Casting Process: Brief History, Capabilities, Casting Processes, Metals processed by casting, Basic Steps, Pattern Making, Sand Casting and Casting Defects

 Casting basics

• Patterns and molds

• Melting and pouring analysis

• Solidification analysis

• Casting defects and remedies

Casting: Brief History

• 3200 B.C. – Copper part (a frog!) cast in Mesopotamia. Oldest known casting in existence

• 233 B.C. – Cast iron plowshares (in China)

• 500 A.D. – Cast crucible steel (in India)

• 1642 A.D. – First American iron casting at Saugus Iron Works, Lynn, MA

• 1818 A.D. – First cast steel made in U.S. using crucible process

• 1919 A.D. – First electric arc furnace used in the U.S.

• Early 1970’s – Semi-solid metalworking process developed at MIT

• 1996 – Cast metal matrix composites first used in brake rotors of production automobile

Complex, 3-D shapes

• Near net shape

• Low scrap

• Relatively quick process

• Intricate shapes

• Large hollow shapes

• No limit to size

• Reasonable to good surface finish

Capabilities

• Dimensions

– sand casting - as large as you like

– small - 1 mm or so

• Tolerances

– 0.005 in to 0.1 in

• Surface finish

– die casting 8-16 micro-inches (1-3 μm)

– sand casting - 500 micro-inches (2.5-25 μm)

Casting Processes

• Sand

• Shell

• Plaster

• Ceramic

• Investment

• Lost foam

• Pressure

• Vacuum

• Die

• Centrifugal

• Squeeze

• Semi-solid

• Single crystal

• Directional solidification

• Slush

• Continuous

Metals processed by casting

• Sand casting – 60%

• Investment casting – 7%

• Die casting – 9%

• Permanent mold casting – 11%

• Centrifugal casting – 7%

• Shell mold casting – 6%

Casting: Basic Steps

• Basic steps in casting are:

– Preparation of pattern(s), core(s) and mold(s)

– Melting and pouring of liquefied metal

– Solidification and cooling to room temperature

– Removal of casting - shakeout

– Inspection (for possible defects)

Pattern Making

• Pattern is a replica of the exterior surface of part to be cast – used to create the mold cavity

• Pattern materials – wood, metal, plaster

Pattern Making

• Pattern usually larger than cast part

Allowances made for:

– Shrinkage: to compensate for metal shrinkage during cooling from freezing to room temp

Shrinkage allowance = aL(Tf – T0)

expressed as per unit length for a given material

a = coeff. of thermal expansion, Tf = freezing temp

T0= room temp

e.g. Cast iron allowance = 1/96 in./ft

aluminum allowance = 3/192 in./ft

• Pattern allowances made for:

– Machining: excess dimension that is removed by machining; depends on part dimension and material to be cast

e.g. cast iron, dimension 0-30 cm, allowance = 2.5 mm; aluminum, allowance = 1.5 mm

– Draft: taper on side of pattern parallel to direction of extraction from mold; for ease of pattern extraction; typically 0.5~2 degrees

Sand Casting

• Green sand mold:

sand + clay + water + additives

• Typical composition (by wt.):

– 70-85% sand, 10-20% clay, 3-6% water, 1-6% additives

• Important properties of molding sand:

– Strength

– Permeability

– Deformation

– Flowability

– Refractoriness

Melting

• For a pure metal:

total heat energy required, H = energy to raise temp of metal to melting point,

Tm + heat of fusion, Hf + energy to raise

temp of liquid metal to pouring temp, Tp

H = rV [cs(Tm – T0) + Hf + cl(Tp – Tm)]

• Heat required for alloys more complex

• Gas fired, electric arc and induction furnaces used to melt metal

• Solubility of gases (hydrogen and nitrogen) in molten metal an issue

• Solubility of H2, S:

S = C exp [-Es/(kq)]

Es = heat of solution of 1 mol of H2

q = absolute temp, C and k are constants

e.g. 1 atm pressure, liquid solubility of H2 in iron = 270 cc/kg; in aluminum = 7 cc/kg

• An important step in casting since it impacts mold filling ability and casting defects

Casting Defects

• Discontinuities

– Hot tears: intercrystalline failure in casting that occurs at a high

temperature within mold; usually forms in sections that solidify

last and where geometrical constraints are present

• Causes: large differences in section thickness, abrupt changes in

section thickness, too many branching/connected sections, mold

has high hot strength and stiffness

• Remedies: through casting and mold redesign

– Cold shut: incomplete fusion of two molten metal flows that

meet inside the mold from opposite directions

• Causes: insufficient superheat, inadequate risers

• Remedies: increase superheat, add additional risers

• Defective surface

– Scabs: thin layer of molten metal that enters gaps in mold and solidifies

• Causes: improper mold design

• Incomplete castings

– Misrun: incomplete casting

• Cause: insufficient superheat

• Remedy: increase superheat

• Inclusions

• Remedy: clean melt before pouring, improve strength of mold

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