The importance of hybrid structural components in the automotive manufacturing industry is steadily growing. Fiber-reinforced plastics (FRP) are now used not only in motorsports but also in passenger vehicles. This trend is driven by the search for ever lighter components with high strength. This involves the creation of hybrid structural components by wrapping steel sheets with FRP. The goal is to maintain the excellent mechanical properties of steel, while further increasing tensile strength and reducing weight through the use of FRP. This enhancement in load-bearing capacity is of particular importance for crash-critical components, ensuring the safety of vehicle occupants.
However, the feasibility of producing such components on a mass-production basis still requires further consideration. The Institute of Forming Technology and Machines (IFUM) at Leibniz University Hannover is investigating the potential for streamlining the number and duration of process steps as part of the research project “Lightweight Hybrid Forming”. The objective is to reduce the production cycle time for hybrid components without compromising their quality.
From a technological standpoint, the greatest difficulty in manufacturing hybrid components today is creating a composite without the use of adhesives or adhesion promoters. This project will address this challenge by applying various surface treatments to galvanized steel sheets to achieve a strong, uniform joint to the FRP without adding time to the manufacturing process.
Electric Vehicles: Reducing Weight While Increasing Crash Safety
The objective of the research project is to gain insights into how the ratio of mechanical properties to component weight can be improved. This is particularly significant in the context of electromobility. With the elimination of the engine block, new crash structures in the front area of vehicles are necessary to ensure occupant safety. At the same time, lightweight construction is becoming increasingly important for electric vehicles, which typically have a relatively high weight.
All findings from this research project will be published publicly. The insights into the mass production feasibility of hybrid components will act as a guide for both car manufacturers and their suppliers, helping them to overcome these challenges without additional research expenditure. Ultimately, the long-term goal is to replace monolithic structural components – such as the bumper on the VW ID3 – with hybrid components, thereby increasing the vehicle’s range through weight reduction.
Combining Without Adhesives: Enhancing Joinability Through Surface Pre-Treatment
Within the scope of the research project, the use of adhesion promoters or adhesives in the production of hybrid components is deliberately avoided. Instead, alternative methods must be employed to achieve a connection between the sheet metal and the FRP. A promising approach is to roughen the steel surface so that the molten plastic can flow into it and solidify upon cooling. The formation of undercuts creates microscopically small interlocking connections that positively affect joint strength. However, it should be noted that surface pre-treatment methods for roughening are generally very time-consuming.
In order to reduce the cycle time in the production of a hybrid component, it is necessary to employ technologies that can achieve high roughness without additional time in the forming and combining sequence. Methods such as sandblasting or laser texturing are not viable options due to their extended process times. An interesting alternative that was examined in detail was the redressing of galvanized steel sheets. To this end, hybrid coupon samples were produced at laboratory scale from redressed steel with various degrees of fineness and organ sheet metal. Their joint suitability was subsequently tested. Tensile shear tests were conducted, which showed that the tensile shear strengths – correlated with joint capability – were lower for redressed sheets than for untreated samples. In summary, redressing did not yield the desired results (see Figure 2).
Promising Pre-Treatment: Resistance Heating Increases Joint Strength
The IFUM then explored a new method of surface pre-treatment using resistance heating, also known as conductive heating. The objective is to induce surface tension in the coating by heating it to just below the melting point (419.5°C for zinc). This process involves clamping a steel sheet between two copper electrodes, as shown in Figure 1, and applying an electric current.
In this manner, sheets can be heated to as high as 1000°C within one second. However, such high temperatures are counterproductive for the pre-treatment of galvanized steel sheets, as they would burn off the zinc coating. Preliminary investigations have shown that target temperatures of 385°C and 415°C are most effective in terms of tensile shear strength, with heating durations of 15 and 60 seconds, respectively. The heating parameters were systematically varied, the roughness parameters were measured, and tensile shear samples were produced. The tensile shear strengths of specimens made from conductively heated steel sheet and organ sheet metal are shown in Figure 2.
The resulting tensile shear strengths were up to 10 MPa for sheets treated at 415 °C for 60 seconds. This represents an approximate fivefold increase in tensile shear strength compared to redressed sheets. However, the surface of the sheets after conductive heating is inhomogeneous. A roughness gradient can be seen, increasing towards the centre of the sheet and thereby forming a potential combining zone. It should be noted that the values indicated in the diagram are averages over the entire surface of the heated sheet. Since the sheet is reinforced with FRP only in its central area, the effective joint strength in that zone is higher, depending on the size of the organ sheet metal cutout. Precise values will be determined using hybrid flat samples produced under process-relevant conditions.
Practical Test: Researchers Develop a Demonstrator Component and Forming Tool
As part of the research project, the initial step was to design a demonstrator component modelled on crash-relevant structural parts used in automobiles. It was also important that standardized post-test procedures could be performed on the manufactured demonstrator component in order to validate the findings from flat sample tests. Consequently, the demonstrator component shown in Figure 3 was developed as a prototype. This component is a hat profile that functions similarly to a vehicle bumper.
In order to produce the demonstrator component using the combined forming–joining process, it is necessary to design, construct, and manufacture an appropriate forming tool. A prototype of this tool is shown in Figure 4.
Currently, IFUM is examining the key mechanical properties of the composite in process-relevant flat samples to achieve the most accurate material characterization possible. Concurrently, strip drawing tests are being conducted to understand the friction conditions between the sheet metal and the tool. Subsequently, the design data, material properties and friction behavior are integrated into a simulation tool to model the forming process. After each simulation, the tool provides feedback on the forming process, thus eliminating the need to build the tool. Subsequent iterations of the press parameters and the geometry of the active elements (die, blank holder, and punch) can then be made until the desired component is produced flawlessly in simulation and the tool can be built. This approach leads to significant savings in cost, material, and time, as well as reducing the need for rework on an already hardened tool.
