Experimental Characterization of Tool Wear Morphology and Cutting Force Proﬁle in Dry and Wet Turning of Titanium Metal Matrix Composites (Ti-Mmcs)

metals

Article Experimental Characterization of Tool Wear Morphology and Cutting Force Proﬁle in Dry and Wet Turning of Titanium Metal Matrix Composites (Ti-MMCs)

Masoomeh Safavi 1, Marek Balazinski 2, Hedayeh Mehmanparast 3 and Seyed Ali Niknam 1,2,*

1 School of Mechanical Engineering, Iran University of Science and Technology, Tehran 13114-16846, Iran; [email protected] 2 Department of Mechanical Engineering, Polytechnique Montreal, Montréal, QC H3T 1J4, Canada; [email protected] 3 Gina Cody School of Engineering and Computer Science, Department of Mechanical, Industrial and Aerospace Engineering (MIAE), University of Concordia, Montréal, QC H3G 1M8, Canada; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: +98-21-7724-0203

Received: 6 October 2020; Accepted: 27 October 2020; Published: 31 October 2020

Abstract: Metal-matrix composites (MMCs) are made of non-metallic reinforcements in metal matrixes, which have excellent hardness, corrosion, and wear resistance. They are also lightweight and may pose a higher strength-to-weight ratio as compared to commercial titanium alloys. One of the MMCs with remarkable mechanical properties are titanium metal matrix composites (Ti-MMCs), which are considered a replacement for super-alloys in many industrial products and industries. Limited machining and machinability studies of Ti-MMCs were reported under different cutting and lubrication conditions. Tool wear morphology and life are among the main machinability attributes with limited attention. Therefore, this study presents the effects of cutting and lubrication conditions on wear morphology in carbide inserts when turning Ti-MMCs. To that end, maximum flank wear (VB) and cutting forces were recorded, and the wear morphologies within the initial period of the cut, as well as the worn condition, were studied under dry and wet conditions. Experimental results denoted that despite the lubrication mode used, abrasion, diffusion, and adhesion mechanisms were the main wear modes observed. Moreover, built-up layer (BUL) and built-up edge (BUE) were the main phenomena observed that increase the tendency of adhesion at higher cutting times.

Keywords: Ti-MMC; machinability; tool wear; turning

1. Introduction Titanium metal matrix composites (Ti-MMCs) are known as a new generation of material that has many advantages over titanium alloys. Generally, it is recognized by reinforcements such as titanium carbide particles (TiC), titanium boride (TiB) and continuous or discontinuous silicon carbide ﬁber (SiC) [1,2]. Among them, reinforcement by hardest refractories, such as TiC particles, may provide excellent properties at high temperatures and with abrasion resistance. The abrasion increases proportionally according to the volume of reinforcements used in the MMC [3]. Due to high strength, low weight, and signiﬁcant strength-to-weight ratio, as well as creep resistance at high temperature, Ti-MMCs are considered a desirable alternative to commercial superalloys (including Inconel and titanium alloys) in a wide range of applications, including aerospace and automotive industries [4–9]. However, due to the abrasive properties of hard reinforcement particles, machining a Ti-MMC is

Metals 2020, 10, 1459; doi:10.3390/met10111459 www.mdpi.com/journal/metals Metals 2020, 10, 1459 2 of 14 difficult and challenging [10,11]. Reinforced MMCs with ceramic particles have several advantages over their base metals, including unique strengths, higher wear resistance at high temperatures, and thermal stability coefficients. Consequently, low surface quality and limited tool life are expected in machining reinforced MMCs. The use of carbide and polycrystalline diamond (PCD) tools in machining MMCs have been observed in the open literature. Many other studies were also performed on turning, followed by horizontal milling and drilling operations [12]. MetalsSeveral 2020, 10, x studies FOR PEER [13 REVIEW–15] show that abrasion is the primary tool wear mechanism in machining3 of 14 MMCs. Abrasion is occurred due to complicated physical, chemical, and thermomechanical interactions parameters were selected according to recommendations made by the cutting tool manufacturer because of the collision between the reinforcing particles and the cutting tool [16]. The rapid tool wear, SECO Inc (Montreal, QC, Canada). The experimental works were repeated twice. which in general appears due to abrasion, is one of the undesirable phenomena in machining Ti-MMCs. Due to the very low thermal conductivityTable 1. The specifications of Ti-MMCs, of the the high inserts level used. of generated temperature in the cutting zone leads to the rapid abrasion of the cutting tool in a tiny area around the cutting edge [17]. The presence ofInsert abrasion Code has been Grade investigated in almost allCoating cutting conditions [8,18,19]. Oxidation CNMG 120404- PVD TiSiN-TiAlN coated carbide with W as and diffusion mechanisms also appearTS2000 due to high-temperatures generated in the machining zone and rapid abrasion in cuttingMF1 tools [20]. As notedthe in core several element studies and [9 ],Co the and permitted Cr as binders levels of maximum CNMG120404- PVD TiSiN-TiAlN coated carbide with W as flank wear (VB) vary between 0.2TS2500 and 0.5 mm for different materials [1,5]. In the case of superalloys, it is advised not toMF4 use cutting tools with VBthe highercore element than 0.3and mm. Co and The Cr cutting as binders time to reach the maximum permitted VB is the useful life of the cutting tool. Figure1 shows the typical tool wear curve that is composedTable 2. of Elemental three distinct composition regions of due titanium to increased metal matrix tool composites wear with (Ti-MMCs) cutting time: (wt%). (I) primary or initial wearChemical (II) steadyAnalysis wear, of Ti-MMC and (III) accelerated Al V C wear, O which Fe was N proposed H by Taylor Ti [ 21]. A longer time to reach maximumMass% VB means better 5.5 tool3.8 performance0.9 0.2 0.004 under 0.2 similar <0.003 cutting Remainder conditions (89. [221) ,22].

Figure 1. Experimental set-up used.

DespiteTo evaluate much the research relationship work inbetween recent decades, cutting factorsforce and governing tool wear initial rate, tool the wear cutting and sizeforces have in notdifferent yet been directions investigated were inrecorded a wide scope using [3 ].a RegardlessKISTLER turning of investigations table dynamometer made on machining (Winterthur, and machinabilitySwitzerland) (Figure evaluation 2) with of Ti-MMCsa sampling [17 frequency,23,24], few of 12 studies kHz. To focused study oninsert the wear adequate mechanisms, cutting tools two usedSEMs in (JEOL, machining JSM-840A Ti-MMCs and [25JEOL]. A JSM review 7800F of the FEG-SEM, literature St-Hubert, indicated that Laval, a Ti-MMC QC, Canada) is classified equipped as an extremelywith Oxford hard X-ray to cut detection material, systems and surprisingly (AZtec EDX) limited for elemental studies were analysis reported and on quantitative machining Ti-MMCsmapping underwere used. different Moreover, machining the JEOL and lubrication JSM was a conditions. field emission In principle, SEM equipped no meaningful with field information emission guns was available(FEG), which on initial provided toolwear, a high and resolution factors governing of 0.8 nm toolat 15 life kV improvement and 1.2 nm at when 1 kV. machining Turning tests Ti-MMCs. were Superficialrepeated twice, knowledge and the on average the relationship values were between used. The the tool initial wear tool measuremen wear morphology,ts were ultimate conducted tool from life, andthe SEM cutting images forces according may lead to to the an ISO3685 inadequate standard. selection As ofnoted cutting earlier, parameters the maximum and decreased flank wear tool (VB) life. Towas remedy used as this the lacktool wear of knowledge, attribute. To the evaluate current studythe tool presents wear morphology, the effects of the cutting chemical and composition lubrication conditionsof the tested on materials initial wear was morphology, studied (Table size, 2). andThis cutting allowed forces a better in the determination turning of Ti-MMCs. of the wear The modes. wear modes,For instance, as well the as presence the relationship of material between composition cutting forces on the and tool wear surface morphology, might be were interpreted investigated. as the signature of adhesion. To detect chemical compositions of the insert surface as a result of generated wear modes, EDX was implemented with particular emphasis on those areas which seem to affect adhesion and abrasion modes. The SEM images of the rake side and flank side of the unused insert TS2500 and EDX of the flank side of the insert surface are presented in Figure 2 and Table 2, respectively. According to Figure 2 and Table 2, the main contributing elements on the insert surface are Al and Ti. The EDX image of the unused flank side of the insert (Figure 2c) comprised of over 99% Al and Ti. The X-axis of Figure 2c represents energy (keV), and the Y-axis is counts.

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2. Materials and Methods Turning was conducted on a cylindrical rod of Ti-MMC with a diameter of 119.6 mm and a length of 140 mm using two grades of coated carbide inserts, TS2500 and TS2000, with commercial codes CNMG 120404-MF1 and CNMG120404-MF4, respectively. According to recommendations by the SECO tools catalog, TS2000 and TS2500 are proposed grades for machining heat-resistant superalloys and titanium alloys. TS2000 is an ideal choice for finishing and semi-finishing of heat-resistant alloys, while TS2500 is recommended for roughing and semi-roughing of heat-resistant alloys. The MF1 is the proposed chip breaker for finishing and semi-finishing operations, and MF4 is proposed for semi-finishing to roughing operations of heat-resistant alloys. The full specifications of the inserts used, as well as the chemical composition of the tested material, are presented in Tables1 and2. An overview of the experimental set-up is presented in Figure1. The fixed levels of cutting speed 70 m/min, feed rate 0.15 mm/rev as well as the depth of cut 0.5 mm were used. The experimental works were conducted in both dry and wet modes. The Bio-lubricant Mecagreen 550 was used as the lubricant in this work. Besides lubrication modes, another variable used in this work was cutting time, which varied within intervals from 0.5 to 8 s. Cutting parameters were selected according to recommendations made by the cutting tool manufacturer SECO Inc (Montreal, QC, Canada). The experimental works were repeated twice.

Table 1. The speciﬁcations of the inserts used.

Insert Code Grade Coating CNMG PVD TiSiN-TiAlN coated carbide with W as the core TS2000 120404-MF1 element and Co and Cr as binders PVD TiSiN-TiAlN coated carbide with W as the core CNMG120404-MF4 TS2500 element and Co and Cr as binders

Table 2. Elemental composition of titanium metal matrix composites (Ti-MMCs) (wt.%).

Chemical Analysis of Al V C O Fe N H Ti Ti-MMC Mass% 5.5 3.8 0.9 0.2 0.004 0.2 <0.003 Remainder (89.2)

To evaluate the relationship between cutting force and tool wear rate, the cutting forces in different directions were recorded using a KISTLER turning table dynamometer (Winterthur, Switzerland) (Figure2) with a sampling frequency of 12 kHz. To study insert wear mechanisms, two SEMs (JEOL, JSM-840A and JEOL JSM 7800F FEG-SEM, St-Hubert, Laval, QC, Canada) equipped with Oxford X-ray detection systems (AZtec EDX) for elemental analysis and quantitative mapping were used. Moreover, the JEOL JSM was a field emission SEM equipped with field emission guns (FEG), which provided a high resolution of 0.8 nm at 15 kV and 1.2 nm at 1 kV. Turning tests were repeated twice, and the average values were used. The tool wear measurements were conducted from the SEM images according to the ISO3685 standard. As noted earlier, the maximum flank wear (VB) was used as the tool wear attribute. To evaluate the tool wear morphology, the chemical composition of the tested materials was studied (Table2). This allowed a better determination of the wear modes. For instance, the presence of material composition on the tool surface might be interpreted as the signature of adhesion. To detect chemical compositions of the insert surface as a result of generated wear modes, EDX was implemented with particular emphasis on those areas which seem to affect adhesion and abrasion modes. The SEM images of the rake side and flank side of the unused insert TS2500 and EDX of the flank side of the insert surface are presented in Figure2 and Table2, respectively. According to Figure2 and Table2, the main contributing elements on the insert surface are Al and Ti. The EDX image of the unused flank side of the insert (Figure2c) comprised of over 99% Al and Ti. The X-axis of Figure2c represents energy (keV), and the Y-axis is counts. Metals 2020, 10, 1459 4 of 14 Metals 2020, 10, x FOR PEER REVIEW 4 of 14

(a) (b)

(c) Figure 2. SEM images of the: ( a)) rake rake side; side; ( (b)) flank flank side; ( c) EDX analysis of the flank flank side of unused insert TS2500TS2500 (wt.%).(wt%). Contribution Contribution of of elemental elemental co compositionsmpositions wt% wt.% of of the the flank flank side with largest contributions of Al = 22.4%22.4% and and Ti Ti = 77.5%77.5% 3. Results and Discussion 3. Results and Discussion The experimental studies were conducted in dry and wet lubrication modes under similar cutting The experimental studies were conducted in dry and wet lubrication modes under similar conditions and cutting tools. cutting conditions and cutting tools. 3.1. Dry Mode 3.1. Dry Mode According to Figures3 and4, after a short cutting time (e.g., 0.5 s), the matrix used in MMCs may According to Figures 3 and 4, after a short cutting time (e.g., 0.5 s), the matrix used in MMCs tend to generate built-up layer (BUL) and built-up edge (BUE). Regardless of the experimental conditions may tend to generate built-up layer (BUL) and built-up edge (BUE). Regardless of the experimental used, abrasion was the main wear mechanism in machining Ti-MMCs [18,26]. This mechanism also conditions used, abrasion was the main wear mechanism in machining Ti-MMCs [18,26]. This depends on the nature and hardness of hard particles used in the metal composite [16]. Abrasion mechanism also depends on the nature and hardness of hard particles used in the metal composite and BUL were two main wear phenomena in turning Ti-MMCs. El-Gallab and Sklad [27] indicated [16]. Abrasion and BUL were two main wear phenomena in turning Ti-MMCs. El-Gallab and Sklad that BUE might cause chipping and instability of cutting operations. Figures3 and4 show that BUL [27] indicated that BUE might cause chipping and instability of cutting operations. Figures 3 and 4 was the main wear mode in insert 1 (TS2000), while abrasion was the main wear mode in insert 2 show that BUL was the main wear mode in insert 1 (TS2000), while abrasion was the main wear mode (TS2500). The compositions wt.% of the worn areas on both inserts are shown in Table3. Even at the in insert 2 (TS2500). The compositions wt% of the worn areas on both inserts are shown in Table 3. low cutting time (0.5), the coating on some areas on the surface of insert TS2500 has been completely Even at the low cutting time (0.5), the coating on some areas on the surface of insert TS2500 has been removed, and the W (tungsten), which is the core element of the insert has been observed. In other completely removed, and the W (tungsten), which is the core element of the insert has been observed. words, abrasion had occurred even at a very low cutting time when insert TS2500 was used. In other words, abrasion had occurred even at a very low cutting time when insert TS2500 was used.

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Figure 3. SEM and EDX images for insert 1 (TS2000) after 0.5 s of cutting. Figure 3. SEM and EDX images for insert 1 (TS2000) after 0.50.5 ss ofof cutting.cutting.

FigureFigure 4.4. SEMSEM andand EDXEDX imagesimages forfor insertinsert 11 (TS2500)(TS2500) afterafter 0.50.5 ss ofof cutting.cutting. Figure 4. SEM and EDX images for insert 1 (TS2500) after 0.5 s of cutting. Table 3. The elemental compositions (wt.%) on the worn areas presented in both inserts in Figures4 and5. Table 3. The elemental compositions (wt%) on the worn areas presented in both inserts in Figures 4 and 5. Table 3. The elemental compositions (wt%) on the worn areas presented in both inserts in Figures 4 and 5. Insert TS2000 Insert TS2500 Elements/Insert Insert TS2000 Insert TS2500 Elements/InsertAbrasion Insert BUL TS2000 Insert Abrasion TS2500 BUL Elements/Insert Abrasion BUL Abrasion BUL Abrasion BUL Abrasion BUL Al (wt.%)Al (wt%) 48.7 48.7 23.8 23.8 6.3 6.3 19.9 19.9 Ti (wt.%)Al (wt%) 49.3 48.7 73.9 23.8 6.3 3.8 19.9 77.9 Ti (wt%) 49.3 73.9 3.8 77.9 W (wt.%)Ti (wt%) 2.3 49.3 2.4 73.9 3.8 89.9 77.9 2.2 W (wt%) 2.3 2.4 89.9 2.2 W (wt%) 2.3 2.4 89.9 2.2 Micro-welds and BUE were formed at the cutting edge in all experimental conditions used Micro-welds and BUE were formed at the cutting edge in all experimental conditions used (Figures 3–5). In general, at the first moments of machining operations, the rake face of the cutting (Figures 3–5). In general, at the first moments of machining operations, the rake face of the cutting tool has an intense interaction with the workpiece. Due to generated thermo-mechanical stresses, the tool has an intense interaction with the workpiece. Due to generated thermo-mechanical stresses, the initial wear behavior may occur during very short periods. Therefore, the tribological friction and initial wear behavior may occur during very short periods. Therefore, the tribological friction and wear can be attributed to areas of two new surfaces, in which the slip speed between the two new wear can be attributed to areas of two new surfaces, in which the slip speed between the two new

Metals 2020, 10, x FOR PEER REVIEW 6 of 14 surfaces at the small edge of the cutting in these regions is close to zero [28,29]. Hence, it seems that the flatness and cleanliness of the contact surface can be attributed to static friction [30]. Adhesive chips on the tool surface may act as a barrier against abrasion wear. However, at a longer cutting time, adhesive chips separated from the cutting tool surface (Figure 5), and eventually, larger BUE formed and consequently, rapid tool wear occurs [26]. Despite the cutting tool used, a gradual increase in flank wear (VB) with high impacts on initial wear was noticed in machining Ti-MMCs (Figure 6). According to Figures 5 and 6, after 60 s of turning operation, insert 2 (TS2000) had reached the maximum allowable flank wear limit (300 microns). EDX results showed that areas with abrasion, tungsten was higher than other elements because the coating layer had been removed by abrasion. After longer cutting times, more VB was seen on both inserts (Figures 5 and 6). The wider regions were attributed to abrasion, which can be related to the effects of reinforcing particles TiC, which are abrasive [8,31]. The scratches and grooves are seen and would prove the abrasion wear mechanism due to the hard and abrasive TiC particles in the workpiece material, which led to the complete removal of the coating. The effects of abrasive particles on temperature generation, tool wear morphology as well as friction need to be understood in a broader scope. However, these topics are Metals 2020, 10, 1459 6 of 14 not within the current scope of this study.

Figure 5. WearWear modes in both inserts after 60 s of cutting.

Micro-welds and BUE were formed at the cutting edge in all experimental conditions used 350 TS2000 (Figures3–5). In general, at the first momentsTS2500 of machining operations, the rake face of the cutting tool has an intense interaction300 with the workpiece. Due to generated thermo-mechanical stresses, the initial wear behavior may occur during very short periods. Therefore, the tribological friction and wear can be attributed to areas of250 two new surfaces, in which the slip speed between the two new surfaces at the small edge of the cutting200 in these regions is close to zero [28,29]. Hence, it seems that the flatness and cleanliness of the contact surface can be attributed to static friction [30]. Adhesive chips on the tool surface may act as a barrier150 against abrasion wear. However, at a longer cutting time, adhesive chips VB VB (µm) separated from the cutting100 tool surface (Figure5), and eventually, larger BUE formed and consequently, rapid tool wear occurs [26]. Despite the cutting tool used, a gradual increase in flank wear (VB) with high impacts on initial wear50 was noticed in machining Ti-MMCs (Figure6). According to Figures5 and6, after 60 s of turning operation, insert 2 (TS2000) had reached the maximum allowable flank 0 wear limit (300 microns). EDX results showed that areas with abrasion, tungsten was higher than other elements because the coating layer had been removed by abrasion. After longer cutting times, 0 10203040506070 more VB was seen on both inserts (Figures5 and6). The wider regions were attributed to abrasion, Time (s) which can be related to the effects of reinforcing particles TiC, which are abrasive [ 8,31]. The scratches and grooves are seen and wouldFigure prove6. Flank the wear abrasion (VB) profile wear for mechanism both inserts. due to the hard and abrasive TiC particles in the workpiece material, which led to the complete removal of the coating. The effects of abrasive particles on temperature generation, tool wear morphology as well as friction need to be understood in a broader scope. However, these topics are not within the current scope of this study. Since TiC is ceramic and can be even used as a coating, as a result of direct contact with the coating layer, coating removal, and scratches on the insert surface may occur. At longer cutting times, the thickness of BUL increased. These areas occurred irregularly as a result of adhesion. High tool wear, which occurred in the first few seconds of machining, was attributed to the initial impact of the cutting tool with the work part elements. Therefore, at the second stage of the tool wear curve, the stable trend of flank wear was observed. As depicted in Figure2, the third wear zone, or accelerated zone, appeared at the higher cutting time and the specific cutting time where the maximum flank wear VB can be achieved at the third wear zone. Higher permitted cutting time means longer tool life at the specific cutting speed used. According to Figure6, the maximum permitted VB (0.3 mm) for insert TS2500 was achieved in less than 60 s, which is very quick. Metals 2020, 10, x FOR PEER REVIEW 6 of 14 surfaces at the small edge of the cutting in these regions is close to zero [28,29]. Hence, it seems that the flatness and cleanliness of the contact surface can be attributed to static friction [30]. Adhesive chips on the tool surface may act as a barrier against abrasion wear. However, at a longer cutting time, adhesive chips separated from the cutting tool surface (Figure 5), and eventually, larger BUE formed and consequently, rapid tool wear occurs [26]. Despite the cutting tool used, a gradual increase in flank wear (VB) with high impacts on initial wear was noticed in machining Ti-MMCs (Figure 6). According to Figures 5 and 6, after 60 s of turning operation, insert 2 (TS2000) had reached the maximum allowable flank wear limit (300 microns). EDX results showed that areas with abrasion, tungsten was higher than other elements because the coating layer had been removed by abrasion. After longer cutting times, more VB was seen on both inserts (Figures 5 and 6). The wider regions were attributed to abrasion, which can be related to the effects of reinforcing particles TiC, which are abrasive [8,31]. The scratches and grooves are seen and would prove the abrasion wear mechanism due to the hard and abrasive TiC particles in the workpiece material, which led to the complete removal of the coating. The effects of abrasive particles on temperature generation, tool wear morphology as well as friction need to be understood in a broader scope. However, these topics are not within the current scope of this study.

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Since TiC is ceramic and can be even used as a coating, as a result of direct contact with the Metalscoating2020 layer,, 10, 1459 coating removal, and scratches on the insert surface may occur. At longer cutting times,7 of 14 Figure 5. Wear modes in both inserts after 60 s of cutting. the thickness of BUL increased. These areas occurred irregularly as a result of adhesion. High tool wear, which occurred in the first few seconds of machining, was attributed to the initial impact of the cutting tool with the350 work part elements.TS2000 Therefore, at the second stage of the tool wear curve, the TS2500 stable trend of flank wear was observed. As depicted in Figure 2, the third wear zone, or accelerated 300 zone, appeared at the higher cutting time and the specific cutting time where the maximum flank

wear VB can be achieved250 at the third wear zone. Higher permitted cutting time means longer tool life at the specific cutting speed used. According to Figure 6, the maximum permitted VB (0.3 mm) for insert TS2500 was achieved200 in less than 60 s, which is very quick. Duong et al. [32] reported that the initial cutting conditions could significantly affect the entire tool life and tool wear150 mechanisms. To interpret the correlation between the cutting forces and tool VB VB (µm) wear morphology and100 tool wear size, the cutting forces in all three directions were measured. The turning cutting forces can be expressed as thrust force, feed force, and cutting force. The overview of cutting force at 60 s, followed50 by the tool wear curve for each insert, is presented in Figures 7 and 8. According to Figure 7a, the maximum permitted tool life was reached after almost 45 s. As shown in Figure 7b, a rapid increase0 in cutting force in all three directions was attributed to elevated tool wear and removal of the cutting edge and BUE on the cutting tool (Figure 5b), which increased the contacting surface between the0 cutting 10203040506070 tool and workpiece, and eventually, more chip removal and higher cutting forces occurred (Figure 7b). Therefore, the cutting force sharply increased when VB Time (s) exceeded 300 microns. According to Figure 8, although maximum allowable VB was not reached after Figure 6. Flank wear (VB) proﬁle for both inserts. 60 s, a gradual increase inFigure cutting 6. forceFlank after wear 45 (VB) s of profile cutting for may both tend inserts. to elevate the flank wear. As shownDuong in Figures et al. [32 6,] reported7b and 8b, that increased the initial VB cutting occu conditionsrred in the could first significantly few seconds affect on thecutting entire tests. tool lifeAdequate and tool selection wear mechanisms. of cutting para Tometers interpret to thedecrease correlation the VB between in the initial the cutting wear forceszone could and tool prolong wear morphologythe tool life. and tool wear size, the cutting forces in all three directions were measured. The turning cutting forcesFigures can be expressed7b and 8b asreveal thrust that force, under feed constant force, and cu cuttingtting parameters, force. The overviewthe thrust of force cutting and force feed at force 60 s, followedtend to increase by the toolwhen wear cutting curve time for increases. each insert, In is ot presentedher words, in although Figures7 andthe 8cutting. According time does to Figure not vary7a, thesignificantly, maximum permittedthe cutting tool forces life was increase reached at after initial almost moments 45 s. As shownof cutting. in Figure Furthermore,7b, a rapid increasewhen the in cuttingpermitted force VB in was all three reached, directions the cutting was attributed force tended to elevated to change. tool wear This and can removal be attributed of the cutting to the edge intense and BUEeffects on of the the cutting worn toolinsert (Figure on the5b), cutting which increasedforce prof theile. contactingWhen the surfaceinsert edge between is worn, the cutting the contacting tool and workpiece,surface between and eventually, the workpiece more chipand removalthe edge and incr highereases. cutting Consequently, forces occurred thicker (Figure chips7 b).removal Therefore, and thehigher cutting resulting force values sharply of increased cutting forces when may VB exceededoccur. Despite 300 microns. the uniform According cutting to conditions Figure8, althoughused, the maximumincreased cutting allowable forces VB was at the not high reacheder cutting after 60 time s, a gradual were observed. increase in Referring cutting force to the after Taylor 45 s of tool cutting life maymodel, tend the to second elevate thetool flank wear wear. regi Ason, shown the steady in Figures zone,6,7 happensb and8b, increasedvery quickly, VB occurred and in infact, the firstthe third few secondsregion is on known cutting as tests.the “accelerated Adequate selection wear zone”, of cutting which parameters starts veryto soon. decrease The rapidly the VB inincreased the initial cutting wear zoneforce couldand tool prolong wear the in toolboth life. inserts was observed.

(a)

Figure 7. Cont. Metals 2020, 10, x FOR PEER REVIEW 8 of 14 Metals 2020, 10, 1459 8 of 14 Metals 2020, 10, x FOR PEER REVIEW 8 of 14

(b) (b) Figure 7. (a) Flank wear (VB); (b) cutting forces profile of insert TS2500 in 60 s of cutting. FigureFigure 7. 7.(a)( aFlank) Flank wear wear (VB); (VB); (b (b) )cutting cutting forces forces profile proﬁle ofof insertinsert TS2500TS2500 in in 60 60 s s of of cutting. cutting.

250 250 TS2000 TS2000 200 200

150 150

100 100 VB VB (µm) VB VB (µm)

5050

0 0

00 10203040506070 10203040506070 Time (s) Time (s) (a))

(b) (b) Figure 8. (a) Flank wear (VB); (b) cutting forces profile of insert TS2000 in 60 s of cutting. FigureFigure 8. (a 8.) Flank(a) Flank wear wear (VB); (VB); (b ()b cutting) cutting forces forces proﬁleprofile of of insert insert TS2000 TS2000 in 60in s60 of s cutting. of cutting.

Metals 2020, 10, 1459 9 of 14

Figures7b and8b reveal that under constant cutting parameters, the thrust force and feed force tend to increase when cutting time increases. In other words, although the cutting time does not vary significantly, the cutting forces increase at initial moments of cutting. Furthermore, when the permitted VB was reached, the cutting force tended to change. This can be attributed to the intense effects of the worn insert on the cutting force profile. When the insert edge is worn, the contacting surface between the workpiece and the edge increases. Consequently, thicker chips removal and higher resulting values of cutting forces may occur. Despite the uniform cutting conditions used, the increased cutting forces Metalsat 2020 the, higher 10, x FOR cutting PEERtime REVIEW were observed. Referring to the Taylor tool life model, the second tool wear9 of 14 region, the steady zone, happens very quickly, and in fact, the third region is known as the “accelerated 3.2. wearWet Mode zone”, which starts very soon. The rapidly increased cutting force and tool wear in both inserts was observed. The cutting tests were performed at 30, 40, 50, 60 and 120 s under wet mode. The wear modes on the3.2. rake Wet Modeface in wet machining must be studied in machinability evaluation of high-resistance alloys [9].The Figures cutting 9–11 tests wereillustrate performed the wear at 30, morpho 40, 50, 60logy and 120on sthe under flank wet face mode. and The the wear rake modes face onof both insertsthe rakeTS2000 face and in wet TS2500 machining after must 120 s be of studied cutting in oper machinabilityation. The evaluation flank sides of high-resistance of both inserts alloys were [9]. worn andFigures could 9not–11 beillustrate allowed the for wear additional morphology tests on after the flank 120 faces cutting, and the although rake face ofthe both rake inserts side TS2000wear seems to beand less TS2500 than after0.3 mm. 120 sAccording of cutting operation. to Figures The 12–14 flank, flank sides wear of both (VB) inserts sharply were wornrose when and could cutting not time increased,be allowed and forthe additional maximum tests allowable after 120 flank s cutting, wear although (VB) was the achieved rake side for wear both seems inserts to be at less around than 70 s. 0.3 mm. According to Figures 12–14, flank wear (VB) sharply rose when cutting time increased, and the Abrasion and BUL were the main wear modes. The slopes of wear profiles in Figures 12–14 indicate maximum allowable flank wear (VB) was achieved for both inserts at around 70 s. Abrasion and BUL that similar to the dry mode, the wet machining of the Ti-MMC was associated with a very short were the main wear modes. The slopes of wear profiles in Figures 12–14 indicate that similar to the steadydry wear mode, zone the wet and machining elevated ofwear the Ti-MMCat short wascutti associatedng times. withThe ause very of shortwet machining steady wear had zone negligible and improvementelevated wear on atthe short useful cutting life of times. both The inserts use ofcompar wet machininged to the had dry negligible mode. In improvementgeneral, it can on be the stated thatuseful regardless life of bothof the inserts cutting compared insert toand the lubrication dry mode. In mode general, used, it can machining be stated that Ti-MMCs regardless is ofassociated the withcutting limited insert tool and life. lubrication mode used, machining Ti-MMCs is associated with limited tool life.

FigureFigure 9. 9.WearWear modes modes on on the the rake faceface of of insert insert TS2000 TS2000 after after 120 120 s of s cutting. of cutting.

Figure 10. The SEM images of wear modes on the rake face of insert TS2500 after 120 s of cutting.

Metals 2020, 10, x FOR PEER REVIEW 9 of 14

3.2. Wet Mode The cutting tests were performed at 30, 40, 50, 60 and 120 s under wet mode. The wear modes on the rake face in wet machining must be studied in machinability evaluation of high-resistance alloys [9]. Figures 9–11 illustrate the wear morphology on the flank face and the rake face of both inserts TS2000 and TS2500 after 120 s of cutting operation. The flank sides of both inserts were worn and could not be allowed for additional tests after 120 s cutting, although the rake side wear seems to be less than 0.3 mm. According to Figures 12–14, flank wear (VB) sharply rose when cutting time increased, and the maximum allowable flank wear (VB) was achieved for both inserts at around 70 s. Abrasion and BUL were the main wear modes. The slopes of wear profiles in Figures 12–14 indicate that similar to the dry mode, the wet machining of the Ti-MMC was associated with a very short steady wear zone and elevated wear at short cutting times. The use of wet machining had negligible improvement on the useful life of both inserts compared to the dry mode. In general, it can be stated that regardless of the cutting insert and lubrication mode used, machining Ti-MMCs is associated with limited tool life.

Metals 2020, 10, 1459 10 of 14 Figure 9. Wear modes on the rake face of insert TS2000 after 120 s of cutting.

Metals 2020, 10, x FOR PEER REVIEW 10 of 14

MetalsFigure 2020Figure, 10 10., x 10. FORThe The PEER SEM SEM REVIEW images images of of wear wear modes modes onon thethe rakerake face of insert TS2500 TS2500 after after 120 120 s sof of cutting cutting..10 of 14

(a) Insert TS2500 (b) Insert TS2000

Figure 11. The SEM images of flank wear on both inserts after 120 s cutting. (a) Insert TS2500. (b) Insert TS2000.

Similar to dry turning, the overview of cutting forces in Figures 12 and 13 showed that under constant cutting parameters, the thrust force and feed force tended to increase when cutting time increased. When the permitted level of flank wear (VB) was reached, the cutting forces had increased almost three times. In other words, although the cutting time did not vary significantly, the cutting forces increased sharply. The rapid increase in cutting force and tool wear in both inserts can be observed and can be attributed to elevated tool wear. As noted earlier in [4], when the cutting tool wears and loses its sharpness, more considerable interaction occurs between the cutting tool and the workpiece, and consequently, larger chip thickness is caused as a result of the worn tool. It is known that turning cutting(a )forces Insert areTS2500 directly affected by the chip thickness (b) Insert and theTS2000 cutting tool geometry [33]. Higher chip thickness tends to increase the cutting force, and as a result of increased cutting FigureFigure 11. 11.The The SEMSEM images of of flank ﬂank wear wear on on both both inserts inserts after after 120 120s cutting. s cutting. (a) Insert (a) InsertTS2500. TS2500. (b) forces, elevated cutting temperatures may occur [33]. The flank wear (VB) of both inserts at various (b)Insert Insert TS2000 TS2000.. cutting times under wet mode were measured from SEM images and presented in Figure 14. Similar to dry turning,500 the overview of cutting forces in Figures 12 and 13 showed that under constant cutting parameters, theTS2000 thrust force and feed force tended to increase when cutting time

increased. When the400 permitted level of flank wear (VB) was reached, the cutting forces had increased almost three times. In other words, although the cutting time did not vary significantly, the cutting forces increased sharply. The rapid increase in cutting force and tool wear in both inserts can be observed and can be300 attributed to elevated tool wear. As noted earlier in [4], when the cutting tool wears and loses its sharpness, more considerable interaction occurs between the cutting tool and the

workpiece, and consequently,VB(µm) larger chip thickness is caused as a result of the worn tool. It is known 200 that turning cutting forces are directly affected by the chip thickness and the cutting tool geometry [33]. Higher chip thickness tends to increase the cutting force, and as a result of increased cutting forces, elevated cutting100 temperatures may occur [33]. The flank wear (VB) of both inserts at various cutting times under wet mode were measured from SEM images and presented in Figure 14. 500 0 TS2000 0 20406080100120140 400 Time(s) (a)

300 Figure 12. Cont.

VB(µm) 200

100

0 0 20406080100120140

Time(s) (a)

Metals 2020, 10, 1459 11 of 14 Metals 2020, 10, x FOR PEER REVIEW 11 of 14 Metals 2020, 10, x FOR PEER REVIEW 11 of 14

(b) (b) Figure 12. (a) Flank wear (VB); (b) cutting force profile of insert TS2000 for 120 s of cutting time. FigureFigure 12. 12. ((aa)) Flank Flank wear wear (VB); (VB); ( (bb)) cutting cutting force force profile proﬁle of of insert insert TS2000 TS2000 for for 120 120 s s of of cutting cutting time. time.

500 500 TS2500 TS2500 400 400

300 300

VB(µm) 200 VB(µm) 200

100 100

0 0 0 20406080100120140 0 20406080100120140 Time(s) Time(s) (a) (a)

(b) (b) Figure 13. ((aa)) Flank Flank wear wear (VB); (VB); ( (bb)) cutting cutting force force profile proﬁle of insert TS2500 for 120 s of cutting time. Figure 13. (a) Flank wear (VB); (b) cutting force profile of insert TS2500 for 120 s of cutting time.

Metals 2020, 10, 1459 12 of 14 Metals 2020, 10, x FOR PEER REVIEW 12 of 14

500 TS2000 TS2500 400

300

VB(µm) 200

100

0 0 20 40 60 80 100 120 140

Time(s) FigureFigure 14. 14. FlankFlank wear wear (VB) (VB) under wetwet mode mode in in both both inserts. inserts.

Similar to dry turning, the overview of cutting forces in Figures 12 and 13 showed that under 4. Conclusions constant cutting parameters, the thrust force and feed force tended to increase when cutting time increased.According When to a thereview permitted of the level literature, of flank limited wear (VB) studies was reached, are available the cutting on machining forces had increased of Ti-MMCs withalmost commercial three times. cutting In other tools words, under although various the cutting cutting time conditions did not vary and significantly, cutting the tools/inserts. cutting Furthermore,forces increased limited sharply. studies The were rapid found increase on the in ma cuttingchinability force and evaluation tool wear of inTi-MMCs both inserts under can various be observed and can be attributed to elevated tool wear. As noted earlier in [4], when the cutting tool cutting conditions. Therefore, to remedy the lack of knowledge observed, this study intended to wears and loses its sharpness, more considerable interaction occurs between the cutting tool and the study the effects of cutting parameters on the machinability attributes of Ti-MMCs when using workpiece, and consequently, larger chip thickness is caused as a result of the worn tool. It is known carbidethat tools. turning According cutting forces to experimental are directly affected results, by thethe chip following thickness conclusion and the cutting can be tool drawn: geometry [33]. • HigherHigh flank chip thicknesswear occurred tends toin increase the first the few cutting seconds force, of andmachining as a result a ofTi-MMC increased under cutting both forces, dry and elevatedwet modes. cutting temperatures may occur [33]. The flank wear (VB) of both inserts at various cutting • timesAbrasive under particles wet mode could were measuredbe understood from SEM as the images essential and presented problem in observed Figure 14. under high-speed 4.machining Conclusions of Ti-MMCs. The matrix behavior and hydrostatic pressure of the cutting edge play prominent roles at low levels of cutting speeds. In this case, the cutting tool acquires sufficient According to a review of the literature, limited studies are available on machining of Ti-MMCs hardness to prevent abrasion and resists the deflection forces. with commercial cutting tools under various cutting conditions and cutting tools/inserts. Furthermore, • limitedDespite studies the wet were cutting found mode on theused, machinability abrasion and evaluation BUL was of the Ti-MMCs main wear under mode various in both cutting inserts. conditions.This was attributed Therefore, to remedyceramic the and lack SiC of particles knowledge that observed, are abrasive this study and intended hard and to studyconsequently the emayffects accelerate of cutting parametersthe coating on removal the machinability process attributesright after of Ti-MMCsthe cutting when process. using carbide Moreover, tools. the Accordingtendency tofor experimental BUL and BUE results, was the intensifie followingd at conclusion higher levels can be of drawn: cutting time. • Regardless of the cutting insert and lubrication mode used, machining Ti-MMCs was associated High flank wear occurred in the first few seconds of machining a Ti-MMC under both dry and •with limited tool life, although better life was observed under wet mode. wet modes. • Other major problems that may tend to hinder the machinability of Ti-MMCs is elevated Abrasive particles could be understood as the essential problem observed under high-speed • temperaturemachining in of the Ti-MMCs. cutting The zone, matrix which behavior was andshow hydrostaticn as sparks pressure and burnt of the cuttingchips in edge the play cutting operations.prominent Thus, roles the at lowtool/insert levels of should cutting resist speeds. the In high this abrasion case, the cuttingand thermal tool acquires loads (i.e., suffi cientCBN). In addition,hardness further to prevent studies abrasion with and respect resists to the the deflection presence forces. of chatter vibration in dry and wet machiningDespite of the Ti-MMCs wet cutting with mode carbide used, abrasiontools and and other BUL commercial was the main tools wear are mode of prime in both importance inserts. • in prospectiveThis was attributed studies. to ceramic and SiC particles that are abrasive and hard and consequently may accelerate the coating removal process right after the cutting process. Moreover, the tendency for Author Contributions:BUL and BUE The was research intensified results at higher in this levels work of were cutting pres time.ented in the B.Sc. project of M.S. The work was initiatedRegardless from a research of the cutting project insert on Machining and lubrication Ti-MMC, mode which used, has machining been initiated Ti-MMCs between was M.B. associated and S.A.N., • faculty memberswith limited of Iran tool University life, although of Science better lifeand wasTechnology, observed and under Polytechnique wet mode. Montreal. The project was supervised by S.A.N. H.M. also contributed to experimental characterizations, signal processing, and article preparation. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by Fonds de Recherche du Québec–Nature et technologies (FQRNT).

Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Metals 2020, 10, 1459 13 of 14

Other major problems that may tend to hinder the machinability of Ti-MMCs is elevated • temperature in the cutting zone, which was shown as sparks and burnt chips in the cutting operations. Thus, the tool/insert should resist the high abrasion and thermal loads (i.e., CBN). In addition, further studies with respect to the presence of chatter vibration in dry and wet machining of Ti-MMCs with carbide tools and other commercial tools are of prime importance in prospective studies.

Author Contributions: The research results in this work were presented in the B.Sc. project of M.S. The work was initiated from a research project on Machining Ti-MMC, which has been initiated between M.B. and S.A.N., faculty members of Iran University of Science and Technology, and Polytechnique Montreal. The project was supervised by S.A.N. H.M. also contributed to experimental characterizations, signal processing, and article preparation. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Fonds de Recherche du Québec–Nature et technologies (FQRNT). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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