Advances in aluminium for automobiles
Making aluminium alloys easier to work with for car manufacturers is the focus of research at Imperial College London, UK, enabling them to exploit the light metal, improving fuel economy and reducing emissions.
Aluminium alloys suffer from low ductility and have a tendency to springback by up to 30% after cold forming or post-forming heat treatment, used after hot stamping, meaning that the materials partly revert to their original shape. A technique called solution-heat treatment, forming and cold-die quenching (HFQ), developed by Professor Jianguo Lin, could alleviate this problem.
Lin has worked with collaborators from the University of Birmingham and technical consultants at Innoval Technology, both in the UK, plus automotive companies and other aluminium alloy technology providers.
The conventional approach for processing aluminium alloys involves three stages – forming the part, carrying out a solution heat treatment and then aging the part. Whatever sequence the first two processes are carried out in, there is an adverse impact on the alloy. If the heat treatment is carried out before the forming process, the alloy becomes difficult to form, whereas if the forming process comes first, the rapid cooling of the part during post-heat treatment can cause distortions.
‘In our process we combine the heat treatment and forming in one operation, and that’s the tricky bit. We use a water-cooled die to do the quenching [in situ],’ explains Lin.
‘The die is made of normal tool steel but we have to put cooling channels in order to keep the die temperature low. If the die temperature is too high, you can’t get rapid cooling and for aluminium alloys, this is a very sensitive issue. If you can’t get rapid cooling, then you may get precipitates forming at grain boundaries, and so the microstructure is affected and then cracking can take place too readily.’
Water-cooled dies are commonly used in the automotive industry for forming safety-critical panel steel parts. Lin says that his group was the first to use this technique to form aluminium alloys and he is patenting the process.
The approach is said to almost completely inhibit springback, enabling part shape and microstructure to be controlled more easily for larger and accurate panels, with improved strength of up to 25%. Lin also claims that more components can be combined in one piece using this technique, for better structural integrity, reducing the need for riveting and welding.
The HFQ technique has so far been trialled at laboratory scale using model structural components. Dr Gary Mahon of Innoval Technology, however, does not foresee any major problems in scaling up production. ‘Some work will be required to get the right sort of tolerance on the part and also uniformity throughout the part but I think that can be overcome,’ he says. ‘Reducing the number of stages in the processing will also make it cheaper to use aluminium alloys.’
Lin is now working on a £2.7m project in collaboration with Jaguar and another six British companies. In the first phase, understanding derived from developing the HFQ technique will be used to produce parts of a particular shape, without seeking to optimise the strength. He says that the technique also offers opportunities for producing parts for the aerospace industry, noting that Airbus has recently joined the research consortium.