Research On Laser Welding-brazing Process Of Titanium/Aluminum Dissimilar Alloys

The composite structure of Titanium-Aluminum dissimilar metals is a common structure in aircraft manufacturing.At present,the commonly used welding methods include diffusion welding,friction stir welding,brazing,etc.However,these methods have the problems of long production period,poor structure adaptability and low mechanical strength,which are difficult to meet the increasingly high manufacturing requirements.The laser welding-brazing method makes use of the difference of melting point of materials,to keep the low melting point metal fused and form brazing interface with the metal at high melting point,so as to realize the connection of heterogeneous metals and effectively reduce the generation of intermetallic compounds.The laser welding-brazing method was used to carry out a systematic study of titanium/aluminum dissimilar metal welding in this paper.First,a single-factor test was performed on the variables involved in the welding process to determine the appropriate process window to obtain a well-formed welding joint.Then,the microstructure of the joint was observed,and the organization morphology and phase composition of the interface reaction layer were analyzed.The growth mechanism of the reaction layer was revealed by combining the thermodynamic and dynamic characteristics during the welding process.Finally,the mechanical properties of the joint were tested,and the influence of microstructure on the joint performance was clarified through the analysis of fracture morphology.On this basis,the association rule between welding process,weld shape,microstructure,and mechanical properties was established.The main conclusions obtained in this study are as follows:(1)Through research on the welding process and weld formation,it was found that the laser offset,laser power and welding speed are the key factors that determine the weld shaping.When the laser offset is too small,which would lead to instability of the molten pool and keyhole,resulting in porosity defect.While excessive offset would lead to insufficient heat input at the bottom of the interface,resulting in unfused defect.Excessively small laser power or excessive welding speed would lead to under welding,while excessive laser power or slow welding speed would cause the weld to collapse seriously.Therefore,a good match of laser offset,laser power and welding speed is a necessary condition to obtain a good weld formation,and on this basis,a process window for laser welding of titanium/aluminum dissimilar metals is proposed.(2)Through the study of the microstructure of the welded joint,it is found that the microstructure of different areas of the weld is significantly different:when the laser offset is 0.25mm,the interface reaction layer is composed of continuous layered IMC(Intermetallic Compound)and discontinuous IMC layer The structure is Ti Al+Ti Al₃;when the laser offset is 0.75mm,the characteristic of the continuous IMC layer at the interface disappears,but the serrated IMC layer composed of Ti Al₃.It can be known from the analysis of welding thermodynamics and kinetics that under high temperature conditions,Ti Al undergoes metallurgical reaction on the titanium side surface to form a layered structure;Ti Al₃ is the preferred phase in Ti-Al intermetallic compounds,and Ti element is solid-liquid during the heating stage The concentration of the front edge has not reached the maximum solubility,and the supersaturated precipitation in the cooling stage forms Ti Al₃.(3)The results of microhardness distribution of joints indicate that intermetallic compounds cause a sudden change in hardness at the interface.The diffused IMC is beneficial to slow down the amplitude of hardness fluctuations at the interface and improve the characteristics of joint brittleness concentration.Through the study of the tensile properties and fracture morphology of the joint,it was found that the maximum tensile strength of the joint reached 204.48Mpa,and the welded joints all fractured at the interface reaction layer.When the interfacial IMC layer is thicker(greater than 10?m),the strength of the joint is low,and the fracture shows cleavage fracture,and the fracture surface is located at the junction of Ti Al₃ and Ti Al.When the thickness of the interface IMC is thin(less than 5?m),the cracks initiate in the middle of the interface and propagate along the upper and lower parts.The joint is in a mixed fracture mode.The thickness of IMC layers could be reduced by adjusting the process parameters,which can improve the joint strength sequentially.