Weight reduction through thixocasting associated with shot peening

摘要

The subject of this paper is a study of weight reduction applied to an automobile wheel disc. After assessing the potential of the main existing and emerging material-process solutions, it turns out that thixocasting of aluminium alloys is a very attractive solution in terms of changing the asymptote "Weight Reduction-Freedom of Style-Cost". The performance levels of thixocast parts are compared with the specifications of automobile manufacturers in the fields of metallurgy, surface finish, fatigue and impact resistance and ambience tests resistance of the protective coating. These performance levels are also compared with those of identical reference parts produced by low-pressure casting or forging. The fatigue resistance observed on our prototypes, which is lower than expected from the thixocast discs (Lf= 120 MPa at 6X10~6 cycles) due to insufficient control of the surface finish, did not enable the required weight reduction to be achieved. In economic terms, for this disc, originally designed to be forged, thixocasting is less competitive than low-pressure permanent mould casting. In order to get closer to the required weight reduction (20 percent compared with the best reference solution), surface prestressing by shot peening was implemented with an Almen intensity F23-F30A, compatible in terms of surface roughness with the protective and decorative coatings certified by vehicle manufacturers. When treated in this way, thixocast discs show high endurance limits, leading to potential weight reduction which may exceed 40 percent but for many of them, no improvement is recorded. This phenomenon can be explained by non-compliant surface finishes before shot peening. It is demonstrated that the residual compressive stresses show the appropriate stability required to resist the thermomechanical stresses to which wheel discs are subjected. It is also verified that shot peening does not alter the coating's properties to provide efficient environmental resistance. These initial results point to several certifiable and industrializable solutions using such prestressing treatments which are well-positioned in terms of "Cost-Weight Reduction-freedom of style". Thixocasting and, even more, rheocasting show the best potential, as long as the surface quality of the parts is under control and complies with specifications.