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 simulation model for crater formation in laser milling

T Dobrev, D T Pham and S S Dimov
Manufacturing Engineering Centre, Cardiff University, Cardiff, CF24 3AA

Abstract

In pulsed laser material removal systems, it is very important to understand the physical phenomena that take place during the laser ablation process. A two-dimensional theoretical model is developed to investigate the crater formation on a metal target by a microsecond laser pulse. The model takes into account the absorption of the laser light, and heating and vaporisation of the target, including an adjustment to compensate for the change of state. A simple numerical technique is employed to describe the major physical processes taking part in the laser milling process. The temperature distribution in the target material during the pulse duration is analysed. The effect of the laser fluence on the resulting crater is investigated in detail. The proposed simulation model was validated experimentally for laser material interactions between a microsecond Nd:YAG laser (λ= 1064 nm) and a stainless steel workpiece. The measured crater depths are in agreement with the model. Such a study is very important for understanding the mechanisms of micro-structuring when laser milling is employed. The results of this research will be used in improving the micro-machining capabilities of the process.

Submitted on May 19, 2008 - 15:12.

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.

Capability study of the Fcubic direct shell process for casting micro-components

J-F. Charmeux (a), R. Minev (a), S. Dimov (a), E. Minev (a), S. Su (a), U. Harrysson (b)

a Manufacturing Engineering Center, Cardiff University, Cardiff, CF24 3AA, UK
b Fcubic, Kallarlyckevagen 6, 42935 Kullavik, Sweden

Abstract

The paper investigates the capability of a new technology, ‘Fcubic’, for a faster and less expensive production of investment casting shells directly from CAD data for the manufacture of micro-components. The technology utilises high resolution 3D printing heads for building shells using zirconia ceramics.

The capabilities of the ‘Fcubic’ process are compared to those of classical two-stage lost wax processes to produce metal micro-components. The tests are carried out on a machine incorporating units for centrifugal and pressure/vacuum casting specially developed to facilitate the replication of components with small features. In particular, this comparative study involved the manufacture of test parts in aluminium/zinc alloys and stainless steel with micro-features in the range of 250 to 700 μm and aspect ratios up to 2.4. The dimensional accuracy and the surface quality of the produced parts were measured. In addition, the production cost of the two different manufacturing routes was assessed to determine the economic viability of the ‘Fcubic’ direct shell technology for casting components incorporating micro-features.

Submitted on May 20, 2008 - 10:28.

Development of an industrial device of blend/filling of heterogenous powders

L. Federzoni (a), M.L. Penaud (a), S. Revol (a), O. Dauchot (b), F. Daviaud (b)
a CEA-Grenoble, DTEN/STN/LT2N, F38054 Grenoble cedex 9
b CEA-Saclay, DSM/DRECAM/SPEC, l’Orme des merisiers, 91191 GIF on Yvette

Abstract

In many industrial fields -- as varied as pharmacy, food, powder metallurgy, ..., the handling of powders or blends of powders is a stage impossible to circumvent during which many difficulties can occur. In particular when it is a question of filling a die by materials finely divided prior to a compression stage, it is very difficult to ensure the transport and the deposition of powder in a controlled, homogeneous and fast way. The situation is even more delicate when one wants to use a blend of powders : at each stage of the process, and in particular during the discharge in the die, segregation may occur. The finer the powders, the more difficult it will be to avoid difficulties. In powder metallurgy, many components are manufactured by the compaction of metal powders obtained by chemical synthesis or atomization means. The powders are deposited in a cavity or die presenting the form which one wants to give to the component. The powder is then compacted under high pressure, and the green parts thus obtained are then sintered.

We have developed a device which ensures the homogeneity of the density distribution of the powder in the die. In addition, our device allows to preserve the composition homogeneity in the case of an initially prepared blend of powders. When two separate batches of powders are used, we show that the blending operation can be carried out simultaneously with the filling.

Submitted on May 19, 2008 - 14:29.

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