| Auto-body structure is an important carrier of automobile lightweight and safety,which is mainly connected by stamping parts and welding process.Resistance spot welding(RSW)is widely used in the automobile manufacturing field,the connection weld quality is an important part to guarantee vehicle performance.Ultrasonic testing is a nondestructive testing method which is widely used to detect resistance spot welding quality.It has many advantages such as simple operation,direct show,harmless.For ultrasonic nondestructive(UT)testing technology of auto-body parts resistance spot welding quality,there are still problems to be solved urgently.Firstly,the vast majority of spot weld ultrasonic testing standard is based on the quantitative evaluation of nugget diameter.But the nugget is a spatial structure,nugget diameter cannot reflect the quality of spot welding.Therefore,the quantitative evaluation of nugget diameter and thickness should be realized,and the comprehensive evaluation method of resistance spot welding should be proposed to form the same effect as the destructive testing.In addition,the major and urgent problem is to realize the automatic on-line detection of auto-body parts resistance spot welding quality,improve detection efficiency,ensure the quality of each weld.For this purpose,this paper carries out the studies on quantitative relationships of weld mechanical property,nugget characteristic,welding parameter,and the ultrasonic automatic testing of auto-body parts resistance spot welding quality.Firstly,effects of welding current,welding time,electrode force,electrode wear morphology and multi-pulse tempering on macro-size,microstructure and defect formation of DP590 steel spot weld are analyzed according to the finite element simulation,spot welding experiment,metallographic test and microhardness test.Six kinds of normal weld,small nugget weld,thin nugget weld,over-burning/deep indentation weld,lack of weld are obtained to provide feature samples for ultrasonic testing of resistance spot welding quality.Then the nonlinear regression models of welding parameter and nugget size are established based on the least square method and Gauss-newton iteration method.It can be seen that the regression models of nugget nugget diameter,nugget thickness and welding current,welding time satisfy the revised index growth model.After that,the nonlinear regression model of nugget size and weld peak load is established according to the tensile-shear test.It shows that the regression model of nugget size and weld peak load can be linearized as a simple equation of nugget volume and weld peak load.And it provide the evaluation basis for ultrasonic testing of resistance spot welding quality.For evaluating the nugget size quantificationally,the material attenuation characteristic of DP590 steel is detected using electromagnetic acoustic resonance method.The absorbing constant and scattering constant of the same material are constant.The quantitative evaluation methods of nugget diameter,nugget thickness and indentation depth are proposed based on ultrasonic reflection and transmission principle,weld nugget ultrasonic detection principle,results of material attenuation detection with electromagnetic acoustic resonance.On this basis,the ultrasonic comprehensive evaluation method of resistance spot welding quality with nugget characteristic and weld mechanical property is proposed to provide evaluation method for ultrasonic automatic testing.The accuracy of these evaluation methods are verified by orthogonal tests.For realizing ultrasonic automatic testing of auto-body parts resistance spot welding quality,an ultrasonic automatic testing system of auto-body parts resistance spot welding quality and an online ultrasonic testing system of resistance spot welding quality are developed.They are suitable for large or medium auto-body parts welded by automatic welding equipment and small auto-body parts welded by fixed spot welding machine,respectively.They meet the requirements such as automatic locating,signal obtaining,eigenvalue extracting,quantitative evaluation of nugget characteristics,detection of weld type and quality,data storage.The validities of these two systems are verified by ultrasonic manual detection and orthogonal test,respectively.In addition,coupling problem is one of the biggest problems which restrict ultrasonic testing in the industrial application.Thus the electromagnetic ultrasonic testing of resistance spot welding quality is studied based on simulation and experiment.It can be seen that the low signal-to-noise ratio affects the signal detection of spot welding defect,the transduction efficiency of electromagnetic acoustic transducer needs to increase enormously to improve detection effect.In this paper,the quantitative intelligent evaluation of nugget diameter and nugget thickness is realized.Then the ultrasonic comprehensive evaluation method of resistance spot welding quality with nugget characteristic and weld mechanical property is proposed.At last,the ultrasonic automatic testing of auto-body parts resistance spot welding quality is realized.It has great engineering significance and practical value on auto-body spot welding detection and vehicle safety travel. |
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