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Characterization of Diamond Tools for Cutting Natural Stones Using Fe-Co-Cu Base Alloy as a Metal Matrix

Berrak Bulut, Onur Tazegül, Murat Baydoğan, Eyüp Sabri Kayalı

İstanbul Technical University - Türkiye

Abstract Diamonds are widely used in industrial applications due to their extremely high hardness. In the cutting operations of natural stone, diamonds are used to reinforce the metal matrix by forming a diamond doped metal matrix composite. During the cutting operation, diamond is getting worn by producing some wear products in the form of small fragments. These fragments then wear and cut the material. In this study, a Fe-Co-Cu based commercial powder was used as a diamond bead matrix and effect of bronze additions on mechanical properties of the matrix were also investigated. Powder was first cold pressed and then the pellets were sintered at 950°C for 1 h in hydrogen atmosphere in a tunnel furnace. Microstructural, physical and mechanical characterizations were performed by SEM-EDS examinations, density measurements, hardness and compression tests. Results showed that bronze additions only resulted in a small decrement in the relative density. However, hardness and compression strength were affected more by the increased amount of bronze addition into the matrix. 1. Introduction

Diamond, an allotropic form of carbon, is the hardest mineral known which is widely used in industrial applications due to its extremely high hardness [1]. Natural stone cutting with Diamond Cutting Tools (DCTs) is one of these applications. The DCTs’ segments are usually being produced by mixing diamond grit with the appropriate metal(s)/alloys powders via powder metallurgy processes [2,3]. Processing of diamond in metal bonds often results in a reaction between the diamond surface and the surrounding metal matrix. The

extent of this reaction depends on specific composition of metal powders, their particle size and distribution, and processing temperature and time [4].

The main factor determining the performance of the tool during service is its wear resistance. A number of operating conditions such as feeding rate, depth of cut, peripheral speed, load, pressure, velocity, cutting mode, rock properties, working conditions etc. govern the wear rate of DCTs [5,6]. Natural stones react differently and show different abrasiveness during cutting processes [7]. For this reason, it is very important to choose the right matrix material in the manufacturing process of DCTs. Depending on the properties of natural stone such as hardness and abrasivity, the metal matrix may contain some metals or alloys such as iron, cobalt, cobalt and tin, cobalt and bronze and tungsten carbide [8]. For a successful cutting operation, wear rates of the matrix and the diamond must be comparable. Otherwise, detachment of diamond from the matrix or embedding of diamond into the matrix may occur. In order to optimize the performance of the tool during cutting, type and amount of the additions are firstly determined by the hardness of natural stone to be cut [9,10].

There are number of studies that have investigated Fe-Co-Cu-Sn based diamond cutting tools. Zeren et al [11] studied sintering of polycrystalline diamond cutting tools. In that study, pressure-sintering experiments during direct sintering of diamond cutting tools were conducted at 730°C for 3–15 min with a pressure of about 350 MPa. For the matrix, cobalt and nickel are used as the bonding agents and the bronze was used as the filling phase in various amounts. They concluded that the matrix firmly supported the diamonds preventing them from damaging during cutting, and the bonding between the matrix and diamond must be strong enough for high performance.

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Selvi et al [12] studied conventional sintering of diamond cutting tools. They compared microstructural, physical and mechanical properties of diamond cutting tools made of Co-Ni-Cu-Sn matrix, produced by Spark Plasma Sintering and Conventional Sintering. Spark Plasma Sintering performed at 850 °C and Conventional Sintering performed at various temperatures (between 950 and 1150 °C) and times (1 to 4 hours). They determined the optimal sintering temperature and time for conventional sintering as 950°C and 3 h. Rosa et al [7] studied diamond-impregnated segmented discs used for hard stone cutting. Different types of matrices were used for manufacturing new segments for the cutting discs. Matrices containing: Co + Bronze (50 wt.%Co + 50 wt.% of a 90Cu–10Sn bronze); Co + bronze + 10 wt.% WC; and Co + bronze + 20 wt.% WC, were produced and their behavior was compared to that of a standard disc available in the market and commonly used by the stone industry for cutting granite. The commercial tool (Co + Bronze) showed the lowest values of resultant force and the highest values of tool consumption or tool wear rate.

The aim of this study is to evaluate the performance of the Fe-Co-Cu based matrix as a diamond bead and the effect of bronze additions on the structural and mechanical properties of the matrix.

2. Experimental Procedure

Commercial free sintering metal powder (W1) with a particle size of 13.62 m was used as matrix material. 30% and 50% bronze (90% Cu+ 10% Sn) by volume were added to W1 powder, which were designated as W1-30 and W1-50, respectively. Chemical composition of W1 powder is given in Table 1. The metal powders were mixed in 360° rotating chamber for 3. The powder mixtures were compacted in a uniaxial cold press by the application of 50 kN force. Green samples were prepared in 7 mm in diameter and 14 mm in length. Conventional Sintering was performed in a continuous tunnel furnace at 950°C under hydrogen atmosphere for 1 h.

Density measurements, hardness and compression tests were performed for physical and mechanical characterization of the samples. Density of the samples was determined by the Archimedes method and divided by the theoretical density to calculate the relative density. Hardness measurements were carried out in the Rockwell

B scale by using Zwick/Roell ZHR hardness tester and the compression tests were carried out using Dartec Universal Testing Machine at a crosshead speed of 1 mm/min. X-ray diffraction (XRD) analysis was carried out by a Bruker D8-Advanced X-ray diffractometer using Cu K radiation. Microstructures of the samples were examined by LeicaTM DM 750M optical microscope and HITACHI TM-1000 Tabletop Scanning Electron Microscope (SEM) equipped with an EDS (Energy Dispersive spectroscopy unit.

Table 1. Chemical composition of the W1 powder. Chemical Composition (wt %)

Fe Co Cu Sn Ni W 40 30 25 3.9 0.5 0.6

3. Results and Discussion

Figure 1 shows XRD patterns of the samples investigated. Identified phases for W1 sample which were sintered at 950°C for 1 h under hydrogen atmosphere are mainly Co3Fe7 and CuSn. By the addition of 30% and 50% of bronze into the matrix powder, the identified phases were essentially the same (Co3Fe7 and CuSn) except some variation in the relative intensities amongst the samples.

Figure 1. XRD patterns of the samples.

Figure 2 shows SEM micrographs of the samples investigated (Figs. 2a-c) and the corresponding EDS

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patterns of the samples (Fig. 2d-f). EDS analysis revealed that the micrograph of sample W1 mainly consists of Fe and Co coming from Fe-Co intermetallic phase as shown in Fig. 1. By adding bronze into the powder, fraction of white regions increased. According to the EDS analysis, the white areas in the SEM micrographs of the samples are bronze-rich regions (Fig. 2a-f). The white areas increased with increasing bronze addition to the matrix (Figs 2a-c) and they are homogeneously distributed in the structure.

Table 2 lists relative density, hardness and compression strength of the samples investigated. The highest relative density was obtained in W1 samples. By adding increment amount of bronze into the powder composition, relative density slightly decreased. Beside density, hardness and compression strength were also the highest in W1 sample. Increasing amount of bronze addition led to more significant decrement in hardness and compression strength than that observed in the relative density. This result is in contrast to the previous study investigated the diamond cutting tools with bronze addition. In this work [13], it was reported that bronze addition resulted in decrement in mechanical properties due to cumulative bronzes in certain regions. However, in

the present work, bronze islands were not in the form of cumulative but uniformly distributed as shown in Fig. 2b and c. Finally it is reasonable to assume that uniformly distributed bronze islands as a softer component in the microstructure decreased hardness and compression strength. Table 2. Hardness and compressive strength values of the samples after sintering.

Samples Relative density,

%

Hardness (HRB)

Compressive Strength (MPa)

W1 98.43 97±0.6 1293±3 W1-30 97.39 86±1.2 1024±3 W1-50 97.30 80±1.7 976±3

In order to evaluate the cutting performance of diamond bead produced by using the matrix material studied in this study is under investigation with cutting marble to test its performance in on-site field cutting operations. It is expected that as the matrix material is softer, the better is the cutting performance due to reduced abrasion resistance in diamond containing cutting tools.

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Figure 2. SEM Microstructures and EDS analysis of sintered samples (a) W1 (b) W1-30 (c) W1-50 (d) EDS analysis of W1 (e) EDS analysis of W1-30 (f) EDS analysis of W1-50. 4. Conclusion

Following results can be inferred from experimental results; 1. The highest relative density, the highest compressive strength and the highest hardness values were obtained in the sample W1. 2. The presence of Co3Fe7 intermetallic phase give rise to mechanical properties in W1 sample. 3. The addition of bronze to W1 sample caused decreases in the mechanical properties (compressive strength and hardness). 4. It is expected that softer matrix with bronze addition to W1 sample may result better cutting performance due to reduced abrasion resistance of matrix, as a result of optimization of wear rates of diamond and matrix in diamond containing cutting tools. References [1] J. Konstanty, Powder Metallurgy Diamond Tools, Elsevier Science Technology, 2006, United Kingdom. [2] A. D. Skury, G. S. Bobrovnitchii, M.G. Azevedo and S. N. Monteiro, Materials Science Forum, 727-728 (2012) 305-309. [3] Z. Nitkiewicz and M. ´Swierzy, Journal of Materials Processing Technology, 175 (2006) 306–315.

[4] S.Y. Luo, Y.S. Liao, J Mater Process Technol, 5 (1995) 296-308. [5] A. Ersoy, S. Buyuksagic and U. Atici, Wear, 258 (2005) 1422-36. [6] W. Tillmann, C. Kronholz, M.Ferreira, A. Knote, W. Theisen and P. Schütte, International Journal of Powder Metallurgy, 47 (2011) 29-36. [7] L.G. Rosa, J.C. Fernandes, C.A. Anjinho, A. Coelho and P.M. Amaral, Int. Journal of Refractory Metals and Hard Materials, 49 (2015) 276–282. [8] H.C.P. Oliveira, S.C. Cabral, R.S. Guimarães, G.S. Bobrovnitchii and M. Filgueira, Materwiss Werksttech, 40 (2009) 907–909. [9] T. Kagnaya, C. Boher, L. Lambert, M. Lazard and T. Cutard, Wear, 267 (2009) 890-897. [10] N.B. Dhokey, K. Utpat, A. Gosavi and P. Dhoka, Int. Journal of Refractory Metals and Hard Materials, 36 (2013) 289–293. [11] M. Zeren and S. Karagoz, J Mater Des, 28 (2007) 1055-1058. [12] E. Selvi, F. Topaloglu, O. Tazegul, E.S. Kayali, Conventional Sintering of Diamond Cutting Tool Used in Natural Stone Cutting, (APMAS), 24-28 April 2013, Antalya, Turkey. [13] Y. Kahraman, Free Sintering of Matrix Materials Which are Used for Diamond Cutting Tools, M.Sc. Thesis, Istanbul Technical University, 2014, Istanbul, Turkey.