Microstructure, properties and natural ageing behavior of friction stir welded dual-phase Mg-Li alloy

In this study, the super-light alpha + beta dual-phase Mg-9Li-3Al-3Zn alloy was successfully jointed by friction stir welding (FSW). The microstructure, mechanical properties as well as corrosion resistance of the welds were specially explored. The natural age-softening of the welds was found and the underlying mechanism for the properties durability was revealed. FSW resulted in significant grain refinement as well as dissolution of alpha-Mg phases into beta-Li phases in the stirred zone (SZ). Meanwhile, Mg17Al12 (lambda) and AlLi (gamma) precipitates were dissolved into the matrix while orderly-arranged MgLi2Al (theta) precipitates were formed, which contributed to a precipitation-strengthening effect. Therefore, the welds exhibited extensive hardening and demonstrated superior tensile strength than base metal (BM). Under the optimal welding parameters, the ultimate tensile strength (UTS) of the weld reached 165.8% of that of the BM. On the other hand, the welds obtained at the various rotation speeds also exhibited lower corrosion rates than that of the BM, and higher rotation speeds led to superior corrosion resistance. Furthermore, owing to increase in gamma precipitate content during natural ageing, the precipitation-strengthening effect in the weld was weakened, resulting in reduction of the strength of the welds. Meanwhile, the increasing gamma precipitate content aggravated the local micro-galvanic coupling corrosion, causing the decrease in corrosion resistance of the welds during natural ageing. This welding method might be useful for obtaining the dual-phase Mg-Li alloys with high mechanical properties and corrosion resistance.