Influence of Cu coating thickness on interfacial reactions in laser welding-brazing of Mg to Ti

Laser welding-brazing Mg/Cu-coated Ti in lap configuration using various coating thickness was performed with AZ92 Mg based filler. The thermodynamic calculation was conducted to help to analyze microstructure evolution. The brazed joint was divided into three regions. Only Ti3Al interfacial layer was generated at the direct irradiation region and grew obviously when thicker Cu coating was adopted. The interfacial reaction layer evolved from Ti3Al to Ti3Al/Ti2Cu to Ti3Al/Ti2Cu + AlCu2Ti at the middle region with the increase of Cu coating thickness. Ti2Cu interfacial layer was observed at the weld toe region and grew evidently as Cu coating thickness increased. The calculation result of chemical potential in AZ92Mg-Cu-Ti system suggested that Cu could promote mutual diffusion between Ti and Al. Ti preferred to react with Al than Cu, indicating Al-Ti compounds were easily produced along the Ti surface. The maximum fracture load was 2314 N with coating thickness of 10.6 μm, which was 55% higher than that with bare Ti sheet and as high as 85% of Mg parent metal. The fracture occurred along the Mg/Ti interface at the direct irradiation region while the crack propagated in Mg-Cu eutectic structure near the interfacial layer at the middle region.