Effects of minor Zr addition on the microstructure and mechanical properties of laser welded dissimilar joint of titanium and molybdenum

Author’s Accepted Manuscript Effects of minor Zr addition on the microstructure and mechanical properties of laser welded dissimilar joint of titanium and molybdenum Lin-Jie Zhang, Guang-Feng Lu, Jie Ning, Qi Zhu, Jian-Xun Zhang, Suck-Joo Na www.elsevier.com/locate/msea

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S0921-5093(18)31409-6 https://doi.org/10.1016/j.msea.2018.10.037 MSA37032

To appear in: Materials Science & Engineering A Received date: 20 August 2018 Revised date: 6 October 2018 Accepted date: 8 October 2018 Cite this article as: Lin-Jie Zhang, Guang-Feng Lu, Jie Ning, Qi Zhu, Jian-Xun Zhang and Suck-Joo Na, Effects of minor Zr addition on the microstructure and mechanical properties of laser welded dissimilar joint of titanium and m o l y b d e n u m , Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.10.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effects of minor Zr addition on the microstructure and mechanical properties of laser welded dissimilar joint of titanium and molybdenum Lin-Jie Zhang1, Guang-Feng Lu 1, Jie Ning1, Qi Zhu 2, Jian-Xun Zhang1, Suck-Joo Na1 1. State key laboratory of mechanical behavior for materials, Xi’an Jiaotong University, Xi’an, 710049, China 2. Technical Center, Jinduicheng Molybdenum Co., Ltd., Xi’an 710077, China

Abstract: The capability to join molybdenum (Mo) and titanium (Ti) is of great interest for applications in aerospace and nuclear energy fields. Despite the importance, no effective methods have been developed to avoid the embrittlement problem of heat affected zone (HAZ) on the side of molybdenum plate to produce high strength joints. In this study, by adding zirconium (Zr) to the molten pool, ultimate tensile strength (UTS) of the joints were increased from about 350 MPa to about 470 MPa which reached more than 90% of that of the Ti base metal (BM). During this process, an ultra-fast diffusion behavior of Zr in HAZ on the side of a Mo plate (HAZMo) was observed. Electron back-scatter diffraction (EBSD) analysis revealed that diffusion distance of Zr in HAZMo was up to 0.75 mm, which was confirmed by the ZrO2 particles presented at both grain boundaries (GB) and the interior of the grains in HAZMo. Formation of ZrO2 particles not only inhibited the growth of recrystallized grains in HAZMo but also suppressed the precipitation of volatile MoO2 at GBs in HAZMo. These results highlight the possibilities for strengthening HAZ of refractory metals with intrinsic brittleness through fusion zone alloying. Keywords: Laser welding; Pure molybdenum; Pure titanium; Dissimilar joint; Minor Zr addition 1 Introduction Mo is a refractory metal with good high-temperature performance, so it is widely applied in the aerospace, nuclear power, and chemical industries [1]. Ti is also generally used therein because Ti, as a lightweight metal, offers good mechanical performance [2, 3]. Fusion welding technology for Mo to Ti is of significance for designing and manufacturing high-temperature hybrid components in aerospace engines and nuclear power equipment, etc. Although Ti shows good weldability, the weldability of Mo is poor. In the fusion welding process of Mo, oxygen is inclined to segregate at GB, which reduces bonding strength of GB [4, 5], and the strength and toughness of the joint. In addition, Mo and Ti demonstrate large differences in physical properties. For example, the fusion point of Mo is about 1,000 °C higher than that of Ti. Moreover, the thermal conductivity of Mo is about six times that of Ti and the coefficient of linear thermal expansion of Mo is only three fifths that of Ti, which results in large structural stress in dissimilar joints between Mo and Ti. Dissimilar joining technology plays crucial roles in manufacturing complicated structures and has attracted many researchers’ attention [6-10]. Recently there are some studies about the dissimilar joining of Mo to other metals [11-13] and the dissimilar joining of Ti to other metals [14-19], but relatively less researches about the dissimilar welding of Mo to Ti, especially for the most widely used fusion welding method. Pouraliakbar H et al. [6] investigated the gas tungaten arc welding of CK45 to AISI304. It was found that in the buttered specimen, the structure of the weld metal was completely austenitic however the microstructure of unbuttered sample was 

Corresponding author. Tel.: 86-29-82663115; Fax: 86-29-82663115 E-mail: [email protected](G-F Lu), [email protected] (L-J Zhang) 1

duplex ferritic-austenitic. Khorrami M S et al. [7] compared the gas tungsten arc welding of low-carbon steel to ferritic stainless steel and found that with the filler metal addition, the UTS of dissimilar joint was improved and the fracture mode changed from mainly cleavage fracture to predominantly ductile fracture. Compared with traditional fusion welding methods, laser welding offers advantages, such as: low heat input and less welding deformation [20-25]. Therefore, this study explored the weldability of fusion welding for dissimilar joints of Mo and Ti by utilising the laser welding method. Many researchers have reported the problem of oxygen segregation in the fusion welding process of Mo and its alloy. Chatterjee A et al. [26] welded TZM with Electron-Beam Welding (EBW) and found that the loss of ductility was due to the formation of Mo-oxide. Babinsky et al. [27] detected oxygen segregation at grain boundaries after recrystallisation of pure Mo by using an atom probe. In welding Ti-Zr-Mo (TZM) alloy using tungsten inert-gas arc (TIG) welding, Tabernig et al. [13] found that argon shielding cannot reduce oxygen segregation at the grain boundary in welding, indicating that oxygen originates from impurities in the BM. To reduce the influences of Mo segregation, researchers have undertaken much work: Tran R et al. [28] investigated the segregation and strengthening/embrittling effects of different metallic dopants at Mo grain boundaries by using density functional theory (DFT) calculations and predicted that Mn, Fe, Co and Nb had reasonable strengthening effects for the Σ5(100) twist grain boundary. Lenchuk O et al. [29] studied the effect of Zr segregation on the cohesive strength of grain boundaries in Mo using DFT calculations and found that there was a strong driving force for segregation of Zr to the grain boundaries in molybdenum if the low-energy insertion sites were available. In welding TZM panels by utilising electron beam welding (EBW), Wang et al. [30] carried out microalloying of FZ by adding Zr foil. The results showed that ZrO2 particles were generated in grains and that the GB was purified which improved the strength and changed the fracture mode of welded joints from one of mixed intergranular and transgranular fractures to transgranular fracturing. In order to assess the feasibility of laser welding of Mo to Ti and improve the strength of welded joints, this work conducted the laser offset welding experiment of Mo to Ti without addition and with Zr foil addition respectively. The microhardness and ultimate tensile strength were tested to evaluate the difference of mechanical properties. And the optical microscope (OM), scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS), EBSD were used to better understand the effects of minor Zr addition. 2 Experimental work The test materials were commercial pure Mo and pure Ti (i.e. TA2) plates with a thickness of 2 mm. Zr foil with a thickness of 0.05 mm was used as an additive. Table 1, Table 2 and Table 3 show the chemical compositions (detected by X Ray Fluorescence) and tensile properties of Mo plate, Ti plate and Zr foil respectively. The microstructure of the BM is shown in Fig. 1 and comprised of rolled grains, while that of Ti consisted of equiaxed grains. Fig. 2a shows a sketch map of the welding process undertaken without Zr foil, while Fig. 2b shows that with Zr foil. An IPG YLS-4000 multi-mode fibre laser was used in these welding experiments. In the welding process, the laser beam irradiated the titanium plates and the distance from the beam spot centre to the Mo/Ti interface was 0.5 mm. Before welding, the specimens were pre-heated to 450 °C. A sliding table with a precision of 0.01 mm was used to realise precision-adjustment of the laser offset and argon was used for shielding the top and bottom surfaces of the joints in the welding 2

process (Fig. 2c). Welding parameters were listed in Table 4. Table 1 Chemical compositions and tensile properties of Mo plate Material

Composition (Wt. %)

Tensile properties

Mo plate

Mo

Rh

S

La

Cl

Si

Al

Fe

σb (MPa)

δ (%)

Bal.

2.4

1.44

0.435

0.24

0.0891

0.067

0.0438

780

4.5

Cu

Cr

Zr

Ni

0.0334

0.027

0.025

0.008

Table 2 Chemical compositions and tensile properties of Ti plate Material

Composition (Wt. %)

Tensile properties

Ti plate

Ti

Al

Fe

Cr

Si

Ni

Cu

Zn

σb (MPa)

δ (%)

Bal.

0.301

0.231

0.0738

0.0319

0.006

0.006

0.004

500

15

S

Mo

Zr

0.003

0.002

0.002 Table 3 Chemical compositions of Zr foil

Material

Composition (Wt. %)

Zr foil

Zr

Hf

Mo

Na

S

Er

As

Cu

Cl

Ti

Bal.

1.2

0.1233

0.11

0.0616

0.0306

0.0234

0.0188

0.017

0.017

Si

Mn

Ta

Ni

Se

Zn

0.016

0.011

0.0106

0.006

0.005

0.003

Fig. 1 Microstructures of the BM of Mo (a) and Ti (b) used in this work.

Fig. 2 Schematic illustration of laser welding without Zr foil (a) and with Zr foil (b) and experimental set-up (c). 3

Table 4 Welding parameters

Specimen

Power (W)

Welding speed

Defocusing

Laser offset

(m/min)

amount (mm)

(mm)

Thickness of zirconium foil (mm)

1

4000

1.5

4

0.5

-

2

4000

1.5

4

0.5

0.05

After laser welding finished, typical cross sections of the joints were cut perpendicular to the welding direction. The standard metallographic samples were prepared, of which the Mo side and Ti side were etched with Mo etchant (i.e. Nitric acid: Sulfuric acid: Water=5:2:3, by volume) and Ti etchant (i.e. Kroll reagent), respectively, and then observed under a Nikon Eclipse MA200 optical microscope (OM) and a LS-JLLH-22 scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) for chemical composition detection. Electron Back Scattered Diffraction (EBSD) was performed to analyse the grain orientation and grain boundary segregation in HAZ. The tensile tests of Mo/Ti joints were performed on an INSTRON universal testing machine at a tensile pulling rate of 1.0 mm/min. The fracture characteristics of the Mo/Ti joints were studied based on SEM observation and EDS analysis. 3 Results and discussion 3.1 Effects of adding Zr on optical microstructures in cross-sections of laser welded Mo/Ti joints Figs 4(a) and 4(b) show optical microstructures in the cross-sections of two welded joints. Figs 4(c) and 4(d) show high-magnification images of microstructures in HAZMo of the two welded joints. As demonstrated in Figs 4(a) and 4(b), when Zr foil was not added, the width of HAZMo was about 1.6 mm, while the width decreased to about 0.9 mm after adding Zr foil. It can be seen from Fig. 4(c) that the initial rolled microstructures (see Fig. 1a) nearly disappeared in the HAZMo of joints without Zr foil and the microstructures were massive equiaxed grains. The size of equiaxed grains near the Mo/ FZ interface reached about 45 µm, and the size of these grains in HAZs gradually decreased with increasing distance from the Mo/FZ interface of the joint without Zr. In Fig. 4(d), although recovery and recrystallisation had, to a certain extent, occurred in the HAZMo of joints after addition of Zr foil, there were residual rolled microstructures presented. In addition, Fig. 4(d) shows that grains in the HAZMo of the joints, after the addition of Zr foil, were small and the grain size near Mo/FZ interface was about 25 µm.

4

Fig. 4 Cross-sections of two welded joints (a, b) and magnified images of regions C and D indicated in panels a and b (c, d), respectively

3.2 Effect of adding Zr on the chemical composition of laser welded Mo/Ti joints Fig. 5 shows the map scanning results of element distribution on cross-sections of the two welded joints: Mo and Ti were distributed uniformly in the FZs. As for the welded joints with Zr, Zr was also uniformly distributed therein. It was worth noting that Ti was only distributed in regions to the right of the Mo/FZ interface, while Zr was detected in regions on both sides thereof, as shown in Figs 5(c) and 5(f).

Fig. 5 Map scanning results of element distribution in a joint without Zr (a, scanning position; b, c, distribution of elements) and a joint with Zr (d, scanning position; e, f, g, distribution of elements)

Fig. 6 shows the line scanning result and point analysis result of element distributions on cross-sections of the two welded joints. Point analysis results in Figs 6(a) and 6(b) show that the Ti contents in FZ of the two joints with, and without, Zr were about 91% and 96% respectively. Line scanning results in Figs 6(a) and 6(b) show that the Ti-content decreased from more than 90% to zero in the Mo/FZ interface of the joints with Zr addition and without Zr addition, respectively. 5

Furthermore, in Fig. 6(b), the Zr content in the FZ of joints with Zr was about 4%, and that at the Mo/FZ interface rapidly decreased from about 4% in the FZ, but did not drop to zero, that is, the Zr content in the HAZ was not zero.

Fig. 6 Line scanning results and point component analysis results of element distribution of the joints without Zr (a) and joint with Zr (b)

3.3 Effect of adding Zr on the HAZ of laser-welded Mo/Ti joints Fig. 7 shows the EBSD result for the HAZMo of the joint with Zr addition. The EBSD scanning region lay close to Mo/FZ interface in the HAZ on the side of Mo plate, as shown in Fig. 7(a). Fig. 7(b) shows grain morphology at the scanning position. It can be seen that grains were recrystallised to a certain extent, while there were still obvious characteristics of residual rolled microstructure. That is, grains elongated in the rolling direction. Fig. 7(c) shows the phase distribution in the scanning region (i.e. red and yellow in Fig. 7c represent Mo and ZrO2, respectively). As shown in Fig. 7(c), ZrO2 particles were distributed at both grain boundaries and the interior of the grains of the HAZMo. This indicated that, during welding, Zr diffused into the unmolten HAZ and combined with elemental O in the HAZMo to generate ZrO2 particles. Fig.8 shows the EBSD result of a welded joint without Zr addition. It can be clearly seen from Fig.8 that there is no ZrO2 particles in the HAZMo of joint without Zr addition. Based on this analysis, when recovery and recrystallisation occurred in the HAZMo during welding, energy on grain boundaries was high and they were mobile so that atoms at the grain boundary were unstable and diffused much faster than atoms in the interior of grains. Meanwhile, owing to the coefficient of linear expansion of Ti being about 1.5 times that of Mo, significant internal tensile stresses, and stress gradients, would be found in HAZMo during welding. This promoted the slippage and migration of grain boundaries and further reduced the activation energy for diffusion across grain boundaries, thus promoting diffusion of Zr into the HAZMo. In addition, 6

oxygen segregation tended to appear at grain boundaries during recrystallisation of Mo [27] and MoO2 was generally generated at grain boundaries; because the free energy of ZrO2 was lower than that of MoO2, Zr was more easily to combine with oxygen in comparison with Mo [30]. Therefore, ZrO2 was formed near grain boundaries. ZrO2 replaced MoO2 with a low fusion point at the grain boundary thus hindering motion of the grain boundary, and inhibiting growth of grains in HAZMo. The ZrO2 particles distributed at the interior of the grains of the HAZMo might be explained as that grain boundaries were continuously keep moving after the formation of ZrO2 particles at grain boundaries, and further research is needed. Apart from this, Fig. 7d shows that ZrO2 which segregated at grain boundaries could be still detected in the region up to 0.75 mm away from the Mo/FZ interface in HAZMo (i.e. red and blue in Fig. 7d represent Mo and ZrO2, respectively). The main reason for this was that Mo has a very high thermal conductivity, being about half the thermal conductivity of copper, therefore, the range of HAZMo was large. Since that the above mentioned diffusion phenomenon of Zr into Mo has not been reported previously, more in-depth investigation on its inherent mechanism in the future is necessary. In addition, after adding Zr foil, there was air gaps at Mo/Zr interface and Zr/Ti interface (as shown in Fig.2b), which might also decrease the heat transfer to the Mo plate from Ti plate to a certain content.

Fig. 7 EBSD results of the HAZMo of a joint with Zr

7

Fig. 8 EBSD results of the HAZMo of a joint without Zr

3.4 TEM observation of the FZ in two laser-welded Mo/Ti joints Fig. 9(a) shows TEM observation results of FZ of a welded joint without Zr. It can be seen that the FZ of the welded joint without Zr was mainly comprised of an acicular α phase and an interspersed β phase, which can attribute to the rapid cooling of the FZ in the welding process. Figs 9(b) and 9(c) show TEM observation results from the FZ of a welded joint with Zr. At this time, matrix of FZ mainly was formed of β phase and Mo2Zr particles were found to be dispersed in the grains. According to the binary phase diagrams of Mo-Zr, Mo-Ti, and Ti-Zr, it is shown that new phases were not formed between Zr and Ti as well as between Mo and Ti, while the Mo2Zr phase was more easily formed between Zr and Mo. The highest initial temperature for the formation of the Mo2Zr phase reached about 2,200 K, while the solidification temperature of Ti was about 1,940 K. Therefore, during solidification of the molten pool, with decreasing temperature, some free Zr and Mo atoms firstly formed an Mo2Zr phase of less than 1 µm in diameter, which provided nucleation sites for the solidification of the molten pool and played a role in dispersion strengthening. Fig 10 shows the grain morphologies and phase distribution of FZ of these two welded joints. It can be seen from the figure that the FZ of welded joint without Zr addition was comprised of acicular α-Ti, while the FZ of welded joint with Zr addition was comprised of columnar β-Ti.

Fig. 9 TEM observation of FZ of welded joints without Zr (a) and with Zr (b, c)

8

Fig.10 The grain morphology of FZ of welded joints without Zr (a) and with Zr (c); the phase distribution of FZ of welded joints without Zr (b) and with Zr (d)

3.5 Effects of adding Zr on microhardness of laser-welded Mo/Ti joints Fig. 11(a) shows the microhardness of both joints: the microhardness of the FZ of a welded joint with Zr was higher than that without Zr, because of solid solution strengthening and dispersion strengthening after addition of Zr into the FZ. The addition of Zr significantly inhibited recrystallisation of grains in the HAZMo, so the HAZ had a small width and fine grains. For these reasons, softening of the HAZMo was relieved after adding Zr, as shown in Fig. 11(b).

Fig. 11 The microhardness distribution on the cross sections of welded joints (a) and microhardness distribution in HAZMo of both joints (b)

3.6 Effects of adding Zr on the tensile strength of laser-welded Mo/Ti joints Fig. 12 shows a comparison of the tensile test results of two welded joints without and with Zr. As shown in Fig. 12(a), the UTS of the joints without Zr foil was 350 MPa, while it increased to 470 MPa after adding Zr foil, and reached more than 90% of that of the BM of Ti. Table 5 shows the tensile properties of welded joints and titanium BM. The elongation rate was too small to measure, while the absorbed energy was calculated from Fig. 12(a) [31-32]. The absorbed energy of joint with Zr was 42.29 J/m3, larger than that of joint without Zr (i.e. 22.14 J/m3). It means that toughness of the joint was significantly improved by Zr addition. Such results indicate 9

that the addition of Zr strengthened the HAZMo of the dissimilar joint. Figs 12(b) and 12(c) compare fracture paths of the two welded joints in tensile test conditions. In the tensile test, the welded joint without Zr foil fractured in the HAZMo close to the Mo/FZ interface, while that with Zr foil fractured near the centre of its FZ. Fig. 11(a) shows that the microhardness of the FZ of a welded joint with Zr was far higher than that of the HAZ and BM. Considering that microhardness can reflect intragranular strength, it can be speculated that joints with Zr probably fractured in the FZ with the highest microhardness because the strength of the grain boundary in the FZ was less than the intragranular strength. After adding Zr foil, grains in the HAZMo were small and maintained their morphology so that grains kept the elongated morphology in rolling direction, and the microhardness improved, indicating an increased strength of the HAZ. Therefore, this correspondingly increased the mechanical performance of the joints and the fracture position changed. Figs 13 and 14 show microscopic morphologies of tensile fractures of the two welded joints. As seen in Fig.13, the fracture of a welded joint without Zr showed the typical morphology of intergranular fracture. It can be seen from Fig. 14 that the tensile fracture of the joint with Zr mainly demonstrated intergranular fracture and small, shallow dimples were found locally. In Fig. 14, the average contents of Zr and Mo in intergranular fractures were 4.58 at% and 4.15 at%, while those on the surface of a dimple fracture were 4.59 at% and 3.51 at%, respectively. The Zr contents in these two fractures were similar, while the main difference was that the average Mo-content in intergranular fractures was 0.64 at% higher than that in dimple fractures.

Fig. 12 The tensile test results of welded joints and Ti BM (a) and fracture paths of two welded joints without Zr (b) and with Zr (c) Table 5 Tensile properties of joints and titanium BM Specimen type

Elongation (%)

UTS (MPa)

Absorbed energy (J/m3)

Joint without Zr

-

350

22.14

Joint with Zr

-

470

42.29

Ti-BM

15

500

279.61

10

Fig. 13 Overall view (a) and typical morphology (b, c) of SEM images of tensile fracture of the welded joint without Zr

Fig. 14 Overall view (a) and typical morphology with point component analysis (b, c) of SEM images of tensile fracture of the welded joint with Zr

4 Conclusions High strength laser welded Mo/Ti dissimilar joint was achieved by adding minor Zr. The following are the major conclusions that can be summarized from the study: (1) After adding 4.6 at% of Zr to the molten pool, the width of HAZMo was decreased from about 1.6 mm to about 0.9 mm, and the grain size in the HAZMo near the Mo/FZ interface decreased from about 45 µm to about 25 µm. (2) The microhardness of the FZ of a welded joint with Zr was higher than that of one without Zr because of solid solution strengthening and dispersion strengthening effects after adding Zr to the FZ. The FZ of a welded joint without Zr mainly comprised an acicular α phase and an interspersed β phase. The matrix of the FZ of a welded joint with Zr mainly consisted of a β phase, and dispersed Mo2Zr particles with diameters of less than 1 µm were found in the grains. (3) After adding about 4.6 at% of Zr to the molten pool, the UTS of Mo/Ti welded joints increased from about 350 MPa to about 470 MPa, reaching more than 90% of that of the BM of Ti. The fracture position of joints under tensile test conditions changed from the HAZMo to the FZ. 11

(4) The mechanism of the strengthening of the HAZMo after adding trace amounts of Zr was that Zr entered the HAZMo through diffusion at grain boundaries and exited at grain boundaries in the form of ZrO2. This inhibited the segregation of volatile MoO2 at grain boundaries in the HAZMo and played a role of inhibiting grain boundary motion in the recrystallisation of the HAZMo, thus inhibiting the growth of grains in the HAZMo. (5) The diffusion distance of Zr in the HAZMo was up to 0.75 mm. Such super-fast diffusion might be attributed to the high-energy grain boundaries and their associated motion during recrystallisation. Moreover, it was related to the significant internal stress and stress gradients generated in the HAZMo during welding, as well as the low Gibbs free energy of ZrO2.

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