BACKGROUND
The invention relates generally to cold spray and, in particular, to the
methods of cold spraying feedstocks including nickel-base alloys.
Bonded surface layers are desired for many applications including
those in which the surfaces experience corrosion, erosion, or high temperature. One
method used for producing bonded metallic coatings on substrates is cold spray
4fe| technology. In cold spray technology (also referred to herein as simply "cold spray"),
particles are mixed with a gas and the gas and particles are subsequently accelerated
into a supersonic jet, while the gas and particles are maintained at a sufficiently low
temperature to prevent melting of the particles. Copper coatings have been deposited
using cold spray in which sufficient bonding was achieved to produce bulk-like
properties. However, higher temperature materials such as stainless steel, nickel,
nickel-based and titanium -based super alloys, are likely to require higher velocities
to produce high quality deposits with limitations of conventional cold spray devices.
In particular, achieving higher particle and deposit temperatures and /or velocities
would be desirable.
In order to attain better properties using higher melting point metals
than copper, cold spray equipment is moving towards higher gas temperatures.
4A However, even high temperature nitrogen gas is difficult to accelerate to velocities
fast enough to make dense deposits of high-melting point materials such as nickel,
iron, or titanium alloys. Therefore, in order to have high enough velocities to make
dense deposits of the high-melting point materials, helium gas is favored compared to
the conventional nitrogen gas. However, using helium gas for cold spraying is
commercially challenging.
Therefore, there is a need for an economical method of making a good
quality bonded deposit of high-temperature melting alloys.
2
BRIEF DESCRIPTION
Briefly, in one embodiment, a method is disclosed. The method
includes introducing a powder feedstock into a cold-spray apparatus, and operating
the cold-spray apparatus to deposit the feedstock. The feedstock includes particles
including a nickel-base alloy having a thermally altered microstructure.
In one embodiment, a method is disclosed. The method includes
introducing a powder feedstock into a cold-spray apparatus, and operating the coldspray
apparatus to deposit the feedstock. The feedstock consists essentially of nickel-
^ ^ base alloy particles having a thermally altered microstructure.
DRAWING
These and other features, aspects, and advantages of the present
invention will become better understood when the following detailed description is
read with reference to the accompanying drawing, wherein:
FIG. 1 illustrates an article with a deposit, according to an embodiment
of the invention.
DETAILED DESCRIPTION
Embodiments of the present invention include the apparatus and
method for producing dense metal deposit on a substrate from solid state impact
fl| deposition with bonded particles using a cold spray device with nickel-base alloy
feedstock.
In the following specification and the claims that follow, the singular
forms "a", "an" and "the" include plural referents unless the context clearly dictates
otherwise.
The term "bonded", as used herein means in contact with and adhered
to. "Bonding" may be between the deposited particles and/or between the deposited
3
particles and the substrate. A "deposit" is a bulk or layer on a substrate. In a specific
embodiment, the deposit is a coating.
Typical cold spray methods use a spray gun that receives a high
pressure gas such as, for example, helium, nitrogen, or air, and a feedstock of deposit
material, such as, for example, metals, refractory metals, alloys, or composite
materials in powder form. The powder granules are introduced at a high pressure into
a gas stream in the spray gun and emitted from a nozzle. The particles are accelerated
to a high velocity in the gas stream that may reach a supersonic velocity. The gas
stream may be heated. Typically the gases are heated to less than the melting point of
^ ^ the particles to minimize in-flight oxidation and phase changes in the deposited
material. As a result of the relatively low deposition temperatures and very high
velocities, cold spray processes offer the potential for depositing well-adhering,
metallurgically bonded, dense, hard and wear-resistant coatings whose purity depends
primarily on the purity of the feedstock powder used.
The powder impacts the substrate at a high velocity. The kinetic
energy of the powder causes the powder granules to deform and flatten on impact
with the substrate. The flattening promotes a metallurgical, mechanical, or
combination of metallurgical and mechanical bond with the substrate and results in a
deposit on the substrate. One advantage of cold spraying methods is the negligible to
nil phase change or oxidation of particles during flight and high adhesion strength of
the bonded particles.
In order to have sufficiently high velocities to make dense deposits of
the high melting point materials, an expensive helium (He) gas is normally used
instead of nitrogen (N2) gas as nitrogen gas is often difficult to accelerate to velocity
fast enough to make dense deposits of high melting point materials such as, for
example, nickel (Ni), iron (Fe), or titanium (Ti) alloys, when used in the conventional
cold spray methodologies. However, spraying with helium is expensive.
Embodiments of the present invention take advantage of benefits conferred by a pretreatment
of the feedstock powder to make it amenable for cold-spraying at less
4
demanding conditions than the conventional helium-based cold spray methods for
depositing a coating of high melting point metals and alloys.
Changing some characteristics of the feedstock microstructure and/or
morphology to effect reduction of particle strength and/or hardness (relative to such
characteristics and properties for particles received after typical powder
manufacturing processes) provides a softer particle feedstock be fed to the spray
apparatus, allowing a softer material to impact and deform at the substrate and thus
forming a dense, high quality deposit. Some embodiments of the disclosed method
include a heat-treatment of the feedstock material that changes the material structure
^ P and property, making the feedstock amenable for cold-spraying at economically
convenient conditions. The disclosed method is different from an in-situ or insidethe-
spray gun heat-treatment of the feedstock material during or just before spraying
out the feedstock. The feedstock material used herein receives its heat-treatment and
thus changes its microstructure, morphology and /or strength/hardness, even before
introduction into the cold spray apparatus. Further, the heat-treatment that is received
by the feedstock material in this application is different than what can be applied
inside a spray gun apparatus. Prior disclosures of inside-the-spray-gun heattreatments
of the feedstock material are limited in the temperature and time duration
of high-temperature treatment of the feedstock material and thereby the
microstructure, morphology, and strength/hardness when compared to the heat-treated
particles of the present application.
^V In one embodiment of the cold spray method presented herein, the
feedstock material comprises a metal, or a metal alloy. Examples include metals such
as nickel, cobalt, titanium, aluminum, zirconium, and copper. Examples of metal
alloys include nickel-base alloys, cobalt-base alloys, titanium-base alloys, iron-base
alloys, steels, stainless steels, and aluminum-base alloys.
Some of the nickel, iron, cobalt, or titanium-base alloys are used in
aviation- and land-based gas turbine engine components and are particularly desirable
to be cold-spray deposited to form a dense coating without undue oxidation. Alloys,
such as so-called "superalloys" commercially available under such trade names as
5
INCONEL®, INCOLOY®, RENE®, WASPALOY®, UDIMET ®, Hastelloy®, and
Mar-M™ materials are some of the non-limiting examples that are particularly
beneficial to be used for the engine components. INCONEL® is a registered
trademark of Huntington Alloys Corporation of Huntington, W. Va. INCOLOY® is a
registered trademark of Inco Alloys International, Inc. of Huntington, W.Va. RENE®
is a registered trademark of Teledyne Industries, Inc. of Los Angeles, Calif.
WASPALOY® is a trademark of Haynes International, Inc. of Kokomo, Ind.
UDIMET® is a registered trademark of Special Metals. Corporation. Hastelloy® is
the registered trademark name of Haynes International, Inc. Mar-M™ is a trademark
^ ^ of Martin Marietta. While different feedstock and deposit materials are included in
^ ^ the invention, the application herein is further described in terms of nickel- base
alloys as the feedstock material as well as deposit material.
A non-limiting example of a nickel-base alloy is alloy 718, having a
specific composition, in weight percent, from about 50 to about 55 percent nickel,
from about 17 to about 21 percent chromium, from about 4.75 to about 5.50 percent
niobium, from about 2.8 to about 3.3 percent molybdenum, from about 0.65 to about
1.15 percent titanium, from about 0.20 to about 0.80 percent aluminum, 1.0 percent
maximum cobalt, and balance iron. Small amounts of other elements such as carbon,
manganese, silicon, phosphorus, sulfur, boron, copper, lead, bismuth, and selenium
may also be present.
Strengthened nickel-base alloys generally include precipitated phases,
^ P such as for example, gamma-prime (y'), gamma- double prime (y")> and hightemperature
precipitates such as, for example, carbides, oxides, borides, and nitride
phases, either singularly or in combination, depending on the alloy composition and
heat-treatments conditions of the alloy. In some embodiments, phases such as delta,
sigma, eta, mu, and/or laves may also be present.
The precipitate phases such as gamma-prime and gamma-double prime
in nickel base alloys are typically dissolved during solution heat-treatments, and re-
•
precipitate during cooling from the solution temperature and during subsequent aging
heat-treatments. The result is a distribution of gamma-prime and/or gamma-double
6
prime secondary phases in a nickel-alloy matrix. High-temperature precipitates such
as carbides, oxides, borides, and nitride phases may not typically dissolve during
solution heat-treatments and may thus remain as precipitates even after solution heattreatment
of the alloys. The general steps involved in these treatments along with
different expected precipitations at each steps are detailed below.
In typical precipitate hardened nickel alloys, the alloys are initially
given a solution treatment (or, in the parlance of the art, the alloys are initially
"solutioned" or "solutionized"), wherein the alloys are heated above the solvus
temperature of the precipitates. The precipitates referred herein may be the 'primary',
J P 'secondary', or 'tertiary' precipitates that form during different stages of temperaturetreatments
rather than the high temperature carbide, oxide, boride, or nitride phases
that may be present even above the solvus temperatures of the
primary/secondary/tertiary precipitates.
Generally the alloys are quenched after solution treatment forming a
supersaturated solid solution phase. In one embodiment, the matrix includes nickelbase
gamma (y) phase. The gamma-phase is a solid solution with a face-centered
cubic (fee) lattice and randomly distributed different species of atoms. In some
alloys, where the high temperature precipitate phases are present, the supersaturated
solid solution phases may still have the precipitates of those high temperature phases.
In one embodiment, in a gamma-prime system like Rene 88® or Waspaloy® for
example, the gamma prime may precipitate quickly even during quenching.
^ P Typically, alloys in the solutioned state, even where precipitation occurs during
quenching, are significantly softer than alloys in the fully processed state, as noted
below. I
In the third step, the supersaturated solid solution phase is heated 1
below the solvus temperature of the precipitates to produce a finely dispersed
precipitate. For example, in a gamma-double prime system, the gamma-double prime
phase may largely precipitate during the aging treatment thereby hardening and
strengthening the alloy.
7
1
Thus, strengthened nickel-base alloys are typically processed by using
designed solution heat-treatment methods that dissolve gamma-prime and/or gammadouble
prime strengthening phases and then allow the optimum reprecipitation of
these phases upon cooling from heat-treatment or after subsequent aging of the
solutioned alloys. The cooling rate, and cooling path imposed on nickel-base alloy
components, along with the aging temperature and times, and inherent properties of
the particular compositions normally influence development of optimum properties in
the nickel-base alloys.
In one embodiment of the invention, a method for preparing an article
mb made of a nickel-base alloy deposits strengthened by the presence of gamma-prime
and /or gamma-double prime phases is disclosed. The method includes the steps of
solution heat-treating a nickel-base alloy powder at a solutionizing temperature above
gamma-prime and/or gamma-double prime solvus temperatures of the nickel-base
alloys. In one embodiment, the method further includes quenching the nickel-base
alloy powders to a temperature less than the gamma-prime and gamma-double prime
solvus temperatures. The quenching may be carried out in one step or in multiple
steps. Normal air quenching or water, oil, or molten salt bath quenching methods
may be used for the quenching.
In one embodiment of the invention, the solution heat-treated and
quenched powders are used as at least a part of the feedstock for the cold-spray
deposition. The solution treatment is normally performed at temperatures sufficiently
^ P high to partially or fully dissolve the strengthening phases, typically on the order of
900°C to 1300°C for nickel-base alloys, typically for a duration of 1 hour to 10 hours.
This solution heat-treatment and quenching alters the microstructure of the nickelbase
alloys and the resultant particles typically have a thermally altered
microstructure.
In one embodiment, the altered microstructure of the nickel-base alloy refers to the
changed microstructure from the atomized state of the nickel-base alloy prior to a
heat-treatment to the atomized powder. A thermally altered microstructure, then,
refers to a microstructure that has microstructural features that differ from the features
8
of the powder prior to heat-treatment as a result of having been exposed to heattreatment.
Non-limiting examples of such features include grain size; grain
morphology; precipitate size, morphology, and size distribution; and degree of
chemical segregation. In one embodiment, the materials are thermally processed
using a heat- treatment that results in the material being softer than it was prior to the
treatment. In one embodiment, the atomized nickel-base alloys are heat-treated to a
temperature of at least half the melting point of the nickel-base alloy for a duration of
at least 5 minutes to develop a thermally altered microstructure. The melting
temperature as defined herein means the incipient melting point of the alloy, wherein
^ ^ a liquid phase begins to appear under equilibrium conditions. w
In one embodiment, the quenched powders, before receiving further
aging heat-treatment, are in a single phase supersaturated solution phase, without
having the presence of any of the gamma-prime or gamma-double prime phase
precipitates. In one embodiment, the quenched powders comprise substantially
solutioned microstructure. As used herein the "substantially solutioned
microstructure" means that the powder particles are in a solution-treated state having
a microstructure characteristic of material having been through a solution heattreatment
and rapid quench. In most embodiments, high temperature phases such as
carbides, oxides, nitrides, and borides, if present in the powder prior to heat-treatment,
persist within the matrix after heat-treatment. In one embodiment, a solution
treatment is a heat-treatment to a temperature where thermodynamics favor existence
^ ^ as a single phase, for a time sufficient to establish equilibrium conditions.
In one embodiment, the solution treated and quenched state includes
matrix phase and precipitate phases that formed during quenching without undergoing
any aging treatment to form post-primary fine precipitates that aid in increasing
strengthening. In one embodiment, a matrix phase of gamma nickel and gammaprime
primary precipitate is present in the solution treated and quenched nickel-base
alloy. In one embodiment, the nickel-base alloys are subjected to slow-quenching
from the solution temperature. Cooling the materials while leaving them in the heat
treatment furnace (a practice known in the art as "furnace cooling") is a typical
method of slow-quenching in these alloys systems. The slow-quenched alloys
9
1
materials typically have coarser grains precipitates and reduced strength compared to
conventionally aged alloys of similar composition.
In one embodiment, the feedstock particles used for the cold spray
include a nickel base alloy. In one embodiment, the nickel-base alloy includes
feedstock particles having at least about 40% of nickel by weight.
In one embodiment, the microstructure of the solution heat-treated and
quenched feedstock powders include coarse grains. As used herein, "grains" are
individual crystals and the grain size refers to size of crystals within a given particle. I
^ ^ In one embodiment, the strength of the nickel-base alloys is reduced by the solution
heat-treatment, relative to the powders before subjecting to the heat-treatment, due to
grain coarsening and/or precipitate dissolution associated with solution heat-treating,.
In one embodiment, the particles of the feedstock materials have average grain size
ranging from about l|im to about 20|xm. Feedstock materials with different particle
sizes can be used in the cold spray method presented herein to form strong and dense
deposits. In one embodiment, the particles used for the feedstock have a median size
in the range from about 1 micron to about 100 microns. In a further embodiment, the
particles have a median size in the range from about 5 microns to about 50 microns.
In one embodiment, the particles obtained after solution heat-treatment and quenching
have a face-centered cubic crystal structure.
As discussed previously, in one embodiment of the cold spray method
g b presented herein, the feedstock material does not melt at the time of spraying. In one
embodiment, the melting point of the feedstock material is above the temperature
experienced by the feedstock material during spraying. In a further embodiment, the §
temperature experienced by the feedstock material is below about 0.9 times the
melting point of the feedstock material.
In one embodiment of the invention, a carrier gas is used for carrying
the feedstock materials for depositing. Because of the change in microstructure and
decreased strength/hardness of the solution heat-treated nickel-base alloys, it is not
necessary to use a helium gas for obtaining a dense deposit of the nickel-base alloys
10
on the article, or to use a very high temperature of the carrier gas or high velocity of
the feedstock material. Therefore, in one embodiment of the invention, a carrier gas
having at least 50 volume% of nitrogen is used for the cold spray. In one
embodiment, the carrier gas includes at least 75 volume% of nitrogen. In one
embodiment, the carrier gas consists essentially of nitrogen. In one embodiment, the
carrier gas used for depositing is essentially free of helium. In one embodiment, the
carrier gas temperature is in the range from about 20°C to about 1200°C. In general,
in the cold spray process, an impact critical velocity of the feedstock material is
defined as below which the particle adhesion to the substrate is not useful for the
^~ intended application. The critical velocity of the feedstock material may depend on
^ ^ the feedstock particles and the substrate nature and properties. In one embodiment,
operating the cold spray device used herein comprises accelerating the feedstock to a
velocity in the range from about 500 m/s to about 1100 m/s.
In one embodiment, the article on which the deposit is formed is
prepared for receiving the deposit. Preparing the article surface for the cold spray
may include cleaning and/or degreasing the surface. In one embodiment, a prepared
region of the article surface is formed by removing the existing material or layer such
as an oxide layer for example, from the surface of the article so that the deposit
formed by directing the feedstock material through cold spray is bonded to the article.
In one embodiment of the invention, an article is provided. The article
may be of any operable shape, size, and configuration. Examples of articles of
^ P interest include areas of components of gas turbine engines such as seals and flanges,
as well other types of articles. The article 10, as shown in FIG. 1 for example, is
formed when a deposit is formed on a substrate 12 of the article 10. The substrate 12
has a depositing surface 14. The deposit 16 is formed on the surface 14 of article 10.
The deposit 16 has a plurality of feedstock particles 18 bonded along their prior
i
particle boundaries 20. A surface of contact between the deposited material 16 and j
the substrate 12 surface 14 is a bond line 22. j
In one embodiment, the article 10 and / or the deposit 16 are heattreated
after the cold spray. Annealing or aging heat-treatments are used to precipitate
11
the gamma prime or gamma-double prime phases in the nickel-base alloy matrix. In
one embodiment, the temperature of the aging is in the range from about 300°C to
about 1000°C. In one embodiment, the temperature of the aging is in the range of
about 400°C to about 850°C. In one embodiment, the precipitates so formed are less
than about 80% by volume of the deposit. In one embodiment, the precipitated
strengthening phases are in the range from about 20 volume % to about 55 volume %
of the deposit.
The heat-treatment may cause the deposit material 16 to interdiffuse to
some degree with the substrate 12 material of the article 10. In one embodiment, the
^ P deposit 16 is solution heat-treated, quenched, and aged to precipitate a desirable
distribution of strengthening phases. In one embodiment, the deposit 16 of article 10
has a density greater than about 95% of theoretical density of the deposit material. In
a further embodiment, the deposit 16 has a density greater than about 99% of
theoretical density.
While only certain features of the invention have been illustrated and
described herein, many modifications and changes will occur to those skilled in the
art. It is, therefore, to be understood that the appended claims are intended to cover
all such modifications and changes as fall within the true spirit of the invention.

WE CLAIM:
1. A method comprising:
introducing a powder feedstock into a cold-spray apparatus, wherein the
feedstock comprises particles comprising a nickel-base alloy having a
thermally altered microstructure; and
operating the cold-spray apparatus to deposit the feedstock.
2. The method of claim 1, wherein the thermally altered microstructure
^ ^ comprises gamma-phase.
3. The method of claim 2, wherein the thermally altered microstructure
further comprises precipitates of gamma prime phase, gamma-double prime
phase, or combinations thereof.
4. The method of claim 1, wherein the particles have an average grain size
in the range from about 1 |^m to 20 |^m.
5. The method of claim 1, wherein the feedstock consists essentially of
particles comprising at least about 40% nickel by weight.
6. The method of claim 1, further comprising exposing the powder feedstock
to a temperature of at least half the melting point of the nickel-base alloy for a
^ P duration of at least 5 minutes to produce the thermally altered microstructure.
7. The method of claim 1, wherein operating the cold spray apparatus
further comprises introducing a carrier gas comprising nitrogen into the
apparatus.
8. The method of claim 7, wherein a carrier gas temperature is in the range
from about 20°C to about 1200°C.
13
9. The method of claim 1, further comprising heat-treating the deposited
feedstock to form a deposit comprising a strengthening precipitate phase
distributed within a matrix phase.
10. The method of claim 9, wherein the matrix comprises nickel-base gamma
phase and the strengthening precipitate phase comprises gamma prime,
gamma-double prime, or combinations thereof.
11. The method of claim 9, wherein the matrix phase has a face-centered
cubic crystal structure.
^W 12. The method of claim 9, wherein the precipitate is less than about 80
volume % of the deposit.
13. The method of claim 12, wherein the precipitate is in the range from
about 20 volume % to about 55 volume % of the deposit.
14. The method of claim 9, wherein heat-treating the deposited feedstock
comprises heating to a temperature in the range from about 300°C to about
1300°C.
15. The method of claim 1, wherein operating the cold spray device
comprises accelerating the feedstock to a velocity in the range from about 500
m/sto llOOm/s.
^ P 16. An article formed by the method of claim 1.
17. A method comprising:
introducing a powder feedstock into a cold-spray apparatus, wherein the
feedstock consists essentially of nickel-based alloy particles having a
thermally altered microstructure; and
operating the cold-spray apparatus to deposit the feedstock.
14
18. The method of claim 17, further comprising heat-treating the deposited
feedstock to form a deposit comprising a strengthening precipitate phase
distributed within a matrix phase.