Experimental Study Of High-strength Aluminum Alloy Stir Friction Welding Based On High-strength And Toughness Welding Tool

High-strength aluminum alloys are widely used in industrial applications because of their high strength,light weight and good workability.However,this type of material is a difficult alloy to weld.It is difficult to obtain good joint quality by using the traditional welding method of fusion welding,which can easily lead to insufficient strength of welded components.Friction stir welding has the advantages of good joint quality,high efficiency and low cost in the joining of such materials,and is effective in improving the welding quality.Due to the strong thermal influence of the welding process,the traditional stir friction welding tools represented by H13 steel are prone to wear and failure and have a short service life.Therefore,this paper designed a new high-strength and toughness cemented carbide welding tool,and the 7075 high-strength aluminum alloy to carry out welding experiments and numerical simulation of the material flow of the joint to study surface quality and the properties of the joint.The main content is as follows:(1)Aiming at the problem that the microphysical field changes cannot be clearly described in the welding process,a finite element analysis model considering the temperature change effect of the material under the effect of thermal coupling is constructed.To study the characteristics of the temperature field,welding force and velocity field during the welding process,and reveal the flow behavior of the material and the influence law of the welding process on each physical field.The results show that temperature field is elliptical distribution.Welding force in the welding tool down to contact the workpiece at the subsequent moment to reach the peak,and welding temperature is higher at high speed or low welding speed,while the change in welding force is the opposite.The material immediately outside of the stirring needle has a larger flow velocity.Rotating speed is in the range of 400～1200r/min,and the material flowability is best when it is 600r/min.The material on the forward side migrates in a rotary flow to fill the cavity behind the welding tool,and flow ability of the material is positively correlated with the formability of the joint.(2)In order to study the evolution of high-strength aluminum alloy stir friction welding head forming and microstructure,the high-strength aluminum alloy stir friction welding process experiments were carried out.Mechanism of the influence of the heat input state on the weld quality is revealed by analyzing the forming results of the joints.Metallographic observation was used to study the characteristics of the microstructure of each region of the joint and to analyze the evolution of the microstructure of the joint under different welding processes.The results show that system heat input is higher under the use of high-strength and toughness GU25 UF welding tool in the rotation speed of1000 ～ 1200r/min high speed.The joints surface pear grooves and skinning defects,so the joint forming quality is poor.In the study of microstructure,it was found that the weld core area consists of uniform and small equiaxed crystals.Deformation of grain morphology in the heat-affected zone,while the grain size in the heat-affected zone is maximum.In addition,grain size is affected by temperature to a greater extent than mechanical stirring,and increases with increasing rotational speed or decreasing welding speed.(3)Based on the test results and fracture morphology characterization of the joint properties under different welding processes to determine the location of the weak areas of joint performance,analysis of the microstructure of the mechanical properties of the joint mechanism of action,and study the failure mechanism and fracture characteristics of the joint.The results show that joint performance is affected by the combination of microstructure and material fluidity,and the degree of influence of fine grain strengthening is higher than that of deformation strengthening.High-strength and toughness GU25 UF at low rotational speed compared to conventional H13 welding tools to obtain high joint performance.Heat-affected zone is the main mechanical properties of the weak zone,and shear fracture of the joint occurs along 45°.Through the crystal tough nest type fracture manifested as a joint plasticity,while coarse intermetallic compoundinduced fracture is brittle fracture characteristics,and joint failure is formed under the action of multiple fracture modes coupled together.