Internal high-pressure forming of tubes and profiles

Integrating joining processes into the forming process not only shortens the process chain but also offers the opportunity to reduce manufacturing tolerances, as the components are joined in a tool that defines the position of the semi-finished parts relative to each other. The industry has established processes such as the hydroforming and joining of lightweight camshafts, which has also been studied at the Fraunhofer IWU. In this process, prefabricated cams are joined onto a tube that is pressurized internally. The In-Mold Assembly (IMA) process, well-known from the production of plastic-metal hybrid components, can also be combined with fluid-based forming. Examples include IHPF-injection molding and the combination of deep drawing, injection molding, and forming with molten material. In hydro-joining or process combinations of forming and injection molding, the connection is usually based on a global interlocking fit, meaning one component encloses the other. If this is not possible, hydroclinching and hydro-punch riveting are used as localized joining processes that can be integrated into the IHPF process.

Process advantages:

• Shortened process chains
• Ensuring low manufacturing tolerances
• Combination of primary forming, reshaping, and joining in one tool/process possible

The trend of using ultra-high-strength materials in innovative vehicle body concepts remains strong. By using ultra-high-strength body components, up to 20 kilograms of mass can be saved in a mid-sized vehicle. This not only reduces the amount of steel required for vehicle production but also decreases fuel consumption and CO2 emissions during the usage phase.

The Process: Press Hardening

A successful example of producing ultra-high-strength body components is the press hardening process for sheet metal. This process, developed at the Fraunhofer IWU, has been adapted for closed profiles using internal high-pressure forming (IHPF). Known as IHPF-press hardening or Hot Metal Gas Forming with Press Hardening (HMGF-PH), this process combines shaping and heat treatment in a single step called press hardening or form hardening. The process involves heating closed profiles above the austenitization temperature, placing them into a cooled forming tool, and then quenching them. This integrated heat treatment produces a martensitic structure, resulting in press-hardened components with extremely high tensile strengths of up to 1800 MPa. Such components can be used as crash-relevant structural parts, such as A- and B-pillar reinforcements, roof frames, bumpers, sills, and in the drivetrain, for example, as camshafts.

Process advantages:

• Production of ultra-high-strength tube-based components with complex geometries
• Combination of structural and material lightweight design

For the production of tube-based hybrid components, a process combination of internal high-pressure forming (IHPF) and injection molding has been used for some time. The research focus at Fraunhofer IWU lies in further developing the process and identifying and overcoming technical limitations.

An initial development involved replacing the liquid working medium with a gaseous medium, in this case nitrogen. Eliminating the liquid medium improves the process's robustness, as plastic is sensitive to moisture. In industry, the bonding between the metal tube and the plastic component is primarily achieved through chemical adhesion promoters. The goal of various research efforts at Fraunhofer IWU is to replace these adhesion promoters by creating a specific surface structure on the tubes and achieving a mechanical interlock between the IHPF and injection-molded components. Additionally, the process has been further developed with respect to the use of thermoplastic fiber-reinforced plastic tubes. In this context, the process management of IHPF and injection molding is particularly important.

In parallel, with the development of the process combination of deep drawing and injection molding, the combination of primary forming, reshaping, and joining in a single process was transferred to another innovative method.

Process advantages:

• Production of metal-plastic hybrid components with complex geometries
• Combination of structural and material lightweight design
• Use of metal and thermoplastic plastic tubes possible

Light metals such as titanium, aluminum, and magnesium offer significant potential for mobility and automotive applications due to their excellent weight-specific mechanical properties. However, there are relatively limited options for economically producing formed components from these materials. Due to their limited formability at room temperature, complex components often require process chains with multiple forming steps and intermediary heat treatments. An alternative is superplastic forming, but this is also costly due to long cycle times, high energy consumption, and the need for protective gas.

One way to improve the formability of many materials is to increase the forming temperature. We are developing temperature-supported process routes for tube and sheet components that enhance formability depending on component geometry or specifically achieve material states that lead to optimized component properties. The developed process technologies enable industry to bring high-performance materials into widespread use. Societal relevance spans all mobility sectors: automotive, aerospace, and micromobility.

The development of temperature-supported forming processes can be implemented across the entire process chain. Based on process-specific property determination, numerical modeling of complex thermomechanical forming processes can achieve very high predictive accuracy, significantly shortening process chain development. Both semi- and fully automated test stands can be combined with various forming presses and other equipment. For component heating, ovens, induction systems, and various contact heating tools and equipment are used, enabling heating processes within the press cycle. Tool-integrated conductive heating processes have also been successfully implemented. Tool temperature control can be achieved using electrical heating elements, oil, and water temperature control devices. Prototype and pre-series production are accompanied by extensive analysis methods to optically, mechanically, or microstructurally characterize components both during and after the process.

Process advantages:

• Improved formability
• Shortened process chains
• Forming of materials with limited room-temperature formability (light metals, ferritic stainless steels, thermoplastic materials, etc.)