Microstructure and mechanical properties of Ti-6Al-4V-5% hydroxyapatite composite fabricated using electron beam powder bed fusion

doi:10.1016/j.jmst.2018.10.025

Abstract

Abstract:

A novel, Ti-6Al-4V (Ti64)/Hydroxyapatite (HA at 5% by weight concentration) metal/ceramic composite has been fabricated using electron beam powder bed fusion (EPBF) additive manufacturing (AM): specifically, the commercial electron beam melting (EBM^®) process. In addition to solid Ti64 and Ti64/5% HA samples, four different unit cell (model) open-cellular mesh structures for the Ti64/5% HA composite were fabricated having densities ranging from 0.68 to 1.12 g/cm³, and corresponding Young’s moduli ranging from 2.9 to 8.0 GPa, and compressive strengths ranging from ～3 to 11 MPa. The solid Ti64/5%HA composite exhibited an optimal tensile strength of 123 MPa, and elongation of 5.5% in contrast to a maximum compressive strength of 875 MPa. Both the solid composite and mesh samples deformed primarily by brittle deformation, with the mesh samples exhibiting erratic, brittle crushing. Solid, EPBF-fabricated Ti64 samples had a Vickers microindentation hardness of 4.1 GPa while the Ti64/5%HA solid composite exhibited a Vickers microindentation hardness of 6.8 GPa. The lowest density Ti64/5%HA composite mesh strut sections had a Vickers microindentation hardness of 7.1 GPa. Optical metallography (OM) and scanning electron microscopy (SEM) analysis showed the HA dispersoids to be highly segregated along domain or grain boundaries, but homogeneously distributed along alpha (hcp) platelet boundaries within these domains in the Ti64 matrix for both the solid and mesh composites. The alpha platelet width varied from ～5 μm in the EPBF-fabricated Ti64 to ～1.1 μm for the Ti64/5%HA mesh strut. The precursor HA powder diameter averaged 5 μm, in contrast to the dispersed HA particle diameters in the Ti64/5%HA composite which averaged 0.5 μm. This work highlights the use of EPBF AM as a novel process for fabrication of a true composite structure, consisting of a Ti64 matrix and interspersed and exposed HA domains, which to the authors’ knowledge has not been reported before. The results also illustrate the prospects not only for fabricating specialized, novel composite bone replacement scaffolds and implants, through the combination of Ti64 and HA, but also prospects for producing a variety of related metal/ceramic composites using EPBF AM.

Key words: Hydroxyapatite (HA), Metal-matrix ceramic composites, Open-cellular (mesh) structures, Electron-beam powder bed fusion (EPBF), Additive manufacturing (AM), Mechanical properties, Microstructure characterization

César A.Terrazas, Lawrence E.Murr, Diego Bermudez, Edel Arrieta, David A.Roberson, Ryan B.Wicker. Microstructure and mechanical properties of Ti-6Al-4V-5% hydroxyapatite composite fabricated using electron beam powder bed fusion[J]. J. Mater. Sci. Technol., 2019, 35(2): 309-321.

Figures/Tables 14

Fig. 1. Pre-alloyed, precursor Ti64 powder. (a) SEM image. (b) OM image of powder particle (interior) section.

Fig. 2. SEM view of Ti64/5%HA powder mixture. (a) and (b) Show low and high magnification views, respectively.

Fig. 3. Comparative precursor powder XRD spectra. (a) Ti64 powder shown in Fig. 1 (a). (b) Ti64/5%HA powder mixture shown in Fig. 2.

Fig. 4. Optical microscope (OM) images of EPBF-fabricated solid components. (a) Ti64 component section observed in the vertical plane (parallel to the build direction). (b) Ti64/5%HA composite section observed in the vertical plane. Dark arrows in (a) point to the grain boundary of the prior β-phase while the white arrows point to two α-lamellae within an α-phase colony. Dark arrows in (b) point to HA precipitated in the grain boundaries of the large grain structure domains while white arrows in (b) point to the precipitated HA particles inside these grain domains.

Fig. 5. XRD spectra for EPBF-fabricated solid component sections observed in the horizontal plane (perpendicular to the build direction). (a) Ti64. (b) Ti64/5%HA composite.

Fig. 6. Comparison of open-cellular, mesh design models (unit cells) and corresponding EPBF-fabricated mesh structures in decreasing order of density. (a) Octet truss (1.12 g/cm3), (b) G structure 3 (G3) (0.81 g/cm3), (c) Rhombic dodecahedron (0.70 g/cm3), and (d) Dode medium (0.68 g/cm3).

Table 1 Comparative EPBF-fabricated solid and mesh structure properties.

Material	Density ρ (g/cm³)	ρ/ρ_s	Porosity^a (%)	Modulus^b E (GPa)	E/E_s	Mesh strut thickness (mm)	Pore size meas. (mm)	Pore size cal. (mm)^c
Solid Ti64	4.43	～1	～0	110	1	-	-	-
Solid Ti64/5%HA	4.16	0.96	4	110	1	-	-	-
Ti64/5%HA Octet-mesh Fig. 6(a)	1.12	0.27	73	8.0	0.073	1.5	0.9	1.3
Ti64/5%HA G3-mesh Fig. 6(b)	0.81	0.19	81	4.0	0.036	0.80	2.1	2.3
Ti64/5%HA Rhom. dodec. mesh Fig. 6(c)	0.70	0.17	83	3.2	0.029	0.65	2.4	2.7
Ti64/5%HA Dode m-mesh Fig. 6(d)	0.68	0.16	84	2.9	0.026	0.60	2.7	2.9

Fig. 7. Mesh strut (for the Dode medium unit cell) section backscatter electron (BSE) image (a), and corresponding strut cross-section OM image for Ti64/5%HA composite (b).

Fig. 8. BSE/EDS image comparison showing magnified views of the strut cross-section shown in the OM image in Fig. 7(b) above. (a) BSE image showing HA dispersoids (white) in Ti64 matrix by atomic number contrast. (b) EDS (characteristic x-ray) map using combined P and Ca elemental x-ray spectra to identify HA dispersoids (white) as in (a).

Fig. 9. Tensile (a) and compressive (b) stress-strain diagrams for solid, EPBF-fabricated Ti64/5%HA composite components.

Table 2 Mechanical property data: Solid Ti64 and Ti64/5%HA; and mesh Ti64/5%HA.

Material	Tensile yield stress YS (0.2%) (GPa)	Ultimate tensile stress (GPa)	Tensile elongation (%)	Vickers hardness (GPa)	Compressive strength (MPa)
Commercial (cast) Ti64 solid^a	0.82	0.90	7	3.1	-
Commercial (wrought) Ti64 solid^a	1.17	1.23	12	3.8	1700^a
Solid EPBF Ti64	0.95	1.11	13	4.1	1300^a
Solid EPBF Ti64/5%HA	-	0.12	5	6.8	875
EPBF Ti64/5%HA Octet mesh Fig. 6(a)	-	-	-	-	10.8
EPBF Ti64/5%HA G3 mesh Fig. 6(b)	-	-	-	-	3.2
EPBF Ti64/5%HA Rhom. Dodec. mesh Fig. 6(c)	-	-	-	-	2.5
EPBF Ti64/5%HA Dode m. mesh Fig. 6(d)	-	-	-	7.1	3.3

Fig. 10. Compression-displacement diagrams for EPBF-fabricated Ti64/5%HA mesh components.

Fig. 11. SEM images of Ti64 tensile fracture surface features. (a) Wrought component. (b) EPBF-fabricated component. (a) and (b) correspond to stress-strain data in Table 1.

Fig. 12. SEM images for tensile fracture surface features for tensile fracture for Ti64/5%HA composite components as in Fig. 9(a); at low (a) and high (b) magnifications.

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