Study On Ultrasonic Nondestructive Testing Of Spot Welding Of Unequal-thickness And Multilayer Low Carbon Sheets

The resistance spot welding of unequal-thickness and multilayer low carbon sheets is more complex than equal-thickness,thus the nuggets quality are difficult to guarantee.However,the quality of solder joint has a great importance on vehicle reliability and security.Almost all car welding production lines don't set spot welding nondestructive testing.Destructive sampling inspection method is widely used to detect solder joint quality.This method is destructive,hysteresis,small sample and testing cost high,so it is difficult to fully reflect the quality of spot welding.What's more,it is unable to meet the industrial development of body welding quality request with high reliability and low cost.Based on it,this thesis uses the conventional ultrasonic nondestructive testing method to study the quality detection and assessment of spot welding for unequal-thickness and multilayer low carbon sheets.The specific contents and research results are as follows:(1)The mathematical model of the spot welding ultrasonic nondestructive testing for unequal-thickness and multilayer low carbon sheets is got.All kinds of information of solder joints in A scanning waveform is obtained.Through the analysis of the relationship between ultrasonic echo acoustic pressure and the nugget size,setting up the mathematical model of the spot welding ultrasonic nondestructive testing.(2)The finite element simulation model of the spot welding ultrasonic nondestructive testing for unequal-thickness and multilayer low carbon sheets is established.Based on sound waves propagation characteristics in the process of ultrasonic nondestructive testing,COMSOL software is used to establish the finite element analysis model of spot welding ultrasonic nondestructive testing for unequal-thickness and multilayer low carbon sheets.The finite element model is built to simulate and analyze the process of spot welding quality nondestructive testing.This thesis studies the acoustic signal characteristic and changing rule of different thickness and different nugget size,studies the influence of nugget diameter detection results when ultrasonic probe position is changing.(3)The experiment of the spot welding ultrasonic nondestructive testing forunequal-thickness and multilayer low carbon sheets is carried out.The resistance spot welding of unequal-thickness and multilayer sheets,the spot welding ultrasonic nondestructive testing and the metallographic test is completed.Comparing with the simulation results of the ultrasonic nondestructive testing and the mathematical model of nugget diameter,this thesis verifies the correctness of the finite element simulation and the mathematical calculation model.The study provides theory and method to guide spot welding quality inspection of unequal-thickness and multilayer low carbon sheets.The results of the study are of great significance to ensure welding reliability and vehicle security.