H. Hoffmann

Method for optimizing the cooling design of hot stamping tools

During the past years hot-stamped components have gained considerable importance in the automotive industry. This is due to the advantageous properties involved, such as good crash behavior and high strength. In the production of hot-stamped components the blank is rapidly cooled by the tool. This exerts an important influence on the final properties and the process time. The tool is actively cooled by cooling ducts through which a medium flows. This article will present a newly developed method in which the design of the cooling ducts can be systematically optimized. A test tool is used to show the results of this method. It is easy to realize an optimized tool for the hot stamping process.

Tensile test of very thin sheet metal and determination of flow stress considering the scaling effect

The reduction in size of mechanical parts is becoming more and more important in metal forming processes. This trend towards miniaturization leads to new requirements regarding the material and the product. The main object of this research is the so-called scaling effect which can occur during the forming process of very thin sheet metals. The goal is to determine and investigate variables which considerably influence the scaling effect. The optimisation of such forming processes needs a more comprehensive use of simulation and finite element analysis. The conventional model used for large and thick sheet specimens is not sufficient yet. Existing research reports on this topic show that among other things the thickness and the grain size of the material influence the scaling effect. Thus it is important to consider these material characteristics in the determination of the flow stress curve. In this paper the flow stress curve is investigated on a micro scale of up to 10μm specimen thickness considering different sheet thicknesses and widths.

Determination of True Stress-Strain-Curves and Normal Anisotropy in Tensile Tests with Optical Strain Measurement

This paper describes a method to measure the flow curve which undergoes necking and normal anisotropy as a function of equivalent strain using an optical measurement system. The flow stress was determined by modifying Siebel and Schwaigerers model. Normal anisotropy was regarded as a function of equivalent strain. Comparing with the experiment of the stretch forming process, the simulation using improved material properties shows the better strain distribution than the results using conventional material properties in high strain areas.

Advanced Wear Simulation in Sheet Metal Forming

A drawback of former wear simulation of deep drawing processes is that the change of wear caused by increasing the number of punch strokes was not considered. Geometry-Update-Scheme (GUS) which has been proposed at Institute of Metal Forming and Casting considers the changes of tool geometry caused by wear through interactive iterations of forming and wear simulation. The wear depth from simulation using GUS shows maximal 0.07 mm of difference from the measurement at a section of worn die-geometry. In this study, only the abrasive wear was considered.

FEM simulation and experimental research on the AlMg4.5Mn0.4 sheet blanking

Both experiment and finite element method (FEM) simulation were done on the AlMg4.5Mn0.4 sheet blanking with 1, 10 and 20% relative clearance, respectively. The cutting force curves and the cutting surface parameters which can be used to describe the quality of the blankings were measured. Simulation was accomplished by MARC Autoforge software package. Calculated cutting forces are always bigger than measured ones. All difference between experiment and simulation is not greater than 20%. It is feasible doing virtual experiment on workstation to estimate the cutting force and to predict the quality of the workpiece for new material using certain blanking technical parameters.

A new approach to determine the wear coefficient for wear prediction of sheet metal forming tools

Accounting for the increase of wear in metal forming tools, it is eminent to have detailed information about the tool lifetime already during the tool design. With the wear simulation tool REDSY—developed at the Institute of Metal Forming and Casting—tool wear can be simulated qualitatively and quantitatively for sheet metal forming processes. The calculations are based on Archard’s wear model, a model using contact mechanics to describe the wear behavior. In this project, a new approach to determine the wear coefficient has been developed using a simple cylindrical cup deep drawing experiment for the wear measurements. Several tool and sheet material combinations were analyzed using a five-stage progressive die tool in a precision automatic punching press in order to achieve a high wear volume in a short period of time. The wear coefficient for the respective material combination could be determined combining the experimental results with simulation. This method is verified by comparing the wear simulation results with actual measurements.

Clean-sheared and rectangular edges through counter-shaving

A various number of products exist which require a clean-sheared edge due to function or surface feel. Traditionally, these parts are produced on fine blanking presses with three separate and distinct movements. Shaving, in particular counter-shaving is a shearing operation to improve the edge quality of a blanked part or punched hole. This paper introduces a progressive die tool to realize the counter-shaving process on a single acting press. Advancing the chip flow through choosing a cutter angle can achieve up to a 85% clean-sheared edge and rectangular edge. Thereby, a sharp-edged transition—without any rollover depth—between the upper surface of the sheet metal and the sheared-edge are formed. The end of the clean-sheared edge is followed by a tear which shapes a burr in the form of a triangular lappet, which can be deburred in a subsequent vibratory grinding process. Experimental results of different process parameters for two sheet metal materials at three thicknesses are discussed and compared to finite element calculations.

Strategies to optimize the part transport in crossbar transfer presses

In order to keep the production of wide body components profitable, even in high-wage countries, the technology of crossbar transfer presses has been implemented into the stamping plants of automobile companies. In particular with large compliant parts, dynamic problems during the transportation have to be seen as one of the limiting factors for the production stroke rate. So far, there are no tools to investigate these problems in order to optimize the material handling. Based on the interaction of different simulation systems, a solution has been developed to predict these problems as early as during the construction process of a die. It becomes possible to calculate the ideal positions for the part holding suction-cups and to analyze the dynamic behavior of the stamping part during transportation, in order to enhance the productivity of the production plant. The results of the generic methodology have been verified by experimental research on compliant sheet metal parts of automotive industry.

Monitoring Phase Transition Kinetics in Austempered Ductile Iron (ADI)

Austempered ductile iron (ADI) is a very attractive material for applications where high strength, good ductility, wear resistance and fatigue strength are required. Thus, it offers design engineers an alternative to steel and aluminium alloys. ADI essentially is a cast ductile iron that undergoes a specially designed austempering heat treatment, which creates a microstructure of high carbon austenite and bainitic ferrite along with graphite nodules. The final proportion of these phases (and thus the mechanical properties) depends on the phase transformation kinetics which is strongly affected by composition, as-cast microstructure and heat treatment parameters (austempering). ADI samples were austempered (heat treated) and the phase transitions were analysed after interrupted austempering. The phase fractions (austenite, ferrite, martensite, etc.) and their relation to bulk properties, like electrical resistivity, magnetic properties and mechanical properties (e.g. strength, hardness), and others were measured using optical and electron microscopy, X-ray and neutron diffraction, Mössbauer spectroscopy, and micro hardness measure¬ment. This combination of complementary techniques allows the correlation of the phase transition kinetics with the resulting properties.

Model based strategies for an optimised ribbing design of large forming tools

A proposal is made for the ribbing of large forming tools. In the software developed for this purpose the ribbing structure is pre-optimised by means of an algorithm based on the power-law approach and then post-optimised with a newly developed algorithm for the reduction of the v. Mises stress. The model, intention and functioning of the software are explained. The optimised ribbing structures are analysed on parameterised test geometries and compared with conventional ribbing strategies in respect of manufacturing suitability, casting defects, mechanical properties and residual stress.