Effect of beam oscillation and oscillating frequency induced heat accumulation on microstructure and mechanical property in laser welding of Invar alloy

In the present work, the 3 mm thick Invar alloy butt joints are connected via circular oscillating welding. The macro appearance, microstructure morphology and mechanical property of the welded joints obtained under different oscillating frequencies are studied. The laser energy distribution is simulated to visually exhibit the range of laser radiation and the degree of energy coincidence for better explaining the formation mechanism of weld geometry. A CFD model for laser oscillating welding process is developed. The powerful vortexes induced by beam oscillation are observed which have a stirring effect on the molten pool and the heat accumulation in the weld center is much severer with increasing oscillating frequency. The influence of beam oscillation on grains around the fusion line is significant but the increase of oscillating frequency is not always beneficial for the microstructure in the weld center. For the situation with a relatively lower frequency (50 Hz), several grains are broken and smaller grain size is obtained. However, the grain refining effect caused by beam oscillation is not obvious as the frequency grows to 100 Hz. This phenomenon is ascribed to the increasing heat accumulation. The tensile test results show that the maximal tensile strength reaches 410.7 Mpa and the elongation is increased by 23.7 % compared with the non-oscillated weld when f = 50 Hz, a mass of equiaxed dimples and a small quantity of serpentine sliding are observed on the fracture surface.