Ultrasonic vibration induced the improvement of mechanical properties and fracture behavior of Inconel 690 alloy joint during laser welding

The ultrasonic vibration (UV) assisted laser welding was employed to weld Inconel 690 alloy. The microstructure evolution, the tensile strength, the elongation and the fracture behavior of the weld joint were studied through a scanning electron microscopy, an electron back-scatter diffraction and a universal testing machine. The results indicated that there produced an equiaxed crystal region in the center of the weld joint where a great deal of the low angle grain boundary distributed due to the cavitation effect caused by adding the UV. In addition, the proportion of the equiaxed crystal tended to increase, the grain size seemed to be finer. Moreover, the tensile strength and the elongation of the weld joint with UV were significantly improved with an increase in the UV because of the grain refinement. The maximum tensile strength increased to be the same as that of the matrix, and the maximum elongation was about 70% of that of the matrix. The joint without UV fractured with necking, nevertheless, the fracture of joint with UV occurred in the dendrite region of the weld joint and happened in the direction of 45° to the tensile direction due to the appearance of the similar direction between slip surface and the direction of the maximum shear stress, and the change in Taylor factor and Schmid factor distribution of the dendrite in the weld joint. Besides, the ductile fracture was still the main fracture feature because of the formation of the dimples and micro-voids.