A New Approach to Direct Friction Stir Processing for Fabricating Surface Composites

crystals

Article A New Approach to Direct Friction Stir Processing for Fabricating Surface Composites

Abdulla I. Almazrouee 1,* , Khaled J. Al-Fadhalah 2 and Saleh N. Alhajeri 1

1 Department of Manufacturing Engineering Technology, College of Technological Studies, P.A.A.E.T., P.O. Box 42325, Shuwaikh 70654, Kuwait; [email protected] 2 Department of Mechanical Engineering, College of Engineering & Petroleum, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait; [email protected] * Correspondence: [email protected]

Abstract: Friction stir processing (FSP) is a green fabrication technique that has been effectively adopted in various engineering applications. One of the promising advantages of FSP is its applicabil- ity in the development of surface composites. In the current work, a new approach for direct friction stir processing is considered for the surface fabrication of aluminum-based composites reinforced with micro-sized silicon carbide particles (SiC), eliminating the prolonged preprocessing stages of preparing the sample and ﬁlling the holes of grooves. The proposed design of the FSP tool consists of two parts: an inner-threaded hollow cylindrical body; and a pin-less hollow shoulder. The design is examined with respect to three important tool processing parameters: the tilt angle of the tool, the tool’s dispersing hole, and the tool’s plunge depth. The current study shows that the use of a dispersing hole with a diameter of 6 mm of and a plunge depth of 0.6 mm, in combination with a tilting ◦ angle of 7 , results in sufﬁcient mixing of the enforcement particles in the aluminum matrix, while still maintaining uniformity in the thickness of the composite layer. Metallographic examination of

Citation: Almazrouee, A.I.; the Al/SiC surface composite demonstrates a uniform distribution of the Si particles and excellent Al-Fadhalah, K.J.; Alhajeri, S.N. A adherence to the aluminum substrate. Microhardness measurements also show a remarkable increase, New Approach to Direct Friction Stir from 38.5 Hv at the base metal to a maximum value of 78 Hv in the processed matrix in the surface Processing for Fabricating Surface composites layer. The effect of the processing parameters was also studied, and its consequences Composites. Crystals 2021, 11, 638. with respect to the surface composites are discussed. https://doi.org/10.3390/ cryst11060638 Keywords: direct friction stir processing; in situ composites; surface composites

Academic Editor: Bolv Xiao

Received: 8 May 2021 1. Introduction Accepted: 30 May 2021 Published: 2 June 2021 Wrought aluminum alloys are considered one of the most signiﬁcant metallic materials in today’s fabrication and manufacturing production, especially in the transportation indus-

Publisher’s Note: MDPI stays neutral tries [1,2]. In general, many aluminum alloys offer an excellent combination of properties with regard to jurisdictional claims in such as high strength-to-weight ratio, corrosion resistance, good formability, and weld- published maps and institutional affil- ability. However, in specific applications where dynamic loading is imposed, aluminum iations. alloys’ fatigue performance is considered a significant drawback. Other properties such as wear resistance are also of concern. Surface treatments such as coating, shot peening and induction friction stir processing (FSP) have been developed as solid-state processes capable of enhancing the surface properties of aluminum alloys [3]. The FSP process was developed based on the principles of friction stir welding (FSW) [3–5]. It involves the use of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. a relatively high-speed rotating non-consumable tool, which consists mainly of a shoulder This article is an open access article with/without a pin to process the sheet/plate surface of the metal. During FSP, a localized distributed under the terms and heat produced by friction is generated at the interface of the rotating tool and the workpiece, conditions of the Creative Commons resulting in metal softening and plasticization; the rotating pin, on the other hand, allows Attribution (CC BY) license (https:// significant stirring and mixing, leading to severe plastic deformation and thus producing creativecommons.org/licenses/by/ microstructure refinement in the stirred zone (SZ) [6–10]. The high-speed rotating tool then 4.0/). traverses along a specified path of interest to process and modify the material’s matrix.

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The microstructural alterations are characterized by equiaxed and ultra-fine grains for many metals and alloys [4,11]. Such grain refinement by FSP has been shown to enhance mechanical properties in several Al alloys, including tensile strength, ductility, and creep and fatigue strength [4,7,12–16]. The processing of other high-temperature materials such as steels [17] and Ti alloys [18] with FSP has also been reported. The application of FSP for the development of surface composite matrixes was first introduced by Mishra et al. [4]. An in situ surface composite was produced by smearing the aluminum surface with reinforcement SiC particles and methanol and applying FSP to produce a surface composite layer. Since then, different methods for adding reinforcement particles have been reported [4,12,19–24]. For example, several studies proposed a new FSP method for making in situ composites, known as the groove method, by filling the reinforcement particles into a machined groove in the base material’s top surface using FSP [25–28]. The groove method can be divided into three different steps: (1) machining the groove with the desired dimensions, (2) processing with a pin-less tool to pack the reinforcement particles, and (3) processing the entrap reinforcement particles by a tool with a pin in the desired processing parameters. The last two steps can be substituted by placing a thin sheet on the groove of the same materials and then applying the process using a tool with a pin to process the reinforcement particles entrapped in the groove by the thin sheet. Another approach for adding reinforcement particles, which eliminates the need for pin-less tool processing, is drilling several holes in a line or lines and then packing them with reinforcement particles, followed by processing with a tool with a pin to develop the surface composites [27,29]. The dimensions and number of grooves and holes play a vital role in acquiring the second phase’s desired volume fraction [7,9]. All the previous methods have used a non-consumable tool. However, a consumable rod can be used [27] with drilled holes placed at different positions along a radial line filled with reinforcement particles to provide excellent results. The aforementioned methods require rigorous preparations of the surface or the consumable rod. More recently, Huang et al. [30] fabricated a surface composite by direct friction stir processing (DFSP) using a hollow tool without preprocessing to add the reinforcement particles. This design allows the in situ implementation of reinforcement particles. Furthermore, the tool’s design in FSW/P plays a critical and decisive role in the welding and processing, as well as the fabrication, of materials and surface composites. Different tool designs have been reported in previous studies [4,9,13,31–33]. Types of tools used for FSW/P are generally categorized into three types, namely, fixed, adjustable, and self-reacting [10]. The first type is the fixed-pin tool, where the tool is a single piece that includes the pin and the shoulder. The second type is the flexible tool with an adjustable pin, which can also be made from another material. The third type is the self-reacting tool, which is similar to the second type but with the addition of a bottom shoulder, and which acts as a backing anvil for the processed piece during the process. These three tool types have been used in FSW/P to weld and process several metallic materials. The work of Huang et al. [30] proposed an efficient tool design for making in situ surface composites. The tool design consists of a pin-less hollow shoulder that is tapered at its lower end. This design was shown to allow the efficient spread and mixing of the reinforcement particles into the matrix surface during FSP. The shoulder is tapered from the center of the tool to minimize unnecessary frictional contact between the shoulder and the metal surface. In this study, a modified design of the FSP tool is proposed for making aluminum-based surface composites reinforced by SiC particles, based on the concept of the tool design reported in [30]. The modification includes several alterations to the tool design, including a two-part design, several hole sizes, and shoulder shape. A new approach for the fabrication of in situ surface composites by FSP has also been proposed in this study. It aims to reduce the probabilities of clogging the hole in the shoulder or any back extrusion during the processing. The role of the new design and technique on the microstructure and microhardness of the matrix surface are studied. Crystals 2021, 11, 638 3 of 14 Crystals 2021, 11, x FOR PEER REVIEW 3 of 15

2. Materials2. Materials and and Methods Methods 2.1. Tool Design and Processing Approach 2.1. Tool Design and Processing Approach In this study, a new design for the tool and a new processing approach are proposed In this study, a new design for the tool and a new processing approach are proposed to carry out the friction stir processing to develop surface composites. A two-part tool is to carry out the friction stir processing to develop surface composites. A two-part tool is fabricated where the two parts are the top part of a hollow cylindrical body and the lower fabricated where the two parts are the top part of a hollow cylindrical body and the lower part of a tool head, as schematically illustrated in Figure1a,b. part of a tool head, as schematically illustrated in Figure 1a,b.

Loading of Enforcemnt particles

7°

6–7.5 (offset distance)

(a) (b) (c)

Loading of enforcement particles

(d)

FigureFigure 1. Schematic 1. Schematic illustration illustration of of the the FSP FSP tool tool and and design andand processingprocessing procedures: procedures: (a ()a upper) upper part, part,and and (b )(b lower) lower part part of of the the FSP FSP tool; tool; (c )(c sectioned) sectioned views views of of the the tilting tilting of of tool tool head head with with respect respect to to the theprocessed processed aluminum aluminum plate; plate; and and ( d(d)) processing processing setup. setup. All All dimensions dimensions are are in in mm. mm.

TheThe top top part part consists consists of a of body a body made made from from H13 H13 steel steel with with an anouter outer diameter diameter of of25 25mm mm andand a 13.5 a 13.5 mm mm hollow hollow body body with with 15 mm 15 mmof th ofreading threading at the at 30 the mm 30 end. mm The end. height The height of the of topthe portion top portion of the oftool the is tool60 mm; is 60 half mm; of half the ofdistance the distance is prepared is prepared for clamping, for clamping, and the and otherthe half other remaining half remaining is used isas used a casing as a for casing the lower for the part. lower The part. lower The portion lower is portion the head, is the containinghead, containing a shoulder a shoulder with a hole with in a holeits midd in itsle middle instead instead of a pin. of aThree pin. Threedifferent different sizes sizesof holesof holeswere wereused usedin experiments in experiments 2, 4, and 2, 4, 6 andmm. 6 The mm. head The also head contains also contains threading threading that fits that intoﬁts the into top the part top of part the oftool’s the body. tool’s body.The uppe Ther upperportion portion of the oflower the lowerpart consists part consists of a 15- of a

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mm thread with a diameter of 11 mm, and then the head body is 25 mm in diameter and 15-mm12 mm thread in thickness with a for diameter the lower of 11 portion mm, and of thenthe lower the head part, body as depicted is 25 mm in in Figure diameter 1b. and The 12total mm length in thickness of the whole for the body, lower assembled portion of from the lowerthe two part, parts, as is depicted 72 mm. inTwo Figure bolts1 b.are The also totalused length to add of more the whole stability body, to the assembled tool during from processing the two parts, by secularl is 72 mm.y tightening Two bolts the are lower also usedpart’s to threaded add more part stability and the to thetop toolpart. during The tool processing can be fabricated by secularly as one tightening part; however, the lower the part’stwo-part threaded design part is easier and the to topmanufacture part. The tooland canallows be fabricatedfor more extension as one part; of however,the tool; thus, the two-partmore powder design can is easierbe loaded to manufacture inside the tool and fr allowsom the for top more part, extension as illustrated of the in tool; Figure thus, 1c. moreThe two-part powder tool can bealso loaded has many inside benefits the tool with from respect the top to the part, flexibility as illustrated of the intool’s Figure use,1c. as Thewell two-part as advantages tool also such has as many the ability beneﬁts to withuse different respect to materials the ﬂexibility and designs of the tool’s for the use, head, as wellsuch as as advantages the different such hole as thesizes ability used toin usethis different experiment. materials The new and design designs also for thefacilitates head, suchhead/tool as the body different reuse, hole reduces sizes the used material in thiss experiment.needed, and Thereduces new the design associated also facilitates costs. head/toolCommercial body reuse, silicon reduces carbide the (SiC) materials particles needed, with andan average reduces size the associatedof 20 μm were costs. used as theCommercial reinforcement silicon particles, carbide as (SiC) shown particles by th withe secondary an average electron size ofimage 20 µ min wereFigure used 2. The as theSiC reinforcement particles were particles, poured asinto shown the cavity by the of secondary the lower electron part of imagethe tool. in FigureThe two-part2. The SiC tool particleswas assembled were poured and clamped into the cavityinto a ofvertical the lower milling part machine of the tool. to Theconduct two-part FSP. toolThe waspro- assembledcessing was and carried clamped out with into aa verticalconstant milling tool rotating machine rate to of conduct 3000 rpm, FSP. using The processinga clockwise wasrotation carried and out a 20 with mm/min a constant travel tool speed. rotating The ratereinforcement of 3000 rpm, SiC using particles a clockwise were fed rotation into the andmatrix a 20 via mm/min gravitational travel force. speed. A The ball reinforcement bearing of 11 mm SiC particlesin diameter were was fed placed into the on matrixthe top viaof the gravitational powder inside force. the A ballhollow bearing tool to of facili 11 mmtate in the diameter dispersion was of placed the reinforcement on the top of par- the powderticles into inside the themetal hollow surface tool to to produce facilitate surface the dispersion composites of the at reinforcementthe top of the particlesmetal matrix. into theTo metalallow surfacethe use to of produce different surface hole sizes, composites the tool at thewas top tilted of theby metal7°, as matrix.illustrated To allowin Figure the ◦ use1c. ofBy different doing so, hole it was sizes, possible the tool for was a part tilted of bythe 7 tool,, as illustratedless than half in Figure of shoulder1c. By surface, doing so, to itbe was in contact possible with for athe part workpiece. of the tool, Offset less than distances half of of shoulder 6 and 7.5 surface, mm were to be used in contact between with the thepoint workpiece. of contact Offset of tool distances with the workpiece of 6 and 7.5 and mm the were center used of the between tool (Figure the point 1c). of Two contact main ofprocessing tool with parameters, the workpiece namely and th thee offset center distance of the tool and (Figure the tilting1c). angle, Two mainwere processingevaluated to parameters,provide the namelybest possible the offset combination distance andof plunge the tilting depth angle, required were to evaluated enhance tothe provide particle the best possible combination of plunge depth required to enhance the particle dispersion dispersion processes with minimum back extrusion effect or tool hole blockage by the processes with minimum back extrusion effect or tool hole blockage by the processed processed material. The use of the two offsets of 6 and 7.5 mm, in combination with a material. The use of the two offsets of 6 and 7.5 mm, in combination with a tilting angle of tilting angle of 7°, was shown to result in plunge depths of approximately 0.9 and 0.6 mm, 7◦, was shown to result in plunge depths of approximately 0.9 and 0.6 mm, respectively. respectively.

Mag = 672 X 30 µm FigureFigure 2.2. Secondary electron electron images images of of the the as-receive as-receivedd commercial commercial silicon silicon carbide carbide particles particles in ag- in aggregategregate state. state.

2.2.2.2. MaterialMaterial andand ExperimentalExperimental ProceduresProcedures T6T6 temperedtempered aluminumaluminum alloyalloy 11001100 platesplates werewere purchasedpurchased inin hothot extrusionextrusion condition.condition. TheThe chemical chemical composition composition of of the the alloy alloy is is given given in in Table Table1 .1. The The aluminum aluminum plates plates were were used used asas thethe basebase forfor fabricatingfabricating the the surfacesurface composite. composite.The Theplates plateswere weresectioned sectioned perpendicular perpendicular toto the the extrusion extrusion direction. direction. The The dimensions dimensions of of the the FSP FSP plates plates were were 100 100× ×150 150× ×6.5 6.5mm, mm, as as schematicallyschematically illustrated illustrated in in Figure Figure1 d.1d. Coupons Coupons were were cut cut transversely transversely from from the the middle middle of of thethe processed processed plate plate for for metallographic metallographic investigations. investigations. AA standardstandard metallographicmetallographic proce-proce- duredure waswas usedused to to prepare prepare the the sample sample for for examination. examination. The The cross-section cross-section of of the the coupons coupons was examined by a metallurgical microscope (AxioImager A1M, ZEISS, Oberkochen, Ger-

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was examined by a metallurgical microscope (AxioImager A1M, ZEISS, Oberkochen, Ger- many). Additionally, a ﬁeld emission scanning electron microscope (FESEM) (JEOL, model: many). Additionally, a field emission scanning electron microscope (FESEM) (JEOL, JSM-7001F, Tokyo, Japan), equipped with an Energy Dispersive X-Ray Spectroscopy (EDS) model: JSM-7001F, Tokyo, Japan), equipped with an Energy Dispersive X-Ray Spectros- detector (Aztec-Energy, Oxford, High Wycombe, UK) was used to examine surface compos- copy (EDS) detector (Aztec-Energy, Oxford, High Wycombe, UK) was used to examine ite layer development in the FSP zone and to identify the elemental distribution across the surface composite layer development in the FSP zone and to identify the elemental distri- surface composite/matrix interface. The EDS analysis was carried out at an accelerating butionvoltage across of the 20 kVsurface and composite/matrix a working distance interface. of 11.5 The mm EDS to allow analysis for was a sufﬁcient carried out depth at of an acceleratingpenetration voltage (1–2 µm). of 20 The kV surface and a working composite’s distance elemental of 11.5 analysis mm to allow was acquiredfor a sufficient from the depthtop of surface penetration to the alloy(1–2 μ matrixm). The toward surface the composite’s plate’s bottom, elemental and horizontally analysis was at theacquired center of fromthe the processed top surface zone. to the alloy matrix toward the plate’s bottom, and horizontally at the center of the processed zone. Table 1. Chemical composition of the 1100 aluminum plate used in the study. Table 1. Chemical composition of the 1100 aluminum plate used in the study. Composition (wt.%) Composition (wt.%)

Si SiFe FeMn MnCu CuTi TiCr CrZn ZnAl Al Al 1100Al 1100 0.1400.140 0.250 0.250 0.001 0.001 0.051 0.051 0.019 0.019 0.001 0.001 0.002 0.002 Bal. Bal.

VickersVickers microhardness microhardness measurements measurements of the of thepolished polished samples samples were were conducted. conducted. In- In- dentationsdentations were were carried carried out outusing using an Innovate an Innovatestst Falcon Falcon 500 500Hardness Hardness Tester Tester (Innovatest, (Innovatest, Maastricht,Maastricht, The The Netherlands) Netherlands) with with a load a load of 1 of00 100gf and gf and a dwell a dwell time time of 20 of seconds. 20 seconds. Micro- Micro- hardnesshardness measurements measurements were were taken taken along along two two lines lines from from the the top top surface surface toward toward the the bot- bottom tomof of the thesamples. samples. The first ﬁrst line line is is vertical vertical in in the the middle middle ofof the the SZ SZ from from the the upper upper surface surface towardtoward the thebottom, bottom, and and the the indentations indentations were were carried carried out out with with increments increments of of 0.1 mmmm inin the the processedprocessed areaarea andand 0.250.25 mmmmin inthe thealloy alloy matrix matrix for for a a total total distance distance of of 3 3 mm. mm. The The other other line lineis is a a horizontal horizontal line line on on the the processed processed area area at a distanceat a distance 150 µ 150m away μm away from thefrom top the surface top of surfacethe SZ,of the with SZ, increments with increments of 0.5 mm, of 0.5 as mm, schematically as schematically illustrated illustrated in Figure in3 Figure. 3.

Horizontal Retreated side Advancing side line

Vertical line

Figure 3. Schematic illustration of the microhardness measurements vertical and horizontal lines. Figure 3. Schematic illustration of the microhardness measurements vertical and horizontal lines.

3. Results3. Results and and Discussion Discussion 3.1.3.1. Processing Processing of the of Surface the Surface Composite Composite As Asa result a result of the of two-part the two-part tool’s tool’s flexibility, flexibility, different different hole holesizes sizesof the of lower the lowerpart of part the oftool the (2, tool 4, and (2, 4,6 andmm) 6 were mm) used were to used examin to examinee particle particle dispersion dispersion efficiency. efficiency. Visual Visualex- aminationexamination showed showed that there that therewas wasa noticeable a noticeable change change in the in thedispersion dispersion characteristics characteristics withwith increasing increasing the the hole hole size size.. Apparently, Apparently, there there was was a a flow flow discontinuity ofof thetheenforcement enforce- mentparticles particles into into the the workpiece workpiece when when processing processing withwith aa 2-mm hole size size since since there there was was a a blockageblockage of ofthe the tool tool hole hole by by a large a large amount amount of ofthe the SiC SiC particles, particles, as shown as shown in Figure in Figure 4b.4 b. ThisThis discontinuity discontinuity can canmost most likely likely be attribut be attributeded to the to thesmall small size size of the of thehole hole used used and and the the enforcementenforcement particles particles adhesion adhesion characteristics. characteristics. The The blockage blockage might might be assisted be assisted by the by thehigh high rotationalrotational speed, speed, which which might might enhance enhance the the mate materialrial flow flow due due to to the the presence presence ofof significantsignif- icantcentrifugal centrifugal forces. forces. Therefore, Therefore, the the workpiece workpiece processed processed by by the the tool tool with with a a 2-mm 2-mm hole hole size sizeshowed showed nono signssigns of reinforcement reinforcement particle particless for for plunge plunge depths depths of ofeither either 0.6 0.6 or 0.9 or 0.9mm. mm. In addition, the use of a tool with a 4-mm hole size resulted in flow irregularity of the In addition, the use of a tool with a 4-mm hole size resulted in flow irregularity of the enforcement particles into the workpiece. This led to the non-uniform distribution of the enforcement particles into the workpiece. This led to the non-uniform distribution of the enforcement particles in the upper region of the matrix. On the other hand, processing with enforcement particles in the upper region of the matrix. On the other hand, processing a 6-mm hole size was shown to promote a continuous flow of the enforcement particles into with a 6-mm hole size was shown to promote a continuous flow of the enforcement par- the workpiece. This resulted in uninterrupted feeding of SiC particles into the aluminum ticles into the workpiece. This resulted in uninterrupted feeding of SiC particles into the matrix. However, there were back-extruded pieces of aluminum formed in the hole when

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aluminum matrix. However, there were back-extruded pieces of aluminum formed in the holealuminum when using matrix. a toolHowever, with hole there sizes were of back-e 4 andxtruded 6 mm, piecesas shown of aluminum in Figure formed5. This inwas the usingparticularlyhole awhen tool evident withusing hole a withtool sizes withan ofoffset 4hole and distance sizes 6 mm, of of as4 and6 shown mm, 6 mm, i.e., in Figure aas plunge shown5. Thisdepth in Figure was of particularly0.9 5. mm. This The was evidentoccurrenceparticularly with of an evidentback offset extrusion with distance an isoffset of believed 6 mm, distance i.e., to a occurof plunge 6 mm, due depth i.e.,to thea of plunge 0.9buildup mm. depth Theof friction-stirred of occurrence 0.9 mm. The of backmaterialoccurrence extrusion into ofthe isback hole believed extrusion by the to occurforging is believed due action to the toof buildupoccurthe tool. due ofThe friction-stirredto easythe buildupaccess to materialof the friction-stirred hole into due the to holethematerial short by the distance into forging the holebetween action by ofthe the the forging processed tool. The action easymaterial of access the andtool. to the The hole easy dueopening access to the toaids shortthe in hole distanceback due ex- to betweentrusion,the short as the dodistance processed the softness between material and the the processed and quantity the hole ofmaterial the opening materials, and aids the as inhole a backresult opening extrusion, of tool aids rotation in as back do and the ex- softnessplungetrusion, depth. and as do the the quantity softness of and the the materials, quantity as of a resultthe materials, of tool rotation as a result and of plunge tool rotation depth. and plunge depth.

(a) (b) (a) (b) Figure 4.4. PhotographsPhotographs of of ( a()a the) the tool tool before before processing, processing, and and (b) blockage(b) blockage of 2 mmof 2 hole mm after hole processing after pro- usingcessingFigure 2 mmusing 4. Photographs hole 2 mm size. hole ofsize. (a ) the tool before processing, and (b) blockage of 2 mm hole after processing using 2 mm hole size.

Figure 5. Photographs of the back-extruded aluminum during processing when the offset between Figure 5. Photographs of the back-extruded aluminum during processing when the offset between Figurethe hole 5. andPhotographs the processing of the material back-extruded is low du aluminumring processing during using processing 4- and when 6-mm the hole offset sizes. between the hole and the processing material is low during processing using 4- and 6-mm hole sizes. the hole and the processing material is low during processing using 4- and 6-mm hole sizes. In general, the processing and tool design parameters, such as the dispersion mechanism,InIn the general, general, tool hole’s the the processing processing size, the tilting and and tool toolangle, design design and parameters, the parameters, offset distance, such such as as thewere the dispersion showndispersion to mecha- signif- mech- nism,icantlyanism, the contribute the tool tool hole’s hole’s in size,the size, blockage the the tilting tilting of angle,tool angle, hole and and and the the back offset offset extrusion distance, distance, buildups. were were shown shown Besides to to signifi- signif-that, cantlytheicantly characteristics contribute contribute in and thein the blockagesize blockage of the of SiCtool of toolenforc hole hole andement and back particlesback extrusion extrusion might buildups. buildups.have Besidesan effect Besides that, on thethat, characteristicsflowthe characteristicsand dispersion and sizeand of of enforcementsize the of SiC the enforcement SiC particle enforcs.ement particlesHowever, particles might only might haveone enforcement anhave effect an oneffect theparticle on flow the andtypeflow dispersionand and size dispersion were of enforcement used, of andenforcement consequently, particles. particle However, thes. currentHowever, only one study enforcementonly was one focused enforcement particle on the type criticalparticle and sizeprocessingtype were and used, sizeparameters were and consequently, used, that and significantly consequently, the current control studythe the current wasflow focused studyand dispersion was on the focused critical of the on processing SiCthe parti-critical parameterscles,processing i.e., hole that parameters size significantly and plunge that controlsignificantlydepth; both the flow parameters control and dispersionthe wereflow shownand of dispersion the to SiC affect particles, theof the SiC i.e.,SiC particle holeparti- sizeblockagecles, and i.e., plungein hole the size tool depth; and hole plunge both and parametersthe depth; buildup both wereof parameters the shown back-extruded to were affect shown thematerial SiC to particleaffect during the blockageprocessing. SiC particle in theTherefore,blockage tool hole into andtheavoid tool the excessive hole buildup and ofamountsthe the buildup back-extruded of ba ofck-extruded the back-extruded material aluminum, during material processing. a balance during is Therefore,processing. required tobetweenTherefore, avoid excessivethe to processed avoid amounts excessive area of and back-extruded amounts the offset of badistance aluminum,ck-extruded between a balancealuminum, the tool’s is required a balancehole and between is the required pro- the processedcessedbetween workpiece areathe processed and surface. the offset area This distance and was the achieved between offset distance by the adopting tool’s between hole a tool and the design the tool’s processed with hole a and6-mm workpiece the hole pro- surface.size,cessed using Thisworkpiece an was offset achieved surface. distance by This of adopting 7.5 was mm, achieved a resulting tool design by adoptingin witha plunge a6-mm a tool depth holedesign of size, 0.6 with usingmm. a 6-mmThis an offset was hole distanceshownsize, using to of provide7.5 an mm,offset the resulting distancebest processing inof a7.5 plunge mm, parame resulting depthters of for in 0.6 thea mm.plunge development This depth was of shown of 0.6 a surfacemm. to provideThis com- was thepositeshown best layer processingto provide in the current the parameters best study. processing for the developmentparameters for of the a surface development composite of a layersurface in com- the currentpositeFigure study.layer 6a in shows the current the aluminum study. matrix’s processed area reinforced with SiC particles as a surfaceFigureFigure6 acomposite 6a shows shows the theat aluminum the aluminum upper matrix’s surface matrix’s processedusing processed a tool area with area reinforced areinforced 6-mm with hole with SiC size particlesSiC and particles a 0.6- as ammas surface aplunge surface composite depth. composite A at well-distinguished the at upperthe upper surface surface usinglayer using of a toolsurface a withtool composites with a 6-mm a 6-mm hole of Al/SiC sizehole and size was a and 0.6-mm shown a 0.6- plungetomm develop, plunge depth. with depth. A no well-distinguished evidence A well-distinguished of porosity layer in layer ofthe surface processed of surface composites zone, composites illustrating of Al/SiC of Al/SiC awas good was shown mixture shown to develop,andto develop, adherence with with no to evidencenothe evidence aluminum of porosityof porosity substrate. in thein theHowever, processed processed as zone, previouslyzone, illustrating illustrating mentioned, a gooda good mixtureusing mixture a andtooland adherencewith adherence a hole to sizeto the the of aluminum aluminum4 or 6 mm substrate. substrate.combined However, However,with a plunge as as previously previously depth ofmentioned, 0.9mentioned, mm resulted using using in a a toollimitedtool with with dispersion a a hole hole size size of ofthe of 4 4Si or or particles 6 6 mm mm combined combinedinto the aluminum with with a a plunge plunge matrix depth depth and of non-uniformof 0.9 0.9 mm mm resulted resulted surface in in limitedcompositelimited dispersion dispersion layer, as of illustratedof the the Si Si particles particles in Figure into into 6b. the the Th aluminum isaluminum is due to matrix thematrix enforcement and and non-uniform non-uniform particle surface flow’ssurface compositeirregularity,composite layer, layer,and asmore as illustrated illustrated aluminum in in Figure material Figure6b. 6b. is This backThis isextruded due due to to the therather enforcement enforcement than being particle particle mixed flow’s flow’swith irregularity,irregularity, and and more more aluminum aluminum material material is is back back extruded extruded rather rather than than being being mixed mixed with with the SiC particles. The high rotational speed and the tilting degree hence plunge depth, aids

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the SiC particles. The high rotational speed and the tilting degree hence plunge depth, in developing the required forging and frictional forces that produce the heat required for aids in developing the required forging and frictional forces that produce the heat re- plasticization and the implementation of reinforcement particles into the matrix’s upper quired for plasticization and the implementation of reinforcement particles into the ma- surface, especially when enough enforcement particles are available. The reinforcement trix’s upper surface, especially when enough enforcement particles are available. The re- particles are stirred and bonded to the upper part of the SZ during FSP. Although the inforcement particles are stirred and bonded to the upper part of the SZ during FSP. Alt- presence of SiC is expected to cause wear to the H13 tool, the H13 tool was visually hough the presence of SiC is expected to cause wear to the H13 tool, the H13 tool was inspected, and there is no evidence of tool wear. This can be attributed to the soft nature visually inspected, and there is no evidence of tool wear. This can be attributed to the soft of the processed material examined in the current study, i.e., aluminum alloy 1100. This nature of the processed material examined in the current study, i.e., aluminum alloy 1100. is also supported by the fact that the FSP tool used in this study had no pin, and thus the This is also supported by the fact that the FSP tool used in this study had no pin, and thus friction stirring action occurred at a shallow depth, resulting in a maximum thickness of the 300frictionµm atstirring the center action of occurred the processed at a shallow zone. depth, resulting in a maximum thickness of 300 μm at the center of the processed zone.

500 µm

(a)

500 µm

(b)

Figure 6. Panoramic image of the cross-section of the processed samples showing the composite Figure 6. Panoramic image of the cross-section of the processed samples showing the composite surfacesurface using: using: (a) a (a tool) a tool with with a hole a hole size size of 6 of mm 6 mm and and a plunge a plunge depth depth of of0.6 0.6 mm; mm; and and (b) (b a) tool a tool with with a a holehole size size of of6 mm 6 mm and and a plunge a plunge depth depth of of 0.9 0.9 mm. mm. In addition, the results show that some variation in the thickness of the developed In addition, the results show that some variation in the thickness of the developed surface composite layer had occurred. Although the surface composite approximately surface composite layer had occurred. Although the surface composite approximately covered the main width of the shoulder, which is around 21 mm, the thickness of the covered the main width of the shoulder, which is around 21 mm, the thickness of the processed layer varied slightly, reaching a maximum thickness of 300 µm at the center of processed layer varied slightly, reaching a maximum thickness of 300 μm at the center of the processed zone, i.e., near the hole, and minimum thickness of approximately 50 µm the processed zone, i.e., near the hole, and minimum thickness of approximately 50 μm near the edge of the tool shoulder. The maximum thickness at the center of the processed near the edge of the tool shoulder. The maximum thickness at the center of the processed zone was most likely achieved due to the high quantity of enforcement particles dispersed zone was most likely achieved due to the high quantity of enforcement particles dispersed at the center during the processing. Some layer depth uniformity can be noticed around at the center during the processing. Some layer depth uniformity can be noticed around the center of the tool, which can be attributed to the high rotational speed of 3000 rpm and the center of the tool, which can be attributed to the high rotational speed of 3000 rpm and the moderate forging force generated from the tool’s shoulder with a tilting angle of 7◦ the moderate forging force generated from the tool’s shoulder with a tilting angle of 7° and plunge depth of 0.6 mm. On the other hand, the workpiece processed with a plunge anddepth plunge of depth 0.9 mm of exhibited 0.6 mm. On a large the other variation hand, in the thickness workpiece along processed the width with of the a plunge processed depthzone. of 0.9 Unlike mm forexhibited the 0.6-mm a large plunge variation depth, in thethickness center along of the the processed width of zone the (near processed the hole) zone.exhibited Unlike for the the minimum 0.6-mm thickness. plunge depth, This isthe a center direct indicationof the processed that back zone extrusion (near the adversely hole) exhibitedinhibits the the minimum dispersion thickness. process atThis this is center a direct region. indication Additionally, that back the extrusion surface adversely irregularities inhibits(inclinations) the dispersion of the process composite at this are center evident, region. which Additionally, suggests that the the surface increase irregularities in the plunge (inclinations)depth was of unnecessary the composite to process are evident, the soft which aluminum suggests matrix. that the increase in the plunge depth wasThe unnecessary dispersion uniformityto process ofthe the soft reinforcement aluminum matrix. particles generally depends on several factors,The dispersion such as the uniformity vertical pressure of the reinforc on theement base metal, particles number generally of passes, depends tool on rotational sev- eraldirection, factors, such travel as speed the vertic of theal rotating pressure tool, on andthe thebase traverse metal, toolnumber speed. of Thepasses, vertical tool pressurerota- tionalon direction, the base metal travel is speed reported of the [34 rotating] to develop tool, aand better the forgingtraverse andtool improvespeed. The material vertical flow pressureand particles on the base dispersion. metal is Additionally,reported [34] to the develop increase a better in the forging number and of passes improve plays mate- a role rial inflow developing and particles a better dispersion. uniformity Additionally, of the distributed the increase second in the phase number particles of passes in addition plays to a roleeliminating in developing the porosity a better and uniformity developing of the refined distributed microstructure second phase [17,21 ,particles34,35]. Furthermore, in addi- tionsignificant to eliminating homogeneity the porosity in theand dispersion developing of refined SiC particles microstructure into the composite [17,21,34,35]. matrix Fur- has thermore,been reported significant [36 ]homogeneity to occur as a in result the ofdispersion changing of the SiC tool particles rotational into directionthe composite between matrixpasses. has been Other reported factors such[36] to as occur the travel as a resu speedlt of of changing the rotating the tool tool rotational and the traversedirection tool between passes. Other factors such as the travel speed of the rotating tool and the traverse

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tool speed have various effects depending on the heating cycles developed during pro- speed have various effects depending on the heating cycles developed during processing, cessing, as a result of deformation processes similar to extrusion and forging [20,35,37]. as a result of deformation processes similar to extrusion and forging [20,35,37].

3.2.3.2. Elemental Elemental Analysis Analysis of of the the Surface Surface Composite Composite OnlyOnly the the samples samples developed developed using using the the tool tool with with a hole a hole size ofsize 6 mm,of 6 mm, a tilting a tilting angle angle of 7◦ andof 7° a plunge and a depthplunge of depth 0.6 mm of were0.6 mm studied. were Figure studied.7 presents Figure the7 presents EDS elemental the EDS mapping elemental of themapping surface compositeof the surface region, composite illustrating region, the distributionillustrating the of major distribution elements of in major the processed elements zonein the (Al, processed Si, C, and O).zone No (Al, noticeable Si, C, and Fe content O). No was noticeable detected inFethe content cross-section was detected of the surface in the composite.cross-section The of processed the surface area composite. was mainly The a processed mixture of area aluminum was mainly matrix a mixture and dispersed of alu- microsizedminum matrix SiC particles, and dispersed as well asmicr someosized other SiC oxides. particles, The dispersedas well as microsizedsome other SiC oxides. particles The candispersed be seen microsized to be uniformly SiC particles distributed can in be the seen aluminum to be uniformly matrix. The distributed presence in of the oxygen alumi- is evidencenum matrix. for the The possible presence formation of oxygen of silicon is eviden or carbonce for oxidesthe possible during formation FSP. Small of pockets silicon ofor oxygencarbon are oxides also present.during FSP. Moreover, Small thepockets concentration of oxygen of are silicon also seemspresent. to beMoreover, well distributed the con- throughoutcentration theof silicon processed seems zone, to indicatingbe well distri a sufficientbuted throughout dispersion ofthe SiC processed particles zone, in the indicat- matrix anding a a lack sufficient of clustering dispersion or agglomerations of SiC particles of in SiC the particles. matrix and a lack of clustering or agglomerations of SiC particles.

25 µm

25 µm 25 µm 25 µm 25 µm

FigureFigure 7. 7.EDS EDS elemental elemental mapping mapping of of the the surface surface composite. composite.

TheThe through-thickness through-thickness distribution of of the the silicon silicon content content is further is further examined examined in Fig- in Figureure 8.8 The. The spatial spatial elemental elemental concentration, concentration, presented presented in terms in terms of counts of counts per second, per second, indi- indicatescates that that the the aluminum aluminum content content is uniform, is uniform, an andd is rich is rich within within the the base base metal metal through through to to the composite/matrix interface. In the processed zone, the SiC content increases with the composite/matrix interface. In the processed zone, the SiC content increases with lower aluminum content, which indicates good dispersion of SiC particles. On the other lower aluminum content, which indicates good dispersion of SiC particles. On the other hand, the Si content is almost zero in the base metal, and increases toward the composite’s hand, the Si content is almost zero in the base metal, and increases toward the composite’s outer layer. Other elements, such as oxygen and carbon, show a negligible change in outer layer. Other elements, such as oxygen and carbon, show a negligible change in con- content through the thickness. tent through the thickness. The variation in silicon throughout the width of the processed zone was also examined, as presented in Figure9. ToTop eliminate surface the presence of foreign particles, the EDS measurements were carried out roughly 100 µm beneath the top layer of the processed Interface zone. The results show that silicon and aluminum are randomly present. The EDS spectrum Line of measurements generally indicates a larger number of counts for aluminum comparedAl to silicon content for all points along the line of measurements.Processed Zone However, at a measurement distance of approximately 110 µm, there is a large peak of silicon contentmatrix presenting higher counts than those for aluminum. The higher silicon content might be a result of the clustering of

Crystals 2021, 11, x FOR PEER REVIEW 8 of 15

tool speed have various effects depending on the heating cycles developed during processing, as a result of deformation processes similar to extrusion and forging [20,35,37].

3.2. Elemental Analysis of the Surface Composite Only the samples developed using the tool with a hole size of 6 mm, a tilting angle of 7° and a plunge depth of 0.6 mm were studied. Figure 7 presents the EDS elemental mapping of the surface composite region, illustrating the distribution of major elements in the processed zone (Al, Si, C, and O). No noticeable Fe content was detected in the cross-section of the surface composite. The processed area was mainly a mixture of aluminum matrix and dispersed microsized SiC particles, as well as some other oxides. The dispersed microsized SiC particles can be seen to be uniformly distributed in the aluminum matrix. The presence of oxygen is evidence for the possible formation of silicon or carbon oxides during FSP. Small pockets of oxygen are also present. Moreover, the concentration of silicon seems to be well distributed throughout the processed zone, indicating a sufficient dispersion of SiC particles in the matrix and a lack of clustering or agglomerations of SiC particles.

25 µm

Crystals 2021, 11, x FOR PEER REVIEW 25 µm 25 µm 25 µm 25 µm 9 of 15

Figure 7. EDS elemental mapping of the surface composite.

The through-thickness distribution of the silicon content is further examined in Fig- Crystals 2021, 11, 638250,000.00 9 of 14 ure 8. The spatial elemental concentration, presented in terms of counts per second, indicates that the aluminum content is uniform, and is rich within the base metal through to 200,000.00the composite/matrix interface. In the processed zone, the SiC content increases with lower aluminumSiC particles. content, Otherwhich Siindicates peaks are good also dispersion shown in of the SiC spectrum, particles. indicating On the other small regions of O Kα1 (counts) hand, the Si contentSiC clusters is almost throughout zero in thethe processedbase metal, zone. and Theincreases presence toward of silicon the composite’s is evident from the line Al Kα1 (counts) outer layer. Otherof measurements, elements, such which as oxygen indicates and sufﬁcient carbon, show dispersion a negligible of SiC particles.change in con- 150,000.00 Si Kα1 (counts) tent through the thickness. C Kα1_2 (counts) cps Top surface 100,000.00 Interface Line of measurements Al 50,000.00 Processed Zone matrix Crystals 2021, 11, x FOR PEER REVIEW 9 of 15

0.00 0250,000.00 100 200 300 400

Distance from top surface (µm) Figure 8. Vertical line EDS200,000.00 spectrum at the center of the surface composite starting from the top surface of the composite layer to the composite/matrix interface.O Kα1 (counts) Al Kα1 (counts) 150,000.00 Si Kα1 (counts) The variation in silicon throughout the width of theC Kα1_2 processed (counts) zone was also exam- cps ined, as presented in Figure 9. To eliminate the presence of foreign particles, the EDS 100,000.00 measurements were carried out roughly 100 μm beneath the top layer of the processed zone. The results show that silicon and aluminum are randomly present. The EDS spectrum generally indicates50,000.00 a larger number of counts for aluminum compared to silicon content for all points along the line of measurements. However, at a measurement distance of approximately 1100.00 μm, there is a large peak of silicon content presenting higher 0 100 200 300 400 counts than those for aluminum. The higher silicon content might be a result of the clus- Distance from top surface (µm) tering of SiC particles.Figure Other8. Vertical Si line peaks EDS spectrum are also at the showncenter of thein surface the spectrum,composite starting indicating from the top small regions of SiC clustersFiguresurface throughout of 8. theVertical composite linelayerthe toproce EDS the composite/matrix spectrumssed zone. at interface. theThe center presence of the of surface silicon composite is evident starting from the top from the line of measurements,surface of the composite which indicates layer to thesufficient composite/matrix dispersion interface. of SiC particles. The variation in silicon throughout the width of the processed zone was also examined, as presented in Figure 9. To eliminate the presence of foreign particles, the EDS measurements were carried out roughly 100 μm beneath the top layer of the processed zone. The results show thatTop silicon surface and alum inum are randomly present. The EDS spectrum generally indicates a larger number of counts for aluminum compared to silicon content for all points Linealong of the measurements line of measurements. However, at a measurement distance of approximately 110 μm, there is a large peak of silicon content presenting higher countsPoint than those A for aluminum. The higher silicon content might be a result of the clustering of SiC particles. Other Si peaks are also shown in the spectrum, indicating small Crystals 2021, 11, x FOR PEER REVIEW 10 of 15 regions of SiC clusters throughout the processed zone. The presence of silicon is evident from the line of measurements, which indicates sufficient dispersion of SiC particles.

200,000.00 Top surface 180,000.00 Line of measurements 160,000.00 Point A 140,000.00

120,000.00

100,000.00 cps O Kα1 (counts) 80,000.00 Al Kα1 (counts) 60,000.00 Si Kα1 (counts) C Kα1_2 (counts) 40,000.00

20,000.00

0.00 0 100 200 300 400 500 Distance from point A (µm)

FigureFigure 9. Horizontal 9. Horizontal line EDS spectrum line EDSof surface spectrum composite of measured surface at composite100 μm below measuredtop surface. at 100 µm below top surface. 3.3. Microhardness The widthwise microhardness profile of the processed zone, along with the average microhardness of the as-received 1100 aluminum, are shown in Figure 10 for three processing conditions. The microhardness indentations were measured at 150 μm below the top surface of the SZ. The average microhardness of the as-received 1100 aluminum was 38.5 Hv. In the case of FSP with negligible SiC dispersion, the results show a reduction in the hardness below the base metal value for all points measured in the processed zone. This indicates that the as-received aluminum, which was initially in cold-worked condition, was exposed to high frictional heat during FSP, resulting in a drop in hardness in the heat-treatable 1100 aluminum alloy. For the workpiece processed using a plunge depth of 0.6 mm, the surface composite’s microhardness was, remarkably, higher than the base metal due to the dispersion-strengthening mechanism attained as a result of the presence of the reinforcement SiC particles in the processed zone [4,35,38,39]. High hardness values, approaching 76 Hv, were recorded near the center of the processed zone, which is consistent with the OM and EDS results, supporting the production of excellent SiC dispersion in the thick composite layer at the center of the processed zone. The increase of hardness extends roughly 5 mm from the center of the processed zone. Beyond 5 mm, a reduction in the hardness was shown to occur, reaching a minimum of approximately 24 Hv at the composite/matrix interface, which is lower than the hardness recorded for the as-received aluminum (38.5 Hv). The reduction in hardness demonstrates that the dispersion of SiC particles into the aluminum matrix became less effective as the distance from the tool center increases. Over such large distances, the hardness is strongly governed by the aluminum matrix. In addition, the hardness profile of the workpiece processed using a plunge depth of 0.6 mm was not symmetrical. The hardness was slightly higher on the retreating side. Additionally, the decrease in hardness below the base metal value was less present on the retreating side. Such non-symmetrical behavior can mostly be attributed to the temperature increase in the advancing side in comparison to the retreating side during processing, as reported by [40,41]. The hardness profile recorded for the workpiece processed with a plunge depth of 0.6 mm was reported by Sharma et al. [26], indicating steep fluctuations in the hardness values from the center towards the composite/matrix interface. The

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3.3. Microhardness The widthwise microhardness profile of the processed zone, along with the average microhardness of the as-received 1100 aluminum, are shown in Figure 10 for three processing conditions. The microhardness indentations were measured at 150 µm below the top surface of the SZ. The average microhardness of the as-received 1100 aluminum was 38.5 Hv. In the case of FSP with negligible SiC dispersion, the results show a reduction in the hardness below the base metal value for all points measured in the processed zone. This indicates that the as-received aluminum, which was initially in cold-worked condition, was exposed to high frictional heat during FSP, resulting in a drop in hardness in the heat-treatable 1100 aluminum alloy. For the workpiece processed using a plunge depth of 0.6 mm, the surface composite’s microhardness was, remarkably,higher than the base metal due to the dispersion-strengthening mechanism attained as a result of the presence of the reinforcement SiC particles in the processed zone [4,35,38,39]. High hardness values, approaching 76 Hv, were recorded near the center of the processed zone, which is consistent with the OM and EDS results, supporting the production of excellent SiC dispersion in the thick composite layer at the center of the processed zone. The increase of hardness extends roughly 5 mm from the center of the processed zone. Beyond 5 mm, a reduction in the hardness was shown to occur, reaching a minimum of approximately 24 Hv at the composite/matrix interface, which is lower than the hardness recorded for the as-received aluminum (38.5 Hv). The reduction in hardness demonstrates that the dispersion of SiC particles into the aluminum matrix became less effective as the distance from the tool center increases. Over such large distances, the hardness is strongly governed by the aluminum matrix.

80 FSP with dispersion using 0.6 mm plunge depth FSP with no dispersion 70 as received

50 Retreating side Advancing side

30 Vickers Hardness (mm) Hardness Vickers

0 -15 -10 -5 0 5 10 15 Distance from the center (mm) FigureFigure 10. 10. VickersVickers microhardness microhardness profile of a horizontal proﬁle line of at athe horizontal upper layer of linethe sample. at the upper layer of the sample.

In addition, the hardness profile of the workpiece processed using a plunge depth of 0.6 mm was not symmetrical. The hardness was slightly higher on the retreating side. Additionally, the decrease in hardness below the base metal value was less present on the retreating side. Such non-symmetrical behavior can mostly be attributed to the temperature increase in the advancing side in comparison to the retreating side during processing, as reported by [40,41]. The hardness profile recorded for the workpiece processed with a plunge depth of 0.6 mm was reported by Sharma et al. [26], indicating steep fluctuations in the hardness values from the center towards the composite/matrix interface. The fluctuations in hardness are strongly related to the SiC agglomeration and formation of clusters/bands of and/or the number of dispersed particles. The volume fraction of SiC particles is expected to influence the microhardness in the processed zone. As the volume Crystals 2021, 11, 638 11 of 14

percent of the SiC particles increases, the microhardness eventually increases. This was reported by Mishra et al. [4], where the microhardness of the surface composites increased by more than 50 Hv when the vol.% of SiC particles was increased from 13% to 27%. The measured microhardness values for the workpiece processed using a plunge depth of 0.6 mm were generally in good agreement with the results of the semi-quantitative elemental mapping presented in Figures8 and9. Figure 11 presents the through-thickness hardness proﬁle at the center of the three cases examined in Figure 10. For the workpiece processed by FSP without particle dispersion, there is a decline in the hardness below the as-received value, reaching a minimum of 28 Hv at approximately 0.4 mm below the top surface. The hardness values gradually increase at increasing depth, reaching the hardness of the as-received material at a depth of 3 mm. For the workpiece by a plunge depth of 0.6 mm, the dispersion of SiC particles at the top surface resulted in an increase in hardness to 76 Hv and a decrease at greater depths, dropping to approximately 45 Hv at the composite/matrix interface (0.5 mm from the top surface). The remarkable increase in the microhardness at the outer processed surface is most likely attributable to the presence of the reinforcement SiC particles, as demonstrated by the ESD mapping presented in Figures8 and9. In addition, a further reduction in hardness to 35 Hv was recorded at depths greater than the composite/matrix interface (between 0.5 and 1 mm). However, the hardness gradually increased to the hardness of the as-received material at a depth of 1 mm. Compared to the workpiece processed using FSP without SiC dispersion, the decrease in hardness in the aluminum matrix region next to the composite/matrix interface was less severe for the workpiece processed using Crystals 2021, 11, x FOR PEER REVIEW SiC dispersion via FSP. This indicates that the amount12 ofof 15 frictional heat absorbed by the

aluminum matrix was greater for the workpiece processed by FSP without SiC dispersion, and thus the reduction in hardness was not only stronger, but also occurred at a greater increase at increasing depth, reaching the hardness of the as-received material at a depth of 3 mm.depth For the below workpiece the by processed a plunge depth surface. of 0.6 mm, It the can dispersion also be of deducedSiC particles that dispersion of SiC on the top at the topsurface surface ofresulted the workpiecein an increase in acted hardness as atolubricant, 76 Hv and a decrease reducing at greater the transition of frictional heat in the depths, dropping to approximately 45 Hv at the composite/matrix interface (0.5 mm from the top aluminumsurface). The remarkable matrix increase due to in FSP, the microhardness and thus at producing the outer processed less losssur- in hardness in the base metal. face is most likelyThe plungeattributable depth to the waspresence shown of the toreinforcement strongly affectSiC particles, the developmentas of uniform thickness demonstratedof the by composite the ESD mapping layer, presented as well in Figures as the 8 and uniformity 9. In addition, of a SiCfurther particle dispersion and hardness reduction in hardness to 35 Hv was recorded at depths greater◦ than the composite/matrix interfacedevelopment. (between 0.5 and 1 For mm). a However, given angle the hardness of tilt gradually ( 7), the increased increase to the in hard- plunge depth resulted in an increase ness of inthe theas-received contact material area at between a depth of 1 the mm. shoulder Compared to and the workpiece the workpiece. processedThis eventually promoted greater using FSP without SiC dispersion, the decrease in hardness in the aluminum matrix region next to depththe composite/matrix in the composite interface was layer, less severe considering for the workpiece that theprocessed back-extrusion using force was not sufficiently SiC dispersionhighto via cause FSP. This matrix indicates plastic that the flowamount into of frictional the tool heat hole. absorbed However, by the the increase in plunge depth aluminumcould matrix have was greater a deteriorating for the workpiece effect processed on by the FSP matrix without SiC microhardness dispersion, due to the high amount of and thus the reduction in hardness was not only stronger, but also occurred at a greater depth belowheat the generated processed surface. during It can processing, also be deduced as that previously dispersion of reported SiC on the by Rathee et al. [42]. However, top surfacethe of reduction the workpiece in acted matrix as a lubricant, microhardness reducing thewas transition shown of frictional to be heat small when the plunge depth was in the aluminum matrix due to FSP, and thus producing less loss in hardness in the base metal. carefully chosen at a value of 0.6 mm, as also demonstrated in Figure 11.

FSP with dispersion using 0.6 mm plunge depth 70 FSP with no dispersion 60 As received 50

30 Vickers Hardness (Hv) Hardness Vickers 20

0 00.511.522.533.5 Distance from top surface (mm)

Figure 11.Figure Vickers 11.microhardnessVickers through-thickness microhardness profile through-thickness of the FSP aluminum alloy. proﬁle of the FSP aluminum alloy.

The plunge depth was shown to strongly affect the development of uniform thickness of the composite layer, as well as the uniformity of SiC particle dispersion and hardness development. For a given angle of tilt (°7), the increase in plunge depth resulted in an increase in the contact area between the shoulder and the workpiece. This eventually promoted greater depth in the composite layer, considering that the back-extrusion force was not sufficiently high to cause matrix plastic flow into the tool hole. However, the increase in plunge depth could have a deteriorating effect on the matrix microhardness due to the high amount of heat generated during processing, as previously reported by Rathee et al. [42]. However, the reduction in matrix microhardness was shown to be small when the plunge depth was carefully chosen at a value of 0.6 mm, as also demonstrated in Fig- ure 11.

Crystals 2021, 11, 638 12 of 14

4. Conclusions A new approach and tool design for direct friction stir processing to eliminate the step of preplacing reinforcement particles into the base metal was proposed for developing a surface composite layer on 1100 aluminum alloy. The FSP tool consists of a two-part hollow body, i.e., a hollow shank and a shoulder. The new processing approach requires less than half of the tool shoulder to contact the surface, allowing the dispersion of reinforcement particles onto the workpiece’s surface through the hole and mitigating the blockage of the hole during processing. Three different sizes of hole were used, but the 6-mm hole was the optimum. The high rotational speed rate, a tilting angle of 7◦ for the tool, and a moderate plunge depth produced the heat required for plasticization, leading to the application of reinforcement particles. These tool design and processing approaches were shown to effectively disperse the microsized SiC particles into the aluminum matrix during FSP, reaching a uniform distribution of SiC particles in the SZ at the top surface of the base metal with no porosity. The Al/SiC surface composite layer has a thickness ranging from 50 µm, near the edge of the processed area, to about 300 µm, at the center of the processed area. The microhardness increased remarkably from 38.5 Hv at the base to about 80 Hv at the composite’s top layer.

Author Contributions: Conceptualization, A.I.A.; methodology, A.I.A., K.J.A.-F. and S.N.A.; soft- ware, A.I.A., K.J.A.-F. and S.N.A.; validation, A.I.A., K.J.A.-F. and S.N.A.; formal analysis, A.I.A., and K.J.A.-F.; investigation, A.I.A., K.J.A.-F. and S.N.A.; resources, A.I.A., K.J.A.-F. and S.N.A.; data curation, A.I.A., K.J.A.-F. and S.N.A.; writing—original draft preparation, A.I.A., K.J.A.-F. and S.N.A.; writing—review and editing, A.I.A., K.J.A.-F. and S.N.A.; visualization, A.I.A., K.J.A.-F. and S.N.A.; supervision, A.I.A., K.J.A.-F. and S.N.A.; project administration, A.I.A. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the support provided by the Public Authority for Applied Education and Training (PAAET) Grants No. TS-18-12. The authors also acknowledge the support provided by Kuwait University General Facility (Grant No. GE 01/07) for sample preparation, OM, EDS analysis, and microhardness measurements. Acknowledgments: The authors acknowledge the help of engineers Ahmad Shehata and Shaji Michael in setting the experiments and preparing samples. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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