Materials and Design 53 (2014) 504–511

Contents lists available at ScienceDirect

Materials and Design

journal homepage: www.elsevier .com/locate /matdes

Microstructures and mechanical property of laser butt weldingof titanium alloy to stainless steel

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.07.044

⇑ Corresponding author. Tel.: +86 010 62334859.E-mail address: jhhuang@ustb.edu.cn (J. Huang).

Shuhai Chen, Mingxin Zhang, Jihua Huang ⇑, Chengji Cui, Hua Zhang, Xingke ZhaoSchool of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China

a r t i c l e i n f o

Article history:Received 8 March 2013Accepted 12 July 2013Available online 24 July 2013

Keywords:Laser weldingTitanium alloyStainless steelIntermetallic compounds

a b s t r a c t

Laser butt welding of titanium alloy to stainless steel was performed. The effect of laser-beam offsettingon microstructural characteristics and fracture behavior of the joint was investigated. It was found thatwhen the laser beam is offset toward the stainless steel side, it results in a more durable joint. The inter-metallic compounds have a uniform thickness along the interface and can be divided into two layers. Oneconsists of FeTi + a-Ti, and other consists of FeTi + Fe2Ti + Ti5Fe17Cr5. When laser beam is offset by 0 mmand 0.3 mm toward the titanium alloy side, the joints fracture spontaneously after welding. Durable join-ing is achieved only when the laser beam is offset by 0.6 mm toward the titanium alloy. From the top tothe bottom of the joint, the thickness of intermetallic compounds continuously decreases and the follow-ing interfacial structures are found: FeAl + a-Ti/Fe2Ti + Ti5Fe17Cr5, FeAl + a-Ti/FeTi + Fe2Ti + Ti5Fe17Cr5

and FeAl + a-Ti, in that order. The tensile strength of the joint is higher when the laser beam is offsettoward the stainless steel than toward the titanium alloy, the highest observed value being 150 MPa.The fracture of the joint occurs along the interface between two adjacent intermetallic layers.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The joining of titanium alloy to conventional structural steel hasmany industrial applications [1,2]. However, it is difficult to weldtitanium alloys to steel due to great differences in thermal, physical,and chemical properties. According to the Fe–Ti phase diagram [3],the solubility of Fe in Ti is very low (0.1 at.%, at room temperature),beyond which, intermetallic phases FeTi and then Fe2Ti (600 HV and1000 HV respectively) begin to form [4]. These intermetallic phasesare highly brittle, causing conventional fusion-welded joints tocrack spontaneously, due to thermal-stress mismatch between thetwo parent materials. Therefore, suppressing the formation of brit-tle intermetallic compounds (IMCs) is the key to realizing reliablejoining. Diffusion bonding [5,6], brazing [7,8], and explosive weld-ing [9,10] of steel and titanium has been investigated for thispurpose. These methods are effective to some extent in controllingthe formation of brittle intermetallic compounds, but their applica-tions are restricted by joint configurations.

Friction stir welding is a novel solid-state welding methodwhich has been considered for joining titanium alloys to steel. Fa-zel-Najafabadi et al. [11,12] achieved defect-free dissimilar lapjoints of CP-Ti to 304 stainless steel by adjusting friction-stir weld-ing parameters. The maximum failure load of the joint reached 73%of that of CP-Ti. Liao et al. [13] investigated the microstructures at

the interface of friction-stir lap joints of pure titanium and steel.Swirling macro- and micro-intermixing zones of titanium and steelwere found along the interface, where tiny Fe–Ti intermetallic par-ticles were mixed with b titanium [13]. The investigations onfriction stir welding of titanium and steel focused mostly on thelap configuration, but it was insufficient to study the buttconfiguration.

Fusion welding, which is widely applied in industry, is con-fronted with a great challenge in joining titanium alloys to steel.In traditional arc welding, it is very difficult to control the moltenpool of titanium–steel mixture, so a mass of intermetallic com-pounds are formed during welding. Recently, the development ofhigh energy beam welding such as laser beam welding and elec-tron beam (EB) wending have made fusion welding of titaniumalloy to steel possible.

To reduce the residual tensile stress of the joint and improvemetallurgical reaction of the molten pool, copper was selected tobe the filler metal for electron beam welding of titanium alloy tosteel [14,15]. Electron beam welding needs to be performed in avacuum environment, which restricts its application. Laser weld-ing, a new joining technology, has great advantages, such as highefficiency, excellent controllability, and ability to be performed ina non-vacuum environment. Hiraga et al. [16] applied a pulse laserlap welding technique to join thin sheets of pure titanium andstainless steel 304. Zhao et al. [17], suggested the laser lap weldingtechnique for joining titanium alloy to steel. They also obtainedfurther understanding of the welding process by FEM model

Fig. 1. Schematic diagram of laser welding of titanium alloy to stainless steel. (a)The process and (b) tensile specimen (thickness: 1 mm).

S. Chen et al. / Materials and Design 53 (2014) 504–511 505

calculation. The laser butt welding of titanium alloy to steel needCu interlayer to improve metallurgical reaction and decrease resid-ual stress. Tomashchuk et al. [18] reported that welding of Ti6Al4 Vto stainless steel AISI 316L through pure copper interlayer carriedout by pulsed Nd:YAG laser. The tensile strength of the welds is upto 359 MPa and is limited by brittleness of CuTi2 + FeTi + a-Ti layersituated next to the solid Ti6Al4 V. Groza et al. [19] weldedX5CrNi18–10 to Ti–6Al–4V through a copper foil, with the thick-ness of 600 lm, as intermediary layer by continuous Nd–YAG laser.The average value of tensile strength reached 328 Mpa. Thus, UsingCu foil as an interlayer leads to acceptable strength characteristicsduring laser welding of titanium alloy to steel.

Laser butt welding of titanium alloy to steel without filler metalis not pay more attention. In fact, direct laser butt welding of tita-nium alloy to steel is convenience in technology. Satoh et al. [20]attempt to weld titanium alloy to stainless steel without filler me-tal by laser welding, but tensile strengths of the joints were ob-served to be lower than the base materials with failure occurringthrough brittle fracture. And, investigation on the microstructuresis insufficient. To improve mechanical property of the joint, it isnecessary to understand deeply the weldability of titanium alloyto steel without filler. Therefore, the weld characteristics andmicrostructure should be investigated systematically.

The present work focuses on laser butt welding of titanium al-loy to stainless steel without a filler metal. To control metallurgicalreactions in the molten pool, the welding experiments were per-formed with various laser beam offsets, either toward the titaniumalloy or toward the stainless steel. The effect of laser beam offseton the microstructural characteristics and mechanical property ofthe joint was investigated.

2. Experiments

The plates of Ti–6Al–4V titanium alloy (100 � 50 � 1 mm3) and201 stainless steel (100 � 50 � 1 mm3) were selected as laserwelding materials and their chemical compositions were shownin Table 1. The plates manufacturing of titanium alloy and stainlesssteel confirm to GB/T3621-94 [21] and GB/T3280-92 [22], respec-tively. The interface sides of the specimens were carefully polishedand then clamped in a butt-weld geometry. A high-power (4 kW)CO2 laser with welding-head focal distance of 200 mm was used.

The laser beam was made to focus on the upper surfaces of thespecimens. All of the specimens were welded at a power of 2000 Wand a welding speed of 2 m/min to ensure full penetration of thespecimens. Influence of the laser beam offsets on the mechanicalproperty and microstructures of the joints were investigated be-cause the formation of the intermetallic compounds is sensitiveto the offsets. The offsets toward stainless steel were defined as po-sitive and those toward titanium alloy as negative, as shown inFig. 1a). In this study, welding with offsets of �0.6 mm,�0.3 mm, 0 mm, 0.3 mm and 0.6 mm were performed.

The microstructures of the joint were observed by scanningelectron microscope (SEM) equipped with an energy-dispersiveX-ray spectrometer (EDS) which has 2% error for middle atomnumber, after standard grinding and polishing procedures. Crystalphases of the joints were identified by microbeam X-ray diffrac-

Table 1Chemical composition of materials used (wt%).

Name Fe C N HTitanium 60.3 60.1 60.05 60.015Name C Si Mn CrSteel 60.15 60.75 5.5–7.5 16.0–18.0

tometer (XRD). To increase diffracted intensity of the phases andanalyze broken position of the joints, the XRD tests were per-formed at the fracture surface of the tensile specimen after tensiletest. The tensile strength of the joints at room temperature wasevaluated by a mechanical testing frame (INSTRON MODEL 1186)at a cross head speed of 1 mm/min. The schematic diagram ofthe tensile specimen is shown in Fig 1b). The tensile test standardsare in accordance with ISO 6892: 1998 [23].

3. Results

3.1. Weld appearance

Fig. 2 shows the effect of the laser beam offsets on the surfaceappearance of weld. When the offset is �0.6 mm (toward titaniumalloy), a durable joining was realized, as shown in Fig. 2a. If the off-sets are 0 mm and �0.3 mm (toward titanium alloy), the speci-mens crackle noticeably during welding and then fracturedspontaneously after welding. Fig. 2b shows surface appearance ofthe weld with a 0 mm offset. A number of cracks appear in the cen-ter of the weld, resulting in spontaneous fracture after welding. Itappears that the weldability is bad when laser beam is offset to-ward titanium alloy.

When the laser beam is offset toward stainless steel, all of thespecimens are joined durably, and the quality of the joint is im-proved significantly. Fig. 2c shows the surface appearance of aweld with a 0.6 mm offset. Compared to the 0.6 mm-offset weld,the surface appearance of weld is relatively smooth. The weld zonewas narrower because stainless steel has higher thermal conduc-tivity (12.1 W/m K, 20 �C) than titanium alloy (5.44 W/m K, 20 �C)[24]. Therefore, laser butt welding of titanium alloy to stainlesssteel has better weldability when the laser beam is offset towardstainless steel than toward titanium alloy.

O Al V Ti60.2 5.5–6.8 3.5–4.5 restN Ni P S Fe60.25 3.5–5.5 60.03 60.06 rest

Fig. 2. Influence of the laser beam offsets on the surface appearance of welds. (a)Offset of �0.6 mm, (b) offset of 0 mm and (c) offset of 0.6 mm.

Fig. 3. Microstructures of the joint at the offset of laser beam toward to stainlesssteel. (a) Macro cross-section, (b) interfacial microstructures and (c) magnificationin the Fig. 3b).

506 S. Chen et al. / Materials and Design 53 (2014) 504–511

3.2. Microstructures of the joint

3.2.1. Offset toward stainless steelFig. 3 shows the microstructures of the joint made by a laser

beam with a 0.6 mm offset toward the stainless steel. A layer ofintermetallic compounds with uniform thickness formed alongthe interface between titanium alloy and stainless steel, as shownin Fig. 5a. The total thickness of the intermetallic compounds wasabout 30 lm. The structures can be divided into two reaction lay-ers, as shown in Fig. 3b. Let the reaction layer adjacent to titaniumalloy be called ‘‘layer I’’ and the one adjacent to stainless steel,‘‘layer II’’. Layer I consisted of two phases in a matrix-and-reticularstructure. Layer II seemed to have complex structures composed offour phases, as shown in Fig. 3c.

To identify phase compositions of the reaction layers, EDS wasperformed on zones A through F in Fig. 3c. The results are shownin Table 2. According to the EDS results, these phases have complexelement compositions. The substitution of the atoms in the latticeshould be considered in identifying phase composition. Since thebinary systems of Ti–V, Ti–Cr, Fe–Cr and Fe–Ni are mutually solu-ble in solid state at high temperature, V and Cr can replace Ti atomsand Cr and Ni can replace Fe atoms in the intermetallic lattice. Itappears that the Fe atoms in the reticular phase (A zone) can be re-placed by Cr and Ni atoms while Ti atoms can be replaced by Vatoms. In contrast, the Fe atoms in the matrix phase (B zone) can

be replaced by Ni atoms and Ti atoms can be replaced by Cr andV atoms. The positions of components of reticular and matrixphases on the Ti–Fe–Al ternary phase diagram [25] are shown byA and B in Fig. 4. The components are close to the eutectic liquidusof FeAl and b-Ti. The reticular and matrix phases seem to be com-posed of FeAl and b-Ti. However, the b-Ti may transform into a-Tiat a low temperature. The crystal structure of the Ti needs to bedetermined by XRD.

The compositions of the dark (C zone) and gray (D zone) phasesare shown by C and D in Fig. 4. It can be deduced that the dark andgray phases are FeTi and Fe2Ti, respectively. In contrast, the lightgrey phase (E zone) and white phase (F zone) have a low content

Fig. 5. Liquidus surface of the Fe–Cr–Ti ternary phase diagram [26].

Table 2EDS results of the joints (at.%).

Position Ti Fe Cr Ni V Al

A 57.51 24.31 5.31 1.78 1.52 9.57B 60.77 20.90 5.58 2.10 1.77 8.87C 49.58 32.18 7.06 3.22 1.14 6.83D 40.92 36.38 9.97 5.18 1.59 5.96E 25.99 51.31 11.84 7.17 0.49 3.13F 12.24 58.87 18.55 5.73 1.01 3.60G 27.35 53.44 13.36 3.47 0.64 1.73H 15.71 57.34 19.17 4.85 0.53 2.40I 4.43 66.06 22.55 4.95 0.69 1.32J – 70.02 21.06 7.97 0.24 0.70

Fig. 4. Liquidus surface of the Fe–Ti–Al ternary phase diagram [25].

Fig. 6. Microstructures of the joint at the offset of laser beam toward to titaniumalloy. (a) Macro cross-section, (b) microstructures at the top of the joint and (c)microstructures at the bottom of the joint.

S. Chen et al. / Materials and Design 53 (2014) 504–511 507

of Al, as shown in Table 2. Zones E and F can then be considered aFe–Cr–Ti ternary system and phase compositions can thus be con-firmed. The locations of these phases on the Fe–Cr–Ti ternaryphase diagram [26] are as shown by E and F in Fig. 5. It can be

deduced that the light grey phase (E zone) and the white phase(F zone) are Fe2Ti and s (Ti5Fe17Cr5), respectively.

It can be seen that gray phases (D zone) and light grey phase (Ezone) are Fe2Ti phases. The contrast difference of the phases iscaused by different average atom numbers between D and C zones.Therefore, interfacial structures of the joint are FeAl + Ti/FeTi + Fe2Ti + Ti5Fe17Cr5.

3.2.2. Offset toward titanium alloyFig. 6 shows the microstructures of the joint made by the laser

beam offset of 0.6 mm toward titanium alloy. Complex structuresappear at the interface. First, partial melting of stainless steel

508 S. Chen et al. / Materials and Design 53 (2014) 504–511

appears at the top of the joint, as shown in Fig. 6a. Second, thethickness gradually decreases from the top to the end of the joint.Third, different interfacial structures appear at the top, middle andbottom of the joint. At the top of the joint in Fig. 6b, two reactionlayers are formed. A grey layer is close to the fusion zone, definedas ‘‘Layer III’’ in this paper. It can be deduced that the phase com-positions of Layer III is same with Layer I because they have thesame morphology. The other white layer is close to stainless steel,defined as ‘‘Layer IV’’. In this layer, a light grey dendritic structuregrows into Layer IV from the interface of Layer III and Layer IV. Alight grey fusion line appears between Layer IV and parent stain-less steel. In the middle of the joint indicated by a rectangle inFig. 6b, the interfacial structures are the same as that made bythe laser beam offset toward stainless steel, as shown in Fig. 3b.The structures consist of two layers of FeAl + Ti/FeTi + Fe2Ti + Ti5-

Fe17Cr5. At the bottom of the joint in Fig. 6c, only one reaction layeris formed between titanium alloy and stainless steel. It is con-firmed that this layer has same structures with Layer I (FeAl + Ti).

According to above observation on the microstructures of thejoint, the phase compositions of Layer IV need to be confirmed.In Layer IV, since the Al content is relatively low, the phase compo-sitions can be identified as a Fe–Cr–Ti ternary system. The compo-sitions of the grey dendritic phase growing into Layer IV (G zone)and white phase of the Layer IV (H zone) in the Fe–Cr–Ti ternaryphase diagram are shown by D and E in Fig. 4. It can be believedthat the grey dendritic phase is Fe2Ti and the white phase is ter-nary intermetallic compound s (Ti5Fe17Cr5). At fusion line (I zone),the content of Ti slightly increases, compared to parent stainlesssteel (J zone).

Therefore, the interfacial structures of the joint are from the topto the bottom confirmed orderly as FeAl + Ti/Fe2Ti + Ti5Fe17Cr5 ?FeAl + Ti/ FeTi + Fe2Ti + Ti5Fe17Cr5 ? FeAl + Ti.

3.3. mechanical property of the joint

To evaluate the mechanical property of the joint, tensilestrength tests were performed. Fig. 7 shows the influence of la-ser-beam offsets on the tensile strength of the joints. Because thespecimens with the offsets of �0.3 mm and 0 mm fracture sponta-neous after the welding, the tensile strength is zero. When the off-set is �0.6 mm, the tensile strength of the joint is only 24.75 MPa.In contrast, when the offsets are 0.3 mm and 0.6 mm, the tensilestrength of the joints increase significantly. When laser beam isoffset to 0.6 mm toward stainless steel, the tensile strength ofthe joint reaches 150 MPa. Therefore, the joints have higher tensile

Fig. 7. Influence of the offset of laser beam on the tensile strength.

strength when the laser beam is offset toward the stainless steelthan the toward the titanium alloy.

3.4. Fracture behavior of the joint

Fig. 8 shows fracture surface of the joint made with laser-beamoffset of 0.6 mm toward titanium alloy. The fracture exhibits typi-cal brittle characteristics. A relatively smooth fracture surface witha river patten appears at the top of the joint. In contrast, the frac-ture at the middle and end of the joint exhibits relatively roughmorphology. In addition, many cracks along the thickness directionof the plate appear at the fracture surface. When the offset is0.6 mm toward stainless steel, the fracture morphology still exhib-its typical brittle characteristics. However, the relatively smoothfracture surface with river patten disappears and all fracture sur-face becomes to relatively rough, as shown in Fig. 9. Also, thecracks along the thickness direction of the plate is more dense thanthat with laser beam offset of 0.6 mm toward titanium.

To confirm phase compositions at the fracture surface, XRD wasperformed on all the samples, as shown in Fig. 10. Because thereare complex element compositions of the joints, distortion of crys-tal lattice is inevitable. Thus, the location of the diffraction peaks ofthe phase compositions may be offset from the location of the stan-dard diffraction peak. It was found that a-Ti and Fe2Ti appear at allof the fracture surfaces. The b-Ti do not appears at the fracture sur-faces, which indicates that the b-Ti in the eutectic structures of b-Tiand FeTi transforms into a-Ti during the cooling of the weld.Therefore, it is confirmed that the Layer I consists of a-Ti and FeTi.

The presence of the a-Ti at all fracture surfaces may indicate-that the joint fractures along the layer which contains a-Ti and FeTi(Layer I or Layer III). However, the presence of the Fe2Ti seems toindicate that the joint fractures along Layer II or Layer IV. There-fore, it can be concluded that fracture only occurs along the inter-face between two adjacent layers.

It should be noted that some of the phases observed in SEMwere not found by the XRD. In general, the XRD results dependon diffracted intensity of the phases. Thus, the XRD were per-formed at fracture surface to increase the diffracted intensity[27]. However, some of the intermetallic compounds in this studyare uncontinuous and inhomogeneous in distribution, such as Ti5-

Fe17Cr5. And, the content is relatively low. Therefore, some of thephases maybe are not found by the XRD, although the XRD wereperformed at fracture surface.

4. Discussion

When titanium alloy is mixed with the stainless steel in the li-quid state, a number of brittle intermetallic compounds are formedinside fusion zone during welding. Meanwhile, a stress mismatcharises in the seam due to great difference in thermal expansioncoefficients between titanium alloy (7.89 � 10�6/�C) and stainlesssteel (16.9 � 10�6/�C) [24]. The specimens fracture spontaneouslywhen a sufficient amount of intermetallic compounds has formed.To improve the weldability of titanium alloy to steel, the Cu inter-layer was used during laser welding [18,19]. On the one hand, theCu interlayer improved metallurgical reaction of weld pool, whichleads to the formation of Ti–Cu intermetallic compounds [19]. Onthe other hand, the interlayer decrease residual stress of the joint[14]. Therefore, the tensile strength of the joint with the Cu inter-layer reached to 359 MPa [18].

Outside using the Cu interlayer, suppressing liquid-state mixingbetween titanium alloy and steel is an important way to realizestrong joining. Satoh et al. [20] attempted to weld titanium alloy tosteel without interlayer by Nd: YAG laser welding, but liquid-statesmixing between titanium alloy and steel is not suppressed fully.

Fig. 8. Fracture surface of the joint with laser beam offset of 0.6 mm toward totitanium. (a) Macro fracture surface, (b) magnification of P1 zone and (c)magnification of P2 zone.

Fig. 9. Fracture surface of the joint with the offset of 0.6 mm toward to stainlesssteel. (a) Macro fracture surface, (b) magnification of P3 zone and (c) magnificationP4 zone.

S. Chen et al. / Materials and Design 53 (2014) 504–511 509

Perhaps samples were processed at laser beam offsets from theinterface toward the Ti plate, which is not good way proved in this pa-per. In our study, the liquid-states mixing is suppressed better by

controlling laser beam offset toward titanium alloy. Thus, laser beamoffsetting toward titanium alloy is an effective way to suppress li-quid-state mixing during welding of titanium alloy to stainless steel.

Fig. 10. XRD patterns of fracture surface of the joint. (a) �0.6 mm, (b) �0.3 mm, (c) 0.3 mm and (d) 0.6 mm.

510 S. Chen et al. / Materials and Design 53 (2014) 504–511

According to the results of the experiment, laser butt welding oftitanium alloy to stainless steel has better weldability when thelaser beam is offset toward stainless steel than toward titaniumalloy. This is closely related to the distribution and compositionsof the intermetallic compounds, which depend on the laser beamoffsets.

The intermetallic compounds along the interface betweentitanium alloy and stainless steel are more uniform in thicknesswhen the laser beam is offset toward stainless steel than whenit is offset toward titanium alloy. This difference in thickness isa result of great difference in thermal conductivity betweenstainless steel (12.1 W/m K, 20 �C) and titanium alloy (5.44 W/m K, 20 �C) [24]. Stainless steel has a higher thermal conductivitythan titanium alloy, which means that the thermal conductionfrom the high temperature zone of the molten pool to the lowtemperature zone is easier when laser beam is offset towardthe stainless steel rather than titanium. Thus, a more uniformtemperature distribution is achieved when the laser beam is off-set toward stainless steel, which is the side with higher thermalconductivity. The low temperature gradient along the interfaceinduces uniform thickness of intermetallic compounds. Theuneven thickness distribution of the intermetallic compoundshas negative influence on the mechanical property of the joint.First, it causes an uneven distribution of residual stress in theseam, compared to the joint with uniform distribution of theintermetallic compounds. Furthermore, stress concentration isinduced inside the seam. The residual stress compromisesmechanical property of the joint. Second, the residual stresscauses cracks to initiate at the top of the joint, where the thickintermetallic compounds layer is. In particular, this causes thespecimens with the �0.3 mm and 0 mm offsets fracture sponta-neously after welding. Third, the uneven thickness distributionof the intermetallic compounds makes it more difficult tocontrol mechanical property of the joint: when the amount inter-metallic compounds at the top of the joint are limited to areasonable level, the bottom of the joint would not have beenreliably welded yet; when the bottom of the joint is welded witha reasonable amount of intermetallics, too much intermetallicswould have formed on the top of the joint. Therefore, the joint

with uniform thickness of intermetallic compounds, made by alaser beam offset toward the stainless steel, has high tensilestrength.

When the laser beam is offset toward titanium alloy, the rela-tively smooth fracture surface with river patten at the top of thejoint indicates that the crack propagation only required low en-ergy, as shown in Fig. 8b. The low propagation energy of the crackleads to low tensile strength of the joint. According to the micro-structures in Fig. 6, slight melting appears at the top of the joint,which indicates the mixing between titanium alloy and stainlesssteel. This proves that the mixing between titanium alloy andstainless steel adversely affects the mechanical property of thejoint. The fracture morphologies when laser beam is offset towardtitanium alloy are similar with that observed by Satoh et al. [20],because similar processing parameters were used. Meanwhile, thephase compositions of this zone (FeAl + a-Ti/Fe2Ti + Ti5Fe17Cr5)greatly differ with other areas (FeAl + a-Ti/FeTi + Fe2Ti + Ti5Fe17

Cr5 or FeAl + a-Ti) of the joint. Therefore, phase compositionmay be another important factor which contributes to low tensilestrength of joint made when the laser beam is offset toward thetitanium alloy.

5. Conclusions

From the investigation on microstructures and mechanicalproperty of laser butt welding of titanium alloy to stainless steel,conclusions were summarized as following:

(1) When the laser beam is offset toward the stainless steel, dura-ble joining is realized. The intermetallic compounds haveuniform thickness along the interface and can be divided intotwo layers: FeTi + a-Ti and FeTi + Fe2Ti + Ti5Fe17Cr5.

(2) When the laser beam is offset by 0 mm and 0.3 mm towardthe titanium alloy, the joints fracture spontaneously subse-quent to welding. Durable joining is obtained only whenthe laser beam is offset by 0.6 mm toward the titanium alloy.From the top to the bottom of the joint, the thickness ofintermetallic compounds continuously decreases and the

S. Chen et al. / Materials and Design 53 (2014) 504–511 511

following interfacial structures are found: FeAl + a-Ti/Fe2

Ti + Ti5Fe17Cr5, FeAl + a-Ti/FeTi + Fe2Ti + Ti5Fe17Cr5 andFeAl + a-Ti, in that order.

(3) The tensile strength of the joint is higher when the laserbeam is offset toward stainless steel than toward titaniumalloy, the highest value being 150 MPa. Fracture occurs alongthe interface between two adjacent layers.

Acknowledgments

The authors appreciate the financial support from the NationalNatural Science Foundation of China (No. 51004009) and the Fun-damental Research Funds for the Central Universities (FRF-TP-12-044A). The authors would like to express their gratitude to Dr. BertLiu, Department of Materials Science and Engineering, The OhioState University, for his help in English revision of this article.

References

[1] Mudali UK, Rao BMA, Shanmugam K, Natarajan R, Raj B. Corrosion andmicrostructural aspects of dissimilar joints of titanium and type 304L stainlesssteel. J Nucl Mater 2003;321:40–8.

[2] Ghosh M, Chatterjee S, Mishra B. The effect of intermetallics on the strengthproperties of diffusion bonds formed between Ti–5.5Al–2.4V and 304 stainlesssteel. Mater Sci Eng A 2003;363:268–74.

[3] Murray JL. Phase diagrams of binary titanium alloys. ASM, International; 1987.p. 99–111.

[4] Akbari Mousavi SAA. Sartangi PF. Effect of post-weld heat treatment on theinterface microstructure of explosively welded titanium–stainless steelcomposite. Mater Sci Eng A 2008;494:329–36.

[5] Kundu S, Roy D, Chatterjee S, Olson D, Mishra B. Influence of interfacemicrostructure on the mechanical properties of titanium/17-4 PH stainlesssteel solid state diffusion bonded joints. Mater Des 2012;37:560–8.

[6] Kundu S, Sam S, Chatterjee S. Interface microstructure and strength propertiesof Ti–6Al–4V and microduplex stainless steel diffusion bonded joints. MaterDes 2011;32:2997–3003.

[7] Yue X, He P, Feng JC, Zhang JH, Zhu FQ. Microstructure and interfacial reactionsof vacuum brazing titanium alloy to stainless steel using an AgCuTi filler metal.Mater Charact 2008;59:1721–7.

[8] Lee MK, Lee JG, Lee JK, Hong SM, Lee SH, Park JJ, et al. Formation of interfacialbrittle phases sigma phase and IMC in hybrid titanium-to-stainless steel joint.Nonferrous Met Soc China 2011;21:s7–s11.

[9] Akbari Mousavi SAA. Farhadi Sartangi P. Experimental investigation ofexplosive welding of cp-titanium/AISI 304 stainless steel. Mater Des2009;30:459–68.

[10] Song J, Kostka A, Veehmayer M, Raabe D. Hierarchical microstructure ofexplosive joints: example of titanium to steel cladding. Mater Sci Eng A2011;528:2641–7.

[11] Fazel-Najafabadi M, Kashani-Bozorg SF, Zarei-Hanzaki A. Dissimilar lap joiningof 304 stainless steel to CP–Ti employing friction stir welding. Mater Des2011;32:1824–32.

[12] Fazel-Najafabadi M, Kashani-Bozorg SF, Zarei-Hanzaki A. Joining of CP–Ti to304 stainless steel using friction stir welding technique. Mater Des2010;31:4800–7.

[13] Liao JS, Yamamoto N, Liu H, Nakata K. Microstructure at friction stir lap jointinterface of pure titanium and steel. Mater Lett 2010;64:2317–20.

[14] Zhang BG, Wang T, Duan XH, Chen GQ, Feng JC. Temperature and stress fieldsin electron beam welded Ti-15-3 alloy to 304 stainless steel joint with copperinterlayer sheet. Trans Nonferrous Met Soc China 2012;22:398–403.

[15] Wang T, Zhang BG, Chen GQ, Feng JC, Tang Q. Electron beam welding of Ti-15-3titanium alloy to 304 stainless steel with copper interlayer sheet. Trans.Nonferrous Met Soc China 2010;20:1829–34.

[16] Hiraga H, Fukatsu K, Ogawa K, Nakayama M, Mutoh Y. Nd: YAG laser weldingof pure titanium to stainless steel. Quart J JWS 2001;19:717–26.

[17] Zhao SS, Yu G, He XL, Zhang YJ, Ning WJ. Numerical simulation andexperimental investigation of laser overlap welding of Ti6Al4V and 42CrMo.J Mater Process Technol 2011;211:530–7.

[18] Tomashchuk I, Sallamand P, Andrzejewski H, Grevey D. The formation ofintermetallics in dissimilar Ti6Al4V/copper/AISI 316L electron beam andNd:YAG laser joints. Intermetallics 2011;19:1466–73.

[19] Groza C, Mitelea I, Ut�u ID, Craciunescu CM. Melted zone morphology by laserwelding of Ti–6Al–4V With X5CRNI18-10. In: 21nd International Conferenceon Metallurgy and Materials. Brno, Czech Republic; 2012, p. 113.

[20] Satoh G, Yao YL, Qiu C. Strength and microstructure of laser fusion-welded Ti–SS dissimilar material pair. Int J Adv Manuf Tech 2013;66:469–79.

[21] GB/T3621-94. Titanium and titanium alloy sheet and plate. The State Standardof the People’s Republic of China, 1994.

[22] GB/T3280-92. Cold rolled stainless steel sheets and plates. The State Standardof the People’s Republic of China, 1992.

[23] ISO 6892: 1998. Metallic materials – Tensile testing at ambient temperature.International Organization for Standardization; 1998.

[24] ASM: ASM metals handbook, In: 10th ed., ASM International, Metals Park(OH); 1990.

[25] Palm M, Lacaze J. Assessment of the Al–Fe–Ti system. Intermetallics2006;14:1291–303.

[26] Ivanchenko V, Pryadko T. Chromium – Iron – Titanium� MSIT. In: Effenberg G,Ilyenko S, editors. Berlin: Springer-Verlag; 2008. p. 1–17.

[27] Kundu S, Anand G, Chatterjee S. Diffusion bonding of 17–4 precipitationhardening stainless steel to Ti alloy with and without Ni alloy interlayer:Interface microstructure and mechanical properties. Metall Mater Trans A2013;44:2196–211.