Dissimilar laser welding of AISI 316L stainless steel to Ti6–Al4–6V alloy via pure vanadium interlayer

Materials Science & Engineering A 622 (2015) 37–45

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Dissimilar laser welding of AISI 316L stainless steel to Ti6–Al4–6V alloy via pure vanadium interlayer I. Tomashchuk n, D. Grevey, P. Sallamand Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS/Université de Bourgogne, IUT Le Creusot-12, rue de la Fonderie, 71200 Le Creusot, France

art ic l e i nf o

a b s t r a c t

Article history: Received 30 July 2014 Received in revised form 27 October 2014 Accepted 31 October 2014 Available online 10 November 2014

Successful continuous laser joining of AISI 316L stainless steel with Ti6Al4V titanium alloy through pure vanadium interlayer has been performed. Three welding configurations were tested: one-pass welding involving all three materials and two pass and double spot welding involving creation of two melted zones separated by remaining solid vanadium. For the most relevant welds, the investigation of microstructure, phase content and mechanical properties has been carried out. In case of formation of a single melted zone, the insertion of steel elements into V-based solid solution embrittles the weld. In case of creation of two separated melted zones, the mechanical resistance of the junction is determined by annealing of remaining vanadium interlayer, which can be witnessed by observing the increase of grain size and decrease of UTS. The two pass configuration allows attain highest mechanical resistance: 367 MPa or 92% of UTS of annealed vanadium. Double spot configuration produces excessive heat supply to vanadium interlayer, which results in important decrease of tensile strength down to 72% of UTS of annealed vanadium. It was found that undesirable σ phase which forms between Fe and V is not created during the laser welding process because of high cooling rates. However, the zones whose composition corresponds to σ homogeneity range are crack-susceptible, so the best choice is to reduce the V content in steel/vanadium melted zone below σ phase formation limit. In the same time, the proportion between V and Ti in Ti6Al4V/vanadium melted zones does not influence mechanical properties as these elements form ideal solid solution. & 2014 Elsevier B.V. All rights reserved.

Keywords: Welding Titanium alloys Electron microscopy X-ray diffraction Fracture

1. Introduction Highly resistant joints between titanium alloys and stainless steel provide many technological advantages in chemical, nuclear and spacecraft industries because of lower cost and weight of the products containing titanium parts. Direct joining of steels with titanium alloys is compromised by the formation of brittle TiFe2 and TiCr2 phases, but mechanical resistance of such welds can be enhanced by insertion of an intermediate metal foil that modifies final phase composition. The most widely used intermediate foils are Cu [1], Ni [2] and their alloys because of their availability and relatively low cost. However, in this case the risk of embrittlement of the weld due to intermetallic phase formation is high enough, and even when welding conditions are optimized, titanium/foil interface appears to be the weakest part of the joint [1,3,4]. Mechanical resistance of such joints rarely overpasses 350 MPa, which is much inferior to mechanical resistance of steels and

n

Corresponding author. Tel.: þ 33 3 85 73 11 23. E-mail address: [email protected] (I. Tomashchuk).

http://dx.doi.org/10.1016/j.msea.2014.10.084 0921-5093/& 2014 Elsevier B.V. All rights reserved.

titanium alloys. Moreover, the properties of Fe–Cu–Ti system [5] indicate that temperature stability of the joint is much inferior to that of steels and titanium alloys because of low temperature transformations between Cu-containing phases. Another group of candidates for joining steels to titanium alloys are the metals that do not form intermetallic phases with Ti: V, Zr, Nb, Mo, Ta and Hf. The most common of them is pure V, which forms continuous solid solutions with Ti [6] and has close fusion temperature (1670 1C for Ti and 1914 1C for V). Fe–V system [7] also shows large regions of continuous solid solutions, but it also contains metastable σ-phase existing in temperature range of 650–1219 1C. Nogami et al. [8] report defect free joining of pure V to 316L stainless steel by electron beam welding and the increase of melted zone hardness comparing to the base metal because of solution hardening. A post-welding heat treatment at the temperature superior to 600 1C leads to important increase of melting zone hardness that can be attributed to solution hardening, formation of σ phase of the Fe–V system and formation of Ni2V3 and NiV3 precipitates. An interesting study [9] dedicated to phase transformation in Fe–V alloys clarifies these observations in showing that σ phase is formed at 700–1200 1C after an

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incubation time (including nucleation, particles growth and formation of clusters) of 4 h at 700 1C while rapid quenching to a low temperature ( o650 1C) induces the separation of initial solid solution into V-rich and V-poor clusters. So σ phase seems difficult to obtain during a rapid welding cycle. Ternary system Fe–Ti–V [10] does not contain ternary phases and presents large regions of continuous bcc solutions. This opens a good perspective for using V as interlayer for welding of steels with titanium alloys. Nevertheless some authors [11–13], report that the insertion of Cr, Ti, Fe shifts the ductile-to-brittle transition point of vanadium alloys above ambient temperature. This effect seems to be stronger if alloy is heated at high temperature (950– 1100 1C). In this case fractures exhibit both intergranular and transgranular cleavage features. Moreover V is very sensitive to impurities as hydrogen, oxygen and nitrogen. So an efficient gas shielding has to be implemented to avoid brittleness of the weld. Very few authors have tested V interlayer for joining titanium alloys to steels. Pure V has been recently tested as potential interlayer for electron beam welding of titanium to stainless steel in combination with Cu–V alloy interlayer 0.7 mm each [14]. Resulted weld showed a weld joint coefficient of 72% relatively to 304L stainless steel (395 MPa) and was composed by Ti, Cu and V-based solid solutions, by σ phase in form of thin layer and by Febased solid solution. Pure V was also used for diffusion bonding of titanium to stainless steels in combination with Cr and Ni in form of 60 mm thick layered foil [15]. UTS of 480 MPa was attained as the formation of σ phase between V and Fe and between Cr and Fe was avoided (here Cr/Ni foil contacts stainless steel to prevent the brittle σ-phase of the V–Fe system and V foil contacts titanium). Laser welding is often used for joining of dissimilar alloys [16– 18] as it provides very local heat supply, rapid heating/cooling gradients and perfect precision in weld realization. It was successfully applied for joining of titanium to stainless steel via copper interlayer during our previous studies [1,19]. Shanmugarajan and Padmanabham [20] attempted to perform keyhole mode laser welding of Ti alloys with steels via V and Ta interlayers, but the welding was not successful because of oxidation problems and brittle phases formation. The present article reports the feasibility of continuous Yb:YAG laser joining of Ti6Al4V to AISI 316L stainless steel through 1 mm pure V interlayer. Three different welding configurations are tested in order to define best welding condition. For the most relevant welds, the relation between mechanical properties, fracture mode and local phase content is discussed.

2. Experimental procedure 2.1. Materials 2 mm thick austenitic stainless steel AISI 316L, 2 mm thick Ti6Al4V and 1.06 mm thick pure V foil were used as raw materials. Their chemical composition and UTS values are listed in Table 1. Table 1 Chemical composition and mechanical resistance raw materials and V foil. Material

Bulk composition (at%) Ti

V

Cr

Fe

Ni

AISI 316L V interlayer

– –

– –

– 100

20.2 –

68.5 –

6.9 –

TA6V

8.65

87.46

3.89

–

–

–

Heat treatment at 450 1C during 24 h.

Welding experiments were performed by Yb:YAG continuous laser of 6 kW maximal power and spot diameter of 200 mm. V foil was fixed on the edge of Ti6Al4V plate with several laser pulses, then Ti6Al4V and steel plates were pressed together in butt configuration and maintained with mechanical fixation. Three different welding methods were tested in this study. In all cases laser was focalized at top surface of the joint and high level of laser power resulted in keyhole mode welding. Welding conditions of most relevant joints are provided below A. one pass welding, with laser beam positioned in the middle of the V interlayer. The laser power is 3 kW and the welding speed is 2 m/min; B. two pass welding. During the first pass the V foil is welded to steel and during the second pass (immediately after the first one) V foil is welded to Ti6Al4V. For both passes, the beam is shifted from joint line at 0.1 mm on titanium or on steel; laser power is 3 kW and welding speed is maintained at 2 m/ min; and C. one pass with use of double spot optical system. The spots are positioned on two sides of foil at 0.1 mm in the base metals. The laser power of each spot is of 2.5 kW and welding speed is 3.5 m/min. Argon has been used as shielding gas with total flow of 15 L min  1 at top and at rear of the joint. Supplementary gas protection device covering the top of the melted zone has been used to minimize the risk of oxidation. 2.3. Characterization methods The cross sections of the welds were polished and attacked by the Keller reagent (5 ml concentrated HNO3, 3 ml concentrated HCl and 9 ml concentrated HF introduced in 190 ml of H2O). The microstructure and chemical composition of the samples were studied by scanning electron microscope JEOL (acceleration voltage of 20 kV, electron beam diameter of 60 mm) with fast EDS analyzer. The initial estimation of local phase content is done using Ti–V [6], Fe–Ti–V [10] and Fe–Cr–V [21] phase diagrams (later confronted with XRD data). Vickers microhardness tests have been carried out with a 15 s load time and a 25 g load. 20 mm  125 mm rectangular cuts of the welds were undergone tensile test. The UTS of the welds were evaluated at room temperature with tensile testing machine (MTS Insight 30 kN) with cross head speed of 8.3  10  5 m s  1. The fractured surfaces from both sides of broken welds were examined. The identification of phase composition on the fractured surfaces and on the polished cuts was carried out by the X-ray diffraction (PANalytical X'Pert PRO) using a cobalt target. The scanning range of 40–1401 with a step scan of 0.01671 and counting time of 200 s per step were used. The shaping of the X-ray beam by application of appropriate slits allowed scanning the whole fracture surface (20  2 mm2). Peak positions and integral intensities of observed reflections were determined by the full profile analysis.

UTS (MPa)

Al

a

2.2. Welding method

3. Results and discussion 630 800 (cold rolled) 400 (annealed a) 950

3.1. General observations Fig. 1 shows typical cross sections of the welds obtained in three above mentioned conditions: one pass (sample A), two pass (sample B) and double spot welding (sample C). Fig. 2 illustrates

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39

melted zone V isle AISI 316

MZ1

Ti6Al4V

vanadium

MZ2

foil AISI 316

MZ1

Ti6Al4V

vanadium MZ2 foil

AISI 316

Ti6Al4V

Fig. 1. SEM images of transversal cuts: sample A performed with one pass (a); sample B performed with two passes (b); and sample C performed with double spot (c) welding configuration.

evolution of Vickers microhardness and dilution of main elements in melted zones at middle height of the welds. The sample A (Fig. 1a) shows almost complete melting of V foil and partial melting of the substrate in the neighborhood of the interfaces, particularly at the top of the weld. The melted zone width evolves from 1.9 mm at the top to approximately 1 mm at the narrowest part, showing a low dilution of the substrates. The global aspect of the melted zone seems rather homogeneous except presence of unmelted V isles situated close to the interfaces (Figs. 1 and 2a). The average chemical composition of melted zone is 85 at% V, 9 at% Fe, 2 at% Ti, 2 at% Cr and 1 at% Ni. Vickers hardness (Fig. 2b) shows average level of 440 HV in the melted zone and decreases down to 100 HV in the zones where V foil remains solid. The sample B (Fig. 1b) shows two melted zones. The one is situated between steel and V (MZ1) has top and bottom widths of approximately 0.8 mm, but only of 0.4 mm width at middle height. Microstructure and chemical composition are rather homogeneous in macroscopic scale (Fig. 2c) with approximately 6 at% V, 20 at% Cr, 68 at% Fe and 7 at% Ni. The second melted zone (MZ2) of the sample B is located between Ti6Al4V and V, its area is slightly superior to the previous one. From a global point of view morphology and the average chemical composition are rather homogeneous (Fig. 2c) with about 11 at% V and Ti content only 10 at % lower comparing to Ti6Al4V. The levels of microhardness in these two melting zones are 350750 HV and decrease to 110 HV in solid V foil (Fig. 2d). The sample C (Fig. 1c) also shows two melted zones. The one situated between steel and V (MZ1) is approximately 0.4 mm thick and another situated between V and Ti6Al4V (MZ2) has average width of 0.6 mm. MZ1 is richer in Fe comparing to other samples (Fig. 2e). Its average composition is 49 at% V, 37 at% Fe, 10 at% Cr and 4 at% Ni, which corresponds to homogeneity region of brittle σ-phase [10]. Fe content and hardness values of 550 750 HV stay constant across all melted zone (Fig. 2e and f). Concerning MZ2,

same remarks as for sample B can be made. V content is slightly more important (15–17 at% V) but it makes no effect on local hardness which always remains 350750 HV. 3.2. Microstructure 3.2.1. Sample A As it was shown in the previous section, the melted zone of sample A contains the mixture of steel elements, Ti and V. It is constituted of solid solution (Cr,V) forming dendrites rich in Cr and V and interdendritic liquid rather rich in Fe, Ti and Ni. The examination of microstructures formed at solid/liquid interfaces and in the middle of melted zone is carried out. The composition of morphological elements determined from EDS analysis is given in Table 2. The interface AISI 316L/MZ (Fig. 3a) formed during interdiffusion of melted V and steel presents layered structure influenced by local convection phenomena. The first layer is supposed to contain γ-Fe and σ phase. It is followed by 30–60 mm thick layer with σ phase composition. However, XRD measurements do not detect it, which is in agreement with previous research run by Ustinovshikov [9] who claims impossibility to form this phase during rapid temperature cycle. So, we suppose that in the zones whose composition corresponds to σ-phase the formation of solid solution (Fe,V)σ occurs. The following layer is very much affected by convection and is composed by (Cr,V) solid solution rich in V. Next to it we found the MZ having close composition, but its microstructure becomes dendritic. It can be noticed that Ti does not diffuse up to inner layers. Only traces of Ti are found in outer layer. The MZ/Ti6Al4V interface (Fig. 3b) contains isle of non-melted V surrounded by (Cr,V) solid solution. No defects were found at this interface. The amount of Ti stays very low even in direct proximity of solid Ti6Al4V side. Furthermore, concentration of steel elements decreases from the center of MZ to this interface. Finally, no voids can be seen at the interface between solid V isle

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600

100

500

MZ

60

HV0.025

at. %

80

V Fe Ti

40 20 0 -1000

400 300 200 100

-600

-200

200

600

AISI 316L

0 -1000

1000

MZ

-500

0

x (µm)

60

V Fe Ti

40

500

HV0.025

at. %

MZ2

MZ1

MZ2

400 300

vanadium foil

200

20

100 -800

-400

0

400

800

0 -1200

1200

-800

-400

x (µm)

0

400

800

1200

x (µm) 600

100

MZ1

MZ2

60

500

HV0.025

at. %

1000

600

MZ1

80

80

500

x (µm)

100

0 -1200

Ti6Al4V

V Fe Ti

40

MZ1

MZ2

300

vanadium foil

200

20 0 -1200 -800

400

100 -400

0

400

800

1200

x (µm)

0 -1200

-800

-400

0

400

800

1200

x (µm)

Fig. 2. EDS analysis and Vickers microhardness measurements at semi-height of the welds (zero point situated in the center of the joint): sample A (a and b); sample B (c and d), sample C (e and f).

and Ti6Al4V; the interdiffusion of Ti and V took place over several mm thick zone. The center of MZ presents dendritic structure (Fig. 3c) and is constituted of continuous solid solutions (Cr,V) where dendrites are rich in Cr and V and interdendritic liquid contains Fe, Ti and Ni.

3.2.2. Sample B Concerning sample B, our attention was attracted not only by the phases present in melted zones but also by microstructure of residual solid V interlayer situated between two separately formed melted zones (Fig. 4). The MZ1 formed between AISI 316L and V foil contains very low amount of V (Table 2). Resulting γ-Fe solid solution has high chemical inertia, which makes difficult to perform valuable etching (Fig. 4a). No other phases were formed. The MZ2 formed between V foil and Ti6Al4V (Fig. 4b) is composed by continuous solid solutions of (Ti,V) having cellular microstructure and containing up to 10 at% V (Table 2). As the dilution of V foil in melted zones is very low, no particular phases other than above mentioned continuous

solutions were formed. No defects were present at solid/MZ interfaces of these melted zones. Fig. 4c presents global view of microstructure of V foil that remained in solid state. Its thickness is close to initial foil (in average 940 mm or 89% of initial foil thickness), which is in accordance with low V amount in melted zones. It can be observed, that due to partial annealing the grain size of V foil becomes about three times bigger in the proximity of interfaces with melted zone. It can be noticed that the thickness of V annealed zone close to Ti6Al4V side is two times higher (about 400 mm) than V annealed zone present at steel/V contact interface. This can be explained by lower thermal diffusivity of Ti6Al4V (2.10  6 m2 s  1) comparing to steel (4.10  6 m2 s  1) and vanadium (10.10  6 m2 s  1). As it will be shown in Section 3.3, this extended annealed zone appears to be the weakest part of the weld.

3.2.3. Sample C In case of sample C, the formation of two melted zone happens simultaneously, which means that V foil reaches higher temperature comparing to case B. Consequently, the melted zones contain higher amounts of V comparing to sample B. The observed

I. Tomashchuk et al. / Materials Science & Engineering A 622 (2015) 37–45

Table 2 EDS analysis of different zones in sample A, B and C. Zone

Chemical composition (at%) Al

Ti

V

Cr

Fe

Ni

  

  0.5

20 50 93

15 10 1

59 36 5

6 4 0.5

Sample A Steel–MZ interface γ-Fe þ(Fe,V)σ (Fe,V)σ (Cr,V) Сenter of MZ (Cr,V) MZ–TiAl46V interface (Cr,V) V (isle)



3

79

3

14

1

 

2.5 

90 100

1 

6 

0.5 

Sample B MZ1-γ-Fe MZ2-(Ti,V)

 6

 84

2 10

20 

70.5 

7.5 

   

   

7 59 38 53

19 8 12 8

67 30 44 35

7 3 5 4

   

   

81 49 47 65

5 12 10 8

13 36 38 25

1 3 5 2

4

76

20







6

76

18







Sample C Steel-MZ1 interface γ-Feþ (Cr,V) (light layer) (Fe,V)0 σ þ (Cr,V) (dark layer) (Fe,V)0 σ (Fe,V)σ MZ1/V interface (Cr,V) (isles) (Fe,V)σ (Fe,V)σ (crack zone) (Fe,V)σ þ (Cr,V) V/MZ2 interface (Ti,V) MZ2/TiAl46V interface (Ti,V)

41

morphologies and their compositions are given in Fig. 5 and Table 2 respectively. The melted zone between AISI 316L and V (MZ1) contains solid solutions which composition corresponds to σ-phase [10]. However, formation of this phase was not confirmed by XRD; also the hardness measurements of MZ1 indicate the absence of σ-phase: the values of microhardness do not exceed 600 HV. Close result was obtained by Nogami et al. [8] during electron beam welding of pure V with steel: as welded sample with 0.2 mm beam shift to steel side and containing 40 at% V had low Vickers hardness and no σ-phase was identified by XRD. However, modification of XRD pattern and increase of weld microhardness up to 1073 HV indicated formation of σ-phase after appropriate heat treatment. As we have mentioned above, these results are in accordance with study of Ustinovshikov [9] who shows that rapid quenching of the melt down to low temperature induces the separation of initial 0 solid solution into V-rich and V-poor clusters (Fe,V)σ and (Fe,V)σ (containing E 50 and E 38 at% V respectively). The interface between AISI 316L and MZ1 (Fig. 5a) is composed of light layer of γ-Feþ(Cr,V) solid solutions, followed by dark layer 0 which composition corresponds to the mixture (Fe,V)σ þ (Cr,V) and 0 by the zones of (Fe,V)σ and (Fe,V)σ solid solutions. The part of melted zone situated next to the MZ1/V interface contains numerous cracks (Fig. 5b). Nevertheless composition and hardness measurements are the same as that in the sound zones (Fig. 2e and f). This cracking can be attributed to conjunction of residual stresses in the proximity of V interface and to increase of ductile–brittle transition temperature in this region induced by high level of Cr and Fe in V matrix [11–13]. The mixture of constituents in this melted zone is imperfect: partially dissolved V isles can be found (solid solution (Cr,V) with 81 at% V). This is the

AISI 316 L (Cr,V)

γ-Fe + (Fe,V)σ

(Cr,V)

(Fe,V)σ

V

Ti6Al4V

(Cr,V)

Fig. 3. Microstructures of sample A: steel–MZ interface (a); MZ–Ti6Al4V interface (b); and center of MZ (c).

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(Ti,V)

γ-Fe

32 µm

10 µm 46 µm

MZ1 (steel/V)

190 µm

MZ2

vanadium foil 360 µm

(V/Ti6Al4V)

Fig. 4. Microstructures of sample B: MZ1 steel/V (a); MZ2V/Ti6Al4V (b); and variation of grain size in unmelted V interlayer (c).

indication of short lifetime of melting zone. No defects are observed at the MZ1/V interface. The composition of this interface corresponds to the mixture (Fe,V)σ þ(Cr,V). The MZ2 formed between Ti6Al4V and V contains continuous (Ti,V) solid solutions and does not present any defects. Local heterogeneities determined by convection insufficient for MZ homogenization can be observed (Fig. 5c and d). Concerning solid V foil, the increase of grain size from 10 mm to about 60 mm is observed through all thickness (Fig. 5e), which corresponds to much more important annealing that in case of sample B. Also, the thickness of remaining solid V is lower than for sample B: 850 mm or 80% of initial foil thickness. 3.3. Tensile test and fractography In this section, tensile properties of the welds are confronted with fractography study and XRD phase identification on broken edges. To estimate the quality of the welds, the UTS values and strength–strain curves of the welds were compared with those of pure V foil annealed at 450 1C (Fig. 6). Examined samples have shown very different behavior during tensile test. Sample A showed the lowest UTS (130 MPa or 32% of annealed vanadium UTS) and elongation (1%) values and fragile behavior. Sample B shows ductile behavior close to that of annealed V: 367 MPa or 92% of annealed vanadium UTS and elongation of 12% (when ε of annealed V was found to be 10%). Sample C also showed ductile behavior, but has weaker mechanical

resistance: 285 MPa UTS and 4% elongation. In Section 3.3.1, the difference in mechanical properties is related to local composition and welding conditions. 3.3.1. Sample A The reconstitution of broken parts of the weld showed that crack occurred in melted zone not far from Ti6Al4V solid interface (Fig. 7a). Chemical composition of this zone corresponds, due to Fe–Cr–V phase diagram, to (Cr,V) solid solution containing 5 at% Fe, 2.3 at% Cr and 2 at% Ti. Both fractured surfaces have the same composition and cleavage morphology (Fig. 7b and c). XRD phase identification of fractured surfaces showed presence of (Cr,V) and (Ti,V) solid solutions and traces of FeTi (Fig. 10a). Fragile behavior can be explained by the shift of ductile-to-brittle transition temperature of vanadium alloys above ambient temperature [11–13]. 3.3.2. Sample B In sample B, the fracture took place at MZ2/V interface (Fig. 8). It can be seen that solid V foil undergoes important deformation: it contains numerous cracks and is visibly elongated. The hat of MZ2 is taken away along with solid V situated beneath. In the same time, MZ1 remains completely intact. On one side of the fracture (near V), XRD analysis shows only V (with slight traces of oxidation, Fig. 10b); on another side (near Ti6Al4V) several zones of (Ti,V) are detected. The reason why the junction broke up close

I. Tomashchuk et al. / Materials Science & Engineering A 622 (2015) 37–45

43

Fig. 5. Microstructures of sample C: steel/MZ1 interface (a); MZ1/V interface (b) V/MZ2 interface (c); MZ2/Ti6Al4V interface (d); and variation of grain size in unmelted V interlayer (e).

to solid Ti6Al4V is related with more important annealing of V foil in this place. This hypothesis is confirmed by bigger grain sizes next to Ti6Al4V than next to solid AISI 316L (Fig. 4c).

Stress (MPa)

400

Vanadium foil

Sample A

300

Sample B

200

Sample C 100 0

0

4

8

12

16

20

ε(%) Fig. 6. Tensile test curves of the samples A, B and C compared with behavior of V foil annealed at 450 1C.

3.3.3. Sample C The double spot weld C showed more complex fracture mode. The reconstitution of fracture sides allows supposing that the top part of MZ1 (steel/V) cracked in the first place and then the crack propagated in the pure V (Fig. 9). Fracture edges reveal the zones composed of deformed V foil and the zones of brittle fracture located in MZ1. Moreover, the XRD analysis of fractured surfaces shows the presence of γ-Fe based solid solutions (Fe,V)σ and (Cr,V) (replacing of σ-phase) and of pure V at steel side and slightly

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fracture

Ti6Al4V

MZ

MZ

AISI 316 L

Fig. 7. Top view of fractured sample A (a), fracture surface at Ti6Al4V side (b); and fracture surface at AISI 316L side (c).

fracture

fracture

V

4 (Ti,V)

1

2

3

3

4 0.5 mm

4

5

7 6 0.5 mm

Fig. 8. Top view of fractured sample B: 1 – solid AISI 316L, 2 – MZ1, 3 – deformed V interlayer, 4 – the top of MZ2, 5 – broken V/MZ2 interface, 6 – undamaged MZ2, and 7 – solid Ti6Al4V.

oxidized vanadium at opposite side (Fig. 10c). The results of XRD analysis of broken surfaces confirm our suppositions about phase content of MZ1. More important grain size of V comparing to sample B (see Figs. 4c and 5e) indicates higher heat input during the welding, as V foil is surrounded by two melted zones. As result, sample C shows ductile tensile behavior due to annealed V, but has lower UTS values because of contribution of fragile fracture in MZ1 and important weakening of V foil due to consequent annealing by two heat sources applied simultaneously.

4. Conclusions The possibility of continuous laser joining of Ti6Al4V alloy with AISI 316L stainless steel through 1 mm V foil has been studied. The main conclusions are presented below.

1. It was found that undesirable σ phase is not created during laser welding process even if chemical composition is in the range of existence of this phase. This can be explained by rapid cooling of melted zone determined by local heat supply and high welding rate. So, the melted zones between steel and V are constituted of a number of solid solutions. However, these solid solutions are found to be crack-susceptible if the quantity of Fe is superior to 30 at%. 2. In case of single pass welding, the creation of a common melting zone (MZ) between all 3 materials is not the best choice as resulting V-rich solid solution becomes fragile because of insertion of Fe, Cr and Ti that shift ductile-tobrittle transition temperature of V alloys above ambient conditions. Consequently, only 130 MPa UTS was reached, which represents only 32% of annealed vanadium UTS. 3. Two pass welding allows best weld quality under condition that Fe–V melted zone does not contain high quantity of V. In this case the fracture during tensile test occurs in remaining V

I. Tomashchuk et al. / Materials Science & Engineering A 622 (2015) 37–45

V 2

1

3

3

4

6

5

7

fracture

(Cr,V)

45

interlayer next to melted zone between V and Ti6Al4V. The mechanical resistance of the weld is determined by partial annealing of this remaining V interlayer. The weld showed tensile behavior close to that of annealed vanadium (367 MPa or 92% UTS). 4. Double spot configuration shows lower mechanical resistance comparing to two pass welding because of high temperature level reached in the working area due to the dual spot application: only 72% UTS (285 MPa) of annealed vanadium is reached. An indication of this weakening is given by important and uniform increase of grain size in remaining V foil.

References Fig. 9. Top view of fractured sample C: 1 – solid AISI 316L, 2 – MZ1, 3 – deformed V interlayer and MZ1, 4 – the top of MZ1, 5 – deformed V interlayer, 6 – undamaged MZ2, and 7 – solid Ti6Al4V.

Intensity (counts)

6400

3600

1600

400

0 50

60

70

80

90

100

110

120

130

110

120

130

2Theta (°)

2500

Intensity (counts)

2000

1500 1000

500

0 50

60

70

80

90

100

2Theta (°)

Intensity (counts)

5000 4000 3000 2000 1000 0 50

60

70

80

90

100

110

120

130

2Theta (°) Fig. 10. XRD spectra of fracture surfaces: sample A (a), B (b) and C (c).

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