Research On Ultrasonic Detection For Connection Of Projection And Spot Welding Of Stainless Steel

Austenitic stainless steel has been widely popularized as an important material for lightweight structural design, and because of its excellent corrosion resistance, processability,surface appearance, good mechanical properties and other characteristics have been widely used in various industries. The projection and spot welding process of austenitic stainless steel reduces the risk of strength reduction and corrosion resistance caused by other welding methods, and this process has the advantages of energy concentration, efficiency and easy to automate, which is an indispensable material processing method. It is widely used in rail vehicles and medical devices fields.There are two kinds of quality evaluation methods used in projection and spot welding.The first is tensile test of welded specimen on the same projection and spot welding process or the actual production of projection and projection and spot welding components, however,this quality evaluation methods is not only low reliability, low efficiency, and wasting. The second is online evaluation based on welding process parameters. However, due to the complexity of the welding process and method for evaluating the quality of projection and spot welding is still not mature evaluation model, hindering its popularization and application. Ultrasonic nondestructive testing technology has the advantages of high sensitivity and easy to realize the field detection. In recent years, many scholars use ultrasonic testing methods to evaluate the quality of projection and spot welding, and obtained some research results.Because of thin plate structure of stainless steel projection and spot weld and special structure characteristics of projection projection and spot weld, there are two major technical difficulties in projection and spot weld ultrasonic testing: First, how to extract the characteristic value which can reflect the internal structure of projection and spot weld;second, how to distinct between the plastic connection area and the fusion core area of theprojection and spot weld. In this paper, based on the solution of the above problems, through the analysis of ultrasonic testing signals to achieve qualitative and quantitative detection of stainless steel projection and spot weld.Establish a stainless steel projection and spot weld ultrasonic testing finite element simulation model, calculated in projection and spot weld internal ultrasonic sound wave propagation with the distribution of travel time and its special internal structure on acoustic scattering law, and generates the ultrasonic echo signal curve. Simulation results show that in the nugget edge region, due to the sharp corner of diffraction effect, the reflection of sound waves in the main direction of acoustic beam relative to the incident wave and the direction of acoustic beam deflection phenomenon. According to the characteristic of ultrasonic echo amplitude, the B-scan characteristic curve along the fusion nuclear cross section is generated,and the characteristic of the echo amplitude characteristic curve is analyzed, which is the theoretical basis for the quantitative analysis of the diameter of the fusion core. In addition,for actual projection and spot welding process in the presence of projection and spot welding indentation, crack, shrinkage holes of the simulation model is set up. Analysis the defects of ultrasonic propagation characteristics, and analyzes the influence of defects on ultrasonic echo and echo amplitude characteristic quantity, sample data are provided for the identification of welding defects.In order to study the influence of the scattering of the echo signal of each frequency band, In this paper, the time-frequency analysis of ultrasonic echo signal of projection and spot welding joint is carried out, we use DB wavelet coefficients to carry out the six level wavelet decomposition of the echo signal, and the different levels of the wavelet decomposition signals are used to calculate the variance as the form of the characteristic quantity. Analysis results show that the ultrasonic echo wavelet decomposition levels of low-frequency signal characteristic quantities can show internal structure characteristics of projection and spot weld, but in the nugget edge gradient change is not obvious; echo signal wavelet decomposition levels of high frequency signal characteristic quantity reflects the projection and spot weld of the structure characteristics of different levels, especially through the five scales of the high-frequency signal features can clearly identify the nuggetand plastic connection region, that nugget relatively coarse grain structure to five scale of the high frequency signal attenuation is most serious. In order to verify the theory, a plate specimen with a molten core crystal structure was prepared, and the experimental results were in agreement with the above analysis. Respectively based on the ultrasonic echo amplitude, frequency, wavelet decomposition when signal eigenvalues are found, in effectively distinguish nugget and plastic connectivity aspects, time-frequency analysis is significantly better than the other two methods.In this paper, we use ultrasonic simulation echo five levels of high frequency signal characteristic value B-scan curve change rate curve to the actual projection and spot welding joint ultrasonic B-scan detection characteristic value curve on the fusion core edge recognition, this method can be used to measure the size of projection and spot welding. By filtering, projection and projection and spot welding ultrasonic C-scan image fusion and edge detection, can give a clear nugget and plastic connection area morphology. By projection and spot welding joint parts metallographic examination showed that, projection and weld nugget size ultrasonic detection result error is less than C-scan step distance, when using the ultrasonic echo signal frequency analysis access feature detection method and fully meet the needs of detection of projection and spot welding joints.