Study On Influence Of Heating Repairing Process On Microstructure And Mechanical Properties Of 6061 Aluminum Alloy Welded Joints

The 6000 series aluminum alloy is widely used in the fabrication of rail car body vehicles.However,some of the problems that result during fabrication and welding include thermal deformation and tensile residual stresses.Torch heating is often used to correct and repair the deformation in weldments after actual production.Since the heat input induced during the repair process will affect the microstructure and properties of materials,it can also affect the driving safety of the train.In this paper,welded joints made from 6061 aluminum alloy sheet which is commonly used in aluminum alloy train bodies was studied.The influence of heat repairing temperature,times and cooling method on residual stress,mechanical properties and microstructure of welded joints of aluminum alloys was studied using experimental and simulation methods.The research results of this thesis have important engineering significance regarding the heating process used in correcting welding induced thermal deformation.A finite element analysis model of the effect of heat repairing on residual stress was established and the influence of different heating schedules on residual stress reduction was analyzed.When the temperature is 250?,the residual stress on the joint is effectively reduced.The longitudinal residual stress can be reduced by up to about 24.45%and the transverse residual stress can be reduced by up to about 9.48%compared with an unheated 6061 weld joint.Heat repairing a weld joint twice is better to reduce the residual stress.The longitudinal residual stress on the joint can be reduced by 6.19%,the transverse residual stress is reduced by 4.77%in the weld zone and 19.22%in the heat affected zone.Water cooling after heat repairing can also reduce the effect of heat input.The longitudinal residual stress can be reduced by about 12.59%and the lateral residual stress is about 4.71%compared with air cooling.The influence of different heating repairing processes on the mechanical properties of welded joints was also researched.The comprehensive performance of welded joints is better when the temperature is 150??250?.After multiple heating repairing,the strength of welding joints increased slightly,but there are low hardness areas near the weld toe with the hardness falling below 60 HV.Water cooling after heating repairing can improve the strength to a certain extent.Water cooling immediately after heat repairing at 200? can increase the tensile strength by about 17.25%compared with an unheated weld joint.The ductility is improved and no area of low hardness has been found after water cooling was used.The influence of different heat repairing processes on the microstructure of welded joints was researched.Heat repairing mainly affects the grain size of the heat affected zone and the distribution of AlFeSi impurity phases.The microstructure near the weld and the fusion line and the Mg2Si strengthening phase are less affected.When the heating repairing temperature is low,the microstructure and precipitation phase characteristics of the welded joints are less different from those which were unheated.When the temperature is high,the grain size of the heat affected zone of the welded joint is increased and the phenomenon of agglomeration of the AlFeSi phase occurs,causing the strength and hardness of the heat affected zone to decrease.The difference between heat repairing a weld joint twice and heat repairing it once is small.A small amount of grain growth occurs near the heat affected zone near the fusion line after heating repairing for three times.Water cooling after heat repairing can reduce the grain size of the heat affected zone and the tendency of the AlFeSi inclusion phase to grow which reduces the adverse effects of heating repairing to a certain extent.