Study On Nugget Shape And Mechanical Properties Of Resistance Spot Welding Of Stainless Steel Sheets With Unequal Thickness

Nowadays,resistance spot welding is an important material connection method in industrial production.In actual production,it is often necessary to spot weld sheets with unequal thickness together.Compared with equal thickness spot welding,the quality problem of unequal thickness spot welding is more prominent.The phenomenon of nugget shifting often occurs in resistance spot welding of unequal thickness sheets,which seriously affects the welding quality.In this paper,the variation of nugget shape and mechanical properties with welding process parameters and the combination of sheet thickness is studied,and the influence of nugget shape parameters on mechanical properties is also studied.In addition,an electrode structure is designed to solve the problem of nugget shifting.In this paper,SUS301 stainless steel sheet with unequal thickness is used as the research object.Five different combinations of sheet thickness are designed.Different welding current,electrode pressure and welding time are used for resistance spot welding.Four kinds of nugget shape parameters,nugget diameter,nugget thickness,nugget absolute offset and nugget relative offset,are measured and calculated by optical microscope.And the tensile shear force at fracture of the spot welded specimen is measured by tensile test.The results show that when the welding current is small,its increase will increase the nugget diameter and thickness,while when the welding current is large,its increase will decrease the nugget diameter and thickness.The increase of the welding current can also reduce the nugget shifting phenomenon to a certain extent.The influence of the second welding current on nugget diameter and offset is greater than that of the first welding current,while the influence of the two welding currents on nugget thickness is similar.The increase of welding current is also beneficial to increase the tensile shear force at fracture of joints,but the change rate decreases with the increase of welding current.The nugget diameter and thickness increase first and then decrease with the increase of the electrode pressure,but the nugget thickness is less affected by the electrode pressure than the nugget diameter.In contrast to the welding current,the increase of electrode pressure will increase the phenomenon of nugget shifting.With the increase of electrode pressure,the tensile shear force of the joint increases first and then decreases.The influence of welding time on nugget size is similar to that of welding current,but the degree of its influence is relatively low.The increase of welding time can not only increase the heat evolution,but also increase the inequality of the heat dissipation of sheets with different thickness,which makes the nugget offset decrease first and then increase.In addition,the increase of sheet thickness gap will also aggravate the phenomenon of nugget shifting.The analysis shows that there is a certain relationship between the nugget shape parameters and the mechanical properties of the joint.The increase of nugget diameter,nugget thickness and the decrease of nugget offset are beneficial to increase the tensile shear force at fracture.The influence of nugget diameter on tensile shear force is the largest,while the influence of nugget thickness is the second.The decrease of nugget offset can also improve the tensile shear force,but its influence is less than that of nugget diameter and nugget thickness.In this paper,a ring section copper electrode is designed to solve the problem of nugget shifting in spot welding of unequal thickness sheets.The electrode is made of copper alloy at the periphery and ceramic material at the inner core.The heat evolution is increased by increasing the current density at the thinner sheet side,and the good thermal insulation performance of the ceramic can reduce the heat dissipation at the same time,which makes the center of the temperature field incline to the thinner sheet side,thus reducing the phenomenon of nugget shifting.In this paper,the finite element numerical simulation of the temperature field distribution of the spot welding joint of unequal thickness sheets proves the influence of the ring section copper electrode on the phenomenon of nugget shifting.