Research On The Optimization Of Projection Welding Of Nut For Automotive

Nut projection welding is widely used in various industries due to its high degree of automation,large-scale welding,high production efficiency,and low environmental pollution,especially in the automotive industry.With advantages,there are also some problems that need to be solved.Due to the small nucleation size,the non-linear dynamic change of the overall resistance,the large fluctuation of process parameters and the transient nature of welding parameter changes,the quality stability of nut projection welding is poor.At the same time,there are also problems such as large fluctuations in welding equipment parameters and unknown process optimization directions.In order to improve the above problems,this paper takes SAPH370 pickled hot-rolled steel sheet and M6 welded square nuts as research objects,reveals the complex relationship between welding parameters and welding quality,and provides a certain process optimization direction.The projection welding of nut is a multi-factor and multi-variable problem.It has characteristics such as scattered test data and inconspicuous regularity.In order to analyze the data,a single-factor test is performed in this paper.The electrode pressure,welding current,and welding time are varied to study the effect of parameter changes on welding quality.In addition,the influence of welding parameters on welding quality is interactive.In order to study the interaction between welding parameters,a compound factor test was added to study the interaction between the welding parameters and its effect on welding qualities of joint mechanics,nugget size,joint hardness,thread deformation,etc.With the increase of welding current,the Pull-out load tends to increase first,then decrease and then increase.Correspondingly,the fracture type of nut welded joints generally shows a transition trend from interface fracture,nut fracture to steel sheet fracture.With the extension of welding time,the Pull-out load of nut welded joints generally does not change much,that is,the impact of welding time on the Pull-out load is small.With the increase of welding current,due to the excessive resistance heat,the heat input is too large,and the thread deformation is easy to occur.When the electrode pressure is large and the welding time is long,thread deformation is liable to occur,because the electrode pressure is too large and the heat input is too much.Therefore,in order to reduce thread deformation,the electrode pressure should not be too large,the welding current should not be too large,and the welding time should not be too long.The matrix structure of the nut and the steel sheet are both ferrite and pearlite.A significant phase change occurred in the heat-affected zone of the nut,and the structure changed from fine ferrite and pearlite to a complex mixed structure of martensite,ferrite,and pearlite.The structure of the heat-affected zone of the steel sheet is ferrite,pearlite and tertiary cementite.The production of tertiary cementite makes the hardness and brittleness of the region increased.Due to the greater brittleness of triple cementite and martensite,welded joints are prone to fracture in the heat-affected zone of nuts and the heat-affected zone of steel sheets.The hardness of the nut and steel sheet is about HV230,and the hardness of the center area of the solder joint floats up and down at 400 HV,which is about 74% higher than the hardness of the nut and steel sheet.As the distance from the center of the nugget increases,the hardness value gradually decreases,showing a rapid downward trend in the heat affected zone(HAZ).Due to the better heat dissipation in the heat-affected zone,the temperature in this area drops faster,resulting in poor plasticity in the area.At the same time,the area has a large stress concentration,which makes it the weakest and most fragile area in the solder joint.Through signal monitoring,it is found that the welding current deviation value of the welding equipment involved in this subject is large,and the welding current deviation value does not change with the extension of the welding time;the welding time deviation value is small.In actual working conditions,the welding parameters should be set in consideration of the deviation of the welding equipment.By modeling the ternary quadratic nonlinear regression of welding parameters and the Pull-out load,the complex relationship between the welding parameters and the welding quality is revealed.It was found that the Pull-out load increased significantly with the increase of welding current.After the welding current increased to a certain extent,the effects of welding time and electrode pressure on the Pull-out load were very small and could be ignored.As for the interaction between welding parameters,it is found that when the electrode pressure is small,the welding time has a large effect on the Pull-out load;when the electrode pressure is large,the welding current has a large effect on the Pull-out load.The interaction effect between electrode pressure and welding current is the largest,the interaction effect between welding current and welding time,and the interaction effect between electrode pressure and welding time are smaller.