Casting “Net Shape” or “Near-Net Shape” Process Advantages:

1. Casting • “Net Shape” or “Near-Net Shape” Process Advantages: • Product is ~finished right out of mold. • High complexity with few steps (usually) • Little if any wasted material (vs. machining) • General Casting Disadvantages: • Expensive and time-consuming patterns/molds/dies: not for low quantities • Solidification issues: shrinkage, porosity, ~low strength, brittleness • Limited alloy selection • Limited precision and surface finish capability • Some methods require many steps (e.g., Investment casting)

2. Casting: Solidification • Grains perpendicular to wall shut-off other grains, so columnar structure naturally develops perpendicular to mold wall. • Grain boundaries tend to be weak  columnar castings tend to be brittle (unless loaded parallel to the column direction, as in turbine blades). • Equiaxed structure usually preferred for strength, can be achieved with innoculating agents and/or fast cool. Kalpakjian Kalpakjian

3. Casting Impurities • Slag/dross: • Metal oxides that form brittle inclusions within casting • Slag floats, so skim off top and/or pour from bottom of ladle • Porosity: trapped gas. Minimize by these methods: • Design part and mold to minimize turbulence of molten metal as it enters mold • Don’t overheat the molten metal (dissolves more gas) • Melt in a vacuum ($$$) • Melt in a protective atmosphere ($$) • Mix in scavenging agents to collect gas bubbles • Pour smoothly (sand casting, permanent mold casting, ) • Pressurize the “pour” (die casting)

4. Fluidity • A measure of how far molten metal (the “melt”) will travel before solidifying. • Determines: • Minimum thickness of sections • Level of fine detail • Runner placement and number • We want melt to fill mold completely, then solidify. • If melt freezes before filling mold, it is called a “misrun” or “cold shunt”. These are most likely in thin sections and far from the pouring cup. http://www.zincdiecasting.umicore.com/TechnicalInfo http://www.castech.fi/CastCAE/CAEvalidation.html

5. Patterns and Molds • Pattern: a model of the actual part, from which the mold is made. • Often made of Wood or Wax. • Mold: an “negative” model of the actual part, into which the melt is poured. • May be made of sand/clay, ceramic, metal…

6. Sand Casting: Patterns DeGarmo

7. Casting: Design Practices • Draft angle (1-3 deg) is needed to allow removal of pattern from mold (sand casting), or removal of part from mold (e.g, die casting) • Upon solidification, thicker sections tend to form cavities inside unless fed by riser or directionally solidified. • Aim for the same wall thickness everywhere or plan solidification direction carefully. • Offset intersection of ribs to achieve uniform thickness. Kalpakjian Kalpakjian Kalpakjian

8. Casting: • Directional Solidification • Porosity and cavities form when melt cannot reach solidifying/contracting regions. • Chills used to initiate local solidification and achieve directional solidification away from the chill. • Risers feed melt opposite to solidification direction. Kalpakjian Schey

9. Porosity in Castings- Dissolved and entrapped air during pouring or injection- Many solutions but still a common problem NADCA www.vidisco.com www.eng.ysu.edu

10. Porosity in Castings: detected after machining Can be a major cause of scrapped parts. Design the supply chain to catch problems quickly. http://www.eere.energy.gov/industry/metalcasting/pdfs/porosity_preven.pdf

11. Casting Defects: Misrun formation (Tschopp, Wang, DeWyse)

12. Other Casting Defects Warped Part: not enough draft angle, part forced out of mold/die. Solidification Cracks: Non-uniform shrinkage in a complex casting http://www.zincdiecasting.umicore.com/TechnicalInfo http://www.zincdiecasting.umicore.com/TechnicalInfo Shrinkage Cavity (External): Melt not fed to areas that solidify last. Need risers or redesign cooling of mold. http://www.panyo.com/ME1/wheel.htm http://www.backyardmetalcasting.com/defects.html

13. Casting: Riser Design Riser supplies molten metal to shrinkage areas to prevent cavities and porosity. Riser must solidify after part: Typically: Triser = 1.25 * Tcasting Chvorinov’s Rule Solidification time = B * (V/A)n B = mold constant n = 1.5 – 2.0 V = volume of casting A = surface area of casting Mold constant: Found from experiments with same metal and mold materials. Use the same constant for part cavity and riser. Worked Example

14. Parting Line Placement • Part Design and Parting Line Placement (Design) are best done simultaneously. • Factors: • Achieve design requirements. • Simplicity of connection between mold sections (flat plane is best). • Minimize number of Mold sections. • Minimize number of Cores for hollow parts (e.g., a cylinder). • Achieve required draft angles and no undercuts. • Aesthetics, including ease of removing Flash. • Tolerances: features that cross a parting line have looser tolerances than • features within a mold section.

15. Sand Casting: Parts of a Sand Mold (expendable mold) Key terms: Flask, Cope, Drag, Sprue, Runner, Gate, Riser, Mold Cavity, Core, Parting Line, Draft (not shown). Kalpakjian

16. Casting: Internal Cavities in Engine Block • Cylinder holes • Cylinder cooling jackets • Intricate cooling passageways http://www.msm.cam.ac.uk/phase-trans/2004/cast.al

17. Casting: Internal Cavities in Cylinder Head • Intake/Exhaust ports • Cylinder head cooling passageways

18. Casting: Internal Cavities in Cylinder Head • Intake/Exhaust ports • Cylinder head cooling passageways

19. Casting: Internal Cavities • Turbocharger housing • Turbine blade cooling passages http://www.tpg.unige.it/research/blade_cooling.jpg

20. Sand Casting: Cores Inline 6 Cylinder Engine GM V-8 Engine http://www.slantsix.org/articles/dutra-blocks/slant-blocks.htm Turbine Housing (~60 cores) www.moderncasting.com/MoreInfo/0206

21. Shell-Molding Process DeGarmo

22. Kalpakjian • Investment Casting • aka “Lost-wax” casting • Unlimited design freedom since draft angles, cores, parting lines, etc., are ~irrelevant • Accurate parts with good surfaces • Many steps • Patterns and molds are expendable • Expensive

23. Investment Casting a Turbine Rotor Kalpakjian/Howmet Corp Wax pattern of turbine rotor Cut-away of ceramic mold applied over over wax pattern Cut-away showing wax melted out of mold. (Metal then poured into mold.) Finished turbine rotor, near-net shape

24. Turbine Blade Casting Kalpakjian Directional solidification Directional solidification for single-crystal blade Single-crystal blade with a spiral attached

25. Single Crystal Silicon “Boule”Directionally solidified from bottom to top as a single crystal (no grain boundaries anywhere).Silicon wafers cut from the boule, made into semiconductor devices (microchips, solar cells, etc.) Kalpakjian

26. Permanent Mold Casting • No pattern is needed, saving time and cost • Mold is machined directly out of cast-iron (adding time and cost) • Mold complexity is limited, 2-3 deg draft angles needed • Molten metal is gravity fed into mold • Good dimensional accuracy and surface finish • Castings cool quickly so strength tends to be good • Molds last 10,000 – 100,000 parts if casting a soft metal (aluminum, zinc) • Special graphite molds ($$) may be made for casting steel parts (unusual) www.aurorametals.com www.offshoresolutions.com

27. Permanent Mold Casting: Aluminum piston Risers As cast After machining Kalpakjian

28. Die Casting • Molten metal is injected at high pressure (2000-30000psi) • Mold machined from tool steel ($$$ and time) • Molds last ~100,000 parts • Difficult to modify once made • Very accurate dimensions, excellent surface finish, intricate details • Aluminum and zinc most commonly cast (steel would erode mold) • Aluminum melts at ~1050F, Zinc at ~700F • Both are ~brittle when diecast • Part size is limited by injection cylinder size (20 lb max) and clamping force (P*A) • No risers needed (hi-pressure runners feed metal) • Slides/cores used to make holes parallel to parting line • Air is vented along parting line, but porosity is often a problem • Very fast production rates possible, fastest of any casting method • Expensive dies/molds and machines: only suitable for mass production

29. Die Casting Advantages • High volume at high speed • Duplicates intricate design details • No pattern • Long mold life: ~100,000 cycles http://www.imaging-resource.com/PRODS/E20/ZMETAL.JPG www.aluminum.org www.kurt.com

30. Die Casting Limitations • Complex and large machinery: expensive • Molds (dies) machined from hardened tool steel: expensive • Molds cannot take extreme heat so “melt” limited to low-melting point alloys: zinc, aluminum, zinc-aluminum, and copper alloys. • Effects of high pressure  limited part size samkwangprecision.en.ec21.com www.atplonline.com

31. Die Casting: Cold-Chamber Process: aluminum alloys Kalpakjian

32. Die Casting: Hot-Chamber Process:zinc alloys(cooler tempsallows plunger and cylinder chamber to stay in melt for auto-refill) Kalpakjian

33. (a) Toggle mechanism- Separating force = Pressure * Area = 400 to 4000 tons (800,000 – 8,000,000 lb)- As in Vise-grip, toggle multiplies clamp force many times. Double Toggle.- Keeps die-halves from separating, minimizing “flash” Kalpakjian NADCA

34. Die (Mold) Design- alignment pins- “slides” make holes perpendicular to die-separation direction. www.toolingtec.com www.toolingtec.com www.toolingtec.com

35. Die (Mold) Design: Cooling Types- Spray Cooling (and lubricant)- Internal Cooling Benefits:- Faster cycle times- Control solidification directions - Longer die life http://www.toshiba-machine.co.jp/english/product/diecast http://www.diecasting.org/design

36. Common Alloys for Die-Cast Parts-Aluminum: A380, 1050F melt- Zinc: Zamac, 720F melt- Both are generally brittle- Steel/Iron rarely done as they require a Graphite mold ($$)Common Alloys for Dies (molds) - Specialty Tool Steels to limit erosion of surface- Typically require Electro-discharge machining (extremely hard).

37. Die Casting: part and runners NADCA

38. Explosion Risk Water trapped under hot metal  Water expands to steam (1500x volume)  Explosion and spray out of the furnace  Possible secondary explosion Avoid water near a casting operation NADCA

39. Costs Comparison for Different Casting Processes Kalpakjian

40. Casting Process Selection

41. References DeGarmo: E.P. DeGarmo et al, Materials and Processes in Manufacturing, Wiley, 2003. Schey: J.A. Schey, Introduction to Manufacturing Processes, McGraw-Hill, 2000. Kalpakjian: http://www.nd.edu/~manufact/index3.htm NADCA: North American Die Casting Association Introduction to Die Casting CD Tschopp, M.A., Wang, Q.G., DeWyse, M.J., “Mechanisms of Misrun Formation in Aluminum Lost Foam Castings,” AFS Transactions, vol 110, pp 1371-1386 (2002).