Behavior of melt flow and porosity formation in laser welding of steel to aluminum with cold-sprayed steel interlayer

A multimaterial car body design, using light materials in combination with steel, has become widespread in recent years in order to reduce a vehicle's weight. The joining of dissimilar materials is one of the most critical issues in this area. In our previous research, we reported on a new dissimilar laser lap joining method, using a cold-spray process. In that procedure, a steel coating was first sprayed onto an aluminum surface, after which a steel sheet, together with the coating, was laser welded. However, it was found that porosity occurred in the weld metal due to melting of the coating. In this study, we investigated the effects of welding parameters on porosity formation in this joining process as well as the behavior patterns of the flow and bubbles in a molten pool. The porosity increased as the laser input energy decreased. It was also discovered that the porosity could be reduced drastically by the melting of the aluminum substrate, together with the steel sheet and coating, even at lower input energy. Electron probe microanalyzer results showed that both the solidification rate, related to the input energy, and aluminum's melting were strongly associated with the generation of flow from the bottom to the surface of the molten pool. This was captured by means of an x-ray in situ observation, using tungsten tracers, behind the keyhole. The bubbles, moving upward and reaching the surface of the pool, were also observed there in the welding conditions, where only a little porosity occurred. These results demonstrated that this flow helped the bubbles move up and go outside the molten pool and, hence, prevented porosity formation.