Development of β-Phase Strengthened Ti Alloys With Improved Mechanical and Tribological Properties via Laser-Material Deposition
Authors: Mohanty Ipsita , Misra Saurav , Kar Sujoy Kumar , Saha Partha 
Published: August 2024

Abstract
Implementing laser-based additive manufacturing (AM) methods over conventional manufacturing routes for making Titanium (Ti) alloy parts gained considerable industrial acceptance. The laser-AM processing of the widely applicable (α+β) type Ti-6Al-4V (Ti64) alloy, including other α-phase dominant Ti-alloys, shows remarkably improved mechanical strength, owing to the solidification process involved during the layer-wise deposition. However, anisotropy in mechanical behavior (due to the growth of prior-columnar β-grains) and a considerably reduced ductility limit their practical applicability. Whereas the β-phase dominant Titanium alloys possess inherent ductility and superior fatigue resistance as compared to the α-type and (α+β) type Ti-alloys. Such a β-phase dominant Ti-Fe-Co system was reported in literature via conventional manufacturing routes, showing a good combination of strength and ductility. However, studies on such alloy systems via laser-AM processing are limited due to the complexity of multi-material systems (as β-phase strengthening involves alloying) in such a complex and non-equilibrium solidification process. Considering Ti64 alloy as the base material, this study investigates the Ti64-Fe-Co systems at two extreme alloying conditions via a laser-material-deposition method. This preliminary investigation aims to study the phase evolution (possible intermetallic phases) and microstructural features as an effect of β-phase strengthening with the potential β-phase stabilizing elements Fe and Co at hypo-eutectoid (low-alloying) and hyper-eutectic (high alloying) compositions. The resulting XRD analysis shows improved β-phase stability with alloying, where the β-Ti phase evolved as a major phase along with the Ti (Fe, Co) phase with an increase in alloy concentration. The resulting backscattered SEM-microstructural features show two distinct regions with the equiaxed dendritic and the matrix region depicting a reversal of high concentrations of Fe and Co upon changing the composition, as evident from the EDS analysis. An improved hardness of twice that of the commercial Ti64 alloy was achieved even at the lower alloying condition, and a further improvement in hardness at higher alloying conditions shows the strengthening effect of the Ti(Fe,Co) phase. The formation of AlTi3 and Ti2Co intermetallic phases formed at the higher cooling rates during laser-material deposition, also contributes to the strengthening effect. The laser-deposited novel Ti-alloys also show improved elastic modulus compared to the commercial Ti64, where identical load-vs-displacement behavior for both alloying conditions indicates the strengthening effect with an increase in alloy concentration without deteriorating the elongation properties. One of the most common issues of poor tribological properties of most Ti-alloys (including Ti64), has also been successfully addressed via this in-situ alloy development work, showing up to 45% improvement in the wear resistance compared to commercial Ti64. A considerable reduction in the specific wear rates with minimized surface deformations for the laser-deposited alloys shows the reduced tendency of abrasive wear, as analyzed from the resulting wear surface morphologies. These findings offer valuable insights for developing metastable β-Titanium alloys with improved strength and tribological properties through laser-based additive manufacturing, aiming for potential applications in aerospace and automobile industries.