Evolution of microstructure in MLX®19 maraging steel during rotary friction welding and finite element modelling of the process

Inertia friction welding (IFW) is a solid-state welding process utilised for joining engineering materials. In this paper, a 2.5D finite element (FE) model was developed to simulate IFW of MLX®19 maraging steel. The predicted results showed a non-uniform temperature distribution, with a decrease in temperature from the periphery to the centre of weld interface. Higher temperature and lower stress distributions were predicted in the weld zone (WZ) and the adjacent regions in the vicinity of the WZ. The von-Mises effective stress, effective strain and strain-rate were investigated at different time steps of the FE simulation. The effective stress was minimum at the weld interface, and the effective strain and strain-rate attained a quasi-steady state status with the on-going IFW after a threshold time (~6.5 s). The simulated results were validated by comparing the predicted flash morphology with an actual IFW weld, and temperature profiles measured at specific locations using embedded thermo-couples. The difference between the experimental and the simulated results was ~4.7%, implying a good convergence of the model. Microstructural characterisations were performed across different regions and the observed features were found to be in agreement with the expected microstructure based on the simulated thermal profiles, which included almost complete (~90%) and partial transformation of martensite to austenite in the WZ and thermo-mechanically affected zone (TMAZ), respectively. Analyses of crystallographic texture, showed that the material (i.e., both transformed austenite and martensite) underwent pure shear deformation during IFW.