| With the light weight and high speed of train,it brings more challenges for the welding technology and reliability of welded structure.FSW is a potential joining technology for the aluminum alloy train body.The 7075 aluminum alloy is used extensively in the fields of aeronautics,astronautics and transports due to its high strength,stiffness and good plasticity.The flat butt welding joint on the thickness of 6 mm 7075 aluminum alloy was formed using friction stir welding technology with different parameters,the size and morphology of precipitated phase among the WNZ,TMAZ,HAZ and BM was quantitatively characterized through a series of micro and nano characterization methods.The species of precipitated phase was accurately characterized through the high solution image observation of different precipitated phase using TEM and the calculation of lattice fringe spacing which contributed to the research of the strength mismatch mechanism of 7075 FSW joint.The post welding heat treatment and multi-stage heat treatment were tested by the characterization of micro-structure and micro-hardness in order to the optimization of heat treatment parameters.Finally the deformation and damage mechanism of FSW joint was studied through the tensile-unloading test and in situ tensile test.The results show as follows:(1)The micro-hardness of 7075 FSW joints presents “W” shape which is heterogeneous distribution influenced by the species and size of precipitated phase,grain size as well as strain hardening effect.(2)The precipitated phases in 7075 mainly include rod shape MgZn2,elliptical shape AlCuMg and Al2 CuMg whose shape is capsule.The precipitated phases in WNZ mainly include AlCuMg and Al2 CuMg,as well as the fine grain strengthening,the micro-hardness is pretty higher which reaches 145 HV.The quantity of precipitated phases AlCuMg and Al2 CuMg in TMAZ is less and the MgZn2 is relative more compared with WNZ,the micro-hardness of TMAZ decrease.Because the MgZn2 is relative more and lacking of the fine grain strengthening and strain hardening effect,the micro-hardness of HAZ decrease further,and the junction of HAZ and TMAZ micro-hardness reaches the lowest of the whole FSW joint which is 125 HV.(3)The precipitate free zone(PFZ)occurs in WNZ,TMAZ,HAZ as well as BM of FSW joints,and the existence of PFZ has some contribution to the softening of the joint.(4)The best parameter of FSW for the 7075 aluminum alloy is as follows: the rotation speed is 800r/min and the welding speed is 300 mm/min.The joint's ultimate strength is 514 MPa which reaches 91% of the BM under the best parameter.The precipitated phases in the joints experienced solution treatment with artificial aging through post welding heat treatment and multi-stage heat treatment.The precipitated phases comes to refining which leads the micro-hardness significantly improve and turn to evenly distribution whose number is 165 HV and 160 HV separately.The best heat treatment parameter and the best multi-stage heat treatment parameter are as follows: the solution temperature is 470 ?and the aging time is 36 h,the first aging time is 12 h and the second aging time is 12 h separately.The strengthening effect of post welding heat treatment is better than the multi-stage heat treatment.(5)During the tensile process of 7075 FSW joint,the micro-cracks first nucleates in the bottom of WNZ and the junction of the HAZ and TMAZ in advance side nucleates next and then the micro-crack of the junction of the HAZ and TMAZ develops to the main crack,which propagates along the junction of HAZ and TMAZ and results in fracture of the joint.The junction of the HAZ and TMAZ is the weakest link of the whole FSW joint. |
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