Patent Application
20180169763
N/A
The present invention relates to the production of high purity metallic powders for use in additive manufacturing and 3D printing machines.
There is a growing demand for fine high quality spherical metallic powders. For example, one major application for such powders is the 3D printing industry, which has been demanding for narrow size cuts of high quality spherical satellite-free Ti-6Al-4V (titanium alloy), of generally between 45 and 250 microns.
This application has raised the bar in terms of quality, as it consumes the highest quality of powders currently available on the market. Many criteria are used to rate the quality of a powder: its sphericity, its particle size distribution, the absence of satellites (significantly small particles that are attached to the main particles). One current problem is that the production capacity of such quality powder is very limited. Another one is that a typical atomization system produces a wide range of particle size, while the industry asks for very narrow and specific cuts.
Numerous methods have been developed over the last years to produce powders by atomization.
For example in U.S. Pat. No. 5,707,419, a method is disclosed whereby plasma torches are used to melt and atomize a titanium wire. In this disclosure, the feed rate for example for titanium is limited to 14.7 g/min and the plasma torches are fixed in position at a 30° angle with respect to the axis of the feed. This 30° angle had been determined as being the optimal angle under certain circumstances. Here, the torches are locked at this specific angle to insure the alignment with the wire. While this method has the advantage of repeatability between runs, as well as minimizing the chances of aiming beside the wire, Applicant's experience demonstrates that this configuration is not optimal. It has been demonstrated that the ideal angle varies with the wire speed as well as the desired particle size distribution.
In PCT Patent Publication No. WO 2011/054113, a method is proposed to improve productivity of the plasma atomization using electrodes for preheating. Using electrodes for preheating is a very complicated process. In this arrangement, there are typically (3) electrodes as well as three (3) plasma torches to ensure a uniform heating. The wire is heated by arcing each electrode to the wire. Therefore, 3 currents are passing through the wire and heat it by resistive heating. This means that 6 power supplies are required to operate, noting that the more power sources there are, the more difficult it is to manage the heat going to the wire, in addition to this also increasing the capital and operating cost significantly.
There are also a number of mechanical inconvenients to this arrangement. For example, for plasma atomization to take place, the torch alignment according to the wire is very critical. All the electrodes and the torches, as well as the wire, must converge at the same exact point. The space around the apex convergence point being very limited, the design of the assembly is therefore dictated by mechanical constraints rather than by the process itself.
Therefore, it would be desirable to have a simplified device to allow for increasing the productivity of plasma atomization. There would also thus be a gain in having a system that allows controlling the particle size distribution.
It would thus be highly desirable to provide a novel apparatus for producing quality powders.
The embodiments described herein provide in one aspect an apparatus to produce metallic powder from a wire by plasma atomization, comprising:
Also, the embodiments described herein provide in another aspect an apparatus to produce metallic powder from a wire by plasma atomization, comprising:
Furthermore, the embodiments described herein provide in another aspect an apparatus to produce metallic powder from a wire by plasma atomization, comprising:
Furthermore, the embodiments described herein provide in another aspect a method to produce metallic powder from a wire by plasma atomization, comprising:
Furthermore, the embodiments described herein provide in another aspect a method to produce metallic powder from a wire by plasma atomization, comprising:
Furthermore, the embodiments described herein provide in another aspect a method to produce metallic powder from a wire by plasma atomization, comprising:
Furthermore, the embodiments described herein provide in another aspect a powder produced by any of the above methods.
Furthermore, the embodiments described herein provide in another aspect a powder produced by any of the above apparatuses.
For a better understanding of the embodiments described herein and to show more clearly how they may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, which show at least one exemplary embodiment, and in which:
In order to produce high quality powders, controlling particle size and maximizing production rate in a plasma atomization reactor, an apparatus P and a method by which torch angle can be adjusted and wire preheated are hereby presented.
As illustrated in
Once preheated, the wire 2 then reaches the apex 8, which is the zone where the wire 2 and the three plasma torches 7 meet for the atomization. The melting atomized particles freeze back to solid state as they fall down into a chamber of the reactor 10. The powder 11 is then pneumatically conveyed to a cyclone 12. The cyclone 12 separates the powder from its gas phase. The powder is collected at the bottom of a canister 14 while clean gas is then sent, via outlet 15, to a finer filtering system (not shown). The canister 14 can be isolated from the cyclone 12 by a gas-tight isolation valve 13.
Now turning our attention to the induction coil 6, the current apparatus P uses an induction coil to preheat the wire, which uses a single power supply and as the heat source does not encumber the apex zone. In this configuration, the wire preheating comes from a single uniform and compact source. Wire temperature can be controlled by adjusting induction power, which is a function of the current in the induction coil 6.
The induction preheating device is illustrated in
The wire guide 5 can be designed to either react with or to be transparent to induction. For example, the wire guide 5 could be made of alumina, or silicon nitride, which are transparent to induction. It could also be made of silicon carbide or graphite, which react with induction. In the latter case, the hot wire guide, heated by induction, will radiate heat back into the wire 2.
For example, when the wire used was a ⅛41 diameter Ti-6Al-4V Grade 23 ELI, the optimal induction frequency for this wire has been found to be between 270 and 290 kHz. The optimum frequency varies with the materials as well as the shape and dimension.
It is known that wire preheating can contribute to a capacity increase. However, the wire cannot be heated higher than its melting point in order to keep it under its solid state. Also, above a temperature of 1000° C., the alloy will convert from α to β phase, which will alter the rigidity of the wire. Depending on the configuration and the distance to be run by the wire, one might choose to maintain the mechanical properties of the wire of its a phase in order to keep the wire stiff and straight. For example, for a ⅛ ″ diameter Ti-6Al-4V Grade 23 ELI, since the wire shall not be brought higher than its melting point prior to atomization by plasma, the maximum capacity increase provided by wire preheating is 2 kg/h. This is equivalent to a capacity increase by a factor of three to four compared to a system without preheating.
As varying the torch angle in relation to the wire 2 will also move the location of the apex 8 (the meeting point of the wire 2 and the three torches 7), this will have an effect on efficiency since the torches 7 have a constant length. In order to avoid such problem, longer torches coupled with spacers can be used. By having longer torches and multiple sizes of spacers, it is possible to attain any angle while keeping the location of the apex 8 at the same place.
Pivoting the torches 7 seems to have an important effect on the plasma atomization process. Prior systems stated that the optimal angle was to be fixed at 30°. Although one could have been tempted to doubt this statement, being able to swivel the torches was not an obvious alternative. Therefore, going with a fixed angle was justified in the case of previous systems. The present arrangement suggests giving flexibility to the system by adding swiveling ball flanges 30 to the design.
Varying the angle of attack between the wire 2 and the plasma jets can affect the atomization in several ways. The major difference between plasma atomization in regards to traditional gas atomization is that heat is supplied by the jet. Therefore, there are two major considerations to take into account; namely heat transfer from the torch 7 to the wire 2, and the atomization by itself.
An important aspect for plasma atomization is the quality of heat transfer between the torches and the wire. Indeed, a proper alignment is required. The angle of attack also has an effect on the heat transfer, in two different ways; steeper (or smaller) the angle is, the surface area (A) that will exchange heat will increase. On the other hand, a shallower (higher) angle will promote a higher exchange coefficient (h).
Q=hAΔT
The equation above is a classic heat transfer formula. The objective is to maximize the value of Q. The angle will have an effect on both the h and the A. From a heat transfer point of view, the optimal angle is the one that maximizes Q for a specific wire feed rate, size and material.
For atomization to occur, some micro droplets must form at the surface of the wire 2 (heating phase). Then a gas flow is used to detach that droplet from the wire 2 and carry it in suspension into the gas phase (atomization phase). It is known that high velocities are required to break the bond between the wire and the droplet. The following equation, taken from water atomization of aluminum literature [5], relates the mean particle size to angle between the melt stream and the argon jet:
D=C/V sin ∝
Although the application is slightly different, the concept is similar; the plasma jets herein replace the water jets and the jets are herein used for heating instead of cooling. Indeed, the formula shows that higher gas velocities are able to detach finer particles, which makes sense as it requires more force to detach a finer droplet from a melting wire. Interestingly, the angle seems to have a similar effect according to this formula.
It becomes clearer that in order to optimize the plasma atomization process, the angle has to be variable in order to adapt to different conditions. The two previous equations have shown how the parameters are intertwined, and that being able to vary the angles constitutes a significant feature of the present apparatus P.
The apparatus P thus includes inter alia (1) an inductive preheating of the wire to increase capacity; and (2) torches that are installed onto the reactor head, using swivelling ball flanges 30 which allow flexibility in regards to the angles that can be reached by the torch alignment, to allow for controlling particle size distribution (powder quality).
While the above description provides examples of the embodiments, it will be appreciated that some features and/or functions of the described embodiments are susceptible to modification without departing from the spirit and principles of operation of the described embodiments. Accordingly, what has been described above has been intended to be illustrative of the embodiments and non-limiting, and it will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the embodiments as defined in the claims appended hereto.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2016/000165 | 6/6/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62171618 | Jun 2015 | US |
