| Resistance spot welding(RSW)is a common welding process for automobile enterprises,which is easy to be integrated and controlled with high efficiency.Low carbon steel still has a long-term accumulated cost advantage in body manufacturing.Because the nucleation process of spot welding is in a closed state and takes place in millisecond,the spot welding process can be well studied by means of finite element analysis.In this paper,through the combination of experimental research and finite element technology,taking low carbon steel sheet as the research object,the nucleation process of resistance spot welding and the influence of changing process factors on the microstructure and properties of joints are explored,which provides some experience for the formulation of low carbon steel spot welding process parameters.In the aspect of experiment,through the analysis of microstructure,mechanical properties and fracture morphology of low carbon steel spot welding joint under different parameters,it is found that the microstructure of low carbon steel nugget zone is mainly lath martensite,and the microstructure of heat affected zone is mainly lath martensite and ferrite;the larger the nugget size is,the greater the shear strength is with increasing welding current and time.When the welding heat input is too large,it is easy to produce spatter and reduce the hardness and shear strength.Using response surface method,the second-order model of maximum shear stress and process parameters of spot welding joint is established.It is found that the influence of welding current and welding time is very significant,but the electrode pressure is not significant.The results show that the maximum shear strength is 7.416 k N when the welding current is 9.42 k A,the welding time is 164.07 ms and the electrode pressure is 2.32 k N.In the aspect of finite element,the contact model of equal thickness low carbon steel plate and conical electrode is established,and the stress distribution in the preloading stage is analyzed.It is found that the change of electrode pressure has no obvious effect on the stress distribution curve and peak position of each contact surface.The thermoelectric model of spot welding of low carbon steel sheet is established.In addition to confirming some conclusions in the experiment,the temperature field and electric field change during spot welding are observed.It is found that the peak position of initial current density is not sensitive to the change of welding current.The current distribution affects the heat flux density,thus affecting the heat generation and temperature field,and the temperature gradient in turn affects the current distribution.With the increase of welding current and welding time,the maximum temperature of nugget zone and the simulated nugget diameter increase. |
PDF Full Text Request