Globularization Mechanism and Near Isotropic Properties in Subcritical Heat-Treated Ti6Al4V Fabricated by Directed Energy Deposition

doi:10.1007/s40195-022-01516-1

Abstract

Abstract:

Microstructure with globular α phase is desirable as it contributes to preferable comprehensive mechanical properties for titanium alloys. However, titanium alloys fabricated by directed energy deposition (DED) are mainly characterized by the lamellar α phase within the basket-weave microstructure, which often leads to severe anisotropy and inferior low cycle fatigue (LCF) properties. To address this, the subcritical annealing and the cyclic annealing were applied to DED Ti-6Al-4V in order to achieve the transformation from the lamellar α phase to the globular α phase. The microstructural characteristics and the globularization behavior of α phase during heat treatment were investigated. The results show that the aspect ratio of α is significantly decreased with the subcritical annealing due to the coarsening of lamellar α. Furthermore, the globular α is obtained with the cyclic annealing as a combination result of the termination dissolution and the side surface growth of the lamellar α. These contribute to a pronounced reduction of 85.4% in the ductility anisotropy, compared with the as-built specimens, and superior comprehensive mechanical properties including LCF are achieved with the formation of globular α.

Key words: Directed energy deposition, Ti-6Al-4V, Subcritical heat treatment, Coarsening, Globularization

Guohao Zhang, Zhiwei Hao, Meng Wang, Xufei Lu, Zhuang Zhao, Qian Wang, Xin Lin, Jing Chen, Weidong Huang. Globularization Mechanism and Near Isotropic Properties in Subcritical Heat-Treated Ti6Al4V Fabricated by Directed Energy Deposition[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(6): 937-948.

Figures/Tables 17

Table 1 Processing parameters of DED

Laser power (W)	Scanning speed (mm s⁻¹)	Layer thickness (mm)	Laser spot diameter (mm)	Hatching space (mm)	Dimension of deposit (mm³)
2000	15	0.5	5	2.5	115(length) × 25(width) × 70(height)

Fig. 1 a Deposition strategy used in DED. b Gage dimensions of room temperature tensile test specimens. c Gage dimensions of the LCF specimens

Fig. 2 Detailed process of the cyclic annealing

Fig. 3 a, b OM and SEM micrographs of the as-built specimens. c, d OM and SEM micrographs of the subcritical annealing for 0.25 h specimens

Fig. 4 a SEM micrograph of the as-built specimen, b corresponding EBSD micrograph in the same position

Fig. 5 Schematic diagram of the coarsening process during the subcritical annealing

Fig. 6 Equilibrium mole fraction of α and β phase with temperature (CALPHAD by JMatPro)

Fig. 7 Microstructure evolution during subcritical annealing at 960 °C with different annealing time: a 0.5 h; b 1 h; c 2 h; d 3 h; e 4 h

Fig. 8 Microstructure characteristics of the α phase during subcritical annealing for different time

Fig. 9 Microstructure under different heat treatments: a, b subcritical annealing at 960 °C for 3 h; c, d cyclic annealing for 3 h between 960 and 820 °C; e, f cyclic annealing assisted with solution-aging treatment

Fig. 10 OM images around the prior-β grain boundaries: a continuous αGB in the as-built specimen; b disappearance of αGB in the cyclic annealing specimen

Table 2 Parameters of α after different heat treatments

	Subcritical annealing	Cyclic annealing	Cyclic annealing + solution-aging treatment
Aspect ratio	2.81	2.02	1.53
Length (μm)	13.31	18.24	11.15
Width (μm)	4.76	9.18	7.36
Mean radius (μm)	3.15	5.49	4.18

Fig. 11 a Schematic diagram of the curvature and phase transformation temperature of the termination and side surface of the α lamellae. b Dissolution and precipitation behavior of the termination and side surface of the α lamellae during one thermal cycle

Fig. 12 Room temperature tensile properties and anisotropic properties of as-built and cyclic annealing specimens

Fig. 13 Fracture morphology of cyclic annealing specimens in different directions: a, b horizontal; c, d vertical

Fig. 14 Micrograph near the fracture surface of cyclic annealing specimen

Fig. 15 LCF properties of the as-built and the CSTA specimens

References 34

[1]	C. Leyens, M. Peters, Titanium and titanium alloys (Wiley-VCH, Cologne, 2003), pp. 1-532
[2]	L.C. Zhang, Y.J. Liu, S.J. Li, Y.L. Hao, Adv. Eng. Mater. 20, 1700842(2018) DOI URL
[3]	B. Baufeld, O.V. Biest, Sci. Technol. Adv. Mat. 10, 15008(2009) PMID
[4]	S.L. Semiatin, S.L. Knisley, P.N. Fagin, D.R. Barker, F. Zhang, Metall. Mater. Trans. A 34, 2377 (2003) DOI URL
[5]	I. Weiss, E.H. Froes, D. Eylon, G.E. Welsch, Metall. Trans. A 17, 1935 (1986)
[6]	Z.N. Wang, X. Lin, L.L. Wang, Y. Cao, Y.H. Zhou, W.D. Huang, Addit. Manuf. 47, 102298 (2021)
[7]	Y.J. Wang, S. Chu, Z.J. Wang, J.J. Li, J.C. Wang, Acta Metall. Sin. -Engl. Lett. 35, 425 (2022)
[8]	G. Lütjering, Mater. Sci. Eng. A 243, 32 (1998) DOI URL
[9]	L.E. Murr, S.A. Quinones, S.M. Gaytan, M.I. Lopez, A. Rodela, E.Y. Martinez, D.H. Hernandez, E. Martinez, F. Medina, R.B. Wicker, J. Mech. Behav. Biomed. Mater. 2, 20 (2009) DOI PMID
[10]	S.Y. Zhang, X. Lin, J. Chen, W.D. Huang, Rare Met. 28, 537 (2009) DOI URL
[11]	J. Yu, M. Rombouts, G. Maes, F. Motmans, Phys. Procedia 39, 416 (2012)
[12]	B.E. Carroll, T.A. Palmer, A.M. Beese, Acta Mater. 87, 309 (2015) DOI URL
[13]	A.J. Sterling, B. Torries, N. Shamsaei, S.M. Thompson, D.W. Seely, Mater. Sci. Eng. A 655, 100 (2016) DOI URL
[14]	G.H. Zhang, X.F. Lu, J.Q. Li, J. Chen, X. Lin, M. Wang, H. Tan, W.D. Huang, Addit. Manuf. 55, 102865 (2018)
[15]	S.L. Semiatin, V. Seetharaman, I. Weiss, Mater. Sci. Eng. A 263, 257 (1999) DOI URL
[16]	N. Stefansson, S.L. Semiatin, Metall. Mat. Trans. A 34, 691 (2003) DOI URL
[17]	N. Davari, A. Rostami, S.M. Abbasi, Mater. Sci. Eng. A 683, 1 (2017) DOI URL
[18]	C.M. Liu, X.J. Tian, H.M. Wang, D. Liu, Mater. Sci. Eng. A 609, 177 (2014) DOI URL
[19]	Z. Zhao, J. Chen, H. Tan, G.H. Zhang, X. Lin, W.D. Huang, Scr. Mater. 146, 187(2018) DOI URL
[20]	S. Cao, Q.D. Hu, A.J. Huang, Z.E. Chen, M. Sun, J.H. Zhang, C.X. Fu, Q.B. Jia, C.V.S. Lim, R.R. Boyer, Y. Yang, X.H. Wu, J. Mater. Sci. Technol. 35, 1578 (2019) DOI URL
[21]	S. Sui, Y.X. Chew, Z.W. Hao, F. Weng, C.L. Tan, Z.L. Du, G.J. Bi, Adv. Powder Mate. 1, 100002(2022)
[22]	R. Sabban, S. Bahl, K. Chatterjee, S. Suwas, Acta Mater. 162, 239 (2019) DOI URL
[23]	Y.J. Wang, J.J. Li, J.W. Li, L. Zhang, J.K. Ma, Z.J. Wang, F. He, J.C. Wang, J. Alloy. Compd. 914, 165322 (2022) DOI URL
[24]	Z. Zhao, J. Chen, S. Guo, H. Tan, X. Lin, W.D. Huang, J. Mater. Sci. Technol. 33, 675 (2017) DOI URL
[25]	S.Y. Zhang, X. Lin, J. Chen, F.Y. Zhang, W.D. Huang, Rare Met. Mat. Eng. 10, 1839 (2007)
[26]	A.J. Ardell, Acta Metall. 20, 61 (1972) DOI URL
[27]	S.L. Semiatin, B.C. Kirby, G.A. Salishchev, Metall. Mater. Trans. A 35, 2809 (2004) DOI URL
[28]	W. Kurz, D.J. Fisher, Fundamentals of Solidification, Fourth Rev (Trans Tech Publications, Switzerland, 1998), pp. 1-13
[29]	I.M. Lifshitz, V.V. Slyozov, J. Phys. Chem. Solids. 19, 35 (1961) DOI URL
[30]	A. Ho, H. Zhao, J.W. Fellowes, F. Martina, A.E. Davis, P.B. Prangnell, Acta Mater. 166, 306 (2019) DOI URL
[31]	J.H. Zhang, Y. Yang, S. Cao, Z.Q. Cao, D. Kovalchuk, S.Q. Wu, E.Q. Liang, X. Zhang, W. Chen, F. Wu, A.J. Huang, Acta Metall. Sin. -Engl. Lett. 33, 1311 (2020)
[32]	C.M. Liu, Y. Lu, X.J. Tian, D. Liu, Mater. Sci. Eng. A 661, 145 (2016) DOI URL
[33]	B10 Committee. ASTM B381-13 Standard specification for titanium and titanium alloy forgings. ASTM Int. (2013)
[34]	Y.M. Ren, X. Lin, P.F. Guo, H.O. Yang, H. Tan, J. Chen, J. Li, Y.Y. Zhang, W.D. Huang, Int. J. Fatigue 127, 58(2019) DOI URL