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01/01/2005
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Scientists have found that if the high temperature ageing process used to strengthen aluminium components is interrupted and the material allowed to undergo secondary ageing at ambient temperature, it becomes 20 per cent tougher.
The secret is all to do with the microstructure of aluminium alloys, writes Roger Bishop. Material treated in this way is said to have a finer structure, engineered at the nano-scale in a way that translates into mechanical property improvements. If the work is fully validated, it could lead to structural castings and vehicle body panels that get stronger the longer they are left out in the sun.
The findings come from the CSIRO Elaborately Transformed Metals (CETM) group in Australia where Dr Roger Lumley says the ‘total-energy-to-rupture’ point can also be extended dramatically (by up to 800 per cent) resulting in safer cars with crumple zones capable of absorbing much more energy as they deform or rupture on impact.
“We have developed two heat treatments, both of which overcome the age-old problem of either increasing the strength of aluminium and reducing its fracture toughness, or vice versa,” says Lumley.
Aluminium alloys are typically age-hardened by a curing process, or aged at high temperatures in a furnace. The most common treatment, which gives the strongest alloys, is called T6 and typically involves six to eight hours of ageing at 150-170°C. CSIRO’s T6i4 heat treatment cuts the high temperature ageing time to about an hour and uses Australia’s warm climate to complete the process.
CETM industry manager Barrie Finnin says even better mechanical properties can be achieved using a variant of the new process, called T6i6. After several hours of secondary ageing, this involves subjecting the material to further high temperature ageing.
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Author
Roger Bishop
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