Study On Microstructures And Mechanical Properties Of Small Shoulder Frictiom Stir Welded Joints Of 2A14Aluminum Alloy

As a solid phase welding,friction stir welding provides a new way to solve light alloy welding.However,friction stir welding requires that the material must be rigidly fixed.It has high requirements on the rigidity of the welding equipment.Especially when welding high-strength aluminum alloy thick plates,the insufficient stiffness of the equipment may result in poor stability in the welding process and un-welding defects at the root of the weld.In view of the above problems,small shoulder friction stir welding was proposed.The load on the welding tool was reduced by reducing the size of the stirring pin and the shoulder.Finally,the rigidity requirement of welding equipment was reduced and the welding of high strength aluminum alloy thick plates was realizedIn this thesis,the 9mm thick 2A14-T4 aluminum alloy was used as the welded material.Considering the heat input and material flow in the welding process,a welding tool structure including a milling three-plane stirring pin and a small-sized shoulder was put forward,and two different sizes of welding tools,type ? and type ?,were designed.This thesis analyzed the load-bearing characteristics of the welding tool during the stable welding process,and calculated the load at the root of the stirring pin.The results showed that the maximum normal stress at the root of the stirring pin was 133 MPa and the maximum shear stress was 35 MPa.The finite element simulation of the stress distribution of the welding tool under different fillet radius was carried out.The simulation results showed that the maximum load of the welding tool appeared at the transition interface between the stirring pin and the shoulder.Increasing the fillet radius could make the load distribution more uniform.Welding tests were carried out using these two welding tools,and the results showed that the welding quality of type II welding tools was more excellent.The effects of welding parameters on material forming,microstructures and mechanical properties were discussed.Weld junctions of the weldments obtained by the selected welding parameters had zero defects such as kissing bonds and grooves.There was an obvious "onion rings" structure in the WNZ,indicating that the milling of the three facets and the thread structure could promote material flow.The precipitated phase at the joints is mainly the rod-shaped ? phase.And the precipitated phases in the WNZ and the TMAZ were broken and dissolved.The size of the recrystallized grains in the upper part of the weld nugget is larger,and the precipitation phase is highly dissolved.The grain size in the lower part is smaller,and the precipitated phase is mainly short rod-shaped.The tensile test results showed that the joint strength obtained under the R500W150P0.3 process parameter condition was the highest,up to 368 MPa,which is 85.8% of the base metal strength.And the failure of the joint mainly occurred in the RS-TMAZ of the joint,showing a 45-degree shear fracture.Under low-rotation speed conditions,the root of the weld was subjected to tensile stress to form equiaxed dimples,and the upper material was subjected to tearing to form torn dimples.The temperature field of friction stir welding of 9mm thick 2A14 aluminum alloy was studied by numerical simulation.With the increase of the diameter of the shoulder,the welding heat input increased obviously.The increase of the welding tool rotation speed made the peak temperature of the weld nugget zone increase significantly,and the increase of the welding speed made the high temperature residence time experienced by the weld nugget zone shorten obviously.The correlation between the temperature field and the microstructures and mechanical properties was studied,which provided a basis for the optimization of FSW joint performance.