Microstructure evolution and mechanical properties of AA2195-T8 Al-Li alloy back gas-shielded VPPA weld

Variable polarity plasma arc welding (VPPA) is an adaptable and efficient method for welding large Al-Li alloy components like rocket fuel tanks. This work investigated the microstructure and properties of AA2195-T8 Al-Li alloy weld using the back gas-shielded VPPA method. The process of microstructure evolution is established to reveal the softening mechanism of the weld. The results show that the mechanical properties of the weld are significantly reduced without back gas shielding, the back of the weld oxidizes to form Li2O with MgO, resulting in back hollows and pores. The addition of back gas shielding during welding solves the above problem. With the addition of back gas shielding, the increase of the strength and elongation for the weld with reinforcement is 33.6 % and 96.7 %, and for the weld without reinforcement is 6.9 % and 28.9 %. There is no significant effect on the microstructure of the weld. The second phases in the weld are mainly Al-Cu and Al-Cu-Mg-Ag eutectic, with a small amount of Al7Cu2Fe and separated eutectic θ phase. The matrix phase in the weld is mainly the composite phase composed of θ and Ω phases. The dissolution of the nanoscale precipitated phase, severe grain boundary segregation, burnout of alloying components, and microscopic defects are the main causes of the reduction in weld properties. In summary, this work provides practical suggestions for optimizing the VPPA welding process to improve the mechanical properties of Al-Li alloy welded joints.