Anti-Galling Cold, Dry Forging of Pure Titanium by Plasma-Carburized AISI420J2 Dies

applied sciences

Article Anti-Galling Cold, Dry Forging of Pure Titanium by Plasma-Carburized AISI420J2 Dies

Tatsuhiko Aizawa 1,* , Tomoaki Yoshino 2, Yohei Suzuki 2 and Tomomi Shiratori 3

1 Surface Engineering Design Laboratory, Shibaura Institute of Technology, Tokyo 144-0045, Japan 2 Komatsu-Seiki Kosakusho, Co., Ltd., Nagano 392-0022, Japan; [email protected] (T.Y.); [email protected] (Y.S.) 3 Faculty of Engineering, University of Toyama, Toyama 930-8555, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-3-6424-8615

Featured Application: Near-net shaping of titanium and titanium alloy workpieces into biomedical parts and devices.

Abstract: A bare AISI420J2 punch often suffers from severe adhesion of metallic titanium as well as titanium oxide debris particles in dry, cold forging of biomedical titanium alloys. This punch was plasma-carburized at 673 K for 14.4 ks to harden it up to 1200 HV on average and to achieve carbon supersaturation in the carburized layer. This plasma-carburized punch was employed in the cold, dry forging of a pure titanium wire into a flat plate while reducing the thickness by 70%. The contact interface width approached the forged workpiece width with increasing the reduction ratio. This smaller bulging deformation reveals that the workpiece is upset by homogeneous plastic flow with a lower friction coefficient. This low-friction and anti-galling forging process was sustained by an in situ solid lubrication mechanism. Unbound free carbon was isolated from the carbon-supersaturated AISI420J2 matrix and deposited as a thin tribofilm to protect the contact interface from mass transfer of metallic titanium. Keywords: low-temperature plasma carburizing; high surface hardness; carbon supersaturation; dry, Citation: Aizawa, T.; Yoshino, T.; cold forging; high reduction in thickness; free carbon isolation; in situ solid lubrication; intermediate Suzuki, Y.; Shiratori, T. Anti-Galling titanium oxide tribofilm formation Cold, Dry Forging of Pure Titanium by Plasma-Carburized AISI420J2 Dies. Appl. Sci. 2021, 11, 595. https:// doi.org/10.3390/app11020595 1. Introduction

Received: 23 November 2020 Pure titanium and titanium alloys have been highlighted as structural biomedical Accepted: 4 January 2021 parts and tools because of their high specific strength and good biocompatibility [1]. Their Published: 9 January 2021 shaping and forging processes were strictly limited to minimum reduction in thickness due to their risk of galling to stainless steel, WC (Co), and monolithic ceramic dies [2]. Die Publisher’s Note: MDPI stays neu- material selection and design is needed to find a way to enable anti-galling manufacturing tral with regard to jurisdictional clai- to shape and forge raw titanium and titanium alloy workpieces into biomedical parts. The ms in published maps and institutio- authors proposed a thick β-SiC-coated SiC die material for anti-galling forging [3]. As nal affiliations. proven by [4], no mass transfer of metallic titanium was seen on the contact interface of a β- SiC-coated punch to a pure titanium workpiece. The debris particles of TiO2 were prevented from depositing onto this coating surface. This anti-galling process was sustained under in situ solid lubrication by the free carbon isolated from the carbon-supersaturated β-SiC Copyright: © 2021 by the authors. Li- coating surface and under the in situ formation of intermediate titanium oxide tribofilms censee MDPI, Basel, Switzerland. on the contact interface. Owing to this galling-free mechanism, a pure titanium wire was This article is an open access article forged under cold and dry conditions with a 70% reduction in thickness [5]. Hence, simple distributed under the terms and conditions of the Creative Commons At- shaping and upsetting can be put into practice in industries by using these β-SiC-coated tribution (CC BY) license (https:// SiC die systems. However, the low toughness of the β-SiC coating becomes a barrier in creativecommons.org/licenses/by/ further applications of this die material system in the near-net shaping and precise forging 4.0/). of raw titanium and titanium alloys into biomedical parts and tools.

Appl. Sci. 2021, 11, 595. https://doi.org/10.3390/app11020595 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 595 2 of 12

Plasma carburizing has developed as one of the most reliable surface treatments to harden tool steel and stainless steel dies [6,7]. After [8], a thick carburized layer with a thickness of 30 µm was formed at 743 K for 72 ks onto austenitic stainless steel substrates. Without the precipitation synthesis of chromium and iron carbides, carbon solute content higher than 2 mass% was attained at the surface of the plasma-carburized layer. This high carbon supersaturation resulted in high hardness, wear toughness, and corrosion resistance. As discussed in [8,9], a fully carbon-supersaturated layer without precipitates was formed by decreasing the holding temperature down to 673 K; however, the carburized layer thickness also decreased down to a few µm. A radio-frequency (RF)–direct current (DC) plasma processing system was developed to build up the experimental setup for low-temperature plasma nitriding and carburizing. The low-temperature nitrogen supersaturation process was performed using this RF–DC plasma processing system [10]. In the case of this RF–DC plasma nitriding with the use of a hollow cathode, a nitrogen-supersaturated layer with a thickness of 60 µm formed at 673 K for 14.4 ks. Its average hardness reached 1400 HV, and its nitrogen solute content became 4 mass%. In the present study, this high-density plasma processing system is utilized to su- persaturate martensitic stainless steel die substrates with carbon. The case-hardened AISI420J2 is employed as a die material, whereas chromium carbide precipitates are used to strengthen the stainless steel matrix. The carburized AISI420J2 punch is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron dispersive X-ray spectroscopy (EDX). For the carbides, the homogeneous carbon mapping by EDX as well as the peak shifts of α’-iron by XRD prove that the AISI420J2 punch is carbon-supersaturated. This punch is ﬁxed into the upper die set for cold, dry forging of pure titanium wires. The power–stroke relationship is monitored in situ to describe this cold, dry forging behavior. The smaller bulging deformation of the forged wires proves that the titanium workpiece is homogeneously upset with low friction. The contact interface between the carburized punch and the titanium workpiece is precisely analyzed by SEM-EDX as well as Raman spectroscopy to describe the free carbon isolation from the carbon-supersaturated matrix of the punch and to investigate the role of isolated carbon as a thin triboﬁlm for anti-galling forging with low friction.

2. Experimental Procedure A low-temperature plasma carburizing system is stated with comments on the hollow cathode device to intensify the carbon ion density as well as the CH radical density. A computer numerical control (CNC) stamper is employed for the dry, cold forging experiment while monitoring the power–stroke relationship. The contact interface between the carburized layer and the titanium workpiece is precisely analyzed by SEM-EDX and Raman spectroscopy.

2.1. Plasma Carburizing System An RF–DC plasma processing system (YS-Electric Industry, Co., Ltd., Yamanashi, Japan) was utilized for plasma carburizing. The plasma carburizing setup was placed onto the DC-biased table, as illustrated in Figure1a. After the diagnosis of the ignited argon plasma in the inside of the hollow [11], the ion and radical densities were increased up to 3 × 1018 ions/m3 in the present setup. Figure1b depicts the ignited plasma under the mixture gas flow. The AISI420J2 punch substrate was covered with a thick plasma sheath in a highly bright condition. In the practical operation, the chamber was evacuated to 0.01 Pa after placing the punch into the hollow cathode. Under the argon flow, the experimental setup was heated up to the holding temperature of 673 K. The specimen was first presputtered by a DC bias of 500 V at 70 Pa under the mixture gas with an argon flow of 160 mL/min and a hydrogen flow of 20 mL/min for 1.8 ks. After presputtering, it was further carburized at 673 K for 14.4 ks under the mixture of argon, hydrogen, and methane Appl.Appl. Sci. Sci. 2021 2021, 11, 11, 595, 595 3 3of of 12 12 Appl. Sci. 2021, 11, 595 3 of 12

furtherfurther carburized carburized at at 673 673 K K for for 14.4 14.4 ks ks under under the the mixture mixture of of argon, argon, hydrogen, hydrogen, and and methane methane (CH(CH(CH44).).4 ).The TheThe flow ﬂow flow rate rate rate of of of CH CH CH44 gas4gas gas was was was 20 20 20 mL/min. mL/min. mL/min. After AfterAfter carbur carburizing,carburizing,izing, the thethe systemsystem system was was was vented vented vented downdowndown to to to 2 2 2kPa kPa kPa in in in nitrogen nitrogen nitrogen and and and the the the specimen specimen specimen was was was cooled cooled cooled in in in a a achamber. chamber. chamber.

FigureFigureFigure 1. 1. 1. High-densityHigh-density High-density plasma plasma plasma carburizing carburizing carburizing system system system with with with the the the use use use of of of a a hollowa hollow hollow cathode. cathode. cathode. ( (a a()a) Schematic) Schematic Schematic view view view of of of the the the experimental experimental experimental setup, and (b) an image of the plasma carburizing system. setup,setup, and and ( (bb)) an an image image of of the the plasma plasma carburizing carburizing system. system.

2.2.2.2. CNC CNC Stamping Stamping System System for for Cold, Cold, Dry Dry Forging Forging Experiments Experiments 2.2. CNCAA computer computer Stamping numerical numerical System for control Cold,control Dry (CNC) (CNC) Forging stamper stamper Experiments (Hoden (Hoden Precision, Precision, Co., Co., Ltd., Ltd., Kana- Kana- gawa,gawa,A Japan) computerJapan) was was numericalutilized utilized to controlto bring bring the (CNC) the upsetti upsetti stamperngng process process (Hoden down down Precision, to to the the Co., specified specified Ltd., Kanagawa, stroke stroke in in reduction,Japan)reduction, was as utilizedas shown shown to in bringin Figure Figure the 2a. upsetting2a. In In this this processstamper, stamper, down four four tounit unit the motors specifiedmotors work work stroke independently independently in reduction, toasto compensate shown compensate in Figure for for eccentric2 a.eccentric In this loading. stamper,loading. The fourThe to unittorquerque motors of of each each work motor motor independently was was summed summed to up compensate up to to calcu- calcu- lateforlate eccentricthe the applied applied loading. power power The and and torque to to monitor monitor of each the motorthe power–stroke power–stroke was summed relationship. relationship. up to calculate The The thecarburized carburized applied AISI420J2powerAISI420J2 and punch topunch monitor was was inserted the inserted power–stroke into into the the upper relationship.upper cass cassetteette The die, die, carburized as as depicted depicted AISI420J2 in in Figure Figure punch 2b. 2b. Both wasBoth theinsertedthe upper upper into and and the the the upper lower lower cassette cassette cassette die, dies dies as were depictedwere fix fixeded in into Figureinto each each2b. bolster bolster Both theof of the upper the CNC CNC and stamper. stamper. the lower In In practicalcassettepractical diesoperation, operation, were fixedthe the stroke intostroke each was was bolsterlowered lowered of at theat a aconstant CNCconstant stamper. velocity velocity In of practicalof 0.1 0.1 mm/s mm/s operation, down down to to the the the specifiedstrokespecified was minimum minimum lowered stroke atstroke a constant (δ (mδ),m ),held held velocity for for 1 1s, s, ofand and 0.1 then mm/sthen moved moved down up up to to to thethe the specifiedoriginal original position minimumposition at at thestrokethe same same (δ mvelocity. ),velocity. held for This This 1 s,δ m andδ mwas was then varied varied moved for for each up each to reduction thereduction original in in the position the thickness thickness at the of of same the the titanium velocity.titanium wires.Thiswires.δ m was varied for each reduction in the thickness of the titanium wires.

Figure 2. The computer numerical control (CNC) stamping system. (a) The CNC stamper with in FigureFigure 2. 2. TheThe computer computer numerical numerical control control (CNC) (CNC) stamping stamping system. system. (a) The (a) TheCNC CNC stamper stamper with within situsitu monitoring monitoring of of the the power–stroke power–stroke relationship, relationship, and and (b ()b the) the upper upper cassette cassette die die set set with with the the in situ monitoring of the power–stroke relationship, and (b) the upper cassette die set with the plasma-carburizedplasma-carburized punch. punch. plasma-carburized punch.

2.3. Contact Interface Analysis The contact interface between the carburized AISI420J2 punch and the titanium workpiece was analyzed to describe the in situ lubrication mechanism. Optical microscopy Appl. Sci. 2021, 11, 595 4 of 12

(Shimazu, Co., Ltd., Kyoto, Japan) was ﬁrst employed to understand the morphology of the contact interface on the punch. Several positions were selected to make pointwise SEM–EDX (JOEL, Tokyo, Japan) analyses. In particular, three regions were selected in the contact interface to describe the difference in the in situ solid lubrication. Raman spectroscopy (Nihon-Kogaku, Co., Ltd., Tokyo, Japan) was utilized to analyze the binding state of iron, chromium, titanium, oxygen, and carbon on the interface.

2.4. Die and Work Materials The AISI420J2 substrate was employed as a die with 24 mm × 14 mm × 5 mm for plasma carburizing. Its chemical composition was as follows: 12.1 mass% chromium, 0.4 mass% carbon, 0.5 mass% silicon, 0.4 mass% manganese, 0.3 mass% nickel, 0.2 mass% potassium and sulfur, and the balance, iron. Its primary temperature was 1223 K (950 ◦C), and its tempering temperature was 973 K (700 ◦C). Pure titanium with the industrial grade of T328H was employed as a work material for the forging experiment. Its chemical composition consisted of 0.001 mass% hydrogen, 0.1 mass% oxygen, 0.007 mass% nitrogen, 0.04 mass% iron, 0.007 mass% carbon, and titanium in balance.

3. Experimental Results The plasma-carburized AISI420J2 punch was characterized by XRD and SEM–EDX to analyze the carbon-supersaturated iron–chromium matrix and its morphology. The cold, dry forging experiment was performed with the use of this punch to reduce thickness by 70%. The variation in the forged titanium wire shape with increasing this reduction ratio was measured, together with in situ monitoring of the power–stroke relationship. Contact interface analyses were made to describe the in situ solid lubrication mechanism by SEM–EDX and Raman spectroscopy.

3.1. Plasma Carburizing of AISI420J2 Dies XRD analysis was employed to describe the carbon supersaturation into the martensitic stainless steel matrix at 673 K. After the ﬁrst-principle calculation of this carbon supersaturation [12], the Fe–Cr lattice expands itself by the carbon solute atom occupation of its octahedral vacancy site. Similar to nitrogen supersaturation [13,14], this lattice expansion is detected by the peak shift of the original α’-lattice to the lower 2θ direction. Figure3 depicts the narrow-scanned XRD diagram in the range of 43◦ < 2θ < 46◦. The original peak of α’ (110) at 2θ = 44.5◦ shifts to two peaks at 2θ = 43.6◦ and 43.7◦, respectively. As reported for the nitrogen supersaturation of AISI420J2 specimens at 673 K, these peaks correspond to the formation of carbon-supersaturated austenitic and martensitic phases (γC and α’C) of Appl. Sci. 2021, 11, 595 5 of 12 AISI420J2, respectively. The SEM–EDX image of the cross section of the carburized punch reveals that the carburized layer thickness at 673 K for 14.4 ks reaches 50 µm.

1800 α’ (110) 1600 1400 γ C 1200

1000 α ’C 800 Intensity /a.u. Intensity 600

400

200

0 43 43.5 44 44.5 45 45.5 46 2θ /degree

Figure 3. XRD diagram of the surface of the plasma-carburized AISI420J2 punch.

Figure 4 depicts the SEM–EDX image of the surface of the plasma-carburized AISI420J2 punch with its element mapping. The case-hardened AISI420J2 punch substrate consists of an iron–chromium binary alloy matrix and chromium carbides. For these precipitates, little carbon agglomerate was detected on the binary alloy punch surface. That is, no free or unbound carbon dots were present on the carburized iron–chromium binary alloy matrix surface.

Figure 4. SEM–EDX analysis of the plasma-carburized AISI420J2 punch at 673 K for 14.4 ks. (a) SEM image of the plasma- carburized AISI420J2 punch surface, and (b) its element mapping.

3.2. Upsetting of Titanium Wire into Thin Flat Plate A titanium wire with a diameter of 1.0 mm and length of 10 mm was employed as a workpiece for cold, dry forging. The stroke (δm) was first controlled to move down from the initial position, where the workpiece contacted the punch surface. Then, it was further moved down stepwise to the specified depth; i.e., δm = 0.1 mm for r = 10% and δm = 0.7 mm for r = 70%. Figure 5a depicts the variation in the cross section for the forged titanium wire with increasing reduction ratio (r). A circular wire was shaped into a flat plate with a thickness of 0.3 mm at r = 70% without significant bulging deformation.

Appl. Sci. 2021, 11, 595 5 of 12

1800 α’ (110) 1600 1400 γ C 1200

1000 α ’C 800 Intensity /a.u. Intensity 600

400

200

0 43 43.5 44 44.5 45 45.5 46 2θ /degree Appl. Sci. 2021, 11, 595 5 of 12 Figure 3. XRD diagram of the surface of the plasma-carburized AISI420J2 punch.

FigureFigure4 depicts4 depicts the SEM–EDXthe SEM–EDX image image of the surfaceof the ofsurface the plasma-carburized of the plasma-carburized AISI420J2 punchAISI420J2 with punch its element with its mapping. element The mapping. case-hardened The case-hardened AISI420J2 punch AISI420J2 substrate punch consists substrate of anconsists iron–chromium of an iron–chromium binary alloy matrixbinary andalloy chromium matrix and carbides. chromium For thesecarbides. precipitates, For these little pre- carboncipitates, agglomerate little carbon was agglomerate detected on was the detect binaryed alloyon the punch binary surface. alloy punch That is,surface. no free That or unboundis, no free carbon or unbound dots were carbon present dots onwere the present carburized on the iron–chromium carburized iron–chromium binary alloy matrix binary surface.alloy matrix surface.

FigureFigure 4. 4. SEM–EDX analysis analysis of of the the plasma-carburize plasma-carburizedd AISI420J2 AISI420J2 punch punch at 673 at 673K for K 14.4 for 14.4ks. (a ks.) SEM (a) image SEM imageof the plasma- of the plasma-carburizedcarburized AISI420J2 AISI420J2 punch punchsurface, surface, and (b) and its element (b) its element mapping. mapping.

3.2. Upsetting of Titanium Wire into Thin Flat Plate 3.2. Upsetting of Titanium Wire into Thin Flat Plate A titanium wire with a diameter of 1.0 mm and length of 10 mm was employed as a A titanium wire with a diameter of 1.0 mm and length of 10 mm was employed as a workpiece for cold, dry forging. The stroke (δm) was first controlled to move down from workpiecethe initial position, for cold, drywhere forging. the workpiece The stroke contac (δm)ted was the first punch controlled surface. to Then, move it down was further from the initial position, where the workpiece contacted the punch surface. Then, it was further moved down stepwise to the specified depth; i.e., δm = 0.1 mm for r = 10% and δm = 0.7 mm δ r δ movedfor r = 70%. down Figure stepwise 5a depicts to the specified the variation depth; in i.e., the crossm = 0.1 section mm for for the= 10% forged and titaniumm = 0.7 mmwire for r = 70%. Figure5a depicts the variation in the cross section for the forged titanium with increasing reduction ratio (r). A circular wire was shaped into a flat plate with a wire with increasing reduction ratio (r). A circular wire was shaped into a flat plate with a thickness of 0.3 mm at r = 70% without significant bulging deformation. thickness of 0.3 mm at r = 70% without significant bulging deformation. Up to r = 30%, the wire was compressed in the axial direction. When r > 30%, its plastic flow became homogeneous in the lateral direction. As shown in Figure5b, the width of the forged wire (Wo) was larger than the width of the contact interface (Wc) for r < 20%. Wc approaches Wo with increasing r. This proves that the titanium workpiece flows in the lateral direction with homogeneous plastic flow velocity along the contact interface. The surface extension rate reached 170% of the original wire surface. Figure6 shows the variation in these Wo and Wc as well as the bulging width (Bg = (Wo-Wc)/2) with increasing r. A monotonous increase in Wo and Wc with r implies that a circular wire widens uniformly with upsetting. In particular, a linear increase in Wc with r reveals that the reduced volume of the wire by compression is compensated by its widening volume in the lateral direction. As shown in the monotonic approach of Bg to zero with increasing r, the bulging deformation of the wire diminishes with r. Af- ter [15], this nearly zero bulging deformation for r > 30% proves that the friction coefficient approaches less than 0.1. Appl.Appl. Sci.Sci. 20212021,, 1111,, 595595 66 of of 1212

FigureFigure 5.5. VariationVariation inin thethe cross cross section section and and plane plane views views of of cold cold dry dry forged forged pure pure titanium titanium wire wire with with increasing increasing reduction reduction in thicknessin thickness (r). (r (a).) ( Variationa) Variation in thein the titanium titanium workpiece workpiece cross cross sections sections with with increasing increasingr, and r, and (b) ( variationb) variation in thein the plane plane view view of Appl. Sci. 2021, 11, 595 7 of 12 theof the forged forged wire wire with with increasing increasingr. r.

Up to r = 30%, the wire was compressed in the axial direction. When r > 30%, its plastic2.5 flow became homogeneous in the lateral direction. As shown in Figure 5b, the width of the forged Wowire (Wo) was larger than the width of the contact interface (Wc) for r < 20%.2 Wc approachesWc Wo with increasing r. This proves that the titanium workpiece flows in the lateral direction with homogeneous plastic flow velocity along the contact interface. The surfaceBg extension rate reached 170% of the original wire surface. 1.5 Figure 6 shows the variation in these Wo and Wc as well as the bulging width (Bg = (Wo-Wc)/2) with increasing r. A monotonous increase in Wo and Wc with r implies that a circular1 wire widens uniformly with upsetting. In particular, a linear increase in Wc with

r reveals Wc, BgWo, /mm that the reduced volume of the wire by compression is compensated by its wid- ening0.5 volume in the lateral direction. As shown in the monotonic approach of Bg to zero with increasing r, the bulging deformation of the wire diminishes with r. After [15], this

nearly0 zero bulging deformation for r > 30% proves that the friction coefficient approaches less than0 0.1. 20406080 Reduction of thickness, r/%

FigureFigure 6. 6. SurfaceSurface area area extension extension with with increasing increasing reduction reduction rate, rr..

FigureFigure 7 depicts the the relationship relationship between between the the applied applied power power (P () andP) and the thestroke stroke (δ) up (δ) toup the to thespecified speciﬁed reduction reduction rate rate for for r =r =10%, 10%, 20%, 20%, 30%, 30%, 50%, 50%, and and 70%, 70%, respectively. respectively. The measured P–δ diagrams for each r superpose on a single master curve; the power increases measured P–δ diagrams for each r superpose on a single master curve; the power increases signiﬁcantly for r > 30% due to the work hardening of pure titanium. significantly for r > 30% due to the work hardening of pure titanium.

9.00

8.00 10%

7.00 20% 30% 6.00 50% 5.00 70% 4.00 Power, P/kW Power, 3.00

2.00

1.00

0.00 0.000 0.200 0.400 0.600 0.800 1.000 Stroke, δ /mm

Figure 7. Relationship of the applied power to the stroke for each reduction in thickness.

3.3. Contact Interface Analysis The cold, dry forging experiments up to r = 70% were continuously performed 10 times to leave a distinct contact interface onto the carburized AISI420J2 punch surface. Figure 8 depicts a microscopic image of the contact interface of the carburized AISI420J2 and the forged titanium workpiece. Many stripes were formed on this interface radially, from its centerline to its ends, together with the lateral plastic flow of the work material. Three positions were selected for SEM–EDX analyses between the centerline of the interface and its end. Region A was located near the end of the interface, region B was in its middle, and region C was near the centerline.

Appl. Sci. 2021, 11, 595 7 of 12

2.5 Wo

2 Wc Bg 1.5

1 Wo, Wc, Wc, BgWo, /mm 0.5

0 0 20406080 Reduction of thickness, r/%

Figure 6. Surface area extension with increasing reduction rate, r.

Figure 7 depicts the relationship between the applied power (P) and the stroke (δ) up to the specified reduction rate for r = 10%, 20%, 30%, 50%, and 70%, respectively. The Appl. Sci. 2021 11 , , 595 measured P–δ diagrams for each r superpose on a single master curve; the power increases7 of 12 significantly for r > 30% due to the work hardening of pure titanium.

9.00

8.00 10%

7.00 20% 30% 6.00 50% 5.00 70% 4.00 Power, P/kW Power, 3.00

2.00

1.00

0.00 0.000 0.200 0.400 0.600 0.800 1.000 Stroke, δ /mm

FigureFigure 7. 7. RelationshipRelationship of of the the applied applied power power to to the the stroke stroke for for each each reduction reduction in in thickness. thickness. 3.3. Contact Interface Analysis 3.3. Contact Interface Analysis The cold, dry forging experiments up to r = 70% were continuously performed 10 times to leaveThe acold, distinct dry contactforging interface experiments onto theup carburizedto r = 70% AISI420J2were continuously punch surface. performed 10 timesFigure to leave8 depictsa distinct a contact microscopic interface image onto of the the carburized contact interface AISI420J2 of punch the carburized surface. AISI420J2Figure and 8 depicts the forged a microscopic titanium workpiece. image of Many the contact stripes wereinterface formed of onthe this carburized interface AISI420J2radially, from and the its centerlineforged titanium to its ends,workpiece. together Many with stripes the lateral were formed plastic ﬂowon this of theinterface work radially,material. from Three its positions centerline were to its selected ends, together for SEM–EDX with the analyses lateral betweenplastic flow the centerlineof the work of Appl. Sci. 2021, 11, 595 material.the interface Three and positions its end. Regionwere selected A was for located SEM–EDX near the analyses end of thebetween interface, the regioncenterline B8 wasof of 12

thein itsinterface middle, and and its region end. Region C was nearA was the located centerline. near the end of the interface, region B was in its middle, and region C was near the centerline.

FigureFigure 8. 8.Optical Optical microscopic microscopic observation observation of of the the contact contact interface interface of theof the carburized carburized AISI420J2 AISI420J2 surface sur- andface the and forged the forged titanium titanium workpiece. workpiece.

FigureFigure9 depicts9 depicts the the SEM SEM images images of of region region A A and and the the element element mapping mapping of of oxygen, oxygen, carbon,carbon, titanium, titanium, chromium,chromium, and iron, iron, respective respectively.ly. For For the the original original precipitate precipitate of (Cr of (Cr and andFe) Fe)oxides oxides and and carbides, carbides, the chromium the chromium and andironiron uniformly uniformly distribute distribute on the on contact the contact inter- interface.face. Carbon, Carbon, titanium, titanium, and and oxygen oxygen distribute distribute in stripes in stripes radially radially along along the the plastic plastic flow ﬂow di- directionrection of of the the titanium titanium workpiece. workpiece. Since Since little little carbon carbon is isdetected, detected, even even in intrace trace levels, levels, on onthe the carburized carburized AISI420J2 AISI420J2 surface surface before before forgin forging,g, these these carbon carbon stripes stripes come come from from the thecar- carbonbon solutes solutes isolated isolated from from the the AISI420J2 AISI420J2 matrix matrix.. Both the titaniumtitanium andand oxygenoxygen stripes stripes are are exclusively present at the positions without carbon content. This implies that the carbon stripes protect the punch surface from adhesion of titanium.

Figure 9. SEM–EDX image and element mapping of region A in the contact interface of the carburized AISI420J2 punch and the titanium workpiece.

Figure 10 also depicts the SEM image of region B with the element mapping in the middle of the contact interface. No change in the chromium and iron maps was detected between regions A and B. The carbon stripes broaden, while the titanium and oxygen stripes become thinner compared with those in Figure 9. The exclusive presence of carbon stripes on titanium and oxygen ones becomes more distinct than those seen in Figure 9.

Appl. Sci. 2021, 11, 595 8 of 12

Figure 8. Optical microscopic observation of the contact interface of the carburized AISI420J2 surface and the forged titanium workpiece.

Figure 9 depicts the SEM images of region A and the element mapping of oxygen, carbon, titanium, chromium, and iron, respectively. For the original precipitate of (Cr and Fe) oxides and carbides, the chromium and iron uniformly distribute on the contact inter- Appl. Sci. 2021, 11, 595face. Carbon, titanium, and oxygen distribute in stripes radially along the plastic flow di- 8 of 12 rection of the titanium workpiece. Since little carbon is detected, even in trace levels, on the carburized AISI420J2 surface before forging, these carbon stripes come from the carbon solutes isolated from the AISI420J2 matrix. Both the titanium and oxygen stripes are exclusively presentexclusively at the present positions at the without positions carbon without content. carbon This content. implies Thisthat impliesthe carbon that the carbon stripes protectstripes the punch protect surfac the punche from surfaceadhesion from of titanium. adhesion of titanium.

Figure 9. SEM–EDXFigure image9. SEM–EDX and element image mapping and element of region mapping A in theof region contact A interface in the contact of the carburizedinterface ofAISI420J2 the carbu- punch and the titanium workpiece.rized AISI420J2 punch and the titanium workpiece.

Figure 10 alsoFigure depicts 10 thealso SEM depicts image the of SEM region image B with of region the element B with themapping element in mappingthe in the middle of themiddle contact of interface. the contact No interface. change in No the change chromium in the and chromium iron maps and was iron detected maps was detected between regionsbetween A and regions B. The A carbon and B. stripe The carbons broaden, stripes while broaden, the titanium while theand titanium oxygen and oxygen Appl. Sci. 2021, 11, 595 9 of 12 stripes becomestripes thinner become compared thinner with compared those in with Figure those 9. The in Figure exclusive9. The presence exclusive of carbon presence of carbon stripes on titaniumstripes and on titanium oxygen ones and oxygenbecomes ones more becomes distinct more than distinctthose seen than in those Figure seen 9. in Figure9.

Figure 10. SEM–EDXFigure 10. image SEM–EDX and element image mappingand element of region mapping B in of the region contact B in interface the contact of the interface carburized of the AISI420J2 carbu- punch and the titaniumrized workpiece. AISI420J2 punch and the titanium workpiece.

Figure 11 showsFigure the 11 SEM shows and the element SEM andmappi elementng of mappingregion C. ofA regionfew titanium C. A few and titanium and oxygen stripesoxygen are seen stripes at the are centerline seen at the of centerline the contact of theinterface. contact Most interface. of region Most C of is region cov- C is covered ered with unboundwith unbound carbon carbonfilms. Iron ﬁlms. and Iron chromium and chromium maps mapsare the are same the same as those as those seen seen in in Figure4; Figure 4; the thepunch punch surface surface at the at the center center in incontact contact is isfree free from from adhesion adhesion of of titanium. titanium. This This proves that proves that thethe punch punch surface surface at at the initial contactcontact withwith thethe titanium titanium workpiece workpiece with with high high static pressure during dry forging is free from galling because of in situ carbon lubrication. Figures9–11 static pressure during dry forging is free from galling because of in situ carbon lubrication. reveal that the interface area overlapped by the carbon ﬁlms gradually decreases from the Figures 9 to 11 reveal that the interface area overlapped by the carbon films gradually decreases from the centerline of the interface to its end. This suggests that formation of free carbon stripes and films on the interface is driven by the applied stress during dry forging.

Figure 11. SEM–EDX image and element mapping of region C on the contact interface of the carburized AISI420J2 punch and the titanium workpiece.

3.4. Raman Spectroscopy The binding state of the tribofilm on the contact interface was analyzed by Raman spectroscopy. Figure 12a shows the Raman spectrum at the center of region B. Three broad peaks around Λ = 220 cm−1, 400 cm−1, and 600 cm−1 are identified by TiOx, or intermediate titanium oxides, including TiO2, after [16]. This proves that the titanium and oxygen stripes coincident with each other consist of TiOx and TiO2. Appl. Sci. 2021, 11, 595 9 of 12

Figure 10. SEM–EDX image and element mapping of region B in the contact interface of the carburized AISI420J2 punch and the titanium workpiece.

Figure 11 shows the SEM and element mapping of region C. A few titanium and oxygen stripes are seen at the centerline of the contact interface. Most of region C is covered with unbound carbon films. Iron and chromium maps are the same as those seen in Figure 4; the punch surface at the center in contact is free from adhesion of titanium. This Appl. Sci. 2021, 11, 595proves that the punch surface at the initial contact with the titanium workpiece with high 9 of 12 static pressure during dry forging is free from galling because of in situ carbon lubrication. Figures 9 to 11 reveal that the interface area overlapped by the carbon films gradually decreases fromcenterline the centerline of the interface of the interface to its end. to This its end. suggests This thatsuggests formation that formation of free carbon of stripes and free carbon stripesﬁlms on and the films interface on the is driveninterfac bye is the driven applied by stress the applied during stress dry forging. during dry forging.

Figure 11. SEM–EDXFigure 11. image SEM–EDX and element image mapping and element of region mapping C on of the region contact C on interface the contact of the interface carburized of the AISI420J2 car- punch and the titaniumburized workpiece. AISI420J2 punch and the titanium workpiece.

3.4. Raman Spectroscopy 3.4. Raman Spectroscopy The binding state of the tribofilm on the contact interface was analyzed by Raman The binding state of the triboﬁlm on the contact interface was analyzed by Raman spectroscopy. Figure 12a shows the Raman spectrum at the center of region B. Three broad spectroscopy. Figure 12a shows the Raman spectrum at the center of region B. Three peaks around Λ = 220 cm−1, 400 cm−1, and 600 cm−1 −1 are identified−1 by TiOx−, or1 intermediate broad peaks around Λ = 220 cm , 400 cm , and 600 cm are identiﬁed by TiOx, or Appl. Sci. 2021, 11, 595titanium oxides, including TiO2, after [16]. This proves that the titanium and oxygen 10 of 12 intermediate titanium oxides, including TiO2, after [16]. This proves that the titanium and stripes coincident with each other consist of TiOx and TiO2. oxygen stripes coincident with each other consist of TiOx and TiO2.

240 260 Upper in region-C TiOx 250 230 240 Center in region-C 220 Unbound C 230 Lower in region-C 210 220 200 210 Intensity/a.u. Intensity /a.u. 190 200 190 180 180 170 170

160 160 200 400 600 800 1000 1200 1400 1600 1800 200 400 600 800 1000 1200 1400 1600 1800 Raman shift, Λ/ cm-1 Raman shift, Λ/cm-1 (a) (b)

Figure 12. RamanRaman spectroscopic spectroscopic analyses of the the titanium-rich titanium-rich an andd the carbon-rich zones in the contact interface. ( aa)) Raman Raman spectraspectra of region B, and ( b) Raman spectra of region C (three positionspositions areare selectedselected forfor measurement).measurement).

IfIf TiO TiO22 werewere only only present present as as titanium titanium oxide, oxide, the the narrow narrow peaks peaks would be detected at −1 −1 −1 ΛΛ == 400 400 cm cm−1 andand 600 600 cm cm−1. A. A broader broader peak peak around around 200 200 cm −1 suggestssuggests that that intermediate intermediate titanium oxides are formed together with TiO debris particles in the titanium and oxygen titanium oxides are formed together with TiO2 debris particles in the titanium and oxygen stripesstripes in in Figure Figure 10.10 .As As analyzed analyzed in in the the literature literature [17,18], [17,18 ],two two broad broad D- D- and and G-peaks G-peaks at atΛ −1 −1 =Λ 1320= 1320 cm cm−1 andand 1600 1600 cm− cm1 in Figurein Figure 12a 12provea prove that that the thecarbon carbon detected detected in Figure in Figure 10 10is unboundis unbound from from titanium, titanium, chromium, chromium, and and iron. iron. The The free free carbon carbon stripes stripes in in Figure Figure 1010 are formedformed by by the the carbon carbon isolation isolation from from the the ca carburizedrburized AISI420J2 matrix of the punch. Figure 12b depicts the Raman spectra of region C at the center of the contact interface. Three positions were selected for analysis in this region. In these three spectra, only two broad peaks are detected at Λ = 1320 cm−1 and 1600 cm−1, respectively. In correspondence to Figure 11, most of the punch surface at the center of the interface is covered with the free carbon film isolated from the carburized punch matrix.

Figure 12b depicts the Raman spectra of region C at the center of the contact interface. Three positions were selected for analysis in this region. In these three spectra, only two broad peaks are detected at Λ = 1320 cm−1 and 1600 cm−1, respectively. In correspondence to Figure 11, most of the punch surface at the center of the interface is covered with the free carbon ﬁlm isolated from the carburized punch matrix.

4. Discussion The formation of a relatively thick carburized layer at 673 K for 14.4 ks is due to the carbon solute diffusion process and carbon supersaturation. After [10,12–14], the carbon and nitrogen supersaturation processes are common to plasma carburizing and nitriding treatments at 673 K. This supersaturation and diffusion of carbon and nitrogen solutes into stainless steel is recognized by the iron lattice expansion and high solute content depth profile in the carburized and nitrided layers. In addition, the grain refinement takes place with this carbon supersaturation process to sustain the carbon solute diffusion through the fine zone and grain boundaries in a manner similar to the nitrogen supersaturation in [10,14]. A solid material with a two-dimensional structure has been identified as a typical solid lubricant; e.g., MoS2 particles and coatings [19] and graphite [20] are frequently employed in industries to reduce the friction and galling in hot forming. In particular, a graphite mold provides a solution for the mold stamping of oxide glasses to optical lenses [21]; the lens surfaces are often contaminated by the dust of graphite. By contrast, DLC-coated dies often suffer from the adhesion of those glasses as well as metallic titanium, as reported in [22]. As analyzed in Figures8–12, the unbound, free carbon film formed in situ during cold forging works as a solid lubricant to protect the die surface from the galling of titanium. Since this solid lubrication film is mainly formed only at the high-pressure contact interface, there is little possibility of its contaminating other work surfaces not only in cold forging but also in warm and hot forging. Isolation of unbound, free carbon solutes from the highly stressed AISI420J2 matrix plays an essential role in the formation of the carbon tribofilm for in situ solid lubrication. Many studies have reported the ordering of carbon atoms in iron and steel under the supersaturated state only in the presence of inner strains [23]. As suggested in [24], the externally applied compressive stress has an influence on carbon–carbon and carbon–iron interactions and induces the separation of supersaturating carbon solutes to ordered carbon and free carbon in stressed steel. This free carbon solute diffuses to the surface by a steep stress gradient and in situ forms a thin film on the contact interface during forging. An intermediate titanium oxide (TiOx) film is also formed in situ together with the in situ solid lubrication by the carbon film. A few TiOx stripes were formed at the center of the contact interface under high compressive stress during forging. With increasing reduction in thickness, the applied stress gradually lowers and the carbon isolation process weakens, so that the TiOx film is formed on the contact interface away from its center line. The plasma carburizing conditions have an influence on this carbon supersaturation into the AISI420J2 matrix. In previous studies on low-temperature DC plasma carburizing [9,10], no iron and chromium carbides were synthesized as a precipitate into stainless steel matrices. However, no statements were found in those papers on the plasma diagnosis of the activated species in plasmas. As suggested in [11], the ratio of CH4 to H2 in the mixture gas of argon, CH4, and H2 has an influence on the CH radical density as well as the ion density of C* {C+,C2+,C3+, and C4+} in the ignited plasmas for low-temperature plasma carburizing. Further study is needed to optimize this ratio for high densification of CH radicals to drive the plasma carburizing process. The plasma-carburized AISI420J2 at 673 K has anti-galling capacity in forging titanium and titanium alloys, enough to be applied to the net shaping of raw titanium workpieces into biomedical parts. Different from β-SiC-coated dies [8,9], this die has sufficient toughness and hardness against local stress concentration as well as eccentric loading during Appl. Sci. 2021, 11, 595 11 of 12

complex shaping. As noted in [25], this die material selection enables the realization of highly precise shaping with dimensional accuracy from titanium wires and bars.

5. Conclusions The plasma-carburized martensitic stainless steel AISI420J2 provides an alternative die selection to put the anti-galling dry, cold forging of titanium and titanium alloy workpieces into practice. The unbound, free carbon solute isolates from this carbon-supersaturated AISI420J2 matrix and deposits on the highly stressed contact interface. This in situ formed free carbon agglomerate works as a solid lubricant to protect the die surface from the mass transfer of metallic titanium and to reduce friction and wear. Owing to this in situ solid lubrication, the titanium wire with a diameter of 1.0 mm is upset and shaped into a thin plate with a thickness of 0.3 mm. The nearly zero bulging deformation at a high reduction ratio of thickness proves that a titanium workpiece makes plastic ﬂow homogeneously along the contact interface with a low friction coefﬁcient of 0.1 or less. The plasma-carburized AISI420J2 die has a high hardness of 1200 HV at the surface and preserves its original ductility and toughness, making it suitable to be used for near- net forging and shaping with local stress transients and eccentric loading in practical production lines. This mechanical property with anti-galling capacity is preferable to the precise metal forming of titanium wires and bars into biomedical parts.

Author Contributions: Conceptualization, T.A. and T.S.; methodology, T.A. and T.Y.; validation, T.A., T.Y., and Y.S.; investigation, T.A., T.Y., Y.S., and T.S.; data curation, Y.S.; writing—original draft preparation, T.A.; writing—review and editing, T.Y., Y.S., and T.S.; project administration, Y.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to express their gratitude to S. Kurozumi (Nano-Film Cota, LLC) and S. Ishiguro (Graduate School of Engineering, University of Toyama) for their help in the experiments. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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