The response to corrosion of Ti-6AI-4V matrix composites manufactured through additive manufacturing

Abstract

This study investigates the electrochemical corrosion and high temperature corrosion behaviour of Ti6Al4V (Ti64) alloy and its composites, TiC/Ti64 and In-situ TiB/Ti64, fabricated using the direct metal deposition (DMD) technique. Titanium alloys, particularly Ti64, are known for their high strength-to-weight ratio and corrosion resistance. However, enhancing their performance in aggressive environments, such as saline and high-temperature oxidative conditions, remains a significant challenge. This research aims to address this by incorporating ceramic reinforcements, TiC and TiB2, into the Ti64 matrix and evaluating their effect on electrochemical corrosion and high temperature corrosion resistance. Electrochemical corrosion tests were conducted in a 3.5% NaCl w/v solution to simulate marine environments. Open Circuit Potential (OCP) and Potentiodynamic Polarization (PDP) measurements demonstrated distinct corrosion behaviours among the materials. TiC/Ti64 exhibited superior corrosion resistance with lower current densities and more stable passive oxide layers, attributed to the chemically inert and dense TiC phase. Conversely, In-situ TiB/Ti64 showed the poorest corrosion resistance due to galvanic interactions at the TiB-matrix interface, which exacerbated localized corrosion. Ti6Al4V showed moderate resistance, with its titanium dioxide (TiO₂) passive layer providing limited protection in chloride-rich environments. High-temperature corrosion studies were performed at 300°C, 600°C, and 900°C in air and environments containing NaCl and Na₂SO₄. These temperatures were chosen because 300°C represents Ti64’s maximum operating limit, 900°C is the target for high-temperature aerospace applications, and 600°C was included to bridge the gap between the two. Thermogravimetric analysis revealed that In-situ TiB/Ti64 displayed the highest high temperature corrosion resistance, due to the formation of stable boron-containing oxides that maintained the integrity of the protective layer. TiC/Ti64 exhibited intermediate resistance, with increased susceptibility at higher temperatures due to potential oxide layer instability. Ti6Al4V, in contrast, experienced significant material loss attributed to the formation of volatile titanium chlorides and sulphates, which disrupted the protective oxide scale. Microstructural characterization, using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD), revealed key insights into the phase distribution and structural stability of the materials. The presence of TiC and TiB reinforcements significantly influenced grain boundary stabilization, phase distribution, and oxide layer adherence. The TiC phase acted as a barrier to defect propagation and oxygen diffusion, enhancing the protective properties of the oxide layer. TiB contributed to oxidation resistance through the formation of boron oxides and its inherent thermal stability, though its susceptibility to galvanic interactions posed challenges in corrosive environments. The study concludes that TiC/Ti64 and In-situ TiB/Ti64 offer promising improvements over Ti64 alloy for applications such as aerospace, marine, and power generation industries, where exposure to aggressive saline and high-temperature environments is inevitable especially below 600°C. In-situ TiB/Ti64 is particularly suitable for high-temperature applications due to its excellent thermal stability and resistance to oxide spallation. TiC/Ti64, while performing well in corrosion resistance, demonstrates limitations at elevated temperatures, highlighting the need for further optimization. These findings underscore the importance of composite microstructure in tailoring titanium alloy properties for specific environmental conditions.