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Modern lightweight design is often based on multi-material components. For example, structured sheet metals can be equipped with a die-cast light metal insert for structural support. To interlock sheet and insert, structures with undercuts are formed into the sheet in a multi-pass rolling process. In a first pass, structured rolls are used to create a structure of channels and ribs. Undercuts are formed in a consecutive pass by flat rolling those ribs. During die-casting, the melt flows into the channels and forms an interlocking connection once solidified. The joint strength is decisively determined by the undercut geometry. The undercuts formed by material displacement increase with the height reduction in the flat rolling pass. However, after a certain amount of material displacement, the channel side edge starts to fold over the channel bottom and forms an inner notch. Those inner notches can be prone to crack initiation and subsequently lead to component failure. To analyze the surface structure regarding channel depth, undercuts and inner notches as well as finally maximize the joint strength, a combined experimental and numerical study was laid out. The surface of 2.0 mm DC04 was structured with up to 0.5 mm deep channels and then flattened with different height reductions. The results from the 2D explicit FE-model suggest that a process optimum for those surface structures with high undercut width but without inner notch exists at 14% height reduction. However, in the experiments inner notches started to form at approx. 8% height reduction with approx. 20 µm wide undercuts for the given experimental setup. In contrast, maximum undercuts of approx. 50 µm form at 26% height reduction, but also cause inner notches with approx. 60 µm length.

Optimization of the Surface Geometry in Structured Cold Rolling for Interlocking of Formed and Die-Cast Metal Components