Research On Enhancing Performance Of External Source/phase Assisted Friction Stir Spot Welded AZ31 Magnesium Alloy Joints

Due to their superior performance, the promising materials of magnesium alloys possess extensive application in the fields of electronics, automobile, aerospace and medical biomaterials. However, their readily oxidization, remarkable deformation, good thermo conductivity, poor weldability and deformation ability make it hard to produce magnesium alloy products by fusion welding process. This paper received AZ31 magnesium alloy as experimental material, investigated the influence of heating tool, liquid nitrogen cooling and nano-Si C particle on the microstructures and mechanical properties of friction stir spot welding(FSSW) AZ31 magnesium alloy joints, and established the inherent relationship between microstructures and mechanical properties to clarify the strengthening mechanisms of the joint welded under different welding conditions. Based on the experiment examination and theoretical analyzation, the main conclusions were obtained as below:The effects of heating tool on the microstructures and mechanical properties of FSSWed AZ31 magnesium alloy joint were investigated systematically by the microstructure examination and mechanical performance testing. The results showed that with an increase in the heating tool temperature, the grain size of stirred zone decreased, the bonded zone was widened, resulting in increasing hardness of stirred zone and tensile shear strength of joints. However, the increasing heating temperature led the grains of thermo mechanically affected zone(TMAZ) to grow and coarsen, weaken its hardness. Moreover, the slope of the Hall-Petch relationship between microhardess and grain size of the TMAZ is larger than that of the stirred zone.The effects of liquid nitrogen cooling on the microstructure and mechanical properties of FSSWed AZ31 magnesium alloy joint were investigated systematically by the microstructure examination and mechanical performance testing. The results showed that as the liquid nitrogen cooling time increased, the interface slope of liquid nitrogen cooling FSSWed(C-FSSWed) AZ31 magnesium alloy joint decreased, the grains of stirred zone and HAZ were refined, thus improved the hardness of stirred zone and HAZ. When the cooling time was as long as 10 s, the HAZ was eliminated absolutely. However, the grain coarsening of TMAZ in combination with the cooling time increasing induced the degradation in the hardness of TMAZ. In addition, the tensile shear load maximums of joints welded with cooling time of 5 s and 7 s were greater than that of conventional FSSWed joint, but the interfacial cracking widened with the increasing cooling time, decreased the tensile shear load maximum of joint.The effects of liquid nitrogen cooling on the microstructure and mechanical properties of FSSWed AZ31 magnesium alloy joint were investigated by the microstructure examination and mechanical performance testing. The results showed that Si C enhancing FSSW joint with dwell time of 3 s was characterized by a typical onion ring structure in stirred zone. The onion ring structure was consisted of alternate Si C-free zones and Si C-rich zones where Si C particles refined the grains. When dwell time was 5 s, onion ring structure disappeared and Si C particles dispersed homogeneously along grain boundaries and inside grains. The tensile and microhardness tests show the microhardness of stirred zone and tensile shear load of Si C enhancing FSSW joint were higher than those of conventional FSSW joint. After heat treatment at 200 ℃ for an hour, grains of stirred zone of FSSW joint grew substantially and thus led to a reduction in tensile shear load and microhardness of stirred zone, while grain size of stirred zone and tensile shear load of Si C enhancing joint was almost invariant but the microhardness of stirred zone increased abnormally.