Promoting directional spreadability of aluminum/stainless steel system by laser-chemical synergistic surface modification strategy

The joining reliability of aluminum/stainless steel (Al/SS) system is frequently constrained by inadequate control of interfacial wetting behaviour. In this study, a novel laser-chemical synergistic surface modification strategy was developed to promote the directional spreadability of Al/SS system and the underlying mechanism was investigated. The wetting behaviour was investigated using a modified sessile drop method. Surface precursor films and interfacial microstructures of the wetted specimens were characterized, and the mechanisms underlying anisotropic wetting were elucidated through thermodynamic modelling and spreading kinetics analysis. The results indicated that laser scanning minimally affected the surface chemical composition but significantly enhanced the surface energy by deepening the groove morphology. The grooved microtextures facilitated spreading in the groove-parallel direction while suppressing it in the perpendicular direction, with the anisotropic effect becoming more pronounced as the scanning times increased. The formation of the bilayer precursor film composed of (Fe,Cr)Al₃ and (Fe,Cr)₂Al₅ through the subcutaneous infiltration mechanism was identified as critical for initiating and sustaining the spreading process. Additionally, steeper grooves amplified capillary action and pinning effects, leading to melt stretching and distortion. These effects reduced the spreading activation energy in the groove-parallel direction while increasing it in the perpendicular direction, thereby intensifying the anisotropic wetting behavior.