Machinability studies of Al–4Mg/in-situ MgAl2O4 nano composites: measurement of cutting forces and machined surface roughness

The present research aims to study the dry turning machinability characteristics of in situ Al-4Mg/MgAl2O4 nanocomposite by High-Speed Steel tool. The influence of various machining process parameters, such as feed rate, depth of cut and cutting speed on the surface roughness and cutting force of the nanocomposites was measured while performing dry turning. From the turning operation results, it is noticed that up to 100 m min−1, the cutting force increased and with further increases in cutting speed, the cutting force starts decreasing up to 150 m min−1. The type of chips and built-up edge (BUE) development were studied using a scanning electron microscope. BUE formations were higher at low cutting speeds (50 m min−1) and lower at high cutting speeds (150 m min−1). At a given depth of cut and feed rate, with an increase in cutting speed, the length of the chip and chip curls increased. Further, higher 2 wt% of in situ MgAl2O4 addition changes long-curled chips to segmental-type chips. With a feed rate of 0.14 mm/rev, the Al-4Mg/1 wt% MgAl2O4 nanocomposite showed the lowest surface roughness value of 2.4 μm proving usage of high speed steel can provide a better surface finish while turning Al-4Mg/MgAl2O4 nanocomposite.


Introduction
Manufacturing of a component includes the removal of material or allowances as one of the major processes while transforming a semi-manufactured component into a completed component.Machining of material is mainly based on different factors such as machining parameters, cutting tool and edge stereotype, workpiece material type and its characteristics, operating environment etc [1][2][3].Considering the different parameters, certain indicators are considered to identify the machining efficiency.The most commonly used indicators used to identify the performance of a machining process are material removal rate, cutting forces, surface roughness values, tool wear and life, shape of the chips etc [4][5][6].Considering different machining operations, turning is proposed to be one of the most commonly used machining operations wherein a cutting tool is used to remove the metal from rotating workpiece to produce the final product.Machining hard materials like composites via turning operations leads to the generation of higher cutting forces and hence the study of cutting forces is considered to be the need of the hour.Identifying a cutting tool for carrying out the machining of hard materials is mainly based on the cutting parameters based on which machining is carried out [7,8].Cutting process parameters also has a major impact on defining the cutting force generated and tool life.
Machining composites is often a tedious process as the hard reinforcement particles reinforced in the matrix metal would lead to severe issues in the machining process such as dimensional and surface concerns of the machined workpiece and short tool life due to wear and tear [9][10][11].This makes the machining of metal matrix composites a tedious process as proper tool material and processing conditions have to be identified to achieve a proper machined surface with prolonged life for the tool [12,13].When it comes to turning a metal matrix composite material, researchers mainly focused on identifying the proper machining parameters along with the surface integrity, cutting forces, chip formation and its analysis along with cutting forces and tool wear and its mechanisms [14][15][16].In the few past years, aluminum situ Metal Matrix Composite (MMCs) have gained significance due to its being free from interfacial reactions, which provides strong bonding with matrix and excellent wettability [17][18][19][20][21]. Any interfacial reactions of the reinforcement with matrix will reduce the load transferring capacity at interface and further it will affect the mechanical properties of MMCs.In ex situ composites the reinforcement phase will be added externally, whereas in situ MMCs, the reinforcement will form due to in situ reactions within the alloy matrix, which is controlled by melting process parameters like melt atmosphere, holding time temperature [12,[22][23][24].
Harini et al [25] Al-MgAl 2 O 4 master alloy are made under UT utilizing Al-2 weight percent Mg alloy and SiO 2 (5 weight percent) precursor; the effectiveness of its grain refinement is examined.Due to ultrasonic cavitation-enhanced wetting between the matrix and MgAl 2 O 4 particles, the grain size of the matrix has decreased by 11-12 folds upon UT.Satish Kumar et al [26] produced Al-2Mg-1Si/MgAl 2 O 4 composites by utilizing stir casting with ultrasonic treatment to help by adding SiO 2 oxide source.In comparison to unreinforced Al-2Mg-1Si, the synthesized composite showed an 82.6% increase in hardness and a 73.4% rise in UTS with the addition of 2 vol% MgAl 2 O 4 particles.
Kara et al [27] produced aluminum metal matrix composite contains 3, 5, and 7 weight percent tungsten carbide (WC) reinforcement, made by stir casting.The addition of WC particles enhanced mechanical qualities but lowered machinability, according to an investigation of tensile strength, hardness, wear resistance, and machinability.
The main concern that limits the use of MMCs in various applications is their poor machinability, which comes from the addition of hard ceramic particulates into soft metal matrix [28].Manna and Bhattacharyya et al [29] studied the effect of cutting feed, speed and depth of cut on the wear of tool and built-up edge (BUE) formation while performing turning process using an uncoated carbide tool on Al-SiC particle reinforced MMC.At high cutting speed and low depth of cut, they noticed the formation of very less amount of BUE.Quan and Zhou et al [30] examined the tool wear mechanisms throughout the machining of SiC-reinforced Al MMCs.The abrasive wear on tool flank surface was observed and they found that carbide tool was mainly apt for machining MMC reinforced with fine size SiC.They also noticed that the size of the SiC particles and their volume fraction in MMCs significantly affected the tool life.Ananda Krishnan et al [31] investigated the machinability of Al-6061/in situ TiB 2 MMCs synthesized through flux addition.They observed that the surface roughness, cutting force and flank wear are increases at higher depth of cut.Sener Karabulut et al [32] examined the effect of milling process parameters cutting force and surface roughness during machining of AA7039/ Al 2 O 3 MMCs by carbide insert uncoated.Regression analysis and artificial neural network were employed to find the cutting force and surface roughness.ANOVA results confirmed that the type of material was the most significant factor that controls surface roughness with 85.24% contribution.The cutting speed and feed rate affect the surface roughness by 5.05% and 7.12% correspondingly.The least effect on surface roughness was observed by depth of cut.
Considering machining process, along with the material removal rate and surface morphology, tool wear, chip morphology and cutting forces have to be given equal importance as these do have effect on the machinability of the material.Studying the chip morphology has a major impact on the machining process as it affects the cutting force, surface integrity, tool and workpiece conditions [33][34][35].Studies have proved that the morphology of chips has a major role in defining the life of cutting tool and workpiece [36][37][38].During the chip removal process, a part of the temperature removed is taken away with the thrown away chip and any variation in the formation of chip size and shape would affect the life of cutting tool and workpiece [39][40][41].
Even though studies have been carried out by researchers to identify the cutting conditions for attaining higher metal removal rate with better surface characteristics, very limited articles are reported on machinability of MgAl 2 O 4 in-situ nanocomposites.Hence, the purpose of the current work is to investigate the role of the cutting parameters, e.g.depth of cut, feed rate and cutting speed on the formation of chips, surface roughness, cutting forces and BUE formation) while doing dry turning of operation on Al-4Mg/ in situ MgAl 2 O 4 nanocomposites.

Materials and methods
Commercial quality magnesium (99.92 wt%) and EC-grade pure aluminum (0.07 wt% Si, 0.1 wt% Fe, 0.005 wt% Ti, and remainder Al) were utilized as the starting ingredients.An oxygen source of around 20 μm in size H 3 BO 3 was utilized for the in situ synthesis of MgAl 2 O 4 .Initially, 4 weight percent of magnesium was dissolved in molten aluminum to create the Al-4Mg matrix alloy.Powdered H 3 BO 3 is pre-weighed to obtain distinct weight percentages (1, 2, and 3) of MgAl 2 O 4 then it is covered with aluminum foil and heated to eliminate moisture and other volatile contaminants.The molten Al-4Mg alloy was kept at 750 °C after the packets of preheated H 3 BO 3 Powders were added.750 °C was the constant temperature for 15 min during the melt.750 °C was the constant temperature for 15 min during the melt.For the purpose of promoting the appropriate solubility of H 3 BO 3 powders and facilitating the in situ reaction, the melt was stirred every five minutes.The melt receives a 5-min ultrasonic wave treatment after being held for 15 min.High-power ultrasonic waves with an intensity of approximately 128 watts per centimeter and a frequency of 20.1 kHz were produced using a magneto restrictive transducer (RELTECH, Russia) and transmitted to molten melt using an SS304 sonotrode (figure 1 schematic depiction).Following ultrasonic treatment, the melt is poured into 20 mm diameter by 120 mm high cast iron molds that have been warmed to 300 °C.A specimen from the cast sample was sectioned and polished for microstructural investigation in accordance with standard metallography procedures.A 2% tetrafluoro boric acid (HBF4) solution was used to electrochemically etch the polished specimens for 60 s at 20 volts DC.After that, the etched specimens were examined using a SEM (Carl Zeiss EVO 18, Germany) connected to an EDS (EDAX) in order to confirm the formation of MgAl 2 O 4 .Vickers microhardness of Al-4Mg/in situ MgAl 2 O 4 nanocomposite composites was measured using a Mutityo hardness tester with a 100 g load and a 15-s dwell time.A tensile test was performed as per standard ASTM E8M with a 0.01 s −1 strain rate.The mean of four test samples was considered as the final hardness and tensile values.

Machinability studies
The dry turning operation experiments were carried out for 1, 1.5, and 2 wt% in situ MgAl 2 O 4 nanocomposite cylindrical samples with a diameter of 35 mm and a length of 250 mm on a lathe machine (Kirloskar, Turnmaster) with a three-jaw chuck.A schematic diagram of the lathe machine integrated with the tool dynamometer is shown in figure 2. The cutting tool made up of high-speed steel with a single point cutting edge was fixed in a tool holder.A new cutting tool is used for each turning operation.The tool geometry and turning operation process parameters followed in the experiment are given in table 1.The turning operation experiments were carried out to find out the surface roughness and cutting force for various depths of cuts (0.5, 1 and 1.5 mm), feed rates (0.14, 0.28 and 0.42 mm rev −1 ), and cutting speeds (50, 100 and 150 m min −1 ).Based on the diameter of the MMC sample and spindle speed, the cutting speed was determined as per the standard chart.The tool holder was connected to a tool dynamometer (Type 9257B) attached to an amplifier from Kistler (Type 5070).Cutting force data were collected using software (DynaWareKistler type 2825A-02).
After each experiment, the machined sample surface roughness (Ra) was measured with a scan length of 50 mm using the surface roughness (Mitutoyo, Surftest SJ-410) apparatus.Surface roughness data were collected using Vision 32 software.Cutting tool morphology and buildup edge formation over the tool tip were analyzed using a scanning electron microscope (SIGMA HV-Carl Zeiss with Bruker).

Microstructure and mechanical properties
The microstructure of in situ MgAl 2 O 4 nanocomposite is explained in a previous study [24].It was observed that the addition of in situ MgAl 2 O 4 nanoparticles by ultrasonic treatment (UT) significantly refined the grain size of the Al-4Mg alloy from 982 μm to 134 μm [24].The grain size of the Al-4Mg/in situ MgAl 2 O 4 nanocomposite plays a very important role in hardness and tensile properties.Figure 3 The microhardness of the in situ MgAl 2 O 4 nanoparticle reinforced significantly increased from 62 HV to 86 HV, an increase of 38%.Significant improvements in yield strength (YS) and ultimate tensile strength (UTS) were observed by means of UT and MgAl 2 O 4 particles.YS of the Al-4Mg alloy increased from 70 MPa to 97 and UTS increased from 170 MPa to 230 MPa, increasing by 38%. and 35%, respectively as reported in table 2. Significant grain refinement and uniform distribution of MgAl 2 O 4 particles by UT, excellent bonding between the particles and matrix lead to good enhancement in the mechanical properties of the Al-4Mg/in situ MgAl 2 O 4 nanocomposite.Hardness and tensile properties are improved without an appreciable reduction in ductility due to the refinement of the Al-4Mg alloy.

Cutting force and surface roughness
Turning operation experiments were performed on the produced composite samples with different wt% of MgAl 2 O 4 reinforcement with respect to cutting force, cutting speed, and surface roughness, as shown in figures 4(a) and (b).It is noticed that the cutting force increased with an increase in cutting speed from 50 m min −1 , to 100 m min −1 and with further increases in cutting speed, the cutting force starts decreasing with an increase in speed up to 150 m min −1 at a constant depth of cut of 1 mm and feed rate of 0.28 mm rev −1 for MgAl 2 O 4 added composites.The existence of hard in situ MgAl 2 O 4 reinforcement significantly refined the grain size and increased the hardness of the composites.Therefore, with an increase in the wt% of MgAl 2 O 4 , the cutting force required for turning the composites also increased.Similar kinds of results were noticed for other combinations of turning parameters.Under dry turning operations, at high cutting speeds, workpiece materials get softened due to more heat generation at the tool-chip interface, which further reduces the cutting force for turning.As shown in figure 5(a), it is observed that at lower speeds, more built-up edges (BUE) form at the tool tip, which leads to more surface roughness.At higher cutting speed (150 m min −1 ), the BUE formation is very low, which gives a good surface finish to the machined surface.At low cutting speeds, the contact area and contact time between the tool and workpiece are higher [27,33].Composite machined samples Surface roughness (Ra) is higher at low cutting speeds as shown in figures 5(b)-(c) and Surface roughness is observed to be lower at relatively higher cutting speeds 150 m min −1 as shown in figure 5(d).
At a constant depth of cut (1 mm) and cutting speed (100 m min −1 ), the cutting force required for turning operations gradually increased with an increase in feed rate as shown in figure 6(a).With the increase in feed rate from 0.14 to 0.42 mm rev −1 , the cutting force was observed to be three times higher.The same trend (increase in cutting force) was observed for all composites with various wt% of MgAl 2 O 4 nanoparticles.Composite workpiece sample offers more resistance to the cutting tool at a higher feed rate along the cutting direction, which could be the reason for the increase in cutting force.While performing the turning operation, at a constant depth of cut (1 mm) and cutting speed, thrust force was observed to be increased with a higher feed rate, which led to more heat generation and vibration, which in turn increased the surface roughness of the machined surface as shown in figure 6(b).
Figures 7(a) and (b) represent the role of depth of cut on machined surface roughness and cutting force is in It is noticed that at a constant feed rate (0.36 mm rev −1 ) and cutting speed (100 m min −1 ), the surface roughness and cutting force tend to increase with an increase in depth of cut (1.5 mm).More depth of cut increases the rate of material removal from the Al-4Mg/in situ MgAl 2 O 4 nanocomposite, which leads to a significant increase in the required cutting force for turning operations.At high depths of cut, high cutting forces led to fracture of in situ MgAl 2 O 4 nanoparticles and the formation of voids, which further increased the surface roughness of the machined surface, as shown in figure 7(b).Similar kinds of results were noticed by Kara et al [27].

Built-up edge formation
While doing a turning operation on in situ MgAl 2 O 4 /Al-4Mg nanocomposite, with a constant depth of cut and feed rate, the effect of cutting speed on built-up edge (BUE) formation over the cutting tool is illustrated in figure 7. It was noticed that, at higher cutting speeds, the BUE height decreases, which leads to an improvement in tool life.The formation of stable BUE over the cutting edge will act as a protective layer that prevents the wear  of the cutting edge.It can be noticed that BUE increases the surface roughness of the sample.Experimental results of the turning operation exhibited that with an increase in cutting speed from 50 to 150 m min −1 , there was a 50% decrease in the height of BUE in comparison with its height at low cutting speeds.Figure 8

Conclusions
Al-4Mg alloys reinforced with 1, 1.5, and 2 wt% in situ MgAl 2 O 4 nanocomposites are developed using ultrasonic cavitation treatment.The present investigation was conducted to analyse the machinability characteristics of Al-4Mg/in situ MgAl 2 O 4 nanocomposites and the conclusions are listed below.3.More cutting force is needed for machining at low cutting speeds (100 m min −1 ) and then decreases at higher cutting speeds (150 m min −1 ).The BUE formation was observed at a low cutting speed (50 m min −1 ), and the height of the BUE decreased with an increase in cutting speed (150 m min −1 ). 4. The surface roughness of the machined surface was significantly reduced with an increase in cutting speed.The surface roughness of the machined surface increased with BUE formation and an increase in wt% MgAl 2 O 4 nanoparticles.
5. Increasing the wt% of MgAl 2 O 4 nanoparticle reinforcement leads to a substantial reduction in chip size, with notable improvements observed at 2 wt%, attributed to higher hardness and reduced ductility in the nanocomposite.

Figure 1 .
Figure 1.Schematic diagram of the ultrasonic treatment experimental setup.
(a) shows the typical SEM image of Al-4Mg/ in situ MgAl 2 O 4 nano composites, figure 3(b) shows the EDS Spectra of MgAl 2 O 4 particles and the matrix alloy.Figure 3 confirms the presence of MgAl 2 O 4 particles and their uniform distribution throughout the matrix.

Figure 4 .
Figure 4. Role of cutting speed on cutting force and roughness of the surface at depth of cut = 1 mm and feed rate = 0.28 mm rev −1 .

Figure 5 .
Figure 5. (a) Shows the formation of built-up edge, (b)-(d) SEM images of the machined surface at a constant depth of cut of 1 mm, feed rate of 0.28 mm rev −1 and at various cutting speeds (b) 50 m min −1 , (c) 100 m min −1 and (d) 150 m min −1 .

Figure 6 .
Figure 6.The role of feed rate on (a) cutting force and (b) surface roughness at 100 m min −1 cutting speed and 1 mm depth of cut.
represents the BUE height machined with various cutting speeds and a constant DOC of 1 mm, feed rate of 0.28 mm rev −1 .The BUE height at various cutting speeds of 50, 100, and 150 m min −1 is 1.2 mm, 0.92 mm and 0.62 mm respectively.Compared to BUE height at low cutting speed of 50 m min −1 , at high cutting speed of 150 m min −1 , BUE height is almost half of it.

3. 4 .
Chips formationThe characteristics of the chips obtained from the turning of Al-4Mg/in situ MgAl 2 O 4 nanocomposite using a High-Speed Steel tool are shown in figures 9(a)-(i).A turning operation was carried out under various cutting speeds, to investigate its effect on chip generation by keeping DOC 1 mm and feed rate 0.28 mm rev −1 constant for 1, 1.5 and 2 wt% MgAl 2 O 4 nanocomposites.Turning 1 wt% MgAl 2 O 4 nanocomposite at a cutting speed of 50 m/min leads to the formation of thin, segmented chips.With further increases in cutting speed from 50 to 100 m min −1 , C-type (figures 9(a)-(b)) chips are formed for the foregoing composite, and spiral coil type chips were observed at 150 m min −1 cutting speed as shown in figure 9(c).It may be due to the low DOC, cutting speed, and feed rate that the composite sample behaved as a brittle one.On the other hand, cutting speeds above 100 m min −1 caused the composite sample to behave as a ductile one.The chips formed while turning 1.5 wt% MgAl 2 O 4 nanocomposite at a cutting speed of 50 m/min are represented in figure 9(d).This shows the development of segmented, short chips at a cutting speed of 50 m min −1 .Turning with a 100 m min −1 cutting

Figure 7 .
Figure 7.The role of (a) depth of cut on cutting force at feed rate = 0.28 mm rev −1 and (b) Feed rate on surface roughness at cutting force = 100 m min −1 .

Figure 8 .
Figure 8.The role of cutting speed on BUE at DOC = 1 mm and feed rate = 0.28 mm rev −1 .

Table 2 .
Mechanical properties of the Al-4Mg alloy and its composites.
1.The SEM/EDS results confirmed the presence and uniform distribution of MgAl 2 O 4 nanoparticles and excellent bonding of MgAl 2 O 4 particles with the matrix, resulting in significant grain refinement in the matrix.2. The in situ MgAl 2 O 4 nanoparticle reinforcement notably increased the microhardness of the Al-4Mg alloy from 62 HV to 86 HV, marking a 38% enhancement.This improvement in hardness correlated with significant increases in yield strength (from 70 MPa to 97 MPa) and ultimate tensile strength (from 170 MPa to 230 MPa), reflecting enhancements of 38% and 35%, respectively, without a substantial reduction in ductility for the Al-4Mg/in situ MgAl 2 O 4 nanocomposite.