| In the aerospace and automobile industry, hybrid structures of titanium and aluminum have great potential for application. However, joining of Al to Ti is difficult due to great differences of their thermophysical and thermochemical performances. Preventing formation of intermetallic compounds is burning question during joining Al to Ti.In this dissertation, formation of intermetallic compounds was controlled effectively with laser welding-brazing method which has the accurate heat input and hybrid characteristic of welding and brazing joints were obtained. Reliable joining of Al/Ti dissimilar alloys was achieved. Laser welding-brazing of Ti/Al dissimilar alloys was investigated detailedly. The parameter rules of influence on weld appearance, interfacial reaction and mechanical property were clarified. It provides a foundation for controlling joint quality. On these bases, the thickness of reaction layer was calculated accurately in the case of thermal cycle with fast heating and cooling characteristic. Furthermore, these results can be used as reference in other dissimilar alloys welding.Ti-6Al-4V alloy and 5A06 Al alloy plates with thickness of 1.5mm were selected as the parent metals. Because Si can control Ti/Al interfacial reaction, the filler wire Al-12Si with a diameter of 2mm was used. Characteristic of laser welding-brazing technology was investigated as heating source by CO2 laser. Because of local heating of laser beam and short time of solid/liquid interaction, study of this technology focus on the influence of laser spot including circular and rectangular, heating zone of laser beam and groove type on weld appearance and tensile strength of the joints. On one side, stabile welding process and good double weld appearance was obtained by rectangular laser spot with uniform energy distribution with V-shaped groove. On the other side, uneven distribution of reaction layer along thick direction of the joint induced by high temperature grade of laser location heating was avoided and whole interfacial reaction of joint was promoted. The cohesion of the interface is higher than the seam and the average tensile strength of joint is 278MPa which is increased at least 39% than the velue of the current results. The phase compositions were identified by micro-beam XRD analysis, SEM and TEM electronic microscope. The microstructural characteristics of the interfacial reaction layer were described. The interfacial microstructures consist of thin continuous layer with nano-particle Ti7Al5Si12 and discontinuous serration-shaped layer with TiAl3. If Ti alloy melted slightly during welding-brazing, interfacial microstructures are very complex. The solid-state phase changes, eutectic, hypoeutectic and hypereutectic reaction layers are formed at the interface, and phase compositions include TiAl3,TiAl,Ti3Alå’ŒTi5Si3.According to the identification of interfacial phase compositions, the analysis of Si diffusion behavior and the formation free energy of intermetallic compounds and physical simulation of interfacial microstructure evolvement, interfacial reaction mechanisms were clarified. It is found that Si gathering phenomena plays an important role for formation of interfacial compounds. Chemical potential prediction model of the ternary alloys was established based on Miedema model of solution enthalpy. The essential reason of Si gathering phenomena was expounded during interfacial reaction by mean of calculation of Si chemical potential in the Ti-Al-Si ternary alloys system. Based on the results of the physical simulation, TiAl3 reaction layer formed by crystallization and Ti7Al5Si12 reaction layer formed by interfacial chemical reaction were confirmed.Considering the two sides of factors including the crystallization of TiAl3 and solid/liquid chemical reaction of Ti7Al5Si12, the mathematical model of calculating thickness of reaction layer was established according to characteristic of thermal cycle during laser welding-brazing. The calculation of reaction layer thickness has been carried out, which indicates that the reaction layer mainly depended on crystallization of TiAl3 intermetallic compounds after the dissolution of Ti parent metal, which differed from traditional growth of reaction layer during furnace brazing. Further, according to thermal cycle calculated by the finite element method, the thickness of reaction layer was predicted in the case of different heat input. Theoretical basis was established to control the thickness of reaction layer in other similar conditions.In order to reveal internal relation between reaction layer morphology and mechanical property of joint, the crack initiation and propagation behavior at the interface was investigated by SEM in-situ observation during tensile test. The tensile test was performed for specimens with different reaction layer morphologies. Optimal interfacial reaction layers with lamella-shaped, serration-shaped and club-shaped morphology were affirmed. Especially, the joint with serration-shaped morphology reaction layer which can prevent crack propagation has the better mechanical property. The result provides a basis for optimization of joint mechanical property.
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