Spot weld bonding − process behavior of three-sheet steel stack‑ups and analysis strategies with online measuring methods

Abstract

Due to the increased demands for reducing CO2 emissions, improving fuel efficiency of modern vehicles has been continuously monitored. The body of a typical compact car design has a weight share of approx. 40%. In addition to increasing torsional stiffness and crash safety of the body, the aim is also to reduce the overall weight at the same time. In order to achieve these individual requirements, the use of three-sheet steel stack-ups with adhesive applications for car body construction is one of the current strategies used in automobile manufacturing. Adhesive applications lead to a change in process behavior of resistance spot welding. The effective weldability lobe is reduced and an adjusted preheat current is necessary to reconstitute the weldability of a component. Depending on squeeze time and electrode force the adhesive will be displaced. For an asymmetric sheet stack-up, the electrical resistance for every faying surface is highly differentiated. During welding, a specific characteristic of the electrical resistance is created for each individual material combination. These characteristics can be analyzed by using an online measurement device. In this manuscript, different sheet stack-ups are examined with regard to their weldability lobes and their process behavior. The individual three-sheet steel stack-ups used are made of low carbon steel (DX51), HSLA-steel (HX340) and UHS-steel (22MnB5). The corresponding characteristics of electrical resistance will be recorded by using an online measurement device. In addition, the process of adhesive displacement during the squeeze time and the initial welding current are discussed on the basis of the electrical energy generated in the component to be welded. The obtained results contribute to a direct verification of the welding process and an automatic detection of possible imperfect welds.