Materials Processing Families

Materials Processing Families Casting Expendable Mold Sand, Shell, Investment, Lost Foam Casting Multiple Use Die, Permanent Mold Casting MaterialRemoval Mechanical, Non Traditional Deformation Hot Bulk Forming, Cold Forming Consolidation Welding, Brazing, Soldering, Mechanical Joining, Adhesive Joining

Materials Processing Families - General Comparison Casting  Can do complex shapes x Subject to internal defects Material Removal  Outstanding dimensional accuracy x Produce scrap as material is processed Deformation  High production rates x Require powerful equipment, special tooling. Consolidation  Can make complex products by assembly. x Joint material(s) may differ from parts.

Manufacturing Processes Chap. 10 - Fundamentals of Casting

Fundamentals of Casting • Definition: • The process and product when molten metal is poured and solidified followed by its removal from a mold. • Advantages • Can produce • complex shapes • Hollow parts • Irregular curved surfaces • Parts difficult to machine. • Cast parts can be very small, (from a zipper tooth) to very large, (a large ship propeller.)

Key considerations in casting: • Solidification / cooling of metal in the mold affected by properties of the metal. • Flow of molten metal in the mold cavity. • Effects of design / type of mold > affects the rate of cooling.

Basic Requirements in Casting • Mold Cavity: • must have proper shape, allowance for shrinkage. • Melting Process: • provide material of proper quality, quantity, temperature, cost. • Pouring Technique: • must be done properly to avoid gas entrapment, filling cavity. • Solidification Process: • controlled to avoid internal porosities/voids. • Mold Removal Ability • Cleaning, Finishing, Inspection Required

Solidification - Pure Metals • Pure Metals have a clearly defined melting point. • Solidify at a constant temperature. • When metal reaches its melting/freezing pt, temp remains constant while latent heat of fusion is given off. • Solidification boundary moves through the metal from walls to center. • When solidification occurs at a pt, cooling resumes.

Solidification - Pure Metals • Initially, rapid cooling occurs at the walls, producing a shell. • Solidification front moves as a plane front. • Grains grow in direction opposite to heat transfer in the mold. • Grains growing at a more favorable angle will grow preferentially, and are called columnar grains. • With less heat transfer driving the solidification, equiaxed coarse grains are formed toward the center .

Solidification -Alloys • Solidification starts at TL and stops at TS. • Between the two temperatures, alloy is mushy with columnar dendrites (3D Structures that interlock once cooled). • Mushy zone: Defined in terms of the temperature difference or freezing range:: FR = TL – TS

Slow cooling rates yield: Coarse dendritic structures Large spacing between dendrites Fast cooling yields: Finer structure with smaller dendrite spacing Smaller grain size Higher strength and ductility of casting Less tendency to crack during solidification Effect of Cooling Rates

Inoculation • Recall that solidification occurs in two stages: Nucleation and Growth

Inoculation • Nucleation: when a stable solid particle forms from within the molten liquid. • Interface surfaces are formed between the newly solidified metal and the parent liquid metal.

Inoculation • Nucleation usually starts at existing surfaces where solidification occurs without a fully wrap-around surface. (i.e. walls, solid impurity particles, called inoculants.) • Since nucleation produces a crystal or fine grain, and fine grains are typically stronger and have better properties, we can promote nucleation by adding impurities before pouring.

Inoculation • Growth: occurs as the latent heat of fusion is extracted from the liquid metal. • We can control the direction, rate and type of growth based on how heat is extracted. • By directional solidification, we can produce a sound casting.

Fluid Flow Stages: • Pouring Basin/Cup: where metal is poured. • Sprue: vertical, tapered channel through which metal goes towards mold. • Runner: channels that carry metal from sprue to mold cavity • Riser: molten metal reservoir to supply metal to compensate for shrinking • Gates: where the molten metal enters the mold. • Vents: provide escape for trapped gases in mold cavity. • Casting: the end result.

Mold Design Characteristics • Must ensure that molten metal reaches every location in the mold. • Must ensure that contaminants are filtered/trapped. • Must minimize premature cooling, turbulence and gas entrapment.

Fluidity of Molten Metal • Capability of Molten metal to fill mold cavities. • Based on two factors: • Molten metal characteristics • Casting parameters

Molten metal characteristics • Viscosity • (fluidity decreases with viscosity) • Surface Tension • (fluidity decreases with ST) • Inclusions • (have adverse effect) • Solidification Pattern of Alloy • (Metals and alloys with short freezing range have better fluidity)

Casting parameters • Mold Design • (layout of mold elements) • Mold Material & Surface Qualities • (higher thermal conductivity decreases fluidity) • Superheat • (higher superheat improves fluidity, delaying solidification) • Rate of pouring • (slower yields lower fluidity) • Heat transfer • (affects viscosity of metal)

Solidification Time • Defined by Chvorinov’s Rule: • f (casting volume, surface area): Ts = C ( Volume / Surface Area)n • C = constant based on mold material, metal properties (including latent heat & temp). • n = value between 1.5 and 2

Example • What is the solidification time of a circular plate, 10 in. diameter and 1 in. thick, if the mold constant is 45 min/in2 and n = 1.6? V plate =  r2h = 78.53 in3 SA plate = 2 r2 + 2rh = 157.07 + 31.41 = 188.48 in2 So, 45 min/in2 ( 78.53 in3 / 188.48 in2)1.6 = 11.1 min. Note: Faster cooling rates produce higher strength castings.

Shrinkage • Due to thermal expansion characteristics, metals contract during cooling, as a result of the following: • contraction prior to solidification • Usually not a problem, as risers compensate. • contraction during phase change • Significant amount of contraction occurs. • contraction of solid metal as temperature drops to ambient (largest amount of shrinkage!) • Not all metals contract during solidification: • gray cast iron expands.

Defects in Castings • Metallic Projections • Cavities • Discontinuities • Defective Surface • Incomplete Casting • Incorrect dimensions/shape • Inclusions

Defects in Castings • Porosity • Caused by shrinkage, gases or both. • Reduces ductility and surface finish. • Can be reduced by: • providing an adequate amount of metal feeding the mold. • Use of chills are a good way of reducing shrinkage porosity • Chills increase rate of solidification in critical regions. • Made of same material as casting, will become part of it.

Defects in Castings • Gases • Are a result of reactions of molten metal with mold material. • Can remove by flushing an inert gas through mold. • Melt/pour metal in vacuum.

Risers • Important Part of the Casting System. • Are added reservoirs to feed liquid to casting as it solidifies. • Must promote directional solidification. (from the extremities toward the riser). Thickest sections are last to freeze, should feed directly into this section. • Must be sized properly to serve purpose and also conserve metal. • Must be properly located. • Can be top, blind, dead (cold), live (hot). • Compensate for shrinkage locally and overall. • Must solidify after the casting.

Risers • Since thickest regions solidify last, risers must feed directly into these regions. • Can be top, side, open, blind, dead (cold), live (hot). • Top: sits at top of casting. • Side: located adjacent to mold cavity. • Open riser: if open to the atmosphere. • Blind: contained completely inside the mold. • Dead: fill with metal that has already flowed through mold cavity. • Live: Receive the last hot metal that enters the mold.

Riser Aids Chills • Used to assist risers in their function, reduce their number. • Speed up cooling of regions in the casting. • Effects: either accelerating solidification of casting or retarding that of the riser. • External Chills • Material w/ high heat capacity & thermal conductivity, placed in mold adjacent to casting to accelerate solidification in various regions. • Internal Chills • Pieces of metal (the same as the casting material) • Placed to absorb heat and promote rapid solidification. • Are in close vicinity to the casting, become part of it. • Are later cut off.

Riser Aids How to slow down cooling of a riser: • Use an open riser instead of a blind riser. • Place insulating sleeves around riser. • Surround riser with material that supplies heat to riser to keep it hot. NOTE: Risers may not always be necessary. • (i.e. alloys with large freezing range). • In die casting, positive pressures provide feeding action to compensate for shrinkage.

Patterns • Almost all expendable mold processes start with a pattern. • It is a duplicate of the part to be cast, modified depending on the casting process and material being cast. • It is used to create the mold cavity which the molten metal will occupy and solidify in.

Patterns • Must include allowances in its design: • For shrinkage: (~ 2% or 1/4” per foot is typ.) • A shrinkage rule is used to make them; larger than std. Rule, has compensation built in. • Location of parting line must be carefully selected. • Perpendicular surfaces must have draft to facilitate part removal: (1 deg. or 1/16” min on any surface. • For machining /finishing allowance: ~1/8”. • For distortion.

Design Guidelines for Parts to be Cast via Expendable Molds 1. It is desirable for parting line to be along a flat plane rather than contoured. Let it be at the corners or edges of the casting if possible to minimize the flash at the parting line. 2. Minimize the use of cores. 3. Ideally, casting should have uniform thickness in all directions. Since it is rarely possible, sections should transition smoothly. 4. Intersections will cause hot spots (areas which cool slower than others, causing abnormal shrinkage.) Definite problem area for serious defects: porosities, shrinkage cavities.

Design Guidelines for Parts to be Cast via Expendable Molds 5. Sections should transition smoothly into each other. Avoid large regions which yield hot spots (they develop shrinkage cavities and porosities). 6. Intersecting ribs contract in opposite directions and promote cracking during solidification. Staggered ribs favors distortion without high residual stresses. 7. Avoid sharp corners: they act as stress raisers and can cause cracking during solidification. Use fillets at these areas(~1/4 typ.) 8. Avoid large flat areas that can warp during cooling due to temperature gradients.

Design Guidelines for Parts to be Cast via Expendable Molds • 9. Provide draft to enable removal of the pattern. 0.5 to 2 degrees typically. • 10. Tolerances should be as wide as possible, within the limits of good part performance. Typical tolerances are +/- 1/32". • 11. Leave machining allowance if such operations are required. Typically .100" to .200". • 12. May need to stress relief part to avoid distortions in critical regions.

End of Chapter 13

Materials Processing Families