Thermodynamic and kinetic models for describing microstructure evolution during joining of metals and alloys

An ability to predict weld microstructure is critical for introduction of new materials, as well as, optimization of existing materials. Complexity of weld microstructure evolution is related to interaction of phase stability, multicomponent diffusion, steep temperature gradients and morphological instabilities during rapid heating and cooling. In the past two decades, computational thermodynamic and kinetic models have been developed to predict these interactions in wide range of alloys. In the first section, a brief introduction of thermodynamic and kinetic models is given. Models for free energy of solid solution and compound phases, as a function of composition and temperature, are discussed. The underlying assumptions of kinetic models, including local equilibrium at the interface and conditions, are highlighted. In the second section, adoption of these models for predicting weld microstructure evolutions is demonstrated with practical examples from structural alloys. The examples focus on the phase transformations that may occur in liquid state (e.g. inclusion formation), during solidification (e.g. solidification range, phase selection, and segregation) and during solid-state transformations (e.g. growth and dissolution of second phases). In the third section, challenges and opportunities facing widespread use of these tools, as well as, validation using high-resolution and in-situ characterization tools are highlighted.