Zuming Liu

Novel postweld heat treatment method for improving mechanical properties of resistance spot weld

Abstract A novel postweld heat treatment method by applying cross-direction welding current (cross-current) was applied on the resistance spot weld. The effect of cross-current on the nugget shape, microstructure and mechanical properties was investigated. The results show that the cross-current postweld heat treatment enhances the efficiency of postweld heat treatment and improves the mechanical performance of nugget. The cross-current promotes the recrystallisation of nugget microstructure, which induced the transformation of microstructure from column crystal to quasi equiaxed grain. The quasiequiaxed grains of martensite in the weld nugget significantly increase the microhardness of weld nugget and the tensile shear force of weld joint.

Dynamic variation of keyhole exit and its inclination in plasma arc welding

The dynamic behavior of keyhole in plasma arc welding determines the weld quality. In this study, the keyhole images are observed during the process, and both the keyhole geometry and its position relative to the torch axis are determined. The deviation distance of the keyhole exit at bottom side of the workpiece from the torch centerline is used to describe the extent of keyhole inclination and is measured based on the captured sequential images of the keyhole exit. Experiments are conducted to study the dynamic variation of keyhole exit and its inclination extent. The interaction between the plasma arc and the molten pool and its influence on the keyhole inclination are analyzed. The results lay solid foundation for monitoring and controlling the thermal process in keyhole plasma arc welding.

Visualization of dynamic keyhole behavior in waveform-controlled plasma arc welding

To overcome the shortcomings of conventional plasma arc welding (PAW), the “controlled pulse keyholing” strategy is proposed and the waveform-controlled keyhole PAW system is developed. To deeply understand the dynamic behaviors of keyhole in this novel PAW process, it is essential to measure and monitor the dynamic variation information of the keyhole geometry in real time. In this study, a vision system is developed to acquire the images of the keyhole from the underside of the workpiece. When a fully penetrated keyhole (open keyhole) is formed, clear keyhole images are captured. Both CCD camera and an efflux plasma voltage sensor are used to measure and characterize the keyhole shape and size during the waveform-controlled PAW process. The dynamic variation features of keyhole shape in a pulse cycle are visualized. The phenomena of “one keyhole in each pulse” and periodic partial-open keyhole transformation are experimentally sensed. The observation results lay solid foundation for controlling keyhole stability and optimizing the process parameters in keyhole plasma arc welding.