into the liquid was also observed. Meanwhile, Mg atoms from the liquid may slightly diffuse into the Ni-alloyed steel A B C D side. Therefore, a thin diffusion or transition layer forms continuously along the interface between steel and fusion zone from the bottom side to the top side of the joint. This transition layer is a solid solution of Ni in Fe (with low content of Mg for transition layer II). This Fe(Ni) solid solution with FCC crystal structure is more favorable for bonding to Mg than having a body-centered cubic (BCC) phase along the interface. Zhang et al. (Ref. 20) used an edge-toedge matching crystallographic model to predict all orientation relationships between crystals that have simple hexagonal close-packed (HCP) and BCC structures, and they found that the lattice mismatching of HCP (Mg) and BCC (Fe) is very large. On the basis of the observation in our study, a diffusion layer composed of Fe and Ni with FCC structure can provide the conditions for the heterogeneous nucleation of α-Mg during solidification. The result is formation of a metallurgical bond between the steel and magnesium alloy. A recent study by Liu et al. (Ref. 4) showed that a nano-layer of Fe2Al5 on steel can also be a transition layer to bond Fe to Mg due to the low energy interfaces and good match of lattice sites between Fe and Fe2Al5 as well as Mg and Fe2Al5. The same behavior was observed for the Fe(Ni) transition layer in this study. Formation of a transition zone was also reported in other studies (Refs. 5–8) using different joining techniques, when an interlayer was used between steel and Mg alloy. These transition layers on steels were reported to make it possible to join Mg and steel. Thirdly, during the solidification process, the AlNi phase with a high melting point (1133°C) precipitates from the liquid and grows in a form of faceted dendrites very close to the interface — Fig. JANUARY 2013, VOL. 92 8-s WELDING RESEARCH Fig. 11 — SEM images of typical fracture surfaces after the tensile shear test. A, B — Fusion zone side at different magnifications; C, D — steel side at different magnifications. Fig. 12 — X-ray diffraction pattern of the fracture surface of the steel side.
Welding Journal | January 2013
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