Resistance Spot Welding Process Of Zn-Al-Mg Coated DC51D Steel And Nugget Formation Mechanism

China has entered a stage of transition from a big steel country to a strong steel country.The proposal of carbon neutrality has made China's steel industry face huge opportunities and challenges.It is especially important to make steel better in the fields of automobiles and home appliances,and to solve the problem of anti-corrosion.Importantly,hot-dip galvanizing has a significant effect on improving the corrosion resistance of steel.Zn-Al-Mg coated steel plate,the coating is different from the traditional alloy coating,it has better corrosion resistance,wear resistance and welding performance.This paper takes 1mm thick DC51D+ZM zinc-aluminum-magnesium coated steel plate as the research object,and passes orthogonal test,regularity test,process window test,as well as electrode wear,nugget formation analysis,macro and microstructure observation,mechanical properties and residual stress detection,etc.Process and mechanism research,summed up the nugget formation and coating changes.The main results of the study include:1)Under the orthogonal test,the best parameter welding current is 10 k A,welding time is 16 cycles,and electrode pressure is 2.2 k N.In the regular test,the diameter of the nugget under the welding current,welding time and electrode pressure,the width of the weld joint,the reduction rate and the tensile shear force all meet the requirements,and the maximum tensile shear force is 8.61 k N.The solder joint microstructure of resistance spot welding is composed of FZ zone,HAZ zone and BM zone.The microhardness value increases from the BM area to the FZ in turn,showing a "W"-shaped distribution,and the microhardness value distribution in different intervals in the thickness direction is all low in the surface and high in the center.2)In the process window test,the tensile shear force showed a trend of first increasing and then decreasing.The maximum value appeared at point G,when the tensile shear force was 8.62 k N.The current range is 6.6?7.4 k A and 10.4?10.6 k A.The maximum welding current range is 3.8 k A,and the minimum is 3.2 k A.Point G is the best welding parameter for the welding window.In the electrode wear test,after 500 times of resistance spot welding,the electrode cap was slightly worn.The appearance has changed,but the nucleation diameter of all solder joints is larger than the specified 5 mm point,and the reduction rate meets the requirement of ? 30%,indicating that the electrode wear condition under this process window meets the requirements of SMTC5111 003-2014.3)When the welding current and welding pressure are the same,as the welding time increases,the diameter of the nugget also increases.When the welding time is 6-8 cycles,the appearance and metallographic observation of the solder joint indicate that the nugget is basically formed;In the process of nucleus formation,Zn is mainly concentrated in the two unwelded parts on the left and right sides of the solder joint through diffusion,and the diffusion is most obvious at 20 cycles.For the weld nugget center,when the welding time is 2 cycles,the element density of Zn,Al,and Mg at the center of the weld nugget is significantly higher.As the welding time increases,the density of the Zn element in the center of the weld nugget decreases.When the welding time increases to 8 cycles,the solder joint is basically formed,and the zinc element is evenly distributed between the two plates.4)Al in the base material zinc-aluminum-magnesium coating reacts with the base Fe to form a Fe-Al phase suppression layer.The Al and Zn elements in the heat-affected zone are distributed between the upper and lower plates.The content of Zn is lower than that of Al,combined with XRD and Analysis of the diffraction spot result of TEM selected area shows that Fe Al phase is formed.Fe Al and Fe3 Al phases exist in the weld nugget area,the coating completely disappears,and the two plates are connected by the base iron.The welding time is 2 cycles,the interface fracture occurs,and the tensile shear force is 1.99 k N.The drawing fracture occurs when the welding time is 4-20 cycles,and the average tensile shear force is 8.24 k N at this time.5)In the horizontal microhardness value,when the welding time is 1-2 cycles,the hardness value is lower,and the hardness value of 4-10 cycles is relatively stable,about310 HV.In the vertical microhardness,the hardness value distribution is close to the horizontal hardness value at 1-2 cycles,showing the same trend.Perform residual stress detection for different welding time groups.When the welding time is short,the residual stress is smaller.As the welding time increases,when the welding time is 12 cycles,the residual stress at this time reaches the maximum of 252 Mpa.