Process model for welding of Al–Mg–Si extrusions Part 1: Precipitate stability

Abstract

The present investigation is part of a larger research programme on process model based alloy design and optimisation of welding conditions for Al–Mg–Si extrusions, with particular emphasis on how changes in the base metal chemical composition and thermal history affect the heat affected zone (HAZ) microstructure evolution. In Part 1, a model for the thermal stability of hardening precipitates is presented, based on an analysis of the thermodynamics and kinetics of the alloy system. It is shown that the initial size of the particles, as determined by the temper condition (i.e. T4, T6, or T7), has a significant effect on the HAZ dissolution kinetics. In contrast, the base metal chemical composition is of less importance, as the metastable solvus is only a weak function of the alloying level. Moreover, implementation of the microstructure model in a dedicated finite element (FE) code shows that asymmetry in the temperature field during welding of hollow extrusions (e.g. T joints) may lead to unexpected changes in the H AZ hardness distribution compared: with that commonly observed in bead on plate welds. The results of the present investigation provide a systematic basis for future research and some new approaches are tentatively indicated.