4M Knowledge base - papers

A Friction Model for Microforming

H.J. Jeon, A.N. Bramley
Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK

Abstract

For the simulation of metal forming processes, input data relating to the tool-workpiece interface is necessary. For microforming applications the tool/workpiece interface conditions tend to dominate the process and it has been found that traditional methods of modeling the interface are not realistic. This paper describes an approach that seeks to describe friction by modelling the geometric surface roughness of the tool as opposed to the use of the traditional empirical friction coefficient or factor. This finite element based model has been validated experimentally in terms of loads and metal flow using the ring test and actual surface measurements. It enables more accurate and also more flexible modeling of friction. As such it will be very suitable for microforming applications.

Submitted on May 19, 2008 - 15:23.

A Synopsis of U.S. Micro-Manufacturing Research and Development Activities and Trends

Kornel F. Ehmann(a)(b)(c)(d)
a: Department of Mechanical Engineering, Northwestern University, Evanston IL, USA
b: Department of Mechanical Science and Engineering, University of Illinois at Urbana/Champaign, USA
c: Department of Mechanical Engineering, Indian Institute of Technology – Kanpur, India
d: Department of Mechanical Engineering, Chung Yuan Christian University, Chung Li, Taiwan

Abstract

Micro-manufacturing in the context of this presentation is defined as the manufacture of components and products in the sub-millimeter to a few-millimeter range with micron size feature characteristics of high accuracy and precision in a wide range of engineering materials by non-lithography based processes. The paper addresses three topics. First, the findings of a worldwide study on micro-manufacturing, conducted by the World Technology Evaluation Center, Inc. (WTEC) will be summarized. This summary will compare U.S. efforts to those in a number of Asian and European countries. Second, a précis of selected ongoing work conducted at U.S. universities with leading programs in the field will be given. Topics to be discussed include the development of miniaturized machine tools for cutting, forming and manipulation as well as the modeling of micro-manufacturing processes. An account of the growing industrial activities related to the development of micro-manufacturing equipment will also be included. Third, emerging directions and challenges in the development of micro-manufacturing technologies will be reflected upon.

Submitted on November 12, 2007 - 16:23.

An advanced approach in simulation of microforming processes

S. Geißdörfer, U. Engel, M. Geiger
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, 91058 Erlangen, Egerlandstr. 11, Germany

Abstract

At microscale, the large ratio between mean grain size of the material and specimen dimension cause an increasing influence of single grain forming behaviour on the overall forming process. Thus the forming behaviour of these parts can no longer be regarded as to be homogeneous. This leads to a change in the material behaviour resulting in a large scatter of forming results, e.g. varying cup height in a cup backward extrusion process or varying spring-back angles in a micro bending process. Moreover, some correlation between the integral flow stress of the workpiece and the scatter of the process factors on the one hand and the mean grain size and its standard deviation on the other hand has been detected in experiments. Conventional FE-simulation which is by its nature size independent, is not able to consider these effects observed when scaling down processes, in particular represented by a reduction of the flow stress, an increasing scatter of the process factors and a local material flow being different to that obtained in the case of macro parts. Therefore, a new simulation model is being developed in order to take into account the identified effects and to determine the scatter of the process factors. The so-called mesoscopic model provides the discretisation of the simulated material into individual objects which represents the grain structure of the real material. To each object an individual flow curve is assigned, calculated on the basis of metal physics given by Hall-Petch and Ashby’s theory. The computational grain structure generation is based on the theory of a Monte Carlo Potts growth law.

The present paper deals with the theoretical background of the new mesoscopic model, its characteristics like synthetic grain structure generation and the calculation of micro material properties - based on conventional material properties. The verification of the simulation model is done by carrying out various experiments with different mean grain sizes and grain structures but the same geometrical dimensions of the workpiece.

Submitted on May 19, 2008 - 14:53.

Impact of Liquid Lubricant on the Flattening Behaviour of Single Asperities

S. Weidel, U.Engel
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, 91058 Erlangen, Germany
S. Weidel, U.Engel
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, 91058 Erlangen, Germany

Abstract

Scaled friction tests show that in micro forming applications friction is increasing significantly when forming processes are scaled down. This phenomenon can be explained by the model of open and closed lubricant pockets characterising the surface topography: the forming load is transmitted from the tool to a lubricated workpiece by three different bearing ratios. These are the real contact area (RCA) as well as open (OLP) and closed lubricant pockets (CLP). The developing hydrostatic pressure which is built up in CLPs takes a part of the external forming load, thus reducing the normal pressure on the asperities leading to a decrease in friction.

With miniaturisation in micro forming applications, the ratio of CLPs is reduced drastically as the surface topographies are mainly invariant to scaling. Thus, the forming load is mainly transmitted by the remaining RCA leading to an increase in friction. Hence, the interface between tool and RCA has to be investigated in more detail for the characterisation of the tribological conditions in microforming. In contrast to the macroscopic approach, where the RCA is assumed is assumed to be flattened completely, in microforming submicron effects within the RCA have to be considered.

In order to investigate the contact state in the RCA a novel, high-resolution experimental set-up has been developed which enables the measurement of the force-displacement characteristics during flattening the surface topography, and Simultaneously, the in-situ observation of the developing real contact area by using a translucent tool. Thus, the deformation behaviour of idealised asperities represented by pyramids with a base area of 120 x120 μm2 and a height of 32 μm can be examined. In-process measurement is complemented by post-process topography analysis.

The present paper will present recent results and experiences obtained by the investigations described above. The detailed knowledge about the evolution of surface topography is relevant in particular to microforming but also for an improved understanding of tribological phenomena in general.

Submitted on November 12, 2007 - 16:23.

categories

Metal Forming

Material Modelling for the Simulation of Microforming Processes at Elevated Temperature

D. D’Addona, R. Teti
Department of Materials and Production Engineering, University of Naples, Naples, Italy

Abstract

The main objectives of this paper are investigations on the usability of artificial neuronal networks for the calculation of material properties at elevated temperatures in case of microforming processes. Modelling of the rheological behaviour of diverse materials subjected to hot forging is attempted through a parallel distributed processing paradigm based on artificial neural network prediction of the metal material response. The evaluation of different feed-forward back-propagation neural networks for flow stress prediction was carried out on the basis of laboratory data of the stress-strain behaviour of nickel base superalloys and mild steel subjected to compression tests with different temperature and strain rate conditions. The results obtained displayed a good agreement with the experimental data, showing that the neural network approach can accurately describe the material flow stress under the considered processing conditions.

Submitted on November 12, 2007 - 16:23.

categories

Metal Forming

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