1. Part II Metal-Casting Processes and Equipment

2. CASTING THE PROCESS OF POURING MOLTEN METAL INTO A MOLD CAVITY Why is casting is preferred over other manufacturing methods? 1. Can produce complex shapes 2. Can contain hollow parts 3. very large parts are produced in one piece 4. can use materials that are difficult to machine 5. Is competitive with other manufacturing processes 6. mechanical properties can equal those of other manufacturing processes

3. EXAMPLES OF CAST PARTS

4. FIGURE II. 1 Examples of cast parts
FIGURE II.1 Examples of cast parts. (a) A die-cast aluminum transmission housing. (b) A tree of rings produced through investment casting. Source: (b) Courtesy of Romanoff, Inc.

5. FIGURE II.2 Cast parts in a typical automobile.

6. Chapter 10 Fundamentals of Metal Casting

7. Factors that affect the events during casting 1. Type of metal 2
Factors that affect the events during casting 1. Type of metal 2. Thermal properties of metal 3. Thermal properties of mold 4. Relation between volume and surface area 5. Shape of mold

8. PURE METALS clearly defined melting or freezing point solidify at a constant temperature the solidification front moves through the molten metal from the mold walls in toward the center Shrinking occurs and might lead to cracking or porosity

9. FIGURE (a) Temperature as a function of time for the solidification of pure metals; note that freezing takes place at a constant temperature. (b) Density as a function of time.

10. SOLIDIFICATION INSIDE A MOLD Mold walls are cooler Solidified skin Grains grow in a direction opposite to that of heat transfer out through the mold

11. FIGURE Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid–solution alloys; and (c) structure obtained by using nucleating agents. Source: After G.W. Form, J.F. Wallace, J.L. Walker, and A. Cibula.

12. FIGURE Development of a preferred texture at a cool mold wall; note that only favorably oriented grains grow away from the surface of the mold surface.

13. SOLIDIFICATION OF ALLOYS dendrites slow cooling rates lead to coarse dendrites as grain size decreases: 1. ductility increases 2. microporosity decreases 3. tendency to crack decreases

14. FIGURE Schematic illustration of alloy solidification and temperature distribution in the solidifying metal; note the formation of dendrites in the mushy zone.

15. FIGURE (a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note that after 11 min of cooling, dendrites reach each other, but the casting is still mushy throughout; it takes about 2 h for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds; note the difference in solidification patterns as the carbon content increases. Source: After H.F. Bishop and W.S. Pellini.

16. FIGURE Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equiaxed dendritic; and (c) equiaxed nondendritic. Source: Courtesy of D. Apelian.

17. FLUID FLOW pouring basin gating system (sprue, runners, gates) traps contaminants by adhering them to walls top riser (feeders) serves as a reservoir for molten metal

18. FIGURE 10. 8 Schematic illustration of a typical riser-gated casting
FIGURE Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification.

19. FLUIDITY OF MOLTEN METAL Capability of molten metal to fill mold cavities molten metal characteristics that influence fluidity 1. Viscosity 2. Surface tension 3. Inclusions 4. Solidification pattern

20. Casting parameters that influence fluidity 1. Mold design 2
Casting parameters that influence fluidity 1. Mold design 2. Mold material 3. Surface characteristics of mold 4. Degree of super heat 5. Rate of pouring 6. Heat transfer

21. TESTS FOR FLUIDITY no test is accepted universally Fluidity is the distance the metal flows along a channel before it solidifies fluidity depends upon 1. thermal properties of metal 2. thermal properties of mold 3. design of channel

22. FIGURE 10. 9 A test method for fluidity using a spiral mold
FIGURE A test method for fluidity using a spiral mold. The fluidity index is the length of the solidified metal in the spiral passage; the greater the length of the solidified metal, the greater is the metal’s fluidity.

23. SOLIDIFICATION TIME function of volume and surface area proportional to volume inversely proportional to surface area a large sphere solidifies at a much slower rate than a smaller sphere T =  d3/d2

24. FIGURE 10. 11 Solidified skin on a steel casting
FIGURE Solidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: After H.F. Taylor, J. Wulff, and M.C. Flemings.

25. DEFECTS (7 basic categories identified by capital letters) A
DEFECTS (7 basic categories identified by capital letters) A. Metallic projections B. Cavities C. Cracks D. Defective surface E. Incomplete casting F. Incorrect dimensions G. Inclusions

26. FIGURE 10. 12 Examples of hot tears in castings
FIGURE Examples of hot tears in castings. These defects occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the molds and cores. Exothermic (heat-producing) compounds may be used (as exothermic padding) to control cooling at critical sections to avoid hot tearing.

27. FIGURE Examples of common defects in castings; these defects can be minimized or eliminated by proper design and preparation of molds and control of pouring procedures. Source: After J. Datsko.

28. FIGURE P10.55

29. POROSITY caused by: 1. Shrinkage 2
POROSITY caused by: 1. Shrinkage 2. Dissolved gases can be reduced by: 1. Provision of adequate quantities of liquid metal 2. Internal or externl chills

30. FIGURE Various types of (a) internal and (b) external chills (dark areas at corners) used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c).