Ultrasonic Signal Analysis And Quality Intelligent Evaluation For Short Cycle Arc Stud Welding Of Stainless Steel

With the rapid development of social economy and the increasing requirement of convenient travel,the speed of railway vehicles is rapidly increasing.In order to improve the airtightness of the whole vehicle and reduce vibration and noise,the stainless steel components of high-speed rail and other railway vehicles are welded with studs through the short-cycle Arc-Stud-Welding(ASW)method to meet the fixed and assembled requirements,so as to minimize the number of holes in the vehicle body.Short-cycle ASW requires short heating time,combines with the stud mechanical movement,and generally without shielding gas,so the welding process is easily influenced by random factors and forms welding defects such as partial welding,necking,splashing,porosity,crack and non-fusion,etc,and resulting in welding quality problems.Therefore,the quality control during welding process and the quality detection about welded joint for stainless-steel short-cycle ASW has great significance to ensure the manufacturing quality of high-speed rail and other railway vehicles.At present,the quality testing and evaluation methods of short-cycle ASW include on-line process testing and evaluation,welded joint destructive and nondestructive testing.The on-line process testing and evaluation is to evaluate the welding quality through the inspection and analysis of the parameters related to welding process and quality.However,since the stud welding process is a nonlinear and multi-parameter coupling complex welding process,the limited detection parameter information is difficult to reliably evaluate the quality of stud welding.Destructive detection is mainly used destructive testing methods such as tensile,bending,shear and torsion to detect ASW specimens.The disadvantage of detective testing is that it can only adopted to detect the welding samples,which not only increases the material consumption and reduces the production efficiency,but also cannot be used to test on the in-service ASW joints.The mainly nondestructive detection methods are visual inspection and X-ray inspection.Visual inspection can only detect on the position and pose of welded stud,but not on the inside joint quality.X-ray can test on the inside quality of ASW joint,but it has safety issues,and has great limitations not only on the actual production,but also on the accessibility and convenience of in-service joint detection.Compared with other methods,ultrasonic nondestructive detection has become the most promising nondestructive detection method for ASW joints due to its advantages of high efficiency,safety,convenience in production and in-service joint.Research shows that the effective connection area and the internal defects size and distribution of ASW joints have a great correlation with its tensile strength,that is,the quality of stud welding.This provides a theoretical and practical basis for evaluating the quality of stud welding through nondestructive detection of the connection status and internal defects of ASW joints.However,for the short-period ASW joints with small diameter stud(≤8mm)and thin plate(≤2mm)which is common in the stainless steel railway vehicles,the traditional ultrasonic detection method is difficult to detect ASW joint due to the internal tiny and porous defects.A new ultrasonic testing method and algorithm is adopted to accomplish the ultrasonic nondestructive testing and quality intelligent evaluation of short-cycle ASW joint,which has important theoretical and practical significance.The FEA model of ultrasonic detection about ASW joint was established to theoretically study the ultrasonic propagation rule in ASW joint and the influence of connection status and defects in ASW joints on ultrasonic echo signals,and the theoretical analysis was verified by adopting the ideal physical model of ASW joint.Based on those researches,an automatic ultrasonic detection and analysis device for accurate positioning and scanning of ASW joints has been developed independently,realizing the precisely positioning ultrasonic C-scan detection with ultrasonic signal transmitting,receiving,analysis and automatic scanning process.It provides a research platform for quantitative ultrasonic detection and quality intelligent evaluation of short-cycle ASW joints,and a patented technology is used to solve the miniaturization of scanning mechanism and the couple between moving probe and workpiece.It lays a good foundation for the application of the device in practical production.The time domain,frequency domain and time-frequency domain analysis of ultrasonic A-scan echo signal for ASW joint were carried.The influence of the connection state of stud welding joint on ultrasonic propagation was revealed and the method of extracting the characteristic value of ultrasonic signal corresponding to the characteristic of ASW joint was established.The results show that the reflection coefficient of ultrasonic pressure varies with the frequency when ultrasonic wave propagates in different connection states of ASW joints.In the high frequency signals decomposed by wavelet packet analysis,there are specific frequency signals respectively sensitive to the weak connection zone and the defect zone of ASW joints,which can be used to accurately identify the weak connection zone and adjacent defects of ASW joint.It lays a theoretical and practical foundation for ultrasonic C-scan detection and quantitative analysis of ASW joints quality.Based on the A-scan echo signal groups detected by ultrasonic C-scan of ASW specimens,the characteristic frequency amplitudes of weak connection zone and defect zone which extracted by wavelet package decomposition form ultrasonic C-scan image which accurately presenting the connection zone and defect zone of ASW joints.Edge recognition and image segmentation algorithm are used to identify the boundary of ultrasonic c-scan image.The influence of ultrasonic beam radius on the ultrasonic echo of the boundary of the fusion zone and the defect zone was studied,and the compensation/correction method for boundary recognition was accomplished.The ultrasonic C-scan image corrected by characteristic boundary compensation has a good agreement with the SEM image of ASW joint,which provides a good basis for the quantitative and intelligent evaluation of ASW joint quality.The influence of the connection state on the tensile strength of stainless steel ASW joint was studied by using the FEA simulation method,which provided theoretical basis for intelligent prediction of ASW joints tensile strength based on ultrasonic testing.The results show that in the process of tensile test,the annular area with specific width at the ASW joint edge is stress concentration zone,and the effective connection area is approximately linear with the tensile strength of ASW joint.By equivalent circular joint and defect area processing about ultrasonic C-scan image of ASW joints,the effective connection area of stress concentration zone can be calculated conveniently,and the tensile strength of ASW joints was evaluated intelligently.Comparing with the actual tensile test,the predicted tensile strength based on ultrasonic testing is in a good agreement with the tested value.The ultrasonic quality detection and intelligent evaluation method and device for stainless steel short-cycle ASW joints developed in this paper have high accuracy and stability,which provides an effective quality assessment method for the quality detection and evaluation of ASW joints in the practical engineering applications.