Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures

doi:10.1016/j.jmst.2021.01.004

Abstract

Abstract:

Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V, which were subjected to a bi-lamellar heat treatment, which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions. The bi-lamellar microstructure was obtained as (i) a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization. For the bi-lamellar treatment with β-homogenization, cooling rates in the range 1-500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 °C. The microstructures were characterized using various microscopical techniques. Mechanical testing with Vickers hardness indentation and tensile testing was performed. The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure, because of the presence of fine α-platelets inside the β-lamellae. Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions. The age hardened bi-lamellar microstructure shows a similar hardness as the very fine, as-built martensitic microstructure. The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition, which has high strength, but poor ductility. After the bi-lamellar heat treatment, the elongation was improved by more than 250 %. Due to the very high strength of the as-built condition, loss of tensile strength is unavoidable, resulting in a reduction of tensile strength of ~18 %.

Key words: Additive manufacturing, Ti-6Al-4V, Targeted heat treatment, Bi-lamellar microstructures, Laser powder bed fusion

Cecilie V. Funch, Alessandro Palmas, Kinga Somlo, Emilie H. Valente, Xiaowei Cheng, Konstantinos Poulios, Matteo Villa, Marcel A.J. Somers, Thomas L. Christiansen. Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures[J]. J. Mater. Sci. Technol., 2021, 81: 67-76.

Figures/Tables 14

Table 1 Measured powder chemical composition (wt%).

	Ti	Al	V	C	Fe	O	N	H
Min	Balance	5.50	3.50	-	-	-	-	-
Actual	Balance	6.42	4.00	0.02	0.17	0.09	0.02	0.0028
Max	Balance	6.50	4.50	0.08	0.25	0.13	0.03	0.0120

Table 2 Most relevant printing parameters.

	Speed (mm/s)	Power (W)	Hatch distance (mm)
Support	650	150	-
Volume	1100	350	0.120

Table 3 Overview of the heat treatments conducted on the as-built Ti-6Al-4V specimens.

	Furnace	Temperature (°C)	Holding time (min)	Cooling rate (K/min)	Atmosphere
β-homogenization	Thermal analyzer	1080	30	1 5 50	Ar
	Horizontal tube furnace			Quench 8 bar ~ 200 Quench 10 bar ~ 500	N₂
	Muffle furnace		8	Water quench	Open air
Inter-critical annealing	Horizontal tube furnace	850 900 950	30	Quench 10 bar ~ 500	N₂
Aging	Horizontal tube furnace	500	1440	-	Vacuum

Fig. 1. LOM micrographs of the as-built martensitic microstructure at different specimen orientations: (a) XZ; (b) XY; (c) XZ. Build direction along Z. Banding is visible in the XZ plane (a) with elongated prior-β grains. In the XY plane (b), the grains appear almost square.

Fig. 2. LOM micrographs of microstructures after initial β homogenization treatment using different cooling rates. Increasing cooling rates result in a microstructure refinement for slow cooling.

Fig. 3. LOM micrographs of microstructures after initial β homogenization treatment using different cooling rates. Primary grain boundary α is formed for all cooling rates. At intermediate cooling rates massive α is formed.

Fig. 4. LOM micrographs of microstructure after full bi-lamellar treatment for three different annealing temperatures for the specimen with a cooling rate of 5 K/min from the β homogenization step. The specimen treated at 950 °C showed the highest fraction of prior β and therefore bi-lamellar structure.

Fig. 5. LOM micrographs of microstructure after full bi-lamellar treatment at 950 °C with different homogenizations. In all cases a bi-lamellar microstructure was formed with the refinement depending on the scale of the specimen after homogenization. The as-built condition is especially chaotic as no β homogenization was performed.

Fig. 6. TEM micrographs of heat treated specimens. The fine secondary lamellae are visible after the inter-critical annealing step.

Fig. 7. Comparison of micro hardness before and after the aging treatment for the homogenized with ~200 K/min and as-built base conditions. After ageing the hardness increased to a similar level of ~375 HV.

Fig. 8. Comparison of tensile properties between the as-built condition and bi-lamellar heat treatments. The bi-lamellar annealing directly from the as-built condition results in a tensile elongation of >12 %.

Fig. 9. Thermo-Calc calculations of the fraction of α and β as a function of temperature for the Ti-6Al4 V alloy (left) and the composition of β as a function of temperature (right).

Fig. 10. Schematic illustration of the effect of the cooling rate after inter-critical annealing on the resulting microstructure within the inter-lamellar β-region, starting from the microstructure at the top of the graph.

Fig. 11. Micrographs of extended inter-critical annealing for as-built base condition. (a) LOM, (b) SEM and (c) sketch. The α platelets inside the β lamellas are clearly visible.

References 28

[1]	B. Dutta, F.H. Froes, 72, 2017, pp.263-274.
[2]	L. Thijs, F. Verhaeghe, T. Craeghs, J. Van Humbeeck, J.P. Kruth, Acta Mater. 58 (2010) 3303-3312. DOI URL
[3]	E. Yasa, J. Deckers, J.P. Kruth, Experimental Investigation of Charpy ImpactTests on Metallic SLM Parts, in: Innovative Developments in Design and Manufacturing: Advanced Research in Virtual and Rapid Prototyping, 2010,pp. 207-214.
[4]	L.E. Murr, S.A. Quinones, S.M. Gaytan, J. Mech, Behav. Biomed. Mater. 2 (2009)20-32.
[5]	T. Vilaro, C. Colin, J.D. Bartout, Metall. Mater. Trans. A 42 (2011)3190-3199. DOI URL
[6]	G.M. Ter Haar, T.H. Becker, Materials (Basel) 11 (2018) 146-161. DOI URL
[7]	M. Simonelli, Y.Y. Tse, C. Tuck, Mater. Sci. Eng. A 616 (2014) 1-11. DOI URL
[8]	B. Vrancken, L. Thijs, J.P. Kruth, J. Van Humbeeck, J. Alloys Compd. 541 (2012)177-185. DOI URL
[9]	Q. Huang, X. Liu, X. Yang, R. Zhang, Z. Shen, Q. Feng, Front. Mater. Sci. 9 (2015)373-381. DOI URL
[10]	G. Lütjering, Mater. Sci. Eng. A 243 (1998) 32-45. DOI URL
[11]	G. Lütjering, J.C. Williams, Springer Berlin Heidelberg, New York, 2007.
[12]	G. Lütjering, Mater. Sci. Eng. A 263 (2002) 117-126. DOI URL
[13]	G. Wegmann, J. Albrecht, G. Lütjering, K.D. Folkers, C. Liesner, Int. J. Mater. Res. 88 (1997) 764-773.
[14]	G. Kasperovich, J. Hausmann, J. Mater. Process. Technol. 220 (2015)202-214. DOI URL
[15]	W. Xu, M. Brandt, S. Sun, Acta Mater. 85 (2015) 74-84. DOI URL
[16]	Y. Chong, T. Bhattacharjee, N. Tsuji, Mater. Sci. Eng. A 762 (2019), 138077. DOI URL
[17]	Q. Huang, N. Hu, X. Yang, R. Zhang, Q. Feng, Front. Mater. Sci. 10 (2016)428-431. DOI URL
[18]	M. Simonelli, Y.Y. Tse, C. Tuck, J. Phys. Conf. Ser. 371 (2012) 1-4.
[19]	W. Xu, M. Brandt, S. Sun, Acta Mater. 85 (2015) 74-84. DOI URL
[20]	S. Cao, R. Chu, X. Zhou, J. Alloys Compd. 744 (2018) 357-363. DOI URL
[21]	J. Mezzetta, J.P. Choi, J. Milligan, Int. J. Precis. Eng. Manuf. Technol. 5 (2018)605-612.
[22]	T. Ahmed, H.J. Rack, Mater. Sci. Eng. A 243 (1998) 206-211. DOI URL
[23]	A.L. Pilchak, T.F. Broderick, JOM 65 (2013) 636-642. DOI URL
[24]	S. Malinov, W. Sha, Z. Guo, Mater. Sci. Eng. A 283 (2000) 1-10. DOI URL
[25]	G.F.V. Voort, Atlas of Time-Temperature Diagrams for Nonferrous Alloys, ASMInternational, 1991.
[26]	Y. Chong, N. Tsuji, The Effect of Initial Microstructure on the Mechanical Properties of Bi-lamellar Ti-6Al-4V, 145th Annu. Meet. Exhib. TMS 2016 (2016) 633-640.
[27]	G. Schröder, J. Albrecht, G. Lütjering, Mater. Sci. Eng. A 319- 321 (2001)602-606.
[28]	C.L. Li, J.W. Won, S.W. Choi, J. Alloys Compd. 803 (2019) 407-412. DOI URL