Aluminium alloys

-          Pure aluminium possesses relatively poor casting characterstics and for this reason castings are produced from aluminium alloys.

-          The principle alloying elements in aluminium base casting alloys are copper, silicon, magnesium, zinc, manganese, chromium, tin, titanium etc. iron may be also be present as an impurity.

-          Copper: copper content of 2 to 5% provides optimum ductility (toughness) whereas greater percentage of copper adds to hardness and strength.

-          Silicon: the general effect of increasing silicon content is to increase the strength (until the eutectic silicon percentage is reached). Ductility is however is lowered.

-          Magnesium: the alloying behavior of magnesium in aluminium is similar to that of copper.

-          Zinc: makes it possible to obtain a maximum of mechanical properties in the as-cast condition.

The effects of alloying elements like Si, MO, Ni, V, CO, W, MO and carbon on steel casting.

1)      Silicon:

a)      Improves oxidation resistance

b)      Strengthens low alloy steels.

c)      Acts as a deoxidizer. 

2)      Molybdenum:

a)      Promotes hardenability of steels.

b)      Makes steel fine grained

c)      Makes steel unusually tough at various hardness levels.

d)     Concentrates tendency towards temper brittleness.

e)      Raises tensile and creep strength at high temperature.

f)       Enhances corrosion resistance in stainless steel.

g)      Forms abrasion resisting particles. 

3)      Nickel:

a)      Increases toughness and resistance to impact.

b)      Lessens distortion in quenching.

c)      Lowers the critical temperature of steel and widens the range of successful heat treatment.

d)     Strengthens steels.

e)      Renders high chromium iron alloys austenitic.

f)       Does not unite with carbon.

4)      Vanadium:

a)      Promotes fine grains in steel.

b)      Increases hardenability.

c)      Imparts strength and toughness to heat treated steel.

d)     Resists tempering and causes marked secondary hardening. 

5)      Cobalt:

Contribute to red-hardness by hardening ferrite.

6)      Tungsten:

a)      Increases hardness

b)      Promotes fine grains.

c)      Resist heat

d)     Promotes heat at elevated temperatures. 

7)      Carbon:

Affects:

a)      Hardness

b)      Tensile strength

c)      Machinability

d)     Melting point.

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)