Effects of high-pressure cooling in the flank and rake faces of WC tool on the tool wear mechanism and process conditions in turning of alloy 718

The exceptional properties of Heat Resistant Super Alloys (HRSA) justify the search for advanced technologies that can improve the capability of machining these materials. One such advanced technology is the application of a coolant at high pressure while machining, a strategic solution known for at least six decades. The aim is to achieve extended tool life, better chip control and improved surface finish. Another aim is to control the temperature in the workpiece/tool interface targeting for optimum cutting conditions. In most of the existing applications with high-pressure coolant media, the nozzles are positioned on the rake face side of the insert and they are directed towards the cutting edge (the high-temperature area). The coolant is applied at high-pressure to improve the penetration of the cooling media along the cutting edge in the interface between the insert and workpiece material (chip) as well as to increase chip breakability. However, the corresponding infusion of coolant media in the interface between the flank face of the insert and the work material (tertiary shear zone) has been previously only scarcely addressed, as is the combined effect of coolant applications on rake and clearance sides of the insert. The present work addresses the influence of different pressure conditions in (flank: 0, 4 and 8MPa; rake: 8 and 16MPa) on maximum flank wear, flank wear area, tool wear mechanism, and overall process performance. Round uncoated inserts are used in a set of face turning experiments, conducted on the widely used HRSA “Alloy 718” and run in two condition tests with respect to cutting speed (45 (low) and 90 (high) m/min). The results show that an increase in rake pressure from 8 to 16MPa has certainly a positive impact on tool life. Furthermore, at higher vc of 90m/min, cutting edge deterioration: due to an extensive abrasion and crack in the wear zone were the dominant wear mechanism. Nevertheless, the increase in coolant pressure condition to 16MPa reduced the amount of abrasion on the tool compared to 8MPa. At the lower cutting speed, no crack or plastic deformation or extensive abrasion were found. When using 8MPa pressure of coolant media on the flank, the wear was reduced by 20% compared to flood cooling conditions. Application of high-pressure cooling on the flank face has a positive effect on tool life and overall machining performance of Alloy 718.


Introduction
Heat resistant super alloys have a self-evident place in the material shelf of the aerospace industry, in particular for various engine applications. Almost 50% of the high-performance components used in the hot sections of a jet engine are made from nickel-based super alloys [1]. Their exceptional material characteristics, such as retained mechanical and thermal properties at elevated temperatures and further, their corrosive and creep resistance, make them very attractive for many high temperature applications as compared to other alloys. Consequently, as the air transportation industry expands, the volumes of advanced materials (such as HRSA) increases. Despite the superior properties of nickel-based superalloys for various specific applications and the contemporary advancements in machining technologies, the material is still regarded as difficult to machine.
The "Alloy 718" -the fifty-year-old "workhorse" material for gas turbine applications -is one such alloy. Alloy 718's ability to retain its mechanical properties at elevated temperatures and good welding properties makes it the natural "first choice" for jet engine components. However, when it comes to machining, Alloy 718 generally exhibits low machinability indexes. This is mainly due to its ability to retain hardness and strength at high temperatures, and its low thermal diffusivity, work hardening and presence of hard abrasive carbides. As a result, accumulated heat in the cutting zone leads to intense tool wear and machining parameters have to be restricted in order to keep the service life of the tools and the material removal rate at industrially reasonable levels.
Various aspects of the heat balance in the cutting zone have been researched. One aspect that is frequently investigated is the effect of elevated temperatures -thermal softening of the chip -which is beneficial to the cutting process due to the lowered cutting forces. However, most research in the field has been directed towards efficient heat transport rather than optimized heat conditions in and around the cutting zone. For the most commonly used chip removing methods (milling, turning and drilling) several means of heat transport concepts have been explored, among which concepts such as Compressed Air; Cryogenic Cooling; Minimum Quantity Lubrication (MQL) and application of conventional cutting coolant under extraordinary pressures, are among the most commonly discussed in literature [2]. Since the machining system (cutting tools, machine tools and fixtures combined) suffers from various forms of heat driven instabilities, the industrial incentive of improved heat transfer rather than optimized heat conditions in and around the cutting zone is obvious. Thus, geometrical shift and drift of the process, driven by thermal effects are considered more significant to achieve a robust production concept than the lowest possible mechanical stress imposed on the system. Despite the fact that a reduced cutting force due to thermal softening is rarely achievable if a certain heat level is not maintained in the cutting zone, thermal effects are considered a more significant driver towards achieving a robust production concept than the lowest possible mechanical stress.
With the exception of the application of conventional coolant as means for heat transfer, most of the mentioned applications have failed in their industrial application. The contribution to the overall process robustness -perceived or real -is frequently expressed as insufficient. The reason is far from scientific: even though all of these cooling methods are potential moderators of the heat flux in the cutting zone and per se contribute to controling the temperature of the cutting tool, most of them usually fail to address the additional requirements of "applied" machining system such as thermal stability of the machinetool structure and chip transport.
It is therefore clearly justifiable to include application techniques of conventional cutting coolant in a system approach when to machine HRSA. In particular, when the process includes traditional tool materials, like cemented carbide (and where applicable high speed steel). Thus, the imminent question is how and where to apply the coolant media in order to reach the most significant effect on lowering tool wear and process capability.
The concept of focussing pressurized coolant to the cutting edge was experimented in 1952 by R J.S. Pigott and A.T. Colwell [3]. They built a high-pressure coolant delivery system with a maximum pressure of about 4 MPa, which improved tool life by about 7-8 times compared to flood cooling. They experimented on the flank face with the aim to bring the coolant in under the chip. Since then, the development of high-pressure technologies (pumps, delivery systems, tools and control units) has led to a considerable increase in available coolant media pressures for machining applications. A notable improvement was Jet-Break technology by Sandvik Coromant. In 1979 they introduced highpressure coolant in turning operations of Titanium specifically for aerospace applications. Results showed significant improvement in the surface properties and MRR for difficult to cut materials by reaching the upper limits of the cutting speed for these materials [4]. During the latter part of the 20th century, coolant media pressure reached values as high as to 360 MPa (and even further) based on equipment used for water jet cutting. However, in later studies typical high pressure applications seem to have settled at 10-50 MPa based on work piece materials, cutting conditions, machine tools and tool holder designs.
The current state of the art, referred to in appendix A, elucidates that most of the scholars have directed their research interests and investigated the impact of the media jets on heat transfer from the cutting zone, when directed towards the rake face (the secondary shear zone). Thus, most aspects of the influence from high pressure media applied to the rake side of the cutting tool has been under scrutiny. Cutting conditions, nozzle diameters/positions, flow rates and cutting tool materials have been investigated with respect to their impact on tool wear, cutting force, chip morphology, surface finish, temperature, machinability, etc.
However, reports on effects from media applied to the additional high-temperature regions of the cutting tool like i.e. the relief side of the cutting edge (the tertiary shear zone) are rare. Thus, the relief side of the cutting edge is one such area that is not yet thoroughly investigated with respect to advanced heat transfer. In particular, the understanding of the fundamental mechanism of using high-pressure cooling on both tool faces (rake and flank -secondary and tertiary shear zones), Fig. 1 (a), is still an existing knowledge gap.
In addition, while machining HRSA using high-pressure coolant, only a limited number of researchers have hypothesized and reported on the formation and existence of the vapour barrier film acting as a barrier for the coolant to reach the vicinity of the cutting edge [5][6][7][8].
This paper aims for a more thorough explanation of the influence of high-pressure coolant pointed to the flank face of the cutting edge in combination with high-pressure cooling applied to the rake face and the resulting tool life and related wear mechanisms, see Fig. 1 (b). Furthermore, emphasis is directed towards thermal effects (such as the vapour barrier) along the flank face and their variations due to changes in pressure conditions.

Workpiece material and cutting insert description
Cast Alloy 718 with an average hardness of 381 ± 21.8 HV was used as the workpiece material for this investigation. A feed rate of 0.2 mm/rev and a depth of cut of 1 mm was kept constant during the machining tests. The spiral cutting length, SCL, as calculated from Eq (1), was approximately 90 m (based on a machined length, l m , of approx. 8 mm). Conditions were kept constant for the entire experimental campaign, where D m -machined diameter and D 1 -initial diameter. The outer diameter of the ring was 742 mm and the inner diameter was 672 mm. The setup can be seen [9].
Round cemented uncoated carbide inserts with the ISO designation RCMX 12 04 00 and grade of H13A were used. These inserts are commercially available with a measured hardness of 1545 ± 14 HV30. In rough machining, round cutting tools play a vital role when machining HRSA, due to their high edge strength and their versatility (number of cutting edges that can be extracted through clever indexing). The insert has a negative face land and is made from cemented tungsten carbide (WC) embedded in a Co-binder.

Machine setup, cutting and high-pressure coolant conditions
A 5-axis vertical CNC machine was used for the face turning operations of the Alloy 718 ring. The machine was programmed for constant cutting speed (the spindle speed increases as the cutting tool moves towards the spindle centre), a more detailed description of the setup can be seen in Ref. [9]. High-pressure coolant was supplied to both the rake face and the flank face through a specially designed tool holder which allows precise focus of the coolant to the cutting edge ( Fig. 2(b-c)). An emulsion consisting of water mixed with 5% oil was used as a coolant at room temperature, recommended by the coolant supplier.
The cutting parameters and pressure conditions are based on the experimental conditions and results discussed in literature given in Appendix A "Current State of the Art". Experiments were restricted to three different flank pressure conditions 0, 4 and 8 MPa ( Fig. 1(b) for the detailed methodology). Two different rake pressures of 8 and 16 MPa were used. Currently, 8 MPa is the basic commercial setup readily sold by different machine tool manufacturers to the end user. Pressures of 16 MPa and above are customized to specific needs and used widely when machining HRSA to reach improved tool life, chip breakability and surface finish.
Since the cutting temperature rises drastically with increased cutting speeds, two cutting speeds 45 (low) and 90 (high) m/min were chosen to investigate the wear behaviour at two distinct levels of process heat generation. The coolant application was arranged through three nozzles with a diameter of 0.8 mm on the rake side and two nozzles with a diameter of 1.2 mm on the flank side.
The cutting parameters and pressure conditions are provided in Table 1. Feed rate, depth of cut, spiral cutting length (SCL) and material removed was kept constant. The cutting conditions were replicated two times, to ensure repeatability and were randomized before experimenting.

Investigation methodology
Different methods were used for tool wear investigation. Light optical microscope, LOM, and scanning electron microscope, SEM, were used to measure and examine the flank wear, material adhesion and wear mechanisms. Moreover, 3-D scanning microscope based on InfiniteFocus variation [10] was used in measurement of maximum flank wear, flank wear area and volume difference measurement. Energy dispersive X-ray spectroscopy, EDX, was used to estimate the chemical composition. The measurement of maximum width of the flank wear land was conducted based on the ISO 1993:3685 standard [11]. For the round insert, a need to set a standard for how to establish the depth of cut line and corresponding wear land had to be established can be seen in Ref. [12]. In addition, the flank wear area was calculated for precise evaluation of the flank wear land for each cutting insert. Evaluation of notch wear and tool-chip contact area was measured based on the contact angle of 34°on the rake face [12].   Analysis by electron backscatter diffraction (EBSD) was performed on cross sections of selected worn cutting tools. A low speed saw with a diamond wafering blade was used to section the tools at about the maximum chip thickness were the maximum width of flank wear was observed. Conventional metallographic methods were applied to polish the cross sections until a deformation-free surface was achieved (0.04 μm colloidal silica was used during the final polishing). The polished cross sections were examined using a LEO Gemini 1550 scanning electron microscope (SEM) equipped with a Nordlys EBSD detector. EBSD maps with 50 nm step size were acquired using an acceleration voltage of 20 kV. Post processing of the datasets was done with Oxford's HKL Channel 5 software [13].

Tool wear investigation
The investigations were focussed on the maximum flank wear (VB max ), flank wear area (VB area ) and wear mechanism observed on the WC cutting tools, by varying flank pressures conditions (0, 4 and 8 MPa) for rake pressure 8 and 16 MPa at cutting speeds 45 and 90 m/ min ( Fig. 1(b)). The findings are explained in the corresponding subsections.

Maximum flank wear and flank wear area
The maximum flank wear is a widely used reference for both research and industry to calculate the service life of the cutting tool. In industrial applications, the VB max is utilized as a fast and reliable way to determine the need for tool replacement. The high-pressure coolant is extensively used with the main purpose to improve the tool life by lowering temperature in the cutting zone and the mechanical effect to break the chips in the rake face, thus, lowering the friction.
The tool failure criteria in this work was set to be the tool breakage, due to that the intention is to understand wear mechanism for the constant cutting conditions. However, according to ISO 1993:3685 standard, the recommendation is VB max as 0.6 mm [11]. The maximum flank wear for v c 45 m/min was observed close to the minimum chip thickness zone, see Fig. 3 (a) and the flank wear was evenly distributed for all the pressure conditions. In case of v c 90 m/min, flank wear wasn't uniformly distributed and VB max was measured in the region the maximum chip thickness zone and as shown in Fig. 3 (b). In addition, VB area was measured to increase the understanding and avoid just one single number (VB max ) value to describe the flank wear. Values of the reached wear levels, extracted from a 3-D scanning microscope, can be found in Table 2.
The maximum flank wear was measured for all the cutting trials (including the replicates, which is shown by the error bar), see Fig. 4. In addition, the flank wear area of the worn zone was measured and values are plotted as shown in Fig. 5. While comparing the effects of varying v c , the difference in mean value of VB max and VB area can be readily seen. The numbers have increased significantly (by approx. 3 to 4 times) for all the cutting conditions. Despite using a high-pressure coolant, the increase in cutting speed reduced the tool life significantly. This is in line with the theory that increased cutting speed leads to rises in the temperature at the cutting edge due to the heat generated in tool-chip and tool-workpiece shear zones, which consequently accelerates tool wear and lowers tool life [6,14].
For v c 45 m/min, an increase in rake pressure from 8 to 16 MPa for constant flank pressure of 8 MPa led to a reduction of the mean values for VB max and VB area by approx. 20 and 17%, respectively. The corresponding wear levels were not significantly influenced by similar cutting conditions by flank pressures of 0 and 4 MPa. For a v c of 90 m/min and a rake pressure of 8 MPa, an increased flank pressure did not lead to a reduction of the maximum flank wear and flank wear area ( Fig. 4 (a) Fig. 5 (b)). However, if the rake pressure is increased to 16 MPa (Figs. 4 (b) and Fig. 5 (b)) the mean VB max and VB area are reduced by approx. of 25 and 20%. The wear levels were not influenced when no flank coolant was used and when the flank pressure was 4 MPa. Finally, at a flank pressure of 8 MPa, the downward wear rate trend was regained.
Based on the obtained tool life, the industrial implication of the results is to rather remain at the lower cutting speeds (at very low tool wear rate) than to increase the cutting speed (at the cost of frequent tool changes).
Observed wear mechanisms from the investigated WC tools used in machining Alloy 718 are elaborated with respect to cutting speed (low and high).

Wear mechanism of WC at low cutting speed
Abrasion and adhesion were observed to be the dominant wear mechanisms on the WC tools disregarding the rake and flank coolant conditions. Increase in rake and flank pressure had no influence on wear mechanism, see Fig. 6. However, the amount of adherence of Alloy 718 and abrasion of WC tools varied due to different coolant pressure conditions on the rake and flank. With all the cutting tools, a more uniform abrasion wear was observed in the form of flank wear.  Further analysis by SEM and EDX revealed the adherence of the workpiece material, Alloy 718 as a uniform grey region on the flank wear land as shown in Fig. 6 (a, b and e), while the white regions (varying spikes) were identified as the WC tool material. Adhesion wear mechanism is observed in all the cutting tools on both flank wear region and tool-chip contact area at the rake face. Considering the mechanical and thermal properties of Alloy 718, the amount of heat and mechanical stress applied to the cutting tool during machining can lead to the frequent chemical bonds between the tool and workpiece materials. This also stands as cause of micro welds to the tool rake face due to thermally controlled diffusion process. Formation and growth of build-up edges is due to adhesion where the material moves from workpiece to cutting tool. Cobalt is widely used as a binder for WC due to its exceptional properties of adhesion and carbide wetting. Cobalt has two allotropic forms. Below 417°C it is hexagonally closed packed and above 417°C cobalt has a face centred cubic, which has positive influence on the adhesion process [15][16][17].
When the coolant touches a heated surface i.e. the cutting tool, temperature higher than the coolant boiling point, the liquid vaporizes immediately and forms an insulating vapour layer. This layer protects the remaining coolant from boiling. This is known as the Leidenfrost effect. The vapour layer acts as barrier for the coolant to reach the proximity of the cutting edge for improved cooling. Traces of vapour layer are seen as a dark region in Fig. 6 (b and c). It is identified by EDX analysis as calcium and sulphur containing layer stemming from the coolant.
The coolant-boiling region formed as a continuous layer below the flank wear region for flank pressure conditions between 4 and 8 MPa, Fig. 6 (b and c). However, continuous formation of the coolant-boiling region did not occur when there was no flank pressure. Instead, the coolant-boiling regions were scattered and did not follow the flank wear pattern. By focussing, the high-pressure coolant media at clearance face so called Leidenfrost effect can be suppressed. Concurrently, increase in flank pressure had a positive impact on the tool life by lowering the flank wear and moving the coolant precipitate layer closer to the cutting edge. Application of high-pressure coolant in the clearance face led to the formation of different zones on the flank face of the tool. The zones can be seen from the cutting edge in Fig. 6 (c) starting from flank wear marked as (1), the no boiling region (2) (visibility of Co and W), followed by the coolant-boiling region (3) (dark region). These zones illustrate the temperature gradient along the cutting tool.
Abrasion and adhesion are the primary and interrelated wear mechanisms observed with rise in temperature. During the machining process, initial stage of wear begin with mechanical wear, abrasion due to the relative interaction of the WC tools with Alloy 718 carbides present in the material. Abrasion is also caused by the burr due to work hardening found on the surface generated in the previous pass due to the side flow of material [15,16] as can be seen in Fig. 6. There was no fracture or cracking observed on any of the tools.
The rake face of all the cutting tools had the build-up layer (BUL) on the tool-chip contact area; one of the tools is shown in Fig. 7 (a). In addition, build-up material (BUM) formation was observed in different areas along the contact zone, as seen Fig. 7 (b) is not the build-up edge (BUE). Normally the BUE is non-stable, breaks periodically and takes away a small amount of tool material, which in turn leads to deterioration of the cutting edge [18]. In this case, the BUM can be from the chips adhered to the tool during the cutting tool disengagement from the workpiece. BUM is scattered along the tool-chip contact area, it   does not represent traditional BUE formation. However, intense formation of BUL could lead to negative effects of the tool life by acting as barrier for the heat to dissipate through the tool. Fig. 7 (c) shows the region of BUM, adhesion of alloy 718 and WC at higher magnification. By use of a 3-D microscope, volume difference analysis was conducted and the resultant micrograph clearly shows the volume adhered to the tool-chip contact area along the cutting edge ( Fig. 7 (d)). The colour variation orange and red being the highest level of material adhered and cyan colour showing the build-up layer on the tool.

Wear mechanism of WC at high cutting speed
Increasing the cutting speed can be beneficial in the production aspect to remove the material at higher rates. Due to the thermal resistance and work hardening property of Alloy 718, it is a disadvantage to machine at higher cutting speeds. Normally, chips carry away most of the heat when machining materials like aluminium and steel. However, chips from Alloy 718 do not dissipate the heat from the toolchip contact zone at the same rate. In addition, the energy created due to friction in the tool-chip contact zone is directly proportional to the cutting speed, hence creating an intense temperature load on the tools [19].
The observed tool wear mechanisms were similar independent of the cutting speed. Nevertheless, the amount of wear such as VB max , VB area , cutting edge deterioration due to extensive abrasion on the tool increased significantly. Apart from BUL, BUM and flank wear, the dominant wear found was the growth of crack in both rake and flank face, disregarding the high-pressure cooling in the rake and flank face almost all the tools showed cracks. These cracks can be seen in the premature stage of the tool breakage. The tools were intensively damaged compared to the lower cutting speed. As shown in Fig. 8, for the rake pressure of 16 MPa and flank pressure of 8 MPa, the crack propagates from the cutting edge across the flank wear zone as can be seen in Fig. 8 (b and c). A grey area can be seen on the flank wear land, which was found to be adhered Alloy 718 material.
When investigating the cutting tool in SEM, it was not possible to see a crack before etching the sample ( Fig. 9(a)). Since the adhered workpiece material obscured the actual flank wear land, the tools were investigated after etching ( Fig. 9(b)). After removing the adhered workpiece material from the flank face, the presence of the cracks was revealed see Fig. 9 (b &c). This could be seen as a kind of confirmation that the cracks are actually present in the tool and are not just part of the adhered layer.
Selected cutting tools from different flank pressure conditions were investigated for cracks in flank wear zone using SEM, micrograph showing the crack propagation for all three conditions can be seen in Fig. 10. For 4 and 8 MPa of flank pressure, the maximum flank wear and area were measured to be lowered compared to no cooling conditions. However, cracks exist in the tools as can be seen in Fig. 10. In addition to adhesion of Alloy 718 and cracks, abrasion in form of sliding led to   removal of cobalt exposing the WC as white zones. Altintas [19], stated that tools start to crack once the principle tensile stress reaches the ultimate tensile strength (UTS) of the tool material. WC has an approx. of 650 MPa as UTS. Furthermore, one needs to consider the accumulation of heat with increase in cutting time, tool wear and in combination with coolant cooling; leads to a thermal gradient on the tool. The cracks found on these tools can be seen as an early stage of plastic failure of the cutting tool. From the results, it can be assumed that by further increase of the cutting speed or SCL, the tool is prone to catastrophic failure. The cutting edge deformation was identified as a common problem in machining HRSA [19]. On the rake face, tool-chip contact area was filled with Alloy 718 material. The cutting edge deterioration due to extensive abrasion and fracture were observed, Fig. 11. Increase in wear occurs as the cutting tool exceeds the deformation resistance due to the thermomechanical stress and at elevated cutting temperature. At this condition, the cutting edge cannot withstand the compressive stress. In case of carbide tools, plastic deformation mechanism occurs due to the increase in the cutting temperature (increase in cutting speed), under high cutting forces.
Machining of Alloy 718 at increased cutting speed induces higher forces and a rise in temperature in the contact zone, which in turn affects structural strength of the cutting tool, thus leading to deteriorate the cutting edge at a faster rate. It is important to retain the strength of the cutting edge at elevated temperatures to avoid a blunt tool, which can change the cutting conditions and damage the workpiece. The loss in structural integrity and edge strength of the tool, could lead to an intense tool wear rate and cause tool breakage in form of cracks, as seen in Fig. 11 (c).

Notch and crater wear
Various researchers like Ezugwu et al. [20,21], Machado and Wallbank [22], and Suárez et al. [23] have investigated machining with the use of high-pressure coolant and reported increases in tool life by lowering flank wear compared to flood cooling or even dry cutting conditions. However, the dominant depth of cut notch wear was observed in their research work for both carbide and ceramic tools. This failure mode was reported to be the most prominent cause for tool failure while machining Alloy 718. Shalaby and Veldhuis [24] mentioned that the probable cause of notch wear can be caused by one effect or a combination of the following: adhesion effects, work-hardened surfaces, stresses, adverse chip flow conditions with chip curling, and temperature gradients. Further, notch wear is the dominant tool failure criterion when machining at higher cutting speeds. However, in some cases notch wear was identified even at lower cutting speeds [23]. In this experiment, no such observation of groove shape or notch wear were found for any of the cutting tools at the depth of cut line (at approx. 3.6 mm as shown in Figs. 3 and 12).
Diffusion wear mechanism is commonly found on the cutting tool in form of crater wear due to the high temperature in the tool-chip contact region. Many studies observed crater wear to be another common tool wear form observed during machining Alloy 718. This was reported by various experts and researchers in the area of machining disregarding  the tool geometry and material such as carbide, ceramics, [25][26][27][28]. In this experiment, none of the tools in tool-chip interface for low cutting speed of 45 m/min showed any form of crater wear. This could be attributed to the insert geometry. The round shape is creating a varying tool-chip area see Fig. 2 (c), thus, reduces the mechanical contact in the secondary shear zone. At higher cutting speed of 90 m/min and disregarding the strength of insert geometry and flank pressure conditions, the tool wear mechanism changed between 8 and 16 MPa. Instead of crater wear, which is one of the common wear mechanism in machining Alloy 718, loss of material was observed on the cutting edge (see Fig. 12 a, b and c), in form of abrasion. However, the shape looks like a "crater valley" than a localized crater wear. For the cutting tools machined with a rake pressure of 16 MPa, abrasion rate and flank wear area were less. In addition, the tool-chip contact area was more constrained and showed less abrasion compared to the tools with a rake pressure of 8 MPa (see Fig. 12).
This result can be correlated to the findings by Ezugwu and Bonney [5] that an increase in the coolant pressure creates access for the coolant jet to penetrate the tool-chip interface. This leads to an efficient cooling and lubrication. Physical formation of coolant wedge due to pressure in the secondary shear zone lowered the tool-chip contact length, cutting force and coefficient of friction. The results show that applying coolant media at a high pressure of 16 MPa on the rake face has a significant effect on the mechanical force to penetrate close to the cutting edge, therefore, reducing the thermo-mechanical load on the rake face, leading to lower the tool wear.
At higher cutting speed in machining Alloy 718, it is highly recommendable to use the maximum available pressure on the rake face. Flank cooling places a vital role in decreasing the tool wear rate. Hence, it is important to have at least a pressure of 8 MPa on the flank face of the tool for improved tool life.

EBSD wear characterization of WC tools at low and high cutting speeds
In order to study the tool wear and its underlying mechanisms in dependence of the machining and coolant conditions, cross sections of four worn inserts were prepared and examined with EBSD regarding the plastic deformation of WC grains. Local misorientation maps of EBSD datasets acquired at the cutting edge can be seen in Fig. 13. The colour of every pixel represents its average orientation difference with a set of surrounding pixels [13]. According to the colour code in Fig. 13 (a), undeformed regions with low/no local misorientation are therefore shown in blue while higher misorientations due to deformation of the WC grains are shown by green, yellow, and red. As can be seen, the majority of all four maps are coloured blue indicating that these areas are free of deformation. However, close to the worn surfaces on the rake and flank faces, green and yellow coloured pixels show significant local misorientation (deformation) within WC grains. The affected zones extend a few rows of WC grains into the tools (below 5 μm). The examined part of the rake face in Fig. 13 (c) shows an exception where no local misorientations can be seen in close vicinity to the worn tool surface. As indicated by the significantly different geometry of the cutting edge (compare with e.g. Fig. 13 a), it is believed that this tool exhibited local chipping of the cutting edge in the region where the tool was sectioned. Brittle fracture of the tool material due to chipping instead of gradual wear might explain the absence of deformation in Fig. 13 (c).
Interestingly, no significant differences between the low and high coolant supply pressures can be observed for the two cutting speeds: The majority of deformation reaches to about the same depth beneath the worn rake and flank face when comparing Fig. 13 (b with d) and when comparing the flank face in Fig. 13 (c with e). The observed reduction in tool flank wear for constant cutting speed when using higher coolant supply pressures (see Figs. 4 and 5) are therefore not likely due to a significant reduction of plastic deformation of WC grains. Instead, the increased cooling is expected to affect other wear mechanisms such as diffusion/dissolution or abrasion, which lead to the reduced overall flank wear.
An increase in cutting speed has an impact on the observed misorientation: Significantly more WC grains which are comparably far from the worn tool surfaces (> 10 μm) exhibit local misorientation for   the tools used with the higher cutting speed of 90 m/min as compared with the tools used for 45 m/min cutting speed (compare Fig. 13 (c and e) with Fig. 13 (b and d). This is likely to be the result of higher thermal and mechanical loads, which are expected to act on the cutting edge when for the higher cutting speed, resulting in a larger zone of the network of WC grains to be deformed during cutting.

Conclusions
This research work has presented the findings from the influence of using flank high-pressure cooling on the cutting tool. Tool wear investigations were focused in different areas such as maximum flank wear, flank wear area, wear mechanism in machining Alloy 718 with cemented tungsten carbide tools. The following conclusions can be drawn: At low cutting speed (45 m/min) with a maximum pressure on the both rake (16 MPa) and flank (8 MPa) reduced the mean VB max and VB area by approx. 22 and 12% compared to 8 MPa of rake pressure and "no cooling" on the flank.
At high cutting speed (90 m/min), with a rake pressure of 16 MPa, mean VB max and VB area reduced by approx. 25 and 19% with a flank pressure of 8 MPa compared to no flank cooling condition.
No depth of cut notch wear was observed in any of the cutting tools including the repetitions.
At the high cutting speed, 3-D measurement results of the tools showed the formation of a crater in the shape of a valley on the cutting edge at a rake pressure of 8 MPa condition. By increasing the rake pressure to 16 MPa significantly decreased the rake wear and abrasion.   13. EBSD results of worn cutting tools: (a) schematic representation of cross section through worn tools with location of EBSD maps indicated; (b)-(e) EBSD local misorientation maps of WC grains for low cutting speed (left column) and high cutting speed (right column) with low and high coolant supply pressures to rake (p Rake ) and flank (p Flank ) in the upper and lower row respectively. The local misorientation maps were generated using 3 × 3 pixel filter size, 5°subgrain exclusion angle. Grain boundaries (> 10°) are shown in white.
At the low cutting speed, abrasion and adhesion were common wear mechanisms; no crack and cutting edge deterioration were observed.
At high cutting speed, drastically increased the tool wear rate (VB max , VB area and rake wear). Cracking of the tool and cutting edge deterioration due to extensive abrasion were dominant tool wear mechanisms.
The EBSD results in the flank face of the tools for high cutting speed indicated significant local misorientation of WC grains as far from the worn tool surfaces (> 10 μm) compared to low cutting speed. Increase in coolant pressure did not have an influence on the WC grain misorientation.
Higher traces of coolant precipitate were found on the tools, machined with a flank cooling. At 8 MPa of pressure on the flank face moved the vapour barrier film to the cutting edge. Thus, enhanced the coolant reachability close to the cutting edge.
Flank coolant at 8 MPa had a substantial positive effect on the tool life.
Based on the investigation, results point in the direction to machine Alloy 718 at low cutting speed with maximum available pressure for both rake and flank to increase the service life of the tool.

Acknowledgement
The authors would like to thank the funding organization Västra Götalandsregionen in association with the PROSAM project. We would also like to thank Sandvik Coromant for the support with cutting inserts. Special thank go to Andreas Gustafsson at University

Appendix A. Current state of the art
Findings from different researcher's over decades in high-pressure coolant application, during machining Ni-base alloy Inconel 718 also known as "Alloy 718" focused mainly on single point cutting tool operations has been discussed below (Inconel 901 was used in the 1990's experiments). Comparisons were made based on the cutting conditions, cutting tool material, high-pressure coolant conditions, the position of the coolant, followed by evaluation and findings, see Table 3. Table 3 Summary of various researchers' investigation in machining Ni-base alloy with high-pressure coolant.   Rough face turning f n = 0.10 mm/rev a p = 2 mm v c = 30 m/min. SCL = 727 m One nozzle in clearance face and two nozzles in the rake face of diameter 0.8 mm, estimated jet speed of 127 m/s. Coolant was oil based emulsion of 10-12% Conventional cooling was set at 0.6 MPa, high-pressure cooling at 8 MPa.
Flank wear decreased more than 30%, cutting force by 10%, and temperature by 50% compared to conventional cooling. Notch wear at the DOC line was the dominant damage to the tool with high-pressure coolant.