Properties of aluminium foundry alloys

Aluminum casting has enjoyed remarkable growth in the automotive industry, where its use has more than doubled over the past thirty years in response to the quest for lighter, more fuel-efficient vehicles. Although the automobile is the predominant application, the foundry also supplies a number of sectors, including the mechanical, rail, electrical and aeronautical industries.

For a large number of products, the choice of production technique (foundry, mechanically-welded assembly, forging, stamping, extrusion, etc.) and material (aluminum, cast iron, steel, magnesium, copper, polymer, etc.) is a question of both technical and economic considerations. This document focuses on the technical aspects of aluminum casting.

Aluminum alloys for casting are quite different from those for wrought products. The absolutely predominant family is Al-Si, due to the excellent casting properties conferred on aluminum alloys by silicon.

The very notion of the mechanical properties (whether static, fatigue or impact) of cast alloys is complex: it only makes sense if we consider the combination of composition + heat treatment + part geometry + casting process. Mechanical properties are strongly influenced by the solidification rate, which is highly dependent on the last two factors. There is a whole range of foundry processes (sand, shell, low-pressure casting, die-casting, lost wax, lost foam, rheomolding...) and for each part the choice must be made according to economic and technical imperatives. Corrosion resistance can be a critical factor in many applications: it depends essentially on the alloy's composition and heat treatment.

The metallurgical quality of the parts (absence of porosity and inclusions, especially oxides) and their modification or refining treatments are also decisive in obtaining good characteristics, especially with regard to ductility and fatigue, two essential points for safety parts.

Cast aluminium alloys can be made from two main types of raw material:

• These are primary alloys, i.e. pure aluminum obtained directly from the pots of an electrolysis plant, to which the constituent elements of the alloy have been added. One method of production assimilated to primary smelting consists in remelting pure aluminum ingots from an electrolysis plant and alloying them with the desired addition metals to produce ingots of casting alloy;

• Recovered, sorted and purified scrap, with the necessary addition of metals, is used to produce refining or secondary alloys, which currently account for around two-thirds of total foundry production.