Interfacial microstructure and corrosion behaviors of TC4/DP780 steel joints by laser welding with H62 interlayer

Abstract

TC4/DP780 dissimilar metals were laser welded with different thicknesses of the H62 interlayer. Joints with good forming and mechanical properties were obtained. The corrosive behaviors of TC4/DP780 joints in 3.5 wt% NaCl solution were investigated by micro-area scanning vibrating electrode technique (SVET). Results showed that TC4/DP780 joints effectively hindered the brittle intermetallic compounds of Ti–Fe through mutual diffusion and the solid solution between the H62 layer metal and TC4 alloy. Cu–Ti intermetallic compounds and H62 Cu-based solid solution formed at the H62/TC4 interface. The intermetallic compounds layer grew dendritically from the dependent TC4 alloy toward the weld. With increased Ti element content and interlayer temperature, the microstructure morphology changed from fine dendrites to coarse dendrites. Corrosion experiments further revealed that with increased thickness of the H62 interlayer, the corrosion resistance of joints initially increased and then decreased. The joints added with a 0.4 mm H62 interlayer had the highest self-corrosion potential and the lowest self-corrosion current density. The corrosion rate of joints was the slowest. DP780 steel formed loose porous Fe2O3, accelerating the metal-corrosion dissolution. The H62 primarily formed due to the corrosion susceptibility of ZnO, leaving the porous structure of the Cu layer and the enabling the formation of CuO. TC4 had the highest corrosion potential and was protected as the cathode. TC4 underwent galvanic coupling corrosion with the DP780 steel and the H62 interlayer metal. Meanwhile, TC4 was oxidized in air to form a dense TiO2 and Al2O3 protective film, which had good restoration and passivation effects.

Keywords

• TC4/DP780 joint
• Immersion corrosion
• Macro electrochemistry
• SVET