Considerations on the metallographic profile of parts made of hard sinterized alloys subjected to electrical discharge machining.

Petrescu, Valentin ; Nemes, Toderita ; Bibu, Marius 等

Abstract: The paper presents some experimental tests on hard sintered alloys, G40 type of metal carbide material during the electric discharge machining process. Metallographic analysis was made to show the cracks from the superficial layer and the influence of the pulse period over the nature of the material during the EDM process. The results showed that for the hard sintered alloys processing the mechanism of the material processing is influenced not only by the thermodynamic processes but also by the cracks in the superficial layer.

Key words: EDM, hard sinterized alloy, metallography.

1. THE EXPERIMENTAL MEASUREMENTS

The experiments have been realized with aluminum samples of hard sintered alloys G40 (20% Co, 50% CW). These experiments have been realized with an EDM installation called ELER-01 and GEP-50F. Electrolytic copper electrodes with a cylindrical pipe shape and the outer diameter [f.sub.e]=10[+ or -]0,1 mm have been used, as shown in figure 1.

The dielectric liquid used was the winter diesel oil 15A STAS 240-88. The liquid's pressure was 0.1 Barr, its increasing value making an easy phenomenon of cleaning the working surface of the processed particles, which influenced in a negative manner the discharge phenomena and increasing the wearing process (Petrescu, 1994; Petrescu, 1995).

The samples have been processed in a coarse regime as showed in figure 2 and have been sectioned before the preparing for the metallographic analysis.

The samples were analyzed macroscopically with a stereo microscope.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The machining processes have been realized with the specific parameters as shown in table 1, during the working regimes, for both coarse and finishing regimes.

For the coarse regime the G40 type, which has a large quantity of binder and a lower hardness, shows an increase area for every crater with a flattening tendency of its shape (figures 3 a, b, c and d). For the toughest processing regime (I = 50A, [t.sub.i] = 95is) a delimitation tendency of the bounding appears and an increase density of the fine composed particles over the processed surface (figures 3 c and d) (Petrescu, Bibu & Deac, 2004).

Starting from the fact that every material has a unique characteristic of stability during the EDM process, for the hard sintered alloys processing it can be observed a selective characteristic of the process, which shows the binder's (Co) erosion with a lower stability than the carbide (CW).

For the finishing regime it can be observed an increase of crater depth after the pulse period of 8is and a current of 6,25 A (figures 4 a and 4 b, compared with figures 4 c and 4 d).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The macroscopic examination of the processed surfaces shows that the material processing is realized through a discontinuous and cumulative process. The shape of the processed surfaces by EDM process is characterized by a complete isotropy comparing with the cutting process, which induce a preferential direction. The roughness and the orientation of the micro lattices of a processed surface by EDM process will be identical on all directions (Konig, 1974; Nemes & Petrescu, 2004).

Some of the processed samples for the different working regimes have been transversal sectioned and analyzed by microscopic analysis.

After the WEDM processing on an ELEROFIL machine, the samples have been rectified on the surfaces needed to be prepared for the microscopic analysis then treated with a water-repellent metallographic paper with different graining.

Next, the samples have been polished with a diamond paste, a special technique used for a cleaner surface then mechanical polished and chemical attacked with Murakami reactant and metallographic analyzed with an EpiTyp2 microscope.

It was proved the existence of a superficial surface layer affected by the EDM process, which increases its depth with the increasing of the values of the EDM processing parameters. This layer appears because of the melting process and the recrystallization process of the melted material remained inside the crater.

The interaction between the work environment and the processed material establishes, based on the microstructure shape, the modifications of the carbide nature in the superficial surface layer compared with the core material of the samples.

The coarse process revealed deeper layers than the finishing process, as showed in figure 5 a, compared with figure 5 b.

Exceeding certain values of the coarse processing lead to the appearance of tiny cracks into the margin layer, orthogonal oriented on the processed surface (figure 5 c), the arc ignition appear usually because of the existent pores into the hard sintered alloys.

During the finishing regime no defects like tiny cracks into the superficial layer were found on the processed samples (figure 5 b).

[FIGURE 5 OMITTED]

The depth of the tiny cracks will increase with the increase of the pulse period. During the 12is pulse periods could not be detected any defect, not even at 1000 x magnification.

The electric discharge caused the heating of some portions of material from the surface layer, which tends to expand. This tendency is blocked by the cold layers of material. This way into the heated volume of material appears recompression strains. When the current pulses stops the heated material is cooled down by working space, which determines the volume's decrease. Surrounding cold layers oppose this tendency and as result in the surface layer appears stretching strains which break the binder determining the appearance of the cracks in the superficial surface layer.

The dependence of the tiny cracks presence and the pulse period is determined by the proportion between the dimensions of the melting zone and vaporization zone of material under the influence of electric discharge. Long pulses lead to large melted areas, the heat propagating into a large volume of material. For short pulses the materials are processed mainly by a vaporization process, the heat didn't have the time to propagate into the material and presenting a limited heating process in a small volume of material.

2. CONCLUSIONS

For the hard sintered alloys processing the mechanism of the material processing is influenced not only by the thermodynamic processes but also by the cracks in the superficial surface layer, which helps the detachment of solid particles. The shape of the processed surface, characterized by a complete isotropy, is determined by the applied working regime. The depth of the superficial surface layer depends also on the working regime. The depth of the cracked layer depends on the pulse energy, the most powerful influence having the pulse period.

3. REFERENCES

Konig, W., Wertheim, R. & Weiss, A. (1974) Funkerosive Bearbeitung fur Hartmetall, Westdeutscher Verlag GmbH, Oplader.

Petrescu, V. et al. (1994) Studies concerning the machining through electro-discharge machining of metallic carbides, Proceedings of "MATEHN '94" conference, Cluj-Napoca.

Petrescu, V. (1995) Researches on the processing of metallic carbides by electro-discharge machining, Proceedings of "TEHNO '95" conference, Timioara, 1995.

Nemes, T. & Petrescu V. (2004) Materials Technology (in Romanian). Publishing House of the Lucian Blaga University, Sibiu.

Petrescu, V., Bibu, M., Deac, C. (2004). The optimisation of the electrod discharge machining process for parts of metallic carbides. Proceedings of the 13th International Metalurgical & Materials Conference METAL 2004, Ostrava, ISBN 80-85988-95-X

Table 1. Processing parameters

Processing Metal-Carbide current pulse period
regimes type [A] [[micro]s]

Coarse G40 25 95
 50 95
Finishing 6,25 8
 12,5 8