Study On Cold Metal Transfer Welding Process Of 5086 Aluminum Alloy Sheet For Maglev Train

The low-speed maglev train will be a significant vehicle among cities due to its light weight,low energy consumption and low noise.The 5086 aluminum alloy sheet is used extensively in the body of the low-speed maglev train.The Cold Metal Transfer?CMT?welding is an adequate joining technology for aluminum alloy sheet because of its no spatter and low heat input.The flat butt welded joint,lap joint and T-joint on the thickness of 1 mm 5086aluminum alloy were formed separately using CMT with different welding processes.The macro morphology of 5086 aluminum alloy joints was observed by stereomicroscope.The microstructure,density of the dislocation and precipitated phases of WM?weld metal?,HAZ?heat affected zone?and BM?base metal?were characterized by a series of micro and nano characterization methods.The micro-hardness and tensile test were tested through microvickers hardness tester and electronic universal tester.The method of the Load/width was proposed to represent the shear strength of lap joint and T-joint.Finally,the FEM were used to simulate the deformation behavior for various 5086 aluminum alloy CMT welded joints by the software of ABAQUS to investigate the most critical weak zone among the joints.The results show as follows:?1?The macro morphologies of the flat butt joints could be divided into complete penetration joints with ladder-shaped and incomplete penetration joints with triangle.The flat butt joint is made up of WM,HAZ and BM.There are the 80 um wide coarse grain zone?CGZ?and different wide fine grain zone?FGZ?in the HAZ.The WM consists of columnar grain and equiaxed grain.The grain sizes of the HAZ are a little bigger than those of the BM.With the increasing of the heat input,the grain sizes of the WM and CGZ increase while the FGZ is no obvious change.The micro-hardness of the joints is almost homogeneous distribution.The micro-hardness of different zones under different welding processes is similar and the value is about75⁹0 HV.There is the fine grain but no dislocation in the WM,the bigger grain and low density dislocation in the HAZ and the biggest grain and high density dislocation in the BM.The precipitated phases in 5086 aluminum alloy welded joints mainly include Mg₂Si,AlMgMn and Mg₂Al₃.For the thickness of 1 mm 5086 aluminum alloy,the best welding parameter of flat butt joint is the welding current of 52⁶1 A and the welding speed of 60⁸0 cm/min,and the ultimate strength of CMT joints reaches maximum 314 MPa which reaches 98%of the BM.The stress states of 5086aluminum alloy butt joint are heterogeneous.The mutation in the stress triaxiality occurs at the junction of WM and HAZ and at the junction of HAZ and BM.With the increasing of loading displacement,the absolute value of the stress triaxiality increases and the maximum value is about 1.0 which occurs at the junction of HAZ and BM.Finally,the mutation in the stress triaxiality also occurs at the BM and subsequent the fracture occurs at the zone.It shows that the most critical weak zone lies in the BM for the 5086 aluminum alloy butt joint.The experiment results and simulation results are in good agreement.?2?The macro morphologies of the lap joints under different CMT parameters are quite different.With the increasing of the heat input,the shapes of the lap joints are from semicircle to oval and the length of effective bonding line increases gradually.At the same time,the grain sizes of the WM and CGZ increase while the change of FGZ is not obvious.Meanwhile,the grains are from the columnar crystal with no orientation to the columnar crystal&the equiaxed crystal with orientation.The welding processes have no influence on the micro-hardness of the lap joint the values of the micro-hardness are 75⁹0 HV.With the increasing of the length of effective bonding line,the shear strength increases first and then decreases.The ultimate shear strength of lap joint reaches maximum 276 N/mm when the length of effective bonding line is 2.75 mm and the corresponding parameter is the welding current of 56 A and the welding speed of 60 cm/min.The mutation in the stress triaxiality occurs at the two ends of effective bonding line.With the increasing of loading displacement,the absolute value increases and the maximum value is 1.3.Hence,the two ends of effective bonding line are the most critical weak zone.The experiment results and simulation results are in good agreement.?3?The welding processes have no influence on the macro morphologies of the T-joint.However,the welding window of the T-joint is very narrow for the thickness of 1 mm 5086 aluminum alloy.Meanwhile,welding deformation,porosity as well as incomplete penetration at the corner occur easily among the T-joint.With the increasing of heat input,the grain sizes of the WM increases while the change of CGZ and FGZ is not obvious.The micro-hardness of the T-joint is almost homogeneous distribution.For the thickness of 1mm 5086 aluminum alloy,the best welding process of T-joint is the welding current of 75 A and the welding speed of100 cm/min,and the ultimate shear strength of CMT joints reaches maximum302N/mm.The mutation in the stress triaxiality occurs at the HAZ of the vertical plate.With the increasing of loading displacement,the absolute value of the stress triaxiality increases and the maximum value is 0.9.Therefore,the HAZ of the vertical plate is the most critical weak zone.The experiment results and simulation results are in good agreement.