Device for atomizing a melt stream by means of a gas

Abstract

The present invention relates to a device for atomizing a metallic, intermetallic or ceramic melt flow into a spherical powder by means of a gas, comprising: a melting chamber (1); a powder chamber (2); an induction coil (3) in the melting chamber (1); a molten material, preferably a molten rod (7) in the induction coil (3); and a spray nozzle (5) arranged in a nozzle plate (4) for a molten flow (8) formed by melting the molten material through the induction coil (3), the spray nozzle connecting the melting chamber (1) with the powder chamber (2), wherein the spray nozzle (5) has a merely converging nozzle profile, the nozzle profile having a nozzle side surface (13) with a circular arc-shaped cross section, whereby the atomizing gas (V) and the molten flow and the droplets generated thereby reach a certain velocity of the atomizing gas (V) to the greatest extent possible, which is preferably less than the speed of sound.

Description

[0001] This patent application claims the priority of German patent application DE 10 2019 214 555.8, the content of which is incorporated herein by reference. Technical Field

[0002] The invention relates to a device for atomizing a metallic, intermetallic or ceramic melt stream into a spherical powder by means of a gas. Background Art

[0003] The background of the invention is that metal powders are used in a large number of technologies in the industrial field to produce components or functional layers, such as sintered steel, hard metals, contact materials, permanent magnets, ceramics and thermal spray coatings.

[0004] In particular, in the field of additive manufacturing and metal powder injection molding, special requirements are placed on the sphericity and particle size distribution of the powder, because these requirements are an essential prerequisite for further processing in the process. Four basic technologies are used to produce spherical metal powders: plasma-assisted wire spraying (see WO 2011 054113 A1), plasma spheroidization of powders with irregular surfaces (see EP 1 689 519 B1), plasma rotating electrode process (see US 5 147 448 A) and gas atomization of liquid melts.

[0005] The last technique can also be applied to some ceramics and involves a number of different methods of creating a liquid melt:

[0006] VIGA (Vacuum Induction Gas Atomization - Induction Melting in a Ceramic Cold Crucible)

[0007] EIGA (Electrode Induction Gas Atomization - Crucible-free induction melting of metallic or intermetallic bars)

[0008] PIGA (Plasma Inert Gas Atomization - Plasma Melting in a Cold Crucible)

[0009] In crucible-based melting processes, the pouring stream to be atomized is produced by tilting the crucible (casting), the bottom outlet of the crucible or the melting rod (electrode). The best melting process is selected according to the specific characteristics of the material to be melted. Thus, for example, refractory and oxygen-philic metals or alloys cannot be melted in ceramic crucibles, because the crucible material cannot withstand the process temperature or is reduced by the melt.

[0010] As Schulz G's professional paper "Laminar sonic and supersonic gas flowatomization-the NANOVAL process" (World Powder Metallurgy and Particulate Materials Conference, Progress in Powder Metallurgy, 1996, No. 1, pp. 43-54) states, all atomization processes of free-falling liquid melts implemented with the aid of an atomizing gas have an impact on the sphericity and gas porosity of the formed powder particles. So far, it has not been possible to produce gas atomized powders without satellite spheres and gas pores on the surface of the powder particles, which is the previous technology using PREP or plasma atomization. However, PREP or plasma atomization has serious disadvantages in terms of economic benefits and is correspondingly much more expensive than gas atomized powders.

[0011] In the gas-assisted atomization process, a distinction is made in principle between different designs of free-fall atomization (free-fall nozzles, close-coupled nozzles) and gas-jet-guided atomization by means of Laval nozzles.

[0012] A device for atomizing a melt flow into spherical powder by means of gas and having such a Laval nozzle is disclosed by WO 2015 / 092008 A1 and comprises a melting chamber, a powder chamber, an induction coil in the melting chamber, and a Laval spray nozzle arranged on a nozzle plate connecting the two chambers, which has a convergent-divergent nozzle inner shape.

[0013] Through this spray nozzle, the melt stream, which is radially surrounded by pressurized atomizing gas and inductively produced from the melt in the melt chamber by an induction coil, is introduced into the nozzle under the action of gravity and is broken up into tiny droplets in and after the Laval spray nozzle. These droplets condense into the powder particles to be produced.

[0014] In this regard, the powder particles produced therefrom need to be improved in terms of satellite sphere formation and gas pore formation. Summary of the invention

[0015] The object of the present invention is to provide an atomizing device for gas atomization of liquid metal and ceramic melt, which device can prevent the formation of satellite spheres and gas pores as much as possible with the greatest economic benefit.

[0016] According to one embodiment, the solution for achieving this object is to use a new type of gas jet guide spray nozzle, which has a nozzle profile that is only convergent, with nozzle sides that are circular in cross section, whereby the atomizing gas and the melt flow and the droplets generated therefrom reach a certain velocity of the atomizing gas to the greatest extent possible, which is preferably less than the speed of sound. Thus, unlike the conventional Laval atomization, the velocity of the atomizing gas does not exceed the speed of sound during the atomization process.

[0017] It has been shown that powder particles of improved quality can be produced with this nozzle profile with maximum economic efficiency in a manner which prevents the formation of satellites and gas voids as much as possible.

[0018] The preferred improved solutions of the present invention refer to other embodiments. Therefore, the surface of the nozzle plate on the nozzle outlet side can be flat and perpendicular to the flow direction of the melt flow. Therefore, a clear edge is formed on the nozzle outlet side, which generates an additional vortex effect that assists the formation of particles from the melt flow.

[0019] According to another preferred embodiment of the present invention, the induction coil can be constructed to be height-adjustable.

[0020] Advantageously, the height adjustability of the coil can be used to vary the injection jet that forms the melt stream up to the free fall height of the nozzle. The melt temperature decreases with increasing fall height, in particular by the output of the injection power, so that the viscosity of the melt changes when it enters the nozzle, thereby specifically controlling the resulting particle size distribution.

[0021] It should be noted that the height adjustability of the coil can be applied to atomizing devices separately from the rest of the invention, while achieving the described advantages, together with other types of spray nozzles.

[0022] According to another preferred embodiment of the present invention, the induction coil can be designed to be tapered toward the spray nozzle, wherein a cylindrical rod of the material to be atomized for forming the melt stream is coaxially arranged in the induction coil.

[0023] The further exemplary embodiment relates to the parameters for designing and dimensioning the nozzle profile, with which parameters excellent quality results are achieved in the powder particles produced. To avoid repetition, reference is made to this exemplary embodiment.

[0024] In order to adapt the atomizing device according to the invention to different applications as rationally and quickly as possible, in another preferred embodiment, the spray nozzle can be arranged in a separate nozzle insert which is detachably arranged in the nozzle plate.

[0025] For more features, details and advantages of the present invention, please refer to the following description of the embodiments in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the axial section of the atomization device.

[0027] The main components of the atomization device shown in the figure are a melting chamber 1, a powder chamber 2 (also called an atomization chamber), an induction coil 3 arranged in the melting chamber 1, and a nozzle plate 4 arranged between the two chambers 1 and 2, on which a spray nozzle 5 is used to connect the two chambers 1 and 2. The nozzle plate 4 is flat on the outlet side 16 and is perpendicular to the flow direction of the melt stream 8. DETAILED DESCRIPTION

[0028] In a melting chamber 1 under argon pressure p1, the material to be atomized in the form of a cylindrical rod 7 equipped with a 45° tip 6 is partially introduced into a conical induction coil 3 with three windings, which is basically known from DE 41 02 101 A1, for example. The conicity of the induction coil 3 corresponds to the conicity of the tip 6 of the rod 7 to be atomized. The tip 6 and in particular the surface of the tip 6 are inductively heated by a central frequency current flowing through the induction coil 3 until a molten phase is formed on the surface. This melt stream 8 flows downward on the conical surface and drips from the tip 6 in the form of a continuous injection stream. The mass flow rate of the injection stream forming the melt stream 8 can be varied within a wide range of 0.4 kg / min to 2.5 kg / min by the inductively coupled electrical power. A melt flow of 0.8 to 1.5 kg / min is particularly suitable for atomization. During atomization, the rod 7 rotates slowly about its axis of symmetry S and moves continuously downward. The corresponding melting rate results from the diameter D of the bar 7, which may be between 30 and 200 mm, and the set lowering speed. A bar diameter D of between 80 and 150 mm is particularly advantageous in terms of process technology.

[0029] The height adjustability ΔH of the induction coil 3 is achieved by means of a linear suspension device 9, which is only schematically shown in the figure, and by means of which the free fall height of the injection stream to the nozzle can be changed, and as mentioned above, the viscosity of the melt when it enters the nozzle can be changed. A spacing of 3 to 100 mm between the spray nozzle 5 and the induction coil 3 is technically reasonable. If the coil spacing is smaller, there is a risk of voltage breakdown from the coil to the nozzle, and if the spacing is larger, there is a risk of the injection stream breaking up before entering the nozzle opening. In addition, a horizontal coil winding is particularly advantageous because, unlike an ascending coil winding, it is possible to prevent the injection stream from being deflected by electromagnetic forces when leaving the coil magnetic field.

[0030] The rotationally symmetrical spray nozzle 5 is located with its center point on the axis of symmetry S of the bar 7 and the coil 3 and at a distance H below the lowest winding in the induction coil 3. The spray nozzle is arranged in a separate nozzle insert 11 which is detachably arranged in the nozzle plate 4 and is indirectly cooled by pressing against the water-cooled nozzle plate 4 with a pressure p1. The melt stream 8 is radially enveloped and compressed by the gas flowing from the melting chamber 1 into the powder chamber 2 and is accelerated to a maximum speed of sound at the nozzle outlet through the circular opening of the spray nozzle 5. The driving force is the positive pressure difference between the gas pressure p1 in the melting chamber and the gas pressure p2 in the powder chamber 2. The pressure difference is at least 0.2 bar and a maximum of 25 bar. A pressure difference in the range of 2 bar to 10 bar is particularly advantageous technically.

[0031] Even in the case of large pressure differences p1-p2, the atomizing gas V in the spray nozzle 5 is accelerated to the speed of sound at most due to the merely converging nozzle shape, since in the supersonic range the converging nozzle profile acts as a diffuser and decelerates the gas again. The greater the pressure difference p1-p2, the faster the limit of the speed of sound in the nozzle profile is reached. The gas flow is therefore not laminar, since the gas pressure immediately adjacent to the nozzle outlet is a function of the pressure and is significantly greater than the ambient pressure p2 in the powder chamber.

[0032] The atomizing gas generates compressive and shear stresses in the jet-like melt stream 8, thereby compressing and accelerating the melt stream. The melting speed in the melt jet decreases radially from the outside to the inside. Immediately after leaving the spray nozzle 5, the compressive and shear stresses are reduced by the melt jet beam 12 breaking up into individual droplets, which condense into spherical powder particles in the atomization chamber. Surprisingly, it is not necessary to use a laminar gas flow for this purpose, or the gas velocity does not need to be greater than the speed of sound. On the contrary, by atomization only in the subsonic range, the sphericity of the powder particles is improved and the gas porosity is reduced compared to the conventional Laval atomization. This is achieved by a nozzle profile that is only convergent, in which the nozzle side 13 with a circular arc cross section is constructed in the form of an incomplete circle, which has a radius R of 2 to 15 mm, preferably 5 mm, and a height h of the spray nozzle 5, which is less than the radius R of the convergence circle. The tangent T on the nozzle outlet is at an angle W of <90° with the nozzle outlet side. In a specific embodiment, the height is 4.5 mm at a convergence radius R of 5 mm. The diameter of the nozzle can vary in the range of 2 to 20 mm. In an embodiment, the nozzle diameter d is 10 mm. With these parameters, the d50 value in the Ti alloy powder reaches 50 μm at a pressure of p1 = 4.5 bar and p2 = 930 mbar.

[0033] Furthermore, the nozzle insert 11 is made of the specific material to be atomized, ie, for example, TiAl or titanium. Its diameter E may be 20 to 200 mm, preferably 140 mm.

[0034] The rod 7 can be, for example, a so-called EIGA electrode, the diameter D of which is at most 150 mm. In the exemplary embodiment shown, a diameter D of 115 mm is selected.

[0035] For the induction coil 3 in the form of a conical internally cooled coil made of copper with a slope of 45°, the inner diameter I of the uppermost winding 14 can be a maximum of 170 mm, specifically, for example, 130 mm, and the vertical spacing G of the uppermost, middle, and lowermost coil windings 14, 15, 10 is 3 to 20 mm, preferably 8 mm. The diameter F of the coil winding tube can be 10 to 30 mm, preferably 16 mm. A rectangular cross section can also be used.

[0036] The distance H between the induction coil 3 and the bottom side of the nozzle 5 is 10 mm.

Claims

1. A device for atomizing a metal or ceramic melt stream into a spherical powder by means of a gas, comprising: Melting chamber (1), Powder room (2), The induction coil (3) in the smelting chamber (1) a molten material, which is a molten rod material (7) in the induction coil (3), and a spray nozzle (5) arranged in a nozzle plate (4) for a melt stream (8) formed by melting the melt through the induction coil (3), the spray nozzle connecting the melting chamber (1) and the powder chamber (2) together, It is characterized in that The spray nozzle (5) has a merely converging nozzle profile with a nozzle side (13) having a circular arc-shaped cross section, whereby the atomizing gas (V) and the melt flow and the droplets generated thereby reach a speed less than the speed of sound, The tangent line (T) on the nozzle outlet makes an angle (W) of <90° with the nozzle outlet side.

2. The device according to claim 1, wherein the surface of the nozzle plate (4) on its nozzle outlet side is flat and perpendicular to the flow direction of the melt stream (8).

3. The device according to claim 1 or 2, wherein the induction coil (3) is height-adjustable.

4. An apparatus according to claim 1 or 2, wherein the induction coil (3) is constructed to gradually narrow conically toward the spray nozzle (5), wherein a cylindrical rod (7) of the material to be atomized for forming the melt flow (8) is coaxially arranged in the induction coil (3).

5. The device according to claim 1 or 2, wherein the cross-sectional profile of the nozzle side (13) is formed by a partial arc with a radius (R) of 2 to 10 mm.

6. The device according to claim 1 or 2, wherein the height (h) of the spray nozzle (5) is 2.5 to 9.5 mm.

7. The device according to claim 1 or 2, wherein the spray nozzle (5) is arranged in a separate nozzle insert (11) which is detachably arranged in the nozzle plate (4).

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