Keywords

1 Introduction

Superalloy materials are predominantly used in manufacturing and engineering construction sectors involved in applications for energy supply. The most common type of superalloys used is nickel-based superalloys. Inconel 718 is a nickel-based superalloy that is most commonly used in aerospace and turbine industries. This alloy is mostly used in the blades of turbine engines and in the nose of aircraft body due to its high temperature resistance, high strength compared to weight, and high tensile strength. Due to it high melting point characteristic, it is used in superheating applications that require materials to withstand at high temperatures.

With the integration of the stated properties, these materials are hard to machine continuously for longer periods of time, and hence, require periodic changing of machining tool due to wear. This, in turn, creates a manufacturing downtime and costs in major industries and production factories due to constant machine breakdown or repair. In order to overcome this problem and achieve efficiency in machining process, research and study are being done on developing new tools and coated materials, and parameter optimization using various lubrication techniques such as cryogenic lubrication, flood lubrication, hybrid lubrication, and liquid nitrogen lubrication. [1].

According to Zhuang et al. [2], cutting fluids provide an effectual way in improving machining of hard-to-machine materials by reducing friction developed at the cutting contact area through effective lubrication that reduces the temperature at the machining zone due to cooling by convectional heat transfer, and hence, reduces tool wear which, in turn, provides improved surface finish [2].

According to Kaynak et al. [3], cutting fluids used in lubrication of machining process contain many harmful chemicals that are difficult to discard and recycle in the environment, also causing skin and lung disease to the operators. Due to strict environment laws, there is less flexibility in using cutting fluids, although various substituent formulants that does not include lead, sulfur, or chloride are made, and they are not yet achieved to be completely harmful. The total incurred costs regarding the usage of cutting fluids are approximated to be several billions per year [3].

Hence, there is a need for an effective and cost-efficient cutting fluid in the industrial market sector associated with environmental and economic impacts. This review is done to find out various suitable cutting fluids and techniques that meet required industrial environmental and economic standards, for effective machining of Inconel 718 material using ceramic inserts and CNC end milling machine as the basis for study, also to obtain longer tool life and smooth surface finish in accordance with various operating parameters.

2 Cooling Techniques

Studies have been done on various cooling techniques for improving the machining of Inconel 718. These techniques depend on economic and technical factors with respect to the machining process in various applications. Technical factors refer to the availability of particular cooling equipment during the machining process such as availability of a particular number of nozzles, nozzle diameter, pressure regulator, and so on. Industries and production units select the techniques for lubrication based on the manufacturing process and cost factors while fabricating or machining the components for the desired end product. Various cooling techniques based on recent research developments include hybrid cooling, cryogenic cooling, flood cooling, minimum quantity lubrication, and high-pressure jet assisted machining (HPJAM). According to Shokrani et al. [4], injection of pressurized fluid on the tool flank and rake surface provided less surface hardness with 10–15% decrease in overall machining force, and hence, obtained results with less surface defects in comparison with flood cooling [4], which provided less surface smoothness due to inability of the lubricant to reach the tool workpiece and chip interface creating a heat transfer barrier (Fig. 1).

Fig. 1
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Flank and rake injection of cooling lubricant in HPJAM

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Similarly, there exist other techniques that provide high production rate and optimum surface finish. In hybrid cooling, a cryogenic setup for spraying liquid nitrogen at the cut zone, in addition with simultaneously spraying pure rapeseed oil for coating the cold tool periodically was found to be effective in improving tool wear without much change in the surface roughness produced, regardless of the change in technique used. The tool life in hybrid cooling technique was found to be 50% greater than MQL, and 33% greater than cryogenic technique [5]. From experimentational studies, inclination of two jets in the direction of rake face and flank face of the machine tool was determined to be the most effective configuration in machining [6].

The experimentation conducted by Chopra et al. [7] determined flood cooling using emulsion-based oil provided better surface finish for shorter life when compared to cryogenic cooling using liquid nitrogen [7]. The cryogenic cooling technique provided best tool life and good surface finish after long machining duration, when compared to other techniques such as dry machining, flood emulsion, and blowing of cold air as a coolant as shown in Fig. 2.

Fig. 2
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Comparison of various lubrication methods on the basis of surface hardness and material removed

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3 Parametric Observations

From Table 1, it can be observed that average cutting speeds used in most experimentations ranges from 30 to 140 m/min, with maximum feed rates up to 0.2 mm/rev. The maximum pressure ranges from 5 to 200 bars. Most of the tool inserts used were made up of coated carbide or ceramic material [8], and the lubricants used mostly included emulsifiers that mostly contain base oil as vegetable oils, along with intermittent use of liquid nitrogen in various experimentations. Higher cutting speeds were observed for ceramic tools and emulsifier lubricants (Figs. 3 and 4).

Fig. 3
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Comparison of lubricants and cutting speed

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Fig. 4
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Comparison of tool inserts and cutting speed

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Table 1 List of studies on effective lubrication parameters
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4 Analytical Study

Emulsified oils such as palm oil mixed with water had higher viscosity compared to synthetic esters. Hence, the lubrication during machining was improved as friction was reduced due to increase in viscosity. Oil–water ratio affects tool life under minimum quantity lubrication. Greater oil content in fluids leads to poor fluidness. Usage of emulsified oils, namely, palm oil and rapeseed oil improve the performance and deposition of lubrication film on the surface of tool or workpiece, due to the presence of greater amount of unsaturated fatty acids in the carbon chain [9]. Use of liquid nitrogen in cryogenic cooling or minimum quantity lubrication provides effective lubrication [10], but these lubricants are difficult to dispose in the environment because of the presence of harmful chemicals present in its chemical composition, and hence, contributes to global warming, skin and lung associated diseases to the operators present in the industrial environment after periodic exposure to nitrogen gas for long durations. Hence, these types of coolants provide less safe working conditions comparatively to other existing coolants. However, they can be used in prescribed amounts as per industrial and environmental standards along with other coolants. According to Rout et al. [11], optimized cutting parameters for improving tool life under minimum quantity lubrication involve lower cutting speeds, less feed rates, and greater quantity of lubrication directed along an inclined axis [11]. As per experimentation conducted by Kaynak et al. [3], optimum and effective oil–water ratio and flow rate of fluid under minimum quantity lubrication are 60:40 and 60 ml/h, on the basis of observations from rake face wear of the tool.

5 Conclusion and Future Scope

Hybrid cooling and high-pressure jet assisted machining are the recent developments in cooling techniques that proved to be effective using combination of more than one property, or by varying external cooling environmental factors. These methods require an additional specifically designed equipment for operation. Among other already existing cooling techniques, cryogenic cooling was observed to provide longer tool life and smooth surface finish after long machining duration [7], whereas flood cooling provides better lubrication and smooth surface finish at lower cutting speeds. At higher cutting speeds, flood cooling provided optimum surface finish and shorter tool life, due to the developmental effect of heat exchange barrier of lubricant present between the workpiece material or cutting tool [12]. Emulsified oils exhibited comparable properties of lubrication to that of synthetic ester and gaseous fluids during machining operation. Emulsified oils tend to cause less harmful effects during machining process, and hence, it is considered to be a viable alternative.

With the use of these reviewed data, research has to be done for finding eco-friendly compatible coolants that improve the machinability of Inconel 718, and machining tool life by using the most appropriate methodology of lubrication.