Phase Field Modelling of Microstructural Evolution in Titanium Alloy Welds

Titanium alloys are finding increasing number of applications in aerospace industry due to their light weight and good mechanical properties. A large number of applications require the welding of similar or dissimilar parts of titanium alloys. The welding in titanium alloy often leads to degraded mechanical properties in the heat affected zones. The complex heat treatment associated with the welding significantly changes the microstructure in the heat affected zones which leads to degraded mechanical properties. An understanding of the microstructure in such heat affected zones is essential to design processes that give stronger welds. Motivated by this, we have studied the microstructure of weld of Ti-6Al-4V alloys using a combined finite element heat transfer model in conjunction with a phase field model for the microstructure of Ti-6Al-4V. Heat transfer calculations are performed for typical welding conditions for Ti-6Al-4V. We also developed a quantitative phase field model to predict the α-β microstructure of titanium alloys. The thermal history at different regions of the heat affected zone is obtained using the heat transfer model. This thermal history is used to predict the microstructure in micron sized regions at different locations in the heat affected zones. Our simulations show that the final microstructures change significantly as we get closer to the weld line. The simulations predict that beta phase is retained in regions very close to the weld. Further away towards the base metal, partially transformed domains of alpha are observed. These results are consistent with published experimental data in titanium alloy welds.