Apparatus for spray deposition

Abstract

An apparatus for producing a three-dimensional object by spray deposition including a powder conveyance line A arranged to channel a flow of spray material entrained in a carrier gas to a nozzle (a first flow). The apparatus also has a process line B arranged to channel a flow of gas to the nozzle (a second flow). The apparatus also has a spray nozzle. The apparatus is such that pressure and temperature parameters of the first flow are controlled in real time, but for the second flow these parameters are not controlled in real time. The two flows merge to cause spray material to be propelled from the nozzle to a substrate to create a three-dimensional object.

Claims

1. Apparatus for producing a three-dimensional object by spray deposition comprising: a powder conveyance first line arranged to channel a flow of spray material entrained in a carrier gas to a nozzle a first flow, the powder conveyance first line consisting essentially of a pressure sensor, a temperature sensor, a pressure dropping element, a spray material feeder supplying spray powder to the carrier gas, and a controller controlling the flow of the spray material entrained in the carrier gas while effective spray deposition on a substrate occurs; a process second line arranged to channel a flow of gas to the nozzle a second flow, the process second line consisting essentially of a pressure element setting air pressure and a heater setting temperature, wherein the pressure element and the heater are set prior to deposition and are not controlled in real time; and a spray nozzle, wherein both the conveying first line and the process second line are upstream of the nozzle; the apparatus being such that the pressure and temperature parameters of the first flow are controlled in real time to maintain pressure within the powder conveyance first line above the pressure in the process second line, the pressure and temperature parameters of second flow are not controlled in real time, and the flow of the process second line merges with the flow of the first line to cause spray material to be propelled from the nozzle to a substrate to create a three-dimensional object.

2. Apparatus according to claim 1, wherein the controller controls the amount of spray material fed into gas moving through the powder conveyance first line.

3. Apparatus according to claim 1, wherein the first and second lines receive gas from a common reservoir.

4. Apparatus according to claim 3, wherein there is a filter which filters gas prior to it entering the reservoir.

5. Apparatus according to claim 1, wherein the gas in each case comprises compressed air.

6. Apparatus according to claim 5, wherein the compressed air is run through a dehumidifier prior to entering the first and second lines.

7. Apparatus according to claim 1, wherein: a) the pressure and temperature sensors for gas in the powder conveyance first line communicate readings to the controller which, in response to the readings, causes adjustments to the pressure and temperature of the gas in the powdered conveyance first line to provide the spray material with desired spray characteristics as it leaves the nozzle; b) the controller controls the amount of spray material fed into gas moving through the powdered conveyance first line; c) there is a common reservoir from which the first and second lines receive gas; d) there is a filter which filters gas prior to it entering the reservoir; e) the gas in each case comprises compressed air; and f) the compressed air is run through a dehumidifier prior to entering the first and second lines.

8. Apparatus according to claim 7, wherein the nozzle is a converging-diverging nozzle arranged to receive air entrained with spray material and to transform that air in terms of increasing its velocity, lowering its pressure and lowering its temperature.

Description

DRAWING

(1) Some preferred embodiments of the invention will be described by way of example and with reference to FIG. 1, which schematically illustrates cold spraying apparatus for forming a three-dimensional object.

DETAILED DESCRIPTION

(2) The apparatus has an electric motor driven air compressor 1 which takes in and compresses atmospheric air. The air is then filtered by an auto-draining filter 2 and stored in a reservoir 3 sized to reduce pressure variations caused by the cycling action of compressor 1. Next in the air train is a refrigeration type de-humidifier 4 which receives air from the reservoir 3 and dehumidifies it. The air at that point is suitable for use as a process gas. The air then splits into two streams each with its own path.

(3) The first path is a powder conveyance line A which supplies air into which spray powder is fed. That line incorporates a controllable pressure dropping element 5, a flow sensor 6 and a powder feeder 7. Spray powder from the feeder 7 is fed into the air stream and conveyed to a supersonic nozzle 8 by a coaxial injector (not shown).

(4) The second path is a main process line B which supplies a flow of process gas (air in this case) used to give velocity to the spray particles. The process line incorporates a pressure control device 9 (eg to reduce the pressure of the air as desired) and a gas heater 10. The heater heats the air to provide it with the desired level of energy.

(5) Air plus spray powder from the powder conveyance A line converges with energised air from the main process line just prior to the nozzle inlet 8a. In the preferred embodiment the nozzle 8 is a De Laval style converging-diverging item, sufficient to transform the low velocity—high pressure—high temperature air from the process line to high velocity—low temperature—low pressure air at its outlet 8b. The high velocity powder stream emanating from the nozzle is used for cold spraying a substrate to form a target three-dimensional item.

(6) In the preferred embodiment shown, powder conveying air from the conveyance line A will only flow into the nozzle injector, and therefore the nozzle 8, if the pressure in that line is higher than the pressure in the main process line B. The pressure control device 9 is provided for this reason; it adjusts (eg reduces) air pressure in the main process line B so that powder can convey from the powder conveyance line A through to the nozzle injector.

(7) The pressure control device 9 is manually adjustable so that a human operator can set it to reduce the impact of variations in compressor pressure. Generally this element need not be servo-mechanical.

(8) An important aspect of the preferred embodiment lies in the manner in which the cold spraying apparatus is controlled. The pressure dropping element 5, which may comprise a servo-mechanical pressure regulator, is controlled by a signal from an electronic controller 11. This controller has a tuned proportional-integral-differential (PID) function implemented in software.

(9) The controller 11 takes a flow pressure measurement signal from the flow sensor 6 as one input, and an external signal as another input. In combination, the flow sensor 6, pressure dropping element 5 and controller 11 form a control system. That system is able to control the flow rate of a powder conveying gas through the conveyance line A to a specific rate. This rate is regulated in reference to an external signal fed in to controller 11.

(10) By controlling the flow rate of the powder conveying gas through the powder conveyance line A, it is possible to maintain the spray powder flow rate above a minimum at which the powder will clog or convey through to the injector and nozzle sub-optimally. It is important to minimise the powder conveying gas to process gas (in this case ‘air’) ratio, to maximise the temperature of the gas supplied to the nozzle 8. This is facilitated by heating the process gas with the heater 10. As a result, the larger the fraction of powder conveying gas mixed into the total gas supplied to the nozzle 8, the lower the gas temperature, and the slower the eventual speed of the projected spray powder.

(11) In the preferred embodiment the cold spray apparatus has a well-functioning control system relying on a minimum number of sensing and controlling features, while maintaining a high level of control and stability. Known high pressure cold spray solutions measure and control gas temperature, pressure and flow rate for the main process line, and sometimes also for a powder conveying line. The inventor has found that simplifying the control system has, in at least the preferred embodiment, the desired benefit of improving stability over many known set-ups as there are fewer control systems to “hunt” and compete with each other.

(12) In the preferred embodiment the correct rate of gas flow is important for achieving high quality material outputs. In this case quality outputs are achieved by tightly controlling the flow rate to specific values in order to maintain consistent particle velocities.

(13) In the preferred embodiment the air through the powder conveyance line A; ie that which transports the spray powder to the nozzle injection point, is not heated. On the other hand, the air in the main process line B is heated to high levels to provide sufficient energy for particle acceleration when passing through the nozzle 8. Therefore, there results a temperature difference between the air in powder conveying line compared to that the main process line. Excessive flow rates for the cooler conveyance line A will reduce the gas temperature at the nozzle's inlet when it combines with the hotter air of the main process line B. This results in a reduction of spray particle velocity. Poor quality results are consequent of poor control over particle velocity; therefore, it is desirable to stabilize and limit the maximum rate of air flowing through the injection tube. The preferred embodiment does so.

(14) In the case of the powder/air feed, the preferred embodiment ensures that there is a minimum level of air flow through the powder conveying line A to lubricate powder as it is conveyed to the nozzle. It is possible to establish a lower mass rate limit (where the flow clogs) and a higher mass rate limit (where the nozzle temperature becomes unduly cooled). This ideal setting above the lower and below the upper limits is used as a target value in the control system which is in turn able to control air flows by measuring volume flow rate or mass flow rate, and using a controllable pressure drop to stabilise the flow to the optimal rate.

(15) In order to simplify the control system, the preferred embodiment controls in real time the parameters for only the powder conveyance line A. This is in distinction to the prior art where the focus is on real time control of the parameters of the main process line. While the parameters of flow in the main process line are not adjusted while the apparatus is cold spraying, they can of course be adjusted/set to the necessary values beforehand.

(16) As the geometry of the nozzle is known, as is the gas pressure and temperature in the main process line, the mass flow can be determined. Using this knowledge, the mass flow in the main process line B is calculated by setting (as opposed to controlling) component parameters. These parameters include one or more of air temperature (eg via heater 10), air pressure and pressure drop (eg at element 5). Setting these parameters allows one to both predict and regulate mass flow in an open loop control sense, without measuring and controlling it directly.

(17) It is preferable to rely on the behaviour of a converging-diverging supersonic nozzle as one parameter for determining parameters for the spray material leaving the nozzle 8. In the preferred embodiment it is important to regulate, in real time, the flow rate through the powder conveying line A for process stabilisation. Tight real time control of the flow rate is then the only requirement for smooth and continuous cold spray process performance, avoiding the need for complex temperature and pressure controls.

(18) An advantage of the elimination of real-time control for the main process line B is that lower flow rate sensors can be used than would otherwise be the case.

(19) In the preferred embodiment it is desirable to ensure that the pressure and temperature of air fed to the nozzle 8 is less time variant. Time variance is in part limited by way of the gas reservoir 3 in the conveyance line B which reduces air pressure fluctuations. If fluctuations are not controlled or averted then time variant changes may lead to undesired fluctuations in the resulting spray particle velocity and thus reduced quality of the three-dimensional object produced.

(20) By controlling flow only on the powder conveyance line A, and allowing air flow in the main process line B to be determined by incoming air pressure, set temperature and nozzle geometry, hunting and instability in the system can be obviated or reduced. The system cost may also be relatively low, involving only one small flow rate control system on the powder feed, and no pressure control on the main process line B.

(21) Real time control of air parameters in the powder conveyance line A is effective for creating cold spray system stabilization. In the preferred embodiment, the controller 11 is used to alter air pressure in order to achieve an identified optimal flow rate. This assists in keeping pressure for the powder conveyance line A lower than that of the main process line B and in keeping variation of air temperature at the nozzle 8 as low as possible.

(22) Controlling the powder conveyance line A rather than the main process line B is also useful because small fluctuations within the powder conveyance line have higher relative impact on the system stability.

(23) In the preferred embodiment it has been found that a useful spray material flow rate out of the nozzle 8 can be achieved by having a 30.5 bar pressure at the supply of the powder conveying section, and a 30 bar pressure at the outlet of the nozzle injector—the pressure drop being 0.5 bar. Assume a system fluctuation where the pressure at the nozzle injector would decrease from 30 bar to 29.5 bar (for some reason), then the pressure drop doubles to 1 bar. The resulting flow rate goes from 0.71K to 1K, an approximate 50% increase. This small 1.7% pressure change (0.5 bar in relation to the 30 bar) can increase the powder conveying flow by a substantial 50%. This high sensitivity, illustrates that maintaining a tight and effective control of the pressure in the powder conveyance line is beneficial for system stability.

(24) While the preferred embodiment uses air for the process gas, in other embodiments alternative gases may be used, for example liquid nitrogen or helium. Air is preferred as it can be continually compressed as it is used, without having to be highly compressed into bottles, meaning that the emergence temperature is not cryogenic and smaller heaters can be used than would otherwise be the case. This may reduce process costs further by reducing energy consumption.

(25) In the preferred embodiment the apparatus measures gas flow rate with a small sensor through the powder feeder 7 and injector, and controls the pressure dropping element 5 to maintain an ideal spray particle mass flow rate flow in the powder conveyance line A.

(26) While some preferred embodiments of the invention have been described by way of example, it should be understood that modifications and improvements can occur without departing from the scope of the following claims.