| Due to its own advantages of high strength,low density,and high corrosion resistance,7075 aluminum alloy has been widely used in many fields such as automotive bodies and aerospace equipment manufacturing.However,the high strength and low ductility characteristics of 7075 aluminum alloy also make it susceptible to cracking defects when connected by mechanical deformation,which affects the quality of the material connection.Hence,in this paper,a self-piercing riveting(SPR)process based on ball-shaped die was proposed to achieve the efficient and reliable jointing of 7075 aluminum alloy sheet materials;The effect of different process parameters on the static mechanical properties of joints was identified by means of response surface design test methods;at the same time,the connection between the mechanical internal locking structure and the mechanical properties of joints was investigated,and a non-destructive testing method for joint quality evaluation was proposed,and the major research contents and results are as show below:Firstly,a finite element simulation model of the self-piercing riveting forming process with a "ball-shaped die" was established to identify the influence of the geometric dimensions of the ball-shaped die,such as die depth and die diameter,on the self-piercing riveting forming process and forming quality.The results show that the mechanical internal locking length of the rivet gradually decreases as the diameter of the die increases,while the remaining bottom thickness increases gradually with increasing die diameter;A ball-shaped die with a depth of 1 mm and a diameter of 10 mm gives a self-piercing riveting joint with excellent overall mechanical properties,The simulation data provides a basis for the optimization of the spherical mould structure and the design of the self-piercing riveting process parameters.Secondly,an experimental study of the "ball-shaped die" self-piercing riveting process was carried out for 7075 aluminum alloy sheet material.Using rivet length,punch speed and punch stroke as the main influencing factors,and joint failure load and energy absorption as the response values,a multivariate non-linear regression model between the influencing factors and the response values was constructed to explore the influence of the influencing factors and their interaction on the response values.The test results show that the significance of the regression model developed is high and the relative error between the predicted and tested values of the model is within 10%.The analysis of the regression model for the failure load revealed that the change in rivet length has the greatest significant effect on the failure load of the joint,followed by punch stroke,with punch speed having the least effect on the failure load,and the interaction between rivet length and punch stroke having the most obvious influence on the failure load;For the energy absorption value regression model,it could be known that the change in rivet length has the greatest influence on the energy absorption value of the joint,with the interaction between rivet length and punch stroke having the most obvious influence on the energy absorption.Finally,the intrinsic link between the mechanical interlocking structure of the “ballshaped die” self-piercing riveting joint and the mechanical properties of the joint was investigated.The results show that the expansion diameter of the rivet tip shows a positive correlation with the failure load,and the expansion diameter of the rivet tip can be used to characterize the strength of the joint.Meanwhile,a method was proposed for the non-destructive testing of the dimensions of the mechanical internal locking structure of the joint.Based on the C-scan image of the joint section with water immersion focused ultrasound,a polynomial was used to describe the distribution of the echo reflection intensity of the rivet tip,and the "extreme point" of the rate of change of the image grey scale value was used as the boundary point for the measurement of the expansion diameter of the rivet tip,the feasibility of the test method was also verified by destructive testing. |
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