Consideration regarding die design for equal channel angular extrusion.

Ghiban, Brandusa ; Dumitru, Florina-Diana ; Ghiban, Nicolae 等

1. INTRODUCTION

In recent years, a growing importance in materials science and engineering was given to severe plastic deformation (SPD) methods. In particular, a number of difficulties connected with residual porosity in compacted samples, processing of large scale billets, impurities from ball milling, and practical application of the given materials were overcome with SPD methods.

The structures formed by the SPD are ultra fine-grained structures of a granular type containing mainly high angle grain boundaries. The interest in grain-size reduction is driven by the possibility to produce ultrahigh strength metals and high strain rate superplasticity, (Eivani & Taheri, 2007).

Presently, among the methods of SPD, equal channel angular extrusion (ECAE) is considered as the most promising for industrial applications.

Equal channel angular extrusion (ECAE) developed by Segal is widely known as one of the techniques to impose severe plastic deformation on bulk materials to produce ultrafine grained materials, without causing a significant change in the dimensions of the processed parts, (Agnew et al., 2005).

The aim of the process is to impart high deformation to the processed materials as they cross the channel. The exit channel is usually manufactured with the same diameter as the entrance channel and hence cross-section of the processed material is not modified and so, there is no geometrical limitation to the deformations that is possible to impart to the processed materials.

It has been suggested that this technology has great advantages over the conventional mechanical attrition of ball milling because it can produce large sized samples free of any residual porosity.

Simple shearing is the key mechanism of the ECAE process. Simple shear can be considered a near ideal deformation method for structure refinement and texture formation in metal working.

Deformation during ECAE is achieved by simple shear in a thin layer at the crossing plane of the channels. Large and uniform strain intensity per pass can be reached in material under low pressure and load without a reduction of the initial billet cross-section. The process can be repeated a number of times in the die because the billet cross-section remains constant, (Tham et al., 2007).

2. MECHANISM OF ECAE

During the ECAE process a metal billet is pressed through a die consisting of two channels, equal in cross section and intersecting at an angle usually between 90[degrees] and 135[degrees]. The billet undergoes essentially simple shear deformation but retains the same cross sectional geometry, so that it is possible to repeat the pressings for a number of passes, each one refining the grain untill the extent which is determined by the material characteristics. High plastic deformations lead to an improvement in the dislocations density in ductile crystalline material and this improvement of the dislocation density is followed by an increase in the strength. Hence with enough accumulation of plastic strain a new structure of submicrometer or even nanometer grain size replaces the former grain size, (Gil et al., 1980).

By multiple passing, very large effective deformation can be developed in bulk products.

The process also leads to the formation of strong crystallographic texture in the material. In ECAE, it is possible to rotate the billet around its longitudinal axis between successive passes, creating different routes.

A nomenclature has evolved in the literature referring to the major variants as Routes A, [B.sub.A], [B.sub.C], and C (meaning no rotations between passes, 90[degrees] back-and-forth, 90[degrees] continuous, and 180[degrees] rotations, respectively), (Iwahashi et al., 1998).

The amount of plastic strain introduced in the materials after ECAE processing depends on the die angle and the number of passes.

3. DIE DESIGN

The ECAE die is manufactured with two channels that usually intersect at an angle with the same cross-section. The material is extruded through the die and it is mainly deformed by a shear mechanism combined with a high hydrostatic pressure which exists within the die channels.

[FIGURE 1 OMITTED]

The simplest shape of these dies is the one containing of two straight channels intersecting with sharp outer and inner corners as shown in Fig. 1a, where [PHI] is the die angle being equal to [alpha] + [beta] which in the case of ECAE, [alpha] = [beta], and so [PHI] = 2a.

Segal's geometry (Fig. 1a) was modified by Iwahashi (Fig. 1b), who proposed an ECAE die with a sharp edge of the inner part of the die and a [PSI]-angle. Luis proposed ECAE dies (Fig. 1c) in which internal radius and external radius can have whatever value and both radii are tangent to the die walls, (Perez & Luri, 2008).

The design of the die took into account the necessity of having separated parts that permit (after dismantling the die) accessing the channel for maintenance if it is needed. These different parts were chosen so as to minimise the number of joints in the channel and to maximise the possibility of access to the channel.

The die is made up of the piston and the die body, which is made out of two parts. The die channel geometry is described by three areas: the input area, the calibrated area and the output area, (Ghiban et al., 2009). The die used in the ECAE experiments has two equal channels with a square cross-section of 10 mm x 10 mm and an intersecting angle of 90[degrees].

The image of our proposed die for ECAE is given in figure 2 (a, b). From figure 2 (a and b) one can observe that the die design has Iwahashi's geometry and has as a drawback the rapid wear of the die and the damaging of the material as is being extruded, because of the sharp edge.

Another drawback is the incorrect flow of the material in the ECAE process due to the fact that the walls of the die are not tangent to the outer radius. One can also observe that the ECAE die is decentred. Also, the thickness of the die body is only 30 mm, which is why the die can not stand in position during ECAE processing.

Another big disadvantage of the die is that the assemblage screws are threaded, and during the ECAE processing they warp and the die can not be opened.

[FIGURE 2 OMITTED]

4. CONCLUSION AND FUTURE RESEARCHES

The considerations made on our proposed die design for ECAE may reveal the following conclusions: the proposed die is not adequate for a proper ECAE processing because the die must be centered, the dimensions must be higher for a good position during ECAE processing and the die must not contain screws, because during the ECAE processing the screws warp and the die can not be opened.

Future researches will be based on designing a new ECAE die with Luis' geometry and also on the material flow behaviour through the die channel.

5. ACKNOWLEDGEMENTS

The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreement POSDRU/88/1.5/S/60203.

6. REFERENCES

Agnew, S.R.; Mehrotra, P.; Lillo, T.M.; Stoica, G.M. & Liaw, P.K. (2005). Texture evolution of five wrought magnesium alloys during route A equal channel angular extrusion: Experiments and simulations, Acta Materialia, Vol. 53, 2005, pp 3135-3146

Eivani, A.R. & Karimi Taheri, A. (2007). An upper bound solution of ECAE process with outer curved corner. Journal of Materials Processing Technology, Vol. 182, 2007, pp 555-563

Ghiban, N.; Serban, N.; Saban, R.; Cojocaru, V.-D.; Ghiban, B. & Ghiban, A. (2009), Equal channel angular extrusion die design for optimum experimental tests made on an AlMgSi alloy, Annals of DAAAM for 2009 & Proceedings of the 20th International DAAAM Symposium, Vol. 20, No. 1, 2009, pp 1547-1548, ISSN 1726-9679

Gil, Sevillano J.; Van Houtte, P. & Aernoudt, E. (1980). Large strain work hardening and textures. Progress in Materials Science, No. 25, 1980, pp 69-134

Iwahashi, Y.; Horita, Z.; Nemoto, M. & Langdon, T. G. (1998) The process of grain refinement in equal-channel angular pressing. Acta Mater., Vol. 46, 1998, pp 3317-3331

Luis Perez, C.J. & Luri, R., (2008). Study of the ECAE process by the upper bound method considering the correct die design. Mechanics of Materials, Vol. 40, 2008, pp 617-628

Tham, Y.W.; Fu, M.W.; Hng, H.H.; Yong, M.S. & Lim, K.B. (2007). Study of deformation homogeneity in the multipass equal channel angular extrusion process, Journal of