Turning of Compacted Graphite Iron using commercial TiN coated Si3N4

under dry machining conditions

J.V.C. Souza1, M. C.A. Nono1, M. V.Ribeiro2, O.M.M. Silva3, M.A. Lanna4

1INPE - Av. dos Astronautas,1.758, S. J. Campo s - SP, CEP. 12245-970, Brazil

2FEG-UNESP – Av.Ariberto Ferreira da Cunha, 333, Guaratinguetá – SP, CEP. 12516-410, Brazi

3CTA-IAE/AMR - Pça. Marechal do Ar Eduardo Gomes, 50 – V. Acácias, S. J. Campo s - SP, CEP. 12228-904, Brazil

4IFI - Pça Marechal do Ar Eduardo Gomes, 50 - Vila das Acácias, S. J. Campos – SP, CEP: 12228-901 - Brazil

vitor@las.inpe.br / candidojvc64@yahoo.com.br

Keywords: Cutting tool coating; Surface roughness; Temperature; Compacted graphite iron.

Abstract. Due to their high hardness and wear resistance Si3N4 based ceramics are one of the most suitable cutting tool materials for machining hardened materials. Therefore, their high degree of brittleness usually leads to inconsistent results and sudden catastrophic failures. Improvement of the functional properties these tools and reduction of the ecological threats may be accomplished by employing the technology of putting down hard coatings on tools in the state-of-the-art PVD processes, mostly by improvement of the tribological contact conditions in the cutting zone and by eliminating the cutting fluids. However in this paper was used a Si3N4 based cutting tool commercial with a layer TiN coating. In this investigation, the performance of TiN coating was assessed on turning used to machine an automotive grade compacted graphite iron. As part of the study were used to characterise the performance of cutting tool, flank wear, temperature and roughness. The results showed that the layer TiN coating failed to dry compacted graphite iron under aggressive machining conditions. However, using the measurement of flank wear technique, the average tool life of was increased by Vc=160 m/min.The latter was also observed using a toolmakers microscope and scanning electron microscopy (SEM).

1. Introduction

The dynamic development in the engineering and technology domains gives the reason to increase the requirements posed to sintered tool materials in regard to their mechanical properties and abrasion wear resistance. Functional properties of many products and their elements depend not only on their capability to carry the mechanical loads by the elements entire cross-section from the material used or on its physical and chemical properties, but very often or mostly on its structure and properties of its surface layers. The contemporary technologies of materials forming employed in the machining, plastic forming, casting, and also plastics forming domains call for using more and more efficient tool materials. These requirements pertain mostly to the extension of life and reliability of tools used in machining processes [1-6]. In machining process the surfaces of Si3N4

cutting tools need to be abrasion resistant, hard and chemically inert to prevent the tool and the work material from interacting chemically with each other during machining. This development was regarded as a significant advance in cutting tool and environmental technology. Therefore the coated are important alternative to Si3N4 cutting tools. Coated are formed basically of one or more thin layers of wear-resistant material, such as titanium nitride (TiN) and others [7]. The increase in

environmental pollution and problems relating to waste disposal of cutting fluids is forcing manufacturers to reduce the volume of oil-based cutting fluids used in large-scale metal cutting operations even to the point of machining dry. For the latter to become widely adopted in industry as a commercially viable alternative to wet machining then the main advantages of cutting fluids, namely, they act as coolants, lubricants and transfer medium for chip removal, need to be obtained in other ways. These advantages can be achieved, in part, by the exploitation of advances in cutting tool materials and surface coatings. The latter have provided some of the most significant advances in dry machining in recent years, particularly through the introduction of physical vapour deposition (PVD) coatings [8].In recent studies have focused on PVD coatings for reducing the volume of cutting fluid used to machine quenched and tempered steels, there is also interest in the dry machining performance of cutting tools used to machine grey cast iron and compacted graphite iron [9-11]. The latter continues to be a material of choice in the automotive industry for cylinder block production in the popular car and truck markets because of its low cost, ease of casting and medium level of machinability. The relatively poor or medium machinability is generally due to the chips are not continuous, the length of contact on the rake face is short and the cutting force and power consumption are low [12]. Clearly, there are strong economic incentives to explore the capabilities of PVD coatings in the dry machining of compacted graphite iron. The aim of this investigation was to conduct turning tests on compacted graphite iron (CGI), which are characterized by a high work hardening rate, high ductility and high thermal conductivity. This paper concentrates on the influences of TiN coated Si3N4 tool, cutting speed, workpiece surface roughness and temperature. It is well known that coatings can reduce tool wear and improve tool life and productivity [13]. Therefore, most of the tools used in the metal cutting industry are coated though coating brings about an extra cost [14].

2. Experimental procedure

2.1. Experiment specimens

The goal of this experimental work was to investigate the effects of cutting parameters on tool wear, temperature and surface roughness, and to establish a correlation between them. In order for this, cutting speed, feed rate and depth of cut were chosen as process parameters. The work material was compacted graphite iron (CGI) in the form of round bars with 83 mm diameter and 372 mm cutting length. This material has had tensile strength of 500 MPa, elastic modulus of 140 GPa, thermal conductivity of 35 W/m-K and 225 BHN. Chemical composition of work material is given in Table 1. The microstructure of the compacted graphite iron is showed in Fig. 1. The matrix is ferrite, with compacted graphite and a few pearlite grains with very fine pearlite.

Table 1. Chemical composition of compacted graphite iron.Element C Si Mg Cr Mn S Ni P Cu Sn Ti

Amount (%) 3.62 2.47 0.09 0.06 0.16 0.02 0.09 0.03 0.72 0.08 0.012

Fig. 1. Microstructure of the compacted graphite iron.

2.2. Machining condition and equipments

The turning tests were conducted in dry conditions on a computer numerical control lathe (CNC - Romi, Mod. Centur 30D). The cutting tool used was TiN coated Si3N4 tool with an ISO designation of SNGN120408 (Fig. 2).The inserts were clamped onto a tool holder of CSRNR 2525 M 12CEA type was used for the cutting experiments. Three levels were specified for each process parameter as given in Table 2. The parameter levels were chosen within the intervals recommended by the cutting tool manufacturer and literature. Flank wear of the insert was measured using a toolmakers microscope. The surface roughness of the machined work piece was measured using a surface roughness meter with a cut-off length of 0.8 mm and sampling length of 5 mm (Mitutoyo Surftest 402 series 178). To measure at temperature work-piece/cutting tools was used an infrared pyrometer.

Table 2. Assignment of the levels to the factorsLevel Cutting speed Vc (m/min) Feed rate f (mm/rev) Depth of cut ap (mm)

1 160 0.50 0.20

2 180 0.50 0.20

3 200 0.50 0.20

Fig. 2. Cutting tool geometry

3. Results and discussions

3.1. Tool life

At each set of cutting conditions, tool wear was observed. Visual examination suggested that TiN coating has been removed of the cutting edge and rake face partially. This is due to stress concentrations that lead to a cohesive failure on the transient filleted flank cutting wedge region. In the Fig. 3, there was a gradual reduction in tool life with increasing cutting speed to 200 m min−1. The flank wear has been defined as a function of cutting length. We find the classical process of tool wear which followed two stages, rapid initial wear, followed by gradual wear. The tool life has been determined as the elapsed time before the flank wear reaches the allowed limit equal to the criterion (ISO 3685). The flank wear value shows a declining of the tool life with increase in cutting speed. At a cutting speed equal to 160 m/min tool life is, respectively, 8.98 min. When the cutting speed is equal to 180 m/min, the tool life is, equal to 7.60 min. For the cutting speed equal to 200 m/min, time decreases to reach 4.71 min. These differences in cut time were associated at TiN coated, that in lower cutting speed receive abrasive impact of the hard abrasive particles to presence in the matrix of the compacted graphite iron. When there is a lubricant film (TiN) on the wear surface, the matrix endures the load, and friction occurs on the lubricant film. Therefore we can view which at increase in cutting speed indicate that the interaction of process energies, including temperature (Fig 4), increases generally and can therefore promote various wear mechanisms, which accelerate tool wear with the consequent reduction in tool life. It is closely associated with each other in the case of machining compacted graphite iron material. In lower cutting speed the workpiece/tool interface is more stable. At lower cutting speed, coated becomes stronger than that formed at higher cutting speeds. At higher cutting speeds, cutting zone temperature increases and this, in turn, softens and decreases strength of coated. This induces less adhesion of coated on the cutting edge and thus high wear of cutting tool is observed during cutting. However, with further increase in cutting speed (200 m/min), the temperature of the cutting zone increases to a level at which cutting edge looses its strength. It is seen from these images that wear predominantly occurred in two regions during the tests: at the rake surface and the cutting edge (Fig 5). For obtain a low tool wear are necessary a synergy among tool, workpiece materials and machining conditions. However this work the combination these data with that of the tool life, we can conclude that the lubricious/protective TiN film the tool surface during cutting to cutting speed equal to 160 m/min.

Fig. 3 Flank wear vs cut length Fig. 4 Temperature vs. cut length.

3.2. Mechanisms of wear

For all cutting speeds in machining compacted graphite iron, the tribological phenomenon of seizure occurs, causing thermoplastic shear of the tool/chip interface. Seizure leads to atomic transfer at the tool/chip interface resulting in dissolution wear. Therefore there are mechanical and dissolution wear at each cutting speed for TiN coated Si3N4 tool. Tool wear was measured after each step machining. The effect of cutting speed is to increase the total wear as well as the percentage of dissolution wear. Toolmakers microscope image of the crater formed from dissolution wear is shown. The crater occurs at a distance away from the cutting edge. There is few wear on the flank face (Fig 5). Thus, mechanical wear remains constant as dissolution wear increases with cutting speeds. The increase in dissolution wear correlates with increase in the tool/chip interface temperature as the cutting speed is increased (Fig 4). There is a strong influence of temperature on dissolution wear rates. However, can be possible make a correlation between the crater wear rate and the solubility of the tool material in the workpiece. Thus, we can conclude the diffusion hypothesis for crater wear and mechanical abrasion to flank wear of cutting tools applies under machining conditions. Consequently the observed increase in the percentage of dissolution wear with cutting speed is attributed to the increase in the equilibrium solubility and diffusion coefficient with temperature rise. Conform (Fig 5) which shown the geometric crater, we can deduce that TiN coated determines where the chip leaves the tool rake face, effectively controlling the tool/chip contact. Agreement Shewmon (1989), we should be concluded that the coating decrease the tool/chip interface temperature by reduction in contact and cutting forces through change in the tribological condition at the tool–chip interface [15].

Fig. 5. Micrographs of wear profile to cutting tools: (a) Vc= 160 m/min; (b) Vc= 180 m/min and (c) Vc =200 m/min.

3.3. Workpiece surface roughness

In the Fig. 6 and 7 shown the workpiece surface roughness values for compacted graphite iron. These values are the averages of three readings. It is seen from Fig. 6 and 7 that cutting speed had a significant influence on the surface roughness produced. This figures shows that the highest surface roughness values are observed at 200 m/min cutting speed. The general trend in the curves in Fig. 6 and 7 is that when cutting speed is increased the surface roughness values increase until a minimum value of 3.45 mm. The increase in surface roughness with increasing cutting speed 200 m/min can be attributed to the increasing cutting tool nose wear, which is the highest cutting

1 mm 1 mm 1 mm

speed employed. However, wear at the cutting edge directly influences the machined surface roughness since the edge is in direct contact with the newly machined surface.

Fig. 6. Surface roughness vs. cut length. Fig. 7. Surface roughness vs. cut length.

4. Conclusions

Turning tests were performed on compacted graphite iron (CGI) using commercial TiN coated Si3N4 cutting tool. Based on the results obtained, the following conclusions can be drawn: — Cutting speed was found to have a significant effect on the flank wear and machined surface roughness values. With increasing cutting speed, surface roughness values increased until a minimum value was reached of Ra=3.45 mm and Ry=23.91mm.— The test results show that acceptable tool life can be achieved when machining compacted graphite iron (CGI) with TiN coated Si3N4 cutting tool with sharp edges using cutting speed and feed rate to 160 m min−1 and 0.20 mm rev−1, respectively, which gave lower cutting forces due to a lower friction coefficient of the TiN top coating layer.— Increase in cutting speed promote the interaction of process energies, including temperature, which in turn accelerates various wear mechanisms, resulting in more severe tool wear and reduced tool life. 5. References[1] L.A. Dobrzański, Fundamentals of Materials Science and Physical Metallurgy. Engineering Materials with Fundamentals of Materials Design, WNT, Warszawa (2002) (in Polish).[2] Y. Sahin and G. Sur, Surf. Coat. Technol. 179 (2004), pp. 349–355.[3] K. Gołombek, L.A. Dobrzański and M. Soković, J. Mater. Process. Technol. 157–158 (2004), pp. 341–347.[4] M. Wysiecki, Contemporary Tool Materials, WNT, Warszawa (1997) (in Polish).[5] Z. Peng, H. Miao, W. Wang, S. Yang, Ch. Liu and L. Qi, Surf. Coat. Technol. 166 (2003), pp. 183–188.[6] L.A. Dobrzański, D. Pakuła and E. Hajduczek, J. Mater. Process. Technol. 157–158 (2004), pp. 331–340.[7] E.P. DeGarmo, J.T. Black and R.A. Kohser, Mater. and Process. in Manuf., Prentice-Hall Inc., New Jersey (1997).[8] W.-D. Münz, Titanium aluminium nitride films: a new alternative to TiN coatings. J. Vac. Sci. Technol. A 4 (1986), p. 2717.[9] H.K. Tönshoff and A. Mohlfeld, PVD-coatings for wear protection in dry cutting operations. Surf. Coat. Technol. 93 (1997), pp. 88–92.[10]. C.E. Bates and E. Eleftheriou, Effects of inoculation on machinability of gray cast iron. Trans. Am. Foundrymen’s Soc. 107 99–122 (1999), pp. 659–669.[11] J. V. C. Souza, S. J. Crnkovic, C. A. Kelly, M. R. V. Moreira, M. V. Ribeiro and O. M. M. Silva, Important aspects on ceramics cutting tools use during machining process, In: 1st International Congress University Cooperation (UNINDU), Brasil (2005).[12] R.O. Marwanga, R.C. Voigt and P.H. Cohen, Influence of graphite morphology and matrix structure on chip formation during machining of gray irons. Trans. Am. Foundrymen’s Soc. 107 99-080 (1999), pp. 595–607.[13] E.O. Ezugwu and C.I. Okeke, Tool life and wear mechanisms of TiN coated tools in an intermittent cutting operation, J Mater Process Technol 116 (2001), pp. 10–15.[14] A. Sharman, R.C. Dewes and D.K. Aspinwall, Tool life when high speed ball nose end milling Inconel 718™, J Mater Process Technol 118 (2001), pp. 29–35.[15] P.G. Shewmon Diffusion in Solids (second ed ed.), TMS Publication (1989).