Effect of Cutting Parameters and Tool Geometry on the Performance Analysis of One-Shot Drilling Process of AA2024-T3

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Article Effect of Cutting Parameters and Tool Geometry on the Performance Analysis of One-Shot Drilling Process of AA2024-T3

Muhammad Aamir 1,* , Khaled Giasin 2 , Majid Tolouei-Rad 1, Israr Ud Din 3, Muhammad Imran Hanif 4, Ugur Kuklu 5, Danil Yurievich Pimenov 6 and Muhammad Ikhlaq 1

1 School of Engineering, Edith Cowan University, Joondalup 6027, Australia; [email protected] (M.T.-R.); [email protected] (M.I.) 2 School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth PO1-3DJ, UK; [email protected] 3 Research Centre for Modelling and Simulation (RCMS), National University of Sciences and Technology (NUST), Sector H-12, Islamabad 44000, Pakistan; [email protected] 4 Department of Mechanical Engineering, CECOS University of Information Technology and Emerging Sciences, Peshawar 25000, Pakistan; [email protected] 5 Department of Mechanical Engineering, Faculty of Engineering, Karamanoglu Mehmetbey University, Karaman 70100, Turkey; [email protected] 6 Department of Automated Mechanical Engineering, South Ural State University, Lenin Prosp. 76, 454080 Chelyabinsk, Russia; [email protected] * Correspondence: [email protected]

Abstract: Drilling is an important machining process in various manufacturing industries. High- quality holes are possible with the proper selection of tools and cutting parameters. This study investigates the effect of spindle speed, feed rate, and drill diameter on the generated thrust force, Citation: Aamir, M.; Giasin, K.; the formation of chips, post-machining tool condition, and hole quality. The hole surface defects Tolouei-Rad, M.; Ud Din, I.; Hanif, M.I.; Kuklu, U.; Pimenov, D.Y.; Ikhlaq, and the top and bottom edge conditions were also investigated using scan electron microscopy. The M. Effect of Cutting Parameters and drilling tests were carried out on AA2024-T3 alloy under a dry drilling environment using 6 and Tool Geometry on the Performance 10 mm uncoated carbide tools. Analysis of Variance was employed to further evaluate the inﬂuence Analysis of One-Shot Drilling Process of the input parameters on the analysed outputs. The results show that the thrust force was highly of AA2024-T3. Metals 2021, 11, 854. inﬂuenced by feed rate and drill size. The high spindle speed resulted in higher surface roughness, https://doi.org/10.3390/met11060854 while the increase in the feed rate produced more burrs around the edges of the holes. Additionally, the burrs formed at the exit side of holes were larger than those formed at the entry side. The high Academic Editor: Jorge Salguero drill size resulted in greater chip thickness and an increased built-up edge on the cutting tools.

Received: 16 April 2021 Keywords: drilling; thrust force; hole quality; surface defects; chip formation; post-machining tool Accepted: 20 May 2021 condition; AA2024-T3 Published: 23 May 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- iations. Aluminium alloys contribute to many manufacturing industries such as the aerospace and automotive industries, where strong, durable, and lightweight products are required [1]. Alloys of aluminium, particularly those from the 2000 series, are commonly used in modern aircrafts’ airframe structures. For instance, AA2024-T3 is installed in aircraft fuselage skins due to its high fatigue resistance [2]. Furthermore, in the aerospace industry, the drilling Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. process is used to assemble different structures, and thus a large number of holes are This article is an open access article required for installing rivets and bolts [3–5]. Poor hole quality can lead to part rejections in distributed under the terms and quality control. Hence, a higher degree of research is required to enhance the holes’ quality conditions of the Creative Commons and minimise the incidence associated with the drilling operation [6,7]. Attribution (CC BY) license (https:// Tool wear, chip formation, and hole quality are key parameters of the drilling process creativecommons.org/licenses/by/ and are strongly inﬂuenced by spindle speed (n), feed rate (f ), tool geometry, machine tool 4.0/). setup, and cutting conditions such as wet/dry or other environmental factors [8–12]. In

Metals 2021, 11, 854. https://doi.org/10.3390/met11060854 https://www.mdpi.com/journal/metals Metals 2021, 11, 854 2 of 13

contrast to wet machining, dry drilling is preferable to reduce the environmental impact of coolants and eliminate the costs of cutting ﬂuids [13]. However, during the dry drilling process, there are more chances of forming a built-up edge (BUE), which can reduce the tool’s life and affect the hole quality [14]. Hence, the proper selection of drilling parameters is essential in combination with the use of suitable drilling tools to avoid deformation of the workpiece or breakage of the cutting tool [15]. Previous studies for improving the drilling operation and producing high-quality holes are given in Table1.

Table 1. Previous work on the effect of drilling parameters: Aluminium/other metals.

Material Cutting Parameters Output Parameters Ref. n = 90, 200, 250, 400 (rpm) Al6061, f = 0.15, 0.2, 0.3, 0.36 (mm/rev) Hole size, Al6351, Point angle = 90◦, 118◦ [16] Thrust force Al7075 HSS drill bit D = 10 (mm) n = 1000, 2000, 3000 (rpm) Thrust force, Al6061 f = 100, 120, 150 (mm/min) Torque, [17] D = 6, 8, 10 (mm) Circularity error Vc = 40, 80, 120 (m/min) Al7075 f = 0.05, 0.1, 0.15 (mm/rev) Thrust force [18] Point angle =120◦, 130◦, 140◦ n = 465, 695, 795 (rpm) f = 18, 20, 26 mm/min Burr height, Clearance angle = 4◦, 6◦, 8◦ Thrust force, Al7075 [19] Point angle = 100◦, 110◦, 118◦ Surface roughness, D = 8, 10, 12 (mm) Circularity error HSS drill bit Vc = 50, 100, 150 (m/min) f = 0.15, 0.2, 0.25 (mm/rev) Thrust force, Al7075 [20] D = 8, 10, 12 (mm) Torque Carbide drill bit n = 600, 800, 1000 (rpm) f = 10, 12, 14 (mm/min) Surface roughness, ﬂank wear, TI-6Al-4V Helix angle = 25◦, 30◦ [21] and drill vibration D = 10 mm Carbide drill bit V = 10, 20, 30 (m/min) Ti-6Al-4V c f = 0.07, 0.14, 0.21 (mm/rev) Burr size, Al7010 [22] D = 6.35 mm Hole size and circularity Al2024. Carbide twist drills

For instance, Reddy et al. [16] reported that n was the most dominant factor that affected the hole diameter following the f during drilling of Al6061, Al6351, and Al7075. In comparison, the impact of point angle on hole diameter was the lowest. Kushnoore et al. [17] conducted drilling experiments on Al6061 and concluded that an increase in drill diameter, n, and f affected the thrust force, torque, and circularity error. Gunay et al. [18] have demon- strated that increasing f led to an increased thrust force while increasing drill point angle resulted in a lower thrust force. Moreover, a low f and high point angle were suggested for attaining minimum surface roughness in the drilling of Al7075. In another study by Sreenivasulu and Rao [19], it was revealed that during the drilling of Al7075, the most inﬂuential parameters on burr height, thrust force, surface roughness, and circularity were f, point angle, and clearance angles, as compared to n and drill diameter. Kyratsis et al. [20] concluded that tool diameter and f affected the thrust force and torque. In contrast, the impact of n on both the thrust force and torque was considerably smaller during Al7075 drilling. Balaji et al. [21] worked on the drilling of Ti-6Al-4V and concluded that high frictional stresses and heat generation increased with the increase in n and helix angle. While the surface roughness was more inﬂuenced by n and f ; however, the helix angle did Metals 2021, 11, 854 3 of 13

not contribute more to the surface roughness. It was further found that surface roughness was affected by the drill vibration. The reasons for the increase in the tool wear were the high n and f. Abdelhafeez et al. [22] conducted drilling experiments on Ti-6Al-4V, Al7010, and Al2024. The study found that the exit burr size was statistically more affected f f Metals 2021, 11, x FORbyMetals PEERin 2021 REVIEW all, 11, x the FOR materials,PEER REVIEW and no signiﬁcant impact of cutting speed and3 of 14 was found3 of 14 on the circularity error and diameter oversize. Furthermore, Pena et al. [23] developed a thresholding algorithm based on internal signals from spindle torque to detect non-desired burr formationet al. [20] during concludedet the al. that drilling[20] tool concluded diameter operations that and tool f affected diameter of Al7075-T6. the and thrust f affected force The and the torque.thrust algorithm force In con- and was torque. successfully In contrast, the impact oftrast, n on the both impact the thrust of n on force both and the torquethrust forcewas considerably and torque was smaller considerably during smaller during developedAl7075 with drilling. an accuracy BalajiAl7075 et drilling. aboveal. [21] workedBalaji 92% et andon al. the [21] was drilling worked expected of on Ti-6Al-4V the drilling to and be of concluded used Ti-6Al-4V as that anand effective concluded that quality control in drillinghigh frictional operations. stresseshigh frictional and heat stresses generation and increased heat generation with the increased increase with in n theand increase helix an- in n and helix angle. While the surfacegle. While roughness the surface was more roughness influenced was bymore n and influenced f; however, by n the and helix f; however, angle the helix angle The abovedid not studiescontributedid more indicate not tocontribute the surface that more mostroughness. to the of surface theIt was previousroughness. further found It workwas that further surface was found rough- conducted that surface rough- either on other metalness alloys was affected or whenness by the wasthe drill affected cuttingvibration. by the The parameters drill reasons vibration. for the The were increase reasons different. in for the the tool increase Moreover,wear were in the tool some wear were studies lack investigationsthe high n and into f.the Abdelhafeez hole high qualityn and et f . al.Abdelhafeez and [22] conducted other et output al. drilling[22] conducted parametersexperiments drilling on inTi-experiments6Al combination-4V, on Ti-6Al with-4V, the Al7010, and Al2024.Al7010, The studyand Al2024. found Thethat studythe exit found burr thatsize thewas exit statistically burr size morewas statistically affected more affected thrust forceby or f in torque. all the materials,by Additionally, f in alland the no materials, significant keeping and impact no significant in of cutting view impact speed the higher andof cutting f was importance foundspeed onand the f was offound the on drilling the process forcircularity high-quality error andcircularity holes, diameter error this oversize and work diameter. Furthermore, investigates oversize Pena. Furthermore, et the al. effect[23] developed Pena of et tool al. a [23]thresh- geometry developed ina thresh- terms of olding algorithmolding based onalgorithm internal based signals on from internal spindle signals torque from to spindle detect non-desired torque to detect burr non-desired burr drill size, spindleformation speed,duringformation the and drilling feedduring operations rate the drilling in of theAl7075-T6. operations one-shot The of algorithmAl7075-T6. dry drilling was The successfully algorithm process was of successfully AA2024-T3 using the uncoateddeveloped with carbide andeveloped accuracy tools. withabove an Therefore, 92% accuracy and was above expected the 92% performance and to be was used expected as an effective ofto be the used quality drilling as an effective process quality was evaluated incontrol terms in drilling of thrustcontrol operations. in force, drilling chip operations. analysis, and tool conditions to assess the surface The above studiesThe indicate above thatstudies most indicate of the previousthat most work of the was previous conducted work either was conducted on either on roughnessother and metal burrs alloys formation.other or when metal the alloys cutting Furthermore, or whenparameters the cutting were the parametersdifferent. hole surface Moreover, were different. defectssome studiesMoreover, were some investigated studies using scanlack electron investigations microscopy.lack into investigations hole quality intoand otherhole quality output and parameters other output in combination parameters with in combination the with the thrust force or torque.thrust Additionally, force or torque. keeping Additionally, in view thekeeping higher in importance view the higher of the importance drilling of the drilling process for high-qualityprocess holes,for high-quality this work investigatesholes, this work the effect investigates of tool thegeometry effect ofin toolterms geometry in terms 2. Materialsof drill and size, Methods spindleof drill speed, size, and spindle feed ratespeed, in theand one-shot feed rate dry in thedrilling one-shot process dry ofdrilling AA2024- process of AA2024- The drillingT3 using the process uncoatedT3 using wascarbide the performeduncoated tools. Therefore, carbide on tools.the a performance verticalTherefore, the manualof theperformance drilling milling process of the drilling machine process using was evaluated inwas terms evaluated of thrust in force,terms chipof thrust analysis, force, and chip tool analysis, conditions and totool assess conditions the to assess the spindle speedssurface ofroughness 1000,surface and 2000, burrs roughness and formation. 3000 and burrsFurthermore, rpm formation. and the feed Furthermore, hole rates surface of defects the 0.04, hole were surface 0.08, inves- defects and 0.14 were mm/rev.inves- All the experimentstigated using werescantigated electron conducted using microscopy. scan electron without microscopy. the use of coolants because dry conditions are

considered2. an Materials eco-friendlier and 2.Methods Materials process and Methods [24,25]. The tools selected were uncoated carbide drill bits. Carbide twistThe drilling drills process areThe highlywas drilling performed process acknowledged on was a vertical performed manual for on a drillingmilling vertical machine manual aluminium usingmilling spin- machine alloys, using especially spin- AA2024-T3dle [ 26speeds], due of 1000, todle 2000, theirspeeds and of enhanced 3000 1000, rpm 2000, and and hardness feed 3000 rates rpm of and and0.04, feed 0.08, toughness rates and of0.14 0.04, mm/rev. 0.08, [27 ].and All T3 0.14 is mm/rev. the temper All the experiments thewere experiments conducted withoutwere conducted the use ofwithout coolants the because use of coolantsdry conditions because are dry conditions are designationconsidered for aluminium an eco-friendlierconsidered alloy, processan eco-friendlier which [24,25]. means The process tools the [24,25].selected alloy The were tools is uncoated solution selected carbide were treated, uncoated drill cold-worked,carbide drill and naturallybits. Carbide aged [twist1].bits. Tabledrills Carbide are2 shows highly twist drillsacknowledged the are technical highly for acknowledged drilling speciﬁcations aluminium for drilling alloys, for aluminium the espe- drill alloys, bits espe- used in this study. cially AA2024-T3cially [26], AA2024-T3due to their [26]enhanced, due to hardness their enhanced and toughness hardness [27]. and T3 toughness is the tem- [27]. T3 is the temper designation per for aluminium designation alloy, for aluminium which means alloy, the which alloy means is solution the alloy treated, is solution cold- treated, cold- worked, and naturallyworked, aged and [1]. naturally Table 2 showsaged [1]. the Table technical 2 shows specifications the technical for specifications the drill bits for the drill bits Table 2. Detailsused in of this drill study. bits.used in this study.

Table 2. Details of Tabledrill bits 2. .Details of drill bits. Specification/DescriptionSpeciﬁcation/DescriptionSpecification/Description TypeType Twist drillTypeTwist drill Twist drill Twist drillTwist drillTwist drill MaterialMaterial Uncoated Material carbideUncoated carbide Uncoated carbideUncoated Uncoated carbide Uncoated carbide carbide Number of flutes Number of flutes02 02 02 02 Number of ﬂutesPoint angle 02140° 140° 02 Point◦ angle 140° ◦ 140° Point angleHelix angle 140Helix angle30° 30° 30° 140 30° Helix angleDrill diameter Drill 30◦ diameter6 mm 6 mm 10 mm 30◦ 10 mm Drill diameter 6 mm 10 mm Overall length Overall length66 mm 66 mm 70 mm 70 mm Overall length 66 mm 70 mm

All the drilling experimentsAll the drilling were experiments performed onwere AA2024-T3, performed which on AA2024-T3, is the commonly which is the commonly All theused drilling alloy in the experiments usedaerospace alloy inindustry the were aerospace [1,28]. performed The industry thickness [1,28]. on of AA2024-T3,Thethe workpiecethickness of material the which workpiece was is the material commonly was 10 mm, and the 10 dimensions mm, and werethe dimensions 200 × 150 mmwere2. 200 A Kistler × 150 force mm2. dynamometer A Kistler force type dynamometer type used alloy9257BA in the was aerospace used9257BA to measure industrywas usedforce tosignals measure [1,28 during]. force The the signals thicknessdrilling during operation. the of drilling the The workpiecedynamom- operation. The material dynamom- was 10 mm, andeter thewas fitted dimensions oneter the wasmachine fitted were bed, on the and 200 machine a support× 150 bed, platemm and was a2 support. mounted A Kistler plate on wasits top force mounted for safety, dynamometer on its top for safety, type 9257BA was used to measure force signals during the drilling operation. The dynamometer was fitted on the machine bed, and a support plate was mounted on its top for safety, which also acted as a fixture to avoid deflection/vibration during the drilling operation. The thrust force was measured using the Kistler’s dynamometer. The quality of the holes was evaluated using a digital microscope, and the tool condition was examined using the optical microscope. The average surface roughness (Ra) of the holes was measured three times at four different locations of the hole edges at 90◦ using the surface roughness tester TR200 (PCWI- precision instrumentation, Australia), and the average readings were considered for evaluation. The Ra values were taken as per the International Organization Metals 2021, 11, x FOR PEER REVIEW 4 of 13

which also acted as a fixture to avoid deflection/vibration during the drilling operation. The thrust force was measured using the Kistler’s dynamometer. The quality of the holes was evaluated using a digital microscope, and the tool condition was examined using the optical microscope. The average surface roughness (Ra) of the holes was measured three times at four different locations of the hole edges at 90° using the surface roughness tester Metals 2021, 11, 854 4 of 13 TR200 (PCWI- precision instrumentation, Australia), and the average readings were considered for evaluation. The Ra values were taken as per the International Organization for Standardization (ISO) code: 4287. The hole surface quality was examined using scanning forelectron Standardization microscopy (ISO) (SEM). code: The 4287. samples The hole were surface cut in quality half and was examined examined under using SEM scanning using electrona Hitachi microscopy SU5000 Chiyoda, (SEM). The Japa samplesn, scanning were electron cut in half microscope. and examined Finally, under the SEM percentage using acontribution Hitachi SU5000 that Chiyoda,each input Japan, parameter scanning had on electron the output microscope. parameters Finally, was theevaluated percentage using contributionANOVA (Analysis that each of input Variance). parameter had on the output parameters was evaluated using ANOVA (Analysis of Variance). 3. Results and Discussion 3. Results and Discussion 3.1. Thrust Force 3.1. Thrust Force Figure 1 shows the thrust force (Fz) generated signals of 6 mm and 10 mm drills for Figure1 shows the thrust force ( F ) generated signals of 6 mm and 10 mm drills for one of the drilled holes. The first regionz of the force signals shows the drill engagement one of the drilled holes. The ﬁrst region of the force signals shows the drill engagement where the tool penetrated the workpiece and Fz began processing. With the drill bit’s ad- where the tool penetrated the workpiece and Fz began processing. With the drill bit’s vancement, the Fz gradually increased then reached a steady state while the drill was in advancement, the Fz gradually increased then reached a steady state while the drill was in full contact with the workpiece. The Fz declined rapidly when approaching the bottom full contact with the workpiece. The Fz declined rapidly when approaching the bottom side side of the workpiece and then reached zero when the drill bit completely exited the work- of the workpiece and then reached zero when the drill bit completely exited the workpiece, piece, indicating the end of the drilling process [29]. indicating the end of the drilling process [29].

FigureFigure 1. 1.Proﬁles Profiles of of average average thrust thrust force force at at 3000 3000 rpm rpm and and 0.14 0.14 mm/rev. mm/rev.

TheThe average averageF Fz zmeasured measured during during the the dry dry drilling drilling process process using using 6 6 mm mm and and 10 10 mm mm drill drill sizesize at at differentdifferent nn andand f selectedselected in in this this study study is is given given in in Figure Figure 2.2 The. The results results showed showed that thatthe the10 mm 10 mm drills drills generated generated a higher a higher Fz comparedFz compared to that to from that fromthe 6 themm 6 drills mmdrills due to due the tolarger the larger contact contact area between area between the drill the and drill the and workpiece. the workpiece. Figure Figure 2 also2 illustratesalso illustrates that Fz thatincreasedFz increased as n and as nf increased;and f increased; however, however, the ANOVA the ANOVA analysis analysis in Table in Table 3 indicated3 indicated that f thathadf thehad highest the highest percentage percentage contribution contribution at 67.33%, at 67.33%, followed followed by the by dr theill diameter drill diameter with a withcontribution a contribution of 29.53%, of 29.53%, while the while influence the influence of n was of insignificantn was insignificant (0.81%). The (0.81%). increase The in increase in F due to the increase in f might be associated with the increase in uncut chip Fz due to thez increase in f might be associated with the increase in uncut chip thickness thickness[30–32]. A [30 confidence–32]. A confidence interval of interval 95% was of us 95%ed in was the used ANOVA; in the therefore, ANOVA; the therefore, influence the of influenceeach process of each parameter process parameteron the response on thes responseswas considered was considered insignificant insignificant if their p if-values their pwere-values estimated were estimated at more atthan more 0.05. than 0.05.

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600 Uncoated carbide drill: 6 mm

500 Uncoated carbide drill: 10 mm

400

300

Average thrust force (N) Average 200

100

0 0.04 0.08 0.14 0.04 0.08 0.14 0.04 0.08 0.14

Feed rate (mm/rev) Feed rate (mm/rev) Feed rate (mm/rev)

1020 2035 3050

Spindle speed (rpm) Spindle speed (rpm) Spindle speed (rpm)

Figure 2. Average thrust force. Figure 2. Average thrust force. Table 3. ANOVA for thrust force. Table 3. ANOVA for thrust force. Source DF Seq SS Contribution Adj SS Adj MS F-Value p-Value Source DF Seq SS Contribution Adj SS Adj MS F-Value p-Value ModelModel 1313 219,473 219,473 99.85%99.85% 219,473219,473 16,882.516,882.5 199.83199.83 0 0 n 2 1791 0.81% 1791 895.5 10.6 0.025 n 2 1791 0.81% 1791 895.5 10.6 0.025 f 2 147,992 67.33% 147,992 73,995.8 875.84 0

f D 12 64,908147,992 29.53% 67.33% 64,908 147,992 64,90873,995.8 768.27875.84 0 0

2-WayD 1 64,908 29.53% 64,908 64,908 768.27 0 8 4782 2.18% 4782 597.8 7.08 0.038 2-WayInteractions Interactions 8 4782 2.18% 4782 597.8 7.08 0.038 n × f 4 566 0.26% 566 141.6 1.68 0.315 n × f 4 566 0.26% 566 141.6 1.68 0.315 n × D 2 103 0.05% 103 51.3 0.61 0.588 n D 2 103 0.05% 103 51.3 0.61 0.588 f ××D 2 4113 1.87% 4113 2056.6 24.34 0.006 Errorf × D 42 3384113 0.15% 1.87% 338 4113 84.52056.6 24.34 - 0.006 - TotalError 174 219,811338 100.00%0.15% -338 -84.5 -- - - n = SpindleTotal speed, f = Feed17 rate, D219,811= Diameter. 100.00% - - - - n = Spindle speed, f = Feed rate, D = Diameter. 3.2. Surface Roughness, Burr Formation, and Hole Surface Damage Analysis 3.2. SurfaceFigure Roughness,3 depicts Burr the surface Formation, roughness and Hole(Ra) Surfaceof holes Damage drilled Analysis in AA2024-T3 alloy. The resultsFigure show 3 depicts that increases the surface in roughness both n and (Ra)f increased of holes drilledRa, irrespective in AA2024-T3 of the alloy. drill The size. resultsHowever, showRa thatof theincreases holes drilledin both using n and the f increased 10 mm drill Ra, bitsirrespective were found of the to drill be almost size. How- similar ever,to those Ra of obtained the holes from drilled drill using bits with the 10 a 6mm mm drill size, bits which were is found in agreement to be almost with the similar findings to thoseof Köklü obtained [33]. from The drill minor bits increase with a 6 from mm thesize, large-size which is in drills agreement was speculated with the tofindings be due ofto Köklü covering [33]. aThe larger minor contact increase area, from resulting the larg ine-size increased drills BUE was formation,speculated whichto be due in turn to coveringincreased a larger the Fz contactand Ra area,[34]. resulting The high inRa increaseddue to the BUE increase formation, in n iswhich attributed in turn to in- the creasedrise in the cutting Fz and temperature, Ra [34]. The whichhigh Ra increases due to the the increase thermal in softening n is attributed of the to material the rise andin cuttinghole deformation temperature, [15 which]. Moreover, increases the the high therfmalincreased softening the of thickness the material of the and chips, hole which de- formationcontributed [15]. to Moreover, the higher the values high of f Raincreased[30]. Table the4 thickness from the ANOVAof the chips, results which shows contrib- that n utedhad to the the highest higher percentage values of contributionRa [30]. Table at 4 65.29%, from the followed ANOVA by resultsf with anshows impact that of n 32.72% had theon highestRa. The percentage influence of contributionD and other at interactions 65.29%, followed were found by f with insignificant an impact as of their 32.72%p-values on Rawere. The more influence than 0.05.of D and other interactions were found insignificant as their p-values were more than 0.05.

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4 Uncoated carbide drill: 6 mm

Uncoated carbide drill: 10 mm 3

2 Surface(µm) roughness 1

0 0.04 0.08 0.14 0.04 0.08 0.14 0.04 0.08 0.14

Feed rate (mm/rev) Feed rate (mm/rev) Feed rate (mm/rev)

1000 2000 3000

Spindle speed (rpm) Spindle speed (rpm) Spindle speed (rpm)

FigureFigure 3. Average 3. Average surface surface roughness. roughness.

TableTable 4. ANOVA 4. ANOVA for forsurface surface roughness. roughness.

Source DF DF Seq Seq SS SS Contribution Contribution Adj Adj SS SS Adj Adj MSMS F-Value F-Value p-Valuep-Value Model 1313 5.091435.09143 99.39%99.39% 5.091435.09143 0.391650.39165 50.3 50.3 0.001 0.001

n 22 3.344693.34469 65.29% 65.29% 3.34469 3.34469 1.672351.67235 214.8 214.8 0 0

ff 22 1.675861.67586 32.72% 32.72% 1.67586 1.67586 0.837930.83793 107.63 107.63 0 0 D 1 0.00061 0.01% 0.00061 0.00061 0.08 0.793 D 1 0.00061 0.01% 0.00061 0.00061 0.08 0.793 2-Way 2-Way Interactions 8 8 0.07026 0.07026 1.37% 1.37% 0.070260.07026 0.008780.00878 1.13 1.13 0.486 0.486 Interactions nn × ff 44 0.056860.05686 1.11% 1.11% 0.05686 0.05686 0.014210.01421 1.83 1.83 0.287 0.287 nn × DD 22 0.007040.00704 0.14% 0.14% 0.00704 0.00704 0.003520.00352 0.45 0.45 0.665 0.665 f × D f × D 22 0.006370.00637 0.12% 0.12% 0.00637 0.00637 0.003190.00319 0.41 0.41 0.689 0.689 Error 4 0.03114 0.61% 0.03114 0.00779 - - ErrorTotal 174 5.122570.03114 100.00%0.61% 0.03114 - 0.00779 - - -- - n = SpindleTotal speed, f = Feed17 rate,5.12257D = Diameter. 100.00% - - - - n = Spindle speed, f = Feed rate, D = Diameter. Another essential characteristic of hole quality was the burrs at the entry and exit of holes.Another Therefore, essential in thischaracteristic study, burrs of aroundhole quality the hole was edges the burrs were at also the thoroughly entry and exit analysed of holes.using Therefore, a digital in microscope. this study, The burrs microscopic around the inspection hole edges also were revealed also thoroughly less formation analysed of burrs usingat the a digital entry than microscope. those formed The atmicroscopic the exit side. inspection Additionally, also bothrevealedn and lessf contributed formation toof the burrsformation at the entry of burrs than aroundthose formed the hole at the edges; exit however,side. Additionally,f was dominant both n overand f ncontributedin increasing to thethe burrsformation due toof the burrs high around insertion the of hole the drilledges; because however, of the f was unstable dominant performance over n [in35 ].in- The creasingreason the for burrs high burrdue formationto the high due insertion to the higherof the ndrillwas because due to theof the high unstable percentage perfor- of the manceelongation [35]. The of thereason AA2024-T3 for high alloy burr [ 29formation]. Figure 4due also to indicates the higher that n thewas drill due diameter to the high had a percentageminor inﬂuence of the elongation on burr size of the and AA2024-T3 height when alloy drilling [29]. theFigure AA2024-T3 4 also indicates alloy. However, that the it drillwas diameter challenging had a to minor evaluate influence burr formation on burr size using and optical height microscopy when drilling accurately; the AA2024- therefore, T3 SEMalloy. images However, were it was used challenging to further investigate to evaluate the burr burr formation formation. using optical microscopy accurately;Figure therefore,5 shows SEM the images borehole were for used 6 and to 10 further mm holes investigate drilled the using burr the formation. same cutting parameters. The SEM graphs reveal that the hole edge quality at the entrance was better than at the exit, regardless of the drill size. Moreover, it was also found that the hole edge quality at the entrance was better in holes drilled using 6 mm drills than those drilled using the 10 mm drills. This was also observed in the remaining drilled holes using different cutting parameters. The larger contact area between the cutting tool and workpiece at the start of the drilling process means that larger thrust force is required before the cutting tool

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Metals 2021, 11, x FOR PEER REVIEW 7 of 13 is in full contact with the workpiece, leading to increased deformations and damage to the hole entry.

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FigureFigure 4.4.Quality Quality ofof holesholes inin termsterms ofof burrsburrs atat 30003000 rpmrpm andand 0.140.14 mm/rev.mm/rev.

Figure 5 shows the borehole for 6 and 10 mm holes drilled using the same cutting parameters. The SEM graphs reveal that the hole edge quality at the entrance was better than at the exit, regardless of the drill size. Moreover, it was also found that the hole edge quality at the entrance was better in holes drilled using 6 mm drills than those drilled using the 10 mm drills. This was also observed in the remaining drilled holes using different cutting parameters. The larger contact area between the cutting tool and workpiece at the start of the drilling process means that larger thrust force is required before the cutting tool is in full contact with the workpiece, leading to increased deformations and damage to the hole entry. Similarly, the hole edge quality at the exit was better in holes drilled using 10 mm drills, as can be seen from Figure 5. It is speculated that the larger cross-sectional area of the 10 mm drills provided more stability and rigidity while drilling throughout the hole, which gave better cutting quality while the tool was exiting the workpiece. This claim can be supported by the fact that the surface deformations and damage observed on the boreholes drilled using 6 mm drills were somewhat more visible than those drilled using the 10 mm drills. Another observation supporting this claim is that aggressive helical feed marks were found on the inner surfaces of some of the holes drilled using the 6 mm drills, as shown in Figure 6. This would imply that the chip collision with the inner walls of the holes was more aggressive when using the 6 mm drills, which could be, as stated earlier, due to less drill stability.

Figure 5. Borehole condition under SEM at 1000 rpm and 0.04 mm/rev. Figure 5. Borehole condition under SEM at 1000 rpm and 0.04 mm/rev.

Similarly, the hole edge quality at the exit was better in holes drilled using 10 mm drills, as can be seen from Figure5. It is speculated that the larger cross-sectional area of the 10 mm drills provided more stability and rigidity while drilling throughout the hole, which gave better cutting quality while the tool was exiting the workpiece. This claim

Figure 6. Borehole condition under SEM at 2000 rpm and 0.08 mm/rev.

Figure 7 shows the surface condition of holes drilled using 6 mm drills at different feed rates. It was observed that increasing f tended to reduce the hole surface quality. This is mainly attributed to the increased chip thickness with the increase in f. For example, when drilling at an f of 0.14 mm/rev, there are more clear signs of smearing and plastic flow due to severe plastic deformation on the upper and lower sides of the hole as well as throughout its thickness. This was also observed in holes drilled using the 10 mm drills and therefore is purely related to the increase in f. Figure 8 shows the borehole condition for holes drilled using 10 mm drills at a different n and a constant f of 0.14 mm/rev. The results show that the edge quality at the hole entry seems to worsen with the increase in n, while the hole edge quality at the exit seemed to be unaffected. The results also showed that increasing n beyond 2000 rpm increased the

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can be supported by the fact that the surface deformations and damage observed on the boreholes drilled using 6 mm drills were somewhat more visible than those drilled using the 10 mm drills. Another observation supporting this claim is that aggressive helical feed marks were found on the inner surfaces of some of the holes drilled using the 6 mm drills, as shown in Figure6. This would imply that the chip collision with the inner walls of the holes was more aggressive when using the 6 mm drills, which could be, as stated earlier, due to less drill stability. Figure 5. Borehole condition under SEM at 1000 rpm and 0.04 mm/rev.

Metals 2021, 11, x FOR PEER REVIEWFigure 6. Borehole condition under SEM at 2000 rpm and 0.08 mm/rev. 9 of 13 Figure 6. Borehole condition under SEM at 2000 rpm and 0.08 mm/rev.

FigureFigure7 7 shows shows thethe surface condition of of holes holes drilled drilled using using 6 mm 6 mm drills drills at atdifferent different feed rates. It was observed that increasing f tended to reduce the hole surface quality. This feedseverity rates. of It deformations was observed due that increasingto plastic flowf tended around to reduce the hole the surface. hole surface This quality. could be This at- is mainly attributed to the increased chip thickness with the increase in f. For example, istributed mainly to attributed the increase to the in increasedthe cutting chip temperatures thickness withand ductility the increase with in thef. Forincrease example, in n, when drilling at an f of 0.14 mm/rev, there are more clear signs of smearing and plastic whenwhich drilling softens at the an materialf of 0.14 and mm/rev, induces there more are th moreermal clear stresses. signs It of is smearing also well andknown plastic that ﬂowflow duedue toto severe plastic deformation deformation on on the the upper upper and and lower lower sides sides of the of the hole hole as well as well as as throughoutthe AA2024-T3 its thickness. alloy possesses This was a relatively also obse rvedhigh inpercentage holes drilled of elongation, using the 10 which mm drillscan affect throughoutthe burr formation its thickness. at the Thishole wasentry also and observed exit sides in [29]. holes For drilled clarification, using the Figure 10 mm 9 reveals drills andand thereforetherefore is purely related related to to the the increase increase in in f. f. the defectsFigure 8in shows the hole the walls borehole of AA2024-T3 condition for from holes 6 mm drilled and using 10 mm 10 drillmm bits.drills at a different n and a constant f of 0.14 mm/rev. The results show that the edge quality at the hole entry seems to worsen with the increase in n, while the hole edge quality at the exit seemed to be unaffected. The results also showed that increasing n beyond 2000 rpm increased the

FigureFigure 7. 7.Borehole Borehole condition condition under under SEM SEM for for holes holes drilled drilled using using 6 6 mm mm drills drills and and 1000 1000 rpm. rpm.

Figure 8. Borehole condition under SEM for holes drilled using 10 mm drills and 0.14 mm/rev.

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severity of deformations due to plastic flow around the hole surface. This could be attributed to the increase in the cutting temperatures and ductility with the increase in n, which softens the material and induces more thermal stresses. It is also well known that the AA2024-T3 alloy possesses a relatively high percentage of elongation, which can affect the burr formation at the hole entry and exit sides [29]. For clarification, Figure 9 reveals the defects in the hole walls of AA2024-T3 from 6 mm and 10 mm drill bits.

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Figure8 shows the borehole condition for holes drilled using 10 mm drills at a different n and a constant f of 0.14 mm/rev. The results show that the edge quality at the hole entry seems to worsen with the increase in n, while the hole edge quality at the exit seemed to be unaffected. The results also showed that increasing n beyond 2000 rpm increased the severity of deformations due to plastic ﬂow around the hole surface. This could be attributed to the increase in the cutting temperatures and ductility with the increase in n, which softens the material and induces more thermal stresses. It is also well known that the AA2024-T3 alloy possesses a relatively high percentage of elongation, which can affect the burr formation at the hole entry and exit sides [29]. For clariﬁcation, Figure9 reveals theFigure defects 7. Borehole in the holecondition walls under of AA2024-T3 SEM for holes from drilled 6 mm using and 6 10 mm mm drills drill and bits. 1000 rpm.

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FigureFigure 8. 8.Borehole Borehole conditioncondition under under SEM SEM for for holes holes drilled drilled using using 10 10 mm mm drills drills and and 0.14 0.14 mm/rev. mm/rev.

Figure 9.9. Hole surface damage analysisanalysis ofof AA2024-T3:AA2024-T3: (a)) 66 mmmm drilldrill bitbit andand ((bb)) 1010 mmmm drilldrill bit.bit.

3.3. Chip Analysis and Tool Condition Chip formation and its breaking mechanism signify the smooth drilling process [36]. Accordingly, the long and continuous chips can easily become tangled around the drill bit flutes and require manual removal, affecting the production [37]. Furthermore, the un- desirable chips increase the formation of built-up edge (BUE), which in return would reduce the quality of holes [38]. Commonly, the reason behind the formation of BUE is the higher value of the coefficient of friction at the tool–chip interface [39]. Figure 10 shows the chips produced during dry drilling of AA2024-T3 using 6 and 10 mm uncoated carbide drills. The chip analysis showed that 10 mm uncoated carbide drill bits had thickened chips compared to the drill size of 6 mm because of the large cross-sectional area covered by its larger size [40]. Figure 10 also shows that, with the increase in both n and f, the length of the chip became shorter, while the thickness of the chip increased with the increase in f and decreased with the increase in n, irrespective of the drill size.

Drill bit: 6 mm Drill bit: 10 mm n ↓ f → 0.04 mm/rev 0.08 mm/rev 0.14 mm/rev 0.04 mm/rev 0.08 mm/rev 0.14 mm/rev

1000 rpm

2000 rpm

3000 rpm

Figure 10. Analysis of chips.

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Figure 9. Hole surface damage analysis of AA2024-T3: (a) 6 mm drill bit and (b) 10 mm drill bit.

3.3. Chip3.3. Analysis Chip Analysis and Tool and Condition Tool Condition Chip formationChip formation and its breaking and its breaking mechanism mechanism signify the signify smooth the smoothdrilling drillingprocess process[36]. [36]. Accordingly,Accordingly, the long the and long continuous and continuous chips can chips easily can become easily become tangled tangled around aroundthe drill the drill bit flutesbit and ﬂutes require and manual require removal, manual removal,affecting affectingthe production the production [37]. Furthermore, [37]. Furthermore, the un- the desirableundesirable chips increase chips the increase formation the formationof built-up of edge built-up (BUE), edge which (BUE), in return which would in return re- would duce thereduce quality the of quality holes [38]. of holes Commonly, [38]. Commonly, the reason the behind reason the behind formation the formation of BUE ofis the BUE is the higher valuehigher of value the coefficient of the coefﬁcient of friction of friction at the at tool–chip the tool–chip interface interface [39]. [Figure39]. Figure 10 shows 10 shows the the chipschips produced produced during during dry drilling dry drilling of AA of2024-T3 AA2024-T3 using using6 and 10 6 and mm 10 uncoated mm uncoated carbide carbide drills. Thedrills. chip The analysis chip analysis showed showed that 10 thatmm 10uncoated mm uncoated carbide carbidedrill bits drill had bits thickened had thickened chips comparedchips compared to the drill to the size drill of 6 size mm of becaus 6 mme because of the large of the cross-sectional large cross-sectional area covered area covered by its largerby its largersize [40]. size Figure [40]. Figure 10 also 10 showsalso shows that, that, with with the theincrease increase in inboth both n nandand f,f, thethe length length ofof the chip became shorter,shorter, whilewhile thethethickness thickness of of the the chip chip increased increased with with the the increase inin f crease inand f and decreased decreased with with the the increase increase in n in, irrespective n, irrespective of the of the drill drill size. size.

Drill bit: 6 mm Drill bit: 10 mm n ↓ f → 0.04 mm/rev 0.08 mm/rev 0.14 mm/rev 0.04 mm/rev 0.08 mm/rev 0.14 mm/rev

1000 rpm

2000 rpm

3000 rpm

Figure 10.Figure Analysis 10. Analysis of chips. of chips.

In the machining process, chips can adhere to the cutting tool to form BUE, resulting in deterioration of the cutting tool [41]. Additionally, according to Rodríguez et al. [24], during dry drilling operations there are chances of both high friction phenomena and chemical diffusion, which cause adhesion, burn marks, and ﬂank wear. Therefore, the higher formation of BUE in drilling affects the hole quality. Depending upon the materials of the cutting tools and different geometries, BUE formation varies under the same cutting conditions. Figure 11 shows the condition of 6 mm and 10 mm drill bits during the drilling process of AA2024-T3, which indicated a BUE on both the tools. The microscopic inspection showed a higher BUE on the 10 mm drill bit compared to the drill size of 6 mm. The greater BUE on the 10 mm drill bit was due to larger chip formation, increased cutting forces, and the contact area between the cutting tool and the workpiece material. In contrast, small drill size led to easy chip evacuation and chip breakage due to the smaller chips. Metals 2021, 11, x FOR PEER REVIEW 11 of 13

In the machining process, chips can adhere to the cutting tool to form BUE, resulting in deterioration of the cutting tool [41]. Additionally, according to Rodríguez et al. [24], during dry drilling operations there are chances of both high friction phenomena and chemical diffusion, which cause adhesion, burn marks, and flank wear. Therefore, the higher formation of BUE in drilling affects the hole quality. Depending upon the materials of the cutting tools and different geometries, BUE formation varies under the same cutting conditions. Figure 11 shows the condition of 6 mm and 10 mm drill bits during the drilling process of AA2024-T3, which indicated a BUE on both the tools. The microscopic inspection showed a higher BUE on the 10 mm drill bit compared to the drill size of 6 mm. The greater BUE on the 10 mm drill bit was due to larger chip formation, increased cutting Metals 2021, 11, 854 forces, and the contact area between the cutting tool and the workpiece material. In11 con- of 13 trast, small drill size led to easy chip evacuation and chip breakage due to the smaller chips.

Figure 11. Post-machining t toolool conditions: ( a) 6 mm drill bit and ( b) 10 mm drill bit.

4. Conclusions Conclusions This study investigated the effect of tool geometry and cutting conditions, such as spindle speed speed and and feed feed rate, rate, for for the the improvem improvementent of ofholes holes drilled drilled in AA2024-T3. in AA2024-T3. The Thefol- lowingfollowing conclusions conclusions could could be bemade made from from the the investigations: investigations: • The thrust thrust force force was was affected affected more more with with the the increase increase in in feed feed rate, rate, while while the the impact impact of spindleof spindle speed speed on thrust on thrust force force was was found found insignificant. insignificant. Therefore, Therefore, the percentage the percentage con- tributionscontributions of feed of feed rate, rate, drill drill size, size, and and spindle spindle speed speed on on thrust thrust force force were were 67.33%, 29.53%, and 0.81%, respectively. • The surface roughness increased increased with with the the in increasecrease in in spindle spindle speed speed and and feed feed rate. rate. The spindle spindle speed speed had had an an impact impact of of65.29% 65.29%,, following following the thefeed feed raterate which which had hada per- a centagepercentage contribution contribution of 32.72%, of 32.72%, while while the the drill drill size size had had insignificant insignificant influence influence with with a contributiona contribution ofof only only 0.01%. 0.01%. • Both spindle speed and feed rate rate were were infl influentialuential on burrs burrs formation; formation; however, however, more burrs were formed with an increased feed rate.rate. Furthermore, it was revealed that entry holes had fewer burrs than holes formed at the exit side. • The 10 10 mm mm size size tool tool covered covered a large a large cutting cutting area area and produced and produced chips chipswith high with thick- high ness,thickness, thus generating thus generating high highthrust thrust force forcecomp comparedared to the to 6 themm 6 size mm tool. size tool.The high The highchip thicknesschip thickness resulted resulted in high in highbuilt-up built-up edges edges on the on drill the drillbits because bits because the small the small drill drillsize enabledsize enabled easy easychip chipevacuation evacuation and breaking and breaking due to due the to short the shortchip size. chip The size. large The built- large upbuilt-up edge should edge should be avoided be avoided for high-quality for high-quality holes. holes. • • Both the the drill drill sizes sizes resulted resulted in in feed feed marks, marks, chip chip debris, debris, deformation deformation due due to chip to chip adhesion,adhesion, and and smearing smearing on onthe the hole hole walls walls of ofAA2024-T3. AA2024-T3. • The study could be further extended to assess other important hole metrics such as deviation of the hole from nominal size, circularity, cylindricity, and perpendicularity. Moreover, drill bits with coatings and other geometric parameters should be used for further improvement of the holes.

Author Contributions: Conceptualization, M.A. and M.T.-R.; methodology, M.A. and K.G.; valida- tion, M.A., K.G., M.T.-R. and D.Y.P.; investigation, M.A., and U.K.; writing—original draft preparation, M.A., M.I.H., M.I. and I.U.D.; writing—review and editing, K.G., I.U.D., M.I., U.K., D.Y.P. and M.I.H. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request. Conﬂicts of Interest: The authors declare no conﬂict of interest. Metals 2021, 11, 854 12 of 13

Nomenclature

ANOVA Analysis of Variance BUE Built-up edge SEM Scan electron microscope D Drill diameter f Feed rate n Spindle speed Ra Surface roughness Fz Thrust force Vc Cutting speed

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