Microstructure And Properties Of Die-casting AZ31 Mg Alloy Friction Stir Welding Joint And Its Evolution Mechanism During Deformation

Magnesium alloy has been widely used in the field of transportation due to its advantages such as low density,high specific strength and good thermal conductivity.However,the joints obtained through traditional methods are prone to welding defects such as porosity and inclusion,which seriously reduce the service life and limit the application of light alloy.Friction stir welding technology can realize the solid state connection of metal materials,which has obvious advantages over the traditional connection methods.However,there is no unified view on the microstructure distribution and grain orientation changes in the friction stir welding process and the evolution mechanism and rule of the joint in the deformation process,which needs to be furtherly studied in order to realize the optimization of the friction stir welding process parameters and the improvement of the joint comprehensive performance.This paper performs the friction stir welding on die-casting AZ31 magnesium alloy.The numerical simulation method of the welding temperature field is simulated and combined with experimental research method,clarify the organization evolution law of the joint,mechanical properties,corrosion resistance,determine the friction stir welding speed and rotating speed on the joint microstructure evolution,mechanical properties and corrosion resistance.At the same time,the experimental study on the tensile,compressive and three-point bending of the welded joints was carried out to obtain the microstructure evolution mechanism and laws of the joints under different deformation conditions,so as to provide theoretical basis for the engineering application of friction stir welding of magnesium alloy.Firstly,the temperature field of friction stir welding was studied,and the friction stir welding model was established using ABQUS.The change rule of joint temperature field with time and joint position was analyzed to provide some guidance for the actual welding process.The results show that heat production of joint increases with the increasing of rotational speed or decreasing of transverse speed.The welding temperature are less than the melting point?650??of magnesium alloys.At the same time,the upper temperature of the joint was significantly higher than the lower zone.Secondly,defect-less and good forming AZ31 magnesium alloy welds was obtained on with different welding process parameters.The results showed base plane texture of{0001}was the strongest in the center of the weld,and the c-axis was parallel to the welding direction.However,it reduces the texture strength and slants the c-axis from the welding direction?WD?to the side?TD?after moving from the center of the weld to the sides of the weld.The joints1400rpm-300mm/min had smaller grain size,and the transition zone of the weld was more uniform.The results of lateral tensile test showed that this joint had the best mechanical properties,and their tensile strength and elongation were252MPa and 16.6%,respectively,reaching 90%and 75%of the base metal.The texture and Schmidt factor analysis of the joint also show that the joint has medium texture strength,but higher Schmidt factor of basal plane slip and twinning.Through fracture observation,it is determined that the crack propagates along the interface on the advancing side of weld in the lateral tensile process,which is because this zone is composed of grains with different grain size and orientation.Thirdly,the annealing process of AZ31 friction stir welded joint was studied.Results show that plastic deformation capacity of the friction stir welded AZ31magnesium alloy joint increases first and then decreases with the increasing of annealing temperature,and maximum elongation was obtained in temperature of200?.Annealing temperature is set at 200?and after annealing treatment at different times to determine the joint best heat treatment process for 200?+30min.It can release the joint internal residual stress,reduce the substructure and improve the microstructure distribution.At the same time,the joint treated by annealing under this condition can obtain the best comprehensive mechanical properties,the tensile strength increased from 241.2MPa to 248.2MPa,and the elongation increased from 7.5%to 11.4%.Then,the corrosion behavior of friction stir welded joint was studied to determine the corrosion mechanism.The electrochemical corrosion results showed that,with the increase of rotating speed,the corrosion rate of the joints decreased and the corrosion resistance became better.For 3.5%Na₂SO₄ solution,the corrosion properties of joints with different rotational speeds have little difference.Electrochemical tests on different areas of the Joint-1400 in3.5%Na Cl solution showed that the center in the weld had good corrosion resistance and the self-corrosion current density was the lowest of 18.61A/cm²,while the AS and RS transition areas had higher self-corrosion current densities?34.39A/cm² and 28.09A/cm²,respectively?.This indicates that the corrosion resistance of the transition zone is poor.The corrosion behavior of friction stir welding joint can be divided into two processes.First,in the short immersion process,the corrosion behavior is mainly originated from local pitting corrosion.Because there are some second phase particles in the local area,corrosion begins to occur at the interface between the second phase particles and the base metal.With the extension of immersion time,the corrosion morphology changed from pitting corrosion to filamentous corrosion and dendritic corrosion.The corrosion path is determined by the crystal orientation,mainly extends along the sample surface with strong{0001}substrate texture.Finally,the deformation behaviors and microstruture evolution mechanism of friction stir welded AZ31 magnesium alloy joints under unidirectional stress and complex stress were analyzed.The surface of friction stir welded joint have obvious concave-convex phenomenon after deformation.After the complex three-point bending deformation,the surface concave-convex phenomenon of friction stir welding joint is the most complex,which is manifested as three concave regions and three convex regions.The region has obvious{0001}basal texture,mainly produced in the process of deformation of?56°{10-11}compression twin and?86°{10-12}extended twin;In the process of compression deformation,due to the change of stress state,deformation of AZ31magnesium alloy friction stir welding head is concentrated in the center of the weld,twin types is composed of some 56°{10-11}compression deformation and a larger number of?86°{10-12}extended twin;However,the stress state in the bending process is relatively complex,and there are three different regions that can produce obvious twins.One is the welding zone on the compressive stress side,which has the{0001}basal plane texture with the c-axis parallel to the welding direction.The twinning mechanism is activated under the compressive stress state.The second and third are the transition zone on the tensile stress side,which has the{0001}basal plane texture with the c-axis parallel to the transverse direction.The twinning mechanism is activated under the tensile stress state.In conclusion,the friction stir welding of die-casting magnesium alloy AZ31was studied in this paper,it systematically stated microstructure evolution mechanism during the welding and past-deformation process,and established the relationship between the properties and microstructure,which provided theoretical basis for improving the joint comprehensive performance,and to provide experimental basis for further expansion of engineering application of magnesium alloys.