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Tatsuya Funazuka, Kuniaki Dohda, Tomomi Shiratori, Syunsuke Horiuchi, [Ikumu Watanabe](https://orcid.org/0000-0002-7693-1675)

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[Effect of Punch Surface Microtexture on the Microextrudability of AA6063 Micro Backward Extrusion](https://mdr.nims.go.jp/datasets/60152574-1d1e-410a-8a8f-c70cb6417250)

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Effect of Punch Surface Microtexture on the Microextrudability of AA6063 Micro Backward ExtrusionCitation: Funazuka, T.; Dohda, K.;Shiratori, T.; Horiuchi, S.; Watanabe, I.Effect of Punch Surface Microtextureon the Microextrudability of AA6063Micro Backward Extrusion.Micromachines 2022, 13, 2001.https://doi.org/10.3390/mi13112001Academic Editors: Xiuqing Hao,Duanzhi Duan and Youqiang XingReceived: 31 October 2022Accepted: 15 November 2022Published: 17 November 2022Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2022 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).micromachinesArticleEffect of Punch Surface Microtexture on the Microextrudabilityof AA6063 Micro Backward ExtrusionTatsuya Funazuka 1,* , Kuniaki Dohda 2, Tomomi Shiratori 1 , Syunsuke Horiuchi 3 and Ikumu Watanabe 41 Academic Assembly Faculty of Engineering, University of Toyama, Toyama 930-8555, Japan2 Department of Mechanical Engineering, Northwestern University, Evanston, IL 60201, USA3 Graduate School of Science and Engineering for Education, University of Toyama, Toyama 930-8555, Japan4 Research Center for Structural Materials, National Institute for Materials Science, Tsukuba 205-0047, Japan* Correspondence: funazuka@eng.u-toyama.ac.jp; Tel.: +81-76-445-6792Abstract: To apply conventional forming processes to microscale processing, the influence of sizeeffects caused by material properties and friction effects must be considered. Herein, the effects oftool surface properties, such as punch surface texture, on microextrusion properties, such as extrusionforce, product shape, and product microstructure, were investigated using AA6063 billets as testpieces. Millimeter-scale, microscale, and nanoscale textures were fabricated on the punch surfaces.Punch texturing was conducted by electrical discharge machining or polishing or using a laser process.The extrusion force increased rapidly as the stroke progressed for all punch textures. Comparing theproduct shapes, the smaller the texture size, the lower the adhesion and the longer the backwardextrusion length. The results of material analysis using electron backscatter diffraction show thatmaterial flowability is improved, and more strain is uniformly applied when a nanoscale-texturedpunch is used. By contrast, when a mirror punch was used, material flowability decreased, and strainwas applied non-uniformly. Therefore, by changing the surface properties of the punch, the tribologybetween the tool and material can be controlled, and formability can be improved.Keywords: aluminum alloy; microextrusion; microtexture; size effect; tribology1. IntroductionOwing to the rapid miniaturization of various products in recent years, the fabricationof microparts that constitute medical devices has been attracting attention. Thus far, mi-croparts have been manufactured mainly by machining; however, research on microplasticforming has also become active owing to requirements such as reduction in the produc-tion cost and flexibility in the product shape [1]. Among such processes, microextrusionprocessing has attracted considerable industrial attention as a micropart-forming processtechnology. When extrusion, which is a conventional macroscale machining technology, isapplied on a microscale, issues in reproducibility and accuracy arise. Engel et al. clarifiedthe effect of small product dimensions on tribology via microscale double-cup extrusiontests and reported the following findings: as the product dimensions decreased, the tool–billet contact area and the pockets that hold the lubricant decrease and the area of directcontact increases, resulting in increased friction [2]. Additionally, a study on suitableprocessing temperatures for microforming showed that stable forming is possible at hightemperatures, where dislocation migration is activated; moreover, the variation in productaccuracy owing to the size effect is reduced [3].Bunget and Ngaile [4], Xu et al. [5], and Lou et al. [6] reported that ultrasonic vibrationcontributed to the improvement of the forming load and material flow performance viaultrasonic microextrusion. Chan et al. [7,8] showed that by considering multiple influ-encing factors, such as material flow pattern, interface condition, and flow stress curves,microforming analysis can be used to accurately predict deformation behavior. In a seriesof studies on microextrusion [9–12], Cao et al. examined the effects of microstructure,Micromachines 2022, 13, 2001. https://doi.org/10.3390/mi13112001 https://www.mdpi.com/journal/micromachineshttps://doi.org/10.3390/mi13112001https://doi.org/10.3390/mi13112001https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/micromachineshttps://www.mdpi.comhttps://orcid.org/0000-0001-8743-2874https://orcid.org/0000-0003-4432-1829https://orcid.org/0000-0002-7693-1675https://doi.org/10.3390/mi13112001https://www.mdpi.com/journal/micromachineshttps://www.mdpi.com/article/10.3390/mi13112001?type=check_update&version=1Micromachines 2022, 13, 2001 2 of 11such as the grain size, shape, and orientation of the billet, and interface conditions, onthe processing. They evaluated the plastic deformation behavior of different grain sizesin microextrusion and showed that the larger the grain size, the more easily the extrudedpart is bent owing to non-uniform deformation and that the difference in grain size af-fects formability. Furthermore, the effectiveness of the hard coatings in stabilizing frictionduring processing was investigated. The effectiveness of applying high-strength, low-friction hard coatings to die surfaces was reported, with the deposition of silicon-containingdiamond-like carbon (DLC-Si) coating being the most effective.In microscale forming, the surface becomes rougher relative to the working scale. Thissurface roughness causes significant changes in the friction and forming behavior [13,14].Moreover, achieving surface properties suitable for the microscale is difficult, and theoptimal tool surface conditions for reducing friction and improving formability shouldbe determined. In the field of microfabrication, microtexturing has been successfullyapplied to reduce friction and increase the lubrication pocket area on tool surfaces [15–17].Microtexturing is expected to stabilize machinability by reducing the tool contact area andretaining the lubricant.The effects of microstructure, lubricant, and die coating on forward–backward microex-trusion techniques for aluminum alloys have been identified [18–20]. Complex material flowin both the forward and backward directions was evaluated experimentally and by simulation;backward extrusion was considerably affected by die and punch friction and microstructure.These studies revealed that grain size control and tribology at the tool–material inter-face significantly affect formability parameters, such as forming force and material flow,in microextrusion. The backward extrusion of microscale parts was performed to realizemicroforming. When the punch surface is provided with an arbitrary surface roughnessusing abrasive paper, the tool contact area reduces, and material flow improves [21]. How-ever, considerable uncertainty exists regarding the effect of the punch surface texture sizeon micro backward extrusion processing; therefore, the effect of the texture size frommillimeter-scale to nanoscale for optimal tool surface design in microforming should be in-vestigated. In shearing tests, certain examples of nanotextured punches have been applied,and the use of nanotextured punches has resulted in high-precision micro-shearing owingto the reduction of the processing-influenced layer [22,23].Herein, millimeter- to nanoscale-textured tools were used to stabilize formability byreducing the tool contact area and retaining the lubricant. Microtextures of various scaleswere applied to the punch surface to investigate the effect of the punch surface properties.The effects of punch surface texture on microextrusion formability were estimated basedon the extrusion force, billet shape after forming, amount of adhesion to the punch, andmicrostructure analysis of the product.2. Materials and MethodsThe microextrusion machine (Micro Fabrication Laboratory, Japan) used in this studyand shown in Figure 1 [21] is a servomotor-driven tabletop screw press that transmits torquedirectly or is amplified by a servomotor to the screw shaft. The screw axis is connected tothe punch via a load cell to control the forming speed and position. The microextrusionmachine, die, punch, and other tools as well as the billet have the same dimensions as thosein the previous study.A schematic of the die and punch used for this study is shown in Figure 2 [21]. Thedie was divided at the center to remove the billet after extrusion. Moreover, the die had acontainer inner diameter ofϕ 1.71 mm. The arithmetic mean roughness inside the containerwas Ra = 0.18 µm. The punch was selected for backward extrusion, and the diameter of theformed part was ϕ 1.47 mm, indicating that a bottomed microtube with a product diameterof ϕ 1.71 mm and wall thickness of 120 µm can be fabricated.Micromachines 2022, 13, 2001 3 of 11Micromachines 2022, 13, x 3 of 11    Figure 1. Photograph of the microextrusion machine [21]. A schematic of the die and punch used for this study is shown in Figure 2 [21]. The die was divided at the center to remove the billet after extrusion. Moreover, the die had a container inner diameter of φ1.71 mm. The arithmetic mean roughness inside the con-tainer was Ra = 0.18 µm. The punch was selected for backward extrusion, and the diameter of the formed part was φ1.47 mm, indicating that a bottomed microtube with a product diameter of φ1.71 mm and wall thickness of 120 µm can be fabricated.        (a) (b) (c) (d) Figure 2. Shape of the die and punch: (a) extrusion die, (b) jig, and (c) punch; (d) schematic of back-ward microextrusion [21]. Figure 3 shows the punches used in this study. Figure 3a shows a mirror-finished punch with a ground surface, and Figure 3b shows a millimeter-textured punch with a constant length of millimeter-sized grooves dug using an electric discharge machine on the mirror-finished punch (Figure 3a). Figure 3c shows a 10 µm-textured punch with 10 µm grooves in Figure 3a formed using abrasive paper with a grain size of 140. Figure 3d shows a 10 µm-textured punch with several micro-sized grooves of approximately 10 µm Figure 1. Photograph of the microextrusion machine [21].Micromachines 2022, 13, x 3 of 11    Figure 1. Photograph of the microextrusion machine [21]. A schematic of the die and punch used for this study is shown in Figure 2 [21]. The die was divided at the center to remove the billet after extrusion. Moreover, the die had a container inner diameter of φ1.71 mm. The arithmetic mean roughness inside the con-tainer was Ra = 0.18 µm. The punch was selected for backward extrusion, and the diameter of the formed part was φ1.47 mm, indicating that a bottomed microtube with a product diameter of φ1.71 mm and wall thickness of 120 µm can be fabricated.        (a) (b) (c) (d) Figure 2. Shape of the die and punch: (a) extrusion die, (b) jig, and (c) punch; (d) schematic of back-ward microextrusion [21]. Figure 3 shows the punches used in this study. Figure 3a shows a mirror-finished punch with a ground surface, and Figure 3b shows a millimeter-textured punch with a constant length of millimeter-sized grooves dug using an electric discharge machine on the mirror-finished punch (Figure 3a). Figure 3c shows a 10 µm-textured punch with 10 µm grooves in Figure 3a formed using abrasive paper with a grain size of 140. Figure 3d shows a 10 µm-textured punch with several micro-sized grooves of approximately 10 µm Figure 2. Shape of the die and punch: (a) extrusion die, (b) jig, and (c) punch; (d) schematic ofbackward microextrusion [21].Figure 3 shows the punches used in this study. Figure 3a shows a mirror-finishedpunch with a ground surface, and Figure 3b shows a millimeter-textured punch with aconstant length of millimeter-sized grooves dug using an electric discharge machine on themirror-finished punch (Figure 3a). Figure 3c shows a 10 µm-textured punch with 10 µmgrooves in Figure 3a formed using abrasive paper with a grain size of 140. Figure 3dshows a 10 µm-textured punch with several micro-sized grooves of approximately 10 µmin depth and 100 µm in pitch on abrasive paper with a grain size of 400. The 5 µm-texturedpunches have several micro-sized grooves of approximately 5 µm in depth and 100 µmin pitch. Figure 3e shows a punch with several micro-sized grooves of approximately100 µm in pitch and 5 µm in depth. The nanometer-textured punch shown in Figure 3ewas produced by applying a nanoscale periodic groove structure (Ra = 0.099 µm) to theground mirror punch shown in Figure 3a using ultrashort pulsed laser machining (LipsWorks Co., Ltd., Ota-ku, Tokyo, Japan). The wavelength, pulse durations, average powerMicromachines 2022, 13, 2001 4 of 11output, and maximum frequency are 515 mm, 180–190 fs, 8.2 W, and 600 KHz, respectively.Figure 3e shows that the grooves are 0.01 µm deep with a pitch of 0.3 µm and are appliedup to 3 mm from the punch tip, as observed using a scanning microscope at 10,000×. Thenanogrooves are oriented parallel to the direction of the punch.Micromachines 2022, 13, x 4 of 11   in depth and 100 µm in pitch on abrasive paper with a grain size of 400. The 5 µm-textured punches have several micro-sized grooves of approximately 5 µm in depth and 100 µm in pitch. Figure 3e shows a punch with several micro-sized grooves of approximately 100 µm in pitch and 5 µm in depth. The nanometer-textured punch shown in Figure 3e was produced by applying a nanoscale periodic groove structure (Ra = 0.099 µm) to the ground mirror punch shown in Figure 3a using ultrashort pulsed laser machining (Lips Works Co., Ltd., Ota-ku, Tokyo, Japan). The wavelength, pulse durations, average power output, and maximum frequency are 515 mm, 180–190 fs, 8.2 W, and 600 KHz, respectively. Figure 3e shows that the grooves are 0.01 μm deep with a pitch of 0.3 μm and are applied up to 3 mm from the punch tip, as observed using a scanning microscope at 10,000×. The nano-grooves are oriented parallel to the direction of the punch.    (a) (b) (c)   (d) (e) Figure 3. Textured punches: (a) mirror surface, (b) millimeter－textured (depth: 100 μm; approxi-mately 2 grooves/mm), (c) 10 μm－textured (depth: 10 μm; ~10 grooves/mm), (d) 5 μm－textured (depth: 5 μm; approximately 10 grooves/mm), and (e) nanometer－textured (depth: 0.01 μm; ap-proximately 3 grooves/μm). The test billets were cut from an A6063 aluminum alloy φ1.70 mm round wire and finished to a length of 4.0 mm. Table 1 lists the shape dimensions, grain size, mechanical properties, and microstructure of the billet [21]. The average grain size of the billets was 23.3 μm. The relationship between the true stress σ and true strain ε of the billet can be expressed by the hardening equation (Equation (1)), which is the power of the plasticity factor F [MPa] and the work-hardening index n. Because the mechanical properties are significantly affected by the microstructure, the mechanical properties should be evalu-ated based on the dimensions. The true stress σ and true strain ε were obtained from mi-crocompression tests using billets 1.70 mm in diameter and 2.5 mm in length [18,19]. The Figure 3. Textured punches: (a) mirror surface, (b) millimeter—textured (depth: 100 µm; approxi-mately 2 grooves/mm), (c) 10 µm—textured (depth: 10 µm; ~10 grooves/mm), (d) 5 µm—textured(depth: 5 µm; approximately 10 grooves/mm), and (e) nanometer—textured (depth: 0.01 µm; ap-proximately 3 grooves/µm).The test billets were cut from an A6063 aluminum alloy ϕ 1.70 mm round wire andfinished to a length of 4.0 mm. Table 1 lists the shape dimensions, grain size, mechanicalproperties, and microstructure of the billet [21]. The average grain size of the billets was23.3 µm. The relationship between the true stress σ and true strain ε of the billet can beexpressed by the hardening equation (Equation (1)), which is the power of the plasticityfactor F [MPa] and the work-hardening index n. Because the mechanical properties aresignificantly affected by the microstructure, the mechanical properties should be evaluatedbased on the dimensions. The true stress σ and true strain ε were obtained from micro-compression tests using billets 1.70 mm in diameter and 2.5 mm in length [18,19]. Thecompression ratio was 80%, and the compression rate was 0.1 mm/s. The plasticity factor,F, and work-hardening index, n, were derived from this microcompression test [18,19,21].σ = F εn (1)Micromachines 2022, 13, 2001 5 of 11Table 1. Dimension and properties of the AA6063 billet [21].Item Value and FigureShape of billet ϕ 1.70 × 4 (mm)Vickers hardness 33.2 (HV)F 169.0 (MPa)n 0.29MicrostructureMicromachines 2022, 13, x 5 of 11   compression ratio was 80%, and the compression rate was 0.1 mm/s. The plasticity factor, F, and work-hardening index, n, were derived from this microcompression test [18,19,21]. σ = F εn (1) Table 1. Dimension and properties of the AA6063 billet [21]. Item Value and Figure Shape of billet φ 1.70 × 4 (mm) Vickers hardness 33.2 (HV) F 169.0 (MPa) n 0.29 Microstructure  Grain distribution  Average grain size 23.3 (μm) Extrusion conditions were set at 0.1 mm/s ram speed and 1.5 mm ram stroke at room temperature, based on previous studies [21]. In this experiment, the extrusion test was repeated four times for each billet to ensure reproducibility. Scanning electron microscopy coupled with electron probe microanalysis (SEM-EPMA, JEOL JXA-8230, Akishima, To-kyo, Japan) was used to observe the surface properties and adhesion of the punch. The acceleration voltage for EPMA was 15 kV, and the measurement range was 2.5 mm (ver-tical) × 1.8 mm (horizontal). Electron backscatter diffraction (EBSD, JEOL JSM-6700F, Aki-shima, Tokyo, Japan) patterns were obtained to analyze the microstructure of the product. Grain orientation distribution and grain size were analyzed using EBSD to measure the effect of using a tool with reduced friction by a grooved punch on the material. A meas-urement of 800 μm × 0.15 mm was obtained from the tip of the extrusion. The specimens were mirror-polished with an abrasive, and the surface static stress was removed by ion milling. The measurement conditions included an acceleration voltage of 20 kV, irradia-tion current of 13 nA, working distance of 15 mm, and magnification of 500×. Step sizes of 0.5 and 0.1 µm were used for measurements at the extrusion tip and local measurements, respectively. 3. Experimental Results and Discussion 3.1. Extrusion Force–Ram Stroke Diagram and Metal Flow during Backward Microextrusion Figure 4 shows the extrusion force–stroke diagrams for the mirror punch, millimeter-textured punch, 10 µm-groove texture punch, 5 µ m-groove texture punch, and punches with different surface properties of the nanotexture. The maximum extrusion load is 5.2 kN for the (a) mirror surface punch, which is the highest, compared to 3.8 kN for the (b) millimeter-textured punch; the force is reduced by the addition of texture. Based on the comparison of the micrometer-scale forces, that is, (c) 4.1 kN for the 10 µm texture and (d) 3.0 kN for the 5 µm texture, the force is reduced by decreasing texture size.  30 μm Grain distributionMicromachines 2022, 13, x 5 of 11   compression ratio was 80%, and the compression rate was 0.1 mm/s. The plasticity factor, F, and work-hardening index, n, were derived from this microcompression test [18,19,21]. σ = F εn (1) Table 1. Dimension and properties of the AA6063 billet [21]. Item Value and Figure Shape of billet φ 1.70 × 4 (mm) Vickers hardness 33.2 (HV) F 169.0 (MPa) n 0.29 Microstructure  Grain distribution  Average grain size 23.3 (μm) Extrusion conditions were set at 0.1 mm/s ram speed and 1.5 mm ram stroke at room temperature, based on previous studies [21]. In this experiment, the extrusion test was repeated four times for each billet to ensure reproducibility. Scanning electron microscopy coupled with electron probe microanalysis (SEM-EPMA, JEOL JXA-8230, Akishima, To-kyo, Japan) was used to observe the surface properties and adhesion of the punch. The acceleration voltage for EPMA was 15 kV, and the measurement range was 2.5 mm (ver-tical) × 1.8 mm (horizontal). Electron backscatter diffraction (EBSD, JEOL JSM-6700F, Aki-shima, Tokyo, Japan) patterns were obtained to analyze the microstructure of the product. Grain orientation distribution and grain size were analyzed using EBSD to measure the effect of using a tool with reduced friction by a grooved punch on the material. A meas-urement of 800 μm × 0.15 mm was obtained from the tip of the extrusion. The specimens were mirror-polished with an abrasive, and the surface static stress was removed by ion milling. The measurement conditions included an acceleration voltage of 20 kV, irradia-tion current of 13 nA, working distance of 15 mm, and magnification of 500×. Step sizes of 0.5 and 0.1 µm were used for measurements at the extrusion tip and local measurements, respectively. 3. Experimental Results and Discussion 3.1. Extrusion Force–Ram Stroke Diagram and Metal Flow during Backward Microextrusion Figure 4 shows the extrusion force–stroke diagrams for the mirror punch, millimeter-textured punch, 10 µm-groove texture punch, 5 µ m-groove texture punch, and punches with different surface properties of the nanotexture. The maximum extrusion load is 5.2 kN for the (a) mirror surface punch, which is the highest, compared to 3.8 kN for the (b) millimeter-textured punch; the force is reduced by the addition of texture. Based on the comparison of the micrometer-scale forces, that is, (c) 4.1 kN for the 10 µm texture and (d) 3.0 kN for the 5 µm texture, the force is reduced by decreasing texture size.  30 μm Average grain size 23.3 (µm)Extrusion conditions were set at 0.1 mm/s ram speed and 1.5 mm ram stroke atroom temperature, based on previous studies [21]. In this experiment, the extrusiontest was repeated four times for each billet to ensure reproducibility. Scanning electronmicroscopy coupled with electron probe microanalysis (SEM-EPMA, JEOL JXA-8230, Ak-ishima, Tokyo, Japan) was used to observe the surface properties and adhesion of thepunch. The acceleration voltage for EPMA was 15 kV, and the measurement range was2.5 mm (vertical) × 1.8 mm (horizontal). Electron backscatter diffraction (EBSD, JEOL JSM-6700F, Akishima, Tokyo, Japan) patterns were obtained to analyze the microstructure ofthe product. Grain orientation distribution and grain size were analyzed using EBSDto measure the effect of using a tool with reduced friction by a grooved punch on thematerial. A measurement of 800 µm × 0.15 mm was obtained from the tip of the extrusion.The specimens were mirror-polished with an abrasive, and the surface static stress wasremoved by ion milling. The measurement conditions included an acceleration voltageof 20 kV, irradiation current of 13 nA, working distance of 15 mm, and magnification of500×. Step sizes of 0.5 and 0.1 µm were used for measurements at the extrusion tip andlocal measurements, respectively.3. Experimental Results and Discussion3.1. Extrusion Force–Ram Stroke Diagram and Metal Flow during Backward MicroextrusionFigure 4 shows the extrusion force–stroke diagrams for the mirror punch, millimeter-textured punch, 10 µm-groove texture punch, 5 µm-groove texture punch, and puncheswith different surface properties of the nanotexture. The maximum extrusion load is5.2 kN for the (a) mirror surface punch, which is the highest, compared to 3.8 kN for the(b) millimeter-textured punch; the force is reduced by the addition of texture. Based onthe comparison of the micrometer-scale forces, that is, (c) 4.1 kN for the 10 µm texture and(d) 3.0 kN for the 5 µm texture, the force is reduced by decreasing texture size.Figure 5 shows the cross-sectional shape of each punch after micro backward extrusionand the backward extrusion length (lb) for each product at a ram stroke of 1.5 mm. Thelb values are 1.95, 2.21, 2.19, 2.64, and 2.60 mm for (a) mirror, (b) millimeter-textured,(c) 10 µm-textured, (d) 5 µm-textured, and (e) nanotextured punches, respectively. Thelb is similar in length; however, the reduction in the textured true contact area possiblyMicromachines 2022, 13, 2001 6 of 11reduces friction and increases lb by facilitating appropriate material flow. The shorterbackward extrusion length of the millimeter-textured and 10 µm-textured punches thanthat of the 5 µm-textured punch may be explained using millimeter-sized grooves inmicroscale machining generating two flows: one in the backward extrusion direction andanother that enters the tool groove. By reducing the texture depth, the Al adhesion of thepunch was broken, and friction was reduced, thereby resulting in smoother plastic flow anda longer backward extrusion length. Therefore, a friction reduction effect can be obtainedby reducing the texture depth in the microscale plastic forming.Micromachines 2022, 13, x 6 of 11    Figure 4. Extrusion force–ram stroke curve in each punch. Figure 5 shows the cross-sectional shape of each punch after micro backward extru-sion and the backward extrusion length (lb) for each product at a ram stroke of 1.5 mm. The lb values are 1.95, 2.21, 2.19, 2.64, and 2.60 mm for (a) mirror, (b) millimeter-textured, (c) 10 µm-textured, (d) 5 µm-textured, and (e) nanotextured punches, respectively. The lb is similar in length; however, the reduction in the textured true contact area possibly re-duces friction and increases lb by facilitating appropriate material flow. The shorter back-ward extrusion length of the millimeter-textured and 10 µm-textured punches than that of the 5 µm-textured punch may be explained using millimeter-sized grooves in mi-croscale machining generating two flows: one in the backward extrusion direction and another that enters the tool groove. By reducing the texture depth, the Al adhesion of the punch was broken, and friction was reduced, thereby resulting in smoother plastic flow and a longer backward extrusion length. Therefore, a friction reduction effect can be ob-tained by reducing the texture depth in the microscale plastic forming.     (a) (b) (c)    (d) (e)  Figure 5. Longitudinal section cross-sectional images of the extrusion: (a) mirror surface, (b) milli-meter-textured, (c) 10 μm-textured, (d) 5 μm-textured, and (e) nanometer-textured punches. Figure 4. Extrusion force–ram stroke curve in each punch.Micromachines 2022, 13, x 6 of 11    Figure 4. Extrusion force–ram stroke curve in each punch. Figure 5 shows the cross-sectional shape of each punch after micro backward extru-sion and the backward extrusion length (lb) for each product at a ram stroke of 1.5 mm. The lb values are 1.95, 2.21, 2.19, 2.64, and 2.60 mm for (a) mirror, (b) millimeter-textured, (c) 10 µm-textured, (d) 5 µm-textured, and (e) nanotextured punches, respectively. The lb is similar in length; however, the reduction in the textured true contact area possibly re-duces friction and increases lb by facilitating appropriate material flow. The shorter back-ward extrusion length of the millimeter-textured and 10 µm-textured punches than that of the 5 µm-textured punch may be explained using millimeter-sized grooves in mi-croscale machining generating two flows: one in the backward extrusion direction and another that enters the tool groove. By reducing the texture depth, the Al adhesion of the punch was broken, and friction was reduced, thereby resulting in smoother plastic flow and a longer backward extrusion length. Therefore, a friction reduction effect can be ob-tained by reducing the texture depth in the microscale plastic forming.     (a) (b) (c)    (d) (e)  Figure 5. Longitudinal section cross-sectional images of the extrusion: (a) mirror surface, (b) milli-meter-textured, (c) 10 μm-textured, (d) 5 μm-textured, and (e) nanometer-textured punches. Figure 5. Longitudinal section cross-sectional images of the extrusion: (a) mirror surface, (b) millimeter-textured, (c) 10 µm-textured, (d) 5 µm-textured, and (e) nanometer-textured punches.3.2. Evaluation of Adhesion to PunchFigure 6 shows the amount of adhesion on the punch surface after processing withpunches with different surface properties: (a) mirror punch, (b) millimeter-textured punch,Micromachines 2022, 13, 2001 7 of 11(c) 10 µm-textured punch, (d) 5 µm-textured punch, and (e) nanotextured punch. EPMAwas used to analyze the punch surface.Micromachines 2022, 13, x 7 of 11   3.2. Evaluation of Adhesion to Punch Figure 6 shows the amount of adhesion on the punch surface after processing with punches with different surface properties: (a) mirror punch, (b) millimeter-textured punch, (c) 10 µm-textured punch, (d) 5 µm-textured punch, and (e) nanotextured punch. EPMA was used to analyze the punch surface. The experimental results showed that the textured punch broke the adherence of Al in the grooves on the circumference. Additionally, when comparing the amount of depo-sition between the two punches with different surface texture sizes, the 5 µm-groove punch shows less deposition than the 10 µm-groove punch, possibly because the pocket into which the material flows shrinks when the groove size is smaller; therefore, the ad-hesion can be broken into smaller pieces [21]. Moreover, the force of friction is reduced by breaking up the adhesion, resulting in a reduction in force. In particular, nanotextured punching causes the least amount of adhesion, which is considered to be due to the fine fragmentation of adhesion at the nanoscale.    (a) (b) (c)    (d) (e) Figure 6. Evaluation of adhesion to punch via EPMA: (a) mirror surface; (b) millimeter-textured; (c) 10 μm-textured; (d) 5 μm-textured; and (e) nanometer-textured punches. 3.3. Microstructure Analysis of the Extrusion Figure 7 shows the inverse pole figure (IPF) map results obtained using mirror, 5 µm-textured, and nanotextured punches. The IPF map can be used to determine crystal orien-tation, which is defined by the crystal plane, by color. Grains at the leading edge of the extrudate flowed out without shearing. The material was sheared longitudinally, and the grain size increased toward the rear end of the material [21]. The measurement position was enlarged from 300 to 600 µm from the tip to exclude the non-steady state during the early stage of extrusion. Compared with the mirror punch, the texture punch was sheared, and the crystal grains were elongated vertically. The microtextured punches were sheared more strongly than the nanotextured punches, resulting in a larger vertical elongation. Therefore, the extrudate length was longer for the microtextured punch. Figure 8 shows the results of the kernel average misorientation (KAM) map, which is a quantitative method for evaluating the residual strain inside a sample based on the crystal orientation difference information. (a) In the specular punch, green and yellow colors with an azi-muthal difference of 1–2° are mostly distributed at the tip of the material, whereas red and yellow colors with an azimuthal difference of 3–5° are mostly distributed at the rear end of the material. By contrast, (b) the 5 µm-textured and (c) nanotextured punch have a uni-form strain exceeding 3–5°. This outcome suggests that the texture punches have low fric-tion, which facilitates material flow; therefore, strain is uniformly introduced to accelerate Figure 6. Evaluation of adhesion to punch via EPMA: (a) mirror surface; (b) millimeter-textured;(c) 10 µm-textured; (d) 5 µm-textured; and (e) nanometer-textured punches.The experimental results showed that the textured punch broke the adherence of Al inthe grooves on the circumference. Additionally, when comparing the amount of depositionbetween the two punches with different surface texture sizes, the 5 µm-groove punch showsless deposition than the 10 µm-groove punch, possibly because the pocket into which thematerial flows shrinks when the groove size is smaller; therefore, the adhesion can bebroken into smaller pieces [21]. Moreover, the force of friction is reduced by breaking upthe adhesion, resulting in a reduction in force. In particular, nanotextured punching causesthe least amount of adhesion, which is considered to be due to the fine fragmentation ofadhesion at the nanoscale.3.3. Microstructure Analysis of the ExtrusionFigure 7 shows the inverse pole figure (IPF) map results obtained using mirror, 5 µm-textured, and nanotextured punches. The IPF map can be used to determine crystalorientation, which is defined by the crystal plane, by color. Grains at the leading edge ofthe extrudate flowed out without shearing. The material was sheared longitudinally, andthe grain size increased toward the rear end of the material [21]. The measurement positionwas enlarged from 300 to 600 µm from the tip to exclude the non-steady state during theearly stage of extrusion. Compared with the mirror punch, the texture punch was sheared,and the crystal grains were elongated vertically. The microtextured punches were shearedmore strongly than the nanotextured punches, resulting in a larger vertical elongation.Therefore, the extrudate length was longer for the microtextured punch. Figure 8 showsthe results of the kernel average misorientation (KAM) map, which is a quantitativemethod for evaluating the residual strain inside a sample based on the crystal orientationdifference information. (a) In the specular punch, green and yellow colors with an azimuthaldifference of 1–2◦ are mostly distributed at the tip of the material, whereas red and yellowcolors with an azimuthal difference of 3–5◦ are mostly distributed at the rear end of thematerial. By contrast, (b) the 5 µm-textured and (c) nanotextured punch have a uniformstrain exceeding 3–5◦. This outcome suggests that the texture punches have low friction,which facilitates material flow; therefore, strain is uniformly introduced to accelerate theprocessing progress. The (a) specular punch has high friction, which hinders material flow,and the strain is considered to accumulate unevenly. Compared to the (b) 5 µm-texturedMicromachines 2022, 13, 2001 8 of 11punch, the (c) nanotextured punch shows negligible red distribution with an orientationdifference of 5◦ or higher; additionally, the green areas, which have a smaller strain, areuniformly distributed. Grain deformation was homogenized by nanotexturing, and thegrain size was refined without the accumulation of local plastic strain. The machining limitis expected to improve because a low-friction machining environment can be realized.Micromachines 2022, 13, x 8 of 11   the processing progress. The (a) specular punch has high friction, which hinders material flow, and the strain is considered to accumulate unevenly. Compared to the (b) 5 µm-textured punch, the (c) nanotextured punch shows negligible red distribution with an ori-entation difference of 5° or higher; additionally, the green areas, which have a smaller strain, are uniformly distributed. Grain deformation was homogenized by nanotexturing, and the grain size was refined without the accumulation of local plastic strain. The ma-chining limit is expected to improve because a low-friction machining environment can be realized.       (a) (b) (c)  Figure 7. IPF map of the extrusion obtained using EBSD: (a) mirror surface, (b) 5 μm-textured, and (c) nanometer-textured punches.        (a) (b) (c)  Figure 8. KAM map of the extrusion via EBSD: (a) mirror surface, (b) 5 μm-textured, and (c) na-nometer-textured punches. 3.4. Comparison of Microtexture and Nanotexture Punches To investigate the anti-adhesion and anti-wear properties of micro- and nanotex-tured punches, a comparison in terms of the first and fifth extrusion cycles was conducted as shown in Figure 9. The nanotexture punch continues to reduce the force, whereas the 5 µm-textured punch tends to increase the extrusion force. Figure 10 shows the results of EPMA measurements of Al element adhesion on the punch surfaces after the first and fifth extrusion cycles. In the case of the 5 µm-textured punch, the EPMA results show increased adhesion in the grooves, and Al adhesion in the recesses of the grooves, which is not observed in the first cycle, is also observed in the fifth cycle. In the case of the 5 µm-textured punch, the grooves are worn out by the fifth cycle of processing, and the in-creased amount of adhesion is considered to have caused the increased extrusion force. Figure 7. IPF map of the extrusion obtained using EBSD: (a) mirror surface, (b) 5 µm-textured, and(c) nanometer-textured punches.Micromachines 2022, 13, x 8 of 11   the processing progress. The (a) specular punch has high friction, which hinders material flow, and the strain is considered to accumulate unevenly. Compared to the (b) 5 µm-textured punch, the (c) nanotextured punch shows negligible red distribution with an ori-entation difference of 5° or higher; additionally, the green areas, which have a smaller strain, are uniformly distributed. Grain deformation was homogenized by nanotexturing, and the grain size was refined without the accumulation of local plastic strain. The ma-chining limit is expected to improve because a low-friction machining environment can be realized.       (a) (b) (c)  Figure 7. IPF map of the extrusion obtained using EBSD: (a) mirror surface, (b) 5 μm-textured, and (c) nanometer-textured punches.        (a) (b) (c)  Figure 8. KAM map of the extrusion via EBSD: (a) mirror surface, (b) 5 μm-textured, and (c) na-nometer-textured punches. 3.4. Comparison of Microtexture and Nanotexture Punches To investigate the anti-adhesion and anti-wear properties of micro- and nanotex-tured punches, a comparison in terms of the first and fifth extrusion cycles was conducted as shown in Figure 9. The nanotexture punch continues to reduce the force, whereas the 5 µm-textured punch tends to increase the extrusion force. Figure 10 shows the results of EPMA measurements of Al element adhesion on the punch surfaces after the first and fifth extrusion cycles. In the case of the 5 µm-textured punch, the EPMA results show increased adhesion in the grooves, and Al adhesion in the recesses of the grooves, which is not observed in the first cycle, is also observed in the fifth cycle. In the case of the 5 µm-textured punch, the grooves are worn out by the fifth cycle of processing, and the in-creased amount of adhesion is considered to have caused the increased extrusion force. Figure 8. KAM map of the extrusion via EBSD: (a) mirror surface, (b) 5 µm-textured, and(c) nanometer-textured punches.3.4. Comparison of Microtexture and Nanotexture PunchesTo investigate the anti-adhesion and anti-wear properties of micro- and nanotexturedpunches, a comparison in terms of the first and fifth extrusion cycles was conducted asshown in Figure 9. The nanotexture punch continues to reduce the force, whereas the5 µm-textured punch tends to increase the extrusion force. Figure 10 shows the resultsof EPMA measurements of Al element adhesion on the punch surfaces after the first andfifth extrusion cycles. In the case of the 5 µm-textured punch, the EPMA results showincreased adhesion in the grooves, and Al adhesion in the recesses of the grooves, whichis not observed in the first cycle, is also observed in the fifth cycle. In the case of the5 µm-textured punch, the grooves are worn out by the fifth cycle of processing, and theincreased amount of adhesion is considered to have caused the increased extrusion force.However, the nanotextured punch shows no significant change in the amount of adhesioncompared to that in the first punching, indicating that the effect of reducing the amount ofadhesion is sustained. This finding suggests that the forces of adhesion and detachmentare repeatedly applied to the textured part of the 5 µm texture, leading to the wear of thetexture and, consequently, an increase in force because the friction reduction effect cannotMicromachines 2022, 13, 2001 9 of 11be maintained. Figure 11 shows 10,000× SEM images of the nanotextured punch beforeand after extrusion. The observation of the side surface of the nanotextured punch showsthat the texture is not worn away and retains its shape. The 5 µm-textured punch wastextured perpendicular to the direction of punch travel, whereas the nanotexture punch wastextured parallel to the direction of punch travel. The microtextured punches applied a forceperpendicular to the edges of the texture ring, and the nanotextured punches applied a forceparallel to the direction of the punch, which may have further damaged the texture ring.Micromachines 2022, 13, x 9 of 11   However, the nanotextured punch shows no significant change in the amount of adhesion compared to that in the first punching, indicating that the effect of reducing the amount of adhesion is sustained. This finding suggests that the forces of adhesion and detachment are repeatedly applied to the textured part of the 5 µm texture, leading to the wear of the texture and, consequently, an increase in force because the friction reduction effect cannot be maintained. Figure 11 shows 10,000× SEM images of the nanotextured punch before and after extrusion. The observation of the side surface of the nanotextured punch shows that the texture is not worn away and retains its shape. The 5 µm-textured punch was textured perpendicular to the direction of punch travel, whereas the nanotexture punch was textured parallel to the direction of punch travel. The microtextured punches applied a force perpen-dicular to the edges of the texture ring, and the nanotextured punches applied a force par-allel to the direction of the punch, which may have further damaged the texture ring. Considering the lifetime of the texture and durability of the force reduction effect, the nanotextured punch is a tool with superior wear resistance and a longer life surface function than that of the microtextured punch. However, reducing the texture size to the nano-level and lower is limited by laser texturing technology, and advanced microsurface creation technology, such as ion beam texturing, is needed.  Figure 9. Extrusion force vs. number of extrusions.     (a) (b)   (c) (d) Figure 10. Evaluation of adhesion to punch via EPMA for 5 μm-textured punch: (a) 1st and (b) 5th extrusion; nanometer-textured punch: (c) 1st and (d) 5th extrusion. Figure 9. Extrusion force vs. number of extrusions.Micromachines 2022, 13, x 9 of 11   However, the nanotextured punch shows no significant change in the amount of adhesion compared to that in the first punching, indicating that the effect of reducing the amount of adhesion is sustained. This finding suggests that the forces of adhesion and detachment are repeatedly applied to the textured part of the 5 µm texture, leading to the wear of the texture and, consequently, an increase in force because the friction reduction effect cannot be maintained. Figure 11 shows 10,000× SEM images of the nanotextured punch before and after extrusion. The observation of the side surface of the nanotextured punch shows that the texture is not worn away and retains its shape. The 5 µm-textured punch was textured perpendicular to the direction of punch travel, whereas the nanotexture punch was textured parallel to the direction of punch travel. The microtextured punches applied a force perpen-dicular to the edges of the texture ring, and the nanotextured punches applied a force par-allel to the direction of the punch, which may have further damaged the texture ring. Considering the lifetime of the texture and durability of the force reduction effect, the nanotextured punch is a tool with superior wear resistance and a longer life surface function than that of the microtextured punch. However, reducing the texture size to the nano-level and lower is limited by laser texturing technology, and advanced microsurface creation technology, such as ion beam texturing, is needed.  Figure 9. Extrusion force vs. number of extrusions.     (a) (b)   (c) (d) Figure 10. Evaluation of adhesion to punch via EPMA for 5 μm-textured punch: (a) 1st and (b) 5th extrusion; nanometer-textured punch: (c) 1st and (d) 5th extrusion. Figure 10. Evaluation of adhesion to punch via EPMA for 5 µm-textured punch: (a) 1st and (b) 5thextrusion; nanometer-textured punch: (c) 1st and (d) 5th extrusion.Micromachines 2022, 13, x 10 of 11      (a) (b) Figure 11. SEM images of the nanotextured punch before and after extrusion (punch side): (a) as received and (b) after extrusion (5th extrusion). 4. Conclusions 1. The tool surfaces were textured from the millimeter to nanometer scale using electri-cal discharge machining, polishing, and an ultrashort pulsed laser. 2. The extrusion force–stroke diagram for the micro-anteroposterior extrusion process increased the extrusion force gradually with an increase in stroke. The extrusion force was reduced by adding microscale texture to the punch surface. 3. The EPMA evaluation of the punch surface adhesion revealed that the punches with no texture and millimeter-scale texture showed more adhesion to the punch, and the amount of adhesion decreased as the texture size reduced. 4. IPF and KAM maps obtained via EBSD show that micro- and nano-textures on the punch surface improved material flow. A more uniform strain on the product was observed, particularly in the case of nano-textures. 5. Repeated experiments showed that the extrusion force and adhesion to the punch increased with increasing extrusion frequency for the microscale texture. For the nan-otextured punches, the extrusion force decreased with increasing extrusion fre-quency, while adhesion to the punches decreased. Future research will include an investigation into texture direction in nanotextured punches and their application to the preparation of biomaterials based on magnesium and titanium. Author Contributions: Conceptualization and writing—original draft preparation, T.F.; methodol-ogy, T.F. and T.S.; validation, T.F., S.H., and I.W.; writing—review and editing, T.F. and S.H.; su-pervision, K.D. All authors have read and agreed to the published version of the manuscript. Funding: This study received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. References 1. Jain, V.K. Microforming. In Microfacturing Processes, 1st ed.; CRC Press: Boca Raton, FL, USA, 2012; pp. 16–18. https://doi.org/10.1201/b13020. 2. Engel, U.; Eckstein, R. Microforming—From basic research to its realization. J. Mater. Process. Technol. 2002, 125, 35–44. https://doi.org/10.1016/s0924-0136(02)00415-6. 3. Egerer, E.; Engel, U. Process Characterization and Material Flow in Microforming at Elevated Temperatures. J. Manuf. Process. 2004, 6, 1–6. https://doi.org/10.1016/s1526-6125(04)70054-7. Figure 11. SEM images of the nanotextured punch before and after extrusion (punch side): (a) asreceived and (b) after extrusion (5th extrusion).Micromachines 2022, 13, 2001 10 of 11Considering the lifetime of the texture and durability of the force reduction effect,the nanotextured punch is a tool with superior wear resistance and a longer life surfacefunction than that of the microtextured punch. However, reducing the texture size to thenano-level and lower is limited by laser texturing technology, and advanced microsurfacecreation technology, such as ion beam texturing, is needed.4. Conclusions1. The tool surfaces were textured from the millimeter to nanometer scale using electricaldischarge machining, polishing, and an ultrashort pulsed laser.2. The extrusion force–stroke diagram for the micro-anteroposterior extrusion processincreased the extrusion force gradually with an increase in stroke. The extrusion forcewas reduced by adding microscale texture to the punch surface.3. The EPMA evaluation of the punch surface adhesion revealed that the punches withno texture and millimeter-scale texture showed more adhesion to the punch, and theamount of adhesion decreased as the texture size reduced.4. IPF and KAM maps obtained via EBSD show that micro- and nano-textures on thepunch surface improved material flow. A more uniform strain on the product wasobserved, particularly in the case of nano-textures.5. Repeated experiments showed that the extrusion force and adhesion to the punchincreased with increasing extrusion frequency for the microscale texture. For the nan-otextured punches, the extrusion force decreased with increasing extrusion frequency,while adhesion to the punches decreased.Future research will include an investigation into texture direction in nanotextured punchesand their application to the preparation of biomaterials based on magnesium and titanium.Author Contributions: Conceptualization and writing—original draft preparation, T.F.; methodology,T.F. and T.S.; validation, T.F., S.H. and I.W.; writing—review and editing, T.F. and S.H.; supervision,K.D. All authors have read and agreed to the published version of the manuscript.Funding: This study received no external funding.Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: Not applicable.Conflicts of Interest: The authors declare no conflict of interest.References1. Jain, V.K. Microforming. In Microfacturing Processes, 1st ed.; CRC Press: Boca Raton, FL, USA, 2012; pp. 16–18. [CrossRef]2. 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Materials 2022, 15, 1682. [CrossRef] [PubMed]http://doi.org/10.1115/1.2386207http://doi.org/10.1007/s00170-012-4661-7http://doi.org/10.1080/10426914.2019.1689262http://doi.org/10.1016/j.ijmachtools.2011.08.013http://doi.org/10.1016/j.precisioneng.2016.09.005http://doi.org/10.1115/1.4045554http://doi.org/10.9773/sosei.59.8http://doi.org/10.9773/sosei.59.101http://doi.org/10.3390/app10144767http://doi.org/10.3390/mi12111299http://doi.org/10.3390/app10082674http://doi.org/10.3390/ma15051682http://www.ncbi.nlm.nih.gov/pubmed/35268912 Introduction  Materials and Methods  Experimental Results and Discussion  Extrusion Force–Ram Stroke Diagram and Metal Flow during Backward Microextrusion  Evaluation of Adhesion to Punch  Microstructure Analysis of the Extrusion  Comparison of Microtexture and Nanotexture Punches  Conclusions  References