Cold Forging

Technology suitable for both serial and small quantity production

Since the most relevant parameters in forming processes describing the material behaviour are the flow stress and the flow curve, it is necessary to perform scaled standard tests to obtain these parameters valid at microscale. Carrying out tensile tests using CuZn15, CuNi18Zn20 [27], copper [28], and aluminium [29] as well as upsetting tests using copper, CuZn15 and CuSn6 [30, 31] two significant effects have been shown: a reduction of the flow stress when increasing the surface to volume ratio as well as an increasing process scatter.

A first approach to describe the decreasing stresses has been done by [32] introducing the so-called surface layer model. Based on the assumption, that grains positioned on a free surface have fewer constraints to fulfil than grains within the material, the local forming behaviour of the surface grains must be different. Dislocations induced by a deformation process are able to pile up at grain boundaries but not at a free surface. Thus, lower hardening occurs in the region of the free surface. Decreasing the specimen size leads to an increasing fraction of surface grains and thus a lower integral flow curve.

In case of a full forward extrusion process, scaled down from an initial specimen diameter of 4 mm down to 0.5 mm, an increase in the relative punch force was detected. A possible explanation for this effect can be the increasing friction with decreasing specimen dimension. Further tests to study the effects of miniaturization on friction were made by Messner, using the ring compression test [33].The increase of friction when decreasing the specimen size was analyzed in a more detailed way by [34] using the double cup extrusion (DCE) test which was first proposed by [35] and applied by [36].

In this test – due to the large plastic deformation being well suited to represent metal forming processes – a cylindrical billet is positioned between a stationary and a moving punch. Theoretically, in case of zero friction (m = 0) both cups are supposed to have the same height, as the friction gets higher, the height of the upper cup is increasing. Thus, the change in the ratio between the upper and the lower cup is able to characterise the change in the friction conditions. If absolute values of the friction factors are requested, the method of numerical identification can be used.

Experimental investigations on the frictional size-effects have been performed by [34] scaling down specimens geometrically similar from a diameter of 4 to 0.5 mm with a ratio of diameter to height D0/H0 = 1. The friction increases with decreasing specimen size from a friction factor about m = 0.02 for the largest specimen up to m = 0.4 for the smallest specimen.

An attempt to describe the frictional behavior on a topographical level is given by a mechanical­rheological model [34] considering the theory of open and closed lubricant pockets. If a forming load is applied to a specimen surface, the roughness peaks start to deform plastically. From this point on, lubricant is either trapped and pressurized within closed areas αCL or squeezed out if a connection to the edge of the surface exists. The forming load can be transmitted into the specimen either by the pressurized lubricant or the flattened asperities.

Due to the scale-invariant production process of a specimen and thus an assumed scale-independent surface topography, the area width where open lubricant pockets appear is constant when scaling down geometry. Additionally, the area of closed lubricant pockets is reduced and thus the real contact area αRC is increased. This leads to an increase in the friction factor. Further independent investigations have confirmed these results [37].

Based on the mechanical-rheological model further investigations have been performed in order to describe the size-dependent friction factor analytically [38]. Using Wanheim/Bay’s [39] friction law and the geometrical boundary conditions, it can be shown that in dependency of the surface topography the friction factor changes as it is in a good agreement with experimentally obtained results.

Investigations on micro extrusion processes with high aspect ratios and large strains have shown a significant dependency of the forming results from the material structure [40]. In case of backward can extrusion process, the cup geometry was chosen with a cup wall thickness of about 8 microns. SEM analysis of the shape building reveals a strong influence of the material structure on the shape building, e.g. by an uneven cup height.

Further investigations using micro hardness measurements to evaluate local material flow also confirmed the above described results. This effect is less distinct in case of fine grained material than in case of coarse grained material. In case of grains being larger than the feature size they are forced to flow into the smaller features and thus in dependency of the size and orientation causes the uneven cup height.

As it was expected from DCE-test results, the increase of the friction factor when scaling down leads to an increase of the ratio between cup height and shaft length for both cases: coarse grained and fine grained material. The minor increase in case of coarse grain, can be explained by the fact that the grain size is in the same range as the feature size. Thus, it is easier for the material to flow into the shaft than into the cup.

[27] Kals, R.T.A.: Fundamentals on the Miniaturization of Sheet Metal Working Processes. Meisenbach, 1999.

[28] Kocanda A.; Prejs, T.: The effect of miniaturization on the final geometry of the bent products. In: Pietrzyk et al. (Eds.): Metal Forming 2000, Proceedings of th 8th International Conference on Metal Forming, Rotterdam, A.A.Balkema, 2000, 375-378

[29] Raulea, L.V.; Govaert, L.E.; Baaijens, F.P.T.: Grain and Specimen Size Effects in Processing Metal Sheets. In: Geiger, M. (Ed.): Advanced Technology of Plasticity 1999. Proceedings of the 6th International Conference on Technology of Plasticity Nuremberg, September 19-24, 1999, Berlin, Springer, Vol. II, 939-944

[30] Picard, P.; Michel, J.F.: Characterization of the Constitutive Behaviour for Very Small Components in Sheet Metal Forming. Proceedings of the 2nd ESAFORM Conference on Material Forming, Guimarães, Portugal, 1999, 169-170

[31] Geiger, M.; Engel, U.; Vollertsen, F.; Kals, R.; Meßner, A.: Metal Forming of Microparts for Electronics. Production Engineering 2 (1994) 1, 15-18

[32] Engel, U.; Meßner, A.; Geiger, M.: Advanced Concept for the FE-Simulation of Metal Forming Processes for the Production of Microparts. Advanced Technology of Plasticity 1996. Proceedings of the 5th International Conference on Techn. of Plasticity Ohio, 1996, Vol. II, 903- 907

[33] Engel, U.; Messner, A.; Tiesler, N.: Cold Forging of Microparts – Effect of Miniturization on Friction. In: Chenot, J.L. et al (Eds.): Proceedings of the 1st ESAFORM Conf. on Materials Forming, Sophia Antipolis, France, 77-80

[34] Tiesler, N.; Engel, U.; Geiger, M.: Forming of Microparts – Effects of Miniaturization on Friction. In: Geiger, M. (Ed.): Advanced Technol. of Plasticity, Proc. Of the 6th Int. Conf. on Technology of Plasticity ICTP 1999, 19-24 Sep 1999, Nuremberg, Germany, Berlin, Springer, Vol. II, 889- 894

[35] Geiger, R.: Der Stofffluß beim kombinierten Napffließpressen. Berichte aus dem Institut für Umformtechnik der Universität Stuttgart Nr. 36, Verlag Girardet Essen, 1976

[36] Buschhausen, A.; Weinmann, K.; Lee, J.Y.; Altan, T.: Evaluation of lubrication and friction in cold forging using a double backward-extrusion process. Journal of Materials Proc. Techn., 33 (1992), 95-108

[37] Tiesler, N.; Engel, U.: Microforming - effects of miniaturization. In: Pitrzyk, M. et al. (Eds.) Proceedings of the 8th International Conference on Metal Forming, Rotterdam, A.A. Balkema, 2000, 355-360

[38] Engel, U.: Tribology in Microforming. Proc. 2nd Int. Conf. On Tribology in Manufacturing Processes ICTMP. Nyborg, Denmark, June 15-18, 2004. Vol. 2, 549-60, to appear in Wear (2005)

[39] Bay, N.: Friction stress and normal stress in bulk metal forming processes. J. Mech. Work. Technol. 14(1987)2, 203

[40] Engel, U.; Tiesler, N.; Eckstein, R.: Microparts - A challenge for forming technology. In: Kuzman, K. (edtr): ICIT 2001 3rd Int. Conf. on Industrial Tools, Celje Slovenia: Tecos 2001, 31- 39

4th congress on Micro and Nano Manufacturing

Micromachines - Supporting the 4M Association

MDPI - Supporting the 4M Association

User login

Metals Processing

  • You must login in order to post into this group.