Influence de l'affinage sur la fissuration a chaud des alliages d'aluminium-cuivre utilises en fonderie

摘要

The hot tearing of aluminium-copper alloys is a serious defect for the foundrymen. The hot tears appear during the solidification (when the alloy is in the mushy state) and propagate between the dendrite arms. So, the alloy reveals an embrittlement due to thin liquid films spreading around the primary solid grains. The origin of the hot tears lies in the inability of the residual liquid to accommodate deformation resulting from the solidification processes. The grain refinement of these alloys allows to decrease their hot tearing susceptibility. The aim of this work is to better understand the hot tearing mechanism and the grain refinement action. In a first step, experimental measurements of hot cracking sensibility have been performed on different aluminum - copper alloys, with and without grain refinement. The influence of copper and magnesium content is also studied. In a second step, the rheological behaviour is studied for solid fraction ranging from 0.81 to 0.98, through tensile deformation, using an apparatus especially developed for this purpose. In order to reproduce the deformation conditions during solidification, small strain rates are imposed. These experiments outline a critical solid fraction which determines the rupture mode of the sample. Below this critical value, the rupture is brittle, and above it the rupture is brittle and ductile with the rupture of solid bridges between grains. A hot tearing model of nucleation and growth of a cavity in the remaining liquid of a solidifying alloy is developed. This model includes the rheological behaviour of the alloy in the semi-solid state, which depends on its morphology and on the solid fraction. Results from this model are in good agreement with experimental data.