자동차용 고강도 강판의 저항 점용접 시 다단통전이 너깃 형성에 미치는 영향

Abstract

With the fast advanced auto industry, prompt and reliable welding processes are required. In more than 90 percent, resistance spot welding process is applied to car body manufacturers. According to the international environment regulations, in the automobile industry, improvement of fuel efficiency became a hot topic and the introduction of high strength steel is accelerated. Based on 2014 developmet of car body, the rate of high strength steel is about 53 percent of total panels of car body. In the high strength steel, increasing alloying elements makes interfacial resistance high and it makes differences of the resistance in between multi layers(more than 3 layers). Because of this reason, imperfect nuggets are builded and bond strengths are lowered. For this reason, there are needs for derivation of optimum welding process different from existing methods. In this paper, observe and examine the short-intensive pre-pulse welding mechanism through the providing existing car body parts.

The method of observing the cross section of the nugget is that cut the center of the nugget, treat the acid and analyze with the optical microscope. The result of applying short-intensive pre-pulse shows that the nugget size of 3 layers welding sheets is about 18% bigger than before and 4 layers welding sheets is about 57% larger than before in contact surface with high-strength steel sheet. Targeting the QTR INR_LH part of the car body in production, after sorting the all spot welding points of 3 and 4 layers welding sheets, research the growth of the nugget size which are applied with the short-intensive pre-pulse 120~140% of the main pulse. As the result, Nugget size looks like bigger than before and 4 layer welding sheets are more efficent than 3 layer welding sheets. In many cases, the car body is composed of thin iron plate and there are cooling effects and radiation effects when spot welding is going hence there are difficulty building the nugget size. But in these cases, applying of the short-intensive pre-pulse make sure that the nugget size growed.