EFFECT OF ALLOYING ELEMENT in die casting

EFFECT OF ALLOYING ELEMENT in die casting

Silicon:-

Advantages of silicon has following advantages of al alloy

1. High wear resistance.

2. Low thermal expansion.

3. Mold filling ability

4. Freedom from hot tearing.

Most alloys used in permanent mold casting are hypo eutectic composition with silicon 4 % to 9.5%.

Hyper eutectic alloys up to 25% is developed with high wear resistance and low thermal expansion. They can retain properties up to the temperature of automobile engine service temperature.

If Al- Si alloys cooled slowly, silicon present will be in coarse needle form giving low strength and poor ductility. Modifying agents are added to the liquid metal to convert the silicon needles into globular form leads to high strength and ductility.

Modifying agents added Sodium, strontium selection of sodium, strontium is on the basis of economic cost and ease with which they are added to molten (alloy) aluminum with the addition of sodium there will be no effective nucleation sites for the silicon phase and fine Silicon grows in solid phase.

Sodium can be added either as pure metal or salt and amount of sodium in melt varies from 0.007 to 0.02% amount varies with section thickness.

Disadvantages of using sodium is that it lost by oxidation and modification structure is not normally retained, when sodium modified metal is melted is retained for longer period in molten metal and can survive successive re melting .But requires longer contact time in molten metal before it become fully effective.

Magnesium:-

Magnesium % should always be less than 9.5% lower magnesium alloys can be pressure die cast. It cannot be die cast at higher levels, because of embroilment at higher levels.

Pressure die casting alloy Al-Si8-Cu3 has Mg-0.3% and some specification limit to 0.1% and Japanese alloys have 0.03% (max)

Most widely used alloy Al-Si-Mg0.3% shows good combination of cast ability, mechanical properties, corrosion résistance.

Higher %Mg gives high tensile strength; lower %gives better ductility .so designers often set their own range of Mg level depending on whether strength or ductility is important.

Combination of Mg & Silicon gives Mg₂Si; solution heat treatment takes magnesium and some silicon into solid solution, which is retained by quenching. subsequent aging allows controlled precipitation of Mg₂Si with increase in tensile strength .10%Mg cannot be pressure die cast as it suffers from hot tearing and is die cast only in unsophisticated shapes.

Iron:-

With 0.2% or less give high strength and ductility in fully heat treated condition range of ductility is 3 to 7%.In process die casting iron helps to present the soldering which causes castings to stuck, to the die.

Iron is attacked by molten aluminum with high turbulence during pouring and is affected by the formation of roughened surface and causes soldering of components to die face.

Iron with 0.6% to 1.2% reduce the rate of solution and hence reduce to plate like structure it affects ductility this is counteracted by manganese (0.5) and Cr.

Copper:-

Up to 2 to 5% produces solid solution strengthening, leading to increase in tensile strength, hardness. But they do not respond to heat treatment , unless copper % is in between 3 to 5% since most pressure die castings are not heat treated ,so it preferable to add more castings for same weight .

Ex: Al-Si10-Cu2-fe specific gravity 2.74

Al-Si8-Cu3-fe specific gravity 2.79

Disadvantages of copper addition corrosion resistance where optimum corrosion resistance is required ,copper % is less (always) than 0.1% this factor leads to selection of Al - Si 12-Fe rather than cheaper Al-Si8-Cu3 .Experiment shows that copper 0.01 to 1% up to 0.24% copper shows corrosion resistance equal to that of copper free alloy . But alloy with copper 1% corrode twice as much.

Manganese:-

Embroilment effect of iron can be counted by Manganese ,which changes the structure of inter metallic phase with increase in Mn % these is corresponding increase in quality of casting , which being more dense than molten aluminum , tends to sink to the bottom of the crucible.

Nickel:-max 0.5%

Added to improve high temperature strength of alloy as in pistons.

Al-Si12-Cu-Mg-Ni

Lead and Tin: - max 0.1%or less

Have very restricted solid solubility in aluminum .if present in excess it adversely affects corrosion resistance. But small additions improve mach inability .so extract lead % tin to 0.1%.

Titanium and Boron:- max 0.2%

In pressure die casting size is not a problem .Ti & B are grain refiners like Al-Ti5-B1. In application where electrical conductivity is a prime consideration, Boron is added to remove Titanium, Vanadium from solid solution as they adversely affect electrical conductivity.