Influence of alloying elements on Aluminum casting alloys

Influence of the most important alloying elements on Aluminum casting alloys:

Silicon
• improves the casting properties
• produces age-hardenability in combination with magnesium but causes a grey color during iodization
• in pure AlCu casting alloys (e.g. Al Cu4Ti), silicon is a harmful impurity and leads to hot tearing susceptibility.

Iron
• at a content of approx. 0.2 % and above, has a decidedly negative influence on the ductility (elongation at fracture); this results in a very brittle AlFe(Si) compound in the form of plates which appear in micrographs as “needles”; these plates act like large-scale microstructural separations and lead to fracture when the slightest strain is applied
• at a content of approx. 0.4 % and above, reduces the tendency to stickiness in pressure die casting.

Copper:
•increases the strength, also at high temperatures (high temperature strength)
• produces age-hardenability
• impairs corrosion resistance
• in binary AlCu casting alloys, the large solidification range needs to be taken into account from a casting/technical point of view.

Manganese
• partially offsets iron‘s negative effect on ductility when iron content is > 0.15 %
• segregates in combination with iron and chromium
• reduces the tendency to stickiness in pressure die casting.

Magnesium
• produces age-hardenability in combination with silicon, copper or zinc; with zinc also self-hardening
• improves corrosion resistance
• increases the tendency towards oxidation and hydrogen absorption
• binary AlMg casting alloys are difficult to cast owing to their large solidification range.

Zinc
• increases strength
• produces (self) age-hardenability in conjunction with magnesium.

Measures influencing microstructural formation are aimed at improving the mechanical and casting properties. In practice, apart from varying the cooling speed by means of different mould materials, additions to the melt are usually used.

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