Pumping up Aluminium Alloys

Pure aluminium-silicon alloys’ mechanical properties are unsuitable for automotive applications. The chemical composition must be modified to increase strength without sacrificing elongation.

There is only one way to increase strength and elongation simultaneously. Grain refinement is the magical word. Refining the grain structure can mitigate the undesirable effects of coarse eutectic silicon particles and large primary α–Al grains. Dislocations – I’ll explain them in a second – also cannot bridge the grain boundary to the next grain.

The grain size in HPDC is quite similar and is only impacted by the die’s cooling capabilities and the casting’s wall thickness. Here comes the role of the alloying elements: Magnesium, Zinc, and Copper. They provide substantial increases in strength and facilitate precipitation hardening.

These elements increase strength through Guinier–Preston (GP) zones, which are small precipitations inside the aluminium matrix. GP zones are associated with the phenomenon of age hardening, whereby room-temperature reactions continue to occur within a material over time, resulting in changing physical properties. As the alloy naturally ages, its ductility will decrease. That is why the ageing process is often sped up through heat treatment. Fine precipitates are formed during the heat treatment, increasing the alloy’s hardness.

The hardness increases as the resistance against deformation increases. Behind this effect are dislocations, which are two-dimensional crystal defects that play a crucial role in the deformation of aluminium alloys. 

Dislocations move through the grains by applying an external force. When such a dislocation reaches a GP zone, it is stopped. Higher stress is needed to move it forward, either climbing around the precipitation or cutting it. This is the effect behind higher strength and lower elongation. So, it is always a trade-off between elongation and strength.

Just adding these three elements isn’t beneficial. Zinc and copper also have negative effects. 

Zinc never stops its natural ageing process, with its self-diffusion temperature at -10°C. This leads to changing properties over the years in the alloy and attached substances, like glue.

Copper delivers a high strength increase, especially after a T6 heat treatment. It is also famous for corroding the Cu2 Phases. The Cu2+ ions can accelerate pitting corrosion in the presence of chloride ions. 

For most structural castings, both elements are limited to 0.03 to 0.05%. This low content reflects the negative aspects. With that low amount of corrosion and ageing, problems are negligible. However, this challenges recycling facilities in producing sustainable HPDC alloys.

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