Local Texture, Mechanical Properties And Fracture Mechanisms Of Friction Stir Welded Magnesium Alloys

Local texture is easily formed in weld zone (WZ) of friction stir welded (FSW)magnesium alloys, which had significant effects on the mechanical properties andfracture behaviors of the joints. In this paper, FSW joints of ZK60-AZ31dissimilaralloys and single AZ31alloy were studied. Electron backscatter diffraction (EBSD) wasused to investigate the effect of local texture on the plastic deformation mechanisms,microstructure evolutions, mechanical properties and fracture behaviors of FSW Mgalloys in different deformation methods, such as tension, compression and bending. Twoparts contents were included in this paper. Firstly, the effects of local texture andinterfacial characterization on the fracture behaviors of FSW joints were studied byusing ZK60-AZ31dissimilar joints as the research object. Secondly, the effects of localtexture on the microstructure evolution and mechanical properties of FSW Mg alloy indeformations were studied by using FSW AZ31alloy as the research object.Two kinds of material arrangements (ZK60or AZ31on advancing side, AS) wereused to understand the FSW weld ability of ZK60-AZ31dissimilar Mg alloys. Theresults showed that:①sound joints of ZK60-AZ31dissimilar alloys with free ofwelding defects were obtained by FSW. The grains in stir zone (SZ) were significantlyrefined, while the grain size had significant differences in the SZ,~9μm in AZ31sideand~3μm in ZK60side. Second phases in ZK60side were broken up and thedistributions were changed from the grain-boundary segregation into a randomdistribution in WZ.②Material arrangements did not significantly affect themechanical properties and fracture positions of ZK60-AZ31dissimilar joints. Ultimatetensile strength (UTS) of the joints was78-82%of AZ31base meital (BM). Yieldstrength (YS) of the joints was higher than that of the AZ31BM, but was lower thanthat of the ZK60BM. The joints consistently fractured in the boundry of transition zone(TZ)/SZ in AZ31side, which was attributed to the abrupt change of hardness in theTZ/SZ boundry as well as the "soft" materials in AZ31alloy.Different sampling design in ZK60/AZ31joint was used to study the effects oflocal texture and interfacial characterization on the transverse tensile properties andfracture behaviors. It was showed that three types of samples with different samplingdesign had no significant differences in tensile properties. Entirety (EN) sample showedthe highest mechanical properties and the SZ sample showed the lowest. But the fracture locations of the three types of samples were significantly different. The crownzone (CZ) and SZ samples fractured in the reverse side (RS), close to the “soft”material of AZ31alloy, while the EN sample fractured along the boundry of TZ/WZ inAS, close to the “hard” material of ZK60alloy. This was mainly attributed to a triplejunction region adjacent to the TZ/WZ boundry in AS. The triple junction region hadsignificant differences of grain size and local texture, where the plastic deformationability and mechanisms were significantly different. Deformation incompatibility wasoccurred in the region, which accelerated fracture in the TZ/WZ boundry nearing the“hard” material of ZK60alloy for the EN sample.Differences of microstructure evolutions and mechanical properties of FSW AZ31Mg alloy in transverse tensile and compressive tests were studied. The results showedthat:①plastic deformation mechanisms and microstructure evolutions of the WZwere significant different in both types of tests. Many {10-12} twins formed in SZ-sideand little in other regions during the tensile test. But little twins were observed inSZ-side and many twins were found in SZ-center and CZ-center during the compressivetest. And most twin variants selection in the regions with the dominant deformationmechanism of twinning during two types of tests was suitable by Schmid factor (SF)criterion.②Severe and complex strain localization was observed in WZ for both typesof tests, which resulted in a “concave-convex” appearance on the ND-TD plane. The“concave-convex” appearances were significantly different on the obverse andreverse sides of the ND-TD plane and that in SZ were more severe compared with theCZ. It was found that the complex local texture in WZ was the fundamental reasons ofstrain localization, and basal slip played a major role in the formation of“concave-convex” appearance in WZ.Three-point bending tests (including Root test and Face test) were applied on FSWAZ31Mg alloy. The effects of local texture on the microstructural evolutions andmechanical properties during bending were studied. The results showed that:①microstructural evolutions in SZ of the Root and Face tests samples were significantlydifferent. Many {10-12} twins were observed in SZ-side but little in SZ-center of theRoot test sample. However, little {10-12} twins were seen in SZ-side and many inSZ-center for the Face test sample.②The YS of FSW samples were larger than theas-received plate during the three-point bending and that of the Face test sample washigher than the Root test sample. But the maximum bending strength of the FSWsamples were lower than that of the as-received plate. For both Root and Face tests samples, the fracture behaviors had little differences. And the fracture was propagatedalong the TZ/SZ interface in AS for the low part of WZ, and located in CZ-side on ASfor the upper part.