Ralf Kolleck

Investigation of the Phase Formation of AlSi‐Coatings for Hot Stamping of Boron Alloyed Steel

Hot stamping of boron alloyed steel is gaining more and more importance for the production of high strength automotive body parts. Within hot stamping of quenchenable steels the blank is heated up to austenitization temperature, transferred to the tool, formed rapidly and quenched in the cooled tool. To avoid scale formation during the heating process of the blank, the sheet metal can be coated with an aluminium‐silicum alloy. The meltimg temperature of this coating is below the austenitization temperature of the base material. This means, that a diffusion process between base material and coating has to take place during heating, leading to a higher melting temperature of the coating.In conventional heating devices, like roller hearth furnaces, the diffusion process is reached by relatively low heating rates. New technologies, like induction heating, reach very high heating rates and offer great potentials for the application in hot stamping. Till now it is not proofed, that this technology can be used w...

Development of Cooled Tools for Press Hardening of Boron Steel Sheets

Two central topics of modern automotive constructions refer to automotive lightweight constructions and safety; this is best indicated by the most various developments in the context of body constructions. The consistent evaluation of cars by means of crash tests has led to the fact that new manufacturing variants combined with the use of high and ultra high strength materials represent a fundamental part of the body in white. One possibility to form such materials is given by the hot forming process. An innovative approach, namely to improve this procedure in economic and technological terms, is represented by the new tool system for tempered forming processes carried out at the Institute Tools & Forming (T&F). The integration of most complex cooling duct geometries by casting integral tube networks into the tool represents an innovative possibility to improve the cooling performance combined with the reduction of the cycle times.

Forming Limits in Sheet Metal Forming for Non-Proportional Loading Conditions - Experimental and Theoretical Approach

The influence of strain paths (loading history) on material formability is well known in sheet forming processes. Sophisticated experimental methods are used to determine the entire shape of strain paths of forming limits for aluminum AA6016‐T4 alloy. Forming limits for sheet metal in as‐received condition as well as for different pre‐deformation are presented. A theoretical approach based on Arrieux’s intrinsic Forming Limit Stress Curve (FLSC) concept is employed to numerically predict the influence of loading history on forming severity. The detailed experimental strain paths are used in the theoretical study instead of any linear or bilinear simplified loading histories to demonstrate the predictive quality of forming limits in the state of stress.

Tools and Technologies for Hot Forming with Local Adjustment of Part Properties

In general hot stamped car body parts show a uniform strength distribution. Especially for safety relevant parts with high requirements concerning crash performance, this uniform strength distribution can cause problems. During a crash a B-pillar e. g. can absorb more energy when the lower part is relatively flexible while the middle and upper part has to be high-tensile to prevent the intrusion into the passenger compartment. Also during the production of hot stamped parts, the high strength causes trouble. When the trimming takes place after the hardening process, the durability of the tool is limited. Thus at the moment the only economic process for trimming of ultra-high-strength steels is laser cutting. This paper presents different approaches to reach local different strength distributions in hot stamped components. In particular the results of a research project of the Institute Tools & Forming, Graz University of Technology are shown where precisely defined areas of different strengths could be obtained in one part. This was achieved by the use of simple and cheap ceramic inserts in conventional press hardening tools.

Investigation of Weld Seam Structures of Tailor Welded Blanks for Hot Stamping

Increasing safety requirements for hot stamped structural car body parts are demanding sufficient mechanical properties of integrated welding seams. Especially conventional tailor welded AlSi-coated 22MnB5 sheets are only fulfilling these requirements when ablated before laser welding and thermo-mechanically treated in a correct way. This paper shows a method that evaluates the hot stamping process of tailor welded blanks by press hardening different sheet thicknesses and thickness combinations in a testing tool. Furthermore, appropriate testing methods for the evaluation of mechanical properties of the welding seam are introduced. The results are ultimately compared with a special developed FEM analysis to predict failure cases in future.

Application of Laser Cladding in Forming Processes—Functionally Graded Coatings

Innovative tooling steels, optimized geometries and thin film coatings are effective methods for improving performance characteristics like abrasive tool wear. Laser cladding is an additional technology with potential to solve both, tool wear and heat transfer problems for deep drawing, cutting, hydroforming and hot forming processes. This technology combines a base material with a highly process orientated coating. The work deals with the application of a functionally graded coating for hot stamping tools. Laser cladding of aluminum bronze on a mild steel substrate is performed and the thermal performance of this configuration is tested.

Failure prediction method for hydro forming simulation of thick walled tubes

The industrial production of thick walled hydro formed steel parts is a process difficult to control. In particular the prevention of cracks in the production of these parts is very important. It is of utmost importance to have a virtual tool to predict forming results. Standard methods for the simulation of hydro formed parts base upon processes using a shell element formulation and implement a forming limit curve (FLC) for crack prediction. But the forming limit curve is limited to the case of linear strain paths. The initial FLC is no longer valid in the case of nonlinear strain paths. Because of the geometric specifications of the investigated parts—thick walls, compact dimensions, high strains—and the known limitations of the forming limit curve—which don’t accord to the hydro forming process— these standard simulation methods are not applicable for the present investigations. A new approach to simulate thick walled hydro formed parts is the use of a volume element formulation in combination with a m...