Friction behaviour in strip draw test of pre-stretched high strength automotive aluminium alloys

Aluminium alloys are being increasingly used in the automotive industry as a means of light-weighting the vehicle body to reduce emissions. The forming characteristics of aluminium alloys, however, differ from those of conventional deep-drawing quality materials, and the drive to reduce vehicle development times has led to reliance on simulation of forming operations to establish feasibility in the early design stages. It is known that friction has a major influence on the forming characteristics of a material and changes as the material plastically deforms and surface roughness increases, causing a transition from hydrodynamic to a mixed friction regime where metal to metal contact is increased. In the simulation environment however, friction is usually assumed to be constant. The development of higher strength alloys requiring higher forming forces suggest that a constant friction value may not always be a valid assumption. In this paper the friction behaviour of three commercially available automotive aluminium grades are compared, at different levels of strain, using a strip draw test. The results show that for a material with a standard mill finish friction is anisotropic, remaining so even at relatively high strains and the anisotropy is independent of the direction of strain. The friction coefficient increases with the level of strain but the use of solid wax lubricants helps to overcome this maintaining a uniform friction at strains up to 10%. For aluminium with an EDT finish the roughening effects due to plastic strain are completely overcome and again a constant friction value is valid. In some alloys the formation of stretcher strains also help to reduce the metal to metal contact promoting hydrodynamic friction and a constant friction coefficient at lower strains.