Slides accompanying 2.008x* video module on Casting, Prof. John Hart, MIT, 2016.
*Fundamentals of Manufacturing Processes on edX: https://www.edx.org/course/fundamentals-manufacturing-processes-mitx-2-008x
3. 2.008x
What is casting?
Why is it a useful and
important manufacturing
process?
How does it compare
and contrast to
processes we have
studied already?
4. 2.008x
à Casting is the process
whereby a part is produced by
solidification (of a molten
metal) to take the shape of a
mold.
à Why casting?
§ Versatile to many types of
metals
§ Potential for rapid and cost-
effective production
§ Wide range of length scales
(mm to m!)
§ Complex part geometries
(including internal cavities)
Engine block by 160SX (talk) - 160SX (talk)'s file, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=7899838
Brass rat by Pigsfly33 at English Wikipedia, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=31980643,
5. 2.008x
Casting and history
Bessemer steel converter (enabled reduction of
carbon content in Iron à steel), 1865
http://www.metmuseum.org/art/collection/search/257580
https://www.metmuseum.org/toah/works-of-art/28.77/
https://en.wikipedia.org/wiki/Bessemer_process#/media/File:Bessemer_converter.jpg (Public
domain)
Left: Bronze statue of a man,
Hellenistic period, mid-2nd-
1st
century B.C., H. 73 in (185.4 cm)
Below: Herakles (Son of Zeus)
6. 2.008x
The engine from the 1903 Wright Flyer had an aluminum crankcase. The Wrights
contracted a local Dayton foundry, the Buckeye Iron and Brass Works, to cast the
aluminum crankcase. Buckeye acquired their raw aluminum from the nearby Pittsburgh
Reduction Company, renamed Alcoa in 1907, the world’s leading producer of aluminum.
http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=5817
7. 2.008x
Global cast iron and steel production
(millions of tons)
World census of casting production:
http://www.afsinc.org/multimedia/contentMCDP.cfm?ItemNumber=16433
8. 2.008x
Agenda:
Casting
§ Classification of casting methods
§ Sand casting
§ Die casting
§ Casting process physics:
§ Fluidity and cooling (with
demonstration)
§ Solidification and microstructure
§ Investment casting
§ Comparison and conclusion
10. 2.008x
Classification of casting processes
General sequence (all casting processes):
§ Pattern/mold making
§ Melt preparation
§ Mold filling
§ Cooling and solidification
§ Removal (‘breakout’) of the parts
Kalpakjian and Schmid, Manufacturing Engineering and Technology
12. 2.008x
Important criteria for
casting materials
§ Melting point and latent heat
§ Density versus temperature
§ Solubility with other elements
§ Diffusion rates
§ Reactivity (especially to oxygen)
§ Outgassing (vapor pressure)
13. 2.008x
3000° C
0° C
1000° C
2000° C
Tungsten Carbide, WC,
Silicon Carbide, SiC
Molybdenum
Alumina Al2O3
Platinum, Pt
Titanium, Ti
IronFE, Plain Carbon Steels, low alloy, stainless
Nickel, Ni
Nickel Allows
Cubic Zirconia, ZrO2
Silicon, Si
Copper, Cu, Bronze, Brass
Aluminum
Magnesium
Zinc, Zn
PTFE (Teflon)
Tin, Sn
HDPE
Nylon
Acetal
Tungsten Carbide (WC)
Silicon Carbide (SiC)
Molybdenum
Alumina (Al2O3;; 2072 oC)
Platinum, Titanium (1668 oC)
Iron and steels, Nickel, Silicon
Copper, Bronze, Brass
Aluminum (660 oC)
Magnesium
Zinc (420 oC)
Tin
16. 2.008x
Sand casting
§ Mold cavity is formed by packing
sand around a pattern.
§ Interior geometry is defined by a
core (disposable).
§ The pattern is removed and the
cavity has the desired shape.
§ Sand for the mold is, for example,
90% sand, 3% water and 7% clay.
Groover, Fundamentals of Modern Manufacturing
17. 2.008x
Sand casting: key attributes
§ Low surface detail;; post-machining often required for
high tolerances.
§ It’s the most common casting method (by total weight);;
can make VERY large parts.
§ Because mold filling is gravity-driven (more to come
later), must pay most careful attention to flow and
shrinkage.
§ It’s a (relatively) labor-intensive process with long cycle
time (why?)
§ 3D printing of molds and complex cores (though one-
time use) can achieve previously impossible geometries.
20. 2.008x
Die casting of aluminum wheel caps
https://www.youtube.com/watch?v=N6ODcxK8_lg
21. 2.008x
Die casting: attributes
§ Pressure: ~1-1000 MPa (how
does this compare to IM?)
§ Cycle time: ~10’s of seconds for
average components (tools/toys)
§ Parts have many similarities to
IM, i.e., ejector pin marks,
parting lines, gates/runners.
§ Dies are endangered by heat-
induced cracking and corrosion
(accelerated at high
temperature) à need tool-grade
steel or other special materials.
22. 2.008x
Die casting: hot chamber method
: cold chamber method
Groover, Fundamentals of Modern Manufacturing
28. 2.008x
Ferrari F12 Berlinetta
Wheelhouses:
sand casting
Frame components:
High pressure die
casting
Top image: screenshot from http://auto.ferrari.com/en_US/sports-cars-models/car-range/f12-berlinetta/#design-360_exterior-5
Bottom image: http://auto.ferrari.com/en_EN/wp-content/uploads/sites/5/2013/07/architecture11.jpg
30. 2.008x
Surface tension and viscosity
Ux(H) = Ux
Ux(y)
Ux(0) = 0
h
Ux
y
U
∂
∂
= µτ
Surface tension, σ Dynamic viscosity, μ
Water (25 C) 0.07 N/m 1×10-3 kg/m-s [Pa-s]
Honey (25 C) ~0.06 ~10
Liquid thermoplastic ~0.03 102-103
Molten aluminum (600 C) 0.90 3×10-3
31. 2.008x
Fluid flow
considerations for
good casting
Kalpakjian and Schmid, Manufacturing Engineering and Technology
à If gravity-driven, pressure ‘head’
must be sufficient to overcome flow
resistance to fully infiltrate mold
cavity.
32. 2.008x
Fluid flow
considerations for
good casting
Kalpakjian and Schmid, Manufacturing Engineering and Technology
à Flow must remain laminar to
prevent air entrainment.avoid
abrupt direction change. Trapped
air leads to ‘dross’ (oxide flakes)
à same as
μ (viscosity)
33. 2.008x
Testing ‘fluidity’ for casting
Al with increasing
content of
reinforcing
particles (SiC)
Kalpakjian and Schmid, Manufacturing Engineering and Technology
Behera et al. “Effect of Reinforcement Particles on the Fluidity and Solidification Behavior of the Stir Cast Aluminum Alloy
Metal Matrix Composites”, American Journal of Materials Science, 2012;; 2(3): 53-61
36. 2.008x
Chvorinov’s rule
§ V = volume of the
casting
§ A = surface area of the
casting
§ C = mold constant,
depends on mold
material and thermal
properties of casting
metal
tsolidify = C ⋅
V
A
"
#
$
%
&
'
n
Kalpakjian and Schmid, Manufacturing Engineering and Technology
37. 2.008x
Solidification: sand cast versus die cast
TM!
SAND
SOLID!
LIQUID!
T0!
0
tsolidify = C ⋅
V
A
"
#
$
%
&
'
2
αsand ~0.01 cm2/s
TM!
METAL
SOLID!
LIQUID!
0
T0!
αsteel ~0.1 cm2/s
αaluminum ~0.9 cm2/s
tsolidify = C ⋅
V
A
"
#
$
%
&
'
à Like injection molding, but
mold has low thermal conductivity
à Lower-bound (assumes
constant mold temp)
39. 2.008x
Solidification of pure metals
§ Metal releases latent heat as it freezes;; this accounts for
up to ~50% of the energy transfer.
§ As a result, solid and liquid co-exist in the mold for a
significant time.
Kalpakjian and Schmid, Manufacturing Engineering and Technology
1/Temperature
40. 2.008x
Formation of cast microstructure
Columnar
Shell zone
(‘Chill’)
grain
y
oy
d
k
+= σσ
Hall-Petch model: smaller grains
give higher strength
σ0 = stress to start dislocation movement
ky = material hardening constant
d = grain size
Grain size is
inversely
proportional to
cooling rate.
Shell has finer
grains à
thinner die
cast parts are
typically
stronger
Kalpakjian and Schmid, Manufacturing Engineering and Technology
46. 2.008x
Casting: general defects
§ Misrun: solidification before complete filling
§ Cold shut: lack of fusion due to premature freezing
§ Cold shot: metal splatter entrapped in casting
§ Shrinkage cavity: depression in surface caused by
solidification shrinkage (or hot tear = internal void)
Groover, Fundamentals of Modern Manufacturing
50. 2.008x
Investment casting: key points
§ Use of wax template enables excellent surface finish
with little/no post-processing.
§ Ceramic shell enables casting of high melting point
metals/alloys.
§ Metal typically poured in vacuum oven (reduces
defects).
§ Very labor intensive à robots!
Why investment casting?
§ Jewelry: complex geometries, high tolerances and fine
features.
§ Jet engine parts: smooth surface finish, compatibility
with high temperature alloys.
52. 2.008x
Image from ATI Aerospace (http://www.slideshare.net/johnpsilk/ati-jet-engine);; see also
http://www.geaviation.com/commercial/engines/genx/;; https://www.youtube.com/watch?v=S1ahHWXGx5Y
53. 2.008x
Investment casting of turbine blades
à Careful control of solidification can give single crystal blades (= very
high strength and fatigue life under cyclic load at high temperature)
Kalpakjian and Schmid, Manufacturing Engineering and Technology
Image at right from http://www.chromalloy.com/files/newspressrelease/7ac07680-0adb-460b-8f27-f7d47745a4c4.pdf
Great article: ‘The metal that brought you cheap flights’ http://www.bbc.com/news/magazine-32749262
55. 2.008x
What casting method was used and why?
Die casting: Small parts, precision
features, good surface finish
Sand casting: Larger parts, rough
surface finish
Investment casting: Complex curves, good surface finish (at right)
1 2
3
Aluminum castings from the sand mold (modified): photograph taken by Glenn McKechnie - Own work, CC BY-SA 2.0,
https://commons.wikimedia.org/w/index.php?curid=109988
An investment cast turbocharger turbine, Nuclear valve by Mark Bolton - CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=10729027,
https://commons.wikimedia.org/w/index.php?curid=6078994
57. 2.008x
Why would I:
à Choose die casting instead of
injection molding?
Need metal (strength, durability)
instead of plastic
à Choose investment casting
instead of die casting?
Higher melting point material,
complex internal cavities, high
precision
à Choose machining instead of die
casting?
Higher tolerances;; better control of
microstructure/properties (can use
wrought/forged material)
58. 2.008x
Reflection: the big four
Sand Investment Die
Rate Medium Low High
Quality Low Medium High
Cost Low Medium High
Flexibility Low-Medium Medium Low
59. 2.008x
Top image from:
http://www.alcoa.com/global/en/innovation/alcoa_micromill.asp
Article: Automotive News, September 14, 2015.