Friction stir welded and deep drawn multi-material tailor welded blanks

Abstract

The ever increasing demand for more resource-efficient and safer vehicles in today's automotive industry makes lightweight construction techniques necessary. However, overcoming contradicting requirements arising from lightweight design and safety remains a challenging task. The extent to which lightweight measures can be applied in order to save fuel, heavily depends on the fact that rising safety requirements have to be met by increasing strength of parts. This contradicting demand for parts with high strength and low weight leads to the development of new production technologies. One example, regarding car body components, is the tailor welded blank (TWB) technology. In tailor welded blanks, materials and thicknesses are locally adapted to meet the needed strength and strain properties while keeping the weight as low as possible. While tailor welded blanks consisting of similar materials with different thicknesses are already used in vehicles, the use of TWBs with dissimilar materials, e.g. steel and aluminum, is still in development due to the problems in joining dissimilar materials. Especially when manufacturing parts made of TWBs through joining and subsequent deep drawing, the joint needs to have very good strength properties in order not to fail during forming. One way to overcome these joining difficulties is friction stir welding. In this paper, a methodology is presented to produce multi-material tailor welded blanks with varying thicknesses through friction stir welding (FSW) and deep drawing in a subsequent step. A newly developed FSW joint configuration is used to weld steel sheets in 1 mm thickness to 2 mm thick aluminum sheets. A welding parameter study is conducted to investigate the influence of the process parameters on the joint quality. Tensile and Nakajima tests show that the joint strength, obtained with optimal process parameters, exceeds the strength of the steel base material. Thus, failure occurs in the steel, whereas the joint remains intact. The friction stir welded blanks were furthermore deep drawn. Two different tool approaches were tested to compensate the different sheet thicknesses during the forming process. Using the more suitable approach, blanks were deep drawn with three different punch geometries to show the potential of friction stir welding for the manufacturing of multi-material tailor welded blanks.