Research On Temperature Field, Microstructure And Mechanical Properties Of Mg Alloy Processed By FSW

In this paper, basing on the principle of that FSW joint is characterized withasymmetric gradient in three dimensions, friction-stirring welding (FSW) experiments ofAZ31magnesium alloy were carried out. The optimized welding parameters (S=1500r/min,F=50mm/min,Z=-3.85mm) were determined according to the results of welding tests. Thevariation principles about welding temperature distribution,macrostructure andmicrostructure, mechanical property of the joint were studied along three dimensions(transverse direction, longitudinal direction, thickness direction) by means of thermocouple,mechanical tests,super depth of focus digital microscope and SEM.It was found that the phenomenon of "two peaks" and "lagging behind" was existed intemperature curves of featured points in the joint. The peak temperature of featured points inthe beginning stage was lower, the one in the middle stage was high, and the one in theending stage was higher. In the direction perpendicular to the longitudinal direction, thefarther of the featured points from the surface, the lower of peak temperature, and theshorter of the lasting time above recrystallization. The temperature of advancing side ishigher than the retreating side. In the thick direction, the nearer of the featured points fromthe surface, the temperature was higher, and the lasting time above recrystallization waslonger.Microstructure of joint shows the significant differences along the transverse andthickness direction. Along the transverse direction, the dynamically recrystallizedmicrostructure in weld nugget zone (WNZ) is uniform and small; There are two thermalmechanical affected zones (TMAZ) beside the weld nugget zone, which are characterizedwith bended and elongated grains. At advancing side, the interface between TMAZ andWNZ is very distinct.While at retreating side, the interface is indistinct and part ofmaterials with some impurities flow from the advancing side to the retreating side, soTMAZ of advancing side is the weakest zone in the joint. Fracture mode is ductile andbrittle mixed fracture. Microstructure of heat affected zone (HAZ)is similar to that of thebase metal(BM), however, the phenomenon of grain growth is not obvious. Along the jointthickness direction, a little lager grains are observed in the most upper zone.Next to theupper zone is a center zone, weld nugget zone (WNZ), with finer and very uniform equiaxeddynamic recrystallization microstructure and a best tensile property. A very thin layer is located at the bottom zone with very fine but nonuniform grains. The size of the grains andstrength of joint is higher at the middle stage, however, the starting and the ending stage ofwhich are smaller and lower respectively.