Research On CMT Welding Process And Numerical Simulation Of TiB₂ Particle Reinforced Aluminum Matrix Composites

The rolled Ti B₂ particle reinforced aluminum matrix composite has the characteristics of uniform structure,fine grain structure,and good strength and toughness.At the same time,this kind of composite has the advantages of convenient preparation,low manufacturing cost,and good processing performance.It has gradually become a hot material in the aerospace and military fields.However,it is difficult to weld Ti B₂ particle reinforced aluminum matrix composites,so exploring new welding processes and welding materials will further expand its application fields and give full play to its performance advantages.In this dissertation,the 5mm thick rolled T6 state Ti B₂/7050aluminum matrix composite is taken as the research object,and the low heat input cold metal transfer（CMT）technology is used to explore and optimize the CMT welding process of the composite by studying the effects of different welding modes and welding parameters on the formation of welded joints;Combined with the experimental parameters and results,the numerical simulation of welding process was carried out to analyze the temperature field and residual stress distribution in the welding process;The microstructures and properties of 6061 aluminum alloy and Ti B₂/6056 composite welding wire filled butt joints in the as welded and T6 heat treated states are compared and analyzed,aiming to provide reference for the connection of Ti B₂ particle reinforced aluminum matrix composites,and to explore the application of composite welding wire welding composites.The process research results show that the limited heat input in CMT mode is not enough to achieve the penetration molding of 5mm thick composite plates.The welding current in CMT+P mode is 170A,and the weld formation effect is good when the welding speed is 6mm/s,which is more suitable for the welding of 5mm thick composite materials;The welding shielding gas flow rate increases from 20L/min to 25L/min,the joint reinforcement decreases,the weld width increases,and the scope of the heat affected zone increases;With the decrease of welding heat input and the increase of welding shielding gas flow,the number and size of weld porosity decrease.The results of numerical simulation analysis show that the temperature gradient at the back end of the heat source is smaller than that at the front end of the heat source.Along the direction of the heat source,the temperature field nephogram is elliptical,and the isotherms at the front end of the heat source are denser than those at the back end;The direction of post weld temperature gradient coexists with the cooling time from the direction along the weld and the direction perpendicular to the weld,and finally completely changes to the direction perpendicular to the weld;The welding residual stress is mainly distributed in the fusion zone and heat affected zone of the joint,and the stress concentration occurs near the weld in the fusion zone.The results of joint microstructure analysis show that the overall microstructure of the joint is typical"as cast"crystalline structure:the"surface fine grain zone"at the side of the fusion zone near the heat affected zone;"Internal columnar crystal zone"at the side of fusion zone close to weld zone;"Central equiaxed crystal zone"inside the weld;The grain size and welding heat affected zone of Ti B₂/6056 composite welding wire joint are smaller than those of aluminum alloy welding wire;There is no obvious change in the grain size of the weld zone of the welding wire joint of the two materials before and after heat treatment.After T6 treatment,Ti B₂ particles on the grain boundary of the weld are compact,Mg₂Si,Al_0.5Fe₃Si_0.5,etcβphase partially dissolved back toα-Al.The results of joint performance analysis show that the hardness curves of the as welded joints of the two kinds of welding wires are"W"type as a whole,with the hardness value of heat affected zone>base metal>weld;The hardness curve of T6 state joint is generally"（?）"type,and the hardness value of base metal and heat affected zone reaches about 170 HV after heat treatment;The joint hardness of composite welding wire is about20 HV higher than that of aluminum alloy welding wire;The hardness of base metal is greatly improved after heat treatment;The welding wire joints of the two materials are fractured at the weld seam,without necking,and the fracture mode is cleavage and intergranular mixed brittle fracture;The average strength of as welded joint of aluminum alloy welding wire is 235 MPa,and that of T6 is 252 MPa,which is 7%higher than that of as welded joint;The average strength of as welded joint of composite welding wire is263 MPa,and that of T6 is increased to 340 MPa,29%higher than that of as welded joint;Composite welding wire has a better effect on improving the joint performance of composite plates.Further development and application of composite welding wire have broad research significance and application prospects.