Friction against superplastic aluminium alloys

The authors of the present paper report the effect of temperature, strain rate and normal pressure on measured coefficients of friction of aluminium alloys 7475 and 5083, representing superplastic alloys designed respectively for high strength and superior formability at temperatures and strain rates within and outside the conditions for superplastic behaviour, deforming variously against a hot die steel tool and an alumina-based high temperature cement tool in air and under nitrogen. It was found that conditions at the interface dictated by the nature of lubrication and atmosphere had little effect on the coefficient of Coulomb friction, which was essentially determined by the particular alloy and the physical structure and the surface finish of the tool surface. Significantly, the coefficient of friction increased approximately exponentially as normal stress decreased and increased essentially linearly with strain rate. The most remarkable feature was the steep minimum in the coefficient of friction at the superplastic forming temperature.