Influence of sampling design on tensile properties and fracture behavior of friction stir welded magnesium alloys

Dissimilar joints of ZK60 and AZ31 alloys were obtained by friction stir welding (FSW). Two types of sampling designs were employed for transverse tensile tests to evaluate the tensile properties and fracture behavior of the joints. One kind of samples was machined excluding the upper part of the weld zone (WZ) and named as the stir zone (SZ) sample. Another kind was machined including the entirety of WZ and named as the entirety sample (EN sample). Both types of samples have strength higher than the base alloy AZ31 but lower than the base alloy ZK60. The tensile properties of the EN sample are slightly better than those of the SZ sample. Significant differences in fracture behaviors were observed between the two types of samples. The fracture of the SZ sample initiated at the boundary of transition zone (TZ)/SZ in retreating side (RS) and propagated toward the SZ. However, the EN sample fractured in advancing side (AS), close to the “hard” material of ZK60 alloy, with more complex fracture morphology. A comprehensive characterization of microstructure and texture distributions was carried out to explain the deformation and fracture behaviors. The grains in the SZ-side are favorably oriented for basal slip or extension twinning, being the weak position during the transverse tensile tests. The grains in the side of crown zone (CZ) have the c-axis tilted toward the normal direction (ND), being harder for extension twinning compared with those in the SZ-side. A triple junction region was clearly observed adjacent to the interface of TZ/WZ in AS, but it was not observed in RS. The grains in the triple junction region were significantly refined but inhomogeneous among the various parts. The texture of the triple junction region was close to that of the CZ-side, but with a tendency to tilt toward that of the SZ-side. This observation confirmed that the triple junction region was located between the SZ-side and CZ-side of the WZ. Significant differences in the grain size and Schmid factor (SF) distributions for slip and twinning were observed among the different parts, which could cause deformation incompatibility, generate stress concentrations and accelerate fracture in the triple junction region in AS.