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 the search for a better 'Weight Reduction / Freedom of Style / Cost' compromise. 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 the ambience test 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 6x106 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 mold casting. In order to get closer to the required weight reduction (20% 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%; but for many of them, no improvement is recorded. This phenomenon can be explained by noncompliant surface finishes before shot peening. It is demonstrated that the residual compressive stresses show the appropriate stability required to resist the thermo-machanical stresses to which wheel discs are subjected. It is 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 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.
Weight reduction through thixocasting associated with shot peening. Part I. Properties of automobiles wheel discs made by thixocasting
Gewichtsreduzierung durch Thixogießen verbunden mit dem Oberflächenverfestigungsstrahlen. Teil 1. Eigenschaften von Fahrzeugradscheiben hergestellt mit dem Thixocast-Verfahren
Light Metal Age ; 61 , 7/8 ; 38-45
2003
8 Seiten, 9 Bilder, 5 Tabellen, 13 Quellen
Article (Journal)
English
Weight reduction through thixocasting associated with shot peening
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