Elsevier

Journal of Manufacturing Processes

Volume 73, January 2022, Pages 895-902
Journal of Manufacturing Processes

PVD multi-coated carbide milling inserts performance: Comparison between cryogenic and dry cutting conditions

https://doi.org/10.1016/j.jmapro.2021.11.033Get rights and content

Abstract

This study experimentally investigates the influence of cryogenic CO2 and dry cutting conditions on the performance of multi-coated PVD tungsten carbide ball nose inserts during high-speed milling of Inconel 718. The areas of focus were the tool life, surface roughness and wear mechanisms. For the cryogenic CO2, a new cooling method approach was applied for a consistent cooling flow deep into the cutting point. The results revealed that the inserts under cryogenic conditions outperformed those under dry cutting where it was possible to improve tool life and surface roughness by an average of 75% and 142.8% respectively. It was also evident that the multicoated tools experienced a hardening effect under cryogenic cutting which increased its hardness as well as its wear resistance. Both cutting conditions showed similar tool failure modes which were dominated by notching and build-up-edge (BUE) on the flank face, as well as flaking on the rake face. However, dry cutting generated severe BUE which accelerated the notching and flaking, as well as the wear rate and caused the inserts to reach the end-of-life in a shorter time. The EDAX element mapping and SEM analysis on the worn inserts proved that adhesion, abrasion and coating delamination were the tool wear mechanisms under both conditions. Yet, the EDAX spectrum detected the presence of the oxygen (O) element which confirmed that the inserts experienced oxidation wear during the dry cutting process and caused them to weaken prematurely. The findings could provide other insights into different approaches of cryogenic cooling applications and their impact in high-speed cutting.

Introduction

High-speed cutting of difficult-to-cut materials such as Inconel 718 using uncoated carbide tools has frequently resulted in rapid tool wear and poor surface finish. Without coating materials, the tools do not have sufficient mechanical, chemical and thermal resistances to withstand higher heat and friction which are abundantly generated. Thus, the tools reach their end-of-life in a shorter time. Consequently, coated carbide tools are more preferred as they offer 100% higher hardness than uncoated tools [1]. Until now, almost 80% of cutting tools used by industries are of the coated type. According to Minato, about 90% of them are tungsten carbide tools as they are highly recommended for cutting ferrous and non-ferrous metals at high-speed [2]. Their superior wear resistance and tribological behavior at high cutting temperatures permits extreme cutting conditions. As previously reported by Ezugwu [3], machining of super alloys can be performed at higher speeds of between 30 and 100 m/min by coated carbide tools, while at only 10–30 m/min by uncoated tools. More recently, Chowdhury et al. [4] reported that monolayer coated carbide tools diminish the intensity of crater wear and prolong tool life longer as compared to uncoated carbide tools. Accordingly, material removal rate can be increased for more economical machining processes.

Generally, coating materials are deposited onto carbide tools to improve wear resistance, increase hardness and reduce friction, which instantaneously reduces the cutting temperatures [5]. Since most machining outputs are temperature dependent, thus the overall machining performance is significantly improved as well. As found by Sharman et al. [6], analysis of variance (ANOVA) confirmed coating materials as the dominant factor that influences and improves tool life of high-speed cutting Inconel 718. To further enhance the mechanical and tribological properties of single-coated carbide tools, multi-coated tools which have been layered with two or more coating materials have been introduced. Thin hard layers are deposited at micro-meter or nano-meter scale alternately until they obtain the total thickness of single-coated tools. Different coating materials are available to provide different cutting tool properties as well as different cutting performances as previously reported. For instance, the longest tool life was obtained with TiAlN coated carbide tools at 90 m/min as compared to CrN [6]. Meanwhile, Kumar et al. [7] found that a AlCrN carbide coated tool offered higher wear resistance than TiAlN coating as it had a higher microhardness at 31.26 GPa as compared to the TiAlN coating at 26.76 GPa. However, when the tool was coated with the alternate layers (TiAlN/AlCrN), it exhibited the highest microhardness at 32.7 GPa. A similar finding was also reported by Liew [8] through a comparison study between TiAlN monolayer and TiAlN/AlCrN multilayer coated carbide tools. Thus, it can be said that the multi coated tools offer higher wear resistance and higher bonding strength compared to monolayer coated carbide tools. As explained by Ghani et al. [9], the number of coating layers can reduce the coefficient of friction and cutting temperature from factors such as mechanical, thermal and chemical reactions which occur during cutting which consequently reduces wear rate as well as produces a better finished surface.

The effectiveness of coated carbide tools could also be optimized by applying correct cutting conditions. This is because most of the mechanical energy used to perform the metal removing process is automatically converted into heat, which inherently increases cutting temperatures. Generally, the higher the cutting speed, the faster the heat generation and temperature increase. For efficient heat dissipation, the application of cutting fluids or coolant in the machining processes is highly recommended. This helps to reduce the cutting temperature as early as possible, while also reducing friction between the cutting tool and the workpiece. Researchers such as Iturbe et al. [10], Yusuf Kaynak [11], Pereira et al. [12] and Fernandez et al. [13] stated that the machinability can be enhanced with the presence of cutting fluid, or coolant through either flood, cryogenic or minimum quantity lubricant (MQL) approaches. While Singh et al. [14] have further improved the cutting process by introducing hybrid nano-cutting fluid in turning of Titanium alloy under MQL condition. As reported, this approach helps to reduce the friction coefficient between surfaces, thus improve surface roughness and cutting force as compared to the base fluid only.

However, in recent times, the application of cryogenic coolants has emerged as a way of enhancing performance. By using a liquid gas such as liquid nitrogen (LN2), carbon dioxide (CO2) or argon during cutting, the coolant largely dissipates the generated heat and cools the cutting area which seems to be highly beneficial for difficult-to-cut materials, especially for those with lower thermal conductivity such as Inconel 718 [15], [16], [17]. Additionally, the coolant is free from any contamination and does not require any cleaning process as it fully evaporates into the atmosphere and leaves no residue once used. Thus, it can be concluded that the advantages of cryogenic cutting fulfill the three main aspects of machining which are: offering a clean and healthy working condition for the machine operator, being sustainable for the environment and increasing productivity of the processes.

Studies in recent years have promoted carbon dioxide (CO2) as a good alternative in cryogenic cutting. It is used to cool the cutting zone and once applied, it totally sublimates into air. In comparison to LN2, CO2 has much lower cooling capacity at −76 °C. According to Pereira et al. [12], this temperature is considered sufficient to tremendously remove cutting heat and at the same time, to reduce the tendency of the workpiece to experience work hardening (due to extreme cooling as occurs with cryogenic LN2 at a temperature of −192 °C) which would increases its hardness. The advantages of cryogenic CO2 in metal cutting in terms of resistance to wear, cutting force or surface roughness have been reported by Jerold and Kumar [18], Sartori et al. [19], Pereira et al. [16] and Fernández et al. [13] when compared in metal cutting. However, past research seemed to apply it more in the turning process rather than milling. Continuous cutting process and fixed tool position in turning provide a steady machining condition with expected constant cutting force and tool tip temperature. Therefore, it can be said that the application of cryogenic CO2 mainly in high-speed milling of Inconel 718 has not yet been fully explored. Milling is one of the most important processes particularly in achieving a specific machined surface quality or dimensional accuracy. Thus, the influence of cryogenic CO2 cooling on the performance of multi-coated PVD tungsten carbide ball nose milling inserts when using high-speed cutting Inconel 718 in terms of tool life, surface roughness, and wear mechanisms has been analyzed in this paper. For cryogenic cooling, a new cryogenic CO2 cooling system was applied for a consistent flow of the cryogen direct into the cutting point. The results were then compared with dry cutting conditions, which had been performed due to its environmental and cost advantages as well as to analyze the performance of multi coated ball nose milling inserts in the absence of coolants. This is because the advanced technology in cutting tools development is also seen to potentially increase the possibility of dry cutting to be conducted at higher speeds, particularly for superalloys. With the right selection of cutting tools, parameters and process, dry cutting is said to be able to give good machining results. For instance, with respect to dry cutting, the multi coated carbide tool was found to be shorter in life when applying cryogenic LN2 in high speed milling of Inconel 718 [20]. Thermal shock which accelerates fatigue and cracks has been found less in dry cutting as the tool does not experience drastic temperature changes as in cryogenic LN2 conditions. As mentioned by Halim et al. [21], this advantage has to be thoroughly explored as dry cutting is also highly favoured due to its low cost and sustainability.

Section snippets

Methodology

The work-piece material was Inconel 718, AMS 5663 grade, after being solution-treated and aged-treated, with dimensions as shown in Fig. 1. Its hardness was 42 ± 2 HRC which had been confirmed using a Mitutoyo Micro Vickers hardness testing machine with a load of 100 kgf or equal to 980.7 N, applied by a diamond indenter for 15 s. Table 1 shows the chemical composition of the work material.

The experiments were carried out on a DMG 635-three-axis vertical CNC milling machine where its spindle

Results and discussions

The experimental results in Fig. 2 evidently confirmed that multi-coated PVD ball nose milling inserts under cryogenic CO2 conditions outperformed those under the dry conditions. For the tool life as shown in Fig. 2a, cryogenic cutting resulted in an average of 75% longer tool life. The consistency and sufficiency of the cooling effect by the cryogen showed its effectiveness in dispersing the heat generated such that it rapidly reduced the temperature. With reference to Halim et al. [21],

Conclusion

This paper experimentally analyzed the influence of cryogenic CO2 cooling on the performance of multi-coated PVD tungsten carbide ball nose inserts during high-speed milling of Inconel 718 as compared to dry cutting conditions.

  • The results proved that the multi-coated (TiAlN/AlCrN) PVD insert gave the best performance in terms of wear resistance under cryogenic conditions where it managed to prolong the tool life and enhance the quality of the machined surface (Ra) by an average of 75% and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Acknowledgement is given to the Government of Malaysia and Universiti Kebangsaan Malaysia for providing the research funds, (Grant No: DIP-2019-026).

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