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Journal of Materials Processing Technology 214 (2014) 2884–2890

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

jo ur nal home p ag e: www.elsev ier .com/ locate / jmatprotec

teel to titanium solid state joining displaying superior mechanicalroperties

slan Miriyev, David Barlam, Roni Shneck, Adin Stern, Nachum Frage ∗

en-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 8410501, Israel

r t i c l e i n f o

rticle history:eceived 22 March 2014eceived in revised form 29 June 2014ccepted 30 June 2014vailable online 8 July 2014

a b s t r a c t

Joint configuration with Ti inserts, which allows the increase of the bonded area of similar materialsat the expense of that of dissimilar ones, is suggested. The force to failure for the specimens with thesuggested configuration was about two fold higher than that for the specimens with the planar interface.Microstructural analysis of the joints, fracture characterization, mechanical testing and finite elementssimulation were conducted in order to elucidate the mechanical behavior of the suggested joint config-uration. Failure of joints with Ti-inserts is governed by the brittle behavior of TiC and �-Ti layers, located

eywords:teelitanium alloyoining

icrostructureracture analysis

at the steel–Ti interface, with cracks initiation within the interfacial layer. The cracks propagate first intothe brittle �-Ti phase and then cause the fracture in the Ti/Ti bond. The results of finite elements analysisare in good agreement with the analysis of the fracture surfaces.

© 2014 Elsevier B.V. All rights reserved.

echanical properties

. Introduction

Joining dissimilar metals by fusion welding techniques mayesult in the formation of brittle intermetallic phases, causing low-uality welds. Solid state welding (SSW) processes are generallyonsidered as an appropriate joining approach for overcoming theroblem of substantial intermetallic phase formation. SSW enables

oining of both dissimilar and similar materials, providing thepportunity to use lighter, stronger and cheaper material com-inations. A very attractive approach for joining relatively largeurfaces by SSW involves using structural transition joints (STJ).uch STJ serve as an intermediate chain between two dissimi-ar metals, as shown schematically in Fig. 1. Trethewey and Little1989) reported that this approach is widely used in shipbuildingndustry for steel/aluminum joining.

Titanium-to-steel joints are extensively used in applicationshere weight saving is important. In their review, Yefei et al.

2012) summarize SSW of titanium alloys to steels over the periodf last ten years. The primary SSW techniques include diffusiononding (DB), explosion welding (EXW) and friction stir welding

FSW), which are used in transportation, power generation, desali-ation plants, in chemical and petrochemical industry, and others.ato et al. (1997) reported on interfacial structures and mechanical

∗ Corresponding author. Tel.: +972 8 6461468.E-mail addresses: nfrage@bgu.ac.il, nfrage@gmail.com (N. Frage).

ttp://dx.doi.org/10.1016/j.jmatprotec.2014.06.026924-0136/© 2014 Elsevier B.V. All rights reserved.

properties of diffusion bonded steel–Ni and steel–Ti. Ghosh andChatterjee (2002) reported on solid-state direct diffusion bondingbetween commercially pure titanium and austenitic stainless steel.It was reported that titanium can be successfully joined by DB tostainless steels (Kundu and Chatterjee, 2008), low alloy steels andhard wearing steels with and without the use of interlayers (Ni,Cu, V) (Church and Wild, 1998; Elrefaey and Tillmann, 2009). Chibaet al. (1995) and Song et al. (2011) have reported that EXW hasproved valuable in many industries as a technology for joining tita-nium to different alloys and for fabrication of STJs. Liao et al. (2010)reported that FSW technique was used for lap joining of pure tita-nium to low-alloy steel. However, the reported tensile strength ofthe assemblies was limited to less than 300 MPa by the presence ofintermetallic compounds at the joint interface.

Recently, the Spark Plasma Sintering (SPS) technique has gainedthe attention of the welding community (Nakamura et al., 2005;He et al., 2012). The mechanism of SPS joining of Ti alloy to low-alloy carbon steel by a thin titanium carbide layer formed at theinterface was reported and discussed recently (Miriyev et al., 2013).The assemblies were fabricated by lap joining of Ti–6Al–4V andAISI4330 steel disks. The average tensile strength of the joint wasabout 250 MPa and was governed by the titanium carbide layer atthe joint interface. However, this relatively low tensile strength is

not satisfactory for a wide range of industrial applications.

The mechanical properties of joints may be improved by spe-cially designed joint configurations, namely by increasing thecontact area of similar metals (Ti–Ti, in the present case) at expense

A. Miriyev et al. / Journal of Materials Process

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Fig. 1. STJ approach for steel to Al bonding.

f the Ti–steel contact area. The present work focuses on the effectf the configuration of the joints on their mechanical properties,nd includes microstructure analysis of the joints, fracture charac-erization, mechanical testing and finite elements simulation.

. Experimental

.1. Joint configuration

Recently, the AISI 4330 steel (ST)–Ti–6Al–4V (TA) joints with planar interface (PLJ) were investigated (Miriyev et al., 2013)Fig. 2a and c). In order to improve mechanical properties of theoint, a joint configuration with TA insert (TIJ), which allows toncrease bonded area of similar materials at the expense of dis-imilar ones, was suggested (Fig. 2b).

Moreover, in such configuration, mechanical locking of theoined materials may be achieved. After joining, the STJ consistsf two types of metallurgical bonded interfaces: ST–TA and TA–TA.he latter provides continuous bonding between the TA-part andhe TA-insert (Fig. 2d).

.2. Materials

Discs (30 mm diameter) were water-jet cut from AISI4330 steelnd Ti–6Al–4V alloy, received in the form of 6 mm and 10 mmlates, respectively. TA inserts were cut from Ti–6Al–4V 21 mm

Fig. 2. Schematic cross-section view of joint configurations b

ing Technology 214 (2014) 2884–2890 2885

diameter rod and machined to a conic shape. The chemical com-positions of ST and TA are listed in Table 1.

2.3. Joining procedure

The mating surfaces of the samples were prepared by a conven-tional metallographic technique with 1 �m diamond paste finish,cleaned with acetone and air-dried. Two types of assemblies wereprepared according to the schematics depicted in Fig. 2a and b.

Each assembly was placed in a graphite die with 30 mm innerand 50 mm outer diameter, and was inserted in a SPS machineequipped with a 50-kN uniaxial press, type HP D5/1 (FCT System,Rauenstein, Germany). Bonding process was carried out at 950 ◦Cfor 3.6 ks under argon atmosphere (10−2 Torr). Uniaxial pressureof 35 MPa was applied and the pulse-mode DC current (pulse 5 msand pause 2 ms) was set.

2.4. Characterization

The bonded specimens were cross-sectioned and preparedfor microstructural analysis using an optical microscope (Zeiss,Germany) and High Resolution Scanning Electron Microscope(HRSEM, JEOL JSM 7400F, Japan), combined with an electron dis-persive spectrometer (EDS) detector. The mechanical properties ofthe similar (ST-ST, TA–TA) and dissimilar (ST–TA) materials jointswere evaluated using the ram tensile test technique according toMIL-J-24445A (ASTM B898-11) standard. The test specimens andram tensile test setup for PLJ and TIJ are shown in Fig. 3.

2.5. Finite elements simulation

Finite elements (FE) analysis was conducted using MSC.MARCsoftware. A 2D model is sufficient to describe the ram-tensile test,

taking into account the axial symmetry of the experiment (Fig. 4band d). The ram and die are simulated by round rigid surfaces. 2Dquad elements were used (Fig. 4e) with 14,120 and 25,250 ele-ments in the model of PLJ and in the model of TIJ, respectively.

efore and after joining, respectively: (a, c) PLJ; (b, d) TIJ.

2886 A. Miriyev et al. / Journal of Materials Processing Technology 214 (2014) 2884–2890

Table 1Chemical composition of steel and Ti-alloy (wt.%).

Element/material C Mn Ni Cr Mo V Ti Al Fe

AISI 4330 (ST) 0.28 0.23 1.31 1.11 0.41 0.06 0.002 0.03 BalanceTi–6Al–4V (TA) 0.01 – – – – 4.2 Balance 6.2 0.07

Fig. 3. Ram tensile test specimen and test setup. (a) Ram tensile specimen for the PLJ; joint area is circled; (b) close-up of the joint area in the PLJ; (c) ram tensile test setupfor PLJ; (d) ram tensile specimen for TIJ; joint area is circled; (e) close-up of the joint area in the TIJ; and (f) ram tensile test setup for the TIJ.

Fig. 4. The FE models of the tests. (a) Ram tensile specimen of the PLJ, rotated counterclockwise by 90◦; (b) model of the PLJ specimen part marked in (a); (c) ram tensilespecimen of the TIJ, rotated counterclockwise by 90◦; (d) model of general view of the TIJ specimen part marked in (c), ST region of the specimen is circled in red; (e) theelements in the vicinity of the ST part, circled in (d). The thickness of the TiC layer along the border is exaggerated by a factor of 10. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of the article.)

A. Miriyev et al. / Journal of Materials Processing Technology 214 (2014) 2884–2890 2887

Fig. 5. Optical microscope images of the bond zone of the PLJ. (a) ST side (etched with Nital solution) and (b) TA side (etched with 2% HF + 8% HNO3 + 90% water).

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ig. 6. Optical microscope image of the bonding zones close to the corner of the STart in TIJ (etched with 2% HF + 8% HNO3 + 90% water).

he materials properties are shown in Table 2. The ST and TA wereodeled as elastic–plastic materials with bilinear extrapolation of

he strain–stress curve, while the TiC layer was assumed to be elas-ic.

. Results and discussion

.1. Microstructure

The bonding zone of a PLJ consists of three main regions (Fig. 5):T, TiC interlayer and TA. The ST microstructure along the planarond zone (Fig. 5a) consists of ferrite and pearlite with evidencesor decarburization close to the continuous 2 �m thick titaniumarbide layer. Adjacent to the TiC interlayer, the microstructuref TA consists of equiaxed � grains (10–20 �m) with residual �hase (Fig. 5b). The formation of this layer (of about 100 �m) isttributed to the presence of carbon, which stabilizes the � phase.he TIJ includes two bonding zones: TA–TA and TA–ST on the pla-ar interface and along conic-shaped regions. The microstructure

f the TA–ST interface is similar to the structures shown in Fig. 5.

The TA–TA bond zone has a mostly continuous and homoge-eous microstructure, with some defects close to the corner of theT part (Fig. 6), whereas TA displays a microstructure with large

able 2aterial properties.

Properties/material Poisson ratio Young modulus (GPa) Yield s

AISI 4330 0.3 210 914

Ti–6Al–4V 0.3 115 737

TiC 0.25 400 –

Fig. 7. Experimental tensile test curves.

(60–70 �m) �-Ti grains. The region of the ST close to the titaniumcarbide layer has a distinct ferritic structure, indicating significantlocal decarburization.

3.2. Mechanical properties and fracture surface characterization

The ram test was conducted for ST–ST, TA–TA, PLJ and TIJ con-figurations. The ST–ST and TA–TA joints failed at 84.7 and 101.9 kN,respectively. The force-displacement curves for PLJ and TIJ jointsare presented in Fig. 7.

Linear force-displacement correlation up to a fracture wasobtained for the PLJ specimen, while significant plastic deforma-tion took place for the TIJ configuration. Failure occurred at about24.7 and 50.4 kN for the PLJ and TIJ specimens, respectively. Thefracture surface of the ram-tensile specimen of TIJ is displayed inFig. 8.

SEM images of the bonding interface are shown in Fig. 9a, wherefour ring-shaped regions may be distinguished. The external ringdepicts the fracture inside the TiC layer (Fig. 9b), followed by awell-defined 50–60 �m thick intermediate zone, consisting of a

ring of coarse cleavage plates (Fig. 9c). River patterns on the cleav-age plates indicate that the fracture propagated from the externalside of the specimen inwards. The fracture mechanism graduallychanges from cleavage to dimple rupture as the crack propagates

trength (MPa) Tensile strength (MPa) Elongation at fracture (%)

1229 61000 14

– –

2888 A. Miriyev et al. / Journal of Materials Processing Technology 214 (2014) 2884–2890

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ig. 8. Ram tensile specimen after the test. Upper part (left) and bottom part (right).

nwards, indicating increasing ductility of the material. The internalbottom) ring depicts the fracture in the TA–TA bond, exhibiting aimple rupture microstructure (Fig. 9d).

The cleavage ring is apparently the fracture through the ringf large carbon-stabilized �-grains (shown in Fig. 6), that grow inront of the corner of the ST part due to carbon dissolution in theA. The size of these grains is close to the width of the cleavageone and their structure and composition gradually returns to �–�tructure, fractured by dimple rupture. The cleavage zone emanatesrom a detachment zone – a 20–30 �m thick ring between the STart and the TA along the conic surface of the ST part (Fig. 9).

The fracture surface in the detachment zone displays a fine inter-ranular fracture (Fig. 9b) typical to the crack propagating throughhe thin TiC layer (path 1 in Fig. 10). The detachment usually ini-

iates from the corner of the ST part along the conic interface andhen deviates to the orientation perpendicular to the tensile stresspath 2 in Fig. 10, observed in the image in Fig. 11). Fig. 11 showshat there is an alternative path for the crack propagation through

ig. 9. SEM images of the fracture surface of the TIJ ram tensile test sample (upper part).

he TiC along the TA–ST planar interface; (c) close-up of brittle cleavage transition to direction of crack propagation; (d) ductile TA–TA failure surface far from the edge. (For inhe web version of the article.)

Fig. 10. Schematic indication of crack paths.

the planar interface perpendicular to the tensile stress (path 3 inFig. 10).

3.3. FE analysis

The experimentally measured forces to failure were used to cal-culate the stress distribution at the breaking points for each typeof joint.

The distribution of axial stress in the PLJ sample (applied force24.7 kN) is shown in Fig. 12a. The stress along the planar inter-face changes from maximum tensile stress at the inner diameter(of about 680 MPa) to a compression at the external side (about−40 MPa). As expected, it represents a combination of tension and

bending states (Fig. 13, red line).

The distribution of the axial stress in the TIJ sample underapplied force of 50.4 kN is shown in Fig. 12b. Preliminary calcula-tions show that the presence of a TiC layer with actual size of about

(a) Typical general view of four ring-shaped zones; (b) close-up of fracture throughuctile dimple rupture in the TA side of the edge (yellow arrows indicate the localterpretation of the references to color in this figure legend, the reader is referred to

A. Miriyev et al. / Journal of Materials Processing Technology 214 (2014) 2884–2890 2889

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Fig. 13. Comparison of the axial stress distribution in the PLJ and TIJ samples under

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ig. 11. SEM image of the fracture surface of the TIJ sample. The crack propagationypes and paths are marked.

�m does not affect the stress map. A small and local effect on thetress map can be observed near the corner of the ST part for thenterfacial layer thickness of 50 �m. This effect is attributed to theending of the rigid TiC layer (Fig. 12c). It is assumed that despitehe fact that this layer is enlarged, the calculation yields a goodpproximation of the stress state associated with an elastic stripmbedded in a large elastic–plastic material. Fig. 12d shows theelative value and direction of the largest principal stress near the

orner of the steel part. The maximum principal stress, located inhe corner, is equal to 312 MPa and indicates the location of possiblenitiation of failure.

ig. 12. The calculated axial stress distribution at the joints. (a) PLJ; (b) TIJ; (c) TIJ, close-ud) the largest principal stress (direction is indicated by red arrows), found at the cornergure legend, the reader is referred to the web version of the article.).

the force to failure. (For interpretation of the references to color in text, the readeris referred to the web version of the article.)

The axial stress profile along the planar joint interface (samplewall) of the TIJ sample, compared with the PLJ sample is shownin Fig. 13. In TIJ, the stress changes from maximum tensile stressof about 1000 MPa at the inner diameter to a compression at theexternal side (about −240 MPa). In the middle of the joint interface,at a stress of about 310 MPa, a sharp v-shape local alteration in theaxial stress occurred. This is due to the change in material behaviorfrom elasto–plastic behavior of the metals to elastic behavior of theTiC layer.

Apparently, the failure of the TIJ sample is governed by the brittle

behavior of TiC and �-Ti layers located at the ST–TA interface. Thesebrittle phases initiate cracks or detachments at the location wherethey sustain the locally highest stress (about 310 MPa at the corner,

p of the corner of the ST part (50 �m thick TiC), part of the specimen is circled; and of the TIJ (area circled in (c)) (For interpretation of the references to color in this

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890 A. Miriyev et al. / Journal of Materials P

ig. 12). This value is close to the bending strength of TiC ceramicsnd, evidently, leads to cracks initiation within the interfacial layer.ost likely, the cracks propagate first into the brittle �-Ti phase and

hen cause the fracture in the TA–TA bond. The results of FE analysisre in good agreement with the analysis of the fracture surfaces.

. Conclusions

. Joint configuration for Ti-alloy/carbon steel bonding with a sim-ple conic-shape Ti insert, that allows increasing the Ti–Ti contactarea at the expense of the steel–Ti interface, was suggested.

. The interaction between carbon, originated in the steel part, andTi leads to the formation of a thin TiC layer at the interface, whilecontinuous and homogenous tough interfaces were obtained inthe regions where similar materials were in contact.

. The force to failure for the specimens with the suggested config-uration was about two fold higher than that for the specimenswith the planar interface.

. The failure is governed by the cracks initiated within the TiCinterfacial layer, followed by their propagation into the brittle�-Ti phase which finally causes the fracture of the Ti–Ti bond.

. The principal stress distribution within the suggested joint con-figuration, predicted by FE analysis, explains the fracture surfacefeatures and indicates the location of failure initiation.

cknowledgements

The authors would express their special thanks to Dr. Ehudalun and Mr. Eran Tuval.

ing Technology 214 (2014) 2884–2890

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