| Different process paramaters have an influence on the friction stir welding(FSW)of AZ40M/AZ61A dissimilar magnesium alloy,different process paramaters,such as the shapes of pin,welding speed and materials position,were carried out to join the AZ40M/AZ61A dissimilar magnesium(6.2mm in the thickness)in the present study.The digital camera,stereoscopic microscope,optical microscope,scanning electron microscope(equipped with EDS),X-Ray diffraction,transmission electron microscope and Electron Back-Scattered Diffraction were employed to study the surface appearance,macro moephology,the development of microstructures and crystallography information of the AZ40M/AZ61A FSW-ed joints.Meanwhile,the micro hardness,tensile properties and corrosion resistance of the FSW-ed joints were studied by the micro hardness tester,universal testing machine and electrochemical workstation,respectively.The results show that tunnel defacts and voids were formed in the FSW-ed joints fabricated by the non-thread pin and the tensile properties of these joints are poor.However,the materials flow is sufficient and the mixture of the two materials is preferable when the pin with threads was employed.Meanw-hile,the tensile properties of FSW-ed joints fabricated by the threads pin are superior than that of FSW-ed joints obtained by the non-thread pin.The affect of welding speed on the machanical properties is obvious as other paramaters are invariant.The tensile properties of the joint under the paramaters of 1200rpm-70mm/min are the most excellent,the tensile strength and elongation are 235MPa and 6%,respectively.However,the tensile properties of the joint fabricated under the paramaters of 1200rpm-50mm/min are the lowest due to the surface crack and tunnel defect,its tensile strength and elongation are 143MPa and 1.66%,respectively.The interface of the two materials in the joints shift with the pin offset.The mixture of the two materials is more sufficient when the AZ40M is placed on the advancing side.Meanwhile,the superior tensile properties can be achieved when the AZ40M is placed on the advancing side and without pin offset.The micro hardness profiles of the cross section of the FSW joints present "W" shape,i.e.,the micro hardness of the two base metal and nuggets are higher than that of the mix zones,which are composed of heat affected zone and thermal mechanical affected zone.Due to the differences of the micro structures,fraction of subgrains,low-and high-grain boundaries,local misorientation,schmidt factor and textures of different zones,the tensile specimen fractures at the nugget tilted to the advancing side,corresponding to the soften zone of the surface hardness profile.Fine and equiaxed grains are formed in the nugget after dynamic recrystallization,and the difference of heat input on the thickness direction leads to the difference of the grains size.The precipitates of the nugget are finer and more homogeneous distribution than the base metal.It can be observed by the transmission electron microscopy that the precipitates are mainly AlgMn5 and Mg17Al12,9and the incoherent interfaces are formed between the precipitates and the matrix.Meanwhile,the high-density dislocations interact with the precipitates,leading to the formation of the regular network of high-density dislocations.And the regular network develops into sub-grains boundaries and high-angle boundaries.It can be inferred that the continuous dynamic recrystallization is occurred in the nugget.The research results of polarization curve and static weightless corrosion test of the AZ40M base metal,2#FSW joint and AZ61A base metal show that the corrosion resistance of the AZ61A base metal is superior than AZ40M base metal and 2#FSW joint in the 3.5 wt%NaCl solution. |
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