Papers by Author: Matthias Weiss

Abstract: Bipolar plates are key components in fuel cells, and their performance strongly depends on the geometry of the microchannels used to distribute reactant gases. Producing channels with sufficient depth in thin metal sheets remains challenging, particularly when cost-effective manufacturing routes are required. This work investigates a multi-stage roller embossing process for forming bipolar plate channels using additively manufactured polymer tools. By dividing the total deformation into multiple forming stages, the process reduces tool deflection that typically limits channel depth in single-pass embossing. Experiments conducted on 0.1 mm stainless steel foil show that the multi-stage approach increases the achievable average channel depth from approximately 0.25 mm in a single pass to approximately 0.34 mm, resulting in a maximum aspect ratio (channel depth to width) of 0.314. These results indicate that combining multi-stage forming with 3D-printed tooling provides a practical route for flexible and low-cost fabrication of metallic bipolar plates, especially for low volume production.

Abstract: To have fuel efficient vehicles with a lightweight structure, the use of High Strength Steels (HSS) and Advanced High Strength Steels (AHSS) in the body of automobiles is increasing. Roll forming is used widely to form AHSS materials. Roll forming is a continuous process in which a flat strip is shaped to the desired profile by passing through numerous sets of rolls. Formability and springback are two major concerns in the roll forming of AHSS materials. Previous studies have shown that the elastic modulus (Young’s modulus) of AHSS materials can change when the material undergoes plastic deformation and the main goal of this study is to numerically investigate the effect of a change in elastic modulus during forming on springback in roll forming. Experimental loading-unloading tests have been performed to obtain the material properties of TRIP 700 steel and incorporate those in the material model used in the numerical simulation of the roll forming process. The finite element simulations were carried out using MSC-Marc and two different element types, a shell element and a solid-shell element, were investigated. The results show that the elastic modulus diminution due to plastic strain increases the springback angle by about 60% in the simple V-section roll forming analyzed in this study.

Abstract: V-sections were roll formed from two grades of steel, and the strain on the top and bottom of the strip near the edge was measured using electrical resistance strain gauges. The channels were bent to a radius of 2 and 15 mm along the centerline. The steel strips were of mild and dual phase steel of yield strength 367 MPa and 597 MPa respectively. The longitudinal bow was measured using a 3-dimensional scanning system. The strain measurements were analysed to determine bending and mid-surface strains at the edge during forming. The peak longitudinal edge strain increased with material yield strength for both profile radii. For the 15 mm radius, the bow was larger in the dual phase steel than in the mild steel. For the 2 mm profile radius, the bow was smaller compared with the 15 mm profile radius and it was similar for both steels. It was observed that the difference between the peak longitudinal edge strain and yield strength to Youngs modulus ratio of the material is an important factor in determining longitudinal bow.

Abstract: Sheets of precipitate hardenable 2024 aluminium have been processed by rolling at liquid nitrogen temperature in order to refine the microstructure. A number of different aging/heat treating procedures have been utilised that have resulted in significantly different mechanical properties. The cryo-rolled material was heat treated at 150 °C for varying times and the resulting mechanical properties evaluated as a function of this holding time. The resulting properties were found to be strongly influenced by precipitates that formed either during the aging step, rolling process or the subsequent heat treatment. The formability of the cryo-rolled and heat treated material has been investigated using a limiting dome height test (Erichsen cupping test).

Abstract: Bending with unloading and reverse bending are the dominant material deformations in roll forming and hence property data derived from bend tests could be more relevant than tensile test data for numerical simulation of the roll forming process. Recent investigations have shown that residual stresses affect the material behaviour close to the yield in a bending test. So, Residual stress introduced during prior steel processing may affect the roll forming process and therefore needs to be included in roll forming simulations to achieve improved model accuracy. Measuring the residual stress profile experimentally is expensive, difficult, time consuming and has limited accuracy. Analytical models are available that allow the determination of residual stress. However, for this detailed information about the pre-processing conditions is required; this information is generally not available for roll forming materials. The main goal of this study is to develop an inverse routine that generates a residual stress profile through the thickness of the material based on pure bend test data.

Abstract: Bending in a V-die has been used to indicate the outcome of bending in cold roll forming, although little direct correlation has been performed. In this work direct comparison of the springback in both processes was performed using six samples of automotive steels in a conventional roll forming line where the transverse springback is measured. A bend of similar radius was formed in a V-die and the springback determined. In general, the springback in V-die forming was greater than in roll forming, in some cases by a factor of 2. The theoretical springback angle was determined for all steels using a simple and approximate analytical equation and compared to the experimental roll forming and bending results. While for the roll forming process good agreement was achieved the theoretical values significantly underestimated springback in the V-bending process.

Abstract: Analytical modelling of deep drawing process is of value in preliminary process design to illustrate the influence of major variables including friction and strain hardening on punch loads, cup dimensions and process limits. In this study, analytical models including theoretical solution and a series of finite element models are developed to account for the influences of process parameters including friction coefficient, tooling geometry and material properties on deep drawing of metal cups. The accuracy of both the theoretical and finite element solutions is satisfactory compared with those from experimental work.