Patent Application
20190024242
The present disclosure relates to a cold spray process which involves deposition of powdered materials through a supersonic nozzle using kinetic energy and plastic deformation upon impacting a target to consolidate the powdered materials. This occurs through a process similar to cold welding, where surface strains on the particle and the impacting substrate expose fresh metal surfaces which then bond.
In order to achieve superior material properties through cold spray deposition, high particle velocity is required. Known cold spray processes use a combination of helium and heat to accelerate metal particles to a predetermined velocity sufficient to obtain good bonding of the metal particles with the substrate. However, helium is an expensive consumable without a reclamation system. Further, the helium gas temperature may need to be raised, increasing the sonic velocity of the gas, in turn increasing the particle impact velocity to a level which would then be sufficient to achieve good particle bonding. This increased heat can cause fouling of the nozzle, leading to a need to replace or maintain the nozzles to bring them back into service.
Alternately, nitrogen can be used in the cold spray process for accelerating particles to sufficient velocities to achieve bonding. The sonic velocity of nitrogen at any temperature is less than half of the velocity of helium at the same spray gas temperature. As with helium, increased temperature can increase the sonic velocity of nitrogen, but very high temperatures, 600-1000° C., may be required for many materials to achieve acceptable velocity at which point many of the attractive characteristics of a cold spray deposit are lost. As in the case of higher temperature helium, it is also more likely to foul nozzles when running very high nitrogen temperatures.
According to the disclosure, a method for cold spray deposition of a material on a substrate comprises the steps of: entraining a metal powder material in a stream of accelerant gas comprising hydrogen; forming a flow of shield gas around the stream of accelerant gas; and impacting the substrate with the stream of accelerant gas whereby the metal powder material is deposited on the substrate.
In a further non-limiting embodiment of the method, the stream of accelerant gas comprises a majority of hydrogen.
In a further non-limiting embodiment of the method, the stream of accelerant gas comprises at least 70% vol hydrogen. In a further non-limiting embodiment of the method, the stream of accelerant gas comprises at least 90% vol hydrogen.
In a further non-limiting embodiment of the method, the stream of accelerant gas has a critical velocity ratio of at least one.
In a further non-limiting embodiment of the method, the stream of accelerant gas has a critical velocity ratio of between 1.5 and 2.0.
In a further non-limiting embodiment of the method, the stream of accelerant gas is at a temperature of less than about 400° C.
In a further non-limiting embodiment of the method, the stream of accelerant gas is at a temperature of less than about 200° C.
In a further non-limiting embodiment of the method, gas mixture leaving an area of impact of the stream of accelerant gas with the substrate has a hydrogen content of less than 5% vol.
In a further non-limiting embodiment of the method, the shield gas is selected from the group consisting of inert gases and gas which is substantially inert with the substrate and metal powder at temperature of the stream of accelerant gas, and mixtures thereof.
In a further non-limiting embodiment of the method, the shield gas is selected from the group consisting of nitrogen, helium, argon, carbon dioxide and mixtures thereof.
In a further non-limiting embodiment of the method, the shield gas is nitrogen.
In a further non-limiting embodiment of the method, the metal powder material comprises particles of aluminum, copper, nickel, iron, tantalum, niobium, cobalt, or mixtures or alloys thereof, where the particle sizes vary from about 5 μm to 40 μm.
In a further non-limiting embodiment of the method, the metal powder material comprises particles of aluminum, copper or mixtures thereof.
In a further non-limiting embodiment of the method, the metal powder material comprises particles of aluminum having a particle size of between 20 and 40 μm.
In a further non-limiting embodiment of the method, the metal powder material comprises particles of copper having a particle size of between 10 and 30 μm.
In a further non-limiting embodiment of the method, the stream of accelerant gas is passed through a nozzle before the impacting step.
In a further non-limiting embodiment of the method, the flow of shield gas is at a higher pressure than the stream of accelerant gas.
In a further non-limiting embodiment, an apparatus for cold spray deposition of a material on a substrate comprises: a nozzle communicated with accelerant gas and metal powder to be deposited on the substrate; a sleeve surrounding the nozzle and defining an annular space around the nozzle, the annular space being communicated with a shield gas.
In a further non-limiting embodiment of the apparatus, the nozzle is defined substantially concentric within the sleeve.
In a further non-limiting embodiment of the apparatus, the nozzle and the sleeve define an outlet for a centered stream of the accelerant gas and a cylinder of shield gas surrounding the centered stream.
In a further non-limiting embodiment of the apparatus, a perforated bulkhead is provided in the annular space to adjust flow characteristics of the shield gas.
A detailed description follows, with reference to the attached drawings, wherein:
Like reference numbers and designations in the various drawings indicate like elements.
The present disclosure relates to cold spray deposition of a material such as a metal powder or particulate on a substrate.
Two streams of gas are fed to cold spray nozzle 18. The first stream 22 passes through powder feeder 14 and conveys a helium with powder mixture to nozzle 18 at a relatively low flow rate. The second stream 24 passes through gas heater 16 at a significantly higher flow rate, and is mixed with the first stream in nozzle 18 to deliver the powder entrained in helium accelerant gas at an elevated temperature and velocity. As set forth above, in order to accomplish higher impacting velocity with either helium or nitrogen, the temperature can be increased but is limited to a point where the nozzle will be fouled. Further, if helium is used, the expensive gas is either lost, or must be recycled using expensive equipment.
Enclosure 62 is also communicated with a source 66 of outside makeup air, and a dust collector 68 is also communicated with spray hood 60. In operation, hydrogen is fed through one line 63 which passes through powder feeder 56 to deliver a stream of hydrogen and entrained powder particles to nozzle 58. Shield gas is fed through stream 61, also to nozzle assembly 58. Nozzle assembly 58 delivers a centered stream of hydrogen accelerant gas with entrained powder for cold spray deposition upon a workpiece, as well as a cylindrical flow of shield gas, in this case, nitrogen, surrounding the flow of hydrogen. The shield gas serves to dilute and safely remove hydrogen gas from the point of application, after impact with the workpiece. This prevents escape of hydrogen at elevated levels or concentrations and helps ensure safety of the process. Shield gas can be fed through line 61 at a significantly higher flow rate or pressure than the hydrogen accelerant gas. With a system as illustrated in
As set forth above, exit velocity (m/s) is a key parameter in obtaining good results with a cold spray deposition. Table 1 below sets forth pressure temperature and exit velocity for several different types of accelerant gas. As shown, helium can generate an exit velocity of 674 m/s at a pressure of 30 bar and a temperature of 20° C. At the same pressure and temperature, nitrogen as an accelerant gas produces an exit velocity of only 397 m/s. This velocity can be increased if either the temperature or pressure are somewhat significantly increased, as shown in Table 1.
Finally, the last line of Table 1 compares exit velocity for a stream of hydrogen as accelerant gas, and shows that exit velocity is significantly higher than it is for helium or nitrogen at the same conditions.
The flow of hydrogen accelerant gas can advantageously comprise a majority of hydrogen. More ideally, the accelerant gas can comprise at least 70% volume of hydrogen, more preferably at least 90% of volume hydrogen.
The stream of accelerant gas can advantageously be fed to the workpiece at a critical velocity ratio, or CVR (the ratio of particle velocity to the critical velocity for particle bonding), of at least one, and more desirably at a critical velocity ratio of between 1.5 and 2.0. Further, such CVRs can be achieved at a temperature of the stream of accelerant gas of about 20° C. when spraying copper particles with an average particle size of 20 μm and a gas pressure of 20 bar. Similarly, relatively high strength aluminum alloy powder such as aluminum alloy 6061, with an average particle size of about 30 μm, can achieve this same CVR range with a hydrogen gas temperature of only 100° C. and a pressure of 20 bar. Even high strength super-alloy powders such as Inconel 625 with average particle sizes of 20 μm can achieve this same CVR range with a hydrogen gas temperature of only 200° C. and a pressure of 35 bar. Generally, temperatures higher than 200° C. are not needed for most metals or alloys of the particle size ranges discussed. However some metals may experience benefits due to higher temperature accelerating gases. Gas temperatures as high as 400° C. or even higher are possible with the heating arrangement described, but increases in temperature will add to the system complexity with higher temperature capably hardware, hose flexibility, and even hydrogen absorption into some metals and alloys.
The shield gas can advantageously be any gas which remains substantially inert at conditions likely to be encountered during the process. More specifically, the shield gas can be selected from the group consisting of inert gases and gases which are substantially inert with the substrate and metal powder at temperatures of the stream of accelerant gas and mixtures thereof. In a further non-limiting aspect of the disclosure, the shield gas can be selected from the group of nitrogen, argon, carbon dioxide, and mixtures thereof, most preferably nitrogen.
The substrate and powder materials can be any metals or ceramic-metal composites including aluminum, copper, nickel, iron, tantalum, niobium, cobalt or mixtures or alloys thereof. In one aspect of the present disclosure, when the particles are aluminum, the particles desirably have a particle size of between 20 and 40 μm. When the metal powder material is particles of copper, these particles can be provided at a particle size of between 10 and 30 μm. Other desirable particle size ranges would be applicable to other metals and alloys where generally higher density and higher strength materials require lower particle size ranges.
Turning to
The embodiment of
In this configuration, a bulkhead 217 can be positioned within annular space and have through-passages or perforations to allow shield gas to pass through bulkhead 217. Such a configuration can help to produce a smooth and uniform flow of shield gas in a cylindrical pattern as desired.
It should be appreciated that the various configurations described and illustrated herein provide for use of hydrogen as an accelerant in a cold spray process to overcome cost and fouling issues in helium-based systems and methods, and to improve quality of results of the cold spray deposition, while maintaining a safe shield of nitrogen or other effectively inert and/or diluting gas around the stream of hydrogen such that the hydrogen is diluted and removed from the impact area with the workpiece after impact is conducted.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
