Interfacial Reactions And Joining Mechanism During Laser Welding-brazing Of Mg To Ti Assisted With Cu Coating

Facing the requirement of environment protection and high energy efficiency,fabricating reliable Mg/Ti hybrid structure would offer tremendous advantages in terms of weight reduction and energy conservation as well as scale their applications.However,Mg and Ti have enormous differences in physical performance and no solubility or reaction is found between them.Therefore,selecting appropriate welding methods and interlayer elements to produce and control interfacial layers became the focus of Mg/Ti joining.Related research indicated that controlling the heat input of welding parameter and the content of interlayer elements were effective way to regulate interfacial reaction.The 1.5-mm-thick AZ31B magnesium alloys and 1-mm-thick Ti-6Al-4V titanium alloys were welded with AZ92 Mg based filler and Cu electroplated interlayer in lap configuration by laser welding-brazing.The element distribution,interfacial microstructure and mechanical properties of joints were studied to illustrate the effect of Cu on Mg/Ti metallurgical bonding.At first,the evolution of interfacial microstructure with various laser power was observed.The results suggested that increasing laser power contributed to the growth of Ti₃Al interfacial layer,benefiting the metallurgical joining.However,higher laser power would give rise to the severe burning loss of Mg,unsatisfying joint appearance and mechanical performance.Then the influence of Cu content on microstructure development and joint fracture load was analyzed.The result showed that only Ti₃Al reaction layer was produced along the interface at the direct irradiation region.This Ti-Al diffusion zone increased with the increase of coating thickness.The interfacial layer at the middle region developed from Ti₃Al to Ti₃Al/Ti₂Cu to Ti₃Al/?Ti₂Cu+AlCu₂Ti?with thicker Cu coating.Ti₂Cu formed when coating thickness excessed 10.6?m at the weld toe region and this Ti-Cu reaction layer grew obviously with the increase of coating thickness.The observation of fracture path indicated crack propagation was hindered by Ti₃Al layer at the direct irradiation region and Ti-Al/Ti-Cu layer at the middle region.However,a mass of??-Mg+Mg₂Cu?eutectic structure exhibiting network-like distribution was produced with much thicker Cu coating,which was detrimental to joint performance.Finally,the chemical potential prediction model and Gibbs free energy prediction model of Mg-Cu-Al-Ti quaternary alloy system were established.Combined with element distribution near the interface,the diffusion behavior of Cu atoms was illustrated clearly and the reason for no concentration of Cu element along the interface at the direct irradiation region was explained.The effect of Cu content on chemical potential of Ti or Al was calculated,suggesting Cu could promote mutual diffusion between Ti and Al.The interfacial reaction was predicted by the Gibbs free energy of quaternary alloy system.A part of calculation results was in accordance with real results,implying the Gibbs free energy of system could predict interfacial reaction to some extent.But the real interfacial products were limited by other conditions,such as welding method and alloy element content.