Metal Forming

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.

Micro-extrusion of ultra-fine grain aluminium
A. Rosochowski (a), W. Presz (b), L. Olejnik (b), M. Richert (c)
a Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow G1 1XJ, UK
b Institute of Materials Processing, Warsaw University of Technology, 02-524 Warsaw, Poland
c Faculty of Non-Ferrous Metals, AGH University of Science and Technology, 30-059 Krakow, Poland

Abstract

Microforming of normal, coarse grain (CG) metals leads to scale problems which originate from the fact that the grain size becomes comparable to the part size. A possible way of dealing with these problems is replacing CG metals with ultra-fine grain (UFG) metals. UFG metals can be produced in bulk by severe plastic deformation (SPD). This paper describes using UFG aluminium 1070 for preliminary trials of micro extrusion of a cylindrical cup. The process of producing bulk UFG aluminium by SPD is explained and the material obtained characterised. The preparation of micro billets for the extrusion operation is discussed. Backward extrusion is carried out for two types of material, CG and UFG. This enables a comparison of the material behaviour and product characteristics.

B. Eichenhueller (a), E. Egerer (b), U. Engel (a)
a Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, D-91058 Erlangen / Germany
b Siemens AG, D-91058 Erlangen / Germany

Abstract

Manufacturing of metallic parts by forming methods is industrially widespread due to several advantages like good surface quality, high accuracy and good efficiency at concurrent high quantity. As a result of the steady miniaturisation of products, large quantities of smallest metallic parts with the above mentioned attributes are needed. Despite the advantages of forming methods, microparts are mainly produced by machining, because of problems caused by so-called size-effects. These effects occur by scaling down geometry and process parameters, leading to the fact that the existing know-how for conventional processes cannot be transferred unrestrictedly to the microscale. One reason for the difference between macro- and microscale is the number of grains within the forming area. At microscale only a small number of grains is directly involved in the forming process, so that the single grain, characterised by its individual size, orientation and position, gains influence on the process. The stochastic distribution of the grain characteristics leads to an inhomogeneous material behaviour and causes an increased scatter of the process parameters. To minimise the effect of inhomogeneous material behaviour, microforming at elevated temperature is applied. Experiments with different materials at elevated temperature show a homogenising effect which leads to a reduced process scattering. This indicates that elevated temperatures are suitable to minimise and control the size-effects at micro-forming processes.

A. Diehl, U. Engel, M. Geiger
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, D-91058 Erlangen / Germany

Abstract

Metal foils attract a large field of applications, e.g. in micro-technology they are being used for sensors, actors, micro-electro-mechanical systems and in medical devices. Conventional sheet metal forming processes are in principle applicable for metal foil forming. Reducing the sheet thickness to the order of micrometers, however, causes various scaling effects. Therefore, the know-how of conventional sheet metal forming cannot be transferred directly to metal foil forming. A known phenomenon during foil forming is the reduction of strength of the material with decreasing thickness due to the increasing share of surface grains with fewer constraints to plastic flow on the overall volume. The opposed phenomenon is the increase of material strength regarding foils with mean grain sizes in the range of the foil thickness or even higher.

In the present paper basic research via scaled free bending tests is performed to investigate size effects in order to provide basic knowledge for the design of the process and of the components, respectively. An important factor in production accuracy of bending processes is the spring-back. In the current research spring-back of aluminium foils (Al 99.5) in dependence of the foil thickness is investigated with foil thicknesses ranging from 25 to 200 microns. Variation of the mean grain size/foil thickness ratio is achieved by different heat treatments. The experimental results are being compared with FE-simulations.

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.