Patent Application
20080099538
The invention relates generally to the field of joining metal parts. More specifically, the invention relates to methods for depositing braze filler material onto a component part joint using cold spray deposition technology.
Brazing is a joining process where a non-ferrous, metal alloy filler is heated to a melting temperature and distributed between two or more close-fitting parts by capillary action. At its liquid temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. The brazed joint comprises different layers, each metallurgically linked to each other.
A variety of alloys of metals, including silver, tin, zinc, copper and others are used as the filler material for brazing processes. Metals such as aluminum can be brazed though aluminum requires more skill and special fluxes. It conducts heat much better than steel and is more prone to oxidation. Some metals, such as titanium cannot be brazed because they are insoluble with other metals, or have an oxide layer that forms too quickly at intersoluble temperatures.
To achieve the highest strength for a brazed joint, the component parts must be closely fitted and the base metals must be exceptionally clean and free of oxides. For capillary action to be effective, joint clearances in the range of between 0.000 to 0.006 inch (0 to 150 μm) are recommended. In most cases, flux is required to prevent oxides from forming while the metal is heated. The most common fluxes for bronze brazing are borax-based.
Current methods of applying braze filler material include paste with flux, foils, or sintered preforms. Pastes have difficulties achieving uniform thickness and controlling contamination. Foils require attachment methods such as tack welding and are limited in use due to geometric constraints. Sintered preforms must account for dimensional changes that takes place during the sintering process and adds to the complexity of the manufacturing process. These methods have many limitations and may prove problematic.
Cold spray deposition is an emerging technology where solid powder particles at or near room temperature are accelerated to velocities in the range of 500 to 1,500 m/sec in a supersonic gas jet. Upon impact with a work piece surface, the high-velocity cold particles plastically deform and bond with the underlying material by a process similar to explosive welding, but on a much smaller scale. Cold spray deposition can be used to deposit a wide range of metals and other ductile materials at high production rates.
Cold spray deposition offers many advantages over other metallization processes. Since the powders are not heated to high temperatures, no oxidation, decomposition, or other degradation of the feedstock material occurs. Because the process occurs essentially at room temperature, copper, aluminum, and many other reactive metals can be cold sprayed in an open-air environment with little or no oxidation.
Powder oxidation during deposition is also controlled since the particles are contained within the accelerating gas stream. Other potential advantages include the formation of compressive residual surface stresses and retaining the microstructure of the feedstock. Also, because relatively low temperatures are used, thermal distortion of the substrate will be minimized. Because the feedstock is not melted, cold spray offers the ability to deposit materials that cannot be sprayed conventionally due to the formation of brittle intermetallics or a propensity to crack upon cooling or during subsequent heat treatments.
It is a challenge to ensure high-strength, consistent brazes in production work due to the required precision and cleanliness. It is therefore desirable to have a method that allows for the precise application of braze filler material using cold spray deposition technology for brazing in production environments.
Although there are various methods for applying braze filler material to a component part interface prior to the application of heat, such methods are not completely satisfactory. The inventors have recognized that it would be desirable to have methods that apply braze filler material using cold spray deposition technology to precisely apply braze filler material having a uniform thickness regardless of the joint contour.
One aspect of the invention provides methods for applying a filler material to a component interface surface. Methods according to this aspect of the invention preferably start with selecting at least one material for the filler material, deriving a nozzle path across the interface surface, and depositing the at least one filler material from the nozzle onto the interface surface so that the filler material plastically deforms without melting and bonds to the joint surface upon impact with the joint surface.
Another aspect of the method is deriving a spray distance for the nozzle path.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Further, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected,” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting, and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.
Embodiments of the invention provide methods using cold spray deposition technology to deposit braze filler and solder materials directly onto a component joint interface surface. The invention allows for the application of filler materials uniformly across complex contours eliminating the need for sintered preforms and controlling joint interface cleanliness.
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In the method of the invention, the braze filler material feedstock may be a powdered metal material of thermal spraying sizing, such as −325 mesh. The powder metal material may be any commercially available powders from industry suppliers of braze filler materials (step 515).
The powdered metal material particles that are used to form the filler materials deposited onto the component 101 joint surface 105 preferably have a diameter in the range of about 5 microns to 50 microns (0.2-2.0 mils). Smaller particle sizes allow for higher particle velocities. Below 5 microns in diameter, the particles risk getting swept away from the surface 105 due to a bow shock layer above the surface 105, i.e., insufficient mass to propel the particle through the bow shock. The narrower the particle size distribution, the more uniform the particle velocity will be. This is because the small particles in the spray/plume will hit the slower, larger ones and effectively reduce the velocity of both.
The particles of the filler material may be accelerated to supersonic velocities using compressed gas, such as helium, nitrogen, other inert gases, and mixtures thereof. Nitrogen is a preferred gas because it is relatively inexpensive and a 100% dense deposit is not required since the deposited filler material will be melted.
The bonding mechanism employed by the method of the invention for transforming the powdered filler material into a deposit is solid state, meaning that the particles plastically deform but do not melt. Any oxide layer that is formed on the particles, or is present on the component surface, is broken up and fresh metal-to-metal contact is made at very high pressures. This aspect is preferable for aluminum brazing to mitigate Al2O3 from the deposition surface.
The powdered metal material used to form the filler material may be fed to the spray gun using any suitable means known in the art, such as modified thermal spray feeders. Fluidized bed feeders and barrel roll feeders with an angular slit may also be used. Any feeder that can deliver the powdered metal at the required feed rate may be employed.
In the method of the invention, the feeders may be pressurized with a gas selected from the group consisting of helium, nitrogen, other inert gases, and mixtures thereof. Feeder pressures are generally 15 psi above the main gas or head pressures, which pressures are usually in the range of from 200 psi to 500 psi, depending on the filler material composition.
The main gas is heated so that gas temperatures are in the range of from 600° F. to 1,200° F. If desired, the main gas may be heated as high as approximately 1,250° F. depending on the material being deposited. The gas may be heated to keep it from rapidly cooling and freezing once it expands past the throat of nozzle 205. The net effect is a temperature on the component joint surface 105 in the range of from 100° F. to 200° F. during deposition depending on the area and thickness of the applied material. The temperature of the underlying substrate remains low enough such that it does not undergo any metallurgical changes due to temperature. Any suitable means known in the art may be used to heat the gas.
To deposit the filler material, the nozzle 205 may pass over the joint surface 105 more than once. The number of passes required depends on the desired thickness of the filler material. A preferred thickness is 0.002 inch. The method is capable of forming a deposit having any desired thickness. When applying braze filler, it is desirable to limit the thickness per pass in order to avoid a quick build up of residual stresses and unwanted debonding between deposit layers.
The main gas that is used to deposit the filler material particles onto the surface 105 may be passed through the nozzle 205 at a flow rate of from 0.001 SCFM to 50 SCFM, preferably in the range of from 15 SCFM to 35 SCFM. The foregoing flow rates are preferred if helium is used as the main gas. If nitrogen is used by itself or in combination with helium as the main gas, the nitrogen gas may be passed through the nozzle 205 at a flow rate of from 0.001 SCFM to 30 SCFM, preferably from 4 to 30 SCFM. Alternatively, the nozzle 205 may have a single inlet which is connected to a valve for switching between two gases (step 520).
The main gas temperature may be in the range of from 600° F. to 1,200° F., preferably from 700° F. to 1,000° F., and most preferably from 725° F. to 900° F.
The pressure of the spray gun may be in the range of from 200 psi to 500 psi, preferably from 200 psi to 400 psi and most preferably 275 psi to 375 psi. The filler material particles are fed from a hopper, which is under a pressure of 10 to 50 psi higher than the specific main gas pressure, preferably 15 psi higher, to the spray gun via line at a rate in the range of from 10 grams/min to 100 grams/min, preferably from 15 grams/min to 50 grams/min.
The filler material is fed to the spray gun using a non-oxidizing carrier gas. The carrier gas may be introduced at a flow rate of from 0.001 SCFM to 50 SCFM, preferably from 8 SCFM to 15 SCFM. The foregoing flow rate is useful if helium is used as the carrier gas. If nitrogen by itself or mixed with helium is used as the carrier gas, a flow rate of from 0.001 SCFM to 30 SCFM, preferably from 4 to 10 SCFM, may be used (step 525).
The nozzle 205 spray distance from the component interface surface is not determined by the deposit thickness. Preferably, the spray distance is in a range of from 10 mm to 50 mm. For example, if a 3 mm thick deposit is required, the nozzle may begin at a starting spray distance of 25 mm. As the braze filler material is deposited, the nozzle distance of 25 mm is maintained from the surface of the deposited braze filler material, not the original component interface surface. In this manner, the same distance is maintained regardless of the deposited braze filler material thickness. The deposition process is insensitive to small variations in nozzle spray distance. For some deposits of less than a predetermined thickness, such as 0.2 inch, the nozzle spray distance may not require adjusting as deposition progresses.
The velocity of the powdered filler material particles leaving the spray nozzle 205 may be in the range of from 825 m/s to 1400 m/s. Preferably from 850 m/s to 1200 m/s. The deposit thickness 207 per pass may be in the range of from 0.001 inches to 0.030 inches.
The nozzle 205 of the cold spray is transitioned across the joint surface 105 at a predetermined, controlled rate. Multiple passes may be performed according to the properties of the filler material, either by controlling the nozzle by hand or by using robotics. The rate and orientation of the nozzle 205 are controlled to achieve a uniform filler material thickness 207 (step 530). Preferably, the spray nozzle is maintained perpendicular to the interface, following the surface contour if any. Alternatively, the nozzle may be stationary and the joint interface surface transitioned across the spray pattern at a controlled rate and distance depending on work piece size.
The invention can use any deposition process that provides sufficient energy to accelerate particles to a high enough velocity such that, upon impact, the particles plastically deform and bond to a surface, building a relatively dense coating or structural deposit. The deposition process does not metallurgically transform the particles from their solid state.
Various techniques to achieve this type of particle deposition have been evaluated and reduced to practice such as cold gas dynamic spraying (cold spray deposition), kinetic metallization, electromagnetic particle acceleration, modified high velocity air fuel spraying, or high velocity impact fusion (HVIF). These are examples of high velocity deposition processes where metallurgical transformation of powder metal particles is not encountered. Preferably, the invention uses the cold spray deposition process. However, the invention may use other cold deposition processes.
The use of any one of these deposition techniques 203 results in uniform filler material thickness over the joint interface 105 contours (steps 535, 540, and 545).
The heating time, power, and other parameters can be set so as to allow different sized components to be brazed. This provides for inline production.
Brazing takes place above 840° F. which is lower than the melting point of the base material. The materials to be joined are commonly heated to about 100° F. above the temperature at which the filler material being used has turned liquid, so that the liquid filler material will cover all of the mating surfaces and then alloy with and form a permanent bond between those surfaces. Brazing may take place in a vacuum (step 570). Gas atmospheres can be used to enhance the vacuum brazing process. Nitrogen or argon not only prevent alloy evaporation, but also protect the part assembly from oxides and discoloration.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
