Homogenizing heat treatment for improving hot workability of the ingots of Al-Mg-Si alloys

Abstract

Hot torsion tests were carried out on the specimens taken out of the semi-continuous cast ingots of Al-Mg-Si alloys for studying the effects of homogenizing heat treatment on their hot workability. Ingot structures after homogenizing heat treatment under various conditions were also examined by metallography and electrical resistivity measurements by the eddy current method. It was shown that appreciable improvements in hot workability were attained by applying homogenizing heat treatment in the following way; (1) to solutionize the as cast ingots at high temperatures above 550°C in order to remove solute segregation due to rapid solidification and to dissolve the second phase particles, (2), subsequently, to cause precipitation of Mg2Si by heating at 400-450°C. The coarse precipitates of Mg2Si thus obtained, which are uniformly distributed, show little dispersion hardening effect. Solid solution strengthening effect of the matrix is also reduced because of decrease in solute concentration during precipitation. Thus, appreciable reduction in flow stress in deformation under the hot working conditions was attained. It was also shown that the curse Mg2Si particles uniformly distributed considerably improved the high temperature ductility of ingots.For the extrusion products to be used under T5 conditions, care must be taken to allow complete solutionizing to take place during deformation. In this case, too coarse Mg2Si particles, produced by slow cooing after high temperature solutionizing of ingots such as by furnace cooling, may not be desirable for the products to attain the optimum mechanical properties.