Study Of Formation Mechanism And Control Of Interface During Forming Ti/Al Dissimilar Alloy By Laser Powder Bed Fusion

Additive manufacturing multi-materials technology can combine the advantages of additive manufacturing and multi-materials,it can directly manufacture parts with complex geometry and multi-function composites.Additive manufacturing has great advantages in the aerospace field which requires high quality,short cycle and high lightweight.Ti/Al dissimilar alloy components have good application prospects because of their light weight,high specific strength,good corrosion resistance.However,the solid solubility between titanium and aluminum is low,the thermophysical properties are greatly different,and brittle intermetallic compounds are easily produced,which difficultly to connect.Laser Powder Bed Fusion（LPBF）has extremely fast heating and cooling speed can well control the formation and growth of Ti/Al intermetallics,and can also realize the integrated manufacturing of complex structural parts.Aiming at the interface control of Ti/Al dissimilar alloys,this thesis takes Ti6Al4V titanium alloy and Al Si10Mg aluminum alloy powder as raw materials,and directly fabricates Ti/Al multi-material structure by LPBF.On the macro-scale,the influence of process parameters on the interface quality and joint performance is discussed,the formation mechanism of interfacial crack is analyzed,and the process window is determined.On the micro-scale,the microstructure and reaction mechanism of Ti/Al interface are analyzed.On the mesoscopic-scale,the finite element model is established to analyze the evolution of titanium alloy and aluminum alloy molten pool.According to the curve of Ti/Al interface temperature changes with time,the growth dynamics model of interface reaction layer is discussed.Finally,the stress evolution and residual stress distribution characteristics of Ti/Al interface are analyzed by the thermal mechanical coupling method.The Ti/Al interface quality is determined by the process parameters of the aluminum alloy.When the laser energy density increases from 34.6J/mm³ to 56.1J/mm³,the interface quality gradually evolves from irregular holes to good quality,pores and transverse cracks.In the proper range of laser energy,the cooperation of laser power and scanning speed is also required to obtain the best interface quality and joint performance.When the laser power is 290-310W and the laser scanning speed is 2000-2200mm/s,the interface quality is without obvious defects,and the longitudinal micro-cracks can be filled by the aluminum alloy melt under the action of gravity and capillary force,the tensile strength of the joint reaches more than 200MPa.Rapid heating and cooling of LPBF determine the unique microstructure of the Ti/Al interface.The titanium side is typical acicular martensite,the aluminum alloy grains in the interface zone and the aluminum alloy heat-affected zone are equiaxed and randomly oriented.The aluminum side is columnar crystals that grow along the forming direction,and the grains are preferentially oriented.Based on the accurate identification of the phase of the Ti/Al interface reaction layer by SEM,XRD and TEM,the interface reaction layer is Ti Al₃,Ti Al+Ti₅Si₃ and Ti₃Al from the aluminum to the titanium.When the molten aluminum alloy wets and spreads on the titanium alloy,the titanium dissolves or melts,and the silicon element in the aluminum alloy gathers at the interface.The gathered silicon atoms react with the titanium atoms in situ to form Ti₅Si₃nanoparticle phase because the lowest Gibbs free energy of Ti₅Si₃.At the same time,a large number of aluminum protons react with the titanium atoms to form Ti Al phase.The nanocrystalline Ti₅Si₃ forms an eutectic layer on the Ti Al matrix.Ti Al₃ nucleates and forms an ordered structure which grows along the direction of the maximum cooling rate.Beyond the fusion line,the supersaturated solid solution-Ti is transformed into Ti₃Al through the diffusion of aluminum atoms in titanium alloy.Because the extreme temperature change and complex thermal cycle in LPBF process,the finite element model of forming Ti/Al dissimilar alloy by LPBF is established,the temperature field of the dissimilar alloy forming process is simulated,and the change curve of interface temperature with time is extracted,so the kinetics of titanium and aluminum reaction is analyzed.Titanium alloy has changed the heat conduction condition of aluminum alloy during forming process.The ratio of length to width of aluminum alloy molten pool increases significantly,and the isotherm shifts greatly to the formed aluminum alloy.The maximum temperature gradient and heat flux of the aluminum alloy molten pool are both shift towards the direction of the laser.Scanning strategy and forming height also affect the temperature distribution characteristics of aluminum alloy molten pool.According to the curve of temperature with time at the interface,the reaction time of Ti and Al is obtained,and the theoretical thickness of the interface reaction layer is calculated.Compared with the test results,the growth mechanism of Ti Al₃ is determined by the melting mechanism.When the process parameters of aluminum alloy are appropriate,Ti and Al react to a certain extent without producing too many brittle intermetallic compounds.Another important factor that affects the forming of Ti/Al dissimilar alloy components by LPBF is the interface stress.Aiming at the problem that the stress is difficult to measure during the forming process of LPBF,the finite element thermal-mechanical coupling method is used to simulate the stress field of LPBF fabricating Ti/Al dissimilar alloys.The results of stress field simulation show that the stress of Ti/Al interface is mainly affected by the residual stress of titanium alloy and the stress during the forming of aluminum alloy.Compared with the strip scanning strategy,the chess scanning strategy can effectively reduce the various stresses in the post forming area and residual stress after cooling.When the forming height of titanium alloy increases,the residual stress on the upper surface of titanium increases,resulting in the increase of residual stress at the Ti/Al interface.Therefore,reducing the residual stress of titanium alloy can effectively control the stress at the Ti/Al interface and achieve integrated complex parts fabriacting by LPBF.