Abstract: With high productivity, easy adjustment of welding parameters and free of weld defects, the variable polarity plasma arc welding process that is made up of variable square wave welding power source is used successfully to weld the wrought magnesium alloy AZ31B. The microstructure characteristics of the weld are investigated by the optical microscope, electron probe microanalysis (EPMA) and X-ray diffraction. At the same time, the micro-hardness of the joint is also measured by the instrument. The results show that the heat-affected zone within joint is not evident, and the grains of fusion zone (FZ) are tiny. Besides, the distribution of element Al and Zn are even in the joint in spite of the vaporizing of Mg. The element seg-regation and Mg17Al12 phase can not be found in FZ. The mi-cro-hardness in FZ exist a little change than that of the parent metal due to the evaporation of Mg element both in longitudinal and transverse direction. In summary, the variable polarity plasma arc welding process is a perfect welding method which can improve the microstructure of joint and increase the weld-ing quality.

Key words: Magnesium alloy, Microstructure, Variable polarity plasma arc