| Dissimilar metals joints have enormous advantages in reduing weight and improving fuel efficiency.Ti/Al joint combind the advantages of Ti(high strength and corrosion resistance)and Al(low weight and low cost)sufficiently.This hybrid structure could be used in aerospace and transportation industries.The challenge of Ti/Al joining was to control the brittle intermetallic compounds(IMC)generated at Ti/Al interface.Because of appropriate energy position,laser welding-brazing was expected to control the IMC layer for obtaining a high-quality Ti/Al joint.Laser welding-brazing technique had attracted increasing attention in Ti/Al joining.In this study,TC4 Ti alloy and 6061 Al alloy were selected as base metal.The problems occurred in brazing zone,heat-treatable zone(HAZ)of Al base metal and fusion zone of TC4/6061 joint had been solved gradually.Firstly,the influence of welding parameters on weld appearance and mechanical properties of dual-spot laser welded-brazed TC4/6061 joint was analyzed.The relationship between IMC layer and fracture behavior of joint was studied.The optimal IMC characteristic of TC4/6061 joint was confirmed.The soften phenomenon occurred in HAZ at Al side in TC4/6061 joint was eliminated by the post-weld heat treatment(PWHT).The microstructure evolution of IMC layer and fusion zone during PWHT was analyzed.Based on microstructure characterization and thermodynamics calculation,the influence of Si on Ti-Al interfacial metallic reaction was analyzed systematically.Finally,laser welding-brazing with filling powder combined with PWHT was selected for control the distribution of IMC layer,microstructure of fusion zone and HAZ synchronously.The strength of TC4/6061 joint was enhanced.This study could provide theoretical guidance for Ti/Al joining.Firstly,the influence of dual-spot laser beam on weld appearance,IMC layer and mechanical property of TC4/6061 joint was analyzed.Compared to single-spot,dual-spot laser beam improved the spreadability of molten wire on TC4,reduced the thickness of IMC layer and improved the homogeneity of IMC at brazing interface.Dual-spot laser beam was beneficial for improving the tensile streng of TC4/6061 joint.For dual-spot laser welded-brazed TC4/6061 joint,the distribution characteristic of IMC layer dominated the fracture behavior of joint during tensile test.0.35?2.10-?m-thick continuous serrate-shaped IMC layer was beneficial for improve interface bonding force between TC4 and weld,and increased the tensile strength of joint.When the IMC layer was continuous 0.45±0.10-?m-thick serrate-shaped,the tensile strength of TC4/6061 joint with reinforcement reached up to 241MPa.The influence of key welding parameters including groove shape,laser power on weld appearance and tensile behavior of joint was analyzed.The generation condition of optimal IMC layer at brazing interface was obtained.PWHT was adopted for eliminating the softening phenomenon occurred in HAZ at Al side of joint,and the tensile strength of joint was enhanced.The influence of heat treatment on microstructures of IMC layer and fusion zone was investigated.During 530?heat treatment,the thickness of IMC layer was increased with the heat treatment time increase.The IMC layer growth mechanism during PWHT was controlled by the interface reaction control mechanism.The phases of IMC was Ti(Al,Si)3,Ti5Si3and Ti7Al5Si12 in welded-brazed sample.After 530?heat treatment,the phase of IMC changed to Ti7Al5Si12 with high density dislocations.This phase transition increased the sensitivity of joint strength to the IMC layer thickness.The Al-Si eutectic in fusion zone was replaced by Si particles after heat treatment.The size of Si particles was increased with the heat treatment time increase.After 530?/2 h+180?/10 h heat treatment,the tensile strength of TC4/6061 joint without reinforcement was 260 MPa,the tensile strength of joint with reinforcement reached up to 300 MPa,which was 94%of that of Al base metal.Based on the numerical simulation and calculation of the Gibbs free energy of Ti-Al-Si ternary system,the formation mechanism of pahses in IMC layer was clarified.The diffused mechanism of Si from weld to brazing interface was clarified.The type of pahses in IMC layer did not change,when the content of Si element in filler metal exceeded 5.0 wt.%.The phases in IMC layer was Ti(Al,Si)3 materix phase,Ti5Si3and Ti7Al5Si12 nano-granular pahses.There were two different formation mechanism of nano-granular phases.The nano-granular pahses in Ti/Ti(Al,Si)3 interface was generated through direct reaction determined by Gibbs free energy.The nano-granular phases in Ti(Al,Si)3 materix phase was generated through precipitation mechanism.The chemical potential of Si(?Si)decreased as the Ti content increased,resulting in the aggregation of Si atoms at the brazing interface.Finally,Al-10Si-Mg powder was induced in TC4/6061 joining through laser welding-brazing with filling powder.The distribution of IMC layer was controlled by laser melting deposition layer.Mg2Si and?'strengthening phases in fusion zone was precipitated through PWHT,and the soften phenomenon occurred in HAZ at Al base metal was eliminated.The tensile strength of TC4/6061 joint without reinforcement was increased from 260 MPa to 291 MPa. |
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