A model for the formation and solidification of grain boundary liquid in the heat-affected zone (HAZ) of welds

The rapid thermal cycle experienced in most welding operations can promote the constitutional liquation of precipitates in certain alloy systems. Grain boundary liquid films form in the subsolidus portion of the heat-affected zone (HAZ) as a consequence of constitutional liquation. Rapid cooling limits the extent of solute diffusion into the matrix from the grain boundary liquid and hence extends its solidification temperature range. Liquation cracking can occur in the HAZ if the grain boundary film exists at a time when the local thermal stresses become tensile. Hence, in order to predict the liquation cracking susceptibility of an alloy under a given welding condition, both the microstructural evolution which centers around grain boundary liquation and the stress generation have to be modeled. This article addresses the issue of microstructural evolution and attempts to present a model for the formation of grain boundary liquid during the heating cycle and its solidifiction during the cooling cycle. The variables which increase the life of the transient grain boundary liquid during the thermal cycle are identified. The onedimensional (1-D) model presented here is an important first step toward the ability to predict liquation cracking susceptibility of an alloy during welding.