Heat Treatment Helps Cars, Aircraft Get Older, Stronger

Researchers at CSIRO, and Australian research institute, have found a way of heat treating aluminum alloys that can boost their initial strength and toughness, and stimulate further improvement in these properties over time. This can be achieved with considerable savings in time and energy over conventional techniques, and in most cases requires no additional equipment.

“It has long been possible to increase the strength of aluminum, but usually at the expense of fracture toughness, or vice versa,” says Dr. Roger Lumley of CSIRO Elaborately Transformed Metals based at Clayton, who led the research team that developed the technique. “We’ve found a way of doing both at the same time.”

The aluminum alloys used in the automotive, building, and aerospace industries are typically age-hardenable. That is, they are strengthened after their initial formation by a curing process known as aging. The most common treatment that gives the strongest alloys is designated T6. Under this regime, the aluminum alloys are typically aged for 6–8 hrs. at 150 – 170°C to generate the tensile properties required for structural applications.

The CSIRO research team discovered that if this high temperature aging process was interrupted after a relatively short period, and the material was allowed to undergo secondary aging at ambient temperature, then alloys of equivalent strength and up to 20 percent tougher could be formed. At the same time, the total energy to rupture can also be improved dramatically – by up to 800 percent.

It’s all to do with the microstructure of the alloy. “What you end up with in the new process is a finer structure, engineered at the nano-scale, in a way that translates into mechanical property improvements,” says Lumley.

Out of this work, CMIT has developed two patented processes, both of which improve properties of age-hardenable aluminum alloys while saving time and money because of the reduced aging period at high temperature.

The T6i4 heat treatment significantly reduces the time of high temperature aging to about an hour, and uses the ambient temperatures provided by Australia’s warm climate to complete the process. In this way, aluminum car body panels can be assembled and painted, and continue to strengthen in the hot sun. The process would continue, albeit at a slower rate, for the life of the vehicle.

“The T6i4 process is a low-cost technology attractive to cost-sensitive environments, particularly the automotive industry where low cost and passenger safety are of high importance,” says Barrie Finnin, business development manager at CSIRO Manufacturing & Infrastructure Technology.

But even better performance from the alloys can be achieved by using the related T6I6 process, where, after several hours of secondary aging at ambient temperature, the material is again subjected to high temperature aging. ‘This can provide significant improvements to mechanical properties over the T6i4 treatment,” says Finnin. “The T6i6 process could quite conceivably be used for aircraft skins and other aerospace applications, or any application where weight reduction is at a premium.”

The potential spin-offs of the new processes for industry are enormous. Not only can aluminum alloy producers boost the strength of their product while making considerable savings in energy, but the processes also allow a faster turnaround time in producing finished components, and may even lead to reduced furnace sizes.

In addition, the CSIRO approach fits in nicely with the work patterns for fabricating car parts. For instance, panels can leave the furnace, be cooled to room temperature, and be fitted and painted, all while strengthening. In addition, the baking cycle used to harden the paint will actually add to the process. The same is true for making alloy wheel hubs.

The technology, which is protected by a series of international patent applications, is now in the proving stage. “Commercial applications are starting to build industry confidence in our innovation, which should encourage further use of aluminum alloys,” added Finnin.

TAGS: Finishing
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