Equal channel angular extrusion die design for optimum experimental tests made on an AlMgSi alloy.

Ghiban, Nicolae ; Serban, Nicolae ; Saban, Rami 等

1. INTRODUCTION

Equal Channel Angular Extrusion (ECAE) was invented in the former Soviet Union by Vladimir Segal in 1977, for which he obtained an Invention Certificate of the USSR, similar to a patent. Researchers in the Texas A&M University's (TAMU) Deformation Processing Laboratory in the Department of Mechanical Engineering have been conducting researches on the ECAE process since 1992. Dr. V. Segal was a research associate in the Lab from 1992 to 1995.

ECAE is an innovative process capable of producing uniform plastic deformation in a variety of materials, without causing significant change in geometric shape or cross section. Multiple extrusions of billets by ECAE permit severe plastic deformation in bulk materials. By changing the orientation of the billet between successive extrusions, complex microstructures and textures can be developed. Changing the chosen billet orientation after each pass, five fundamental equal-channel angular extrusion routes are defined and utilized to obtain different textures and microstructures, (Miyahara et al., 2005).

The technique is able to refine the microstructure of metals and alloys, thereby improving their strength according to the Hall-Petch relationship. ECAE is unique because significant cold work can be accomplished without reduction in the cross sectional area of the deformed work piece. In conventional deformation processes like rolling, forging, extrusion, and drawing, strain is introduced by reduction in the cross sectional area. ECAE produces significant deformation strain without reducing the cross sectional area. This is accomplished by extruding the work piece around a corner. For example, a square cross section bar of metal is forced through a channel with a 90 degree angle. The cross section of the channel is equal on entry and exit. The complex deformation of the metal as it flows around the corner produces very high strain. Because the cross section remains the same, a work piece can be extruded multiple times with each pass introducing additional strain. Die design is critical because of the large forces required, (Fukuda et al., 2004).

This paper attempts to offer an optimum solution to the most important problem regarding the equal channel angular extrusion technique, namely the die design for this process, applied to the ECAE of an AlMgSi alloy.

2. ECAE COMMON RESEARCH METHOD

In the ECAE procedure, a sample is extruded through a die with two channels intersecting at an angle 2[PHI] and with equal cross-sections. During each extrusion, the cross-section of the sample remains unchanged and consequently this procedure can be repeated a great number of times, resulting in the accumulation of large strains through the alloy. The key parameters of this technique are mainly the geometry of the device, the temperature of extrusion and the number of extrusions [N.sub.E]. The deformation path (i.e. the route) is also an important parameter since it is now well established that the resulting mechanical properties for a given number of extrusions depends upon the angle of rotation between each pass, (Dupuy & Blandin, 2002).

The schematic diagram of the ECAE deformation is shown in fig. 1. Previous studies conducted found that the highest and most uniform strain distribution is achieved when the half-angle between the two die channels ([PHI]) and the angle at the outer intersection of the channels ([PSI]) are 90[degrees]and 0[degrees], respectively (as shown in fig. 1). The effective strain introduced in the materials depends on both the inner and outer channel intersection angles. By taking into consideration the outer intersection of the channels, the effective strain for N pass extrusion can be explained in a complex analytical equation, (Tham et al., 2007):

[epsilon] = N (1/[square root of 3][2 cotan ([PHI] + [PSI]) + [PSI] cosec ([PHI] + [PSI])] (1)

where the outer intersection of the channels is represented by an arc and designated as an angle 2[PSI], as illustrated in figure 1.

ECAE has been applied to various materials but most studies have concerned aluminium alloys: pure aluminium and binary Al--Mg were firstly investigated but in the recent past, commercial Al alloys have received increasing attention. For pure Al and binary alloys, the ECAE processing is frequently carried out at room temperature whereas for industrial alloys, an increase of the extrusion temperature is generally required. In any case, grain sizes of about 1 um or less can be produced. Such refinements of the microstructures improve the room temperature yield stress via the Hall--Petch relation but it may also decrease the temperature and/or increase the strain-rate for which superplastic properties can be obtained, (Dupuy & Blandin, 2002).

[FIGURE 1 OMITTED]

3. ECAE OPTIMUM DIE DESIGN

Honeywell started the scale-up efforts of ECAE in 1997 with the construction of the first production die. Today, several large-scale die sets for a few standard billet sizes are in normal operation for Al, Cu and, occasionally, pure Ti using presses with 1000 and 4000 tonnes capacity. Most of these dies have been in use on a weekly basis for 6 years. The mass of the largest ECAE billet is 32.7 kg for Al alloys and, most recently, 110 kg for Cu and Cu alloys. As a comparison, the largest reported ECAE processed Al billet obtained with a die channel angle of 105[degrees] has a mass of 6.7 kg whereas the mass of the most typical 10mmx10mmx60mm Al billet used also for present research is 0.016 kg, (Ferrasse et al., 2008). After several attempts with distinct values of the angle 2[PHI] we arrived to the conclusion that the optimum values for the angle in the case of some experimental tests made on an commercial AlMgSi alloy concerning the structure and the mechanical properties of the samples after ECAE, are 90[degrees], 100[degrees] and 110[degrees], as shown in figure 2 (a, b and c).

[FIGURE 2 OMITTED]

4. CONCLUSIONS AND FUTURE RESEARCHES

From figure 2 (a, b and c) one can observe that a wide interval of the angle between the two die channels is taken in consideration (90[degrees]-110[degrees]). The die channel geometry is described in all the cases by three areas: the input area, the calibrated area and the output area. It can clearly be seen that the input area section is a bit wider than the calibrated area section of the die channel. This is because the elastic deformation of the aluminium samples was taken into consideration for multiple successive extrusions of billets. The three taper alignment pins (holes) and the six assemblage screws (holes) should ensure the accurate alignment, respectively the correct installation oh the two parts of the die for each case. Future researches will be based on the study of the microstructural evolution of the billets made from an commercial AlMgSi alloy (10mmx10mmx60mm), that will be subject to multiple extrusions and also on the study of the influence of the main parameters of the process over the mechanical properties of the extruded samples. Also, the material flow behavior through the die channel and especially through the calibrated area will be studied and the flowing curves of the material particles and the equations that defines the phenomenon will be established.

5. REFERENCES

Dupuy, L. & Blandin, J.J. (2002). Damage sensitivity in a commercial Al alloy processed by equal channel angular extrusion. Acta Materialia, Vol. 50, 2002, pp 3251-3264

Ferrasse, S.; Segal, V.M.; Alford, F.; Kardokus, J. & Strothers, S. (2008). Scale up and application of equal-channel angular extrusion for the electronics and aerospace industries. Materials Science and Engineering, Vol. A493, 2008, pp 130-140

Fukuda, Y.; Oh-ishi, K.; Furukawa, M.; Horita, Z. & Langdon, T.G. (2004). The application of equal-channel angular pressing to an aluminum single crystal. Acta Materialia, Vol. 52, 2004, pp 1387-1395

Miyahara, Y.; Matsubara, K.; Horita, Z. & Langdon, T.G. (2005). Grain refinement and superplasticity in a magnesium alloy processed by equal-channel angular pressing. Metallurgical and materials transactions, Vol. 36A, July 2005, pp 1705-1711

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 Materials Processing Technology, No. 192-193, 2007, pp 121-127