# Laser-Based Additive Manufacturing of Zirconium
## 4 Discussion  
Laser Engineered Net Shaping (LENS(tm)) is a powder-based additive manufacturing technique. It has several features such as controlled inert environment, focused high power laser, and in situ alloying and composite deposition. These features are ideal for the processing of specialty materials like superalloys and NiTi alloys, high melting point materials such as tantalum as well as in the fabrication of specialized designs such as three-dimensional porous or bimetallic components. In the current research, zirconium, a reactive metal, was processed using the LENS(tm) technique on a Ti6 Al4V alloy. Initial SEM scans revealed the presence of some porosity in spite of extensive process optimization. The parameters thus need to be further optimized for forming dense Zr coatings or 3D structures. Nevertheless, it was shown that Zr can be additively manufactured using a direct energy deposition technique such as LENS(tm). LENS(tm) processing of Zr produced a zoned microstructure. It consisted of equiaxed grains that were less than a micron in size. Some samples were subjected to a laser scan on the surface after initial LENS(tm) deposition. However, such a laser pass did not affect the grain size. In the samples without any laser pass, there were two distinct zones found in the Zr region. The top region had equiaxed grains, while the region beneath it and directly above the Ti6 Al4V substrate had similar grain but with embedded particles, presumably of another phase of Zr. The samples that were laser-passed after initial deposition had an additional layer between the embedded phase layer and the top equiaxed grained layer. This layer also showed similar embedded particles. In both types of Zr depositions, there was a secondary, patchy phase seen along with the equiaxed grains. It seemed to agglomerate and spread more evenly after the laser pass on the surface. Figure 8: LENS(tm)-processed Zr cylindrical/tube geometry. Phase analysis of the LENS(tm)-deposited Zr samples showed the formation of both \(\alpha\) and \(\beta\) phases of Zr. The feedstock powder was purely \(\alpha\) (hcp) phase. However, the microstructure showed a single type of equiaxed grains in the upper regions. In the regions close to the interface, the secondary embedded phase was found. We hypothesize that the upper regions of the equiaxed grains in both types of samples are in the \(\alpha\)-Zr phase. Near the regions closer to the Ti6 Al4V substrate, owing to the stabilizing effect of Ti, there is \(\beta\)-Zr (bcc) phase formation. Without any stabilizing agent, Zr by itself also phase transforms from \(\alpha\) phase to \(\beta\) phase at approximately 850 \({}^{\circ}\)C. Hence, the microstructure shows \(\alpha+\beta\) structure. This \(\alpha+\beta\) structure was also similar to the optical micrographs of a heat-treated Ti-39.6% Zr alloy reported by [PERSON] et al. [22]. Therefore, the formation of a \(\beta\) phase after laser processing can be attributed to the \(\alpha\)-Zr phase powder's melting and solidification process as well as the effect of micro alloying occurring from the Ti6 Al4V substrate. Another possibility of the embedded phase would be the formation of intermetallic phases of Ti-Zr, Zr-V, and Zr-Al. However, EDS imaging on these particles, as seen in Figures 5 and 6, showed that the particles were depleted in Al and V. The particles closest to the Ti6 Al4V substrate showed low Ti signal on EDS mapping as well. There were also no strong peaks of any intermetallic phase detected by X-ray diffraction. Thus, there is a strong possibility of the formation of a \(\beta\) phase of Zr, which is supported by XRD results and EDS imaging. From Figure 4b,c, it was observed that the diffraction signal was stronger for the samples that had a laser pass on the surface. Thus, the \(\alpha\) and \(\beta\) phases of Zr seemed to be more oriented after the laser remelting process. Laser surface scanning of the LENS(tm)-deposited Zr had some effect on the hardness of Zr. Hardness increased from 268 HV0.1 to 328 HV0.1.