Mechanical Properties of Titanium/Nano-Fluorapatite Parts Produced by Laser Powder Bed Fusion

Abstract

Laser powder bed fusion (L-PBF) has attracted great interest in recent years due to its ability to produce intricate parts beyond the capabilities of traditional manufacturing processes. L-PBF processed biomedical implants are usually made of commercial pure titanium (CP-Ti) or its alloys. However, both alloys are naturally bio-inert, and thus reduce the formation of apatite as implants are put into the human body. Accordingly, in an attempt to improve the bioactivity of the materials used for making orthopedic implants, the present study decomposed fluorapatite material (FA, (Ca₁₀(PO₄)₆F₂)) into the form of nano-powder and mixed this powder with CP-Ti powder in two different ratios (99%Ti + 1%FA (Ti-1%FA) and 98%Ti + 2%FA (Ti-2%FA)) to form powder material for the L-PBF process. Experimental trials were conducted to establish the optimal processing conditions (i.e., laser power, scanning speed and hatching space) of the L-PBF process for the two powder mixtures and the original CP-Ti powder with no FA addition. The optimal parameters were then used to produce tensile test specimens in order to evaluate the mechanical properties of the different samples. The hardness of the various samples was also examined by micro-Vickers hardness tests. The tensile strength of the Ti-1%FA sample (850 MPa) was found to be far higher than that of the CP-Ti sample (513 MPa). Furthermore, the yield strength of the Ti-1%FA sample (785 MPa) was also much higher than that of the CP-Ti sample (472 MPa). However, the elongation of the Ti-1%FA sample (6.27 %) was significantly lower than that of the CP-Ti sample (16.17%). Finally, the hardness values of the Ti-1%FA and Ti-2%FA samples were around 63.8% and 109.4%, respectively, higher than that of the CP-Ti sample.

Keywords: biocompatibility; biomedical implants; fluorapatite material; laser powder bed fusion.

Figure 1

Machining of tensile test specimens.

Figure 2

Dimensions of tensile test specimens.

Figure 3

SEM images showing surface morphologies of different powder types: (a) CP-Ti, (b) FA, (c) Ti-1%FA, and (d) Ti-2%FA.

Figure 4

XRD patterns of different powder types.

Figure 5

XRD patterns of different powder types in main peak range of FA.

Figure 6

SEM surface images of as-built CP-Ti samples fabricated using different scanning speeds and hatching spaces: (a) v = 600 mm/s—h = 100 µm; (b) v = 800 mm/s—h = 70 µm; (c) v = 1000 mm/s—h = 70 µm; (d) v = 600 mm/s—h = 65 µm; (e) v = 800 mm/s—h = 50 µm; (f) v = 1000 mm/s—h = 50 µm.

Figure 7

SEM surface images of as-built Ti-1%FA samples fabricated using different scanning speeds and hatching spaces: (a) v = 600 mm/s—h = 70 µm; (b) v = 800 mm/s—h = 70 µm; (c) v = 1000 mm/s—h = 70 µm; (d) v = 600 mm/s—h = 45 µm; (e) v = 800 mm/s—h = 45 µm; (f) v = 1000 mm/s—h = 45 µm.

Figure 8

SEM surface images of as-built Ti-2%FA samples fabricated using different scanning speeds and hatching spaces: (a) v = 600 mm/s—h = 70 µm; (b) v = 800 mm/s—h = 70 µm; (c) v = 1000 mm/s—h = 70 µm; (d) v = 600 mm/s—h = 45 µm; (e) v = 800 mm/s—h = 45 µm; (f) v = 1000 mm/s—h = 45 µm.

Figure 9

Stripe scanning strategy employed in the L-PBF process.

Figure 10

L-PBF samples processed using optimal parameters: (a) CP-Ti, (b) Ti-1%FA, and (c) Ti-2% FA.

Figure 11

As-built surface morphology of (a) CP-Ti, (b) Ti-1%FA, (c) Ti-2%FA.

Figure 12

Comparisons between surface roughness of as-built samples of three different material.

Figure 13

OM images showing cross-sections of various as-built samples: (a) CP-Ti, (b) Ti-1%FA, (c) Ti-2%FA.

Figure 14

Relative densities of as-built samples.

Figure 15

XRD patterns of L-PBF powders and as-built samples.

Figure 16

EDS analysis results for as-built samples: (A) CP-Ti, (B) Ti-1%FA, and (C) Ti-2%FA.

Figure 17

Measured hardness values of different samples.

Figure 18

Fracture samples after tensile tests: (a) Cp-Ti, and (b) Ti-1%FA.

Figure 19

Stress vs. strain response of CP-Ti and Ti-1%FA samples.

Figure 20

SEM images showing cross-section fracture surfaces of tensile specimens: (A) CP-Ti, and (B) Ti-1%FA.