| In order to solve the problems exist in the riveting process of aircraft ribbed structural parts,such as complex processing procedures,poor fatigue performance,easy falling off of rivets and weight gaining,backfill friction stir spot welding is taken as the research object of this research,the 2.0mm-thick 7050-T62 aluminum alloy long truss and the 2.0mm-thick2524-T3 aluminum alloy are used to generate new material RFSSW.In this research,the forming mechanism of the joint is revealed by studying the material fluidity inside the joint;the law of the influence of welding process parameters on the performance of RFSSW joints under multi-variable coupling constraints is explored,the optimal welding process parameters are obtained,the purpose of reducing the weight and improving the mechanical properties of the structure are achieved.Secondly,the best process parameters is used to weld component-level typical structural parts in this experiment,the experiment studies the tensile,shear and fatigue properties of the component-level test parts,improves the welding process and analyzes the quality stability of the test parts' joints.The results show that the solder joints of RFSSW are divided into the following four zones: nugget zone(NZ),thermal-mechanical affected zone(TMAZ),heat affected zone(HAZ)and base metal(BM).It is divided into the pin affected zone(PAZ)and the sleeve affected zone(SAZ).During the penetration process,the aluminum-clad metal in the nugget zone forms a multi-layer structure.Distributed continuously on the joint surface of the solder joints and form a cladding aluminum layer,which high in center and low on two sides on both sides.The penetration process and backfilling process verify that there are horizontal,vertical and rotating flows inside the material.The best process parameters of 7050-T62 and 2524-T3 aluminum alloy RFSSW are welding speed 2000 rpm,penetration depth 2.3mm,penetration time 3.15 s,refilling time3.85 s,residence time 1s.Under these process parameters,the tensile force of the joint is20.63 k N,the shear force is 8.98 k N,compared with riveting,the tensile of RFSSW joints is increased by 257.53%,and the shearing force is increased by 53.77%.The fracture of tensile test part is in the form of quasi-cleavage fracture.The fracture of shear test part shows two modes: weld nugget peeling and plug fracture.The fracture form of the riveted joint is rivet fracture.In the hardness distribution of RFSSW joints,NZ has the highest hardness value,with an average hardness value of 157 HV,followed by BM of 7050-T62 with 153 HV and2524-T3 of BM with 138 HV.The average hardness value of HAZ of 7050-T62 is 130 HV,the average hardness value of HAZ of 2524-T3 is 123 HV,and the minimum hardness value of hook-shaped defects is 100 HV.The tensile strength of the RFSSW joint of the component-level tensile test parts is381.11 MPa and the shear force is 27.27 k N.Compared with the riveted structure,the shear force of the RFSSW joint is increased by 51.42%.The average number of failure cycles for RFSSW joints is 24663,while number of the riveted joints is 23609.The fracture of tensile test part is in the form of quasi-cleavage fracture,the fracture of the component-level shear test parts is weld nugget peeling fracture,which is a mixed mode of ductile fracture and cleavage fracture;the fracture morphology of component-level fatigue test parts is composed of the crack initiation,extension zone and instantaneous fracture zone,which shows a mixed fracture mode consist of cleavage fracture and ductile fracture as a whole. |
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