The Influence of Preplating Ni Parameters on Microstructure and Mechanical Properties of Gleeble Tests of DH Galvanized Steel

The liquid metal embrittlement (LME) cracks form in galvanized DH steel during resistance spot welding because of the low melting temperature of Zn layer. Zn penetrates into steel substrate through the liquid Zn infiltration and the rapid solid‐state diffusion. The crack propagation path obtained by electron probe microanalyzer shows that the path is along the distribution of Zn. The preplating technique in this study is supposed to reduce the LME sensitivity of DH galvanized steel. It has been proved by transmission electron microscopy observation that the preplated Ni layer shows columnar‐like grain morphology. The liquid Zn would form high‐melting‐point intermetallic phase Ni 5 Zn 8 together with Ni when diffusing through the channels of columnar Ni layer. The higher current density preplating technique produces more nucleation sites, introducing more tinier and denser columnar Ni crystals. Then, there would be more channels for the Ni–Zn intermetallics to form, preventing Zn from penetrating into the substrate. The mechanical properties of galvanized plates and preplated plates have been evaluated by the Gleeble tests. The preplated ones possess superior elongation. Correspondingly, the higher current density preplating sample performs the longer displacement. Therefore, the preplating technique, especially the higher current density one, can be employed to alleviate the LME phenomenon.