Laser forming of Ti-6Al-7Nb alloy porous parts for medical devices
A group of six papers presented at the conference focussed on aspects of the processing of titanium powder feedstocks by Additive Manufacturing (AM) techniques, with the majority of these papers featuring biomedical applications.
One paper in this category was delivered by Hideshi Miura, Kyushu University, Japan on behalf of his co-authors, Hyungoo Kang and Yoshinori Itoh, and described a project aimed at processing Ti-6Al-7Nb porous scaffolds for medical applications by laser forming.
Ti-6Al-7Nb has been developed as a more suitable biomaterial than Ti-6Al-4V, because of toxicity concerns with vanadium.
As a means of overcoming fabrication issues in producing complex, close tolerance parts in this material by conventional manufacturing technologies, interest has grown in the application of a number of routes using powder feedstocks, including laser forming, for their manufacture.
In laser forming, a thin layer of powder is melted by laser scanning. Following this, a further powder layer is provided and the same procedure repeated, so that 3-dimensional structures can be built layer by layer.
The experimental laser forming apparatus used in this work is illustrated in Fig. 1. To prevent oxidation during forming, the laser sintering was carried out in a vacuum chamber at 10-1-10-2 Pa. The laser source was a 20W pulsed YVO4 and the focussed beam diameter was approximately 50µm. The required honeycomb structures were built on a titanium base plate.
The average size of the Ti-6Al-7Nb powder used in this study was 26µm and its composition is given in Table 1. As a pre-alloyed powder, the oxygen content was very low.
In previous work, this group had reported that, in building such honeycomb structures, two-step irradiation of each powder layer had led to better results than single-step irradiation. In this current work, the number of irradiation steps/layer was further increased markedly to 20 or 40. Initially, a laser beam of low power (8W) was used to pre-sinter each layer. Next, a higher power beam (20W) was scanned 20 or 40 times on the same pattern to melt the pre-sintered layer. To control energy density, scanning velocity was increased in direct proportion with the increase in numbers of scanning passes.
Honeycomb structures were built with designed edge length of each hexagon set at 1000, 800, 600, 500 and 400µm. Fig. 2 shows SEM images of the formed structures.
It was found that more precise and finer structures were produced as the number of irradiation passes/layer was increased. The lower right-hand image in Fig. 2 shows the finest structure.
Next, compressive tests were carried out on the honeycomb structures, built using 40 irradiation passes/layer, in order to evaluate mechanical properties. Elastic moduli in the x, y and z axes are plotted in Fig. 3. These measured Young’s moduli are almost the same as reported values for human cancellous bone (1.2 – 4.6 GPa). Fig. 4 shows the measured values of density and compressive strength. The compressive strength values were similar to or higher than those reported for human bone (21 – 116 MPa).
These results indicated that these honeycomb structures could be suitable for use as biomedical scaffolds.
Finally, biocompatibility was evaluated by culturing of mouse osteoblast cells. Fig. 5 shows SEM images of cell-cultured samples after 14 and 28 days. In the finest structure with 300 µm holes, the extracellular matrix (ECM) formed chord-like structures inside the holes, which could promote proliferation of osteoblasts.
Fig. 6 shows a magnified view around the hole after 21 days. More ECM was observed in the smaller pore structure than in the larger one.
Overall, it was concluded that these laser formed honeycomb scaffolds showed good mechanical compatibility with human bone and had excellent biocompatibility.
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