U. Engel

Microforming: from basic research to its realization

Abstract The production of miniature parts is gaining importance due to the trend of miniaturization which is increasingly determining the development of products ranging from mobile phones and computers to medical products. The application of conventional manufacturing processes for the production of such microparts is possible, but there are problems that result from the small dimensions. This fact applies also in the field of metal forming, however, in the meantime many research projects in several countries could improve this situation. This paper gives a review of the problems associated with miniaturization, the way of solution starting from basic research, and the results showing the progress of microforming today.

Size effect in the FE-simulation of micro-forming processes

Abstract Micro-forming is very important for the manufacturing of micro-parts. Because of the empirical process design that characterizes this technology today, the application of the specific advantages is rather restricted. In order to improve this momentary situaation the numerical process simulation seems to be a suitable instrument which can be used additionally to accelerate the process design. Quite in contrast to conventional metal forming, where numerical process simulation is already successfully applied, the micro-forming process is characterized by some scaling effects which have to be considered in an advanced process simulation. Therefore a strategy for FE-simulation is suggested and applied for upsetting of small cylindrical specimens. This strategy is based on the experimental determination of the input data with conventional upsetting tests of specimens with dimensions corresponding to the micro-forming process. In order to perform a size dependent numerical metal flow simulation for dry upsetting of cylindrical specimens the flow curves and the friction coefficient were determined with respect to the scaling effects. Finally the numerical results were verified by experimental data of the tool load and the shape of the specimens.

Excimer Laser Micro Texturing of Cold Forging Tool Surfaces - Influence on Tool Life

Abstract In bulk metal forming the tribological state is significantly dependent on the surface topography of tool and workpiece. To provide the process with an improved tribological behaviour in some cases the billets surface is prepared by shot blasting. Micro texturing of the tool surface provides an additional opportunity to optimise the frictional conditions. This paper deals with the investigation of micro texturing of the surface of TiN-coated tools with regard to their tribological behaviour in the field of cold forging. The micro textures are applied by excimer laser radiation. Tool life investigations in an industrial press shop yield that the tool surface can be improved significantly leading to higher tool life.

A theoretical study on wear simulation in metal forming processes

Abstract One of the main reasons of tool failure in industrial application of metal forming technologies is wear. Typical for this kind of failure is that it affects not only the costs of the process but also the tolerances of the formed parts. The only way to control these features is to develop methods which allow prediction of wear and which are suited to be used in the design stage in order to optimize the process. Starting with a survey of prediction methods, a new concept for the simulation of wear is presented in this paper.

Strength improvement of cemented carbides by hot isostatic pressing (HIP)

HIP treatment after sintering increases the strength of the investigated cemented carbide alloy by a factor of two whereas hardness, fracture toughness, and work of fracture remain unchanged. HIP does not affect the microstructural parameters of the carbide skeleton and the binder phase, but the residual pores are eliminated entirely. Failure of both the as-sintered and post-densified material occurs by a pure Griffith mechanism. The strength-flaw size relationship is established experimentally and is shown to obey exactly Griffiths basic strength equation. The strength is controlled by the largest microstructural defects, i.e. pores in the as-sintered material, and coarse WC grains and inclusions in the HIP-treated specimens.

A Basic Study of the Influence of Surface Topography on Mechanisms of Liquid Lubrication in Metal Forming

Abstract Applying a transparent tool technique the tool/workpiece interface in plane strip drawing of aluminium is studied. The strips are provided with macroscopic lubricant pockets, and the compression and eventual escape of trapped lubricant by the mechanisms Micro Plasto Hydro Dynamic Lubrication (MPHDL) and Micro Plasto Hydrostatic Lubrication (MPHSL) is observed and quantified experimentally with respect to the lubricant pocket parameters, shape, volume, and angle to the edge. The two mechanisms have proved to depend very differently upon these parameters. The level at which the hydrostatic pressure is stabilised is shown to be independent of the volume of a pocket and the MPHDL mechanism is therefore solely dependent on the angle to the edge, which is shown both experimentally and theoretically by a fluid mechanic analysis. For the MPHSL mechanism a dependency of the volume is observed.

Estimation of tool life in bulk metal forming based on different failure concepts

Abstract For an economic production with a high process reliability, an increased tool life is required. Therefore, the necessity exists to give a reliable estimation of the tool life during the design of new tool systems. Especially for cold forging processes, the failure caused by surface fatigue is characteristic. Different approaches are suitable to calculate the time period until the initiation of fatigue cracks. The aim of this paper is to assess the applicability of different failure concepts for a closed cold forging die. The critical, process-dependent load is quantified and localized by using a finite element method. Based on the resulting stress–strain distributions, the damage parameters are calculated yielding different estimates of tool life that are compared with practically experienced data.

New Developments for the Qualification of Technical Surfaces in Forming Processes

Abstract In modern sheet metal forming, blank topographies are characterized by various quite different and rather sophisticated structures. This is due to the various deterministic or semi-deterministic methods of surface texturing produced by skinpass rolling. The conventional surface parameters determined from profile data are not capable of characterizing such surfaces, in particular with respect to their functional behavior. Thus, there is a strong demand for the development of new parameters which can only be determined via the measurement of area. With special respect to the application of deterministically textured topographies in metal forming, appropriate 3d surface parameters can be derived from the mechanical rheological model recently developed at the Institute of Manufacturing Technology. These are the maximum ratio of closed void area and the closed void volume, both of which result from a special evaluation of the bearing ratio curves. Industrial applications show that these parameters can be related to the functionality of the surface and hence may be regarded as very suitable for the characterization and qualification of complex surface structures.

Process Characterization and Material Flow in Microforming at Elevated Temperatures

The application of metal forming to the manufacturing of microparts is a very interesting technology with several advantages, but the use of the technology is still not widespread. This is due to the fact that the know-how of conventional forming cannot be simply transferred to the micro scale because of so-called size effects. One size-effect problem can be found in grain constellation, which affects the forming result and may be different for each micropart. Among other effects, these size effects result in a significant increase of scatter and forming behavior that is difficult to be predicted exactly. One possibility to counteract the observed inhomogeneous material character is to perform the process at elevated temperatures. Results gained by upsetting and backward can extrusion show that temperature influences the forming result, tending to homogenize the material flow. Further, temperature also leads to a reduced scatter of process parameters such as the force-stroke characteristic. Overall, this study indicates that, in microforming, the forming result can be improved by using an elevated process temperature.

3D-surface parameters and their application on deterministic textured metal sheets

In sheet metal forming the surface structuring of steel sheets is applied by several methods during skin pass rolling. The function of the surface is to lower the friction in the forming process and to achieve a high glossiness of the finished product after painting. In the past, two technologies have been developed to texture the rolls with a three dimensional deterministic structure: the laser texturing surface and the electron beam texturing surface. Due to their deterministic feature such topographies are totally different from those usually applied and thus, they require adequate methods to characterize them. Appropriate parameters can be derived from the so called mechanical-rheological model which is about to be developed at the LFT. Basic parameters are the material area ratio as well as the open void area ratio and the closed void area ratio. In the present paper the theoretical background of the new surface parameters as well as the method of measurement and the program SAM (Surface Analysis Module) will be explained. To demonstrate the capabilities of the method, different kinds of surfaces are characterized using the newly developed parameters. For practical use in industrial application the change of the surface in a cold rolling process for the manufacturing of steel sheets was investigated. It can be shown by the new parameters that there is a strong influence of the mills wear process on the structure of the rolled steel sheet. Another application concerns the surface deformation in a deep drawing process.