Modelling of grain boundary liquation in Al-Zn-Mg weldments

The present investigation is concerned with the modelling of grain boundary liquation (i.e. local melting) within the subsolidus region of the heat affected zone (HAZ) during welding of Al-Zn-Mg alloys. As a starting point, the low melting grain boundary constituents in two commercial extrusions have been identified and documented by means of optical and transmission electron microscopy. Based on a closer examination of the kinetics of particle dissolution during continuous heating, a mathematical formulation of the underlying diffusion problem is presented. The results from the numerical simulations show that the occurrence of local melting depends on the interplay between two main variables, i.e. the heating rate and the initial size of the grain boundary precipitates. In an actual welding situation, these variables are interrelated to the applied heat input and the thermal history of the base material, respectively.