# Oxidation-Reduction of Ti-6 Al-4V in Direct Energy Deposition Subject to Minimum Argon Consumption
## 1 Introduction  
Manufacturing technologies have been emerging with increasing demand for high-quality metal components, reduced production costs, and lead times. Metal additive manufacturing (MAM) techniques have been gaining interest over the past few decades for their ability to print three-dimensional shapes based on layer-by-layer deposition concepts to produce complex shapes that are nearly impossible to achieve with conventional methods [1]. As this method eliminates the need for tooling and machining, low scrap rates (less than 10%) are achieved in MAM [2]. These qualities make additive manufacturing (AM) a promising technology in the automotive, medical, defense, and aerospace industries [3]. With their high specific strength, low density, and excellent corrosion resistance, titanium alloys, especially Ti-6 Al-4V, are capturing a large portion of the aerospace industry today [4]. However, scrap rates of 80% to 90% have been reported due to poor conductivity and the tendency for strain hardening during conventional processing methods, including the casting, forging, and rolling of Ti-6 Al-4V products [5]. Additionally, because of its high strength, low conductivity, and resistance, the post-processing of this alloy is expensive and laborious. The advantage that AM has over conventional processing methods is the substantial savings in material and lead time [6]. There are a variety of high-deposition rate processes that may be used for manufacturing large-scale AM parts, where the heat may be provided by laser, electron beam, or arc-based processes. Direct energy deposition (DED) is an AM technique where a laser beam interacts with feedstock powder to form a melt pool, where melting and re-solidification occur rapidly to form the desired shape with a 3D CAD model consistently [7; 8]. However, contamination of titanium with atmospheric air, particularly oxygen, is an important consideration in the DED process, where temperatures are beyond 1600 \({}^{\circ}\)C. Titanium's high affinity and reactivity with oxygen at elevated temperatures increase its brittleness, making the allowable oxygen concentration strictly limited [9; 10; 11]. Areas where the temperature exceeds 600 \({}^{\circ}\)C need to be shielded to avoid oxidation of the surface. Hence, a high-purity shielding environment is essential to decrease the oxygen concentration in the critical areas for depositing this alloy with the DED process to prevent embrittlement [12]. For this reason, an environment consisting of inert gases, i.e., chemically inactive gases, such as argon or nitrogen, is created for the DED process to allow the building of parts that meet exacting standards. DED can take place in this inert gas chamber without the risk of contamination by reactive gases, such as oxygen and carbon dioxide [13; 14; 15; 16]. However, purging an enclosed chamber would not only require a sophisticated chamber but also significantly increase the cost of shielding gas [17]. Creating a vacuum chamber is not a viable solution as it requires robust chambers to withstand the vacuum forces. Additionally, the chemical composition of the alloy can be compromised in the low-pressure environment, causing aluminum in the composition to evaporate [18; 19]. Moreover, gas evacuation and post-processing can become too expensive, and the chamber itself can restrict the size of the print. A local or trailing shielding gas technique is another method of shielding generally used for powder/wire feeding additive manufacturing systems, which overcomes most of the shortcomings of a fixed chamber [20]. In local shielding, a shielding environment is created around the deposition process to protect the newly deposited layers from oxidation. Applying local shielding increases the flexibility of the process and does not restrict the part size. For this purpose, argon is used at high flow rates of up to 195 L/min to reduce the O2 concentration at elevated temperature zones (_critical areas_) [21].