The effect of evaporation on laser-induced wetting and spreading behaviors of Mg alloy on Cu-coated steel substrates

This work systematically investigated the effect of evaporation on the non-isothermal wetting behavior of liquid Mg on Cu-coated steel substrates using a laser-induced dynamic droplet method. The results indicated that wetting behavior was significantly governed by laser power, which controlled both the evaporation rate and temperature. At low laser powers (<2500 W), evaporation consumed the energy available for wetting, resulting in a non-wetting state. Conversely, as the temperature increased, intensified Cu diffusion into the Mg melt promoted the formation of Mg₂Cu intermetallic compounds (IMCs) and reduced surface tension, thereby improving wettability. However, when the laser power exceeded 3000 W—especially at temperatures above the boiling point of Mg—evaporation increased dramatically. This process reduced the droplet volume and caused the contact radius (CR) to retract. Although the dissolution of the Cu coating and the formation of Mg₂Cu stabilized the contact angle (CA) at approximately 23°, the intense evaporation severely hindered further spreading. This study reveals a complex interplay among evaporation, temperature, and interfacial reactions that critically controls the wetting dynamics of Mg on Cu-coated steel. These findings provide crucial insights for optimizing Mg-based alloy joining and coating technologies under non-isothermal conditions.