Effects of Strain Rate and Temperature in Forming Processes of Metals

In metal forming processes, the behavior of the metals undergoing large plast~c deformation is atyected by the dynamic or strain rate effects of the material and inertia effect. Since both of' the matenal and mechanical effects are correlated each other in the forming processes, they are effectively treated by rhe finite element methods which can take account of strain rate effect. Usually the effect of strain rate on the flow stress of metals is not significant at low temperature and becomes significant as the temperature rises. When the strain rate effect is significant, the forming pressure is lowered by decreasing the strain rate. In isothermal forming, in which the tools are heated to the same temperature as the billet in order not to cool down the billet, a low forming velocity is employed to reduce the forming load. The deformed shape of the material is also affected by the strain rate effect. In general, deformation is dispersed when the strain rate effect is significant. This effect is most utilized in superplastic forming in which a-curence of necking is avoided and a very large deformation is attained. Since this phenomenon is usually caused at a very low strain rate, it is desirable to develop metals which exhibit significant strain rate erfects at high strain rate. As an example, a superplastic aluminium alloy which causes a significant strain rate effect at a high strain rate is introduced. Risumi : Au cours des procedes de mise en forme, les metaux subissant de fortes deformations plastiques son[ soumis aux effets des grandes vitesses de deformation et aux effets d'inertie. Ces effets &ant dependants les uns des autres, on peut utiliser les methodes de simulation aux 6lements finis pour reproduire les etats dkformes A grande vitesse. En general, on constate que ce n'est que lorsque la temperature augmente significativement que l'effet de la vitesse de deformation devient non negligeable. La pression exercee lors du formage est alors diminuee par la chute de la vitesse de deformation. En condition de tbrmage isotherme, I'outil est port6 2 la m&me temperature que la billette afin de ne pas retioid~r celle-ci, dans de telles conditions on utilisera une faible vitesse de formage de maniere h reduire la charge appliquee. La forme finale de la piece sera elle aussi intluencee par la vitesse de deformation. Les plus fortes dispersions sont constatees par les fortes vitesses. Cet effet est largement exploit6 dans le procede de formage superplastique oh I'kcoulement plastique homogene est obtenu pour de uks fortes valeurs d'ecoulement plastique. Compte tenu que ce phenom2ne n'est exploit6 qu'h basse vitesse de deformation, on comprend I'interet de developper des metaw pour lesquels cette proprikte serait obtenue A grande vitesse de deformation. Cest dans cet objectif que nous presentons ce travail realis6 sur un alliage d'aluminium A comportement superplastique et presentant I'effet de vitesse de defomlation aux grandes vitesses.