Correlation between solidification parameters and weld microstructures

Solidification behaviour controls the size and shape of grains, the microstructure, the extent of segregation, the distribution of inclusions, the extent of defects such as porosity and hot cracks, and ultimately the properties of weld metal. In the past, theories related to single crystal growth technology and the solidification of castings have been extrapolated to describe weld metal solidification. However, within the past two decades there have been several new developments in solidification theory. Furthermore, welding involving high energy beam processes and significantly greater cooling rates have received much emphasis. Traditional solidification theories as they apply to castings are not appropriate in describing solidification under these latter conditions. This paper provides a comprehensive review of the solidification behaviour of a weld pool incorporating recent advances in solidification theories. Basic principles from ingot, single crystal, and rapid solidification processes are used to understand and explain observed weld metal microstructures. Recent results on weld modelling, including heat and fluid flow, as they relate to weld pool geometry and solidification behaviour of the weld metal are reviewed. Finally and most importantly, solidification parameters that control weld pool solidification such as temperature gradient and growth rate, and the resultant interface undercooling, are correlated with weld metal microstructure.