| To vigorously develop High-speed train with advantages of high speed,safety,comfort,all-weather transportation,environmental friendliness and sustainability is not only the strategy of making a strong country clearly stated in the 14th Five-Year Plan,but also a solution to our country under energy and environmental constraints.It is also the only way and the inevitable choice for the contradiction of insufficient supply of transportation capacity.As High-speed train has strict requirements for lightweight,airtightness,and corrosion resistance,at present,all parts of the train body are made by aluminum alloy profiles.As the core technology in rail vehicle manufacturing,the welding of aluminum alloy is directly related to the quality of the train and driving safety.However,when welding aluminum alloys by fusion welding,the main weldability problems are:high pore sensitivity,high tendency of hot cracks,prominent joint softening and deformation problems,which greatly affect the performance and manufacturing cost of joints.Benefiting from the advantages of high efficiency,low cost,high quality,and environmental protection,friction stir welding(FSW)will surely replace traditional fusion welding and become the dominant connection technology for the new generation of aluminum alloy High-speed Trains.Although FSW is considered to be an ideal welding method for aluminum alloys,the FSW of aluminum alloy profiles for High-speed train bodies still has some problems,such as back support during butt welding,hook defects during butt lap welding,and hole defect problem.This hinders the industrialization process of FSW in the manufacturing of High-speed train to a certain extent.This paper takes several typical parts of the High-speed train body as the research object,carries out a systematic study on FSW process adaptability,joint structure and mechanical properties,reveals the formation rules of typical defects,and gives effective avoidance measures from the welding process and welding tools,and establishs safety assessment method for FSW components,which has important application value for promoting the application of FSW in train body manufacturing and improving the quality of FSW parts.For the 6005A aluminum alloy thin profiles(4mm)used for side walls,roofs,and bottom plate,the process,joint structure and properties of Bobbin tool friction stir welding(BTFSW)under butt joint were studied.Process research shows that the nugget area of BTFSW presents a dumbbell shape,and the crystal grains appear as fine equiaxed crystals after dynamic recrystallization.There are two basic forms of "S" and "(?)" shapes in the nugget zone.And the advancing side edge is prone to hole defects.Microstructural research shows that:the hole defect is located at the intersection of the upper shoulder drive area,the lower shoulder drive area,and the stirring pin drive area.When the three plastic metal flow speeds cannot match each other(low welding speed or high rotation speed),they will meet and a cavity is left to form a hole defect;The "Z"-shaped line defect is composed of Al2O3 and AlSi.Among them,AlSi is the crystalline phase of 6005A aluminum alloy during the smelting process,and Al2O3 comes from the alumina at the butt surface.The mechanical test results show that when the j oint contains hole defects,cracks are initiated at the tip corners of the holes and propagate in the nugget zone along the 45° direction under the dominance of the shear stress.The strength of the joint depends on the size of the hole;when the j oint does not have hole defects,the existence of the "Z"-shaped line will not induce cracks.The fracture occurs in the heat-affected zone adjacent to the nugget zone,and the joint strength can reach more than 80%of the base metal strength.A new type of tool was designed to effectively suppress the occurrence of holes defects,and orthogonal experiments were designed using the response surface method to establish a binary quadratic regression equation between the process parameters of the new tool and the joint strength.The optimal process parameters were obtained.For the 16mm 6005A aluminum alloy profile used for the corbel,the process of the single-shoulder FSW in the butt-lapped form,the structure and properties of the joint are studied.The results show that no matter what kind of welding process is used,there are Hook defects in the FSBLW joint.It is the result of the migration of the original lap surface under the dual action of heat and force.The Hook defect originates from the edge of the lap surface and ends at the boundary of the thermo-mechanical affected zone/nugget zone,it appears as a crack under the light microscope,and the grains on both sides are obviously coarsened,and the tip of the hook defect is obtuse;The longer the stirring tool,the larger the angle between the Hook defect and the lap surface and the smaller the size,which helps prevent crack propagation;The welding speed increases,the size of the hook defect decreases,but the rotation speed has little effect on the size;the larger the size of the Hook defect,the lower the tensile strength of the joint,The small size defect joint fractures in the heat-affected zone,and the fracture has a large number of dimples,a typical ductile fracture;while the larger size Hook defect joint tensile fracture is at the edge of the weld nugget zone,a ductile-brittle composite fracture.Using the theory of linear elastic fracture mechanics,the fatigue expansion evaluation model of Hook defect is established,and the critical expansion stress amplitude relationship with smaller error is obtained,and the Hook defect is evaluated according to the non-expansion condition. |
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