Technology Development for Forming and Joining of Lightweight Components

Implementing lightweight design solutions has been gaining importance for many years, but still the trend of rising vehicle weight is not reversed, so that efforts must be intensified. Exploiting the full weight reduction potential requires optimizing material and design. Thus, complex geometries have to be formed from materials, which are difficult to process and innovative joining technologies for producing high quality multi-material joints without auxiliary materials are needed. Extending formability is possible via temperature- and strain rate-dependent effects. Depending on the material, high or low values can be beneficial. Typical automotive steel or aluminium alloys can be formed by conventional methods (e.g. deep drawing) or by hydroforming at room temperature, but formability increases at elevated temperature. Magnesium however, is formed at elevated temperature only. For high strength steel (e.g. 22MnB5) press hardening is used: tube and sheet metal parts are formed at temperatures up to 780°C and cooled down rapidly to reach maximum strength. Process variants using contact heating accelerate the heating process and enable tailored heating to adjust finally the local strength of the finished part. For 7xxx aluminium alloys, sophisticated temperature dependent process routes help combining formability and final strength. The target microstructure is reached via heating, rapid cooling, and aging the component. The forming step can be integrated in the cooling by using cold dies or it can be performed in a short time after quenching in a fluid. In the first case, lubrication and tool wear are challenging, while in the latter case distortion can occur. To avoid these issues, Fraunhofer IWU uses a flat quenching die before cold forming the part. For 5xxx and 7xxx aluminium alloys, also very low temperatures improve formability. So-called cryogenic forming exploits this effect. Benefit from high strain rate is exploited e.g. in electromagnetic forming (EMF), a process using the energy density of pulsed magnetic fields for contact-free application of forces to electrically highly conductive tube and sheet metal parts. The advantage of this technology is the forming of sharp edges. Size limitations can be overcome by sequential forming or by combining EMF and conventional forming. In such process combinations the overall workpiece geometry is realized e.g. by deep drawing and critical details are calibrated locally by EMF. This technology can also be used for joining: crimping allows joining metal to non-metal components, while magnetic pulse welding provides high-quality metallically bonded joints without significant heating of the parts and thus avoiding temperature induced problems e.g. intermetallic phases. Benefits from low strain rates (and high temperatures) are exploited in superplastic forming, a technology which is especially interesting for titanium alloys. The Fraunhofer IWU assists companies in the development of these technologies considering process, equipment and product properties to come to the optimum solution for the client-specific manufacturing task.