Sebastian Härtel

Numerical simulation of non-circular spinning: a rotationally non-symmetric spinning process

This paper describes the peculiarities encountered in the numerical modeling of non-circular spinning processes using motion-controlled roller tools and applying the Finite Element Method (FEM). This process is suitable for producing non-circular, hollow components in small to medium-sized production lots. Numerical simulation can be used to optimize the process. Therefore, it is necessary to make a realistic sheet thinning and wrinkling calculation by using the FEM. This can be achieved through the definition of the real kinematics, a suitable flow curve and an optimal sheet meshing strategy using solid elements. An optimal sheet meshing strategy is particularly necessary in order to realistically calculate the process within an acceptable computing time. Reference experiments with the rotationally non-symmetric mandrel types, the “Tripode” and “Pagoda”, were carried out to compare simulations and experiments. A comparison of the reference experiments with the “Tripode” mandrel demonstrated that it is possible to simulate non-circular spinning with a deviation of less than 5% with respect to minimum sheet thickness. It is also possible to predict wrinkling in critical, non-circular spinning processes. This has been confirmed by comparing the “Pagoda” reference experiment with the FEM simulation.

Novel Approach for the Determination of the Taylor-Quinney Coefficient

Within this study, a new method for the determination of the Taylor-Quinney coefficient is presented. The coefficient was identified by measuring the force-displacement-behavior as well as the temperature change resulting from an adiabatic compression test. In order to deduce the global temperature increasing of the specimen from the local measured temperature a suitable specimen geometry was designed with the use of numerical simulation. The resulting specimen allows a friction-free compression and therefore precludes a temperature increase through friction. Finally, the Taylor-Quinney coefficient of C35 steel (1.0501) was experimentally determined in the initial state as well as after a heat treatment.

Development of a Quenching-Partitioning Process Chain for Forging Components

The aim is to realize a Q&P (Quenching and Partitioning) process for a hot forged component made of low-alloyed advanced high-strength steel (AHSS) 42MnSiCr. One advantage of this steel is the low alloy concept which is cost-effective. After forging, the component is cooled down to room temperature with a subsequent heat treatment to achieve the characteristic microstructure with martensite and retained austenite. The material is annealed and then quenched to just above the martensite finish temperature (MF-temperature). Hence, in the martensitic matrix about 10 to 15% retained austenite is included. Finally, the Q&Ped material is artificially aged at 250 °C to support the diffusion process of carbon from the over-saturated martensite into the austenite. Thereby, mechanical properties of 2000 MPa for tensile strength with fracture strains of 10% can be achieved. This paper provides details of the process and material behavior for a reduction of the process chain. The goal is to develop a technology for the quenching and partitioning treatment of forged components by using the thermal energy from forging. Ideally, the quenching step should be performed in the forming dies just above the MF-temperature with additional holding on the temperature level. The majority of forged parts have different cross sections. Therefore, the cooling conditions are inhomogeneous in each cross section of the components. This cooling behavior was analyzed in laboratory tests with a forged part. Furthermore, the heat transfer coefficients were determined for different cooling media (water, air). The cooling technology was experimentally and numerically simulated in a first step for the conventional process chain (forging, cooling to room temperature, austenitisation, quenching, artificial ageing) and correlated with the microstructural evolution in combination with the component’s mechanical properties.

Non-circular Spinning

The increasing trend of the individualization of products demands an advancing widening of the range of varieties of the goods on offer while at the same time lot sizes decrease. To handle the challenges this implies manufacturing needs to use technologies which allow for a flexible and effective production especially in the lower lot size segment. One process which fulfils these requirements is spinning. Low tool costs and short set-up times enable a flexible and cheaper production in the middle and lower lot size segment compared to conventional forming processes (e.g., deep-drawing). One way to further increase the economic relevance and flexibility of spinning is expanding the range of parts to be produced by non-circular spinning. In the following the process development of the socalled “non-circular spinning” with motion controlled rollers will be described.