Microstructure and fatigue behavior of variable polarity plasma arc welded Al-6.2Mg alloys

5 mm-thick Al-6.2 Mg aluminum alloys joints with defect-free and excellent appearance of the positive side are obtained by variable polarity plasma arc (VPPA) vertical up welding with optimized parameters. The keyhole formed in the welding process and the variable current mode are the key to guaranteeing deeper penetration and improving the quality of welds for aluminum alloys, respectively. The micro-hardness of weld zone has slight variation on the whole, but local softening and laminar distribution happened. It is consistent with the lamellar segregation in the weld caused by crystallization with change of chemical content. Disperse phase beta(Mg2Al3) and second phase (Fe, Mn) Al-6 in different regions have various particle sizes, shapes and distribution, which have significant effects on the properties of the joints. Fatigue behaviors of the welded joints are characterized under constant loading frequency at a stress ratio of 0.1 and the tests of static tensile strength are also carried out. Compared with the joints welded by traditional tungsten inert gas welding, those joints welded by VPPA have higher endurance limit of 141 MPa and tensile strength of 347 MPa, verifying that VPPA welding is significant to improve the properties of the joint effectively.