WO2008056345A1

WO2008056345A1 - Deposition of powders

Info

Publication number: WO2008056345A1
Application number: PCT/IE2007/000109
Authority: WO
Inventors: Joseph Stokes; Lisa Looney; Saleem Hashmi; Mahbub Hasan; Michael Tyrrell
Original assignee: Dublin City University
Priority date: 2006-11-10
Filing date: 2007-11-09
Publication date: 2008-05-15
Also published as: IE20070818A1

Abstract

A powder feed apparatus for a spray deposition apparatus, has two powder chambers (2, 3), each having an outlet (6). A needle-shaped bolt (4, 5) adjusts powder flow rate from each chamber by adjusting effective size of the outlet (6). A mixing chamber (15) communicates with the powder chambers (2, 3), for receiving and mixing different powders received from the powder chambers. The apparatus has a through hole (16) for fitting onto a pick-up shaft (18) of a spray gun supply, and it fits into a single-powder hopper (51) of a conventional feed apparatus. An inlet gas pressure tube (10) connected to the mixing chamber (15) is for delivery of gas to assist powder mixing and fluidising. The inlet gas pressure tube (10) delivers gas under sufficient pressure to force mixed powders into a spray gun pick-up shaft (18).

Description

"Deposition of Powders"

INTRODUCTION

Field of the Invention

The invention relates to the co-deposition of two or more materials to form a coating.

Thermal spraying is a popular technology due to its flexibility and the high quality of coatings produced compared to other hard facing techniques. Thermal spraying contains a number of processes namely; spray and fuse, low pressure plasma, detonation gun, electric arc, plasma arc, flame and high velocity oxy-fuel (HVOF). These processes use rod, wire or powder as their starting material to produce coatings. Another technology is cold spray, which solely uses powders within its applications. In these powder-based technologies, a gun melts and projects powder particles onto a substrate to produce a coating. Often, coatings are deposited in a monolithic manner. Functionally graded coatings, in which the microstructure and properties vary gradually from the bond-coat to the top-coat, offer better properties and lower residual stress build-up in the components compared to monolithic coatings.

At present, in spraying technology, it is known to provide an apparatus having multiple chambers and associated feed units to deliver powders to a spray gun.

The invention is directed towards providing an improved spray deposition apparatus for the co-deposition of materials

SUMMARY OF THE INVENTION

According o the invention, there is provided a spray deposition system powder feed apparatus comprising: a plurality of powder chambers, each having an outlet; an adjustment means for each chamber, each for adjusting powder flow rate from its associated chamber; a mixing chamber communicating with the powder chambers, for receiving and mixing different powders received from the powder chambers; and a mixing chamber outlet for delivery of mixed powders to a spray gun supply.

In one embodiment, the apparatus comprises means for fitting onto a pick-up shaft of a spray gun supply.

In one embodiment, the apparatus comprises means for fitting into a single-powder hopper of a conventional feed apparatus.

In one embodiment, the apparatus further comprises an inlet gas pressure tube connected to the mixing chamber for delivery of gas to assist powder mixing and fluidising.

In one embodiment, the inlet gas pressure tube comprises means for delivering gas under sufficient pressure to force mixed powders into a spray gun supply pick-up shaft.

In one embodiment, the chamber feed adjustment means' are independent.

In another embodiment, at least one feed adjustment means comprises an actuator for varying effective size of the outlet of the associated chamber.

hi one embodiment, said actuator is cone-shaped at an end for engagement with the chamber outlet, and is mounted with respect to the outlet so that longitudinal actuator movement varies the outlet effective size.

hi one embodiment, said cone-shaped end is tapered at an angle in the range of 55° to 65°.

In one embodiment, said angle is approximately 60°.

In one embodiment, each chamber is cone-shaped adjacent its outlet. In one embodiment, the actuator is driven by a stepper motor.

In one embodiment, each powder chamber communicates with the mixing chamber via a supply line which is bent at one or more locations.

In one embodiment, the supply line enters the mixing chamber at an angle in the range of 72° to 78° to vertical.

In one embodiment, the angle is approximately 75°.

In one embodiment, the inlet gas pressure tube is located so that it ejects a gas into the mixing chamber to assist the fluidisation of a powder mixture and generate a pressure difference between the mixing chamber and nitrogen flowing through the pick up shaft.

In another aspect, the invention provides spray deposition system comprising a spray gun, a powder supply pick-up shaft connected to the gun, and any feed apparatus as defined above connected to the pick-up shaft.

hi a further aspect, the invention provides a method of feeding a plurality of mixed powders in a spray deposition system comprising a spray gun, a powder supply pickup shaft connected to the gun, and a feed apparatus connected to the pick-up shaft, said feed apparatus comprising a plurality of powder chambers, each having an outlet; means for adjusting powder flow rate from each chamber; a mixing chamber communicating with the powder chambers for receiving and mixing different powders received from the powder chambers; a mixing chamber outlet communicating with the pick-up shaft; and an inlet gas pressure tube connected to the mixing chamber, the method comprising: pouring a different powder into each powder chamber, adjusting the adjustment means to set a desired flow rate of powder from each powder chamber, and injecting a gas into the mixing chamber via the inlet gas pressure tube at a pressure to assist mixing of the powders and delivery of mixed powder into the pickup shaft. In one embodiment, there is a ratio of between 2.2:1 and 2.3:1 between the inlet gas pressure tube pressure and pressure of gas flowing in the pick-up shaft.

In one embodiment, the ratio is approximately 2.25:1.

In one embodiment, the gas in both the inlet gas pressure tube and the pick-up shaft is nitrogen.

DETAILED DESCRIPTION OF THE INVENTION

Brief Description of the Drawings

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:-

Fig. 1 is a set of diagrams showing the main elements of a feed apparatus of the invention;

Fig. 2 shows the apparatus incorporated into a conventional hopper for attachment to a spray gun; and

Fig. 3 is a diagram showing relationship between needle displacement and powder flow rate.

Description of the Embodiments

A spray deposition system performs co-deposition of materials using only a single feed apparatus, thus benefiting not only plasma arc, but also other thermal spray and cold spray pro cesses .

Referring to Fig. 1, in one embodiment, the powder feed apparatus 1 comprises two powder chambers 2 and 3 each having a needle-shaped bolt 4 for controlling flow rate of powder from the respective chamber. An inlet pressure tube 10 extends from a top plate 11 and a base plate 12. Powder flow tubes 14 extending from the outlets of the chambers 2 and 3 feed powders into a mixing chamber 15. Mixing occurs due to the powder flow rate and powder flow angle (75°) generated within the tubes 14. The inlet gas pressure tube 10 is also connected to the mixing chamber 15. The mixing chamber 15 directs the mixed powder into a pick-up shaft (which exists in conventional hopper arrangements) where the powder is carried by nitrogen gas towards a spray gun, which may be of the thermal spray or cold spray types.

Each bolt 4 has a conical end 5 engaging a chamber outlet 6, whereby vertical position of the bolt 4 governs the outlet cross-sectional area. A combination of gravity, gas- pressurised chamber 15, powder flow-ability, orifice opening and bolt 4 and chamber outlet taper angle causes powder to flow through each outlet 6, the flow rate being determined by the vertical position of the bolt 4. Each bolt 4 is rotated for vertical movement by a stepper motor assembly 20 having bearings 21, the bolts 4 being threaded where they pass through the top plate 11. The taper angle of the bolt end 5 is 60° and of the bottom of each chamber 2 and 3 is 60°.

The lowermost end of the apparatus 1 has a horizontal through hole 16 to accommodate a pick-up shaft of a convention spray apparatus. The manner in which the feed apparatus 1 fits into a conventional spray apparatus is shown in Fig. 2. The apparatus 1 fits into a hopper 51 of the type conventionally used for storing and feeding a single powder. The apparatus 1 sits upon the existing hopper 51 conventional pick-up shaft 18 so that its inlet opening communicates with the mixing chamber 15. The combination of the feed apparatus 1 and the hopper 51 provides a complete feed system 50.

Fig. 2 also shows representations of the different powders, namely Powder A in chamber 2 and Powder B in the chamber 3. It also shows different chamber outlet settings and hence different flow rates to the mixing chamber 15.

The feed apparatus of the invention offers a more cost effective way of producing functionally graded coatings without changing any of the features of the original hopper powder feed unit. The apparatus suits the Sulzer METCO DJP powder feed unit, and is also applicable to many hopper systems used for powder deposition processes (whether thermal or cold spray). The apparatus fits onto the pick up shaft which already exists in single hopper units, and therefore it is interchangeable with a number of deposition manufacturer's hopper unit, hi addition, the apparatus is standalone and does not require any modification of the original hopper equipment.

The two cylindrical shaped powder chambers 2 and 3 are used to contain two dissimilar powders. The needle bolts 4, aligned centrally in the middle of these two powder chambers, are used to vary independently the flow rate of each of the two powders. Each essentially acts as a continuously variable valve which, when rotated, moves upwards and downwards on a screw thread, allowing powder to flow out through the hole 6 at the end of the chamber. Their movement is calibrated in order to control the amount of the powder flow, using permanent indicators on the top plate to measure the angle of rotation of each bolt and indicated on a host computer via instrumentation and control.

The top plate 11 has eight 8 mm diameter holes, two 10 mm diameter holes and a single 7 mm diameter hole (Fig. 1). The 8 mm diameter holes are used to feed (pour in) the two dissimilar powders initially before deposition. The 10 mm diameter through holes contain roller bearings, thus controlling their rotated movement, while a 7 mm diameter hole was used to hold the central inlet pressure tube. The inlet pressure tube maintains a differential pressure in the device which fluidises and forces the powder mixture through a pick-up shaft hole, into the nitrogen gas flow inside the pick-up shaft.

The base plate 12 has three 7 mm diameter holes. The central hole accommodates the inlet pressure tube 10, while the other two holes facilitate attachment of a two powder flow tube 14 arrangement from the two powder chambers 2 and 3.

The two powder flow tubes 14 are of the same dimensions and they carry the powders from the powder chambers 2 and 3 to the mixing chamber 15, where the powders mix. hi order to increase the degree of powder mixing, each powder flow tube 14 has two bend sections (75°) and they are positioned very close to each other inside the mixing zone 15.

The apparatus 1 mixes two powders in the mixing zone 15. Mixing occurs due to the powder flow rate and powder flow angle (75°) generated within the tubes 14. The inlet pressure tube 10 in the chamber 15 position fluidises the mixed powder and directs the mixed powder into a pick-up shaft via both fluidisation and relative pressure difference (2.25:1) between the mixing chamber 15 inlet pressure tube 10 and the nitrogen gas flowing through the pick-up shaft 18. The combined mixed powder is carried by the flowing nitrogen gas to the deposition gun.

The pick-up shaft 18 is part of a conventional hopper feed system. Initially two dissimilar powders are poured into the two powder chambers. The apparatus 1 is then placed into a conventional powder feed hopper. The percentages of each powder are controlled by turning the needle-shaped bolts 4 using the stepper motors 20 (controlled by Labview™ software). The powders then flow from the powder chambers 2 and 3 to the mixing chamber 15 through the powder flow tubes 14, and then mix in the mixing chamber 15. The powder mixture is then forced through the pick-up shaft hole into the nitrogen gas flow by a combination of gravity, flow momentum, and direction, and the nitrogen gas pressure difference that exists between the inlet pressure tube 10 to the pick-up shaft 18.

According to a simulation model and experimentation results, the nitrogen gas pressure ratio of the inlet pressure tube 10 to pick up shaft 18 is preferably maintained at 2.25:1 in order to achieve mixing of the powders in the mixing chamber 15 and to force the mixed powder into the nitrogen carrier gas flowing in the pick up shaft 18. The preferred range of ratios is 2.20:1 to 2.30:1 as verified experimentally.

Optimized taper angles of 60° (more generally preferably within the range 55 to 65°) are preferred on both the needle tips 5 and chamber convergence zones to ensure proper flow-ability of all types (various size, shape and density) of powders used. Optimized taper angles of 75° (more generally preferably within the range 72 to 78°) are preferred on the powder flow lines and mixing chamber convergence zones to ensure proper mixing of all types (various size, shape and density) of powders used.

The following describes the qualification procedure used to assess the functionality of the apparatus 1. Powder flow bench tests were carried out to calibrate the powder flow for various vertical movements of the needle bolts 4 which were coupled with two linear actuators and controlled with Lab VIEW software via a PC.

Three powder types (Stainless Steel - SS, Inconel 625 - Nl, Nickel Chromium - N2) were used to test the apparatus 1 as they varied in particle shape and density and composition (as shown in Table 1 below).

Table 1: Selected stainless steel/nickel base alloy powders used in the verification testing.

Bench Tests of Powder Flow:

The bench tests were carried out to calibrate the movements of the bolts inside the powder chambers 2 and 3, labelled as chambers 'A' and 'B' respectively. These needle-shaped bolts 4 move upwards and downwards according to the user's requirement inside the chamber (controlled by the Lab VIEW software). When the bolts 4 are in a fully closed position or zero position, no powder flows. With the increase of the vertical movement, powder starts to flow from the chamber into the mixing zone. Initially the dual feed powder holder was placed inside the powder hopper and then the needle shaped bolts were placed inside both chambers. The stainless steel powder was poured into the chamber 'A' and the hopper cover was attached. After that the linear actuators were coupled with the needle shaped bolts. Variation of vertical movement was carried out controlled by Lab VIEW to calibrate the flow of powder flowing through the hole at the bottom of the chamber. During this process, powder particles were collected into a pre-weighted container at each stage of vertical increment starting at the bottom of the powder flow tube. The mass of powder flow was measured over a 10 second time period (which means the needle shaped bolt was opened for 10 seconds at every stage of vertical increment) and the weight of the powder collected was calculated, subtracting the weight of the container from the total weight of both container and powder. For each step vertical increment three readings were taken. Next, the two nickel base alloys were poured separately into the chamber 'B' and the above procedure was repeated. To verify the results, chamber 'B' was filled also with stainless steel and chamber 'A' was filled with the nickel base alloys to justify if there was any difference between the two chambers results.

Fig. 3 shows the combined graphical representation for chamber 2 ("Chamber A") and chamber 3 ("Chamber B"), for each of the powders used. The results show how vertical increment (vertical movement of the needle via rotation, which opens the chamber orifice) relates to powder flow rate for each powder type. This result of 4 mm increment was also valid for Chamber B for all powders.

Although the graph shows that a 4mm vertical increment of bolt 4 is the optimum level for 100 % particle flow for all three different powders, it also shows that a difference exists in the mass of powder flow found between each other. The reason behind this is, the different densities/mass between the powders (as the volume may be the same), and the difference shapes of the each powder particles material type between. However the difference is not significant. Considering a 4 mm vertical decrement as the base point (100% powder particle flow), the graph shows the average mass of powder flow when the needle shaped bolt is closed (0mm) to fully open (4mm) and the associated average mass of powder flow across this range.

Experimental sprayed functionally graded coatings results

A stainless steel substrate was functionally graded coated with stainless steel (SS) powder deposited as base coat up to Inconel 625 (Nl) as a top coat and similarly another one was coated with stainless steel (SS) powder deposited as a base coat up to Nickel Chromium (N2) as a top coat using the dual feed automated system. Chemical composition of the different layers was determined using the energy dispersive X-ray (EDS) spectroscopy. Three analyses at each point (i.e. Bottom layer, Middle layer and Top layer) were carried out during the chemical composition analysis using SEM (scanning electron microscopy) technique for each sample. Each element detected at these points were measured and compared to the desired composition of the starting powder. Verification of the design was not about attaining measured element composition values equal to that of the desired values (as elements get burnt off or change phase during spraying), but rather the trend (increasing or decreasing in composition) was seen as important, as shown in Table 2 below.

Table 2: Average range of Powder Chemical composition compared for FGM coatings.

(T) Increased, (I) decreased and (-) Not detected

*12 out of 15 Elements showed the same trend in composition FGM coating change as that of bulk powder materials

This confirms while the exact compositions were not attained never the less the proposed design has the potential of producing FGM which is a new venture for Thermal and Cold spraying. The reason the proportions are not the same as the stating powder compositions (that is less) is that the cumulative wt % of these elements are dependent on each other, for example if one element is increased/decreased then other elements will be change proportionally (in the range of 100 %), hence the expected composition is in fact lower than would be expected. However the trend from: high to low or low to high composition for each element is maintained almost throughout (12/15 elements) the functionally graded deposit.

The invention achieves in a simple manner excellent control of the relative flow of even very dissimilar powders through the apertures in the base plate with effective mixing of the powders. Also, the gas over-pressure is sufficient to fluidise and force the powder mixture completely and continuously into the pick-up line.

Another advantage of the invention is that the powders are mixed inside the "hopper" before they are carried through to a nitrogen gas flow line.

The apparatus requires use of only one existing feed hopper of any powder thermal or cold spray process (where a two-powder double chamber is placed inside it), for the deposition of two dissimilar powders. Also, there is only one over-pressure gas line to control the powder flow from the chambers to the outlet. The apparatus stands alone inside the hopper powder feed unit and no modification to the existing hopper is required. Thus, the apparatus offers an easier and more cost-effective way of depositing functionally graded coatings compared to prior systems used so far for thermal and cold spray.

The invention is not limited to the embodiments described but may be varied in construction and detail. For example, there may be more than two powder chambers, depending on the intended application.

Claims

1. A spray deposition system powder feed apparatus comprising:

a plurality of powder chambers (2, 3), each having an outlet (6);

an adjustment means (4, 5, 20) for each chamber, each for adjusting powder flow rate from its associated chamber;

a mixing chamber (15) communicating with the powder chambers (2, 3), for receiving and mixing different powders received from the powder chambers;

a mixing chamber outlet for delivery of mixed powders to a spray gun supply.

2. A feed apparatus as claimed in claim 1, wherein the apparatus comprises means (16) for fitting onto a pick-up shaft (18) of a spray gun supply.

3. A feed apparatus as claimed in any preceding claim, wherein the apparatus comprises means for fitting into a single-powder hopper (51) of a conventional feed apparatus.

4. A feed apparatus as claimed in claims 1 or 2, further comprising an inlet gas pressure tube (10) connected to the mixing chamber (15) for delivery of gas to assist powder mixing and fluidising.

5. A feed apparatus as claimed in claim 4, wherein the inlet gas pressure tube (10) comprises means for delivering gas under sufficient pressure to force mixed powders into a spray gun supply pick-up shaft (18).

6. A feed apparatus as claimed in any preceding claim, wherein the chamber feed adjustment means (4, 5, 6, 20) are independent.

7. A feed apparatus as claimed in any preceding claim, wherein at least one feed adjustment means comprises an actuator (4) for varying effective size of the outlet (6) of the associated chamber (2, 3) .

8. A feed apparatus as claimed in claim 6, wherein said actuator (4) is cone- shaped at an end (5) for engagement with the chamber outlet, and is mounted with respect to the outlet (6) so that longitudinal actuator movement varies the outlet effective size.

9. A feed apparatus as claimed in claim 8, wherein said cone-shaped end (5) is tapered at an angle in the range of 55° to 65°

10. A feed apparatus as claimed in claim 9, wherein said angle is approximately 60°.

11. A feed apparatus as claimed in any of claims 7 to 10, wherein each chamber (2, 3) is cone-shaped adjacent its outlet (6).

12. A feed apparatus as claimed in any of claims 7 to 11, wherein the actuator is driven by a stepper motor (20).

13. A feed apparatus as claimed in any preceding claim, wherein each powder chamber communicates with the mixing chamber (15) via a supply line (14) which is bent at one or more locations.

14. A feed apparatus as claimed in claim 13, wherein the supply line (14) enters the mixing chamber at an angle in the range of 72° to 78° to vertical.

15. A feed apparatus as claimed in claim 14, wherein the angle is approximately 75°

16. A feed apparatus as claimed in any of claims 4 to 15, wherein the inlet gas pressure tube (10) is located so that it ejects a gas into the mixing chamber (15) to assist the fluidisation of a powder mixture and generate a pressure difference between the mixing chamber (15) and nitrogen flowing through the pick up shaft (18).

17. A spray deposition system comprising a spray gun, a powder supply pick-up shaft (18) connected to the gun, and a feed apparatus (1) as claimed in any preceding claim connected to the pick-up shaft.

18. A method of feeding a plurality of mixed powders in a spray deposition system comprising a spray gun, a powder supply pick-up shaft connected to the gun, and a feed apparatus connected to the pick-up shaft, said feed apparatus comprising a plurality of powder chambers (2, 3), each having an outlet; means ( 4, 5, 20) for adjusting powder flow rate from each chamber; a mixing chamber (15) communicating with the powder chambers (2, 3) for receiving and mixing different powders received from the powder chambers; a mixing chamber outlet communicating with the pick-up shaft; and an inlet gas pressure tube (10) connected to the mixing chamber, the method comprising: pouring a different powder into each powder chamber, adjusting the adjustment means to set a desired flow rate of powder from each powder chamber, and injecting a gas into the mixing chamber via the inlet gas pressure tube at a pressure to assist mixing of the powders and delivery of mixed powder into the pick-up shaft.

19. A method as claimed n claim 18, wherein there is a ratio of between 2.2:1 and 2.3:1 between the inlet gas pressure tube pressure and pressure of gas flowing in the pick-up shaft.

20. A method as claimed in claim 19, wherein the ratio is approximately 2.25:1.

21. A method as claimed in any of claims 18 to 20, wherein the gas in both the inlet gas pressure tube and the pick-up shaft is nitrogen.