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
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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)