Technological aspects concerning severe plastic deformation by equal channel angular pressing.

Chirita, Constantin ; Comaneci, Radu ; Zaharia, Luchian 等

Abstract. The paper presents some considerations regarding Severe Plastic Deformation (SPD) with reference to the Equal Channel Angular Pressing (ECAP). The properties of the materials obtained depend on the plastic deformation behavior during ECAP, which is governed mainly by the die geometry, the material itself and the processing conditions. As the mechanical properties of the severely deformed material are directly related to the deformation history, understanding the phenomena associated with strain and forces development in the ECAP process becomes important. A comparative study of the forces in the process of severe plastic deformation is analyzed by using two type of dies with 1200 and 900 between channels. It is also presented a method to determine the friction in the first channel.

Keywords: severe plastic deformation (SPD), equal channel angular pressing (ECAP), deformation load, data acquisition.

1. INTRODUCTION

The principle of ECAP is presented schematically in the three-dimensional illustration in Figure 1 (Fukuda et al.,2002). ECAP die contains two channels, equal in cross-section, intersecting at an angle near the centre of the die. The test sample is machined to fit within these channels and it is pressed through the die.

[FIGURE 1 OMITTED]

Measurement and estimation of the strain developed during ECAP is the first step to correlate process control variables, internal parameters and resulting properties. The total strain a of workpiece in N passes through the die is given by Eq.(1) (Fukuda et al., 2002; Hartley et al., 1979):

[[epsilon].sub.N] = N / [square root of 3][2ctg ([phi]/2 + [psi]/2) + [psi] cosec ([phi]/2 + [psi]/2)] (1)

where the significance of terms are revealed in Figure 2. For average strain rate the following relation is derived:

[??] = 1 / [square root of 3][2ctg ([phi]/2 + [psi]/2) + [psi] cosec ([phi]/2 + [psi]/2)] v[square root of 2] / w[PHI](2)

where w [mm] is a diameter or width of the workpiece and v[mm/s] is the punch speed (S.K. Hyoung, 2006).

[FIGURE 2 OMITTED]

2. EXPERIMENTAL PROCEDURE

The material used for this study was aluminium cpm. The specimens were machined at the10x10x60 [mm].

ECAP testing was carried out with 8.75 mm/s at room temperature through route [B.sub.C] (the sample was rotated by 90[degrees] in the same sense around the longitudinal axis after each pass). This processing route was adopted because it leads most expeditiously to equiaxed grain structures consisting of high angle boundaries. In order to reduce the friction, a lubricant containing zinc stearat was used.

3. EXPERIMENTAL EQUIPMENT

Experimental assembly includes:

--hydraulic press with maximum load of 750 kN, Figure 3, a;

--1000 bar hydraulic drive system;

--load cell with nominal load of 300 kN and signal conditioner;

--pressure and displacement transducers;

--National Instruments PCI 6023E Data Acquisition Card DAQ;

--virtual instruments for DAQs.

The dies. The facility for ECAP processing the samples was based on a system of two symmetrical half dies, machined from two blocks of Romanian 165VMoCr115 steel, heat treated to achieve a nominal hardness of 60 HRC. Equal cross-section channel of 10x10 [mm.sup.2] were machined.

Previous practical experiences had shown that a calibrated length of 15 mm after the corner followed by an enlargement of the exit channel could reduce friction and thus lower the pressing load. The two half dies were assembled and held together with 3 bolts and 6 screws having a diameter of 12 mm.

In order to analyze the evolution of deformation forces with the angle die during ECAP two types of dies (900 and 1200) assembled on hydraulic press were used.

The die with angle of 1200 between channels is presented in Figure 3, b.

[FIGURE 3 OMITTED]

4. EXPERIMENTAL RESULTS

The evolution of deforming loads for different number of passes during ECAP process are presented in Figure 4 and 5:

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

In Figure 5 we can observe the decrease of forces thanks to the continuum reducing of friction in the first channel, so an experimental relation for tangential stress can be derived.

Summarizing the evolution of forces in ECAP process, it obtains the following curve (Figure 6).

[FIGURE 6 OMITTED]

The tangential stress is:

[tau] = [DELTA]F/A (3)

where [DELTA]F = [F.sub.1]-[F.sub.2] and A=4a([s.sub.2]-[s.sub.1]) is the contact area between die and workpiece in the first channel. So:

[tau] = [F.sub.1] - [F.sub.2] / 4a([s.sub.2] - [s.sub.1]) (4)

[F.sub.1], [F.sub.2], [s.sub.1], [s.sub.2] can be determinated from the experimental curves and a is the dimension of transversal section. If we consider the constant friction law [tau] = m x k, where 0 [less than or equal to] m [less than or equal to] 1 is a frictional factor and k = [[sigma].sub.Y]/2 (after Tresca), with relation (3) it is possible to determine frictional factor m. For aluminium [[sigma].sub.Y] = 100 MPa (after Segal), so for the first pass results m = 0,128 ... 0,132.

5. CONCLUSIONS

The evolution of deforming forces in ECAP process using data acquisition is presented. Considering the constant friction law, the tangential stress at the interface and the friction factor is experimentally determinated.

6. REFERENCES

Fukuda, Y.; Ohishi, K.; Horita, Z. & Langdom, T.G. (2002) Processing of a low carbon steel by equal-channel angular pressing, Acta Materialia vol. 20, 2002, pp. 1359-1368.

Furukawa, M.; Nemoto, M. & Horita, Z. (2001). Processing of metals by equal-channel angular pressing, Journal of Mat. Science, vol. 36, 2001, pp. 2835-2843.

Hartley, P.; Sturgess, C.E.N. & Rowe G.W. (1979). Friction in Finite-Element Analyses of Metalforming Processes, Int. J. of Mech. Science, vol. 21, 1979, pp. 301-311.

Hyoung, S.K. (2006). Analytical and Numerical Modeling of Strain and Strain Rate in Equal Channel Angular Pressing, Key Engineering Materials vol. 306-308, 2006, pp. 965-970.

Segal, V.M. (1995). Materials processing by simple shear, Mat. Sci. Eng, A 197, 1995, pp. 157-164.