Effect of Combined Use of Brazing or Soldering around the Nugget on Tensile Shear Strength of Resistance Spot Welded Lap Joint

Abstract

Resistance spot welding is used in the assembly of automobile bodies. The use of high tensile strength steel plates in automobile bodies is expanding, and there is a need to develop a welding process that improves the joint strength per resistance spot weld point. To improve the joint strength per resistance spot weld, resistance spot welding can be combined with other joining methods, such as the WeldBond method. As an additional joining method around the nugget, other than using adhesives as in the WeldBond method, brazing can be used to obtain a strong joint without melting the base metal. In this paper, it was proposed that the joint strength can be enhanced by using resistance spot welding and brazing or soldering based on the rule of mixture. And, based on this concept, effect of additional joint around a nugget, such as brazing or soldering, on rotational deformation of joint part and tensile shear strength of resistance spot welded lap joint was examined.First, three types of lap joint specimens that were using only resistance spot welding, using only brazing and using both joining methods (called WeldBraze) were prepared. A high-strength steel sheet HT590 with a thickness of 1.0 mm was used. For the brazing filler metal, lead-free solder (composition: Sn-0.3Ag-0.7Cu) was used. Next, tensile shear tests were conducted using all the test specimens. In general, rotational deformation of the joint occurs due to the offset of the load axis for the tensile shear test of the lap joint. It is necessary to investigate the effect of the rotational deformation on joint strength. Therefore, a tensile shear test was performed using a test specimen in which the welded joint part was sandwiched between two steel blocks to suppress rotational deformation.As a result, it was confirmed that the new joining method using both brazing and resistance spot welding increased the joint strength and the amount of displacement up to fracture as compared with the case of brazing or resistance spot welding only.So, it was clarified that the factor of the strengthening of the joint was the rule of mixture, and the increase of displacement up to the fracture was affected by the suppression of rotational deformation.In conclusion, it was found that the joining method using both brazing and resistance spot welding was effective in improving the joint strength.