Trace Elements in Al-Si Foundry Alloys

Abstract

Aluminium alloys based on the aluminium-silicon system are widely used for automotive and aerospace applications as this industry seeks competitive light materials to replace e.g. cast iron and steel. Al-Si alloys have low density, hence manufactured parts are light weight; they possess good strength and high ductility. The alloys can be made from higher purity raw materials such as primary aluminium or by recycling of aluminium alloy scrap. In both processing routes undesired impurity elements may enter the alloy along the process chain and eventually will be found downstream in the final Al alloy. Recent estimates predict a decrease of the quality of raw materials used for the electrolytic reduction of Al and an increased use of scrap metal. Consequently, higher levels of some trace elements must be tolerated in future Al-Si based alloys as there currently exist no cost efficient method for removal. This thesis deals with the effect of the trace elements Calcium, Nickel, Phosphorus, Vanadium and Zinc on the solidification and microstructure of hypoeutectic Al-Si foundry alloys.

It was found that P is the key element influencing the structure of the Al-Si eutectic. Higher P concentrations promote nucleation of eutectic Si at lower undercoolings resulting in a coarse plate-like Si morphology as opposed to P lean alloys which possess a refined plate-like Si. In addition, the element P represents the link to the alteration of the macroscopic growth mode of the Al-Si eutectic which itself can be applied to describe the differences of Si morphology.

Ca can be added to Al-Si alloys in order to induce a reversion of the coarsening of Si crystals in the presence of higher amounts of P. It is concluded that the precipitation of Al2Si2Ca intermetallics and calcium phosphides removes P before the start of the eutectic reaction preventing Si nucleation on heterogeneous substrate particles such as AlP. In this case, the structural transformation of the eutectic Si from flakes to fine plates is caused by an increase of the velocity of eutectic solid-liquid interface. A definite flake-to-fibrous transition, in other words chemical modification cannot be achieved with Ca additions. It is emphasized that with respect to the mechanism that is active, there is a distinction between a refined and modified morphology of the Al-Si eutectic.

An investigation of solidification path and microstructure of high purity Al-Si alloys shows that Ni partitions into the Al3Ni phase which is well distributed in the Al-Si eutectic and forms a close system with eutectic Si crystals. In commercial Al-Si alloys where Fe is inevitably present in concentrations sufficient to form a variety of different intermetallics Ni binds Fe in the Al9FeNi phase. The solidification path remains unchanged as the Ni-rich intermetallics precipitate in the eutectic reaction together with Al, Si and other intermetallic phases when 300 ppm Ni or more are added.

V and Zn additions in the range of 50 to 600 ppm each have no influence on the solidification sequence of hypoeutectic Al-Si alloys. Zn remains in solid solution in the Al matrix and does not form any intermetallic phase. V tends to partition into known ternary Fe-rich phases. Traces of V were detected in ß-Al5FeSi and α-Al8Fe2Si phase particles. It was found that in A356 alloys most of the V was consumed by the formation of the pre-eutectic polyhedral Si2V phase when the V concentration was increased to or above 0.2 wt%.