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Optimisation of Inertia Friction Welding Steel to 6061 Aluminium
In: Aachen Germany,. Wiley-VCH; 2008. p. 1972-1978.
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Abstract
There is considerable interest in the automotive industry in producing hybrid structures involving aluminium and steel components, to reduce weight and CO2 emissions. However, fusion welding is very difficult to apply to such dissimilar materials due to the rapid formation of a brittle intermetallic interface layer. An alternative approach is to use solid state friction welding techniques. Both direct drive and inertial friction welding (IFW) can be readily applied to axisymmetric drive train components. In studies on direct drive friction welding steel to aluminium weld strengths as high as 70-80% of the aluminium parent have been obtained. Although to date no work has been published on the inertia welding method, this process has the advantage over direct drive friction welding, of being simpler and cheaper to implement, while allowing a higher energy input to be delivered to the weld over a shorter time, potentially reducing the susceptibility for interfacial reaction. In this paper the effect of key process variables on the integrity of IFW joints produced between a typical steel (C45) and aluminium alloy (6061) were explored. This included investigating the effects of the welding pressure, flywheel energy and momentum (which control the heat input and weld time), as well as the influence of surface preparation and post-weld heat treatment. Under optimised conditions weld strengths of 80% parent were obtained, which is higher than in any previous work. The HAZ was quite limited and extended only 0.2 mm into the aluminium side of the weld with no effect on the steel. The weld strength was found to be closely related to the area of interfacial adhesion and HAZ softening, being most affected by the applied pressure and prior steel surface treatment. When friction welding materials with such dissimilar flow stresses at temperature, no plastic deformation occurred in the steel. This meant it was not possible to fully eject prior oxide or contamination layers from both sides of the weld with flash formation, as would normally occur in a conventional friction weld, making it impossible to obtain 100% adhesion. Owing to the very short weld times, little interfacial reaction was found to have occurred by TEM investigation. However, post-weld solution treatment resulted in rapid interfacial reaction and a dramatic loss of joint properties.