Friction Drilling of Difficult-To-Machine Materials Are Product Quality and Tool Life

metals

Article Friction Drilling of Diﬃcult-to-Machine Materials: Workpiece Microstructural Alterations and Tool Wear

Shayan Dehghan * , Mohd Idris Shah b. Ismail * , Mohd Khairol Anuar b. Mohd Ariﬃn and B. T. Hang Tuah b. Baharudin Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, UPM Serdang 43400, Selangor, Malaysia * Correspondence: [email protected] (S.D.); [email protected] (M.I.S.b.I.)

Received: 3 July 2019; Accepted: 23 July 2019; Published: 29 August 2019

Abstract: Difficult-to-machine materials are metals that have great toughness, high work-hardening, and low thermal conductivity. Friction drilling of difficult-to-machine materials is a technically challenging task due to the difficulty of friction drilling, leading to excessive tool wear, which adversely affects surface integrity and product performance. In the present study, the microstructural changes of workpieces and tool wear for friction drilling of AISI304, Ti-6Al-4V, and Inconel718 are characterized. It helps to have an in-depth understanding of heat generation mechanics by friction and the mechanism of the friction drilling process. The study contributes to providing an enhanced microstructural characterization of workpiece and tool conditions, which identifies the material behavior and shows how it affects the bushing formation quality and drilling tool performance. The results reveal that the abrasive wear is mostly observed in the conical region of the tool, which has maximum contact with hole-wall. Moreover, the low thermal conductivity of Ti-6Al-4V increases frictional heat generation severely, and reduces product quality and tool life subsequently.

Keywords: Friction drilling; Microstructure; Tool wear; Diﬃcult-to-machine materials; AISI304; Ti-6Al-4V; Inconel718

1. Introduction The fact that the hole-making process is one of the most important operations in industry is undeniable. Friction drilling, as a green, non-traditional, hot-shear machining technology, established by Streppel and Kals [1], is a clean and chipless hole-making process with no cutting fluids that can fulfill the needs of dry machining [2]. However, it has not received any significant attention until last decade, when Miller [3] pioneered the use of this methodology for sheet metal hole-making. During this process the temperature increases, the material structure becomes soft, and as Chow et al. [4] explained, the bushing is formed with a thickness approximately three times larger than the original workpiece thickness. The main contribution of friction drilling is to increase the effectiveness of the thread length, as well as screw coupling, which is used for load clamping in joining applications. Several researchers successfully applied friction drilling in this direction and showed this process is an excellent alternative for hole stamping and nut welding [5], such as for screw coupling [6] and joining of dissimilar materials [7]. The main act in this process is friction leading to the temperature rising in the process, increasing the material ductility and extruding onto both sides of the drilled workpiece. Miller et al. [8] showed that in general, high heat generation changes the material properties and microstructural characteristics. Moreover, Miller et al. [9] found that friction increases erosion and wear dramatically. In other words, material degradation, erosion, and wear have significant effects on the quality and quantity of the process.

Metals 2019, 9, 945; doi:10.3390/met9090945 www.mdpi.com/journal/metals Metals 2019, 9, x FOR PEER REVIEW 2 of 15

Metalscharacteristics.2019, 9, 945 Moreover, Miller et al. [9] found that friction increases erosion and wear dramatically.2 of 14 In other words, material degradation, erosion, and wear have significant effects on the quality and quantity of the process. The term “difficult-to-machine materials” refers to metals that have unique metallurgical properties, The term “difficult-to-machine materials” refers to metals that have unique metallurgical such as great toughness, high work-hardening, and low thermal conductivity. A wide range of properties, such as great toughness, high work-hardening, and low thermal conductivity. A wide applications for difficult-to-machine materials can be found in industry, from automotive to aerospace, range of applications for difficult-to-machine materials can be found in industry, from automotive to and from nuclear to medical. The performance of these materials in machining makes them interesting aerospace, and from nuclear to medical. The performance of these materials in machining makes for researchers [10]. Chow et al. [4] claimed that the attractive advantages of austenitic stainless steel them interesting for researchers [10]. Chow et al. [4] claimed that the attractive advantages of AISI304 are excellent corrosion resistance, a high work-hardening rate, modest thermal conductivity, austenitic stainless steel AISI304 are excellent corrosion resistance, a high work-hardening rate, high temperature oxidation, and good formability. Later, Lee et al. [11] demonstrated that this material modest thermal conductivity, high temperature oxidation, and good formability. Later, Lee et al. [11] usually is accompanied by low productivity, poor surface quality, and short tool life. The wide demonstrated that this material usually is accompanied by low productivity, poor surface quality, applications of nickel-based alloy Inconel718 in high temperature conditions with creeping, corrosion, and short tool life. The wide applications of nickel-based alloy Inconel718 in high temperature and thermal shock resistance encouraged Yang et al. [12] to bring up its usage in extreme environments, conditions with creeping, corrosion, and thermal shock resistance encouraged Yang et al. [12] to bring such as aerospace and aircraft industries, gas turbine blades, seals, and jet engines. Shokrani et al. [13] up its usage in extreme environments, such as aerospace and aircraft industries, gas turbine blades, completed a survey on the applications of some difficult-to-machine materials, including austenitic seals, and jet engines. Shokrani et al. [13] completed a survey on the applications of some difficult-to- stainless steel AISI304, titanium alloy Ti-6Al-4V, and nickel-based alloy Inconel718 in different fields. machine materials, including austenitic stainless steel AISI304, titanium alloy Ti-6Al-4V, and nickel- In a previous study [14], Zhu et al. considered the application of Ti-6Al-4V, which is readily regarded as based alloy Inconel718 in different fields. In a previous study [14], Zhu et al. considered the a difficult-to-machine material, in the aircraft industry due to the good compromise between mechanical application of Ti-6Al-4V, which is readily regarded as a difficult-to-machine material, in the aircraft resistance and tenacity, together with its low density and excellent corrosion resistance. Therefore, industry due to the good compromise between mechanical resistance and tenacity, together with its one might say that the most significant issues addressed in friction drilling of difficult-to-machine low density and excellent corrosion resistance. Therefore, one might say that the most significant materials are product quality and tool life. issues addressed in friction drilling of difficult-to-machine materials are product quality and tool life. The process of friction drilling involves six different stages, as shown in Figure1. At the beginning, The process of friction drilling involves six different stages, as shown in Figure 1. At the the drilling tool comes into initial contact with the workpiece. Dehghan et al. [15] found out that beginning, the drilling tool comes into initial contact with the workpiece. Dehghan et al. [15] found the peak point of stress also occurs in this step. At the second stage, the workpiece is softened, out that the peak point of stress also occurs in this step. At the second stage, the workpiece is softened, extruded, and pushed sideward and upward by the drilling tool. Initial bulging on lower surface of extruded, and pushed sideward and upward by the drilling tool. Initial bulging on lower surface of the workpiece, leading to the bushing formation, also starts in this step. At stage three, the drilling the workpiece, leading to the bushing formation, also starts in this step. At stage three, the drilling tool enters the softened workpiece and the conical region is encompassed by molten and softened tool enters the softened workpiece and the conical region is encompassed by molten and softened material. In sequence, the drilling tool pierces the workpiece and the bushing formation begins. At the material. In sequence, the drilling tool pierces the workpiece and the bushing formation begins. At fourth stage, the drilling tool penetrates inside the workpiece and pushes aside the workpiece material. the fourth stage, the drilling tool penetrates inside the workpiece and pushes aside the workpiece While the contact interface between tool and workpiece changes from conical to cylindrical regions, material. While the contact interface between tool and workpiece changes from conical to cylindrical the bushing formation is completed. The process of boss formation is completed in stage five, when the regions, the bushing formation is completed. The process of boss formation is completed in stage five, drilling tool shoulder pushes the softened back-extruded workpiece material downward. Finally, at the when the drilling tool shoulder pushes the softened back-extruded workpiece material downward. sixth stage, the drilling tool retracts and leaves the completed drilled hole. Finally, at the sixth stage, the drilling tool retracts and leaves the completed drilled hole.

Figure 1. Process stages involved in friction drilling.

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The tool wear in friction drilling is categorized into three mechanisms, which are named abrasive wear, adhesive wear, and oxidative wear [9]. The material removal from the drilling tool by hard abrasive phases in the workpiece material is defined by abrasive wear. In friction drilling, abrasive wear occurs as circular grooves in the conical and center regions of the drilling tool. The contact between the two sliding surfaces in the friction drilling process, which generates heating, causes material adhesion from the workpiece and drilling tool to each other. However, most material transfer occurs from the workpiece to the drilling tool. The material adhesion from the workpiece to the drilling tool is named adhesive wear. High heat generation in the workpiece–tool interface increases the potential for oxidization on the drilling tool surface. An oxide layer normally covers the surface of the drilling tool, preventing material adhesion and reducing the adhesive wear tendency. Lee et al. [11] discovered that insufficient friction drilling performance and excessive tool wear are the most important challenges and obstacles in friction drilling of difficult-to-machine materials. Hence, the improvement of friction drilling performance and increased tool life are two critical issues that should be developed more. To improve friction drilling performance, a comprehensive study on the correlation between associated physical processes of friction drilling and thermo-mechanical behaviors of the process parameters is necessary. Mutalib et al. [16] referred to tool wear as one of the noticeable parameters in all of the manufacturing processes. Since tool wear affects the characteristics and tolerances that are achievable, it is a significant concern that must receive more attention. The excessive tool wear in friction drilling of difficult-to-machine materials shows why a fundamental study on tool condition and microstructural analysis of tool wear for different workpiece materials is needed for further development. Due to the importance of excessive heat generation, which is close to the melting point, and high plastic strain, Eliseev et al. [17] and Lee et al. [11] highlighted the necessity of studying microstructural changes to investigate effects of workpiece material properties, heat generation, and plastic strain. In addition, Ozek and Demir [18,19] showed that microstructural analysis assists in examining the effects of different spindle speeds and feed rates on surface roughness, plastic strain distribution, and cracks on drilled holes. On the other hand, microstructural analysis is a powerful tool for analyzing different tool wear mechanisms occurring in different regions of the drilling tool. Miller et al. [8] conducted the preliminary study on friction drilling of stainless steel, aluminum, and titanium with a microstructural alterations approach. They stated that the material adhesion to the drilling tool has a wrecking effect on bushing surface quality and tool life. In addition, they found that the generated heat during the friction drilling is dissipated through the drilling tool, workpiece, and surroundings. After that, Miller et al. [9] studied tool wear for AISI1015 and categorized different tool wear mechanisms for the friction drilling process. They also suggested more studies on tool wear and process parameters that may affect the drilling tool performance. Lee et al. [20] applied friction drilling to machining of IN-713LC cast super alloy with different spindle speeds and feed rates. They found that hardness is greater near the hole-wall. They also observed that higher spindle speed causes better roundness. Subsequently, Chow et al. [4] studied friction drilling characteristics of AISI304, such as tool wear, surface roughness, and micro hardness of drilled-holes. In order to obtain higher product quality, they found optimal process parameters for friction drilling of AISI304 with 2 mm thickness. The optimal process parameters were friction angle of 30◦, friction contact area ration of 50%, feed rate of 100 mm/min, and spindle speed of 3600 rpm. Lee et al. [11] conducted experiments using uncoated and PVD AlCrN-coated and PVD TiAlN-coated tungsten carbide tools for friction drilling of austenitic stainless steel, where PVD is referred to as physical vapor deposition. Their results showed that coated drilling tools suffered less tool wear than uncoated drilling tool. However, at high spindle speed the drilling tool coating increases heat generation and increases tool wear. Elissev et al. [17] carried out experiments to investigate modification of the AA2024 microstructure produced by friction drilling. They characterized the different zones of materials around the drilled-hole. They also found that frictional heat generation causes grain structure recrystallization. Metals 2019, 9, 945 4 of 14

Due to the wrecking effect of insufficient or extreme heat generation on product and tool life, the microstructure of drilling tools and workpieces is important to analyze. The present work focuses Metals 2019, 9, x FOR PEER REVIEW 4 of 15 on the microstructural characterization of AISI304, Ti-6Al-4V, and Inconel718 as workpiece materials thatthat areare subjectedsubjected toto highhigh temperaturetemperature andand largelarge deformationdeformation inin thethe frictionfriction drillingdrilling process.process. TheThe tool condition, including wear, degradation, and surface chemistry, for drillingdrilling of didifferentfferent workpieceworkpiece materials is alsoalso studied.studied. In In addition, a comparative study on microstructural changes and tooltool conditions forfor frictionfriction drillingdrilling ofof AISI304,AISI304, Ti-6Al-4V,Ti-6Al-4V, and and Inconel718 Inconel718 is is conducted. conducted. The findingsfindings of this study are expected to cont contributeribute useful knowledge regarding the mechanism of friction drilling for didifficult-to-machinefficult-to-machine materials that have high resistance to corrosion corrosion and and wear. wear. TheyThey cancan alsoalso helphelp toto enhanceenhance the applicability of the hole-making process in didifferentfferent industries, wherewhere thethe didifficult-to-machinefficult-to-machine materialsmaterials areare widelywidely used.used.

2. Materials and Methods 2. Materials and Methods As shown in Figure2, a three-axis computer numerical control (CNC) milling machine (OKUMA As shown in Figure 2, a three-axis computer numerical control (CNC) milling machine (OKUMA MX-45VA, Aichi, Japan) was used in this study. The SEM (HITACHI, S3400N, Tokyo, Japan) used MX-45VA, Aichi, Japan) was used in this study. The SEM (HITACHI, S3400N, Tokyo, Japan) used was to observe the microstructure of drilled holes and tools condition. The hole-wall thickness was was to observe the microstructure of drilled holes and tools condition. The hole-wall thickness was measured with a micrometer. To analyze chemical composition changes of the drilling tool surface, measured with a micrometer. To analyze chemical composition changes of the drilling tool surface, energy dispersive spectroscopy (EDS) analysis was also conducted. The diﬃcult-to-machine materials energy dispersive spectroscopy (EDS) analysis was also conducted. The difficult-to-machine chosen for workpieces were AISI304, Ti-6Al-4V, and Inconel718 with thicknesses of 3 mm. materials chosen for workpieces were AISI304, Ti-6Al-4V, and Inconel718 with thicknesses of 3 mm.

Figure 2. Experimental setup of friction drilling process.

The chemicalchemical compositioncomposition and thermo-mechanicalthermo-mechanical properties of the workpieceworkpiece materialsmaterials are shown in Table Tables 11 and and Table2. The 2. drillingThe drilling tool tool was wa mades made using using tungsten tungsten carbide carbide in in a cobalta cobalt matrix. matrix. TheThe chemicalchemical compositioncomposition of the drilling tool is alsoalso presentedpresented in TableTable3 3.. Moreover,Moreover, thethe geometrygeometry and dimensiondimension of thethe drillingdrilling tooltool isis shownshown inin FigureFigure3 3.. TheThe drillingdrilling tooltool usedused inin thisthis studystudy hashas aa shankshank regionregion ofof 35 35 mm, mm, shoulder shoulder region region of of 5 mm,5 mm, cylindrical cylindrical region region of 10 ofmm, 10 mm, conical conical region region of 9 mm,of 9 centermm, center region region of 1 mm,of 1 shankmm, shank diameter diameter of 8 mm,of 8 mm shoulder, shoulder diameter diameter of 14 of mm, 14 mm, center center angle angle of 90 of◦, and90°, conicaland conical angle angle of 37 of◦. 37°.

Table 1. Chemical compositions of AISI304, Ti-6Al-4V, and Inconel718 (wt.%).

Materials Cr Fe Mn Ni Si Al Ti V Co Mo Nb AISI304 18~20 66~74 2 80~10.5 1 ------Ti-6Al-4V - - - - - 6 90 4 - - - Inconel718 17~21 17 - 50~55 - - - - 1 2.8~3.3 4.8~5.5

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Table 1. Chemical compositions of AISI304, Ti-6Al-4V, and Inconel718 (wt.%).

Materials Cr Fe Mn Ni Si Al Ti V Co Mo Nb AISI304 18~20 66~74 2 80~10.5 1 ------Ti-6Al-4V - - - - - 6 90 4 - - - MetalsInconel718 2019, 9, x FOR 17~21 PEER REVIEW 17 - 50~55 - - - - 1 2.8~3.3 4.8~5.55 of 15

TableTable 2.2. Thermo-mechanicalThermo-mechanical propertiesproperties ofof AISI304,AISI304, Ti-6Al-4V,Ti-6Al-4V, and and Inconel718. Inconel718.

MechanicalMechanical and Thermal and thermal Properties properties AISI304 AISI304 Ti-6Al-4V Ti-6Al-4V Inconel718 Inconel718 UltimateUltimate tensiletensile strength, strength, MPa MPa 505505 950 950 1375 1375 YieldYield strength,strength, MPa MPa 215215 880 880 1100 1100 Young’sYoung’s modulus,modulus, GPa GPa 198198 123 123 298 298 Thermal conductivity, WW 16.2 5.8 11.4 Thermal conductivity,m ◦K 16.2 5.8 11.4 Melting point, ◦Cm°K 1400~1455 1604~1660 1260~1336 Melting point, °C 1400~1455 1604~1660 1260~1336

Table 3. Chemical composition of tungsten carbide (wt.%). Table 3. Chemical composition of tungsten carbide (wt.%). Materials Cr Ni C Si Ti Fe Ta W Co Materials Cr Ni C Si Ti Fe Ta W Co WCWC (Tungsten (Tungsten carbide) carbide) 0.100.10 0.13 0.13 9~12 9~12 0.15 0.15 1.96 1.96 1.93 1.93 5.02 57~67 7~12 7~12

FigureFigure 3.3. KeyKey dimensionsdimensions ofof thethe frictionfriction drillingdrilling tool.tool.

ToTo evaluateevaluate thethe relationship between the the micros microstructuraltructural changes changes and and mechanical mechanical properties properties of ofthe the workpiece workpiece material, material, micro-hardness micro-hardness measurem measurementent of of the the drilled drilled hole hole was performed onon thethe adjacentadjacent areaarea nearnear thethe drilleddrilled hole-wall.hole-wall. The threethree testedtested pointspoints werewere nearnear toto thethe hole-wallhole-wall withwith didifferentﬀerent distances,distances, asas shown shown in in Figure Figure4. 4. The The micro-hardness micro-hardness measurement measurement was was conducted conducted using using a a micro-hardnessmicro-hardness machinemachine 401MVD401MVD (Wilson(Wilson andand Wolpert,Wolpert,Aachen, Aachen, Germany)Germany) underunder VickersVickers scalescale HV0.1HV0.1 andand 100100 gfgf testtest load.load.

Figure 4. Schematic of selected points to measure hardness.

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Table 2. Thermo-mechanical properties of AISI304, Ti-6Al-4V, and Inconel718.

Mechanical and thermal properties AISI304 Ti-6Al-4V Inconel718 Ultimate tensile strength, MPa 505 950 1375 Yield strength, MPa 215 880 1100 Young’s modulus, GPa 198 123 298 W Thermal conductivity, m°K 16.2 5.8 11.4 Melting point, °C 1400~1455 1604~1660 1260~1336

Table 3. Chemical composition of tungsten carbide (wt.%).

Materials Cr Ni C Si Ti Fe Ta W Co WC (Tungsten carbide) 0.10 0.13 9~12 0.15 1.96 1.93 5.02 57~67 7~12

Figure 3. Key dimensions of the friction drilling tool.

To evaluate the relationship between the microstructural changes and mechanical properties of the workpiece material, micro-hardness measurement of the drilled hole was performed on the adjacent area near the drilled hole-wall. The three tested points were near to the hole-wall with different distances, as shown in Figure 4. The micro-hardness measurement was conducted using a Metalsmicro-hardness2019, 9, 945 machine 401MVD (Wilson and Wolpert, Aachen, Germany) under Vickers scale HV0.16 of 14 and 100 gf test load.

Figure 4. Schematic of selected points to measure hardness.

3. Results and Discussion In this section, the obtained results are discussed regarding two factors: workpiece microstructural alterations and wool wear.

3.1. Workpiece Microstructural Alterations The microstructural analyses of workpieces and the drilling tool are studied in the following. Figure5 shows high magnified optical views of the workpiece materials’ cross-sections, which were AISI304, Ti-6Al-4V, and Inconel718. Differences in the material displacement are evident, as in the bushing shape, surface quality of the hole-wall, and petal shape. The friction-drilled hole of AISI304 is relatively uniform in shape and exhibits a smooth bore surface finish. However, some circular grooves on the hole-wall surface, which imply material removal, can be observed. The circular grooves confirm the extreme interaction between the conical region of the drilling tool and the drilled hole-wall. Moreover, the hole-wall thickness is about 2.6 mm. In contrast, because of the low thermal conductivity of Ti-6Al-4V, which causes ineffective heat generation through the workpiece–tool interface during the friction drilling, the bushing formation is not sufficient. This ineffective heat generation causes material melting, and subsequently bushing formation quality is reduced. As can be observed from Figure5, due to the excessive heat generation the melted material could not back-extrude and the boss is not formed. This excessive heat generation also has a wrecking effect on petal formation. The low hole-wall surface quality, insufficient hole-wall thickness, and extremely high bushing height implies that the heat generated is not sufficient and the workpiece material is almost melted. The hole-wall thickness of Ti-6Al-4V is smaller than AISI304 and it is about 1.1 mm. This confirms the insufficient heat generation throughout the Ti-6Al-4V drilled hole and the poor product quality. According to titanium’s color, which changes to blue when the material temperature exceeds the melting point, the blue strip on drilled hole and golden color on the lower region of the hole-wall for Ti-6Al-4V is caused by extremely high heat generation in this region of the drilled hole. This causes a chemical reaction between Ti and O elements. Blue and golden colors are the results of Ti2O3 and TiO, respectively. In comparison with Ti-6Al-4V, the friction-drilled hole of Inconel718 is more uniform in shape than AISI304. High shear strength and high work-hardening of Inconel718 cause sufficient frictional heat generation for friction drilling of this material. The uniform boss and petal formations imply that the heat generation and material softening are more effective and sufficient. The hole-wall thickness is about 1.9 mm. Although the hole-wall thickness is smaller than AISI304, the bushing height is considerably higher than AISI304. Furthermore, the surface quality of the hole-wall for Inconel718 is higher than AISI304 and Ti-6Al-4V. To analyze the surface quality of the drilled holes, Figure5 shows close-up views of two zones along the hole-wall, marked by zones (a) and (b) for scanning electron microscopy (SEM). From zone (a) it can be stated that plastic strain on the hole-wall for Ti-6Al-4V is higher than AISI304 and Metals 2019, 9, 945 7 of 14

Inconel718. Due to the high heat generation for Ti-6Al-4V, which causes melting of the workpiece material, plastic deformation is severely high. This severely high deformation, which causes surface delamination, confirms the low bushing formation quality for friction drilling of Ti-6Al-4V. In contrast, high strain hardening of Inconel718 results in more sufficient flow material and high bushing formation quality. Moreover, zone (b) presents the surface quality of the hole-wall. It is revealed that the worst quality hole-wall surface with severe surface delamination belongs to Ti-6Al-4V. In contrary, because of the high shear strength of Inconel718, the quality of the hole-wall surface is significantly better than AISI304 and Ti-6Al-4V. Furthermore, the carbide element, which is one of the hard particle elements of Inconel718, has a significant effect on the high quality of the hole-wall surface. Metals 2019, 9, x FOR PEER REVIEW 7 of 15

Figure 5. Cross-sectionCross-section and and close-up close-up view of zones ( a)) and and ( b) of friction drilled-holes.

The microstructure views of the petal regions for drilleddrilled holes are also shown inin Figure6 6.. AsAs cancan be observed from the microstructure microstructure view of the the petal petal region region for for AISI304, AISI304, insufficient insufficient heat heat generation, generation, which results in ineineffectiveffective material softening, ruptures the workpiece material and causes material removal from the hole-wall surface in the petal region. In In other other words, words, since heating is not generated throughout thethe workpieceworkpiece su sufficientlyfficiently and and workpiece workpiece materials materials do notdo not soften soften properly, properly, in the in third the stage,third stage,where where the drilling the drilling tool pierces tool pierces the softened the softened material, material, the material the material is ruptured. is ruptured. Moreover, Moreover, the existing the existingporosities porosities in the petal in regionthe petal of AISI304 region confirmsof AISI304 insu confirmsfficient piercing insufficient of the piercing softened of material the softened by the materialdrilling tool.by the Regarding drilling tool. the threading,Regarding whichthe threading, is the secondary which is process the secondary after hole-making, process after and hole- the making,insufficient and hole-wall the insufficient surface ofhole-wall the petal surface region of the drilledpetal region hole for of AISI304,the drilled this hole area for of AISI304, the hole-wall this areais not of e fftheective hole-wall for clamping is not effective and disabling for clamping the thread. and disabling the thread. Because of the the low low thermal thermal conductivity conductivity of of Ti-6Al Ti-6Al-4V,-4V, which causes severe heat generation along the drilled hole, the petals are not formed properly. A A microstructural view view of of the the petal region in Figure 66 shows shows excessive excessive plastic plastic strain strain and and low low hole-wall hole-wall surface surface quality quality of Ti-6Al-4V of Ti-6Al-4V in the petalin the region. petal region.In comparison with AISI304, the petal region of Inconel718 is formed properly and material removalIn comparison from the petal with region AISI304, is negligible. the petal Thisregion means of Inconel718 the softened is formed workpiece properly is pierced and properly.material removalSome circular from grooves,the petal which region result is negligible. from contact This betweenmeans the the softened rotational workpiece drilling tool is pierced and the properly. hole-wall Somesurface, circular are also grooves, observed which on the result hole-wall from contact surface. be Ittween can be the stated rotational that the drilling drilled tool hole and effective the hole- area wallof Inconel718 surface, are for also threading observed includes on the the hole-wall petal region. surfac Thise. It meanscan be stated that clamping that the ofdrilled Inconel718 hole effective is more areaefficient of Inconel718 than AISI304 for andthreading Ti-6Al-4V. includes the petal region. This means that clamping of Inconel718 is more efficient than AISI304 and Ti-6Al-4V.

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Figure 6. Microstructure view in the petalpetal regions of drilled-holes.

In orderorder toto evaluateevaluate thethe heatheat aaffectedffected zonezone hardnesshardness ofof thethe drilleddrilled hole,hole, thethe measurementmeasurement is performed on the adjacentadjacent areaarea nearnear thethe drilleddrilled hole-wall.hole-wall. In general, the hardness of the adjacentadjacent area nearnear thethe drilled drilled hole hole is higheris higher than than the basethe materialbase material hardness hardness of austenitic of austenitic stainless stainless steel AISI304, steel i.e.,AISI304, 226 HV i.e.,. 226 It canHV. beIt can noted be noted that thethat closest the closes pointt point to theto the drilled drilled hole-wall hole-wall has has higherhigher hardness, as displayeddisplayed in in Table Table4. A 4. shorter A shorter distance distance from the from drilled the hole-wall, drilled dramatichole-wall, increase dramatic in temperature, increase in andtemperature, rapid temperature and rapid reduction temperature result reduction in fine grain result size in fine and grain high hardness.size and high Therefore, hardness. fine Therefore, grain size andfine highgrain hardnesssize and high are obtained hardness when are obtained an excessive when temperature an excessive gradient temperature occurs gradient near the occurs hole-wall. near Fromthe hole-wall. the view ofFrom heat the treatment, view of an heat excessive treatment, temperature an excessive gradient temperature changes the gradient microstructure changes of the materialmicrostructure and makes of the the material grain and size makes finer. However,the grain size the finer. hardness However, of the the measured hardness points, of the whichmeasured are farpoints, from which the drilled are far hole-wall, from the drilled steadily hole-wall, decreased. steadily The slow decreased. temperature The sl reductionow temperature makes reduction the grain sizemakes coarser the grain and reduces size coarser the hardness. and reduces Thermal the conductivityhardness. Thermal of Ti-6Al-4V conductivity has a significant of Ti-6Al-4V role inhas the a hardnesssignificant changes role in forthe the hardness friction changes drilling process.for the friction As illustrated drilling in process. Table4, As the illustrated lower hardness in Table occurred 4, the atlower the closerhardness region occurred to the at hole-wall. the closer With region the to distance the hole-wall. from the With hole-wall, the distance the hardness from the is hole-wall, increased gradually.the hardness After is increased penetration gradually. and drilling After toolpenetration retraction and from drilling the drilled tool retraction hole, the from temperature the drilled of thehole, closer the temperature areas to the of hole the edgecloser is areas increased. to the hole However, edge is the increased. temperature However, does the not temperature rise significantly does whennot rise the significantly distance of when the drilledthe distance hole of edge the increases.drilled hole Inedge other increases. words, In due other to words, the low due thermal to the conductivitylow thermal conductivity of Ti-6Al-4V (kof =Ti-6Al-4V5.8 W/mK), (k = i.e., 5.8being W/mK), almost i.e., threebeing timesalmost lower three than times stainless lower steelthan AISI304stainless (ksteel= 16.2 AISI304 W/mK), (k = the16.2 heating W/mK), phase the heating takes morephase time. takes Therefore, more time. the Therefore, higher heat the higher generation heat generation needs a longer cooling time, and the longer cooling time prolongs the needed time for growth of the grain size. Thus, the hardness is increased gradually when the tested points are away from the drilled hole edge. As can be observed, for Inconel718, the region closer to the hole edge has

Metals 2019, 9, 945 9 of 14 needs a longer cooling time, and the longer cooling time prolongs the needed time for growth of the grain size. Thus, the hardness is increased gradually when the tested points are away from the drilled hole edge. As can be observed, for Inconel718, the region closer to the hole edge has lower hardness. This can be attributed to the low thermal conductivity of Inconel718 (k = 11.4 W/mK), which is lower than AISI304 (k = 16.2 W/mK), as a result of slow heating and cooling of the hole edge. The low thermal conductivity of Inconel718 causes the cooling to take a long time, and subsequently large grains are formed along the hole edge, resulting in low hardness.

Table 4. Hardness of selected points for different materials. Metals 2019, 9, x FOR PEER REVIEW 9 of 15 Workpiece-Materials Tested Points Hardness, HV lower hardness. This can be attributed to the low1st th pointermal conductivity of Inconel718 239 (k = 11.4 W/mK), which is lower than AISI304 (k = 16.2 W/mK), as a result of slow heating and cooling of the hole edge. AISI304 2nd point 233 The low thermal conductivity of Inconel718 causes the cooling to take a long time, and subsequently 3rd point 228 large grains are formed along the hole edge, resulting in low hardness. 1st point 379 Ti-6Al-4VTable 4. Hardness of selected2nd pointpoints for different materials. 396 Workpiece-materials 3rd Tested point points Hardness, HV 410 1st point1st point 239 283 Inconel718 AISI304 2nd2nd point point 233 341 3rd point3rd point 228 459 1st point 379 Ti-6Al-4V 2nd point 396 3.2. Tool Wear 3rd point 410 1st point 283 Figure7, Figure8 and FigureInconel7189 display the drilling2nd tools,point which drilled 341 one hole for each workpiece material (AISI304, Ti-6Al-4V, and Inconel718) in the3rd point center and conical 459 regions. Circular grooves in the conical region imply material removal from the drilling tool and abrasive wear. It also confirms maximum3.2. interaction Tool Wear of the drilling tool with the hole-wall in the conical region. The material from center regionFigure of the 7, drilling Figure 8, tool and isFigure also 9 removed, display the but drilling not thetools, same which as drilled for the one circular hole for grooveseach in the conical region.workpiece In the material first stage,(AISI304, during Ti-6Al-4V, initial and contact Inconel7 between18) in the center the tool and and conical workpiece, regions. Circular heat generation grooves in the conical region imply material removal from the drilling tool and abrasive wear. It also and materialconfirms softening maximum are stillinteraction insuffi ofcient. the drilling Therefore, tool with stress the andhole-wall required in the thrustconical forceregion. for The penetration are extremelymaterial high. from center region of the drilling tool is also removed, but not the same as for the circular Due togrooves the severe in the conical heat generationregion. In the duringfirst stage, the during friction initial drilling contact between of Ti-6Al-4V the tool and caused workpiece, by low thermal conductivityheat ofgeneration Ti-6Al-4V, and material plastic softening strain are on still the insufficient. drilling toolTherefore, is much stress greaterand required than thrust tools force that drilled for penetration are extremely high. AISI304 and Inconel718.Due to the severe On heat the generation other hand, during the the carbide friction elementdrilling of Ti-6Al-4V that exists caused in Inconel718 by low thermal causes more severe materialconductivity removal of Ti-6Al-4V, and circular plastic groovesstrain on the on dril theling drilling tool is toolmuch than greater AISI304. than tools that drilled The toolAISI304 profiles and Inconel718. of a new On tool the other and hand, tools the after carbid drillinge element of that AISI304, exists in Ti-6Al-4V, Inconel718 causes and Inconel718more are comparedsevere in Figure material 10 removal. The materialand circular removal grooves on from the drilling tools andtool than material AISI304. adhesion on tools represent The tool profiles of a new tool and tools after drilling of AISI304, Ti-6Al-4V, and Inconel718 are the abrasivecompared wear andin Figure adhesive 10. The material wear on removal conical from and tools center and material regions. adhesion Although on tools wear represent on thethe tools that drilled AISI304abrasive and wear Inconel718 and adhesive is wear in the on conical form ofand material center regions. removal, Although circular wear grooveson the tools on that the tool that drilled AISI304drilled AISI304 are worse and Inconel718 than Inconel718. is in the form In of contrast, material removal, wear on circular the tool grooves hat on drilled the tool Ti-6Al-4V that is in the form ofdrilled material AISI304 adhesion. are worse than It is Inconel718. worth mentioning In contrast, wear that on the the weartool hat on drilled the toolsTi-6Al-4V hat is drilled in the AISI304, form of material adhesion. It is worth mentioning that the wear on the tools hat drilled AISI304, Ti- Ti-6Al-4V,6Al-4V, and Inconel718 and Inconel718 are are about about 0.85%, 0.85%, 3.84%, 3.84%, and and 1.26%, 1.26%, respectively. respectively.

Figure 7. MicrostructuralFigure 7. Microstructural views of views drilling of drilling tool attool conical at conica andl and center center regions regions for for friction friction drilling drilling of of AISI304. AISI304.

Metals 2019, 9, 945 10 of 14 Metals 2019, 9, x FOR PEER REVIEW 10 of 15 Metals 2019, 9, x FOR PEER REVIEW 10 of 15

Figure 8. Microstructural views of the drilling tool in conical and center regions for friction drilling Figure 8. Microstructural views of the drilling tool in conical and center regions for friction drilling Figure 8. Microstructuralof Ti-6Al-4V. views of the drilling tool in conical and center regions for friction drilling of Ti-6Al-4V. of Ti-6Al-4V.

Figure 9. Microstructural views of the drilling tool in conical and center regions for friction drilling Figure 9. MicrostructuralFigure 9. Microstructural views of the views drilling of the tooldrilling in conicaltool in co andnical centerand center regions regions for for friction friction drilling of Inconel718. of Inconel718. Metalsof 2019 Inconel718., 9, x FOR PEER REVIEW 11 of 15

Figure 10. Profiles of new tools and after drilling of AISI304, Ti-6Al-4V, and Inconel718. Figure 10. Proﬁles of new tools and after drilling of AISI304, Ti-6Al-4V, and Inconel718. Figure 11 shows energy dispersive spectroscopy (EDS) analysis of the drilling tool before drilling. A large amount of Si is observed on the drilling tool surface. This is attributed to the coating of the drilling tool surface with SiO2.

Figure 11. EDS analysis of new drilling tool surface.

Metals 2019, 9, x FOR PEER REVIEW 11 of 15

Metals 2019, 9, 945 11 of 14

Figure 10. Profiles of new tools and after drilling of AISI304, Ti-6Al-4V, and Inconel718. Figure 11 shows energy dispersive spectroscopy (EDS) analysis of the drilling tool before drilling. Figure 11 shows energy dispersive spectroscopy (EDS) analysis of the drilling tool before A large amount of Si is observed on the drilling tool surface. This is attributed to the coating of the drilling. A large amount of Si is observed on the drilling tool surface. This is attributed to the coating drilling tool surface with SiO2. of the drilling tool surface with SiO2.

Figure 11. EDS analysis of new drilling tool surface. Metals 2019, 9, x FOR PEER REVIEWFigure 11. EDS analysis of new drilling tool surface. 12 of 15

Figure 12,12, Figure 13,13, and and Figure Figure 1414 displaydisplay SEM SEM mapping mapping and and EDS EDS analysis analysis of of center center and and conical conical regions for for drilling drilling tools tools that that drill drill different diﬀerent workpiece workpiece materials materials (AISI304, (AISI304, Ti-6Al-4V, Ti-6Al-4V, and and Inconel718). Inconel718). As observed in Figure 11,11, the Fe element, which is presented on the drilling tool surface that drills AISI304, indicates the transfer of this element from the workpiece to the drilling tool. This material transfertransfer confirms conﬁrms the the adhesive adhesive wear. wear. Moreover, Moreover, the the presence presence of of the the O O element on on the surface of of the drilling tool is evidence of oxidization in the center and conical regions of the drilling tool, where the heating was generated.

Figure 12. ScanningScanning electron electron microscope microscope (SEM) (SEM) mapping of the drilling tool in the center and conical regions for friction drilling of AISI304.

The SEM mapping and EDS analysis of the drilling tool that drilled Ti-6Al-4V is indicated in Figure 13.13. The The large large amounts amounts of of Ti Ti and Al elements confirm conﬁrm transfer transfer of of material material from from the the workpiece toto the the drilling drilling tool tool and and adhesive adhesive wear. wear. In addition, In addition, the larger the larger amount amount of Si element of Si element in the center in the region center comparedregion compared to the conical to the conicalregion shows region that shows interact thation interaction between between the hole-wall the hole-wall surface and surface the drilling and the tooldrilling in the tool conical in the region conical is more region severe is more than severe the center than theregion, center and region, material and removal material from removal the conical from regionthe conical is more region severe is morethan severethe center than region. the center In comparison region. In with comparison Figure 12, with it is Figure also indicates 12, it is that also adhesiveindicates wear that adhesive on the drilling wear ontool the that drilling drills toolTi-6Al-4V that drills is more Ti-6Al-4V severe isthan more the severe drilling than tool the that drilling drills AISI304.tool that This drills is AISI304. related to This the is lower related thermal to the lowerconduc thermaltivity of conductivity Ti-6A-4V than of Ti-6A-4VAISI304, which than AISI304, causes excessive heat generation in the workpiece–tool interface and material adhesion to the drilling tool surface. Furthermore, the presence of the O element on the drilling tool surface confirms the oxidization and oxidative wear. As can be seen, due to the higher adhesive wear on the drilling tool that drills Ti-6Al-4V compared with the drilling tool that drills AISI304, oxidative wear on the drilling tool that drills Ti-6Al-4V is less than the drilling tool that drills AISI304.

Metals 2019, 9, 945 12 of 14 which causes excessive heat generation in the workpiece–tool interface and material adhesion to the drilling tool surface. Furthermore, the presence of the O element on the drilling tool surface conﬁrms the oxidization and oxidative wear. As can be seen, due to the higher adhesive wear on the drilling tool that drills Ti-6Al-4V compared with the drilling tool that drills AISI304, oxidative wear on the drilling tool that drills Ti-6Al-4V is less than the drilling tool that drills AISI304. Metals 2019, 9, x FOR PEER REVIEW 13 of 15

Metals 2019, 9, x FOR PEER REVIEW 13 of 15

FigureFigure 13.13. Scanning electron microscope (SEM)(SEM) mappingmapping ofof thethe drillingdrilling tooltool inin thethe centercenter andand conicalconical regionsregionsFigure forfor 13. frictionfriction Scanning drillingdrilling electron ofof Ti-6Al-4V.microscopeTi-6Al-4V. (SEM) mapping of the drilling tool in the center and conical regions for friction drilling of Ti-6Al-4V. AsAs displayed in in Figure Figure 14, 14 from, from the the SEM SEM mapping mapping and and EDS EDS analysis analysis of the of dr theilling drilling tool that tool drills that drillsInconel718, Inconel718,As displayed it is revealed it in is Figure revealed that 14, the thatfrom oxidative the the oxidativeSEM wearmapping wear is greater an isd greater EDS than analysis than other other of drilling the drillingdrilling tools. tool tools. However, that However, drills the Inconel718, it is revealed that the oxidative wear is greater than other drilling tools. However, the thepresence presence of Ni of Niand and Fe Feelements elements confirms conﬁrms the the ad adhesivehesive wear. wear. It It can can be be stated stated that the maximummaximum presence of Ni and Fe elements confirms the adhesive wear. It can be stated that the maximum adhesiveadhesive wearwear is is observed observed on on the the drilling drilling tool thattool drillsthat Ti-6Al-4V.drills Ti-6Al-4V. This means This thatmeans Ti-6Al-4V that Ti-6Al-4V elements adhesive wear is observed on the drilling tool that drills Ti-6Al-4V. This means that Ti-6Al-4V canelements easily can transfer easily totransfer the drilling to the drilling tool surface. tool su Onrface. the On other the other hand, hand, due due to the to the lower lower tendency tendency of elements can easily transfer to the drilling tool surface. On the other hand, due to the lower tendency Inconel718of Inconel718 elements elements to transferto transfer to theto the drilling drilling tool tool surface, surface, the the maximum maximum oxidative oxidative wear wear is found is found on of Inconel718 elements to transfer to the drilling tool surface, the maximum oxidative wear is found the drilling tool that drills Inconel718. onon the the drilling drilling tool tool that that drills drills Inconel718.Inconel718.

Figure 14. SEM mapping of the drilling tool in the center and conical regions for friction drilling of FigureFigure 14. SEM 14. SEM mapping mapping of the of drilling the drilling tool in tool the in center the center and conical and conical regions regions for friction for friction drilling drilling of Inconel718. of Inconel718. Inconel718. 4. Conclusions 4. Conclusions In this paper, an enhanced microstructural characterization of workpiece and tool conditions for frictionIn this drilling paper, of an difficult-to-m enhanced microstructuralachine materials characis provided.terization The microstructural of workpiece and changes tool conditions of AISI304, for frictionTi-6Al-4V, drilling and of Inconel718 difficult-to-m andachine tool wear materials are analyzed is provided. and Thecompared. microstructural Details of changes microstructural of AISI304, Ti-6Al-4V,alterations and of workpiece Inconel718 and and tool tool wear wear help are to identifyanalyzed the and material compared. behavior Details and how of microstructural it affects the alterationsbushing formation of workpiece quality and and tool drilling wear helptool performanceto identify the for materialfriction drilling behavior of difficult-to-machineand how it affects the bushing formation quality and drilling tool performance for friction drilling of difficult-to-machine

Metals 2019, 9, 945 13 of 14

4. Conclusions In this paper, an enhanced microstructural characterization of workpiece and tool conditions for friction drilling of difficult-to-machine materials is provided. The microstructural changes of AISI304, Ti-6Al-4V, and Inconel718 and tool wear are analyzed and compared. Details of microstructural alterations of workpiece and tool wear help to identify the material behavior and how it affects the bushing formation quality and drilling tool performance for friction drilling of difficult-to-machine materials. The experimental results show that bushing shape, surface quality of the hole-wall, and petal formation of Inconel718 and AISI304 are significantly better than Ti-6Al-4V. The low thermal conductivity of Ti-6Al-4V and high shear strength and work-hardening of Inconel718 have the main effects on product quality and tool wear. Moreover, it is found that the abrasive wear is mostly observed in the conical region of the drilling tool that has maximum contact with the hole-wall. The microstructural analysis and surface chemistry of the drilling tool show that the tool wear on the drilling tool that is used for friction drilling of Ti-6Al-4V is more severe than other tools. The abrasive wear on the tool that drilled AISI304 is in the form of material removal and the abrasive wear on the tool that drilled Ti-6Al-4V is in the form of tool destruction. Moreover, the abrasive wear on the tool that drilled Inconel718 is in the form of circular grooves. A higher amount of adhesive wear occurs from Ti-6Al-4V to the drilling tool and a lower amount of adhesive wear occurs from Inconel718 to the drilling tool. The maximum oxidative wear is also mostly observed on the drilling tool that drilled Inconel718.

Author Contributions: Conceptualization, S.D.; methodology, S.D.; M.I.S.b.I., and M.K.A.b.M.A.; formal analysis, S.D.; investigation, S.D.; resources, S.D. and M.I.S.b.I.; data curation, S.D.; M.I.S.b.I., and B.T.H.T.b.B.; writing—original draft preparation, S.D.; writing—review and editing, S.D., M.I.S.b.I., M.K.A.b.M.A., and B.T.H.T.b.B. Acknowledgments: This research was funded by Fundamental Research Grant Scheme (FRGS/1/2015/TK03/UPM/02/2) under the Ministry of Higher Education of Malaysia (MOHE), and Putra Grant (GP-IPS/2015/9452600) under Universiti Putra Malaysia (UPM). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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