Novel heat treatment solution for die cast aluminium
The automotive industry could benefit from a novel heat treatment process for high-pressure die cast (HPDC) aluminium components, enabling doubled mechanical strength and higher fatigue resistance.
Scientists at the Light Metals Flagship of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) believe such cost effective HPDC components could replace sand-cast and permanent mould cast aluminium alloys and wrought products in some applications.
Age hardening enhances a range of material properties, but was previously not suitable for HPDC parts, ‘Traditionally, much longer times and high temperatures are used for solution treatment of aluminium alloys, often only 10ºC away from the solidus temperature (when melting begins),’ explains Dr Roger Lumley, who led the research.
‘For example, for permanent mould or sand castings, treatment may be for 6-12h at 540ºC. This produces vacancies that are retained in the super-saturated solid solution when the alloy is quenched. The solution then decomposes at lower temperatures producing fine scale precipitates within the grains, causing strengthening.’
In HPDC parts, however, precipitation causes trapped gas pores to expand, blister and distort.
The new technique at CSIRO heats widely used HPDC alloys for 10-15 minutes at 430-480ºC. This produces at least a partial solid solution of alloying elements such as silicon, copper and magnesium, which is then quenched into water and aged to a temper of T4 (22ºC), T6 (150ºC) or T7 (200ºC).
‘The silicon particles appear to pin defects simultaneously, preventing their expansion,’ says Lumley.
The resulting alloys experience different levels of improvements, but overall heat-treated HPDC products are said to have superior properties to sand-cast and most permanent mould cast aluminium alloys. They show doubled mechanical strength, high fatigue resistance close to wrought products, up to doubled energy absorption and a 20% increase in thermal conductivity.
Researchers have also discovered a range of new alloy compositions, made from scraps of other HPDC alloys such as A360 and A380, with rapid strengthening behaviour.
Geoff Scamans of Innoval Technology Ltd, Banbury, UK, a consultancy for manufacturers and end-users of aluminium, comments, ‘The CSIRO are good at providing simple practical solutions to industry’s problems.
"The new process is, in some ways, analagous to continuous heat treatment of sheet that requires considerably less thermal exposure compared to batch heat treatment of coils. There is also some similarity with press quenching of extrusion alloys to get a sufficient level of solutionising compared to formal solution heat treatment. The novelty is in the application of these well-understood principles’.
Die casters can now make complex HPDC parts for crash sensitive structural components, load carrying and safety applications, as well as engine blocks and transmission housings which remove heat efficiently and quickly.
Lumley adds, ‘It’s an attractive option because the HPDC process is more cost effective than other manufacturing methods in mass production. And the net increase in design strength post-treatment may allow castings to be made using up to 30% less metal’. This provides significant cost reduction per part, as well as reduced vehicle fuel consumption.
Scamans echoes the potential for ‘market penetration’. He says, ‘Automotive castings are probably the biggest single use of aluminium at the moment, and the market is growing rapidly. Improved mechanical and fatigue properties will help maintain this’.
Industrial trials have been conducted on parts up to 30kg. The team now wants to undertake a component case study with collaboration from an original equipment manufacturer.