casting (mit 2.008x lecture slides)

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CastingMIT 2.008x

Prof. John Hart

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What is casting?

Why is it a useful and important manufacturing process?

How does it compare and contrast to processes we have studied already?

2.008xà 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=7899838Brass rat by Pigsfly33 at English Wikipedia, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=31980643,

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Casting and history

Bessemer steel converter (enabled reduction of carbon content in Iron à steel), 1865

http://www.metmuseum.org/art/collection/search/257580https://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-1stcentury B.C., H. 73 in (185.4 cm)Below: Herakles (Son of Zeus)

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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

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Global cast iron and steel production (millions of tons)

World census of casting production: http://www.afsinc.org/multimedia/contentMCDP.cfm?ItemNumber=16433

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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

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Casting:

2.Classification of casting methods

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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

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Sand casting

Groover, Fundamentals of Modern Manufacturing

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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)

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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)MagnesiumZinc (420 oC)Tin

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Casting:

3. Sand casting

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Sand casting

https://www.youtube.com/watch?v=HSOtZj2Y8is

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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.


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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.

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Voxeljet: 3D printed sand molds

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Casting:

4. Die casting

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Die casting of aluminum wheel caps

https://www.youtube.com/watch?v=N6ODcxK8_lg

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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.

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Die casting: hot chamber method

: cold chamber method


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Hot Wheels!

Image: http://www.amazon.com/Hot-Wheels-9-Car-Gift-Styles/dp/B006EFMSSM

2.008xWhich processes?

20 for $15 (Amazon);; how is that possible?

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DFM: Integral rivets!

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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

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Casting:

5. Fluidity and cooling

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Surface tension and viscosity

Ux(H) = Ux

Ux(y)

Ux(0) = 0

h

Ux

yU∂

∂= µτ

Surface tension, σ Dynamic viscosity, μWater (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

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Fluid flow considerations for good casting


à If gravity-driven, pressure ‘head’ must be sufficient to overcome flow resistance to fully infiltrate mold cavity.

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Fluid flow considerations for good casting


à Flow must remain laminar to prevent air entrainment.avoidabrupt direction change. Trapped air leads to ‘dross’ (oxide flakes)

à same as μ (viscosity)

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Testing ‘fluidity’ for casting

Al with increasing content of reinforcing particles (SiC)

Kalpakjian and Schmid, Manufacturing Engineering and TechnologyBehera 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

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Demonstration: Cooling in sand casting vs. die casting (‘wood’s metal’)

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Solidification time

Time [s]

Tem

pera

ture

[°C

]20

Sand Casting

10

30

40

50

60

70

80

Die Casting

2000 400 600 800 1000 1200

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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 ⋅VA"

#$

%

&'n


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Solidification: sand cast versus die cast

TM!

SAND

SOLID!

LIQUID!

T0! 0

tsolidify =C ⋅VA"

#$

%

&'2

αsand ~0.01 cm2/s

TM!

METAL

SOLID!

LIQUID!

0T0!

αsteel ~0.1 cm2/sαaluminum ~0.9 cm2/s

tsolidify =C ⋅VA"

#$

%

&'

à Like injection molding, but mold has low thermal conductivity

à Lower-bound (assumes constant mold temp)

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Casting:

6. Solidification and microstructure

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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.


1/Temperature

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Formation of cast microstructure

Columnar

Shell zone (‘Chill’)

grain

yoy d

k+=σσ

Hall-Petch model: smaller grains give higher strengthσ0 = stress to start dislocation movementky = material hardening constantd = grain size

Grain size is inversely proportional to cooling rate.

Shell has finer grains àthinner die cast parts are typically stronger


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Alloys: dendrites and the ‘mushy zone’


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Casting:

7. Defects

2.008xWhat causes the voids?

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Shrinkage!

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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)


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Casting:

8. Investment casting

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Investment (‘lost wax’) casting

Image from http://www.custompartnet.com/wu/investment-casting

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An industrial investment casting process

Screenshot from: https://www.youtube.com/watch?v=cptlGzWYFEk

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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.

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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

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Investment casting of turbine bladesà 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 TechnologyImage 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

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Casting:

9. Comparison and conclusion

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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=109988An 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

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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)

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Reflection: the big four

Sand Investment Die

Rate Medium Low High

Quality Low Medium High

Cost Low Medium High

Flexibility Low-Medium Medium Low

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Top image from: http://www.alcoa.com/global/en/innovation/alcoa_micromill.asp

Article: Automotive News, September 14, 2015.

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Continuous casting of aluminum sheet(Alcoa ‘micromill’ process)

https://www.youtube.com/watch?v=AH2QcNGM87whttp://www.alcoa.com/global/en/innovation/alcoa_micromill.asp

Furnace (molten Al)

Continuously moving sheet

2.008xReferences1 Introduction

Photo of Fire Hydrant by User: KeystonePetPlace on Pixabay. This work is in the public domain.

Photo of Die Cast BMW Car by User: RockyHorror on Pixabay. This work is in the public domain.

Photo of Cast Elbow Pipes by User: Dyanap on Pixabay. This work is in the public domain.

Photo of Wheel Rims by User: Pashminu on Pixabay. This work is in the public domain.

Photo of Jet Engine Turbine by User: Michael Schwarenberger on Pixabay. This work is in the public domain.

Photo of Engine Block by 160SX (160SX (talk)'s file) on Wikimedia. (CC BY-SA) 3.0

Photo of Propeller Blade by User: seehund on Pixabay. This work is in the public domain.

Photo of MIT Class Ring by User: Pigsfly33 on Wikimedia. (CC BY-SA) 3.0

Image of Herakles Statue © 2000–2016 The Metropolitan Museum of Art

Image of Bronze Statue of a Man © 2000–2016 The Metropolitan Museum of Art

2.008xReferencesImage of Bessemer Converter Diagram from "Discoveries & Inventions of the Nineteenth Century" by R. Routledge, published 1900. This work is in the public domain.

Image of Wright Brothers Engine from the National Air and Space Museum © The Smithsonian.

Image of Wright Brothers Flyer from the National Air and Space Museum © The Smithsonian.

Image of US and China Casting Production © American Foundry Society

Image of Global Cast Iron and Steel Production © IKB Deutsche Industriebank AG

2 Classification

Casting Process Classification: Figure II.3 from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Sand Casting Mold Features: Figure 10.2 b) from "Fundamentals of Modern Manufacturing (4th Edition)" by Groover. © John Wiley & Sons Inc. (2010).

2.008xReferences3 Sand Casting

Video of Sand Casting © 2006 - 2016 ChinaSavvy. All Rights Reserved.Sand Casting Mold Features: Figure 10.2 b) from "Fundamentals of Modern Manufacturing (4th

Edition)" by Groover. © John Wiley & Sons Inc. (2010).

4 Die Casting

Video of Die Casting © 2016 Die Castings China

Hot Chamber Die Casting: Figure 11.13 1) and 2) from "Fundamentals of Modern Manufacturing (4th Edition)" by Groover. © John Wiley & Sons Inc. (2010).

Cold Chamber Die Casting: Figure 11.14 1) and 2) from "Fundamentals of Modern Manufacturing (4th Edition)" by Groover. © John Wiley & Sons Inc. (2010).

Image of Ferrari F12 Berlinetta by Ferrari S.p.A. Copyright 2016. All Rights Reserved.

Image of Ferrari Aluminum Chassis by Ferrari S.p.A. Copyright 2016. All Rights Reserved.

2.008xReferences5 Fluidity / Cooling

Gated Casting with Risers: Figure 10.8 from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Spiral Test Casting: Figure 10.9 from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Image of Changing Fluidity Index © 2012 Scientific & Academic Publishing

Temperature Distribution at Mold Wall: Figure 10.10 from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

6 Solidification

Temperature Evolution over Time: Figure 10.1 a) from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Change in Specific Density: Figure 10.1 b) from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

2.008xReferencesCasting Cross-Section: Figure 10.2 b) from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Phase Diagram: Figure 10.4 from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

7 Defects

Shrinkage and Cavity Formation: Figure 10.8 2) and 3) from "Fundamentals of Modern Manufacturing (4th Edition)" by Groover. © John Wiley & Sons Inc. (2010).

Shrinkage Allowances: Table 12.1 from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Common Casting Defects: Figure 11.22 a) - d) from "Fundamentals of Modern Manufacturing (4th Edition)" by Groover. © John Wiley & Sons Inc. (2010).

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8 Investment Casting

Image of Investment Casting Process © 2009 CustomPartNet

Video of Investment Casting Process ©2011 Wisconsin Precision Casting Corporation

Image of Jet Engine Materials © 2016 ATI. All Rights Reserved.

Single Crystal Turbine Blade: Figure 11.25 b) and c) from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Image of Wax Patterns for Turbine Blades © SouthComm Publishing Company 2015.

9 Conclusion

Photo of Threaded Faucet Connection by User: byrev on Pixabay. This work is in the public domain.

Photo of Bearing by User: petitgiovanni on Pixabay. This work is in the public domain.

2.008xReferencesPhoto of Toy Truck by User: da_hammer on Pixabay. This work is in the public domain.

Photo of Sand Cast Parts by User: Graibeard on Wikimedia. (CC BY-SA) 2.0

Photo of Investment Cast Turbocharger Turbine Blades by MarkBolton at English Wikipedia. (CC BY-SA) 3.0

Photo of Valve for Nuclear Power Station by MarkBolton at English Wikipedia. (CC BY-SA) 3.0

Compatison of Casting Methods: Table 11.2 from "Manufacturing Engineering & Technology (7th Edition)" by Kalpakjian, Schmid. © Upper Saddle River;; Pearson Publishing (2014).

Image of Micromill © 2016 Alcoa Inc.

Article on F150 Production © Crain Communications, Inc.

Image of Ford F150 Chassis © 2014 Sustainable Enterprises Media, Inc.

Image of Ford F150 © 2016 Ford Motor Company

Image of Micromill Plant © 2016 Alcoa Inc.

Video of Micromilling © 2016 Alcoa Inc.