Study on Microstructure and Microhardness of Oscillating Laser Welded Lap Joint of TC21 Titanium Alloy

The lap tests of 2.3 mm thick TC21 titanium alloy plates were carried out by using oscillating laser welding. The macroscopic formation, microstructure and microhardness of the weld seams under the oscillating amplitude of 1-2.5 mm and the oscillating frequency of 25-150 Hz were studied. The results show that under the laser power of 4 kW, welding speed of 2 m/min, with the increase of oscillating amplitude and oscillating frequency, the spatter on the weld surface decreases, the porosity decreases and the macroscopic morphology of the cross-section changes from nail shape to cone shape. When the oscillating amplitude is 1.5mm, with the increase of the oscillating frequency, both the weld penetration depth and weld width slightly decrease and then tend to be stable, while the weld width of the lap joint surface decreases. When the oscillating frequency is 50 Hz, increasing the oscillating amplitude has little effect on the weld width, while the weld depth and the weld width of the lap surface are significantly reduced. The microstructure of the weld zone of the welded joint is composed of lamellar α, residual β and a very small amount of α" phase. The heat-affected zone can be divided into four sub-zones of the semi-melting zone, the complete phase transformation zone, the partial phase transformation zone and the annealing zone in sequence. In the welded joints, the hardness of the weld zone is biggest. As the distance from the base metal increases, the hardness first decreases and then increases. The lowest point is in the annealing zone. Moreover, under the same oscillating amplitude, the hardness of the weld zone and the minimum hardness of the joint with a large oscillating frequency are lower than those with a small oscillating frequency. Under the same oscillating frequency, the hardness of the weld zone and the minimum hardness value of the joint with a large oscillating amplitude are lower than those with a small oscillating amplitude.