Factors which determine the selection of a casting alloy and the casting process to be employed.

Alloyed metals possess:

a)      Higher tensile strength

b)      Better high temperature strength

c)      Better corrosion resistance

d)     Improved machinability and workability

e)      Lower  melting point

f)       Improved castability

Special casting techniques:

Classification:

a)      Metal mold casting

1)      Gravity or permanent mold casting

2)      Die casting – A) hot chamber process

b)                                  B) cold chamber process

3)      Slush casting

4)      Pressed or carthias casting

c)      Non-metallic mold casting

1)      Centrifugal casting:

-          True centrifugal casting

-          Semi centrifugal casting

-          Centrifuge casting

2)      Carbon dioxide molding

3)      Investment mold casting or lost-wax process

4)       Shell molding

5)      Plaster molding

6)      Mercast process

d)     Continuous casting

1)      Reciprocating molds draw casting

2)      Stationary mold

3)      Direct sheet casting

Casting can made in:

a)      Permanent molds: made up of ferrous metals and alloys (steel, gray cast iron etc.)

b)      Temporary refractory molds: made up of refractory sands and resins.

Aluminum Alloys And Foundry Practice

• Casting properties of aluminium alloys:

1)      The alloys should posses minimum solidification shrinkage so that foundry yield is at maximum and the problem of hot tear and micro porosity are minimized.

2)      The alloy should have adequate fluidity in the molten condition.

3)      The should not posses a tendency to hot tear or crack.

4)      The alloy should face minimum problem as regards drossing and gas absorption.

5)      It should possible to produce pressure-tight casting.

• Melting of aluminum alloys:

a)      Crucible furnace

b)      Reverberatory furnace

c)      Pot furnace

d)     Induction heating furnace

• Drossing:

Drossing is the formation of the aluminum oxide and other oxides which are collected on the surface of the molten metal.

Melting with minimum dross-formation occurs when;

a)      Melting is fast

b)      Metal is protected from products of combustion (in the melting furnace).

• Gas absorption:

Aluminium alloys can absorb harmful quantities of hydrogen gas and consequently give rise to gas or pin hole or gas porosity.

• Fluxing and flushing:

Purpose:

a)      To remove dissolved hydrogen from the melt.

b)      To separate dross fro the molten metal

c)      To entrap dross.

Both solid fluxes (containing aluminium and zinc chloride) and gaseous fluxes (i.e. nitrogen, helium, argon, and chlorine) are used to flush the molten metal.

Flushing is carried out by skimming off surface dross and then bubbling gaseous fluxes through the molten metal.

• Grain refinement:

Grain size in cast aluminium alloy can be refined by:

a)      Chilling

b)      Temperature adjustment (i.e. pour at the possible lowest pouring temperature).

c)      Late addition (such as those of boron, titanium, chromium. Columbium and sodium ) to the melt.

• Process of the casting aluminium alloys:

1)      Sand casting

2)      Permanent mold casting(GDC)

3)      Die casting(PDC)