Grain refinement and laser energy distribution during laser oscillating welding of Invar alloy

Laser oscillating welding method was employed to welding of Invar alloy so as to achieve grain refinement and decrease internal defects. Effects of frequency and amplitude of beam oscillation with sinusoidal trajectory on weld appearance and solidification microstructure were researched. The essential effect of beam oscillation could be attributed to change the laser energy deposition distribution on the processing surface. Optical metallographic and electron backscattered diffraction analysis results demonstrated that beam oscillation could achieve the transformation of solidification microstructure from oriented dendrite grain to equiaxed dendrite grain. Further, the application of beam oscillation might retard solidification cooling speed and strengthen the formation of γ -fiber texture that is conducive to the improvement of ductility. According to the acquisition of dynamic feature pictures with high-speed camera, the local rapid movement of laser spot resulted in a more stable keyhole without fluctuation and the intensification of melt flow. Temperature curves calculated by finite element method simulation showed that lateral laser oscillation made the temperature distribution of molten metals nearby the center of weld more uniform even negative temperature gradient. The comprehensive description of grain refinement mechanism caused by beam oscillation was presented on the basis of the experiment and simulation analyses. Keywords: Grain refinement, Laser oscillating welding, Invar alloy, Solidification microstructure, Finite element method