PROCESS MODEL FOR WELDING OF AL-MG-SI EXTRUSIONS PART-1 - PRECIPITATESTABILITY

The present investigation is part of a larger research programme on pr ocess model based alloy design and optimisation of welding conditions for Al-Mg-Si extrusions, with particular emphasis on how changes in th e base metal chemical composition and thermal history affect the heat affected zone (HAZ) microstructure evolution. In Part 1, a model for t he 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 th e HAZ dissolution kinetics. In contrast, the base metal chemical compo sition is of less importance, as the metastable solvus is only a weak function of the alloying level. Moreover, implementation of the micros tructure model in a dedicated finite element (FE) code shows that asym metry in the temperature field during welding of hollow extrusions (e. g. T joints) may lead to unexpected changes in the HAZ hardness distri bution compared with that commonly observed in bean on plate welds. Th e results of the present investigation provide a systematic basis for future research and some new approaches are tentatively indicated. (C) 1997 The Institute of Materials.