| Energy conservation and environmental protection are very important for society development.In automobile production,increasing the use of aluminum alloys to reduce the weight of vehicle is an important means to achieve energy saving.6082 aluminum alloy has been widely used in automobile manufacturing due to its low density,high specific strength,good impact resistance and excellent weldability and extrusion formability.However,the joints obtained by traditional fusion welding techniques usually consist of welding defects and their strengths need to be improved.Friction Stir Welding(FSW)is a solid-state joining technique,which has been proved to be relatively effective in producing sound joints of aluminum and magnesium alloys.In this paper,6082 hot extruded aluminum plates with a thickness of 5 mm were welded by FSW.Sound weld joints with excellent mechanical properties were obtained by optimization of process parameters.The influence of welding parameters on microstructure and mechanical properties were investigated.The joints obtained under the optimum conditions were selected and subjected to conduct post-weld heat treatment.The effects of heat treatment on the microstructure and mechanical properties of the joint were analyzed.Finally,friction stir spot welding(FSSW)technology of 6082 alloy was investigated,and the influence of processing parameters on the mechanical properties of spot joint was examined.The results of FSW experimental works show that defect-free joint can be obtained by improving the pre-weld treatment methods and optimizing the welding parameters.The joint is composed of four vegions: base metal(BM),heat affected zone(HAZ),thermomechanically affected zone(TMAZ),nugget zone(NZ).The microstructure of NZ consists of fine equiaxed grains due to dynamic re-crystallization and its grain size becomes finer.After FSW,the coarse ?-(AlFe MnSi)phase still exists and the needle-like ??? phase is dissolved into the matrix.The average grain size of the NZ decreases with the welding speed increasing.When the rotation speed of 1200r/min and welding speed of 200mm/min,the average grain size is the smallest(3.8mm).The hardness distribution of the joint along the cross-section is shown as a W-shape curve: the hardness of BM is the highest,the hardness of NZ is higher than HAZ but lower than BM.When ?/? ratio is kept in certain range,the joint hardness does not change much.At the rotation speed of 1200r/min and welding speed of 40mm/min,the overall hardness of the joint decreases due to the high heat input.The ultimate tensile strength(UTS)reaches its peak value(236MPa),which is 74.0% of the base material strength under the conditions of rotation speed of 1200r/min and welding speed of 200mm/min.The tensile strength of the joint increases first then decreases with the increase of rotating speed and welding speed.The post-weld heat treatment study indicates that,after solution treatment,the equiaxed grains in NZ grow coarse.The coarse ?-(Al FeMnSi)phase remained on the matrix.Most of ?-Mg2 Si and ???phase are dissolved in the matrix after solution treatment.After artificial aging treatment,the fine needle-like ??? phase re-precipitate from the matrix.The hardness of NZ increases from 70.5HV to 88.0HV after solution treatment at 530? for 120 min.Artificial aging at 170?,the hardness of NZ increases first then decreases with the increase of time,and reaches its peak at 5h,which is 121.3HV.The ultimate tensile strength(UTS)of solution-treated and T6-state joint is improved to 282 MPa and 358 MPa,and elongation is 14.2% and 6.4%,respectively.SEM fracture morphologies indicate that the fracture mechanism of solution-treated joint is ductile fracture,T6-state joint is brittle fracture with some ductile fracture characteristics.The results of FSSW experimental works show that the joints consist of stir zone(SZ),TMAZ,HAZ and BM,and "Hook" zone is formed near SZ.The mechanical properties of the spot joint increase with increasing the distances between the end of the "Hook" to the edge of the Keyhole and the upper surface of the plate.This can be achieced by optimizing welding parameters and post-weld heat treatment.The hardness distribution of the joint along the cross-section is symmetrical,and the hardness of BM is higher than that of SZ.The hardness value of SZ increases with the increasing of dwell time.After T6 treatment,the hardness distribution becomes uniform,and the average hardness value of SZ increases from 67.8HV to 109.5HV.The fracture load of the joint increases first and then decreases with the increasing of the rotation speed,and increases with the increasing of dwell time.When the rotation speed is 1400r/min and the dwell time is 20 s,the fracture load of joint reaches its peak value(3.56KN).After T6 heat treatment,this value is improved to 5.34 KN. |
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