Laser forming of Ti-6Al-7Nb alloy porous parts for medical devices

February 22, 2012

In the last of our exclusive reports from the international conference on “Powder Processing, Consolidation and Metallurgy of Titanium”, held at the University of Queensland, Brisbane, Australia on 5-7 December, 2011, Dr David Whittaker reviews a presentation focused on the processing of Ti-6Al-7Nb via Additive Manufacturing.


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.

fig_1

Fig. 1 Schematic illustration of the experimental

apparatus 

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.

table_1

Table 1 Composition of the Ti-6Al-7Nb used in this study

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.

fig_2

Fig. 2 Honeycomb structures of Ti-6Al-7Nb fabricated by the present laser-forming process. Length below each photo shows the designed edge length of hexagonal walls,and the scan speed and number of the irradiation times are shown in the left of the images

fig_3

Fig. 3 Relationship between hole diameter and Young’s

modulus

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.

fig_4

Fig. 4 Relationships between hole diameter and density and

compressive strength

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.

fig_5

Fig. 5 SEM micrographs of honeycomb structures on which osteoblasts were cultured for 14 days and 28 days, respectively

fig_6

Fig. 6 SEM image of osteoblasts on

honeycomb structures after 21-day culture

(upper: 700μm hole diameter, lower: 300μm)

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.

 

 

 


 

Author: Dr David Whittaker is a consultant to the Powder Metallurgy and associated industries. Contact +44 1902 338498 email: [email protected] 

Further reading:

Titanium and titanium alloy Powder Injection Moulding: Matching application requirements

Mixing titanium MIM feedstock: Homogeneity, debinding and handling strength

Titanium powder injection moulding (Ti-PIM): A review of the current status of materials, processing, properties and applications

Production of fine titanium powders via the Hydride-Dehydride (HDH) process

Titanium parts by powder injection moulding of TiH2-based feedstocks

Advanced metal powder injection moulding for multilayered micro porous titanium components

 

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February 22, 2012

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