A die-casting process has been developed which could cut the weight of car components by 20% while simultaneously increasing strength by the same value.
The new process is called thixotropic casting. It uses a property which enables certain metals and alloys to become soft and runny before hardening into a more solid form.
The process involves stirring alloys as they cool, allowing them to solidify as round particles which flow smoothly when reheated.
It avoids defects which occur during turbulent flow, such as oxide particles being introduced into the solidifying metal as a result of reaction with the atmosphere. The process also saves energy as there is no need for the alloy to be fully melted before forming.
The thixotropic behaviour of aluminium was first discovered in 1970 but it is only recently that manufacturers have used it for car component manufacture. Honda is using its own thixotropic process, thixo-cast, in the production of its new petrol/electric hybrid, the Insight, launched at last week’s Frankfurt Motor Show.
The rear outrigger is a section of the Insight’s body structure where three structural frames meet from three different directions. The deep box-shape of the outrigger, on to which the suspension fits, is difficult to form. Conventional die-casting methods produce walls which are too thick and heavy.
Honda uses the thixo-cast method – a first in body frame construction, the company claims – to form the outrigger by pouring half-solidified aluminium into the cast. The poured metal creates a fine structure which allows wall thickness to be cut 22%, increases strength 20% and cuts the weight of the casting 20%.
According to Dr Helen Atkinson of the University of Sheffield’s department of Engineering Materials: `Thixoforming has advantages over processes like die-casting because the metal flows into the die in a smooth way rather than the turbulent flow that occurs when fully liquid metal is injected.’
Her work has included forming a rotor for an electric motor from copper alloy, which cannot be done by die-casting. These rotors have a much higher electrical conductivity than aluminium rotors and therefore makes the motor more efficient.
The next step for the university will be the development of alloys specifically for thixoforming, which could open up applications in the aerospace industry.