Friction stir spot welding of rare-earth containing ZEK100 magnesium alloy sheets

Microstructure and Properties of Mg-Gd-Y-Zn-Mn High-Strength Alloy Welded by Friction Stir Welding

Mg-Gd-Y-Zn-Mn (MVWZ842) is a kind of high rare earth magnesium alloy with high strength, high toughness and multi-scale strengthening mechanisms. After heat treatment, the maximum tensile strength of MVWZ842 alloy is more than 550 MPa, and the elongation is more than 5%. Because of its great mechanical properties, MVWZ842 has broad application potential in aerospace and rail transit. However, the addition of high rare earth elements makes the deformation resistance of MVWZ842 alloy increase to some extent. This leads to the difficulty of direct plastic processing forming and large structural part shaping. Friction stir welding (FSW) is a convenient fast solid-state joining technology. When FSW is used to weld MVWZ842 alloy, small workpieces can be joined into a large one to avoid the problem that large workpieces are difficult to form. In this work, a high-quality joint of MVWZ842 alloy was achieved by FSW. The microstructure and properties of this high-strength magnesium alloy after friction stir welding were studied. There was a prominent onion ring characteristic in the nugget zone. After the base was welded, the stacking fault structure precipitated in the grain. There were a lot of broken long period stacking order (LPSO) phases on the retreating side of the nugget zone, which brought the effect of precipitation strengthening. Nano-α-Mn and the broken second phase dispersed in the matrix in the nugget zone, which made the grains refine. A relatively complete dynamic recrystallization occurred in the nugget zone, and the grains were refined. The welding coefficient of the welded joint exceeded 95%, and the hardness of the weld nugget zone was higher than that of the base. There were a series of strengthening mechanisms in the joint, mainly fine grain strengthening, second phase strengthening and solid solution strengthening.

Comparative study on twinning characteristics during two post-weld compression paths and their effects on joint enhancement

Friction stir welding (FSW) has promising application potential in Mg alloys. However, the texture distribution in stir zone (SZ) is usually complicated for Mg alloys, which deterioriates the joint performance. In this study, the texture distribution in SZ was tailored by applying two kinds of post-weld compression deformation along normal direction (ND) or welding direction (WD) of the FSWed AZ31 Mg alloy plates. The twinning behavior and texture change in the various regions of SZ were then evaluated by electron back scatter diffraction (EBSD) characterization. The effect of texture change on the joint performance was discussed in terms of Schmid factors (SFs) for basal slip and extension twinning. The results showed that profuse extension twins were formed through the whole SZ for the sample subjected to compression along ND, whereas they were observed mainly in SZ-side for the sample compressed along WD. Most of the twins were present in the forms of twin bands or chains. The directions of the twin bands or chains were related to the habit plane traces of selected twin variants. The ND post-weld compression had better strengthening effects on the joints compared to the WD compression, and the underline mechanism was discussed.