Effect of externally applied magnetic field on the tool wear and surface morphology of Inconel 718 in turning operation

This study examined the effect of magnetic field with single point coated‐carbide tool during turning process of Inconel718 (In718). After the turning process, tribological properties (surface roughness and tool wear) and chip morphology were analyzed. Two cylindrical Neodymium (N52) grade permanent magnets are used to apply the magnetic field during the machining. Cutting speed and magnetic field intensity along with constant feed and depth of cut are used to analyze tribological properties along with chip morphology. Experimental results show that 80 m/min cutting speed and 426 Gauss magnetic field has the best results in term of lowest surface roughness and tool wear. Lorentz forces developed due to the applied external magnetic field help to improve surface roughness compared to nonmagnetic field machining. Developed fine particles during the turning process get repulsed in case of magnetic field machining which helps to improve tool wear.

presence of hard abrasive carbide particles in the microstructure makes the cutting difficult.So, In718 is included in the group of "Difficult to cut materials." 3][4] Different types of coating materials such as CrN, ZrN, CrAlN, and TiAlN are used for carbide tool to cut In718.Most of the coating is frequently deposited with the physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods.Among different coatings used, TiAlN coating is found better due to its antifriction effect. 5Surface roughness and chip temperature is found to depend on the TiAlN coating thickness.Zhao and Liu 5 have reported that after cutting of In718 with carbide tool as coating thickness increased from 1.6 to 3 μm, the surface roughness spoiled and temperature increased by 42 • C. The Aluminum (Al) percentage is most important in TiAlN coating.It affects thermal conductivity of tool.Thick TiAlN coating reduces heat dissipation into the tool body as compared to thin coating; hence chip temperature increases with thicker TiAlN coating. 6Poor surface quality (surface roughness) of component is the most common major problem that occurs during the turning process of In718. 7Different techniques are used to improve surface roughness in the turning operation of In718 such as the use of cutting fluid, flood cooling, minimum quantity lubrication (MQL), 8,9 MQL with nanoparticles, 10 cryogenic, 11 and compressed air (oxygen and nitrogen) cooling, 12 heat treatment etc. MQL is found better than flood cooling for improving surface roughness, besides comparatively lowering environment pollution. 8It is also reported that in MQL 11 and cryogenic cooling method surface roughness, tool wear and temperature in cutting zone is reduced significantly. 8,9But on the other hand, cryogenic cooling method increases machining costs due to increased volumetric use of cryogenic fluid and high power.Also, its effect on workpiece properties and environment is under investigation. 9Use of magnetic field is one of the alternative solutions considered for improving the surface finish of work piece in turning operation.5][16] It is an environment friendly and low-cost method. 17,18The magnetic field is produced with electromagnet and permeant magnet.In an electromagnet, the magnetic field intensity is controlled by changing current (alternative current (AC) or direct current (DC)) in the coil.Researchers in the past have applied magnetic field on the workpiece or cutting tool or simultaneously on both to see its effect on the surface roughness, tool wear, vibration, and temperature.Most of the researchers have used ferromagnetic material as work piece and found that surface roughness, tool wear, and vibrations all have minimizing effect.Initially, Bagchi and Ghosh 12 studied the effects of a magnetic field on tool life during turning operation of mild steel and they found that tool life increase with increase in the magnetic field strength. 19,20Bagchi and Ghosh 12 applied 5-7 mT magnetic field on the cemented carbide tool during turning operation of ferromagnetic materials (Austenitic Stainless Steel) and found that the tool life increases by 1.2-1.5 times, while surface roughness improves by 10%-40%. 12The authors have also reported that although both oxygen and nitrogen as cooling media improves surface roughness, nitrogen is comparatively better.Further, magnetic field is found even better than liquid nitrogen for enhancing the surface finish. 12The electromagnetic field influences tool wear during a cutting operation depending on its strength.Due to the electromagnetic field, the finer fragments of chips get adhered with the tool rake face and may act as solid lubrication between tool and workpiece.It helps to increase the tool life. 19,20The chip morphology is very important to understand the effect of various parameters in the machining process.The Chips observed by Mohamad et al. 22 under scanning electron microscope (SEM) reviled serrations formed on them when cutting was performed in the presence of magnetic field.The surface roughness was also observed to be better than that without a magnetic field.As In718 is a popular material choice for making critical components used in aerospace and automotive industry, the surface roughness of such parts is of prime significance.To achieve the perfect assembly of machined parts and to avoid the premature failure of component, measurement of Ra value is very important.
In this research, experimental tests are performed on the nickel-based superalloy Inconel 718.During the tests feed and depth of cut are kept constant.Speed and magnetic field strengths are varied over three levels.The main objective of the study is to analyze the effect of machining parameters and magnetic field on the surface roughness under tested conditions.

Workpiece and cutting tool
The properties and chemical composition of In718 material are listed in Tables 1 and 2, respectively.The microstructure of In718 was obtained with the help of SEM which is as shown in Figure 1.The average grain size observed through a microscope and measured by the averaging method is 5.The PVD TiAlN coated-carbide tool is used in this reserach and its details properties are shown in Table 3.The designation of the tool insert is TNMG 160408 MT TT 5080 and it is mounted on a tool holder, MTLNL 2525 M06 (Make-TaeguTec).The nose angle and nose radius of the cutting inserts are 60 • and 0.8 (mm), respectively.The material is cured and it is measured hardness is 350 HB.The cutting insert is presented in Figure 2.

Generation and application of magnetic field
In this experiment two N52 grade (NdFeB/Neodymium) permanent magnets of size ø25 × 25 mm are used to apply the magnetic field on the tool which is shown in Figure 3.The details specification as shown in Table 4.It is the strongest magnet maximum energy product (BH)max.The magnet is fixed on the tool holder in three different positions which are shown in Figure 4.The magnetic field is measured near the tip of insert at different positions which is shown in Figure 4.The position of the magnet is decided by measuring the magnetic field strength at various locations on the tool holder.And it is finalized where the maximum magnetic field strength is obtained.

Experimental setup and cutting conditions.
Turning operations were performed on a CNC lathe (Make: Ace Micromatic Ltd., Model: Super Jobber 500LM).Experimental methodology is shown in Figure 5. Initially, an Inconel 718 bar of 32 mm diameter and 45 mm length is cut by a power hacksaw and fixed in the hydraulic chuck.The cutting insert is screw fixed to the tool holder.The TNMG 160408 MT TT 5080 cutting tool is selected as it gives better Ra value.Three different cutting speeds (V) 40, 60, and 80 m/min are employed.Feed rate (f) 0.15 mm/rev, and depth (d) 0.15 mm are kept constant.The Ra value is measured using the Mitutoyo SJ-210 surface roughness tester which is as shown in Figure 6.Sampling length is selected 2.5 mm for each work piece for surface roughness measurement.Experiments are performed on three different work pieces for each experimental setting.During machining synthetic coolant is used the details specification of coolant as shown in Table 5. Surface  roughness is measured at three different locations on each machined surface.In this way total 36 experiments are performed under room temperature conditions.Experimental parameters are shown in Table 6.The surface morphology of the turned specimens is observed under trinocular microscope model SM-9 (Figure 5).Flank wear is measured using tool maker microscope (Make-Mitutoyo, least count 0.005 mm).

Surface roughness
Experimental results of surface roughness measured at three different locations on each machined surface of all 36 tests are shown in Table 7.
From the tabulated results, it is clearly observed that the surface roughness of the tested workpiece decreases with increase in the intensity of magnetic field.Further even with increasing speed also the surface roughness of the specimen gets improved in the presence of increasing magnetic field (Figure 8).This improvement in the surface finish can be linked to heat generated inside the workpiece on account of the Lorentz forces.
An eddy current is produced inside a In718 work piece when it is rotated in external applied magnetic field.Development of this eddy current generates magnetic field within In718 workpiece as it is conductive material.This magnetic field developed by eddy current is in opposite direction to that of applied external magnetic field as shown in Figure 7.These two opposite magnetic fields produce forces that resist the change in magnetic flux.This force is called as Lorentz force. 14,20,21quation of Lorentz force (F) is given by where Vis the cutting speed of the work piece, q is the electronic charge carried by the work piece, E is the electric field, and B is the magnetic field.
From Equation (1), it can be observed that the Lorentz force depending upon cutting speed of the work piece, 21 also the Lorentz force is directly proportional with the magnetic field intensity.
From Figure 8, it can be observed that for magnetic field machining, with increase in the cutting speed and external applied magnetic field surface roughness decreases.On the other hand, even though increase in cutting speed in the absence of magnetic field surface roughness increases.Equation (1) shows that Lorentz force is directly proportional to cutting speed and external applied magnetic field.With increase in the cutting speed and external applied magnetic field, Lorentz force increases which ultimately develops small amount of heat inside the IN718 workpiece. 19This developed small amount of heat helps to generates short/continuous ribbon types of chips from workpiece.
Chip morphology at 135 Gauss with cutting speed 40 m/min, 263 Gauss with cutting speed 60 m/min and 426 Gauss with cutting speed 80 m/min is shown in Figure 9.At higher cutting speeds, continuous ribbon-shaped chips are formed and at lower cutting speeds, plastic deformation causes thick, continuous tangled chips to form.As cutting speed increases short ribbon types chips are produced; hence, surface roughness is improved.It can reflect that such kind of chips are easily removed from the cutting zone or they cannot be in contact with machined surface which results high surface finished work surface is obtained.Whereas, in the case of nonmagnetic field, from Figure 8 it can be observed that as cutting speed increases the surface roughness increase.Due to high cutting speed, the temperature of In718 increases during turning operation and also because of In718 low thermal conductivity property, heat is not easily dissipated.Hence surface roughness of machined In718 increases with increasing cutting speed.It is observed that in case of nonmagnetic field (Figure 10) the chips are, long snarled.It can reflect that such kind of chips are not easily removed from table the cutting zone or they will be in contact with machined surface which results poor surface finished work surface is obtained.The surface morphology of the workpiece under different cutting conditions is presented in Figure 11.The surface morphology shows that the hard abrasive carbide particles.Higher temperature was produced at the cutting zone as the cutting speed increased.In718 shows good strength at an elevated temperature which increase with cutting speed.At higher cutting speed, the feed rate increases which leads to reduction in machining time.Heat dissipation does not take place effectively due to this reduced machining time and hence at higher cutting speed surface roughness decreases.
In Figure 12, thin and thick feed marks are observed.During nonmagnetic field machining, high cutting forces and tool pressure causes workpiece get plastically deformed at outer surface.At the same time heat generated during turning process gets stored in IN718.As IN718 has lower thermal conductivity, this stored heat is unable to dissipate easily during turning process.As soon as cutting forces and tool pressure get removed, heat stored inside material causes swelling of the material during solidification.This leads to large material recovery volume which causes large noticeable thick feed marks on machined surface 25 as shown in Figure 13.
Whereas from Figure 12B, 13, it seems that at 426 Gauss magnetic field machining thin feed marks are observed.This is owing to fact that, development of Lorentz forces in material in the presence of external applied magnetic field tend to reduces forces and pressure applied by tool.This Lorentz forces helps to reduce the rate of plastic deformation of the F I G U R E 13 Generation of thin and thick feed marks on surface after swelling and recovery.

Tool wear
Tool wear is measured at each tool corner after each machined surface of all 12 tests and is shown in Table 8.The tool wear of the turned specimens are measured under tool maker microscope (Make-Mitutoyo, least count 0.005 mm).These results demonstrate that tool wear decreases with increasing magnetic field strength and cutting speed.In case of nonmagnetic machining tool wear increases with increase in cutting speed, which is shown in Figure 14.Basically, while turning of In718 with TiAlN coated tool, one of the major characteristics of tool wear is adhesion.Metal melting is followed by metals adhering to the tool, which leads to adhesive tool wear. 18SEM images of tool for magnetic and nonmagnetic field are shown in Figure 15A, B. From these figures, it can be observed that, for nonmagnetic field turning operations, the adhesion of material seems to be more compared with the magnetic field turning operation.As a result, tool wear is more in without magnetic field machining which is shown in Figure 16.
Figure 17 shows the BUE formation and adhesion of fine particles on the tool.The fine particles are produced during turning operation. 26It is found that the wet machining produces more fine particles than dry machining. 27It depends upon workpiece material, tool material, thermal energy, plastic deformation, cutting temperature, cutting parameter, macroscopic, and microscopic friction.The nanoparticles are produced due to friction between the workpiece and tool, and friction between the tool rake face and chip (Figure 18).As cutting speed increases temperature in the primary and secondary shear zone increases (Figure 19).This temperature helps to separate some developed fine particles.So, these nano particles are deposited on the tool nose radius, rake face and on the workpiece surface.Hence tool wear is more at high-speed nonmagnetic field turning operation.Figures 15 and 16 also shows the adhesion on tool rake and flank side and it is observed that the adhesion is more at high-speed nonmagnetic field compared to magnetic field machining.Magnetic line of forces generated around tool, repulse these nano particles from tool rake side and from workpiece surface as depicted in Figure 20 which majorly contributes to decreases tool wear.F I G U R E Effect of magnetic field on nano particles. 28,29

TA B L E 4 F
Specification of N52 magnet.I G U R E 4 Different positions of N52 magnet (A), (B), and (C). (A) First position of N52 magnet, (b) second position of N52 magnet, and (c) third position of N52 magnet.

F I G U R E 5 F I G U R E 6
Experimental methodology.Surface roughness tester setup.

F I G U R E 8
Variation of surface roughness (Ra) with magnetic field at different speeds.F I G U R E 9 Chip observed with magnetic field at different speeds. (A) 40 m/min, (B) 60 m/min, and (C) 80 m/min.

F I G U R E 10
Chip observed with nonmagnetic field at different speeds.(A) 40 m/min, (B) 60 m/min, and (C) 80 m/min.

F I G U R E 11
Surface morphology at cutting speed 80 m/min for nonmagnetic field.F I G U R E 12 Thick and thin feed marks at speed 80 m/min of the workpiece under different test conditions.(A) 0 Gauss, (B) 426 Gauss.

F I G U R E 14
Variation of tool wear with magnetic field at different speeds.F I G U R E 15 (A) SEM image of adhesion of material on tool rake face.(a) 0 Gauss, 80 m/min.(b) 426 Gauss, 80 m/min.(B) SEM image of adhesion of material on tool flank side.(a) 0 Gauss, 80 m/min, (b) 426 Gauss, 80 m/min.
U R E 16 SEM image of tool wear at (A) 426 Gauss and 80 m/min speed.(B) 0 Gauss and 80 m/min speed.F I G U R E 17 SEM image of BUE formation and adhesion of fine particle on tool.

F
I G U R E Chip, nano particles and BUE formation in turning operation. 28F I G U R E Schematic of heat zone in turning operation.
24operties of In718.23Chemicalcomposition of In718.24 Details specification of coolant.Experimental parameters.Experimental results of surface roughness.
TA B L E 5

speed, m/min Magnetic field intensity in Gauss Tool wear in mm
Experimental results of tool wear in mm.