Study On Quantitative Ultrasonic Test And Quality Evaluation For Laser Welded Lap Joint Of Stainless Steel

With the rapid development of the urban rail transit in China,the demand for the quality of railway vehicles is getting higher.The stainless steel body is widely used in the manufacture of various railway vehicles with its characteristics of corrosion resistance,high safety and light weight.At present,the welding of the stainless steel body mainly adopts resistance spot welding and laser deep welding(laser welding).Laser welding is mainly used to weld the sidewall of the stainless steel body.As a partial penetration way,the stainless steel skin covers outside of the frame and the laser beam welds the inside of the frame.Compared with the traditional resistance spot welding,laser welding improves the appearance of the stainless steel body.The study on the laser welding technology has been paid more attention in the field of the rail transit.However,the fluctuation of the welding parameters and the instability of energy transferred from the laser beam to the workpiece affect the formation of welded joints.If the two steel sheets are not tightened,the base metal will be melted inhomogenously,resulting in the defects such as incomplete fusion and penetration.Therefore,it is very important to establish an effective method to evaluate the quality of the laser welded joint.The mainly evaluating methods for the quality of the laser welded joint are the online evaluation based on the process parameters and the destructive inspection after welding.The former only provides the limited information of the welding quality,which is unable to reflect the joint quality accurately.The latter obtains the geometric parameters precisely,such as weld width and depth,causing material wastage and low efficiency.Therefore,it is an important research direction to analyze the quality of welded joints by the nondestructive testing method.The ultrasonic testing technology is suitable for the detection of laser welded lap joints for the thin sheets with its high penetrating capability and sensitivity.To realize the quantitative detection for the laser welded lap joint,in this paper,the ultrasonic testing process of the laser welded joint is studied thoroughly,and the effective evaluation of the welding quality is performed,providing the reliable guarantee for the actual production application.The ultrasonic test of the laser welded joint is simulated by COMSOL Multiphysics software.Both the transient distribution of acoustic field and A-scan signals are analyzed when the ultrasonic probe is located over the base metal,the fusion boundary and the fusion center zones.The intensities of the front two echoes have different characteristics when the probe is located over different zones,and using the amplitude of the first echo is the most effective and direct judgment on the fusion boundary.The transverse scan of ultrasonic test is simulated,and the variation of the amplitude of the first echo is analyzed.As the probe moves from the base metal zone to the fusion center,the amplitude of the first echo decreases gradually,and then the maximum value appears.The ultrasonic scanning tests of the actual laser welding and contrast specimens are carried out,which prove that the maximum value point of the amplitude during the numerical simulation is the fusion center of the lap plate.The fusion center provides a basis for evaluating the small weld width.Based on the time and frequency domain analysis of the actual ultrasonic signals for the laser welded joint,change rules of the A-scan waveform and its corresponding frequency spectrum characteristic curve are studied.In the time domain,the semi-attenuation method is used to evaluate the weld width,that is,with the movement of the probe from the base metal to the fusion zone,the position of the probe is considered as a boundary of the weld width when the amplitude of the first echo is half of the base metal zone.In the frequency domain,the first main frequencies are different when the probe is over the base metal and the fusion zones.The amplitudes of the first main frequencies are extracted during the transverse scanning process.The position of the probe is considered as a boundary of the weld width when the two amplitudes are equal.The semi-attenuation and frequency domain analysis methods for evaluating weld widths are verified by the experiment.The semi-attenuation method has larger errors in the evaluation of weld widths.When the weld widths are smaller,the frequency domain analysis method also has larger errors.The small weld width is defined when the weld width is smaller than the focusing diameter of the probe.The actual focusing diameter of the probe is deduced,and the mathematical model of calculating small weld widths is established based on the time domain signals.The mathematical model of calculating small weld widths is used to correct the errors of the frequency domain analysis method.The results show that the model improves the accuracy of evaluating weld widths by the frequency domain analysis method,and the highest accuracy occurs when the probe is located over the fusion center.In this paper,ultrasonic C-scan for the laser weld is carried out.The C-scan imaging is performed using the amplitudes of the first echoes and the first main frequencies.The imaging results show that the C-scan images reflect the fluctuation and morphology of the fusion boundaries at the lap plates roughly,but the characteristics of fusion centers and boundaries are unable to recognized quantitatively.Therefore,the characteristic extraction method of a laser weld is proposed.The characteristic extraction of a weld is mainly to extract the characteristic points of the fusion center and two boundaries for each transverse scanning line,and then add them towards the direction of the weld length.The curve fitting is made for the scanning points of the amplitudes on a transverse scanning line.Finally,the only fusion center and two boundary characteristic points on the transverse scanning line are confirmed,realizing the characteristic extraction of the weld at the lap plate.A concept of the equivalent weld width is proposed.The equivalent weld width not only reflects the effective connection area at the lap plate,but also makes an effective evaluation on the tensile shear performance of the weld.The automatic recognition system of laser welds is developed.The system detects the fusion center and boundaries at the lap plate quantitatively,which is of great significance to guarantee the quality of the laser weld.The process parameters of laser welding have certain influences on the formation of the weld depth.Moreover,there is a certain quantitative relation between the weld width and depth.Therefore,the laser weld depth is predicted using the multivariate linear regression and BP neural network models.The thickness of the completely penetrated plate,the laser power,the welding speed,the defocusing distance and the evaluated value of the weld width by ultrasonic test are selected as the input parameters.The prediction results show that the predicted values by the multivariate linear regression model have larger errors when the weld depths are smaller.On the whole,the prediction accuracy of the BP neural network model is higher,and it has a stronger anti-interference ability and higher engineering application value.