High-Temperature Diffusion Bonding of Ti–6Al–4V and Super-Duplex

Journal of Manufacturing and Materials Processing

Article High-Temperature Diﬀusion Bonding of Ti–6Al–4V and Super-Duplex Stainless Steel Using a Cu Interlayer Embedded with Alumina Nanoparticles

Kavian O. Cooke 1,* , Anthony Richardson 1, Tahir I. Khan 1 and Muhammad Ali Shar 1,2 1 Faculty of Engineering and Informatics, University of Bradford, Richmond Road, Bradford BD7 1DP, UK; anthony.richardson@cranﬁeld.ac.uk (A.R.); [email protected] (T.I.K.); [email protected] (M.A.S.) 2 King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia * Correspondence: [email protected]

Received: 21 December 2019; Accepted: 9 January 2020; Published: 12 January 2020

Abstract: In this study, Ti–6Al–4V alloy was diffusion bonded to super-duplex stainless steel (SDSS) using an electrodeposited Cu interlayer containing alumina nanoparticles to determine the effects of bonding parameters on the microstructural evolution within the joint region. The results of the study showed that the homogeneity of the joint is affected by the bonding time and bonding temperature. The results also showed that when a Cu/Al2O3 interlayer is used, Ti–6Al–4V alloy can be successfully diffusion bonded to SDSS at temperatures above 850 ◦C. The combination of longer bonding time and high bonding temperature leads to the formation of various intermetallic compounds within the interface. However, the presence of the Al2O3 nanoparticles within the interface causes a change in the volume, size, and shape of the intermetallic compounds formed by pinning grain boundaries and restricting grain growth of the interlayer. The variation of the chemical composition and hardness across the bond interface confirmed a better distribution of hard phases within the joint center when a Cu/Al2O3 interlayer was used.

Keywords: stainless steel; titanium; diﬀusion bonding; nanoparticles; intermetallic compounds; grain size reduction

1. Introduction Advanced alloys of titanium and stainless steel are widely regarded as high-strength, corrosion-resistant materials that find applications in industries such as biomedical, aerospace, automotive, and oil and gas. The growth in the demand for higher-performing multicomponent structures and mechanical systems necessitates the combination of these advanced alloys into superstructures containing multiple materials of uniquely differing chemical, thermal, and thermomechanical properties [1,2]. Currently, structures made with materials of similar types are joined using traditional welding technologies such as arc welding [3], laser welding [4], spark plasma welding [5], ultrasonic welding [6], etc. The welding of multiple components of structures is still an area that requires extensive research to identify technologies that are capable of welding dissimilar materials without adverse effects given the differences in the properties of the materials [7]. Techniques such as friction stir welding (FSW) [8], friction stir spot welding (FSSW) [9], and ultrasonic welding (USW) [10] have shown potential for producing multicomponent structures; however, there still exist numerous limitations that constrain the successful application of these technologies in industry. Diffusion bonding is another technique that has demonstrated promise for combining dissimilar metals into hybrid structures while constraining the volume of intermetallic compounds that form at the joint interface [11,12]. There are two variants of the diffusion bonding process: The first is transient

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the joint interface [11,12]. There are two variants of the diffusion bonding process: The first is transient liquid phase bonding, during which liquid may form due to the direct melting of the interlayer or due liquid phase bonding, during which liquid may form due to the direct melting of the interlayer or to eutectic reaction between the interlayer and the base metal [13]. The second variant is solid-state due to eutectic reaction between the interlayer and the base metal [13]. The second variant is solid- diffusion bonding, in which bonding occurs below the melting points of the base metals or interlayer. state diffusion bonding, in which bonding occurs below the melting points of the base metals or Unfortunately, the persistent formation of a continuous layer of intermetallic compounds at the joint interlayer. Unfortunately, the persistent formation of a continuous layer of intermetallic compounds interface limits the potential applications of solid-state diffusion bonding. at the joint interface limits the potential applications of solid-state diffusion bonding. Recent studies on the transient liquid phase diffusion (TLP) bonding of dissimilar alloys [11,14] Recent studies on the transient liquid phase diffusion (TLP) bonding of dissimilar alloys [11,14] have shown that the addition of nanoparticles to the interlayer has the capacity to constrain the have shown that the addition of nanoparticles to the interlayer has the capacity to constrain the size size of the intermetallic compounds that form at the interface while disrupting the shape of the of the intermetallic compounds that form at the interface while disrupting the shape of the intermetallic layer. In these studies, the results show that when nanoparticles were added to the intermetallic layer. In these studies, the results show that when nanoparticles were added to the interlayer, the intermetallic compounds that formed at the interface were shorter and more disperse interlayer, the intermetallic compounds that formed at the interface were shorter and more disperse within the joint region [11]. However, the studies were all conducted on relatively low-melting-point within the joint region [11]. However, the studies were all conducted on relatively low-melting-point nonferrous alloys. nonferrous alloys. This study investigated the diffusion bonding of Ti–6Al–4V and SDSS using an electrodeposited This study investigated the diffusion bonding of Ti–6Al–4V and SDSS using an electrodeposited Cu coating containing 40 nm Al2O3 particles as a method of disrupting the volume, shape, and size Cu coating containing 40 nm Al2O3 particles as a method of disrupting the volume, shape, and size of the intermetallic layers that form at the interface during bonding. The results of this study are of the intermetallic layers that form at the interface during bonding. The results of this study are significantsignificant since it provides a method of controllingcontrolling or modifying the type and form of intermetallic compounds that form duringduring didiffusionffusion bondingbonding ofof dissimilardissimilar alloys.alloys. 2. Experimental Procedure 2. Experimental Procedure 2.1. Materials 2.1. Materials Table1 shows the composition of the materials studied. Prior to bonding, the Ti–6Al–4V sample Table 1 shows the composition of the materials studied. Prior to bonding, the Ti–6Al–4V sample was coated with an electrodeposited copper coating (Cu) containing α-Al2O3 nanoparticles having an averagewas coated diameter with an of electrodeposited 40 nm. The electrodeposition copper coating process (Cu) containing was carried α out-Al2 asO3 described nanoparticles in apreviously having an publishedaverage diameter article of [11 40]. nm. A scanning The electrodeposition electron microscopy process was (SEM) carried micrograph out as described and energy-dispersive in a previously spectroscopepublished article (EDS) [11]. spectrum A scanning of the electron interlayer micr confirmedoscopy (SEM) the uniform micrograph distribution and energy-dispersive of nanoparticles spectroscope (EDS) spectrum of the interlayer confirmed the uniform distribution of nanoparticles within the coating (see Figure1). The bonds produced using a 5 µm thick Cu/Al2O3 interlayer were comparedwithin the tocoating bonds (see formed Figure using 1). 25Theµ mbonds thick produced Cu foil to using assess a the 5 µm role thick of the Cu/Al particles2O3 interlayer in the bonding were processcompared and to the bonds properties formed of using the joint 25 µm interface. thick Cu foil to assess the role of the particles in the bonding process and the properties of the joint interface. Table 1. Chemical composition (wt %) of the alloys studied. Table 1. Chemical composition (wt %) of the alloys studied. Alloys Al Mn V Cr Ag Cu Si Ni Mg Ti Fe C Mo Alloys Al Mn V Cr Ag Cu Si Ni Mg Ti Fe C Mo SDSS 1.1 0 0 25 0 0.43 0.02 7 0 0 Bal. 0.03 4 SDSS 1.1 0 0 25 0 0.43 0.02 7 0 0 Bal. 0.03 4 Ti–6Al–4V6 0.23 4 0 0.86 0 0.15 0 2.7 Bal. 0 0 0 Ti–6Al–4V 6 0.23 4 0 0.86 0 0.15 0 2.7 Bal. 0 0 0

Al2O3 particles

Figure 1. ((A)) Cu/Al Cu/Al2OO33 coatingcoating deposited deposited on on the the titanium; titanium; ( (BB)) EDS EDS spectrum spectrum of of the the highlighted highlighted region conﬁrmingconfirming thethe elementselements presentpresent inin thethe CuCu/Al/Al22O3 coating.coating.

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2.2. Sample Preparation and Bonding Process Samples of 10 mm diameter and 7 mm length were prepared from bars of Ti–6Al–4V and duplex stainless steel. The Ti sample was coated for 10 min to deposit the Cu/Al2O3 coating. A schematic of the process is presented in Figure 2. The coating process was published in a previous article [11]. A hole of 1.5 mm diameter by 3 mm deep was drilled into the Ti sample to accommodate a K-type thermocouple. The faying surfaces were prepared by grinding progressively from 320 to 2500 grit size SiC, polished to 1 µm grit size using particle-impregnated carrier paste, and then cleaned in an acetone bath. Prior to bonding, the samples were assembled at room temperature and then placed on the lower platen within the induction coil, and an ungrounded K-type thermocouple was inserted into the hole located approximately 1 mm from the bonding surface. Once a vacuum of 2 × 10−3 Torr was achieved, the assembly was heated to the bonding temperature of 800 °C, 850 °C, or 900 °C. The samples were loaded with 5 MPa and heated to the bonding temperature at a rate of 65 °C/min, then held at that temperature for bonding time of 15, 30, 45, or 60 min. Samples that were diffusion bonded for 15 min or less failed during preparation. Following the diffusion bonding process, the samples were cooled under vacuum to room temperature. Each bonded sample was sectioned transversely to the bond- line by an abrasive saw, following which each haft was mounted in Bakelite. Three samples were J. Manuf. Mater. Process. 2020, 4, 3 3 of 14 prepared for each condition studied. The mounted specimens were prepared by grinding progressively on silicon carbide papers to 2500 grit, followed by a final polish to 1 µm finish. 2.2. SampleHardness Preparation testing of and the Bonding cross section Process of the joints was performed using a Leitz microhardness tester. Indentations were made at 100 µm spacing using a diamond tip indenter to which a 0.1 kg Samples of 10 mm diameter and 7 mm length were prepared from bars of Ti–6Al–4V and duplex load was applied for 30 s, after which the length of the diagonals was measured, and the hardness stainless steel. The Ti sample was coated for 10 min to deposit the Cu/Al O coating. A schematic number calculated. Microscopic examination of the bonded joints was 2performed3 using a Leitz of the process is presented in Figure2. The coating process was published in a previous article [ 11]. optical microscope and a scanning electron microscope (SEM) (FEI Quanta 400, Oxfordshire, UK) A hole of 1.5 mm diameter by 3 mm deep was drilled into the Ti sample to accommodate a K-type equipped with an INCA X-sight X-ray. Quantitative compositional analyses were carried out using thermocouple. The faying surfaces were prepared by grinding progressively from 320 to 2500 grit energy-dispersive spectroscopy (EDS) and a Bruker X-Ray diffractometer (XRD) from 2θ ranging µ fromsize SiC,10° to polished 80° and to measuring 1 m grit time size using1 s per particle-impregnated step. carrier paste, and then cleaned in an acetone bath.

FigureFigure 2. 2. SchematicSchematic of of the the coating coating deposition deposition and and bonding bonding process. process.

Prior to bonding, the samples were assembled at room temperature and then placed on the lower 3. Results and Discussion platen within the induction coil, and an ungrounded K-type thermocouple was inserted into the hole located approximately 1 mm from the bonding surface. Once a vacuum of 2 10 3 Torr was achieved, 3.1. Effect of Bonding Time on the Bond Interface × − the assembly was heated to the bonding temperature of 800 ◦C, 850 ◦C, or 900 ◦C. The samples were The influence of bonding time on the microstructure formed within the joint region during loaded with 5 MPa and heated to the bonding temperature at a rate of 65 ◦C/min, then held at that diffusiontemperature bonding for bonding was assessed time of by 15, varying 30, 45, the or 60 bonding min. Samples time from that 15 were min ditoff 60usion min. bonded Figure for3A 15shows min theor lessSEM failed micrograph during preparation. of a sample Following bonded at the 850 di ff°Cusion for 30 bonding min. The process, joint theappears samples to wererepresent cooled a homogenousunder vacuum bond to room between temperature. the Ti–6Al–4V Each bonded alloy sampleand SDSS. was However, sectioned transverselya thick reaction to the layer bond-line was by an abrasive saw, following which each haft was mounted in Bakelite. Three samples were prepared for each condition studied. The mounted specimens were prepared by grinding progressively on silicon carbide papers to 2500 grit, followed by a final polish to 1 µm finish. Hardness testing of the cross section of the joints was performed using a Leitz microhardness tester. Indentations were made at 100 µm spacing using a diamond tip indenter to which a 0.1 kg load was applied for 30 s, after which the length of the diagonals was measured, and the hardness number calculated. Microscopic examination of the bonded joints was performed using a Leitz optical microscope and a scanning electron microscope (SEM) (FEI Quanta 400, Oxfordshire, UK) equipped with an INCA X-sight X-ray. Quantitative compositional analyses were carried out using energy-dispersive spectroscopy (EDS) and a Bruker X-Ray diffractometer (XRD) from 2θ ranging from 10◦ to 80◦ and measuring time 1 s per step.

3. Results and Discussion

3.1. Effect of Bonding Time on the Bond Interface The influence of bonding time on the microstructure formed within the joint region during diffusion bonding was assessed by varying the bonding time from 15 min to 60 min. Figure3A shows the SEM micrograph of a sample bonded at 850 ◦C for 30 min. The joint appears to represent J. Manuf. Mater. Process. 2020, 4, 3 4 of 14

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 4 of 14 a homogenous bond between the Ti–6Al–4V alloy and SDSS. However, a thick reaction layer was observed at the interface on the Ti side of the bond. Al O particles appear to have formed clusters observed at the interface on the Ti side of the bond. Al22O33 particles appear to have formed clusters andand segregatedsegregated toto thethe graingrain boundaryboundary regionsregions withinwithin thethe TiTi sideside ofof thethe bond.bond. The highlighted regionregion presentedpresented inin FigureFigure3 3AA shows shows an an additional additional reaction reaction layer layer at at the the SDSS SDSS interface. interface. WhenWhen thethe bonding bonding timetime waswas increasedincreased toto 4545 min,min, thethe widthwidth ofof thethe interfaceinterface alsoalso increasedincreased withwith thethe formationformation ofof twotwo additional phases within the reaction layer containing dispersed Al O particles. Microcracking was additional phases within the reaction layer containing dispersed Al22O33 particles. Microcracking was alsoalso observed in this this layer layer of of the the interface. interface. The The fracture fracture may may have have occurred occurred during during the thecutting cutting of the of thesample; sample; these these crack crack patterns, patterns, however, however, suggest suggest that th thate reaction the reaction layer may layer be may brittle. be Further brittle. increase Further increaseof the bonding of the bonding time to time60 min to 60resulted min resulted in the information the formation of four of fourdistinct distinct reaction reaction regions regions at the at theinterface, interface, primarily primarily on the on Ti the side. Ti side.While Whileinterdiffusion interdiﬀ ledusion to the led formation to the formation of a wide of region, a wide labelled region, labelledA, the nanoparticles A, the nanoparticles appear appearto have tosegregated have segregated to form to a formthick a layer thick at layer the atSDSS the SDSSinterface. interface. EDS EDS analysis of the dark grey particles showed that the Al O particles reacted with Cu and Fe [15,16]. analysis of the dark grey particles showed that the Al2O3 2particles3 reacted with Cu and Fe [15,16].

FigureFigure 3.3. EEffectﬀect ofof bondingbonding timetime onon thethe jointjoint zonezone duringduring didiffusionﬀusion bondingbonding ofof Ti–6Al–4VTi–6Al–4V andand SDSSSDSS

usingusing aa CuCu/Al/Al2O2O33 interlayer.interlayer. Each Each sample sample was was bonded bonded at at 850 850◦C °C for for (A )( 30A) min,30 min, (B) 45(B min,) 45 min, or (C or) 60 (C min.) 60 min. With increasing bonding time, Kirkendall effects were observed to play an important role in the homogeneityWith increasing of the bonding interfaces. time, AsKirkendall identified effects in Figure were3 observed, the formation to play of an voids important occurred role at in the the Cuhomogeneity/Al2O3/SDSS of interface,the interfaces. and the As volume identified of voids in Figure present 3, in the this formation area increased of voids as the occurred bonding at time the increased.Cu/Al2O3/SDSS The formationinterface, and of these the volume voids was of voids attributed presen tot in the this interdi area ffincreasedusion of as Cu the and bonding Fe near time the bottomincreased. of the The TiCu formation2 layer. Theof these presence voids of was the voidsattribut formeded to atthe the interdiffusion SDSS interface of wasCu attributedand Fe near to the dibottomfferences of the in the TiCu rate2 layer. of diff Theusion presence between of Cu the in voids Fe and formed Fe in Cu. at the Continued SDSS interface interdiff wasusion attributed during the to ditheff usiondifferences bonding in processthe rate is of believed diffusion to resultbetween in anCu accumulation in Fe and Fe of in voids Cu. Continued at the SDSS interdiffusion interface and blocksduring thethe di diffusionffusion of bonding Fe, causing process the growthis believed of the to TiCu result2 layer. in an accumulation of voids at the SDSS interface and blocks the diffusion of Fe, causing the growth of the TiCu2 layer. 3.2. Effect of Bonding Temperature on the Interface 3.2. EffectThe eofff ectBonding of bonding Temperature temperature on the Interface on the microstructural evolution within the joint region was evaluatedThe effect at bonding of bonding temperatures temperature of 800 on◦ theC, 850microstructural◦C, and 900 ◦evolutionC for 30 min within bonding the joint time. region Samples was bondedevaluated at temperaturesat bonding temperatures below 800 ◦C of failed 800 °C, during 850 preparation°C, and 900 for°C testing.for 30 min For bonding a bonding time. temperature Samples ofbonded 800 ◦C, at three temperatures distinct reaction below layers 800 °C were failed observed during at thepreparation interface asfor shown testing. in Figure For a4 A.bonding These regionstemperature were of identified 800 °C, three by the distinct differences reaction in layers the shades were ofobserved the layers at the formed. interface The asmicrograph shown in Figure also shows4A. These evidence regions of incomplete were identified bonding by at the the differences SDSS/Cu/Al 2inO 3theinterface shades due of tothe the layers presence formed. of several The cavitiesmicrograph/voids. also On shows the other evidence hand, of the incomplete Ti/Cu/Al2 bondingO3 interface at the showed SDSS/Cu/Al evidence2O3 ofinterface uniform due bonding to the betweenpresence theof several Cu/Al2 cavities/voids.O3 interlayer and On the Tiother base hand, metal. the This Ti/Cu/Al dissimilarity2O3 interface may showed be attributed evidence to the of diuniformfferences bonding in the dibetweenffusion the coe ffiCu/Alcient2O of3 Cuinterlayer in Fe and and Cu the in Ti Ti. base The metal. literature This shows dissimilarity that within may the be temperatureattributed to rangethe differences studied, thein the diff diffusionusivity of coefficient Cu in Ti shows of Cu fastin Fe impurity and Cu in di ffTi.usion The whenliterature compared shows tothat other within metals the temperature [17]. Further range increase studied, of the the bonding diffusivity temperature of Cu in Ti to shows 850 ◦ Cfast led impurity to the formation diffusion ofwhen a uniform compared bond to other at both metals the SDSS [17]. /FurtherCu/Al2O increase3 and Ti of/Cu the/Al bonding2O3 interfaces, temperature as shown to 850 in °C Figure led to4 theB. Theformation Al2O3 ofnanoparticles a uniform bond appear at toboth be the distributed SDSS/Cu/Al along2O the3 and grain Ti/Cu/Al boundaries2O3 interfaces, on the Ti as side shown of the in bond.Figure When 4B. The the Al bonding2O3 nanoparticles temperature appear was furtherto be distributed increased along to 900 the◦C, grain the width boundaries of the reaction on the Ti zones side atof the bond. Ti/interlayer When the interface bonding increased temperature in thickness, was furthe andr increased a third reaction to 900 °C, layer the waswidth observed of the reaction at the zones at the Ti/interlayer interface increased in thickness, and a third reaction layer was observed at the SDSS/interlayer interface (see Figure 4C). According to the Ti–Cu phase diagram, bonding within the temperature range of 870–900 °C would allow for the formation of a eutectic liquid of the form L

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SDSS/interlayer interface (see Figure4C). According to the Ti–Cu phase diagram, bonding within J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 5 of 14 the temperature range of 870–900 ◦C would allow for the formation of a eutectic liquid of the form J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 5 of 14 L = TiCu4 + Ti Cu2. Evidence of the formation of the eutectic liquid is shown in Figure5A. Additionally, = 𝑇𝑖𝐶𝑢 +𝑇𝑖 𝐶𝑢 . Evidence of the formation of the eutectic liquid is shown in Figure 5A. at higher bonding temperatures the α β Ti microstructure forms at the Ti/Cu/Al2O3 interface (see =Additionally, 𝑇𝑖𝐶𝑢 +𝑇𝑖 at higher𝐶𝑢 . Evidence bonding of temperaturesthe −formation the of 𝛼−𝛽 the eutectic 𝑇𝑖 microstructure liquid is formsshown at thein Ti/Cu/AlFigure 5A.2O3 Figure5B). Additionally,interface (see atFigure higher 5B) bonding temperatures the 𝛼−𝛽 𝑇𝑖 microstructure forms at the Ti/Cu/Al2O3 interface (see Figure 5B)

FigureFigure 4. 4.SEM SEM micrographmicrograph of of Ti–6Al–4VTi–6Al–4V and and super-duplex super-duplex stainless stainless steel steel di diffusionﬀusion bonded bonded for for 30 30 min min

usingFigureusing a a4. Cu Cu/Al SEM/Al2 Omicrograph2O33 interlayerinterlayer of atat Ti–6Al–4V thethe followingfollowing and temperatures:super-duplextemperatures: stainless ((AA)) 800800◦ °C,steelC, ((BB diffusion)) 850850 ◦°C,C, ((bondedCC)) 900900 ◦°C. forC. 30 min using a Cu/Al2O3 interlayer at the following temperatures: (A) 800 °C, (B) 850 °C, (C) 900 °C.

Figure 5. SEM micrographs of (A) the eutectic structure form at the interface for samples bonded at FigureFigure900 °C; 5.5. ( BSEMSEM) Microstructure micrographsmicrographs on of the ((AA)) Ti thethe side eutecticeutectic of the structurestructure interface. formform atat thethe interfaceinterface forfor samplessamples bondedbonded atat 900 °C; (B) Microstructure on the Ti side of the interface. 900 ◦C; (B) Microstructure on the Ti side of the interface. 3.3. Effect of Interlayer Composition on the Bond Interface 3.3.3.3. EEffectffect of Interlayer CompositionComposition onon thethe BondBond InterfaceInterface The influence of the interlayer composition on the microstructure of the joint region was investigatedTheThe influenceinfluence by comparing ofof thethe interlayerinterlayer bonds formed compositioncomposition using an on electrodeposited thethe microstructuremicrostructure Cu coating of thethe containing jointjoint regionregion α-Al waswas2O 3 investigatedinvestigatednanoparticles byby (Figure comparingcomparing 6A,B) bondsbonds and a formed formed25 µm Cu usingusing foil an anwas electrodepositedelectrodeposited used as the interlayer CuCu coatingcoating (Figure containing containing 6C,D). Figureα α-Al-Al22O O6A33 nanoparticlesnanoparticlesshows the presence (Figure of6 6A,B)A,B) six separate andand aa 2525 distinct µµmm CuCu layers, foilfoil waswas where usedused each asas thethe layer interlayerinterlayer is distinguishable (Figure(Figure6 6C,D).C,D). by the FigureFigure shade6 A6A of showsshowsthe specific thethe presencepresence region. of ofThese sixsix separateseparate areas were distinctdistinct labelled layers,layers, P2 where–Pwhere7. The eacheach elemental layerlayer isis distinguishablecompositiondistinguishable of byeachby thethe phase shadeshade was ofof thetheidentified specificspecific using region.region. EDS TheseThese and areasisareas presented werewere labelledlabelled in Table PP 222–P–P. The77. The EDS elemental analysis shows composition that the of composition eacheach phasephase waswasof P 1 identifiedidentifiedis that of usingusing the EDSsuper-duplex EDS and and is is presented presented stainless in in Tablesteel Table 2beca. 2 The. Theuse EDS EDSof analysisthe analysis high shows showsFe and that that Cr the thecontent composition composition and oflow P of 1 CuPis1 thatisconcentration. that of theof super-duplexthe super-duplex stainless stainless steel because steel beca of theuse high of Fethe and high Cr contentFe and andCr lowcontent Cu concentration.and low Cu concentration.TheThe dark dark grey grey region region P P22is is aa Cu-richCu-rich phasephase ofof TiCuTiCu22 whichwhich containscontains 6767 wtwt %% CuCu andand 2828 wtwt %% Ti.Ti. PP33also alsoTheappears appears dark grey to tobe be regiona aCu-rich Cu-rich P2 isphase phasea Cu-rich with with phase a acomposition composition of TiCu2 of whichof 33 33 wt wt contains % % Ti Ti and and 67 58 58 wt wt wt % % % Cu Cu Cu and and and 28 is is possiblywt possibly % Ti. thePthe3 also same same appears TiCu TiCu2 2 (seeto (see be Figure Figurea Cu-rich6 B).6B). Similarphase Similar with changes changes a composition were were observed observed of 33 forwt for region% region Ti and P P4 58,4, which which wt % hadCu had and a a composition composition is possibly oftheof 41 41same wt wt % TiCu% Ti Ti and2 and(see 55 55Figure wt wt % Cu,% 6B). Cu, while Similar while P5 similarlychangesP5 similarly were showed showed observed a marginal a marginal for region change change P4, in which the in Ti thehad content. Ti a compositioncontent. The EDS The analysisofEDS 41 analysiswt suggests% Ti suggestsand that 55 wt both that % P Cu,both4 and while P4 P and5 are P 5P similarly TiCu5 are TiCu compounds showed compounds (seea marginal Figure (see Figure6 changeA). The 6A). in Ti-rich Thethe TiTi-rich phase content. phase P 6 wasThe P 6 identifiedEDSwas analysisidentified as Ti suggests2 Cuas Ti into2Cu that which into both which Cu P4 had and Cu di P ffhad5 used.are diffused. TiCu The compounds general The general trend (see showed trend Figure showed that6A). the The that Ti Ti-rich content the Ti phase withincontent P6 thewaswithin reaction identified the reaction layers as Ti increased layers2Cu into increased progressivelywhich Cu progressively had from diffused. the fr steelom The the side general steel of theside trend bond of the showed towards bond towardsthat the the Ti base Tithe content Ti metal. base Thewithinmetal. region theThe labelledreaction region Playerslabelled7 contained increased P7 contained spherical progressively particlesspherical fr distributed omparticles the steel distributed within side theof the grain within bond boundary towardsthe grain region. the boundary Ti EDSbase analysismetal.region. The EDS showed region analysis the labelled presence showed P7 of thecontained Al presence2O3 particles spherical of Al as2O well3particles particles as Cu, distributedas Ti, well and as Fe. Cu, within Ti, and the Fe. grain boundary region.The EDS Ti/SDSS analysis joint showed bonded the using presence 25 µm of Cu Al 2foilO3 particlespresented as in well Figure as Cu, 6C Ti,shows and the Fe. presence of five distinctThe reactionTi/SDSS regionsjoint bonded identified using by 25 theµm differences Cu foil pres inented shade, in Figure similar 6C to shows those theidentified presence when of five the distinctCu/Al2 Oreaction3 interface regions was identifiedused. EDS by analysis the differences of the region in shade, labelled similar P8 tosuggests those identifiedthat this phasewhen theis a Cu/Alternary2O 3compound interface wasof the used. form EDS Tix Cuanalysisx-Fex. The of theregion region labelled labelled P9 is P believed8 suggests to that be compound this phase TiCuis a ternarygiven the compound high Ti content of the form recorded TixCu (seex-Fe Figurex. The region6D). A labelled summary P9 ofis believedthe phases to identifiedbe compound is listed TiCu in givenTable the2. The high low Ti 0.54–1.95content recorded wt % of Fe(see recorded Figure 6D). within A summarythe reaction of thelayer phases confirms identified that phases is listed P3, inP4 , TableP5, and 2. PThe7 are low likely 0.54–1.95 binary wt compounds % of Fe recorded formed within from re theactions reaction between layer Ticonfirms and Cu. that EDS phases maps P of3, Pthe4, P 5, and P7 are likely binary compounds formed from reactions between Ti and Cu. EDS maps of the

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 6 of 14 bond formed using Cu foil as the interlayer showed that the distribution of the elements Fe, Cu, and Ti within the reaction layer (Figure 7) led to the formation of the ternary intermetallic compound labelled at P8: (Ti33Cu67 − xFex; 1 < x < 2.5), T2 (Ti40Cu60 − xFex; 5 < x < 17), T3 (Ti43Cu57 − xFex; 21 < x < 24), τ (Ti37Cu63 − xFex; 6 < x < 7), and τ1 (Ti45Cu55 − xFex; 4 < x < 5) [18]. EDS line scans presented in Figures 8 and 9 show a visual representation of the variation of the composition across the joint zone and confirm the high concentration of Ti and Cu within the reaction layers. The Ti–Cu phase diagram shown in Figure 10A suggests that within the range of composition listed in Table 2, the compound TiCu2 would form with Cu content varying between 33 wt % and 48 wt %. The compound labelled P8 was only found at the interface when Cu foil was used as the J.interlayer. Manuf. Mater. This Process. phase2020 has, 4, 3 a higher concentration of Fe and is believed to be a ternary intermetallic6 of 14 phase.

Figure 6. EffectEﬀect of interlayer compositioncomposition on on joint joint microstructure microstructure during during di diffusionﬀusion bonding bonding of of Ti–6Al–4V Ti–6Al–

and4V and super-duplex super-duplex stainless stainless steel. steel. The The sample sample was was bonded bonded at at 850 850◦ C°C for for 30 30 min. min. ((AA)) SampleSample bonded using a Cu/Al Cu/Al2O3 coatingcoating electrodeposited electrodeposited onto onto Ti–6Al–4V; Ti–6Al–4V; (B) Detail of Region 1; ( C) Sample bonded using 25 µµmm Cu foil; (D)) DetailDetail ofof RegionRegion 2.2.

Table 2. EDS assessment of the composition of the joint interface (wt %). Table 2. EDS assessment of the composition of the joint interface (wt %). Phase Al Ti V Fe Cu C Cr Ni Possible Phase Phase Al Ti V Fe Cu C Cr Ni Possible Phase P1 - 0.22 - 61.57 0.68 3.51 24.45 5.73 P2P 1 1.86- 28.64 0.22 1.03 - 0.54 61.57 67.34 0.68 3.51 - 24.45 - 5.730.59 TiCu2 P3P 2 2.431.86 33.78 28.64 1.24 1.03 1.26 0.54 58.45 67.34 2.34 - - 0.59 - TiCu TiCu2 2

P4P 3 0.322.43 41.31 33.78 0.73 1.24 1.26 - 55.54 58.45 2.34 2.1 - - - TiCu TiCu2 5 PP 4 1.360.32 41.55 41.31 1.40 0.73 1.13 - 51.32 55.54 2.89 2.1 - - TiCu TiCu P6 5.53 79.00 4.83 0.45 10.18 - - - Ti2Cu P5 1.36 41.55 1.40 1.13 51.32 2.89 TiCu P7 7.49 47.08 1.95 40.47 1.47 1.08 0.46 TiCu/Al2O3 P6 5.53 79.00 4.83 0.45 10.18 - - - Ti2Cu P8 1.35 41.55 1.40 1.13 51.32 2.89 0.36 - Tix Cux-Fex P7 7.49 47.08 1.95 40.47 1.47 1.08 0.46 TiCu/Al2O3 P9 2.19 54.19 1.91 0.81 37.77 2.58 0.29 0.26 Ti2Cu P8 1.35 41.55 1.40 1.13 51.32 2.89 0.36 - Tix Cux-Fex

These phasesP9 are2.19 believed 54.19 to 1.91have formed 0.81 37.77due to 2.58the dissolution 0.29 0.26 of approximately Ti2Cu 38 wt % of Cu in the cubic Ti–Fe lattice. The Ti–Cu–Fe ternary phase diagram presented in Figure 10B confirms the likelihoodThe Ti/SDSS of the joint formation bonded of using intermetallic 25 µm Cu co foilmpounds presented within in Figurethe temperature6C shows range the presence of 800–900 of five°C [19]. distinct reaction regions identified by the differences in shade, similar to those identified when The shape of the compound formed at the interface when the Cu foil was used as the interlayer the Cu/Al2O3 interface was used. EDS analysis of the region labelled P8 suggests that this phase is a appears to be a continuous series of brittle plates, which are stacked as shown in Figure 6D. When ternary compound of the form TixCux-Fex. The region labelled P9 is believed to be compound TiCu given the high Ti content recorded (see Figure6D). A summary of the phases identified is listed in Table2. The low 0.54–1.95 wt % of Fe recorded within the reaction layer confirms that phases P 3,P4, P5, and P7 are likely binary compounds formed from reactions between Ti and Cu. EDS maps of the bond formed using Cu foil as the interlayer showed that the distribution of the elements Fe, Cu, and Ti within the reaction layer (Figure7) led to the formation of the ternary intermetallic compound labelled at P : (Ti Cu xFex; 1 < x < 2.5), T (Ti Cu xFex; 5 < x < 17), T3 (Ti Cu xFex; 21 < x < 24), 8 33 67 − 2 40 60 − 43 57 − τ (Ti Cu xFex; 6 < x < 7), and τ (Ti Cu xFex; 4 < x < 5) [18]. 37 63 − 1 45 55 − J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 14

J. Manuf.compared Mater. to Process. the bond2020, formed4, 3 using the Cu/Al2O3 coating as the interlayer, the nanoparticles are seen7 of 14 to occupy positions around grain boundaries as shown in Figure 6C.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 14 compared to the bond formed using the Cu/Al2O3 coating as the interlayer, the nanoparticles are seen to occupy positions around grain boundaries as shown in Figure 6C.

Figure 7. EDS maps of Ti–6Al–4V and SDSS joint bonded at 850 °C for 45 min with 25 µm Cu foil. Figure 7. EDS maps of Ti–6Al–4V and SDSS joint bonded at 850 ◦C for 45 min with 25 µm Cu foil. EDS mappingEDS mapping of elements of elements (A) iron, (A) ( Biron) Titanium, , (B) Titanium, and (C and) Copper. (C) Copper.

EDS line scans presented in Figures8 and9 show a visual representation of the variation of the composition across the joint zone and conﬁrm the high concentration of Ti and Cu within the reaction layers. The Ti–Cu phase diagram shown in Figure 10A suggests that within the range of composition listed in Table2, the compound TiCu 2 would form with Cu content varying between 33 wt % and 48 wt %. The compound labelled P8 was only found at the interface when Cu foil was usedFigure as the 7. interlayer. EDS maps Thisof Ti–6Al–4V phase has and a SDSS higher joint concentration bonded at 850 of °C Fe for and 45 min is believed with 25 µm to beCua foil. ternary intermetallicEDS mapping phase. of elements (A) iron , (B) Titanium, and (C) Copper.

Figure 8. (A) SEM micrograph of a region on the Ti–6Al–4V and SDSS joint bonded at 850 °C for 45

min with Cu/Al2O3 coating. EDS line scanning analysis of elements (B) Ti, (C) Fe, and (D) Cu.

The EDS line scans presented in Figure 9 show that a large portion of the reaction layer formed on the Ti side of the bond was promoted by the diffusion of Cu into the Ti base metal, given that Cu diffuses faster into Ti than it does into SDSS. Similar findings were observed when the Cu/Al2O3 interlayer was used (see Figure 9). The reaction layer was found to have a higher percentage of Cr and Fe, which may suggest that Fe and Cr diffused faster into the Cu/Al2O3 coating than into the Cu foil. When the process was modelled using Thermo-Cal, the results showed that within the bonding

temperatureFigure 8. (A range) SEMSEM studied micrographmicrograph (850–900 of of a a region region °C) four on on the thebinary Ti–6Al–4V Ti–6Al–4V phases and andare SDSS SDSSpossible; joint joint bonded Cu bonded3Ti at2, 850Cu at 2◦850Ti,C for °CTi2 45Cu,for min 45 and Cuwithmin4Ti3 ,with Cuas /shownAl Cu/Al2O3 coating.in2O 3Figure coating. EDS 10A, EDS line support line scanning scanning the analysis EDS analysis data of elements. Similarof elements observations (B) Ti,(B) ( CTi,) Fe,(C) and wereFe, and (D reported) Cu.(D) Cu. by Pardal et al. [3] who studied dissimilar metal joining of Ti and duplex stainless steel using Cu as a transition metalTheThese interlayer. EDS phases line scans areA plot believed presented of the toGibbs have in freeFigure formed energy 9 show due profile to th theat shown a dissolution large in portionFigure of 10C,D approximately of the indicates reaction 38 thatlayer wt several %formed of Cu oninstable thethe cubicTi phasesside Ti–Fe of arethe lattice. likelybond Thewasto form Ti–Cu–Fe promoted in the ternary byTi–Cu the phasediffusbinaryion diagramsystem of Cu for presentedinto the the bonding Ti in base Figure temperaturemetal, 10B given conﬁrms rangethat theCu diffuseslikelihoodstudied. faster ofThe the sequenceinto formation Ti than of ofphase it intermetallic does formation into SD compounds goesSS. Similarfrom CuTi within findings2 to the Cu temperature4wereTi3, Cu observed3Ti2, and range Cuwhen of2Ti. 800–900 Thethe growthCu/Al◦C[219 O].3 interlayerof Thethese shape wasphases used of is the believed(see compound Figure to be9). formed promotedThe reaction at the by interface inlaterdiffusionyer was when found between the to Cu have foilCu wasaand higher usedTi into percentage as the the reaction interlayer of Cr andappears Fe, which to be amay continuous suggest that series Fe of and brittle Cr diffused plates, whichfaster into are stackedthe Cu/Al as2 shownO3 coating in Figure than into6D. the When Cu foil.compared to the bond formed using the Cu/Al2O3 coating as the interlayer, the nanoparticles are seen to occupyWhen positions the process around was modelled grain boundaries using Thermo-Cal, as shown in the Figure results6C. showed that within the bonding temperature range studied (850–900 °C) four binary phases are possible; Cu3Ti2, Cu2Ti, Ti2Cu, and Cu4Ti3, as shown in Figure 10A, support the EDS data. Similar observations were reported by Pardal et al. [3] who studied dissimilar metal joining of Ti and duplex stainless steel using Cu as a transition metal interlayer. A plot of the Gibbs free energy profile shown in Figure 10C,D indicates that several stable phases are likely to form in the Ti–Cu binary system for the bonding temperature range studied. The sequence of phase formation goes from CuTi2 to Cu4Ti3, Cu3Ti2, and Cu2Ti. The growth of these phases is believed to be promoted by interdiffusion between Cu and Ti into the reaction

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 8 of 14

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 8 of 14 layer. The XRD spectrum presented in Figure 11 confirms the presence of Cu2Ti within the bonded region. J.layer. Manuf. The Mater. XRD Process. spectrum2020, 4, 3 presented in Figure 11 confirms the presence of Cu2Ti within the bonded8 of 14 region.

Figure 9. (A) BSE micrograph of a region on the Ti–6Al–4V and SDSS joint bonded at 850 ◦C for 45 min Figure 9. (A) BSE micrograph of a region on the Ti–6Al–4V and SDSS joint bonded at 850 °C for 45 with 25 µm Cu foil. EDS line scanning analysis of elements (B) Ti, (C) Fe, and (D) Cu. min with 25 µm Cu foil. EDS line scanning analysis of elements (B) Ti, (C) Fe, and (D) Cu. Figure 9. (A) BSE micrograph of a region on the Ti–6Al–4V and SDSS joint bonded at 850 °C for 45 min with 25 µm Cu foil. EDS line scanning analysis of elements (B) Ti, (C) Fe, and (D) Cu.

FigureFigure 10.10. ((A)) Ti–CuTi–Cu binarybinary phasephase diagramdiagram highlightinghighlighting thethe compositioncomposition ofof thethe reactionreaction layerlayer formedformed duringduring thethe bondingbonding process;process; ((BB)) Cu–Fe–TiCu–Fe–Ti isothermal isothermal section section at at 850 850 °C;C; ((CC)) PhasePhase fractionfraction asas aa functionfunction Figure 10. (A) Ti–Cu binary phase diagram highlighting the composition◦ of the reaction layer formed of temperature; (D) Gibbs free energy curve with respect to the concentration of Cu at a bonding during the bonding process; (B) Cu–Fe–Ti isothermal section at 850 °C; (C) Phase fraction as a function temperature of 850 ◦C.

J. Manuf. Mater. Process. 2020, 4, 3 9 of 14

The EDS line scans presented in Figure9 show that a large portion of the reaction layer formed on the Ti side of the bond was promoted by the diffusion of Cu into the Ti base metal, given that Cu diffuses faster into Ti than it does into SDSS. Similar findings were observed when the Cu/Al2O3 interlayer was used (see Figure9). The reaction layer was found to have a higher percentage of Cr and Fe, which may suggest that Fe and Cr diffused faster into the Cu/Al2O3 coating than into the Cu foil. When the process was modelled using Thermo-Cal, the results showed that within the bonding temperature range studied (850–900 ◦C) four binary phases are possible; Cu3Ti2, Cu2Ti, Ti2Cu, and Cu4Ti3, as shown in Figure 10A, support the EDS data. Similar observations were reported by Pardal et al. [3] who studied dissimilar metal joining of Ti and duplex stainless steel using Cu as a transition metal interlayer. A plot of the Gibbs free energy profile shown in Figure 10C,D indicates that several stable phases are likely to form in the Ti–Cu binary system for the bonding temperature range J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 9 of 14 studied. The sequence of phase formation goes from CuTi2 to Cu4Ti3, Cu3Ti2, and Cu2Ti. The growth of these phases is believed to be promoted by interdiffusion between Cu and Ti into the reaction layer. of temperature; (D) Gibbs free energy curve with respect to the concentration of Cu at a bonding The XRD spectrum presented in Figure 11 confirms the presence of Cu Ti within the bonded region. temperature of 850 °C. 2

FigureFigure 11. XRDXRD spectrum spectrum of ofthe theTi–6Al–4V Ti–6Al–4V and andSDSS SDSS joint bonded joint bonded at 850 at°C 850for 45◦C min for with 45 min Cu/Al with2O3 Cucoating./Al2O 3 coating.

3.4.3.4. HardnessHardness TheThe influenceinfluence ofof bondingbonding temperaturetemperature onon variationvariation ofof thethe hardnesshardness acrossacross thethe bondingbonding interfaceinterface waswas evaluatedevaluated andand isis presentedpresented inin FigureFigure 1212.. TheThe hardnesshardness valuesvalues werewere measuredmeasured acrossacross thethe jointjoint µ startingstarting atat 300300 µmm fromfrom thethe jointjoint center.center. The resultsresults showshow thatthat thethe hardnesshardness ofof thethe TiTi basebase metalmetal µ fluctuatedfluctuated betweenbetween 320320 andand 420420 VHNVHN upup toto 100100 µmm fromfrom thethe jointjoint centercenter asas thethe bondingbonding temperaturetemperature waswas increasedincreased fromfrom 800800 toto 900900◦ °C.C. TheThe hardnesshardness withinwithin thethe jointjoint centercenter waswas alsoalso observedobserved toto decreasedecrease fromfrom 634634 toto 405405 VHNVHN whenwhen thethe bondingbonding temperaturetemperature waswas increasedincreased fromfrom 800800 toto 900900 ◦°C.C. TheThe hardnesshardness ofof thethe SDSSSDSS basebase metal metal was was found found to to be be higher higher than than that that of of the the Ti Ti base base metal, metal, with with a hardness a hardness value value of 387of 387 VHN. VHN. The The result result of of the the hardness hardness test test shows shows (see (see Figure Figure 12 12A)A) that that the the hardness hardness within within thethe jointjoint centercenter decreaseddecreased withwith increasingincreasing bondingbonding temperature.temperature. TheseThese findingsfindings werewere attributedattributed toto increasedincreased didiffusionffusion ratesrates atat higherhigher bondingbonding temperaturestemperatures leading leading to to homogenization homogenization of of the the bond bond region. region. TheThe eeffectffect ofof bondingbonding timetime onon variationvariation ofof thethe hardnesshardness acrossacross thethe jointjoint interfaceinterface waswas alsoalso studiedstudied µ (see(see FigureFigure 12 12B).B). Similarly, Similarly, the the hardness hardness was was measured measured across across the the joint joint starting starting at 300 at 300m fromµm from the joint the centerjoint center as a function as a function of bonding of bonding time. time. The hardnessThe hardness within within the joint the joint center center increased increased from from 554 to554 752 to VHN752 VHN when when the bonding the bonding time was time increased was increased from 30 from min of30 bonding min of timebonding to 60 time min. to The 60 maximum min. The hardnessmaximum of hardness 750 VHN of was 750 recorded VHN was at the recorded center ofat thethe joints center bonded of the forjoints 60 min.bonded The for hardness 60 min. at The the centerhardness of theat the bond center is believed of the bond to have is believed been caused to ha byve been the formation caused by of the the formation reaction layerof the made reaction up layer made up of binary and ternary intermetallic compounds as well as Al2O3 nanoparticles dispersed at the joint interface [20]. Finally, the impact of interlayer composition on variation of the hardness measurements (see Figure 13) across the joint region was evaluated by comparing the bonds made with Cu/Al2O3 and Cu foil interlayers. The results show that when the Cu/Al2O3 interlayer was used, higher hardness values were recorded in the joint region when compared to samples bonded using Cu foil. The results show that the hardness within the joint center was marginally higher when Cu/Al2O3 coating was used as the interlayer due to the dispersion of hard nanoparticles at the interface.

J. Manuf. Mater. Process. 2020, 4, 3 10 of 14

of binary and ternary intermetallic compounds as well as Al2O3 nanoparticles dispersed at the joint interface [20].J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 14 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 14

(A) (B)

Figure 12. (AFigure) Hardness 12. (A) Hardness measurements measurements for for the the samplesample bonded bonded for 30 for min 30 as mina function as a of function bonding of bonding temperature; (B) Hardness measurements for the sample bonded at 850 °C as a function of bonding temperature;time. (B) Hardness measurements for the sample bonded at 850 ◦C as a function of bonding time.

Finally, the impact of interlayer composition on variation of the hardness measurements (see Figure 13) across the joint region was evaluated by comparing the bonds made with Cu/Al2O3 and Cu (A) (B) foil interlayers. The results show that when the Cu/Al2O3 interlayer was used, higher hardness values wereFigure recorded 12. ( inA)the Hardness joint region measurements when compared for the sample to samples bonded bonded for 30 min using as Cua function foil. The of bonding results show that thetemperature; hardness (B within) Hardness the joint measurements center was for marginally the sample higherbonded whenat 850 Cu°C /asAl a2 Ofunction3 coating of bonding was used as the interlayertime. due to the dispersion of hard nanoparticles at the interface.

Figure 13. Hardness measurements as a function of interlayer composition for samples bonded at 850 °C for 30 min.

3.5. Growth Kinetics of Interfacial Phases The width of the reaction layer was measured as a function of bonding time and bonding temperature and plotted as shown in Figure 14. The data indicate that as the bonding time increased, the width of the reaction layer also increased as predicted by the power law shown in Equation (1). Similar behavior was observed when the bonding temperature was increased from 800 to 900 °C. This response is believed to be consistent with the changes in the rate constant as the bonding temperature increased. The width of the reaction layer increased with increasing bonding temperature and can be estimated by the mathematical relationship shown in Equation (1): 𝑑=𝑘𝑡 (1)

where d is the thickness of the reaction layer, k is the rate factor, t the diffusion time, and n the time ( ) exponent. Further, 𝑘=𝑘𝑒 , where T is the bonding temperature (K), Q is activation energy Figure 13. 13. HardnessHardness measurements measurements as as a function a function of ofinterl interlayerayer composition composition for forsamples samples bonded bonded at 850 at 2 (kJ/mol), R is the real gas constant (8.314 J/K mol), and 𝑘 is the growth constant (m /s). The growth °C850 forC 30 for min. 30 min. ◦ kinetics of the intermetallic layer is controlled by interdiffusion (volume diffusion); therefore, the 3.5. Growth Growth Kinetics Kinetics of of Interfacial Interfacial Phases The width of the reaction layer was measured as a function of bonding time and bonding temperature and plotted as shown in Figure 14.14. The The data data indicate indicate that that as as the the bonding bonding time time increased, increased, the width of of the reaction layer layer also also increased increased as as predicted predicted by by the the power power law law shown shown in in Equation Equation (1). (1). Similar behavior was observed when the bonding te temperaturemperature was increased from 800 to 900 °C.◦C. This response is believed to be consistent consistent with the chan changesges in the rate rate constant constant as as the the bonding bonding temperature temperature increased. The width of the reaction layer increased with increasing bonding temperature and can be estimated by the mathematical relationship shown in Equation (1): 𝑑=𝑘𝑡 (1) where d is the thickness of the reaction layer, k is the rate factor, t the diffusion time, and n the time ( ) exponent. Further, 𝑘=𝑘𝑒 , where T is the bonding temperature (K), Q is activation energy

2 (kJ/mol), R is the real gas constant (8.314 J/K mol), and 𝑘 is the growth constant (m /s). The growth

J. Manuf. Mater. Process. 2020, 4, 3 11 of 14 increased. The width of the reaction layer increased with increasing bonding temperature and can be estimated by the mathematical relationship shown in Equation (1):

d = ktn (1) where d is the thickness of the reaction layer, k is the rate factor, t the diffusion time, and n the time ( Q ) exponent.J. Manuf. Mater. Further, Process. 2020k =, 4, kxo FORe− RT PEER, where REVIEW T is the bonding temperature (K), Q is activation energy11 of 14 2 (kJ/mol), R is the real gas constant (8.314 J/K mol), and ko is the growth constant (m /s). The growth kinetics of the intermetallic layer is controlled by interdiinterdiffusionffusion (volume didiffusion);ffusion); therefore, the 1/2 didiffusionffusion time is estimatedestimated toto bebe tt1/2 (i.e., n == 0.5). On On the the other other hand, if the growth kinetics was controlled by interfacial diffusion, the time exponent would be n = 1 [21,22]. controlled by interfacial diffusion, the time exponent would be n = 1 [21,22].

Figure 14. Variation of the width of the reaction layer as a function of the diﬀusion bonding conditions.

TheFigure results 14. Variation of the experimental of the width study of the were reaction used tolaye assessr as a the function kinetics of leadingthe diffusion to the bonding formation of the Ticonditions.2Cu reaction layer at the interface. A summary of the growth kinetics of the reaction layer is shown in Table3. The activation energy was calculated to be 111.4 kJ /mol for a bonding temperature of 850 ◦TheC. This results is similar of the to experimental findings published study were in the used scientific to assess literature the kinetics which suggest leading that to the the formation activation 2 energyof the Ti forCu the reaction formation layer of at Ti the2Cu interface. is approximately A summary 122.1 of kJ the/mol growth [16]. Inkinetics this study, of the the reaction calculation layer ofis theshown activation in Table energy 3. The was activation performed energy based was on calculated a simplified to be di ff111.4usion kJ/mol model, for which a bonding assumes temperature diffusion inof a850 single °C. phaseThis is which similar is influenced to findings by published the thickness in the of the scientific reaction literature layer formed which during suggest the dithatffusion the bondingactivation process energy andfor the formation increases inof theTi2Cu rate is coeapproximatelyfficient. The 122.1 width kJ/mol of the [16]. diffusion In this layer study, is alsothe believedcalculation to beof driventhe activation by the di energyffusion was of Cu performed into the Ti based base metal,on a simplified leading to thediffusion formation model, of various which compounds.assumes diffusion The Thermo-Cal in a single phase model which predicted is influenced the possible by the formation thickness of of several the reaction other compounds layer formed at theduring interface; the diffusion however, bonding the limitation process of and the simplifiedthe increases diff usionin the modelrate coefficient. is that only The a single width phase of the is assumeddiffusion tolayer have is formedalso believed at the interface.to be driven by the diffusion of Cu into the Ti base metal, leading to the formation of various compounds. The Thermo-Cal model predicted the possible formation of several other compounds at the interface; however, the limitation of the simplified diffusion model is that only a single phase is assumed to have formed at the interface.

Table 3. Growth kinetics of the reaction Cu2Ti layer during the diffusion bonding process.

Bonding Temperature (K) Interlayer Thickness (µm) Rate Coefficient k (µm/s1/2) Q (kJ/mol) 1073 10 0.236 110.3 1123 15.4 0.362 111.4 1173 63.2 1.482 102.6

3.6. Mechanism of Bond Formation The diffusion bonding process involves several stages [13]. The first stage involves initial contact between the faying surfaces and the interlayer, followed by deformation of the surface asperities under the effects of an externally applied load. The combination of heat and load increases the area

J. Manuf. Mater. Process. 2020, 4, 3 12 of 14

Table 3. Growth kinetics of the reaction Cu2Ti layer during the diﬀusion bonding process.

Bonding Temperature (K) Interlayer Thickness (µm) Rate Coeﬃcient k (µm/s1/2) Q (kJ/mol) 1073 10 0.236 110.3 1123 15.4 0.362 111.4 1173 63.2 1.482 102.6

3.6. Mechanism of Bond Formation The diffusion bonding process involves several stages [13]. The first stage involves initial contact between the faying surfaces and the interlayer, followed by deformation of the surface asperities under theJ. eManuf.ffects Mater. of an Process. externally 2020, 4,applied x FOR PEER load. REVIEW The combination of heat and load increases the area of 12 contact of 14 between the base metals and the interlayer and closes some of the voids or cavities present at the interlayerof contact/base between metal the interfaces. base metals and the interlayer and closes some of the voids or cavities present at theThe interlayer/base second stage metal of bonding interfaces. occurs due to interdiffusion between the base metals and the interlayer,The leadingsecond stage to the of formation bonding ofoccurs an intermetallic due to interdiffusion compound between at the interface,the base metals which and promotes the theinterlayer, closing of leading remaining to the voids formation and cavities. of an intermet The thirdallic stage compound of bonding at the involvesinterface, di whichffusion promotes of Cu into thethe SDSS closing and of Ti–6Al–4V remaining voids base metals,and cavities. leading The to third a eutectoid stage of bonding reaction involves as predicted diffusion by theof Cu equation into the SDSS and Ti–6Al–4V base metals, leading to a eutectoid reaction as predicted by the equation β Ti = α Ti + Ti2Cu for bonding temperatures of 850 ◦C and below. The EDS analysis presented in −𝛽 − 𝑇𝑖− = 𝛼 − 𝑇𝑖+𝐶𝑢 for bonding temperatures of 850 °C and below. The EDS analysis presented Table2 shows that intermetallic compounds such as CuTi 2 and Cu2Ti are also formed at the interface duringin Table the bonding2 shows process.that intermetallic A schematic compounds of the mechanism such as CuTi involved2 and in Cu the2Ti bond are formationalso formed is shownat the in Figureinterface 15. Theduring results the bonding show that process. the width A schematic of the reaction of the mechanism layer increases involved with increasingin the bond bonding formation time. is shown in Figure 15. The results show that the width of the reaction layer increases with increasing Similarly, the literature shows that the strength of the bond increases with increasing thickness of the bonding time. Similarly, the literature shows that the strength of the bond increases with increasing reaction layer. When a bonding temperature of 900 C is used, a eutectic liquid forms at the interface, thickness of the reaction layer. When a bonding temp◦ erature of 900 °C is used, a eutectic liquid forms as illustrated in Figure 15E, and solidifies isothermally in the subsequent stages of the bonding process. at the interface, as illustrated in Figure 15E, and solidifies isothermally in the subsequent stages of At temperatures below 900 C, solid-state bonding is achieved. the bonding process. At temperatures◦ below 900 °C, solid-state bonding is achieved.

FigureFigure 15. 15.A A schematic schematic of of thethe mechanismmechanism involved in in bo bondnd formation formation of of Ti–6Al–4V Ti–6Al–4V and and super-duplex super-duplex stainless steel using a Cu/Al2O3 interlayer: (A) Initial surface contact and asperity deformation; (B,C) stainless steel using a Cu/Al2O3 interlayer: (A) Initial surface contact and asperity deformation; Interdiffusion between interlayers and base metals; (D) Formation of the reaction layers; (E) Eutectic (B,C) Interdiﬀusion between interlayers and base metals; (D) Formation of the reaction layers; (E) Eutectic melting which occurs in samples bonded at 900 °C; (F) Isothermal solidification and homogenization melting which occurs in samples bonded at 900 ◦C; (F) Isothermal solidiﬁcation and homogenization of of the interface. the interface. 4. Conclusions In this study, Ti–6Al–4V alloy was diffusion bonded to super-duplex stainless steel (SDSS) using an electrodeposited Cu interlayer containing alumina nanoparticles to determine the effects of bonding parameters on the microstructural evolution within the joint region. • The results of the study showed that SDSS and Ti–6Al–4V can be successfully diffusion bonded using a Cu interlayer embedded with alumina nanoparticles at temperatures above 800 °C. • The combination of longer bonding time and high bonding temperature leads to the formation of various Ti–Cu intermetallic compounds within the interface. The use of a Cu interlayer prevented the formation of the more brittle intermetallic compounds of Fe–Ti and Ti–C. • The addition of Al2O3 nanoparticles was successful in restricting the growth of a continuous intermetallic layer and caused a change in the volume, size, and shape of the intermetallic compounds formed by pinning grain boundaries and restricting grain growth.

J. Manuf. Mater. Process. 2020, 4, 3 13 of 14

4. Conclusions In this study, Ti–6Al–4V alloy was diffusion bonded to super-duplex stainless steel (SDSS) using an electrodeposited Cu interlayer containing alumina nanoparticles to determine the effects of bonding parameters on the microstructural evolution within the joint region. The results of the study showed that SDSS and Ti–6Al–4V can be successfully diffusion bonded • using a Cu interlayer embedded with alumina nanoparticles at temperatures above 800 ◦C. The combination of longer bonding time and high bonding temperature leads to the formation of • various Ti–Cu intermetallic compounds within the interface. The use of a Cu interlayer prevented the formation of the more brittle intermetallic compounds of Fe–Ti and Ti–C. The addition of Al O nanoparticles was successful in restricting the growth of a continuous • 2 3 intermetallic layer and caused a change in the volume, size, and shape of the intermetallic compounds formed by pinning grain boundaries and restricting grain growth.

Author Contributions: Formal analysis, K.O.C. and M.A.S.; Funding acquisition, T.I.K.; Investigation, K.O.C., and A.R., and M.A.S.; Methodology, M.A.S.; Project administration, T.I.K.; Resources, T.I.K.; Supervision, T.I.K.; Writing—original draft, K.O.C.; Writing—review and editing, K.O.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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