Effect Of Weld Processes And Post-weld Heat Treatments On Properties Of Dissimilar Aluminum Alloy FSW Joints

Aluminum alloys have many excellent properties and are widely used in various industries to provide convenience for people’s lives.However,the performance of a single aluminum alloy is often not suitable for complex using environments,and to some extent,wasting of resources.Dissimilar aluminum alloys connections can solve these problems.Due to their own characteristics,dissimilar aluminum alloys are often difficult to weld,so researchers are looking for new ways to connect dissimilar aluminum alloys.The invention of friction stir welding（FSW）provides a new method for dissimilar aluminum alloys’welding.In this paper,4mm thick 6061-T6 aluminum alloy and 7075-T651 aluminum alloy are used as welding materials,and different welding processes and post-weld heat treatments are combined to optimize the connection processes of dissimilar aluminum alloys.The effects of welding processes and different post-weld heat treatments on the surface morphology,macro-microstructure,phase composition,mechanical properties and corrosion resistance of welded joints were analyzed.According to the FSW principle,the working mode of the stir pin and the joint properties and microstructure measured after welding,the plastic heat production of the weld metal was studied.The metal migration law in the weld was studied by the distribution of the dissimilar metal flow field in the weld cross sections.It was found that the weld bead performance is better when the 6061-T6 aluminum alloy is placed on the welding advance side.Under the appropriate parameters,the tensile strength of the weld is over 87%of the 6061-T6 aluminum alloy base material,and the welds have almost no flash,and the surface appearances are beautiful.The microhardness of the welded joints is generally"Z"-shaped,and there is a cliff-type drop at the junction of dissimilar metals.At the same time,different speeds and preheating time have a great influence on the weld hardness distribution.The grain size of the weld fusion line adjacent to the 6061-T6 aluminum alloy side is significantly larger than that of the fusion line adjacent to the 7075-T651aluminum alloy side.It was found by XRD analysis that the second phase present in the weld were Mg₂Si,AlFe₆Si but not MgZn₂.The microstructure,mechanical properties and corrosion resistance of the weld are greatly changed after heat treatments.After the heat treatments,the tensile strength of the joints are improved to different extents.The second phase precipitates mainly on the grain boundaries.As the temperature increases,the number decreases and the size increases.The weld nuggets are equiaxed grains.Appropriate heat treatments process parameters can make the tensile strength of the joints comparable to the tensile strength of the 6061-T6 aluminum alloy base material,and the microhardness of the joints are improved overall and the distributions are more uniform.After heat treatments,the equiaxed grains in the weld nugget zone become more uniform,and the corrosion tendency of the welded joints decreases,but the corrosion rate varies.The analysis of the FSW principle shows that the plastic heat production can be regarded as the work done by the stir pin to overcome the forward resistance.The ratio of the rotational speeds to the welding speed is not proportional to the welding heat input.If the stir pin’s speed is too high or too low,and the preheating time is too long or too short,it is not conducive to improving the welding performance.Proper stir pin rotation speeds and preheating time can optimize the degree of softening of the metal and the friction of the metal against the plastic metal,which is beneficial to the sufficient mixing of the metal in the weld.The metal flow in the weld is divided into four directions,and the welding processes have a great influence on the flow state of the weld metal.