Influence of galvannealed zinc coating on refining microstructure and enhancing mechanical performance of laser brazed DP600 and DP980 steels

Laser brazing is a critical process for class-A automotive joints, with coating properties significantly affecting joint quality. However, the direct relationship between coating variations and the resulting microstructure and mechanical performance in laser-brazed advanced high-strength steels (AHSS) remains largely unexplored. This study systematically investigates how galvannealed (GA) coating thickness and Fe content within the coating influence laser brazing of DP600 and DP980 steels, including similar and dissimilar configurations. It reveals that variations in coating, rather than bulk steel chemistry, govern joint behavior. Thicker Zn coatings enhanced wetting but introduced surface imperfections, while a lower Fe content increased the cooling rate during DP980 brazing directly refining both the Cu braze microstructure and Fe(Si) interfacial reaction layer. The DP980 joint achieved the highest fracture load (4.4 kN for a 15-mm-wide strip), attributed to its refined braze grains and improved interfacial integrity. The dissimilar DP980-DP600 joint exhibited a refined microstructure compared to the DP600 braze, though the DP600/Cu boundary limited its joint strength due to incomplete wetting. Micro-indentation tests confirmed enhanced yield and tensile strengths in the DP980 braze. This study opens pathways for investigating tailored coating chemistries and thicknesses as design parameters for optimizing braze microstructure and mechanical integrity in AHSS assemblies.