1. Production and Casting of Metals ISEN 3723 Dr. Darrell R. Wallace Department of Mechanical & Industrial Engineering College of STEM Youngstown State University

2. SOLIDIFICATION OF METALS Casting Processes

3. Cooling Curves Kalpakjian, Manufacturing Engineering and Technology, 6 th Ed.

4. Phase Diagram and Cooling Curve

5. (Inverse) Lever Rule

6. Chvorinov’s Rule Predicts solidification time for molded shapes t s =B(V/A) n Where: t s = solidification time (seconds) B = mold constant V= volume of cast part A = surface area of cast part

7. Grain Structure in Casting Chill Zone – Rapid cooling near the surface creates many nucleation sites and many small, randomly oriented grains. Columnar Zone – Directionally oriented grains radiating inward from the surface of the part. Equiaxed Zone (not shown) – Randomly oriented, spherical crystals. (isotropic properties)

8. Pouring Ladles

9. Challenges of Molten Metal Hot metal readily forms oxides (dross or slag) –Can be carried into the mold –Can be controlled by pouring methods –Control of temperature and atmosphere can slow creation of slag

10. Challenges of Molten Metal Dissolved gases –Porosity –Can be controlled by: Vacuum degassing Gas flushing “Killing” - Reacting trapped gas with material that will form buoyant compound that will float to surface –Oxygen removed from copper by adding phosphorous –Oxygen removed from steel by adding aluminum or silicon

11. Challenges of Molten Metal Temperature Control –Temp too Low Misruns Cold shuts –Temp too High Excessive mold wear Higher reactivity of molten metal Penetration defects (excessive flash or entrapped sand)

12. SHAPE CASTING PROCESSES Casting Processes

13. Common Shape Casting Processes

14. Effect of Casting Process on Properties

15. Basic Casting Terminology

16. Runners and Gates Should distribute metal to all parts of casting(s) Should control flow rate Should be laminar flow

17. Pattern and Cores

18. Gate Filtering

19. Solidification Shrinkage

20. Risers

21. Provide additional material to fill in as shrinkage occurs Live risers receive hot metal directly entering the mold Dead risers are filled by hot metal that has already passed through the mold Blind risers are closed to the atmosphere Open risers penetrate through the mold cavity Side risers feed the part through the runner / gate system Top risers are attached directly to the part

22. Risers

23. Riser Design Risers should take longer to cool than the casting: For a cylindrical riser:

24. Risering Aids Internal Chills – Extract or absorb heat through the mold walls External Chills – Thermal sinks placed inside the mold cavity (become part of the finished casting) Insulating Sleeves – Limit heat transfer out of the riser Exothermic Materials – Add heat to the riser

25. Shrinkage Allowance The amount by which the size of a mold must differ from the final cast part is called the allowance. Shrinkage Allowance accounts for shrinkage of the material as it cools from the solidification temperature to room temperature. DL=  Twhere:  = coeff. of thermal expansion  T= T solidification – T room Typical allowances:

26. Draft Allowance Draft allows the cast part to be removed from the mold more easily. Draft taper is typically at least 1 degree.

27. Finish Allowance Where a part requires a precise surface or dimensional tolerance, most castings will have to be machined. In these cases, extra material must be cast so that it may be machined to size. The extra material provided is called a finish or machining allowance.

28. Combined Allowances

29. Design Considerations in Castings Location / Orientation of Parting Plane affects: –Number of cores –Method of supporting cores –Gating –Casting weight –Dimensional accuracy –Ease of molding

30. Good Parting Line Choice

31. Eliminating Cores

32. Effect of Alternate Parting Lines

33. Section Change Transitions Where the section thickness of a cast part changes, two problems can arise: –Stress concentrations –Hot spots

34. Section Change Transitions

35. Hot Spots

36. Hot Spots and Shrinkage Cavities

37. Risers and Hot Spots

38. Rib Placement

39. Minimum Section Thicknesses

40. Sand Casting

44. Sand Casting Patterns Patterns may be made from a variety of materials: –Wood –Metal –Hard Polymers –EPS (Styrofoam)

45. Types of Sand Casting Patterns One-Piece

46. Types of Sand Casting Patterns One-Piece (Using a Follow Board)

47. Types of Sand Casting Patterns Split Pattern

48. Types of Sand Casting Patterns Match-Plate

49. Types of Sand Casting Patterns Cope-and-Drag Split Patterns

50. Types of Sand Casting Patterns Loose Piece Pattern

51. Types of Sands Sands can be comprised of: ordinary silica (SiO2), zircon, olivine, or chromite and may be compounded with additives to meet four requirements: –Refractoriness –Cohesiveness –Permeability –Collapsibility

52. Green Sand Contains bonding agents and water Typical “green sand” is: –88% silica –9% clay –3% water

53. Mullers ContinuousBatch

54. Sand Parameters Grain Size – measured by sifting sand through sieves Moisture Content – measured with moisture meter Clay Content – measured by weighing a sample of sand before / after washing Permeability – AFS permeability number measured using “standard rammed sample” (Green) Compressive Strength – measure of mold strength before pouring Hardness – resistance of packed sand to penetration

55. Permeability Tester

56. Sand Hardness

57. Desirable Properties of Sand- Based Molding Materials Inexpensive in bulk quantities Retains properties through transportation and storage Uniformly fills flask or container Can be compacted or set by simple methods Sufficient elasticity to remain undamaged during pattern removal Can withstand high temperatures and maintain dimensional stability until solidification Sufficiently permeable to allow gases to escape Sufficiently dense to prevent metal penetration Sufficiently cohesive to prevent wash-out of mold material into the pour stream Chemically inert to the metal being cast Can yield to solidification and thermal shrinkage, preventing hot tears and cracks Recyclable

58. Sand Casting Defects Sand Expansion Defects – occur on large, flat portions of castings where large expansion must occur in one direction. Can be alleviated by: –Careful selection of sand geometry (some sands “slide” past each other more easily –Use of low-expansion sands (zircon or olivine) –Additional clay added to absorb expansion –Volatile additives in sand mixture (burn off and create space) Voids or Blows – due to trapped gas and low sand permeability Penetration – overly fluid pour traps sand particles in melt Hot tears or cracks – high solidification shrinkage resisted by mold with poor collapsibility

59. Sand Packing Hand ramming Lift / Drop (“jolting”) Squeezing –Flat plate –Flexible-diaphragm

60. Automatic Match Plate Molding

61. Flaskless Molding

62. Common-Runner Flaskless Pours H-process – long, horizontal runners connect multiple cavities –Large variations in fill temperature Stack Molding – Molds are stacked and share a common vertical sprue

63. Sand-Cast Parts

64. Green Sand Casting Summary

65. SodiumSilicate-CO 2 Molding Sand has 3%-6% sodium silicate (“water glass”) added as binder. –Mixed in a standard muller –Addition of high-concentration CO2 gas causes binder to harden in a matter of seconds: –Very strong mold, but poor collapsibility

66. No-Bake / Air Set / Chemically Bonded Sands A variety of other binders that cure at room temperature can be used to hold sand together Provide greater mold strength than green sand. Added cost and time.

67. Shell Casting Individual grains of sand are pre-coated with a phenolic resin and heat-sensitive liquid catalist. Sand spread on top of a heated (450 – 600 deg. F) metal pattern. Heat bonds the material near the pattern. Excess sand is poured off. Thin shell is removed and placed in an oven for further curing.

68. Shell Casting

70. Shell-Mold Casting Summary

71. V-Process (Vacuum Molding)

72. Eff-Set Casting Wet sand with minimal clay is packed around a pattern Pattern is removed and surface is sprayed with liquid nitrogen to freeze surface Molten metal is poured while mold is still frozen. Low binder cost and excellent shakeout, but not being used commercially.

73. Cores and Core-Making

74. Cores to Make Holes

75. Dump-type Core Box (Dry Sand Cores)

76. Chaplets

77. Three-Segment Flasks

78. Undercuts with a Core

79. Plaster Casting Expendable Plaster Molds –Can only be used for low-melting alloys –Excellent dimensional characteristics

80. Antioch Process Variation of plaster mold casting where mold is comprised of 50% plaster and 50% sand. Molds are cured in an autoclave to reduce solidification time and improve permeability Foaming agents can be added to increase volume and permeability by 50-100%

81. Ceramic Mold Casting Similar to plaster mold casting Ceramic material has higher temperature resistance. Excellent dimensional and surface finish characteristics. “Shaw Process” uses slurry with volatiles mixed in. Partially set mold is “fired” with a torch to set the ceramic and create a fine network of microfractures that offer excellent permeability

82. Summary of Ceramic Mold Casting

83. Investment Casting

84. Lost Foam Casting

85. Die Casting

86. Permanent Mold Casting