Trends in microstructure modeling in weld metals

Various physical processes, such as thermochemical reactions in liquid, solidification, and solid state transformations, control the microstructure development in weld metals. Some fundamental knowledge of the effects of these physical processes on weld microstructure development already exists. However, generalized and integrated models encompassing the current understanding are just evolving. Such models are needed in the design of successful welding procedures for new alloy systems and advanced materials. The principles, methodology, and future directions of modeling weld microstructure development are described in this paper, with examples in low-alloy steel, stainless steel, and Ni-base superalloys. In low alloy steels, the nucleation and growth of oxide inclusions in the melt was modeled as a function of the welding process and composition. This inclusion model has been recently coupled with solidification and numerical heat and mass transfer models. Recent advances in theoretical and physical modeling of the solidification process will be reviewed in this paper with regard to predicting the solidification modes, grain structure development, segregation effects, and nonequilibrium solidification in welds. In nickel-base superalloy welds, the effects of solidification and solid state transformations on microstructure development will be described. In these welds, the final microstructure was found to be dependent on the cooling rates and solidification modes. The weld microstructure was investigated with the help of advanced analytical techniques such as atom-probe field-ion microscopy. The result addresses the importance of advanced analytical techniques in modeling the solid state transformation