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Microstructure evolution and deformation resistance of heavy-thickness Ti-6Al-4V narrow-gap welded joints
Materials Letters 250 (2019) 116–118
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Microstructure evolution and deformation resistance of heavy-thickness Ti-6Al-4V narrow-gap welded joints Tao Yang a,⇑,1, Dezhi Xu a,1, Weilin Chen b, Ruixin Yang a, Shixiong Lv c a
School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China Nuclear Power Institute of China, Chengdu 610213, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China b
a r t i c l e
i n f o
Article history: Received 17 April 2019 Received in revised form 27 April 2019 Accepted 30 April 2019 Available online 2 May 2019 Keywords: Heavy-thickness Ti-6Al-4V plates Welding Microstructure Deformation resistance
a b s t r a c t In this work, 60 mm thick Ti-6Al-4V plates were joined by narrow-gap multi-pass gas tungsten arc welding (GTAW) technology to investigate microstructure and deformation resistance of joints. Depending on cooling condition, mixed weld microstructure consists of stable a phase, martensite a0 phase and few retained b phase. Hardness of joined parts is higher than base metal (BM) mainly contributed by martensite strengthening. A simplified model was established to reveal microstructure evolution during welding thermal cycle. It can also demonstrate that different thermal action effect is responsible for distinct microstructure forms especially martensite a0 , which varies hardness in different regions of joints. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Ti-6Al-4V alloy is the preferred manufacturing material of submersible equipment used for marine exploration in deep-sea areas. Due to the underwater hyperbaric work environment, heavythickness Ti-6Al-4V plates are necessary to ensure sufficient deformation resistance of submersible for safety. Moreover, as the weaknesses of various key structures, joined parts between thick plates are expectedly reinforced to possess better nondeformability as well. As achievements in previous research, microstructure played the key role in Ti-6Al-4V joint properties. Balasubramanian et al. compared tensile strength and fatigue crack growth behavior of gas tungsten arc, electron beam and laser beam welded Ti-6Al4V joints and discovered that refining weld microstructure can most greatly enhance mechanical properties [1]. Babu et al. demonstrated that optimized microstructure of GTAW weldments after introducing current pulsing can bring strengthening effect on mechanical properties of Ti-6Al-4V joint [2]. Rao et al. performed multi-pass bead-over-bead electron beam welding and the more relaxed microstructures and dislocations effectively improved hardness distribution and fracture toughness of Ti-6Al-4V joints ⇑ Corresponding author at: 111, 1st Section, Northern 2nd Ring Road, Chengdu, Sichuan 610031, China. E-mail address: [email protected] (T. Yang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.matlet.2019.04.130 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
[3]. In this paper, for heavy-thickness Ti-6Al-4V plates, we have employed narrow-gap multi-pass GTAW technology to obtain high-quality joints. Furthermore, microstructures and deformation resistance were investigated. A simplified model was developed to reveal microstructure evolution mechanisms of joints during welding thermal cycle as well.
2. Experimental procedures 60 mm thick Ti-6Al-4V plates (6.06 wt% Al, 3.92 wt% V, 0.30 wt % Fe, 0.013 wt% C, 0.014 wt% N) were multi-pass welded using GTAW method and Ti-4Al-2V filler wire (4.50 wt% Al, 1.82 wt% V, 0.20 wt% Fe, 0.033 wt% C, 0.032 wt% N). Equiaxed microstructure of base metal (BM) is shown in Fig. 1(b). A double-side narrowgap groove type with 1 mm middle root face was designed for GTAW. Metallographic structure was accessed by optical microscope (OM) using corrosive liquid by 10% hydrofluoric acid, 30% nitric acid and 60% H2O. Hardness distribution was obtained through HVS-30 Vickers hardness tester. X-ray diffraction (XRD) patterns were collected using Empyrean diffractometer. Element content was accessed by SU8010 scanning electron microscope (SEM). JEM-2100 transmission electron microscope (TEM) was used to deeply take insight into microstructure. TEM specimens were mechanically grinded to 50 lm thickness and automatically milled by Gatan Model 695 precision ion polishing system.
T. Yang et al. / Materials Letters 250 (2019) 116–118
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Fig. 1. (a) Macrograph of the whole joint. (b)–(d) microstructures of BM, FZ and HAZ respectively. (e) CCT curve. (f) XRD patterns of FZ and HAZ. (g) alloying element content of weld seam along depth.
3. Results and discussion The macrograph of the whole Ti-6Al-4V joint (Fig. 1(a)) shows that the I-type weld seam has excellent uniformity. The cover layers and filled layers in fusion zone (FZ) are mainly composed by huge columnar grains whose dimensions can reach up to millimeter level due to the great sensitivity of grain coarsening for titanium alloys [4]. The narrow back layer in the middle of weld seam is full of fine equiaxed grains. The lower weld current mode was employed here to prevent root face burnthrough which simultaneously constrained coarsening as well. According to continuous cooling transformation (CCT) curve (Fig. 1(e)), the critical cooling rate to form full martensite a0 for Ti-6Al-4V is 410 °C/s [5]. However, due to the higher linear energy of GTAW, the actual cooling rates of FZ and heat affected zone (HAZ) were both between 20 and 410 °C/s. The transformed am (massive a) is approximately identical to a0 + a mixed structure. It is noteworthy that martensite a0 in FZ (Fig. 1(c)) is basket weave type while it is acicular shape in HAZ (Fig. 1(d)). This is greatly connected with the distinct thermal cycle condition which will be discussed next. Moreover, alloying element content of weld seam (Fig. 1(g)) especially Al has evidently exceeded nominal component of filler metal since the thermal action process simultaneously provided diffusion path for alloying agents from BM to FZ as illustration in Fig. 1(a). The additional Al can bring about solid solution strengthening effect due to its brilliant solute ability in a phase. Microstructure types would deeply influence joint performance. Non-deformability of the whole joint was characterized by hardness distribution (Fig. 2). The outstanding higher hardness in the middle and upper layers are connected with the strengthening mechanisms of microstructure refining and strain hardening respectively. In general, both FZ and HAZ are harder than BM mainly caused by martensite reinforcement. It can directly prove that deformation resistance of joined parts has been brilliantly improved. Solid solution strengthening in FZ can also do favor to hardness enhancement. However, HAZ is commonly about 20 HV harder than FZ, which is owing to distinction of microstructure types. Mechanical properties are highly related to microstructure while microstructure transformation continuously varies over time [6]. In order to further explore the relationship between
Fig. 2. Hardness distribution of the whole welded joint.
microstructure and non-deformability of Ti-6Al-4V joint, a schematic illustration of microstructure evolutions was developed (Fig. 3(a)). GTAW can heat Ti-6Al-4V to point 1 far above liquid line (1655 °C). During solidification stage, b grains competitively grew to columnar formation along fusion boundary until they met from two sides (point 2). According to CCT curve, both FZ and HAZ were experienced mixed transformation mode to produce (a0 + a) phase with few retained b phase. Besides, martensite grains took place aging to precipitate granular a (point 4). Finally, parallel martensite laths combined with colony b were formed in FZ as a bundle and adjacent bundles had an orientation angle (Fig. 3(b)). It is noteworthy that retained b phase contains continuous or discontinuous layers. This is intimately related to the nonequilibrium transformation under fast cooling rate in FZ, which can be considered as the effect of complete or incomplete grain boundary wetting as well [7]. The discontinuity of softer b phase can reduce its volume fraction in FZ, which is beneficial for deformation resistance [8,9]. When heating temperature reached up to 450 °C, reverse transformation from a to b would occur [10]. While HAZ was relatively far
118
T. Yang et al. / Materials Letters 250 (2019) 116–118
Fig. 3. (a) Microstructure evolutions of FZ and HAZ. (b) and (c) morphologies of martensite a0 in FZ and HAZ respectively (red circles were regions for selected area electron diffraction (SAED) patterns).
away from welding heat source, temperature peak did not exceed 975 °C. A few parent a phase of BM was retained at the end of thermal cycle (point 6). Furthermore, martensite a0 grew as acicular shape in HAZ (Fig. 3(c)), connected with the distinct linear energy condition and alloying element content from FZ [11]. This chaotically interlaced structure is more favorable to resisting deformation compared with the more ductile martensite bundles [12,13]. It is the main reason for the higher hardness in HAZ.
Declaration of Competing Interest
4. Conclusions
References
(1) The heavy-thickness Ti-6Al-4V joint welded by narrow-gap multi-pass GTAW reveals a homogeneous I-type weld seam. Both FZ and HAZ are a0 + a mixed microstructure combined with few retained b phase while they have morphological difference. Element migration occurred during welding thermal action as well. (2) Hardness distribution of the whole joint demonstrates that deformation resistance of joined part is better than BM caused by martensite reinforcement. (3) Distinct thermal cycle characteristics in different regions of joints can vary microstructure evolutions, which is responsible for distinct microstructure types especially martensite a0 as well. Besides, the interlaced martensite structure can contribute to higher hardness compared with the bundle structure.
The authors declare that there is no conflict of interest regarding the publication of this article. Acknowledgements This work was supported by National Natural Science Foundation of China under Grant No. 51605399 and National Key R&D Program of China (2016YFB1200504-A-03).
[1] T.S. Balasubramanian, V. Balasubramanian, M.A.M. Manickam, Mater. Des. 32 (2011) 4509–4520. [2] N.K. Babu, S.G.S. Raman, R. Mythili, S. Saroja, Mater. Charact. 58 (2007) 581– 587. [3] K.P. Rao, K. Angamuthu, P.B. Srinivasan, J. Mater. Process. Technol. 199 (2008) 185–192. [4] M.J. Hu, J.H. Liu, Trans. Nonferrous Met. Soc. China 19 (2009) 324–329. [5] P. Xu, L. Li, C. Zhang, Mater. Charact. 87 (2014) 179–185. [6] Q. Chu, M. Zhang, J. Li, F. Yan, C. Yan, Mater. Lett. 231 (2018) 134–136. [7] A.S. Gornakova, B.B. Straumal, A.N. Nekrasov, A. Kilmametov, N.S. Afonikova, J. Mater. Eng. Perform. 27 (2018) 4989–4992. [8] X. Tan, Y. Kok, Y.J. Tan, M. Descoins, D. Mangelinck, S.B. Tor, K.F. Leong, C.K. Chua, Acta Mater. 97 (2015) 1–16. [9] N. Kashaev, V. Ventzke, V. Fomichev, F. Fomin, S. Riekehr, Opt. Laser. Eng. 86 (2016) 172–180. [10] J.W. Elmer, T.A. Palmer, S.S. Babu, W. Zhang, T. DebRoy, J. Appl. Phys. 95 (2004) 8327–8339. [11] S. Lathabai, B.L. Jarvis, K.J. Barton, Mater. Sci. Eng. A 299 (2001) 81–93. [12] J. Tao, S. Hu, L. Ji, Mater. Charact. 120 (2016) 185–194. [13] M. Yang, H. Zheng, B. Qi, Z. Yang, J. Mater. Process. Technol. 243 (2017) 9–15.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Microstructure evolution and deformation resistance of heavy-thickness Ti-6Al-4V narrow-gap welded joints Tao Yang a,⇑,1, Dezhi Xu a,1, Weilin Chen b, Ruixin Yang a, Shixiong Lv c a
School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China Nuclear Power Institute of China, Chengdu 610213, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China b
a r t i c l e
i n f o
Article history: Received 17 April 2019 Received in revised form 27 April 2019 Accepted 30 April 2019 Available online 2 May 2019 Keywords: Heavy-thickness Ti-6Al-4V plates Welding Microstructure Deformation resistance
a b s t r a c t In this work, 60 mm thick Ti-6Al-4V plates were joined by narrow-gap multi-pass gas tungsten arc welding (GTAW) technology to investigate microstructure and deformation resistance of joints. Depending on cooling condition, mixed weld microstructure consists of stable a phase, martensite a0 phase and few retained b phase. Hardness of joined parts is higher than base metal (BM) mainly contributed by martensite strengthening. A simplified model was established to reveal microstructure evolution during welding thermal cycle. It can also demonstrate that different thermal action effect is responsible for distinct microstructure forms especially martensite a0 , which varies hardness in different regions of joints. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Ti-6Al-4V alloy is the preferred manufacturing material of submersible equipment used for marine exploration in deep-sea areas. Due to the underwater hyperbaric work environment, heavythickness Ti-6Al-4V plates are necessary to ensure sufficient deformation resistance of submersible for safety. Moreover, as the weaknesses of various key structures, joined parts between thick plates are expectedly reinforced to possess better nondeformability as well. As achievements in previous research, microstructure played the key role in Ti-6Al-4V joint properties. Balasubramanian et al. compared tensile strength and fatigue crack growth behavior of gas tungsten arc, electron beam and laser beam welded Ti-6Al4V joints and discovered that refining weld microstructure can most greatly enhance mechanical properties [1]. Babu et al. demonstrated that optimized microstructure of GTAW weldments after introducing current pulsing can bring strengthening effect on mechanical properties of Ti-6Al-4V joint [2]. Rao et al. performed multi-pass bead-over-bead electron beam welding and the more relaxed microstructures and dislocations effectively improved hardness distribution and fracture toughness of Ti-6Al-4V joints ⇑ Corresponding author at: 111, 1st Section, Northern 2nd Ring Road, Chengdu, Sichuan 610031, China. E-mail address: [email protected] (T. Yang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.matlet.2019.04.130 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
[3]. In this paper, for heavy-thickness Ti-6Al-4V plates, we have employed narrow-gap multi-pass GTAW technology to obtain high-quality joints. Furthermore, microstructures and deformation resistance were investigated. A simplified model was developed to reveal microstructure evolution mechanisms of joints during welding thermal cycle as well.
2. Experimental procedures 60 mm thick Ti-6Al-4V plates (6.06 wt% Al, 3.92 wt% V, 0.30 wt % Fe, 0.013 wt% C, 0.014 wt% N) were multi-pass welded using GTAW method and Ti-4Al-2V filler wire (4.50 wt% Al, 1.82 wt% V, 0.20 wt% Fe, 0.033 wt% C, 0.032 wt% N). Equiaxed microstructure of base metal (BM) is shown in Fig. 1(b). A double-side narrowgap groove type with 1 mm middle root face was designed for GTAW. Metallographic structure was accessed by optical microscope (OM) using corrosive liquid by 10% hydrofluoric acid, 30% nitric acid and 60% H2O. Hardness distribution was obtained through HVS-30 Vickers hardness tester. X-ray diffraction (XRD) patterns were collected using Empyrean diffractometer. Element content was accessed by SU8010 scanning electron microscope (SEM). JEM-2100 transmission electron microscope (TEM) was used to deeply take insight into microstructure. TEM specimens were mechanically grinded to 50 lm thickness and automatically milled by Gatan Model 695 precision ion polishing system.
T. Yang et al. / Materials Letters 250 (2019) 116–118
117
Fig. 1. (a) Macrograph of the whole joint. (b)–(d) microstructures of BM, FZ and HAZ respectively. (e) CCT curve. (f) XRD patterns of FZ and HAZ. (g) alloying element content of weld seam along depth.
3. Results and discussion The macrograph of the whole Ti-6Al-4V joint (Fig. 1(a)) shows that the I-type weld seam has excellent uniformity. The cover layers and filled layers in fusion zone (FZ) are mainly composed by huge columnar grains whose dimensions can reach up to millimeter level due to the great sensitivity of grain coarsening for titanium alloys [4]. The narrow back layer in the middle of weld seam is full of fine equiaxed grains. The lower weld current mode was employed here to prevent root face burnthrough which simultaneously constrained coarsening as well. According to continuous cooling transformation (CCT) curve (Fig. 1(e)), the critical cooling rate to form full martensite a0 for Ti-6Al-4V is 410 °C/s [5]. However, due to the higher linear energy of GTAW, the actual cooling rates of FZ and heat affected zone (HAZ) were both between 20 and 410 °C/s. The transformed am (massive a) is approximately identical to a0 + a mixed structure. It is noteworthy that martensite a0 in FZ (Fig. 1(c)) is basket weave type while it is acicular shape in HAZ (Fig. 1(d)). This is greatly connected with the distinct thermal cycle condition which will be discussed next. Moreover, alloying element content of weld seam (Fig. 1(g)) especially Al has evidently exceeded nominal component of filler metal since the thermal action process simultaneously provided diffusion path for alloying agents from BM to FZ as illustration in Fig. 1(a). The additional Al can bring about solid solution strengthening effect due to its brilliant solute ability in a phase. Microstructure types would deeply influence joint performance. Non-deformability of the whole joint was characterized by hardness distribution (Fig. 2). The outstanding higher hardness in the middle and upper layers are connected with the strengthening mechanisms of microstructure refining and strain hardening respectively. In general, both FZ and HAZ are harder than BM mainly caused by martensite reinforcement. It can directly prove that deformation resistance of joined parts has been brilliantly improved. Solid solution strengthening in FZ can also do favor to hardness enhancement. However, HAZ is commonly about 20 HV harder than FZ, which is owing to distinction of microstructure types. Mechanical properties are highly related to microstructure while microstructure transformation continuously varies over time [6]. In order to further explore the relationship between
Fig. 2. Hardness distribution of the whole welded joint.
microstructure and non-deformability of Ti-6Al-4V joint, a schematic illustration of microstructure evolutions was developed (Fig. 3(a)). GTAW can heat Ti-6Al-4V to point 1 far above liquid line (1655 °C). During solidification stage, b grains competitively grew to columnar formation along fusion boundary until they met from two sides (point 2). According to CCT curve, both FZ and HAZ were experienced mixed transformation mode to produce (a0 + a) phase with few retained b phase. Besides, martensite grains took place aging to precipitate granular a (point 4). Finally, parallel martensite laths combined with colony b were formed in FZ as a bundle and adjacent bundles had an orientation angle (Fig. 3(b)). It is noteworthy that retained b phase contains continuous or discontinuous layers. This is intimately related to the nonequilibrium transformation under fast cooling rate in FZ, which can be considered as the effect of complete or incomplete grain boundary wetting as well [7]. The discontinuity of softer b phase can reduce its volume fraction in FZ, which is beneficial for deformation resistance [8,9]. When heating temperature reached up to 450 °C, reverse transformation from a to b would occur [10]. While HAZ was relatively far
118
T. Yang et al. / Materials Letters 250 (2019) 116–118
Fig. 3. (a) Microstructure evolutions of FZ and HAZ. (b) and (c) morphologies of martensite a0 in FZ and HAZ respectively (red circles were regions for selected area electron diffraction (SAED) patterns).
away from welding heat source, temperature peak did not exceed 975 °C. A few parent a phase of BM was retained at the end of thermal cycle (point 6). Furthermore, martensite a0 grew as acicular shape in HAZ (Fig. 3(c)), connected with the distinct linear energy condition and alloying element content from FZ [11]. This chaotically interlaced structure is more favorable to resisting deformation compared with the more ductile martensite bundles [12,13]. It is the main reason for the higher hardness in HAZ.
Declaration of Competing Interest
4. Conclusions
References
(1) The heavy-thickness Ti-6Al-4V joint welded by narrow-gap multi-pass GTAW reveals a homogeneous I-type weld seam. Both FZ and HAZ are a0 + a mixed microstructure combined with few retained b phase while they have morphological difference. Element migration occurred during welding thermal action as well. (2) Hardness distribution of the whole joint demonstrates that deformation resistance of joined part is better than BM caused by martensite reinforcement. (3) Distinct thermal cycle characteristics in different regions of joints can vary microstructure evolutions, which is responsible for distinct microstructure types especially martensite a0 as well. Besides, the interlaced martensite structure can contribute to higher hardness compared with the bundle structure.
The authors declare that there is no conflict of interest regarding the publication of this article. Acknowledgements This work was supported by National Natural Science Foundation of China under Grant No. 51605399 and National Key R&D Program of China (2016YFB1200504-A-03).
[1] T.S. Balasubramanian, V. Balasubramanian, M.A.M. Manickam, Mater. Des. 32 (2011) 4509–4520. [2] N.K. Babu, S.G.S. Raman, R. Mythili, S. Saroja, Mater. Charact. 58 (2007) 581– 587. [3] K.P. Rao, K. Angamuthu, P.B. Srinivasan, J. Mater. Process. Technol. 199 (2008) 185–192. [4] M.J. Hu, J.H. Liu, Trans. Nonferrous Met. Soc. China 19 (2009) 324–329. [5] P. Xu, L. Li, C. Zhang, Mater. Charact. 87 (2014) 179–185. [6] Q. Chu, M. Zhang, J. Li, F. Yan, C. Yan, Mater. Lett. 231 (2018) 134–136. [7] A.S. Gornakova, B.B. Straumal, A.N. Nekrasov, A. Kilmametov, N.S. Afonikova, J. Mater. Eng. Perform. 27 (2018) 4989–4992. [8] X. Tan, Y. Kok, Y.J. Tan, M. Descoins, D. Mangelinck, S.B. Tor, K.F. Leong, C.K. Chua, Acta Mater. 97 (2015) 1–16. [9] N. Kashaev, V. Ventzke, V. Fomichev, F. Fomin, S. Riekehr, Opt. Laser. Eng. 86 (2016) 172–180. [10] J.W. Elmer, T.A. Palmer, S.S. Babu, W. Zhang, T. DebRoy, J. Appl. Phys. 95 (2004) 8327–8339. [11] S. Lathabai, B.L. Jarvis, K.J. Barton, Mater. Sci. Eng. A 299 (2001) 81–93. [12] J. Tao, S. Hu, L. Ji, Mater. Charact. 120 (2016) 185–194. [13] M. Yang, H. Zheng, B. Qi, Z. Yang, J. Mater. Process. Technol. 243 (2017) 9–15.