Study On Microstructure And Mechanical Properties Of Pulsed Laser Welded Joint For Titanium Alloy And Aluminum Alloy

The composite structure of titanium alloy and aluminum alloy is able to combine the excellent properties of these two materials and has great value and wide application prospects in modern industries such as aerospace and automobile.The welding technology of titanium alloy and aluminum alloy has also become the key that has significant research value and significance.However,it is difficult to connect titanium alloy and aluminum alloy with a traditional welding method because of the different thermophysical properties and the brittle intermetallic compounds,especially the Ti-Al intermetallic compounds at the interface,which gravely affects the mechanical properties of titanium alloy/aluminum alloy joint.Thus,reducing or avoiding the Ti-Al intermetallic compounds in the joint is the primary work of the research on titanium alloy/aluminum alloy welding technology.In this paper,the finite element method?FEM?was used to calculate the temperature field distribution and welding thermal cycle during the welding process and the characteristics of temperature field and thermal cycle were studied when the laser beam focus on titanium alloy and aluminum alloy,respectively.The results indicated that the distribution of the temperature on the titanium alloy and the aluminum alloy was asymmetrical and the thermal cycle curves on both sides also had a lot of difference.At the same time,the temperature gradient between the titanium alloy and the aluminum alloy at the interface was more than 2500°C,which could cause the different physical state of the two alloys at interface.On this foundation,characteristics of temperature distribution in the joints with Nb or V interlayer materials were calculated and analyzed,it was found in the study that the intermediate layer material could be melted partially under proper welding conditions,and the titanium alloy/intermediate layer interface was brought to a temperature that could promote atomic diffusion while the the temperature in titanium alloy did not exceed its melting temperature.Base on the study about the characteristics of temperature field distribution.Titanium alloy and aluminum alloy were welded without interlayer materials by laser with a beam offset to both sides of the interface,respectively.The effects of welding process parameters on interface forming,microstructure and mechanical properties were investigated.The results showed that the laser energy could be controlled accurately by the method of adjusting the beam offset,laser power,pulse width and pulse frequency.In the same time,the formation of interface,the distribution of intermetallic compound and the thickness of compound layer could be controlled effectively.The brittle intermetallic compounds in titanium alloy/aluminum alloy laser welded joints were mainly TiAl,TiAl₂ and TiAl₃,which were the main factors affecting the mechanical properties of joints.And the most harmful compound to the joint was TiAl₃.Under the conditions of this experiment,the maximum tensile strength of the joint reached 174 MPa when the beam was biased to the titanium alloy,and the maximum tensile strength of the joint reached 162 MPa while the laser beam was focusing on the aluminum alloy.And no matter which side the beam was focused on,the joints fractured in the compound layer.On the basis of the study on distribution characteristics of temperature field during welding process and joints without intermediate layer,two intermediate layers which were Nb and V were added to joints for separating the mixing of titanium alloy and aluminum alloy,respectively.A titanium alloy/aluminum alloy laser welded joint with unmelted intermediate layer material was obtained and the relationship between the formation of interface,microstructure and mechanical properties was also described clearly.The results showed that the intermediate layer of Nb could effectively separate the mixing and reaction between titanium alloy and aluminum alloy and avoid the production of Ti-Al intermetallic compounds.A reaction layer was formed at the Nb intermediate layer/aluminum alloy interface and the compounds in the reaction layer were mainly Nb₂Al and NbAl₃.And a diffusion layer was realized at the titanium alloy/Nb intermediate layer interface.The joint fractured in the reaction layer at the Nb intermediate layer/aluminum alloy interface and the highest tensile strength of the joint was 289MPa,which was 75%of that to aluminum alloy.V has the same function as the layer of Nb,which could decrease the formation of Ti-Al compounds,but the mixing of titanium alloy and aluminum alloy was not avoided completely due to the physical properties and process adaptability and therefore the mechanical properties of the joint were not greatly improved,the maximum tensile strength reached 192 MPa.