Axisymmetric deviation influence on structural and mechanical characteristics of some copper semifinished products.

Ghiban, Nicolae ; Serban, Nicolae ; Ghiban, Alexandru 等

1. INTRODUCTION

Nowadays, the equilibrium of flowing speeds and the control of the deformation nonuniformity in order to obtain quality products, in the case of extrusion process of the non-rounded products, there are matters of experience. The projecting and the technological practice activities are still made in an empirical way, using the results obtained from the experience of some manufacturers, and also the ones that are obtained from industrial tests, using the "step by step" technique, (Altan et al., 1994).

The projecting of the die, in the case of extrusion process of the non-rounded products, is not only an operation of placing the die cavity (porthole) inside the block, (Ulysse, 2002), but also an operation of establishment of the geometrical and technological parameters correlation which determinates the materials flow dynamics in the analyzed case study, (Zienkiewicz & Taylor, 2000), (Hartley et al., 1992).

The purpose of this paper is to show, by different methods, experimental as well as theoretical, the problems still not fully solved in the matter of extrusion of the non-rounded products. Also, the authors tried to establish some correlations between technological parameters by which one can operate in a practical, easy and efficient way, in comparison with other researchers (Altan et al., 1994), (Ulysse, 2002).

2. MATERIALS AND RESEARCH METHOD

The experimental researches were made with a hydraulic press of 1000 tones force, which is usually used for the extrusion of brass, bronze, copper and copper--chromium bars, and which possesses an horizontal design and can work with two or more profiles simultaneously. For the extrusion of the analysed profiles, the following billet dimensions were used: for brass [PHI] 174 x 400 mm, and for copper [PHI] 145 x 300 mm. For the billets extrusion, the tools were heated at a temperature of 250-280[degrees]C, and after the end of the extrusion process their cooling was made slowly for avoiding material cracking. The billets heating was made in an induction furnace of low frequency, as follows: the brass billet was heated at 850[degrees]C and the copper billet at 950[degrees]C.

[FIGURE 1 OMITTED]

From the non-rounded extruded profiles (figure 1, a and b) there were made samples for mechanical tests at environmental temperature, (Ghiban, 1997).

The mechanical tests were made using a sensitivity of 1%. The yield strength was measured using an extensometer on the samples, with an accuracy of the measurement of 0.01%. The tensile strength, the yield strength and the elongation were measured.

Metallographic analysis was made using an Reichert microscope at magnifying powers of 500-2000 times, in the section of the extruded samples, using an ammonium persulphate solution for the metallographic attack.

3. RESULTS AND INTERPRETATION

In order to highlight the influence of incorrect positioning of the die cavity (deviation) on the structure and on the mechanical properties of the extruded products, there were made experimental researches with three positions of the die cavity for each profile, as follows: for the brass profile, there were made experiments with the deviations [y.sub.1] = -30 mm (non-optimum position), [y.sub.2] = 0 (optimum position) and [y.sub.3] = +10 mm (non-optimum position of the die cavity); and for the copper profile, with the deviations [y.sub.1] = -40 mm (non-optimum position), [y.sub.2] = 0 (optimum position) and [y.sub.3] = +30 mm (also a non-optimum position of the die cavity), (Ghiban, 1997), (COSMOS, documentation, 1997).

The samples cuted from each profile were utilized in order to highlight the microstructural transformations which depend on the extrusion conditions. As can be seen from figure 2, the brass samples have a dual phase structure, made of a mixture of [alpha] solid solution (Zn in Cu solid solution, with a FCC lattice) and [beta]' solid solution (a solid solution based on the electronic compound CuZn, with an electronic density of 3/2, and with a BCC lattice). The dual phase structure of brass is highly influenced by the extrusion conditions: in the case of an optimum position of the die cavity (no deviation, figure 2, a), it can be seen an homogeneous distribution of the two solid solutions; as the deviation grows, a nonuniform distribution of the two phases is observed, and even a Widmannstatten structure in some areas (figure 2, b).

[FIGURE 2 OMITTED]

The copper samples have a monophase structure, with a FCC lattice, mackled with distinct color shades grains, figure 3, a and b. The monophase structure of the copper samples (oxidized) is also highly influenced by the extrusion conditions; the medium conventional diameter of the grain being dependent of the values for the deviations from linearity and coaxiallity (misalignment); while for the samples without deviation, the medium conventional diameter is 0.0315 mm, at a deviation of y = +30 mm the diameter is 0.0287 mm, and for y = -40 mm the inhomogeneous granulation generated an medium conventional diameter of 0.0268 mm.

[FIGURE 3 OMITTED]

The variation of mechanical characteristics during extrusion with different values of deviation is suggestively given in the figure 4 (a, b, c for the brass samples) and figure 5 (a, b, c for the copper samples).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

4. CONCLUSIONS

The microstructural analysis showed the dependence of structural modifications on the extrusion conditions. In the case of brass samples, the dual phase structure of brass is described by a uniform distribution of the two solid solutions (when the deviation is zero) and by a nonuniform distribution of the phases, with an Widmannstatten aspect (for both samples with deviation). In the case of copper samples, the structure is homogeneous (with the medium conventional diameter of 0.0315 mm when the deviation is zero) and inhomogeneous (with the medium conventional diameter of 0.0268 mm when the deviation is y = -40 mm, respectively with the medium conventional diameter of 0.0287 mm when the deviation is y = +30 mm).

The analysis of mechanical characteristics values showed that a correlation may be described between linear deviation--coaxiallity and the mechanical characteristics of the semifinished products; when the linear deviation grows it also grows the tensile strength and the hardness, but the plasticity decreases in value. In the case of brass samples is trot out a growth of the tensile strength and hardness values of approximately 5% and a decrease of elongation of approximately 5.5% for a deviation y = +10 mm and a growth of the same characteristics with approximately 10% when the deviation is y = -30 mm. In the case of copper samples, the mechanical characteristics modification is 12-14% when the deviation is y = -40 mm, and approximately 10-11% when the deviation is y = +30 mm.

5. REFERENCES

Altan, T.; Oh, S. & Gegel, H. (1994). Metal Forming--Fundamentals and Applications, ASM, 1994

COSMOS M 1.65 (1997). Documentation

Ghiban, N. (1997). Studies and experimental researches about the non-rounded extruded profiles, Doctoral Thesis, University Politehnica Bucharest

Hartley, P.; Pillinger, I. & Sturgess, C.E.N. (1992). Numerical Modelling of Material Deformation Processes, Springer--Verlag

Ulysse, P. (2002). Extrusion die design for flow balance using FE and optimization methods. International Journal of Mechanical Sciences, Vol. 44 (2002), pp 319-341

Zienkiewicz, O.C. & Taylor, R.L. (2000). Finite Element Method, Elsevier Butterworth-Heinemann, ISBN 0-7506-5049-4