Additive manufacturing of Ti-6Al-4V lattice structures with high structural integrity under large compressive deformation

doi:10.1016/j.jmst.2018.10.029

Abstract

Abstract:

Additively manufactured Ti-6Al-4V lattice structures have found important niche applications. However, they often show insufficient compressive ductility or insufficient structural integrity. In this study, a batch of 45 octahedral Ti-6Al-4V lattice structures was manufactured in three different strut diameters (0.5, 1.0, 1.5 mm) by selective electron beam melting (SEBM). The influence of post-SEBM annealing on the compressive deformation characteristics of the lattice structure was investigated. The as-built Ti-6Al-4V lattices fragmented when the compressive strain reached 13%-23% depending on strut diameter. Annealing at 950 °C (β transus temperature: 995 °C) only slightly improved the compressive ductility of the lattice structures. However, annealing at 1050 °C (β-annealing) fundamentally changed the compressive deformation mode of the lattice structures. The resultant compressive stress-strain curve was featured by a long smooth plateau and no facture occurred even after significant densification of the lattice structure had taken place (>50% of compressive strain).

Key words: Selective electron beam melting, Ti-6Al-4V, Lattice structures, Ductility, Annealing, Beta annealing

Kun Yang, Jian Wang, Liang Jia, Guangyu Yang, Huiping Tang, Yuanyuan Li. Additive manufacturing of Ti-6Al-4V lattice structures with high structural integrity under large compressive deformation[J]. J. Mater. Sci. Technol., 2019, 35(2): 303-308.

Figures/Tables 10

Fig. 1. Octahedral unit.

Fig. 2. As-built SEBM Ti-6Al-4V lattice samples with strut diameters of (a) 0.5 mm, (b) 1.0 mm, and (c) 1.5 mm. (a?), (b?) and (c?) are SEM images of the unit cell structure in each lattice. (a??), (b??) and (c??) are cross-sectional views of selected struts from each lattice structure.

Table 1 Comparison of strut diameters and porosity levels by design and after SEBM.

Designed strut diameter (mm)	Manufactured strut diameter (mm)	Deviation in strut diameter (mm)	Designed lattice porosity (%)	Porosity of manufactured lattice (%)	Deviation in lattice porosity (%)
0.5	0.71 ± 0.03	0.21 ± 0.03	94.43	91.25 ± 0.03	-3.18 ± 0.03
1.0	1.12 ± 0.01	0.12 ± 0.01	86.67	84.28 ± 0.02	-2.39 ± 0.02
1.5	1.56 ± 0.02	0.06 ± 0.02	72.10	71.05 ± 0.05	-1.05 ± 0.05

Fig. 3. Strut surface roughness of Ti-6Al-4V lattice structures in different conditions.

Fig. 4. Representative optical microstructures of struts in the as-built and annealed states. (a): As-built, (b) annealing at 950 °C for 2 h, (c-c?) annealing at 1050 °C for 2 h.

Table 2 Comparison of microstructure features for the lattices in the different conditions.

Designed strut diameter (mm)	α-lath thickness (μm)
Designed strut diameter (mm)	As-built	950 °C × 2 h	1050 °C × 2 h
0.5	0.92 ± 0.06	2.73 ± 0.53	6.08 ± 1.33
1.0	0.87 ± 0.07	2.89 ± 0.73	6.11 ± 0.85
1.5	0.90 ± 0.05	2.87 ± 0.60	5.98 ± 0.77

Fig. 5. Compressive stress-strain curves of Ti-6Al-4V lattices with different strut diameters in as-built and annealed conditions.

Table 3 Compressive mechanical properties of Ti-6Al-4V lattice structures in as-built and annealed conditions.

Strut diameter (mm)	Condition	Compressive strength (MPa)	Compressive strain (%)	Elastic modulus (MPa)
0.5	As-built	1.13 ± 0.05	23.58 ± 1.24	101.43 ± 4.88
	950 °C × 2 h	1.11 ± 0.06	29.07 ± 2.76	108.01 ± 4.83
	1050 °C × 2 h	1.07 ± 0.02	>50%	101.20 ± 4.79
1.0	As-built	8.06 ± 0.03	15.04 ± 0.88	1095.43 ± 12.73
	950 °C × 2 h	7.60 ± 0.04	17.51 ± 0.58	1073.77 ± 25.84
	1050 °C × 2 h	7.43 ± 0.17*	>50%	1010.06 ± 16.54
1.5	As-built	35.29 ± 1.45	13.41 ± 0.70	4196.55 ± 143.42
	950 °C × 2 h	33.22 ± 0.84	15.15 ± 1.01	4047.90 ± 70.58
	1050 °C × 2 h	34.09 ± 0.94	>50%	3799.90 ± 70.58

Fig. 6. Evolution of compression deformation of Ti-6Al-4V lattice structures (strut diameter: 1.0 mm) in the as-built and annealed states. (a): As-built, (b) annealing at 950 °C for 2 h, (c) annealing at 1050 °C for 2 h.

Fig. 7. SEM side-view images of deformed lattice (strut diameter: 1.0 mm) structures. (a, b) As-built, when first fragmentation of struts occurred (compressive strain: 15%). (c) Annealed at 950 °C for 2 h, compressive strain: 60%. (d) Annealed at 1050 °C for 2 h, compressive strain: 60%. Arrows indicate occurrence of micro-cracks near lattice nodes.

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