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 af fected zone (HAZ) during welding of Al-Zn-Mg alloys. As a starting point, t he low melting grain boundary constituents in two commercial extrusions hav e been identified and documented by means of optical and transmission elect ron microscopy. Based on a closer examination of the kinetics of particle d issolution during continuous heating, a mathematical formulation of the und erlying diffusion problem is presented. The results from the numerical simu lations 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 t he grain boundary precipitates. In an actual welding situation, these varia bles are interrelated to the applied heat input and the thermal history of the base material, respectively.