4 79.4 64.7 64.7 79.4 60.4 54.6 75.3
-4 68.3 41.6 70.7 60.8 63.7 63.7 60.8 70.7 41.6 68.3
-2 77.8 70.6 70 78 65.6 65.6 78 70 70.6 77.8
2 77.8 70.6 70 78 65.6 65.6 78 70 70.6 77.8
4 68.3 41.6 70.7 60.8 63.7 63.7 60.8 70.7 41.6 68.3
6 75.3 54.6 60.4 79.4 64.7 64.7 79.4 60.4 54.6 75.3
8 71.8 60.8 65.8 63.3 70.4 70.4 63.3 65.8 60.8 71.8
10 56.4 58.8 44.1 60.5 77.2 77.2 60.5 44.1 58.8 56.4
12 78.5 77.2 67.7 84.7 81.3 81.3 84.7 67.7 77.2 78.5

47



-10 -8 -6 -4 -2 2 4 6 8 10
-12 61.8 64 82 66.2 54.4 54.4 66.2 82 64 61.8
-10 73.1 65.8 45.2 58.6 68 68 58.6 45.2 65.8 73.1
Along Y- direction

-8 64.5 61.1 68.3 70.8 69.1 69.1 70.8 68.3 61.1 64.5
-6 56.3 66.7 59.7 51.7 79.1 79.1 51.7 59.7 66.7 56.3
-4 70.4 69.7 82.8 49.9 59.7 59.7 49.9 82.8 69.7 70.4
-2 75.8 54.7 58.2 63.7 64 64 63.7 58.2 54.7 75.8
2 75.8 54.7 58.2 63.7 64 64 63.7 58.2 54.7 75.8
4 70.4 69.7 82.8 49.9 59.7 59.7 49.9 82.8 69.7 70.4
6 56.3 66.7 59.7 51.7 79.1 79.1 51.7 59.7 66.7 56.3
8 64.5 61.1 68.3 70.8 69.1 69.1 70.8 68.3 61.1 64.5
10 73.1 65.8 45.2 58.6 68 68 58.6 45.2 65.8 73.1
12 61.8 64 82 66.2 54.4 54.4 66.2 82 64 61.8

48


Figure 11 shows is the graphical representation of the above, ie., variation in the degree of
homogeneity in deformation with heat treatment. The higher the heat treatment temperature, the
more is the homogeneity in the material. In other words, the average hardness is decreased and
the homogeneity in the material is expanding with increasing the heat treatment temperatures.

a. b.

49


c. d.
Fig 11. Barchart showing degree of homogeneity,
a. deformed, b. heat treated 2000C, c. 3500C and d. 4500C
Figure 12 shows the effect of heat treatment temperature on the hardness of the cold-worked
sample. The hardness decreases with increasing temperature.

Figure 12. Variation of microhardness with heat treatment temperature.

A mathematical model is developed using the method of least squares. Considering the three
average microhardness values as H1, H2 and H3 and corresponding temperatures as T1, T2 and T3
the following equations have been derived:
(1)
(2)
are determined from
experimentally obtained microhardness values H1, H2 and H3 at temperatures T1, T2 and T3 .

50


Temperature(T) Micro hardness (H) Log (H) T log(H) T2
200 80 1.903089987 380.6179974 4000
350 67 1.826074803 639.1261811 122500
450 64 1.806179974 812.7809883 202500
, , ,

Substituting these values in equations (1&2) we will get the following equations.
5.535344764 = 3 log (a) + 1000 log (b) (3)
1832.525167 = 1000 log (a) + 36500 log (b) (4)
Equations (3 & 4) are solved and the following relation is obtained between hardness and
temperature.
H = (94.98141844) (0.999084977)T
Using the above equation one can determine the suitable heat-treating temperature to
obtain a particular hardness value in a deformed Al-8%Mg alloy. The obtained equation is
checked for validation by determining hardness measurements experimentally at temperatures
1500C and 3000C. The experimentally measured and mathematically calculated hardness values
at 150C and 3000C are shown in table (7). A good agreement is observed between the
experimentally measured and mathematically calculated hardness values.

Temperature 0C Hardness Error percentage
Experimental Mathematical Model
150 87 83 4.5
300 77 72 6.5

Table 7: Validation of the mathematical model showing error percentage

4. CONCLUSIONS

Present investigation is prediction of homogeneity in deformation based on microhardness
measurements. The results obtained are summerized as follows:
(1) Cast structures show more inhomogeneity in deformation than the homogenized ones.
(2) Homogenization yield stable phases. Formation of Al3Mg2 during homogenization enhances
hardness of the resultant matrix.
(3) Higher the heat treatment temperature, lower is the hardness and more the uniformity in
hardness / homogeneity in matrix.
(4) A mathematical model has been developed and validated to design the temperatures to get
required hardness

Acknowledgements
The authors are grateful to the Department of Metallugical Engineering, IIT Madras, Chennai
and Naval Science and Technologcal Laboratoy, Viskhapatnam for providing facilites in
carrying out this work. Special thanks are due to Sri Sambhi Reddy, Scientist-E, NSTL and Sri P
Mallikarjuna Rao, Senior Research fellow, IIT Madras, Chennai for their valuable suggestions.

51


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