Effects of Spray Parameters and Post-spray Heat Treatment on Microstructure and Mechanical Properties of Warm-Sprayed Ti-6Al-4V Coatings

Journal of Thermal Spray Technology, Dec 2016

Warm spray is a novel thermal spray technique that allows the formation of dense and relatively pure Ti-6Al-4V coatings due to its capability to control the temperature of the propellant gas by diluting the combustion flame with an inert gas such as nitrogen. Recently, its combustion pressure has been increased from 1 to 4 MPa aiming to further increase particle velocity to over 1000 m/s. Two series of coatings with combustion pressure of 1 and 4 MPa and various nitrogen flow rates were prepared in this study. Effects of combustion pressure and nitrogen flow rate on the microstructure and mechanical properties of the Ti-6Al-4V coatings were systematically studied. Miniature tensile specimens with a total length of about 9 mm were used for static tensile tests. It was found that the spray parameters affect both the porosity and oxygen content of the coatings significantly and had remarkable effects on their mechanical properties. High level of porosity in the Ti-6Al-4V coatings reduced the effective cross-sectional area of the mini-specimens and caused a drop in their tensile strength and Young’s modulus. Subsequent heat treatments were found effective in significantly recovering the mechanical properties of the as-sprayed coatings.

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Effects of Spray Parameters and Post-spray Heat Treatment on Microstructure and Mechanical Properties of Warm-Sprayed Ti-6Al-4V Coatings

Effects of Spray Parameters and Post-spray Heat Treatment on Microstructure and Mechanical Properties of Warm-Sprayed Ti-6Al-4V Coatings R. M. Molak 0 1 2 H. Araki 0 1 2 M. Watanabe 0 1 2 H. Katanoda 0 1 2 N. Ohno 0 1 2 S. Kuroda 0 1 2 R. M. Molak 0 1 2 0 Plasma Giken Co. Ltd. , Toda, Saitama , Japan 1 Kagoshima University , Korimoto, Kagoshima , Japan 2 National Institute for Materials Science (NIMS) , Tsukuba , Japan Warm spray is a novel thermal spray technique that allows the formation of dense and relatively pure Ti-6Al-4V coatings due to its capability to control the temperature of the propellant gas by diluting the combustion flame with an inert gas such as nitrogen. Recently, its combustion pressure has been increased from 1 to 4 MPa aiming to further increase particle velocity to over 1000 m/ s. Two series of coatings with combustion pressure of 1 and 4 MPa and various nitrogen flow rates were prepared in this study. Effects of combustion pressure and nitrogen flow rate on the microstructure and mechanical properties of the Ti-6Al-4V coatings were systematically studied. Miniature tensile specimens with a total length of about 9 mm were used for static tensile tests. It was found that the spray parameters affect both the porosity and oxygen content of the coatings significantly and had remarkable effects on their mechanical properties. High level of porosity in the Ti-6Al-4V coatings reduced the effective cross-sectional area of the mini-specimens and caused a drop in their tensile strength and Young's modulus. Subsequent heat treatments were found effective in significantly recovering the mechanical properties of the as-sprayed coatings. mechanical properties; microstructure; mini- specimen; oxygen content; porosity; tensile test; warm spraying - There are two main application areas of titanium alloys in the aerospace industry, i.e., airframes and aero engines, driven by their superior properties such as high strength, fatigue resistance and creep resistance at low-to-moderate temperatures in combination with low density and low modulus (high flexibility). Because of difficulties in casting and welding of titanium alloys due to the high affinity to oxygen and the high solid solubility of oxygen (about 14.5%) (Ref 1), however, there is a demand for direct fabrication of metal parts in near-net shapes (NNS), which can produce complex components with minimal waste of material (Ref 2). In aerospace industry, for example, the buy-to-fly ratio, which is the ratio of the mass of material needed to machine a part to the mass of material in the finished component, may be in the range of 1.5:1 for turbine blades to over 22:1 for complex shapes structural components. For compressor and ring sections, the ratio is approximately 12:1 and an analysis performed by Pratt and Whitney indicated that the ratio could be reduced by 41% to 7:1 by using cold spray for parts manufacturing (Ref 3). Another practical application of thermal spray is to repair local damage on finished parts (Ref 4, 5). It allows reclamation components, which otherwise have to be removed prematurely because of corrosion, erosion, abrasive wear or fatigue. Currently, many of these worn titanium parts cannot be reclaimed due to the lack of means to restore them for service. For instance, various welding processes have been developed, but tend to induce undesirable thermal stresses that can lead to premature failure. Hence, there is a great expectation for thermal spray technology as a potential method for both production and repair of Ti-based materials. Because of the high oxygen affinity of titanium and the possibility to form the hard and brittle a-case, it is necessary to avoid oxidation of titaniumbased deposits as much as possible. One of the techniques that has explored is low-pressure plasma spray (LPPS). It suppresses the oxidation of the feedstock powder through operation in an inert atmosphere under low-pressure conditions. The LPPS equipment costs are rather high. Another potential and newer technique is cold spraying (CS), in which metallic powder is accelerated by a supersonic gas jet generated by expansion of high-pressure gas through a convergent divergent nozzle. By using inert gas as a propellant, it is possible to essentially eliminate oxidation of the feedstock powder during processing. Even though the most recent CS equipment operates with a gas pressure of 4 MPa and a temperature close to 1000 C, it is still dif ficult to form dense coatings of especially high-strength materials such as Ti-6Al-4V, because of the high critical velocity required to form bonding between deposited particles. To achieve a sufficient particle velocity, helium as a propellant gas has to be used (Ref 6, 7). The shortage and high cost of helium however, a gas recycling facility is required for industrial scale production, which would be a large initial investment. Recently, a novel spray process called warm spraying (WS) (Ref 8-10), whic (...truncated)


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R. M. Molak, H. Araki, M. Watanabe, H. Katanoda, N. Ohno, S. Kuroda. Effects of Spray Parameters and Post-spray Heat Treatment on Microstructure and Mechanical Properties of Warm-Sprayed Ti-6Al-4V Coatings, Journal of Thermal Spray Technology, 2017, pp. 627-647, Volume 26, Issue 4, DOI: 10.1007/s11666-016-0494-5