CHAPTER 10

1. Fundamentals of Metal Casting CHAPTER 10

2. Topics • Introduction • Solidification of Metals • Fluid Flow • Fluidity of Metals • Heat Transfer • Defects

3. Overview of Casting • Casting uses the idea that a liquid metal can take the shape of any vessel containing it. • When the metal cools it has taken the shape of its container • Casting is one of the most versatile manufacturing processes

4. Advantages of Casting • Low cost and quick • Easier to manufacture • Can produce intricate shapes and internal openings • Can produce parts in one piece • Best suited for composite components

5. Introduction Important Considerations • Flow of Molten Metal • Solidification and Cooling • Type of Mold Material

6. Solidification of Metals • Involves liquid metal turning back in to solid metal • The process is different for Pure metals and alloys • Can be divided into two steps: • Formation of stable nuclei • Growth of crystals Pure Metals • Have a clearly defined melting point • Temperature remains constant during freezing • Solidifies from the walls of the mold toward the center of the part

7. Grain Structure for Pure Metals • Two types of grains are formed for a pure metal • Fine equiaxed grains • Columnar • Rapid cooling at the walls produces fine equiaxed grains • Columnar grains grow opposite of the heat transfer throughout the mold following the chill zone Equiaxed Grains • If crystals can grow approximately equally in all directions – equiaxed grains will grow. • Large amounts of under cooling is needed near the wall of the mold.

8. Illustration of Cast Structures

9. Alloys • Solidification in alloys begins when the temperature drops below the liquidus TL and is complete when it reaches the solidus, TS.

10. Alloys • Within the TL and TS Temperature range, the alloy is like a slushy with columnardendrites

11. Effects of Cooling Rates • Slow cool rates results in course grain structures (102 K/s) • Faster cooling rates produce finer grain structures (104 K/s) • For even faster cooling rates, the structures are amorphous (106 – 108 K/s) • Grain size influences strength of a material • Smaller grains have higher ductility and strength • Smaller grains help prevent hot tearing and/or cracks in the casting

12. Fluid Flow Basic casting system: • Fluid is pored though a pouring basin • Flows though the gating system into the mold cavity Schematic of typical riser-gated casting. Fig : Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification.

13. Fluid Flow Sprue – is a vertical channel though which the molten metal flows downward in the mold Runners– channels that carry the molten metal from the sprue to the mold cavity Gate– is the portion of the runner though which the molten metal enters the mold cavity Risers – serve as reservoirs to supply the molten metal necessary to prevent shrinkage. Principles of fluid flow • Bernoulli’s Theorem • Continuity Flow Characteristics: turbulence is an important consideration in gating systems.

14. Flow Characteristics Reynolds Number is used to quantify this aspect • 0 < Re < 2000 => laminar flow • 2000 < Re < 20 000 =>mixture of laminar and turbulent flow • Re > 20 000 => severe turbulence Techniques for minimizing turbulence • Avoid sudden changes in flow direction • Dross or slag can be eliminated by vacuum casting • Use of filters eliminates turbulent flow in the runner system

15. Fluidity of Molten Metal Fluidity of Molten Metal : The capability of molten metal to fill mold cavities is called fluidity. The following influence fluidity • Characteristics of molten metal • Viscosity • Surface tension • Inclusions • Solidification pattern of the alloy • Casting parameters • Mold design • Mold material and its surface characteristics • Degree of superheat • Rate of pouring • Heat transfer Note: Castability – describes the case with which a metal can be cast to obtain a part with good quality.

16. Fluidity of Molten Metal Fig : 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 the length of the solidified metal, the greater is its fluidity.

17. Heat Transfer Important consideration in casting • Heat flow in the system • Complex • Depends of flow characteristics Solidification Time • A function of the volume of a casting and its surface area • Solidification time = C volume 2 surface area • Effects on solidification time • Mold Geometry • Skin thickness

18. Heat Transfer • Shrinkage – causes dimensional changes and, sometimes cracking, is the result of the following: • Contraction prior to solidification • Contraction during phase changes • Contraction as temperature drops to ambient temperature Fig : Solidified skin on a steel casting. The remaining molten metal is poured out at times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which based on this principle

19. Metallic projections Fins Flash Massive projections Swells Rough surfaces Cavities Internal or external Blow holes Pin holes Shrinkage cavities Discontinuities Cracks Cold or hot tearing Cold shunts Defects Fig : 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.

20. Defective surface (i) Folds Laps Scars (iv) Adhering sand layers Oxide scale Incomplete casting (i) Misruns (ii) Insufficient volume (iii) Runout – due to loss of metal from mold (iv) Temperature too low when metal is poured (v) Metal is poured to slow Incorrect Dimensions or Shape Improper shrinkage allowance Pattern mounting error Irregular contraction (iv) Deformed pattern Warped casting Inclusions – form after melting, solidification and molding Non-metallic Harmful Stress raisers Reduce the strength of the casting May react with: Environment Crucible Mold material Slags and other foreign material entrapped in the metal can become inclusions too.

21. Casting Defects Fig : Examples of common defects in castings. These defects can be minimized or eliminated by proper design and preparation of, olds and control of pouring procedures.

22. Fig : 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). POROSITY:Methods of removal of porosity by (a)Internal and (b)External chills

23. THE END