Inhomogeneous Microstructure And Properties Of Rotary Friction Welded 2024 Aluminum Alloy Joints

Rotary friction welding exhibits outstanding characteristics of high-efficiency,high quality and energy-saving,and it is particularly suitable for welding rotary body structural parts.However,due to the uneven distribution of heat generation and pressure on the welding interface along radius during the welding process,the overall performance of the joint is uneven and contains weak areas,which affects the service safety of the rotary friction welded joints and becomes the bottle-neck of their application in the equipment manufacturing field.Therefore,it owns important theoretical meaning and engineering value to conduct systematic research on the inhomogeneity of rotary friction welded joints,which can clarify the welding mechanism of rotary friction welding and reveal the influence of the uneven radial thermomechanical coupling on the microstructure and performance of the joint during the welding process,further to instruct how to make the technique methods of solving the inhomogeneity problem of joints.This thesis takes rotary friction welded 2024 aluminum alloy joints as the research object.The method of slicing joint along radius is applied and optical microscope,scanning electron microscope,electron probe micro-analyzer,focused ion beam,transmission electron microscope,universal material testing machine,DIC system,electrochemical workstation and other equipments are also used to carry out research and analysis on the inhomogeneity of the microstructure,mechanical properties and corrosion properties of the as-welded and aged joints,in order to reveal the distribution principle of microstructure evolution and properties,and further to explore the method of solving the inhomogeneity problem of joints.The main findings are listed as follows:It showed that the as-welded joint under the severe thermo-mechanical coupling effect displayed inhomogeneous microstructure.The different evolution of texture causes inhomogeneity of mechanical properties in macroscopical way.The texture evolution principle of the as-welded joints from BM to DRZ is as follows: Copper ? S ? Brass(strong)? Brass(weak)in 0.25 R position;Copper ? S ? Rotated Goss ? Brass(weak)in 0.75 R position.Moreover,there are many slidable slip systems of the textures in 0.25 R position and the Schmid factor of them is small,which means the texture shows hard orientation and high strength.As for the texture in 0.75 R position,the amount of slidable slip systems is small and the Schmid factor is large,suggesting soft orientation and low strength of the texture.The mechanical properties of the as-welded joint are unevenly distributed along radius.The optimal mechanical properties appear in 0.25 R position,while the mechanical properties in 0.75 R position is the worst.The highest tensile strength in 0.25 R position is 423 MPa and the corresponding elongation is 9.25%.During tensile test,the micro-strain curves in different positions are M-shaped,the strain concentration occurs in heat affected zone(HAZ)and thermo-mechanically affected zone(TMAZ),and the strain at 0.75 R is the largest.The corrosion performance is also significantly different along radius.After the immersion corrosion,joints mainly suffer from pitting and intergranular corrosion.The corrosion behavior becomes more and more serious with the increasing of radius.Center and 0.25 R positions tend to corrode lightly,while 0.5R and 0.75 R positions corrode severely.TMAZ near dynamic recrystallized zone(TMAZ-D)and base metal(BM)have good corrosion resistance,while dynamic recrystallized zone(DRZ),TMAZ near HAZ(TMAZ-H)and HAZ have poor corrosion resistance.Aging treatment has a homogenizing effect on the uneven distribution of the tensile strength of the sliced samples.After artificial aging,the maximum difference in tensile strength of the sliced samples is only 9MPa,and the average strength is above 400 MPa.With the increase of time of natural aging,the tensile strength of the sliced samples gradually increase and are homogenized,but the tensile strength in 0.75 R position remained relatively low.Moreover,aging treatment also has a homogenizing effect on the corrosion resistance of the joint.Artificial aging causes the uniform distribution of elements in intragranular and grain boundary,which reduces the tendency of intergranular corrosion.Therefore,the inhomogeneity of corrosion resistance along radius gradually disappears.DRZ,TMAZ,HAZ and BM mainly suffer from pitting corrosion.Moreover,with the increase of natural aging time,the inhomogeneity of corrosion behavior is also alleviated.BM exhibits the best corrosion resistance.The corrosion type of DRZ,TMAZ,and HAZ gradually changed to pitting corrosion.