Characterisation of Intermetallic Phases in Fusion Welded Commercially Pure Titanium and Stainless Steel 304

metals

Article Characterisation of Intermetallic Phases in Fusion Welded Commercially Pure Titanium and Stainless Steel 304

Timotius Pasang 1,*, Stevin Snellius Pramana 2 , Michael Kracum 3, Wojciech Zbigniew Misiolek 3 , Mona Aziziderouei 1, Masami Mizutani 4 and Osamu Kamiya 5

1 School of Engineering, Computer and Mathematical Sciences, Auckland University of Technology, Auckland 1020, New Zealand; [email protected] 2 School of Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK; [email protected] 3 Loewy Institute & Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015, USA; [email protected] (M.K.); [email protected] (W.Z.M.) 4 Joining and Welding Research Institute (JWRI), Osaka University, Osaka 567-0047, Japan; [email protected] 5 Faculty of Engineering Science, Department of System Design Engineering, Akita University, 1-1 Tegatagakuen-machi, Akita 010-8502, Japan; [email protected] * Correspondence: [email protected]

Received: 3 October 2018; Accepted: 19 October 2018; Published: 24 October 2018

Abstract: A series of trials to fusion weld commercially pure titanium (CPTi) to stainless steel 304 (SS304) have been conducted using laser beam welding (LBW) and gas tungsten arc welding (GTAW). Neither technique produced adequate weld joints with LBW showing a more promising result, while GTAW yielded separation of the workpieces immediately after welding. Cracking and fracturing took place mainly on the SS304 side, which was explained by the differences in the materials’ thermal properties. Various intermetallic phases formed during welding that were identiﬁed using energy dispersive X-ray spectroscopy (EDS) and electron backscattered diffraction (EBSD) technique and were compared with an isothermal ternary phase diagram of Fe-Cr-Ti. Their corresponding hardness values are reported and correlated with alloy compositions.

Keywords: laser beam welding; gas tungsten arc welding; commercially pure titanium; stainless steel 304; phase diagram; hardness

1. Introduction Dissimilar welding is increasingly popular in industrial applications for various reasons, such as weight reductions, functionalities, design compatibilities and cost reductions. Various manufacturing companies have taken advantage of dissimilar joints in automotive, aerospace, nuclear, smelter, construction, petrochemical, medical and micro-electronics industries. Challenges are the mismatch in mechanical properties, melting temperatures, crystal structures, solubility and, perhaps more importantly, the thermal properties such as thermal conductivity, thermal expansion and thermal diffusivity. It is known that fusion welding of titanium to stainless steel is very challenging without an interlayer [1]. This is often mitigated by incorporating brittle intermetallic compounds in the softer matrix phase [2,3]. Various attempts have been made in the past to join titanium and stainless steel. Most of the studies, however, have focused on the joint between Ti6Al4V and the stainless steel,

Metals 2018, 8, 863; doi:10.3390/met8110863 www.mdpi.com/journal/metals Metals 2018, 8, 863 2 of 12 e.g., SS304 or SS316. Some of the common methods to join dissimilar metals are (i) fusion welding, including electron beam and laser beam welding, and (ii) solid-state welding, such as diffusion bonding, explosive welding, friction welding and, more recently, friction stir welding. They are summarised as the following. Direct studies of CPTi and stainless steel welds were performed by Shanmugarajan and Padmanabham on autogenous (no filler added) welds as well as with interlayers such as vanadium (V) and tantalum (Ta) using a CO2 laser welding technique [4]. While the autogenous welds resulted in immediate fracture after welding, regardless of welding speed and beam position, the welds with interlayers did not show a significant strength improvement with a maximum of 44 MPa ultimate tensile strength (UTS) and a small degree of ductility obtained with Ta as an interlayer. Various phases were observed on the fracture surface including Fe0.2Ni4.8Ti5 (on both the CPTi and stainless steel sides), NiTi (on the Ti side) and Cr2Ti (on the stainless-steel side). The authors claimed that the thickness of the intermetallic layer was more than 10 µm thick at the location of the fracture; hence, it fractured in a brittle manner. However, no metallographic evidence was provided. In another study, Satoh et al. conducted laser welding of CPTi and SS316 and performed tensile tests on samples with 0, 150 and 300 µm offset to the Ti side from the Ti-SS interface; however, the strength was far lower than those of the base material [5]. The fracture surfaces were brittle. The authors presented metallographic evidence of the weld profile, energy dispersive X-ray spectroscopy (EDS) line scan of various areas, as well as electron backscattered diffraction (EBSD) and scanning electron micrographs (SEM). They suggested that the fracture occurred along the CPTi and SS interface weld pool. Based on their study, they also suggested that the upper weld pool (face surface) had a rich Ti content and the lower weld pool (closer to the root surface) had a low Ti content. Chen et al. studied the weldability of Ti6Al4V and stainless steel 201 by offsetting the laser beam to either side of the weld line [6]. They reported better weldability and a more durable weld when the beam was offset on the stainless-steel side than on the titanium side due to the higher thermal conductivity of stainless steel compared with titanium and resulting in a narrower weldment. The interfacial structures of the joint were FeAl + Ti/Fe2Ti + Ti5Fe17Cr5, FeAl + Ti/FeTi + Fe2Ti + Ti5Fe17Cr5 and FeAl + Ti. The presence of the α-Ti at all fracture surfaces may indicate that the joint fractured along the layer that contained α-Ti and FeTi. It was concluded that the fracture occurred between two adjacent intermetallic layers, i.e., FeTi + α-Ti and FeTi + Fe2Ti + Ti5Fe17Cr5. Akbarimousavi and GohariKia investigated dissimilar welding between CPTi and AISI 316L using continuous friction welding [7]. They reported the presence of various phases and intermetallic compounds such as FeTi, Fe2Ti, Fe2Ti4O, Cr2Ti and α-Ti at the interface, which reduced the tensile strength. They reported that hardness values increased sharply at the interface zone where the aforementioned phases were present. Ghosh and Chatterjee performed solid-state diffusion bonding of CPTi to stainless steel 304 and observed intermetallic phases such as σ, Fe2Ti, Cr2Ti and FeTi in the interface area [8,9]. The authors claimed that these phases were brittle and responsible for weakening the interface although no hardness values were provided. However, a tensile strength of around 225 MPa (vs. CPTi = 319 MPa and SS304 = 569 MPa) was achieved in this experiment. Other phases such as α-Fe, β-Ti and Fe2Ti4O were also present at the weld interface. Despite the above-mentioned attempts, there is still a lack of direct metallographic evidence of phases within the weld zone, particularly from that of fusion welding. In this study, autogenous welding of CPTi and SS304 was performed using two fairly different welding techniques with different welding speeds and magnitude of heat input. Gas tungsten arc welding (GTAW) and laser beam welding (LBW) were employed to compare the effect of welding speed, power density and cooling rates on the formation of undesired intermetallic phases with the assumption that GTAW is lower in all the above-mentioned welding parameters. Clear metallographic evidence such as phases present and their chemical compositions are elaborated and correlated with their hardness, grain structure and fracture surfaces. Metals 2018, 8, 863 3 of 12

2. Experimental Procedures

2.1. Materials The materials used in the reported investigation were commercially pure titanium (CPTi) (Grade 2 Ti) and stainless steel 304 (SS304). The thickness of the sheet used in this investigation was 1.6 mm. The nominal composition of the materials is given in Table1.

Table 1. Chemical composition of commercially pure titanium (CPTi) and stainless steel 304 (SS304) (wt.%).

Material Ti Fe H N O C Cr Ni Mn CPTi 99 <0.015 <0.03 <0.25 <0.1 - - - SS304 - 72 - - - - 18–20 8–10 <2.0 Note: for SS304: p < 0.045; Si < 1.0; S < 0.03.

2.2. Welding Procedures Two welding techniques were employed in this study, i.e., GTAW and LBW, both with the intention of full penetration along the centre line. GTAW was performed with a direct current electrode negative (DCEN) of 50 A, a voltage of 10 V coupled with argon gas shielding of 12–16 L/min with a travelling speed of around 0.3 m/min (5 mm/s). The tungsten electrode used in the experiment was a thoriated one with a diameter of 2.4 mm sharpened to 0.8–1 mm tip ﬂat. In comparison, the laser welding was conducted with a TruDisk 16002 machine (Trumpf, Ditzingen, Germany) with a power of 3 kW, the focus beam diameter at focal position of around 0.28 mm, travelling speed of 6 m/min (100 mm/s) with argon gas shield of 30 L/min. Laser welding was performed at the Joining and Welding Research Institute (JWRI) in Osaka University. In both cases, the length of the weldments was about 100 mm long to allow sufﬁcient sampling.

2.3. Metallography A number of cross-sections were cut from the joint materials for metallographic examination. They were cut perpendicular to the welding line and mounted using a conductive bakelite resin. The mounted samples were ground and polished down to 0.05 µm colloidal silica on polishing cloths. For backscattered electron images (BSEI) (Hitachi High Technologies, Schaumburg, IL, USA) and EDS analyses (EDAX Inc., Mahawah, NJ, USA), the samples were examined in the un-etched condition. However, for the light optical microscopy, the samples were etched with two different etchants, i.e., Kroll’s and Kalling’s reagents for CPTi and SS304, respectively. To improve the electron backscatter diffraction (EBSD) (EDAX Inc., Mahawah, NJ, USA) signal, an attack polish of 0.05 µm colloidal silica and 30% hydrogen peroxide was used for ﬁnal polishing before EBSD analysis.

2.4. Microhardness Hardness tests were conducted on the polished and etched samples using a micro-Vickers (Leco AMH 55, St. Joseph, MI, USA) with a load of 300 g for bulk hardness measurements, and with 25 g load for hardness measurements of phases, layers or other small/narrow features.

2.5. Fracture Surface Examination A scanning electron microscope (SEM), Hitachi SU70 (Hitachi High-Technologies Corporation, Minato-ku, Tokyo, Japan) was employed to study the fracture surfaces following ultrasonic cleaning for 10 min using ethanol.

2.6. Electron Microscopy and Grain Structure A Hitachi 4300SE FEG scanning electron microscope was used for EBSD analysis. An EDAX Hikari camera (EDAX Inc., Mahawah, NJ, USA) and OIM software (OIMDC 6/OIM Analysis 6, Metals 2018, 8, 863 4 of 12

Mahawah, NJ, USA) were used to collect patterns. Inverse pole figure (IPF) and phase maps were Metals 2018, 8, x FOR PEER REVIEW 4 of 12 produced from the scans and overlaid with Image Quality (IQ) grayscale maps. Mahawah, NJ, USA) were used to collect patterns. Inverse pole figure (IPF) and phase maps were 3. Resultsproduced and Discussionfrom the scans and overlaid with Image Quality (IQ) grayscale maps. Figure1 presents LBW and GTAW samples after welding. The LBW sample remained intact 3. Results and Discussion after welding. The width of the weldment was 1–2 mm (Figure1a). For the GTAW (Figure1b), Figure 1 presents LBW and GTAW samples after welding. The LBW sample remained intact cracking wasFigure observed 1 presents on theLBW weldment and GTAW joint samples line about after 10–20welding. mm The behind LBW thesample weld remained pool (GTAW intact torch) after welding. The width of the weldment was 1–2 mm (Figure 1a). For the GTAW (Figure 1b), while the torch was still in motion, and the whole sample was completely fractured shortly after the cracking was observed on the weldment joint line about 10–20 mm behind the weld pool (GTAW weldingtorch) process while hadthe torch been was completed. still in motion, Shanmugaran and the whole and sample Padmanabhan was completely [4] reported fractured similar shortly results with extensiveafter the welding cracking process on autogenous had been completed. welding Sh betweenanmugaran CPTi and and Padmanabhan SS304, and [4] the reported joint piecessimilar broke while beingresults removedwith extensive from cracking the welding on autogenous fixture following welding between laser welding. CPTi and Chen SS304, et and al. [the6] observedjoint pieces similar resultsbroke when while the being laser removed was placed from at the the welding centre, fixture and thefollowing fracture laser was welding. worse Chen when et al. it [6] was observed offset to the titaniumsimilar side. results Satoh when et al.the [ laser5] reported was placed a slightly at the centre, better and result the fracture where was the worse welded when samples it was offset remained to the titanium side. Satoh et al. [5] reported a slightly better result where the welded samples intact,to but the following titanium side. the tensile Satoh et test, al. the[5] strengthsreported a wereslightly fairly better low, result i.e., where between the 41welded and 81 samples MPa. remained intact, but following the tensile test, the strengths were fairly low, i.e., between 41 and 81 The metallographic cross-sections of both welds imaged using optical microscopy are presented MPa. in Figure2. The weld profiles from LBW and GTAW were completely different, having a fairly narrow The metallographic cross-sections of both welds imaged using optical microscopy are presented weld zonein Figure on the 2. The former weld and profiles a fairly from wide LBW on and the GTAW latter. Figurewere completely2a shows different, a poorly having mixed a fusion fairly zone (FZ) innarrow LBW withweld zone a very on narrow the former heat-affected and a fairly zonewide (HAZ).on the latter. The faceFigure surface 2a shows had a apoorly width mixed of around 1 mmfusion and became zone (FZ) narrower in LBW with towards a very the narrow root surface.heat-affected Figure zone2b (HAZ). indicates The theface fusion surface zone had a on width the CPTi side. Theof around HAZ 1 was mm fairlyand became wide narrower as indicated towards by the root white surface. arrows Figure and 2b the indicates dashed the line fusion separating zone it from theon the fusion CPTi zone. side. The Higher HAZ magnificationwas fairly wide opticalas indicated micrographs by the white of selectedarrows and areas the aredashed presented line in separating it from the fusion zone. Higher magnification optical micrographs of selected areas are Figuresseparating3 and4 . it Cracks from the were fusion present zone. atHigher the boundary magnification between optical FZ micrographs and HAZ orof baseselected metal areas (BM) are of the presented in Figures 3 and 4. Cracks were present at the boundary between FZ and HAZ or base SS304 side. Higher magnification of the crack indicated a smooth path when it propagated along the metal (BM) of the SS304 side. Higher magnification of the crack indicated a smooth path when it brightpropagated area and became along the fairly bright rugged area and along became the fairly dark, rugged dendritic along area. the dark, dendritic area.

FigureFigure 1. Optical 1. Optical macrographs macrographs showing showing commercially commercially pure pure titanium titanium (CPTi) (CPTi) and and stainless stainless steel steel 304 304 (SS304) interface(SS304) following interface welding following by welding (a) laser by beam (a) laser welding beam welding (LBW) and(LBW) (b )and gas (b tungsten) gas tungsten arc welding arc welding (GTAW) . (GTAW).

Figure 2. Low magniﬁcation optical micrographs showing weld proﬁles of (a) LBW and (b) GTAW.

The dashed white line on (b) has been added to illustrate the boundary between fusion and heat-affected zones. Note: insets show schematic welding process for LBW and GTAW. Metals 2018, 8, x FOR PEER REVIEW 5 of 12

Metals 2018Figure, 8, 863 2. Low magnification optical micrographs showing weld profiles of (a) LBW and (b) GTAW. 5 of 12 The dashed white line on (b) has been added to illustrate the boundary between fusion and heat- Metals affected2018, 8, x zones.FOR PEER Note: REVIEW insets show schematic welding process for LBW and GTAW. 5 of 12 Hardness measurements from selected areas of both LBW and GTAW are given in FiguresHardness3Hardnessb and4c measurementsmeasurements. Note the hardness fromfrom selected of the areas BM of of CPTiboth LBW was aroundand GTAW 300 are HV givengiven and aroundinin FiguresFigures 210 3b3b HV forandand SS304. 4c.4b. Note HardnessNote thethe hardnesshardness values on ofof thethethe weldmentsBMBM ofof CPTiCPTi variedwas around signiﬁcantly. 300 HV Forand LBW,around the 210210 hardness HVHV forfor ofSS304.SS304. the FZ Hardness values on the weldments varied significantly. For LBW, the hardness of the FZ reached a reachedHardness a value values of on up the to 1660weldments HV in varied the bright signific areasantly. (Figure For LBW,3b). the Manikandan hardness of et the al. FZ [ 10 reached] reported a value of up to 1660 HV in the bright areas (Figure 3b). Manikandan et al. [10] reported hardness hardnessvalue of values up to of1660 up HV to 1392 in the HV bright in the areas FZ following (Figure 3b). their Manikandan explosive weldinget al. [10] of reported titanium/stainless hardness values of up to 1392 HV in the FZ following their explosive welding of titanium/stainless steel, while steel,values while of up others to 1392 reported HV in the hardness FZ following values their of less explosive than 1000 welding HV in of thetitanium/stainless interface [3,7]. steel, At the while small others reported hardness values of less than 1000 HV in the interface [3,7]. At the small area (shown areaothers (shown reported in black hardness box) of values the GTAW of less sample,than 1000 the HV hardness in the interface measured [3,7]. at CPTiAt the was small between area (shown 520 and inin blackblack box)box) ofof thethe GTAWGTAW sample,sample, thethe hardnesshardness measured at CPTi waswas betweenbetween 520520 andand 545545 HVHV 545 HV (Figure4c). This was comparable to the hardness of the bright layer adjacent to the CPTi with (Figure(Figure 4c).4b). ThisThis waswas comparablecomparable toto thethe hardnesshardness of the bright layer adjacent to thethe CPTiCPTi withwith aa a hardness of 535 HV. This value was signiﬁcantly different from the Ti base metal (BM) found in the hardnesshardness ofof 535535 HV.HV. ThisThis valuevalue was significantly different from the Ti base metal (BM)(BM) foundfound inin thethe LBW sample (~300 HV); hence, this area was the heat-affected zone. This hardness value of the Ti LBWLBW sample sample (~300(~300 HV);HV); hence,hence, thisthis area was the heat-affected zone. This hardnesshardness valuevalue ofof thethe TiTi BMBM BM was not found in the current study of this GTAW sample (Figure4c). It was again noted that the waswas notnot foundfound inin thethe currentcurrent study of this GTAW sample (Figure 4b).4c). It was again notednoted thatthat thethe highesthighesthighest hardness hardnesshardness in inin the thethe GTAW GTAWGTAW sample samplesample was was in in the the brightbright phase,phase, i.e., upup toto to 14201420 1420 HVHV HV (Figure(Figure (Figure 4b).4c).4c).

FigureFigure 3. Optical3. Optical micrographs micrographs of of LBW LBW showing showing (a )(a a) distincta distinct solidiﬁcation solidification pattern pattern with with the the dashed dashed line Figure 3. Optical micrographs of LBW showing (a) a distinct solidification pattern with the dashed boxline indicating box indicating the area the of area interest of interest for hardness for hardne measurements,ss measurements, chemical chemical composition composition analysis analysis and and phase line box indicating the area of interest for hardness measurements, chemical composition analysis and identiﬁcation,phase identification, and (b) the and hardness (b) the distribution hardness distribution within the dashedwithin linethe boxdashed area. line The box compositions area. The of phase identification, and (b) the hardness distribution within the dashed line box area. The labelscompositions A-O are overlaid of labels in A-O the are phase overlaid diagram in the (Figure phase diagram 6) [11]. (Figure 6) [11]. compositions of labels A-O are overlaid in the phase diagram (Figure 6) [11].

Figure 4. Optical micrographs of GTAW showing (a) low magniﬁcation and (b) higher magniﬁcation Figure 4. Optical micrographs of GTAW showing (a) low magnification and (b) higher magnification

of theof the “weld “weld zone” zone” with with the the dashed dashed lineline box indicati indicatingng the the area area of of interest interest for for hardness hardness measurements, measurements, chemicalchemical composition composition analysis analysis and and phasephase identiﬁcation,identification, and and ( (cc)) the the hardness hardness distribution distribution within within the the dasheddashed line line box box area. area. Note Note that that hardness hardness values values around around the the cracks cracks are are comparable comparable toto thosethose adjacentadjacent to theto fracture the fracture surface surface area, area, i.e., i.e., 970 970 to 1300 to 1300 HV. HV.

Metals 2018, 8, x FOR PEER REVIEW 6 of 12

Figure 4. Optical micrographs of GTAW showing (a) the “weld zone” with the dashed line box indicating the area of interest for hardness measurements, chemical composition analysis and phase identification, and (b) the hardness distribution within the dashed line box area. Note that hardness

Metalsvalues2018, 8 ,around 863 the cracks are comparable to those adjacent to the fracture surface area, i.e., 970 to 13006 of 12 HV.

The backscattered electron image together with the chemical compositions across the FZ of the LBW sample is presented in Figure 55a,b,a,b, respectively.respectively. InIn addition,addition, thethe hardnesshardness valuesvalues fromfrom FigureFigure3 3 are correlated with with the the measured measured chemical chemical compositio compositionsns at atdifferent different locations locations (Table (Table 2).2 Based). Based on onthe isothermalthe isothermal ternary ternary phase phase diagram diagram of Fe-Cr-Ti of Fe-Cr-Ti at 550 °C at [10], 550 ◦theC[ possible10], the phases possible formed phases are formed identified are inidentified Figure in6. FigureThe nickel6. The concentration nickel concentration was no wast considered not considered when whenidentifying identifying the phases. the phases. The Theintermetallic intermetallic phases phases formed formed closest closest to the to Ti the BM Ti (point BM (point B in BFigure in Figure 3) were3) were both both cubic cubic FeTi FeTi and Feandy(Ti Fe1−xyCr(Tix1)− withxCrx )space with group space Pm group3m andPm 3Imm3andm, respectively.Im3m, respectively. The band Theof high band hardness of high (1650 hardness HV) (1650was identified HV) was as identified hexagonal as P hexagonal63/mmc (FeP16−x3Cr/mmcx)2Ti (Fe(Point1−x CrC).x )In2Ti their (Point work, C). InManikandan their work, et Manikandan al. reported theet al. highest reported hardness the highest values hardness belonged values to FeTi belonged and Fe2Ti to [10]. FeTi Further and Fe away2Ti [10 from]. Further the Ti BM, away the from Ti at.% the Tiincreased BM, the creating Ti at.% increasedFeTi and creating(Fe1−xCrx FeTi)2Ti with and (Feslightly1−xCr xlower)2Ti with hardness. slightly A lower soft hardness.dendritic region A soft containingdendritic region ~53 at.% containing Ti, 8 at.% ~53 Cr, at.% 34 Ti,at.% 8 at.%Fe and Cr, 5 34 at.% at.% Ni Fe (points and 5 at.%E and Ni F) (points was identified E and F) as was a combinationidentified as aof combination three phases—FeTi, of three phases—FeTi, Fey(Ti1−xCrx) Feandy(Ti (Fe1−1x−CrxCrxx))2 andTi, with (Fe1− xaCr hardnessx)2Ti, with of a~700 hardness HV. Anotherof ~700 HV. unique Another intermetallic unique intermetallic phase of Ti5 phaseFe17Cr of7 (hexagonal, Ti5Fe17Cr7 (hexagonal,P63/mmm) wasP63 /foundmmm )at was position found atG positiontogether Gwith together (Fe1−xCr withx)2Ti (Fe when1−xCr thex)2Ti Ti when content the decreased Ti content decreasedwith increasing with increasingFe, consistent Fe, consistent with the withreport the by report Chen by et Chenal. [6]. et al.The [6 ].hardest The hardest interm intermetallicetallic phase phase (>1000 (>1000 HV) HV)was wasdetermined determined to be to (Febe1 (Fe−xCr1−x)x2CrTi,x while)2Ti, while a mixed a mixed phase phaseof FeTi of and FeTi Fe andy(Ti1 Fe−xCry(Tix) 1crystallised−xCrx) crystallised with a hardness with a hardness of 600–900 of HV.600–900 The HV.correlation The correlation between between hardness hardness and compositio and compositionn for the LBW for the sample LBW sampleis presented is presented in Figure in 7,Figure where7, where500 < 500HV << HV1000 < 1000was wasthe region the region where where FeTi FeTi + Fe +y Fe(Tiy1(Ti−xCr1−xx) Crformed,x) formed, while while (Fe1 (Fe−xCr1−x)x2CrTi xwas)2Ti wasfound found for hardness for hardness >1000 >1000 HV. HV.

Figure 5. (a) Backscattered electron image and (b) chemical composition (at.%) of the area along the scanned line of LBW weld proﬁles. Metals 2018, 8, x FOR PEER REVIEW 7 of 12

Figure 5. (a) Backscattered electron image and (b) chemical composition (at.%) of the area along the scanned line of LBW weld profiles.

Metals 2018Table, 8, 863 2. Correlation between hardness, chemical composition and possible phases across the LBW7 of 12 profile. The label A-O corresponds to Figure 3, and the corresponding chemical composition is overlaid in Figure 6 [11]. Table 2. Correlation between hardness, chemical composition and possible phases across the LBW proﬁle. TheLocation label A-O correspondsHV to Figure Possible3, and the Ph correspondingases chemicalTi compositionCr Fe is overlaidNi in Figure6[11A]. 298; 302 αTi 99.9 0.0 0.1 0.0 B 870 FeTi+Fey(Ti1−xCrx) 68.8 5.5 22.6 3.0 Location HV Possible Phases Ti Cr Fe Ni C 1650 (Fe1−xCrx)2Ti 29.4 13.1 50.8 6.8 A 298;D 302 1240 FeTi+(FeαTi1−xCrx)2Ti 99.9 37.5 11.2 0.0 45.2 0.1 6.2 0.0 B 870 FeTi + Fe (Ti Cr ) 68.8 5.5 22.6 3.0 E 645 FeTi + Fey(Tiy 1−1x−Crx x) x+ (Fe1−xCrx)2Ti 52.7 8.2 33.9 5.2 C 1650 (Fe1−xCrx)2Ti 29.4 13.1 50.8 6.8 F 710 FeTi + Fey(Ti1−xCrx) + (Fe1−xCrx)2Ti 54.8 7.8 32.5 4.9 D 1240 FeTi + (Fe1−xCrx)2Ti 37.5 11.2 45.2 6.2 1−x x 2 5 17 7 EG 645 946 FeTi + Fey(Ti (Fe1−xCrCrx)) +Ti+Ti (Fe1−xFeCrx)Cr2Ti 52.7 24.1 15.0 8.2 54.2 33.9 6.7 5.2 FH 710 870 FeTi + FeTi Fey(Ti + 1Fe−xyCr(Tix)1− +xCr (Fex)1 −+ x(FeCrx1)−2xTiCrx)2Ti 54.850.4 9.4 7.8 34.9 32.5 5.3 4.9 GI 9461370 (Fe1−xCrx)2Ti (Fe +1 Ti−x5CrFex17)2CrTi 7 24.1 31.6 13.8 15.0 48.6 54.2 6.0 6.7 H 870 FeTi + Fey(Ti1−xCrx) + (Fe1−xCrx)2Ti 50.4 9.4 34.9 5.3 J 895 FeTi + Fey(Ti1−xCrx) + (Fe1−xCrx)2Ti 57.9 6.6 31.7 3.9 I 1370 (Fe1−xCrx)2Ti 31.6 13.8 48.6 6.0 K 920 FeTi + Fey(Ti1−xCrx) + (Fe1−xCrx)2Ti 58.0 7.4 29.6 5.0 J 895 FeTi + Fey(Ti1−xCrx) + (Fe1−xCrx)2Ti 57.9 6.6 31.7 3.9 KL 920 1660 FeTi + Fey(Ti1− FeTi+(FexCrx) + (Fe1−x1Cr−xxCr)2Tix) 2Ti 58.0 33.5 11.9 7.4 47.4 29.6 7.2 5.0 LM 1660 660 FeTi FeTi + +Fe (Fey(Ti1−1x−xCrCrxx))2 Ti+ (Fe1−xCrx)2Ti 33.558.6 11.9 7.1 29.2 47.4 5.1 7.2 MN 660 300 FeTi + Fey(Ti1−xCrx SS) + + (Fe FeTi1−x Crx)2Ti 58.6 32.1 12.4 7.1 49.6 29.2 5.9 5.1 N 300 SS + FeTi 32.1 12.4 49.6 5.9 O 212;O 210 212; 210 SS SS 0.1 0.1 18.7 18.7 72.1 72.1 9.2 9.2

Figure 6. Isothermal (550 ◦C) ternary phase diagram of Fe-Cr-Ti adapted from [11] where labels B-O Figure 6. Isothermal (550 °C) ternary phase diagram of Fe-Cr-Ti adapted from [11] where labels B-O correspond to the positions in Figure3. correspond to the positions in Figure 3.

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Figure 7. Variation of hardness withwith compositioncomposition forfor thethe LBWLBW sample.sample.

The high hardness of of the the bright bright areas areas (1200–1660 (1200–1660 HV HV)) suggests suggests they they are are the the most most brittle brittle areas areas of ofthe the weld weld zone. zone. Despite Despite the the presumed presumed brittleness brittleness of of the the bright bright phase, phase, cracking cracking (or fracturing) took placeplace betweenbetween thisthis hardhard phasephase andand thethe softersofter SS304SS304 side.side. To identifyidentify thethe phasesphases presentpresent inin thethe weldweld zonezone andand theirtheir correspondingcorresponding grain orientations, electron backscattered diffraction diffraction (EBSD) (EBSD) was was perfor performed.med. EBSD EBSD analysis analysis considered considered only only binary binary Fe- Fe-TiTi compounds compounds assuming assuming Cr Crand and Ni Ni are are solid solid substitutions substitutions for for either either Fe Fe or or Ti. Ti. As As described described earlier, earlier, FeTi andand FeFeyy(Ti(Ti11−−xxCrCrxx) )crystallised crystallised in in the samesame cubiccubic crystalcrystal system,system, while while Fe Fe2Ti,2Ti, (Fe (Fe1−1x−xCrCrxx)2Ti andand TiTi55Fe17CrCr77 werewere hexagonal. hexagonal. In In addition, addition, only only a limited a limited number number of references of references have have reported reported ternary ternary Fe- Fe-Cr-TiCr-Ti compounds. compounds. Crystallographic Crystallographic information information taken taken from from Yan Yan et et al. al. [12] [12] (Inorganic CrystalCrystal Structure Database (ICSD) code 155674) and Thompson et al. [[13]13] (ICSD(ICSD codecode 633925)633925) werewere usedused toto indexindex PP633/mmcmmc FeFe2Ti2Ti and and PmPm33mm FeTi,FeTi, respectively. respectively. On On the the CPTi CPTi side, it can be observed that a FeTi phasephase formedformed along the interface of both LBW and GTAW weldmentsweldments whilewhile onon thethe SS304SS304 side,side, thethe boundary consisted of mixed phases. The FeTi layerlayer adjacent to CPTi on LBWLBW waswas thinnerthinner andand thethe grains much smaller than those observed on GTAW (Figures(Figures8 8 and and9 ).9). More More elongated elongated grains grains were were generally observedobserved forfor thethe FeFe22Ti phase. Upon etching,etching, aa range range of of dendrite dendrite sizes sizes was was observed. observed. In In the the FZ FZ of bothof both welds welds (Figures (Figures 10 and 10 and11), two11), cleartwo areasclear wereareas evident, were evident, i.e., one showingi.e., one ashowing high density a high of dendriticdensity of phase, dendritic and a plain/smoothphase, and a area.plain/smooth The grain area. structure The grain in these structure two typesin these of weldstwo types showed of welds some showed differences. some Largedifferences. grains Large were observedgrains were in observed the CPTi regionin the CPTi of the region GTAW of weld,the GT likelyAW weld, due to likely the high due to heat the input. high heat At the input. boundary At the interface,boundary the interface, FeTi phase the FeTi on the phase LBW on sample the LBW was sample fairly narrowwas fairly and narrow had a smallerand had average a smaller grain average size, butgrain on size, GTAW but wason GTAW about tenwas times about thicker ten times and thicker had larger and grains.had larger grains.

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FigureFigure 8. Electron8. Electron backscattered backscattered diffraction diffraction (EBSD) (EBSD) phase phase map map with imagewith image quality quality (IQ) overlay (IQ) overlay showing Figure 8. Electron backscattered diffraction (EBSD) phase map with image quality (IQ) overlay Figureshowing 8. intermetallicsElectron backscattered present in diffractionthe CPTi/SS304 (EBSD) weld phase zone. map Scan with step image sizeµ isquality 1.5 µm. (IQ) The overlay EBSD intermetallicsshowing intermetallics present in thepresent CPTi/SS304 in the CPTi/SS304 weld zone. weld Scan zone. step Scan size step is 1.5 sizem. is The 1.5 EBSDµm. The patterns EBSD of FeTishowingpatterns and Fe ofintermetallicsTi FeTi are and also Fe shown.2 Tipresent are also in shown.the CPTi/SS304 weld zone. Scan step size is 1.5 µm. The EBSD patterns of2 FeTi and Fe2Ti are also shown. patterns of FeTi and Fe2Ti are also shown.

Figure 9. Corresponding EBSD inverse pole figure (IPF) maps with IQ overlay from Figure 8. FigureFigure 9. 9.Corresponding CorrespondingEBSD EBSD inverseinverse pole figure ﬁgure (IPF) (IPF) maps maps with with IQ IQ overlay overlay from from Figure Figure 8. 8. Figure 9. Corresponding EBSD inverse pole figure (IPF) maps with IQ overlay from Figure 8.

Figure 10. SEM image of LBW showing the fusion zone (FZ) and cracking on the SS304 side where Figure 10. SEM image of LBW showing the fusion zone (FZ) and cracking on the SS304 side where FigureFigure(A) showing 10. 10.SEM SEM extensive image image of mainlyof LBW LBW FeTi showing showing dendrite thethe phase, fusionfusion and zone (B )(FZ) (FZ) showing and and cracking a cracking fairly smooth on on the the (FeSS304 SS3041–xCr sidex) side2Ti where patch where (A) showing extensive mainly FeTi dendrite phase, and (B) showing a fairly smooth (Fe1–xCrx)2Ti patch (A)(surroundedA showing) showing extensive byextensive mainly mainly mainlyFeTi dendrites. FeTi FeTi dendrite dendrite The inset phase, phase, shows and and (theB) (showing Bgeneral) showing LBWa fairly a profile. fairly smooth smooth (Fe1–xCr (Fex)2Ti patchCr ) Ti surrounded by mainly FeTi dendrites. The inset shows the general LBW profile. 1−x x 2 surrounded by mainly FeTi dendrites. The inset shows the general LBW profile. patch surrounded by mainly FeTi dendrites. The inset shows the general LBW proﬁle.

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FigureFigure 11. 11.SEM SEM images images of of GTAW GTAW showing showing ((aa)) thethe fusion zone zone and and fracture fracture locations locations on on the the SS304 SS304 side, side, andand (b )(b intermetallic) intermetallic compounds compounds including including aa layerlayer adjacentadjacent to to the the CPTi. CPTi. Figure 11. SEM images of GTAW showing (a) the fusion zone and fracture locations on the SS304 side, ForFor GTAWand GTAW (b) intermetallic welds, welds, the the compounds fracture fracture occurred includingoccurred amainly layermainly adjacent along to thethe CPTi.centreline centreline of of the the weld. weld. SEM SEM images images showingshowing fracture fracture surfaces surfaces are are given given inin FigureFigure 1212.. It can be be seen seen that that the the fracture fracture surface surface was was almost almost For GTAW welds, the fracture occurred mainly along the centreline of the weld. SEM images entirelyentirely cleavage, cleavage, indicating indicating a brittlea brittle weld weld joint. joint. A A similar similar observation observation was was reported reported by by Satoh Satoh etet al.al. [5] showing fracture surfaces are given in Figure 12. It can be seen that the fracture surface was almost as[5] well as aswell by as Chen by Chen et al. et [6 al.] where [6] where brittle brittle characteristics characteristics were were observed observed along along with with river river patterns. patterns. As As entirelymentioned cleavage, earlier, indicating cracking a brittle was weld taking joint. place A similar during observation welding, was and reported fracturing by Satoh occurred et al. mentioned[5] as well earlier, as by cracking Chen et al. was [6] takingwhere brittle place char duringacteristics welding, were and observed fracturing along occurredwith river immediatelypatterns. immediately after welding. For LBW, the welded pieces were intact after welding and survived afterAs welding. mentioned For earlier, LBW, thecracking welded was pieces taking were place intact during after welding, welding and and fracturing survived occurred throughout throughout wire cutting for the preparation of the dog-bone samples, but completely separated prior wire cuttingimmediately for the after preparation welding. For of LBW, the dog-bone the welded samples, pieces were but completely intact after separatedwelding and prior survived to tensile to tensile testing. Therefore, tensile testing was not performed. However, fracture surfaces were testing.throughout Therefore, wire tensile cutting testing for the waspreparation not performed. of the dog-bone However, samples, fracture but completely surfaces separated were available prior for available for examination. Fractures took place at the weld zone on the SS304 side (also reported by examination.to tensile Fractures testing. Therefore, took place tensile at the testing weld zonewas not on theperformed. SS304 side However, (also reported fracture bysurfaces Satoh were et al. [5]). Satoh et al. [5]). Figure 3a also shows cracking on the weld zone at the SS304 side. Upon examination Figureavailable3a also showsfor examination. cracking Fractures on the weld took zoneplace at the weld SS304 zone side. on Uponthe SS304 examination side (also reported of the fracture by of the fracture surfaces, one can see a mixture of fracture mechanisms from cleavage to fairly rugged surfaces,Satoh one et al. can [5]). see Figure a mixture 3a also of shows fracture cracking mechanisms on the weld from zone cleavage at the SS304 to fairly side. Upon rugged examination features. One features.of the fractureOne of thesurfaces, reasons one as can to whysee a crackingmixture of and fracture or fracturing mechanisms took from place cleavage in the SS304 to fairly side rugged may be of the reasons as to why cracking and or fracturing took place in the SS304 side may be related to relatedfeatures. to the One thermal of the properties.reasons as to Table why 3cracking provides and the or thermal fracturing properties took place of CPTiin the and SS304 SS304 side includingmay be the thermal properties. Table3 provides the thermal properties of CPTi and SS304 including thermal thermalrelated conductivity to the thermal and properties. the thermal Table 3expansion provides the coefficient. thermal properties The therma of CPTil expansion and SS304 coefficient— including conductivitywhichthermal relates andconductivity the the change thermal and in volume expansionthe thermal due coefﬁcient.to expansion a change coefficient. Thein temperature—of thermal The expansion therma stainlessl expansion coefﬁcient—which steel iscoefficient— twice as high relates theas change titanium,which relates in volumesuggesting the change due that to in athermal volume change duestress in temperature—ofto isa changemore easily in temperature—of built stainless up on steelthe stainless SS304 is twice steelside. as is hightwice asas titanium,high suggestingas titanium, that thermalsuggesting stress that isthermal more easilystress is built more up easily onthe built SS304 up on side. the SS304 side.

FigureFigureFigure 12. 12.SEM 12. SEM SEM images images images showing showing showing thethe the fracturefracture fracture surfacesurface of of ( (aa––ccc)) ) LBWLBW LBW andand and ( d(d (–d–f)f– )GTAWf )GTAW GTAW samples. samples. samples. Gas Gas Gas poresporespores are are visible are visible visible on on (ond ();d (); and );an extensivean extensive extensive cleavage cleavage cleavage fracture fracturefracture is shown shown inin in ((e (e,ef,f);,f); );porosities porosities porosities due due due to to incomplete to incomplete incomplete solidiﬁcationsolidificationsolidification are are evidentare evident evident in in (inc (,c f(,).cf,).f).

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Table 3. Thermal properties of CPTi and SS304 [14,15].

Properties CPTi SS304 Density (g/cm3) 4.5 7.8 Melting point (◦C) 1665 1400–1455 Thermal conductivity (W m−1k−1) 16–22 14–19 Thermal expansion coefﬁcient (m/m/◦C) 9 × 10−6 16 × 10−6 Thermal diffusivity (m2/s) 6.9 × 10−6 4.2 × 10−6 Speciﬁc heat capacity (J/g-◦C) 0.523–0.69 0.500

4. Conclusions Based on the experimental results, metallographic observations, phase identiﬁcations and fractography examination, the following points are concluded:

1. Neither LBW nor GTAW was able to produce a sound weld joint of CPTi and SS304. As a joint was nearly made using LBW, this will be the focus for future investigations. The samples from GTAW separated immediately after welding was completed. 2. Fractures were observed mainly on the SS304 side and are hypothesised to be due to the higher thermal expansion coefficient of SS304 compared to CPTi. 3. The fracture surface was predominantly covered by brittle fracture features, e.g., cleavage and fairly flat. 4. Hardness was found to have increased significantly from 210–300 HV in the base metal to 1660 HV and 1420 HV in the weld zones in LBW and GTAW, respectively.

5. The hardest phase was identiﬁed to be (Fe1−xCrx)2Ti with hardness values up to 1660 HV observed.

Author Contributions: T.P. initiated the project, designed the experiments, and prepared the original draft; M.M. performed welding; M.A. conducted metallography and scanning electron microscopy; S.S.P. conducted phase identification and revised the paper; M.K. performed EBSD and edited the paper; W.Z.M. oversaw the entire project and provided direction; O.K. initiated and conceived the project with T.P. Funding: This research received no external funding. Acknowledgments: One of the authors (T.P.) would like thank Lehigh University for an appointment as a Loewy Visiting Professor financed through the endowment from the Loewy Family Foundation. The authors also thank Manabu Tanaka of JWRI, Osaka University for the use of their laser welding facilities. Conflicts of Interest: The authors declare no conflict of interest.

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