| In recent years,with the rapid development of China’s automobile industry,people pay more attention to the environmental pollution and the energy consumption.Investigation shows that nearly 70 percent fuel consumption is on automobile body itself in case of no load,and 3 percent to 7 percent of fuel will be saved if the weight is reduced by 10 percent.Therefore,the automobile lightweight is significant for energy-saving and emission-reduction.The CR340/590DP high strength steel developed by Pan steel Group is of high strength and good plasticity and toughness,as the car plate,it can effectively reduce the vehicle weight and cut down the fuel consumption.But at present the steel spot welding process has not been done a systematic research and been lack of relevent data.The 1.2mm thick CR340/590DP high strength steel is chosen as welding material,and then analyse the influence of process parameters on the properties of the joints to define the range of the parameters.Design parameters through orthogonal experiment and then do the spot welding test.Obtain the optimal operating technical parameters through orthogonal experiment tests,do the spot welding test.According to the tear test,shear test and metallographic test to analyze splash,shrinkage,cracks and other defects that may occur during spot welding tests,measure the nugget size and the shear force of the joints.Judge the fracture failure mode of spot welded joints by analyzing fracture morphology after tensile shear test.Take the spot weld nugget diameter,the tensile shear load and the indentation depth as welding quality standard,to obtain the optimization of process parameters combination of the welding current,welding time and electrode pressure,and then obtain the insequence of these factors importance.Describe the relationship curve and process window of orthogonal factor and index.Get hardness of welded spot through hardness testing.Do the single factor experiment on this basis,to verify the optimization process parameters and analyze the influence of single process parameters on the microstructure and mechanical properties of spot welded joints.The results of orthogonal experiment show that the microstructure of nugget zone is gross lath martensite.The microstructure of heat affected zone is ferrite,martensite and a little bainite.Hardness of base material is 200HV,hardness of nugget zone is far more 405HV,and heat affected zone isn’t intenerated.When the welding current is 8.5kA,the welding time is 360ms,and the electrode pressure is 3.6kN,the joints obtain the maximum shear tensile strength.The sequence of factors importance of welded joints are respectively welding current,welding time,electrode pressure.Tensile shear of welded joints substantially increase with the increase of welding current(only excessive welding current cause internal splash of welded joint):however,with the increase of welding time and electrode pressure,they firstly increase then decrease and all in a gentle change degree.When the welding current is 5.5kA,the welding time is 240ms and the electrode pressure is 4.2kN,the joints obtain the minimum indentation depth.The sequence of factors importance of indentation depth are respectively welding current,electrode pressure,welding time.According to single-factor expriment,the change trendency of both tensile shear and nugget size again the process parameters change are consistent.The plastic ratio is more than 40 percent,the plasticity is nice.And the maximum tensile shear is up to 15.25kN.The change trendency of the optimum process parameters and tensile shear change with the change of technological parameters are basically consistent with the results of orthogonal test.Microstructure of wleded joint specimens widen with the increase of welding current or welding time,while it affecte little by electrode pressure. |
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