Sven Hildering

Methodology for the Analysis of the Process Behaviour of Advanced High Strength Steels in Bending and Shearing Operations

In the automotive sector the application of advanced high strength steels (AHSS) for structural and safety relevant components plays an important role. Typical manufacturing processes concerning these parts are bending and cutting operations. However, the forming and cutting potential of these steel grades is different compared to conventional steels, as the process behaviour is changing. For an improved workpiece quality the fundamental knowledge of the damage and failure mechanisms is essential. This study presents a methodology for the analysis of AHSS in bending and out-of-plane shearing operations. Two micro alloyed high strength steels are investigated within this work. First results are presented concerning material characterisation by tensile tests, the material performance in air bending tests and the development of a modular punching tool. The study is closed by summarizing the damage behaviour along the process chain considering both bending and cutting. This shows the applicability of the presented methodology for analysing the process behaviour with respect to occurring failure.

Influence of Process Errors on the Tool Load in Microblanking of Thin Metal Foils With Silicon Punches

The trend toward miniaturization of metallic microparts results in the need of high-precision production methods. Major challenges are, for example, downsizing of tools and adequate positioning accuracy within blanking. Starting from a novel approach for tool miniaturization and its realization, the aim of this study is showing the assessment of tool sensitivity against process errors. Etched silicon punches were used for blanking copper foils, where outbreaks occurred at the cutting edge. Hence, tool stresses during blanking were analyzed by finite element (FE) method in dependency of defined positioning and process errors and evaluated concerning tool stresses and sheared edge quality.

Tool Load Sensitivity Against Multidimensional Process Influences in Microblanking of Thin Metal Foils With Silicon Punches

The continuous trend towards miniaturization of metallic micro parts of high quality at low costs results in the need of appropriate production methods. Mechanical manufacturing processes like forming and blanking meet these demands. One major challenge for the application of them are so called size effects. Especially the downsizing of the required manufacturing tools and adequate positioning causes higher effort with increasing miniaturization. One promising approach for downsizing of tools is the transfer of knowledge from microsystems technology. This study shows the process behavior of etched silicon punches in microblanking operations. For the application as tool material especially the brittle material behavior and sensitivity against tensile stresses have to be considered. These mechanical loads favor wear in form of cracks and breaks at the cutting edge of the punch and decrease tool life. In a special test rig these wear phenomena were observed in microblanking of copper foils. Although high positioning accuracy between tools and workpiece can be assured within this test rig, scatter of tool life is observable. Therefore, a finite element analysis of the tool load in the microblanking process with special respect to tensile stresses was performed. Within the 3D finite element model multidimensional positioning errors like tilting between punch and die were integrated. Their influence on the tool load in form of increasing tensile stresses is evaluated with respect to the type and magnitude of positioning error. Furthermore, the effects of small outbreaks at the cutting edge on the process behavior and tool load are analyzed.Copyright

Prediction of damage in small curvature bending processes of high strength steels using continuum damage mechanics model in 3D simulation

Sheet metal bending of modern lightweight materials like high-strength low-alloyed steels (HSLA) is one major challenge in metal forming, because conventional methods of predicting failure in numerical simulation, like the forming limit diagram (FLD), can generally not be applied to bending processes. Furthermore, the damage and failure behaviour of HSLA steels are changing as the fracture mechanisms are mainly depending on the microstructure, which is very fine-grained in HSLA steels composed with different alloying elements compared to established mild steels. Especially for high gradients of strain and stress over the sheet thickness, as they occur in small curvature bending processes, other damage models than the FLD have to be utilised. Within this paper a finite element (FE) 3D model of small curvature bending processes is created. The model includes continuum damage mechanics model in order to predict and study occurring failure by means of ductile coherence loss of the material and crack formation with respect to influencing process parameters. Damage parameters are determined by inverse numerical identification method. The FE-model is strain based validated considering the deformation field at the outer bending edge of the specimen by using an optical strain measurement system. The Lemaitre based damage model is calibrated against the experimental results within metallographic analysis adapting the identified damage parameters to the bending process und thus adjusting the crack occurrence in experiment and simulation. Using this model the bendability of common HSLA steel, used for structural components, is evaluated with respect to occurring damage and failure by numerical analysis.

Development of a Damage Prediction System for Bending and Cutting of High Strength Steels

Abstract. The application of modern high strength low alloyed steels (HSLA) and advanced high strength steels (AHSS) for structural and safety relevant components in the automotive industry offers the advantage of combining low specific weight with high material strength. Typical manufacturing processes for these steel grades are bending and cutting operations. The forming and cutting potential of these innovative steel grades is different to conventional steels as the process and the damage behaviour is changing. In bending operations cracks occur at the outer bending edge, whereas in cutting operations delamination can appear at the sheared edge. These damages, even though they are small, can initiate the component to fail. For a reliable use of such materials in industrial application a method for the process design is essentially needed. In particular, damages have to be predicted at an early stage. In industrial application damage is detected by a trial-and-error approach causing significant work and a high failure rate. A system for an offline assessment of the risk of failure is unknown so far. In the scope of this work, a method is presented to describe the damaging behaviour of both, bending and cutting operations, by theoretical metamodels. In order to generate a database experiments were carried out using different high strength steels. The main influence factors have been varied, such as the rolling direction, the punch-to-die clearance and the cutting contour in the cutting operation. The bending was investigated using an air-bending process varying the bending angle, the bending radius and the rolling direction. To calculate further sampling points a finite element model has been developed and validated against the experimental data. The damage criterion of Lemaître has been applied. The necessary parameters were determined by reverse identification by means of the major strain for the bending operation and by the punch force-punch stroke curve for the cutting operation. To build up a system for the prediction of the damage the gained data basis was approximated by mathematical functions. An error analysis was carried out showing good accordance. In doing so, a metamodel for the occurrence of damages could be established. The functions are implemented in a software tool which allows the user to determine the failure probability for a given parameter set.

Use of Monocrystalline Silicon as Tool Material for Highly Accurate Blanking of Thin Metal Foils

The trend towards miniaturisation of metallic mass production components combined with increased component functionality is still unbroken. Manufacturing these components by forming and blanking offers economical and ecological advantages combined with the needed accuracy. The complexity of producing tools with geometries below 50 μm by conventional manufacturing methods becomes disproportional higher. Expensive serial finishing operations are required to achieve an adequate surface roughness combined with accurate geometry details. A novel approach for producing such tools is the use of advanced etching technologies for monocrystalline silicon that are well‐established in the microsystems technology. High‐precision vertical geometries with a width down to 5 μm are possible. The present study shows a novel concept using this potential for the blanking of thin copper foils with monocrystallline silicon as a tool material. A self‐contained machine‐tool with compact outer dimensions was designed to avoid ten...