| As a kind of widely used light alloy material with excellent properties,the 6000 series aluminum alloy has great potential to become the raw material for the lightweight manufacturing of automobiles.But it is difficult for 6000 series aluminum alloy to obtain a high quality joint by using traditional welding method,especially for aluminum alloy sheet.Therefore,it is necessary to study the joining technology of 6000 series aluminum alloy sheet.In recent years,a new type of welding technology called pinless friction stir welding has been used to obtain high quality and stable joint of aluminum alloy sheet.However,there is a lack of systematic study on the pinless friction stir welding process of 6000 series aluminum alloy sheet.In this paper,1mm 6061 Aluminum Alloy sheet was chosen as the welding material and pinless FSW was applied to accomplish welding.The main processing parameters which affect performances of the pinless FSW connector include spindle speed,welding speed,load experiment design.Butt joint and lap joint are obtained by using pinless FSW of different processing parameters.Testing the mechanical properties and cross sectional microhardness of welds.The defects in weld zone were observed by metallographic microscope.Electron backscatter diffraction(EBSD)was used to observe the grain size and shape.Scanning electron microscopy(SEM)was used to observe and analyze the fracture surface.The main conclusions are as follows:The pinless FSW butt joint of 6061 Aluminum Alloy sheet performance beautiful.With the increase of welding speed,the tensile strength of the weld increases first and then decreases,and the peak value is 198 MPa,which reaches 73.4% of the base metal.The elongation increases with the increase of spindle speed,and the maximum value is20%.The lower pressure has a certain influence on the tensile strength,and the optimum downward pressure is 0.1mm in this study.Natural aging can effectively improve the mechanical tensile properties.The microhardness distribution of the cross section of the joint is "W" or "1" shape,and the joint with a higher minimum microhardness has a higher tensile strength.The fracture location of the tensile specimen is located at the center of the weld.Optical microscopy results showed no weld penetration defects on the back of the joint.The optimum welding process is:spindle speed 1500 rpm,welding speed 300mm/min,lower pressure 0.1mm,natural aging after welding for more than one week.The pinless FSW lap joint of 6061 Aluminum Alloy sheet performance beautiful.The tensile properties increased with the increase of welding speed,and the maximum value is 199.5MPa,reaching 73.9% of base metal.The results of the microhardness test show that the microhardness of the upper and lower plates of the pinless FSW lap joint is "W" or "1".The hardness decreasing area of the upper plate is wider than the lower plate.The joint with a higher minimum microhardness has a higher tensile strength.The tensile sample is broken in the heat affected zone at the retreating side of the upper plate.The results of optical microscopy show that the fusion area between the upper plate and lower plate of the weld with good mechanical tensile properties is wider.The Hook defects are easy to appear on the advancing side of the joint that is welded in a high rotating speed.The optimum technological parameters is: 1500 rpm,v300mm/min,lower pressure 0.1mm.EBSD was used to observe the best performance pinless FSW butt joint and lap joint.The results show that grains on the plate on the weld center position is isometric crystal which undergos a severe plastic deformation.TMAZ is simultaneously affected by plastic deformation and thermal cycling,and the grain size is uneven.The HAZ is only heated by cyclic action,and the grain size is the biggst.The grains of the lower plate are bulky that is grown by heating.The tensile fracture SEM results show that the high elongation butt joint is ductile tensile fracture,and the lap joints are brittle fracture. |
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