Joint Formation And Mechanical Properties Of Non-weld-thinning Friction Stir Welded 6082-T4 Aluminum Alloys

As a state-of-the-art solid-state joining technology,friction stir welding can avoid cracks and pores caused by fusion welding,and it has advantages of small welding distortion and residual stress.As such,friction stir welding has been applied to a variety of industrial fields.However,a plunge depth is needed to obtain desirable formation,which results in weld thinning phenomenon.Weld thinning is detrimental to the formation and mechanical properties of joints.Thus,it remains an urgent issue to solve.Based on “single welding,double forming” concept,a novel tool is designed to achieve high-quality friction stir welded joint and realize non-weld-thinning friction stir welding.The surfaces of joints in all the parameters are smooth without flash defects,showing good surface formations.This reduces the workload of post-weld milling and increases production efficiency.The three-dimensional morphology of joint shows that the color transition is smooth and the thickness difference of base metal and joint is small.The maximum weld thinning is only 0.02 mm,which satisfies the requirement of non-weld-thinning friction stir welding.Non-weld-thinning friction stir welded joints has two shoulder affected zones,which are inner shoulder affected zone and outer shoulder affected zone.The joints in the majority of parameters have defect-free macrostructures,depicting that non-weld-thinning friction stir welding can enhance the internal formation in a wide range of processing window.The microhardness distribution of non-weld-thinning friction stir welded joints is W-shape,in which the minimum microhardness point locates in HAZ.With the increase of welding speed and the decrease of rotational velocity,the overall microhardness of joint increases,and the width of HAZ is narrower.The maximum ultimate tensile strength of non-weld-thinning friction stir welded joint is 231 MPa,which reaches 91% of base metal.The joint efficiency of non-weld-thinning friction stir welded joints is higher than that of conventionally friction stir welded joints.This indicates that non-weld-thinning friction stir welding can relieve the negative effects of weld thinning,and obtain high-quality joint.The flow velocity of non-weld-thinning friction stir welded joint decreases when the distance from weld center increases.Materials around the pin flow downwards and materials away from the pin flow upwards.The accumulation of plasticized materials provided by inner shoulder is limited,which is unable to prevent materials from flowing out completely.Outer shoulder can restrict overflowing materials and avoid weld thinning.The forming mechanism of non-weld-thinning friction stir welding can be summarized as “single welding,double forming”.Inner shoulder and pin attribute to the first forming to ensure the internal formation of joint and outer shoulder attributes to the second forming to restrict and refill the overflowing materials.Meanwhile,outer shoulder can also promote the first forming and further improve internal formation,which prevents the internal defects.