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A mathematical model is developed to investigate numerically the interaction mechanism between the ultrasonic vibration and the plasma arc in a plasma arc welding (PAW) process variant, i.e. ultrasonic assisted keyholing PAW (U-PAW). It is found that when ultrasound is exerted into the plasma arc from the tungsten electrode, the arc pressure is greatly increased, but the increment at different regions in arc space is varied. Near the anode surface, the difference between the peak value of the plasma arc pressure in U-PAW and the arc pressure in PAW is much larger than the additional acoustic pressure, indicating that the plasma arc pressure in U-PAW is not simply a superposition of plasma arc pressure and ultrasonic pressure. Experimental measurements of plasma arc pressure at anode are conducted in both PAW and U-PAW, and the numerical simulation results are in good agreement with the measured ones. This work lays foundation for understanding the underlying phenomena in U-PAW.

The distribution of the residual stress in the weld joint of HQ130 grade high strength steel was investigated by means of finite element method (FEM) using ANSYS software. Welding was carried out using gas shielded arc welding with a heat input of 16 kJ/cm. The FEM analysis on the weld joint reveals that there is a stress gradient around the fusion zone of weld joint. The instantaneous residual stress on the weld surface goes up to 800 ~ 1000 MPa and it is 500 ~ 600 MPa, below the weld. The stress gradient near the fusion zone is higher than any other location in the surrounding area. This is attributed as one of the significant reasons for the development of cold cracks at the fusion zone in the high strength steel. In order to avoid such welding cracks, the thermal stress in the weld joint has to be minimized by controlling the weld heat input.