Numerical simulation of β to α phase transformation in heat affected zone during welding of TA15 alloy

Cellular automaton model based on thermodynamic and evolution kinetic analysis was employed to simulate β to α phase transformation upon welding cooling in the heat affected zone (HAZ) of TA15 alloy. In this model, the effect of cooling rate on the phase transformation undercooling, solution solubility, nucleation rate as well as crystallographic characteristic related anisotropic interface moving mobility is quantitatively characterized. Simulation results indicate as the cooling rate increases from 0.6 °C/s, then successively to 1.5, 5.0, 16.7 °C/s, α phase plates grow finer and faster due to larger undercooling and degree of supersaturation, therein the according average thickness of α plates is 9.2, 7.4, 2.1, 1.5 μm, but the final transformation percent decreases due to shorter transformation time, which is 33.8%, 26.5%, 10.3%, 4.6%. With welding parameters constant, the temperature field, grain size distribution, and constituent phase and morphology are acquired in the whole HAZ. The modeled α phase growth kinetics and grain size are compared with the experiments and proved to be effective, which lays a solid foundation for integration of welding process, multi-scaled microstructure and mechanical properties in both theory and practice.

► Non-equilibrium CCT model for HAZ of welded joint is developed.
► Effect of cooling rate on phase transformation kinetics is characterized.
► Temperature field, grain size distribution, and constituent phase and morphology of HAZ are acquired.
► Moderate heat input is suggested to be best for titanium welding.