SONOTRODE FOR ULTRASONIC ATOMIZATION OF METALS AND THEIR ALLOYS

Abstract

 The invention relates to a sonotrode for high-temperature application in ultrasonic atomization of metals and their alloys, characterized in that it includes:
a body (1) made of a material having a thermal conductivity greater than 150 W / m ^∗ K, and a core (2), constituting a high-temperature tip of the sonotrode, made of a material having a melting point or thermal decomposition temperature of at least 1200°C,
wherein the body (2) and the core (1) are connected mechanically or by diffusion or by both methods combined.

Description

[0001] The invention relates to a sonotrode for ultrasonic atomization of molten metals and their alloys having a melting point greater than 600 °C.

[0002] One of the methods of producing high-quality metallic powders is the method of ultrasonic atomization. Such powders find application as a raw material for 3D printing in additive technologies, powder metallurgy (sintering) and special-purpose metallic coatings.

[0003] In the ultrasonic atomization method, the material is atomized due to the instability of the capillary waves in the liquid. After overcoming the viscous forces, droplets are periodically ejected and then the process repeats after the instability is again realized. This technique is commonly used at low temperatures, i.e. for atomization of water-based solutions or organic solvents, or for atomization of fusible metals, especially tin solders. Atomization of alloys at temperature above the melting point of aluminium (>600 °C) on the sonotrode without cooling features is impossible in standard systems, due to sonotrode decalibration and cavitation damage.

[0004] To assure the sonotrode durability under metal atomization temperatures in the range of 600 - 3000 deg C, the present invention uses a combination of different materials to effectively take advantage of their different properties at elevated temperatures, such as thermal expansion, strength, melting point and thermal conductivity.

[0005] The subject of the invention is a sonotrode for ultrasonic atomization of metals and their alloys, comprising a body made of a material with a thermal conductivity greater than 150 W/m*K and a core item constituting a high-temperature sonotrode tip, made of a material having a melting or thermal decomposition temperature of at least 1200 °C. The body and core are connected mechanically, or by diffusion, or by both methods together. Preferably, the body and core connection includes: interference or threaded connection or Morse taper or heat shrink fitting or soldering. Preferably, some or all of the side surface of the body has a coating having a hardness greater than 250 HV and a thickness of up to 3 mm.

[0006] Preferably, the sonotrode body is combined outside with a water jacket cooling, preferably on at least 30% of the side surface of the sonotrode. The cooling liquid can be a pure water or any fluid having the following properties: specific heat > 1200 J / kg * K and a vapor pressure less than 70 kPa at 90 0C at a pressure of 100 kPa. Preferably, the cooling fluid is pressurized.

[0007] Preferably, the surface of the high-temperature sonotrode tip, at least in the area of the core, is textured, in particular profiled, fluted, cut or drilled to a depth of up to 3 mm.

[0008] The sonotrode according to the invention is intended to jointly perform the following functions:

• Atomization of molten material takes place on the core; the material can be melted on the core by means of e.g. an electric arc, plasma, electron beam or by any other heat source utilizing electricity (e.g. induction heating) or their combination.
• The sonotrode atomizes the molten material at its hot end (i.e. its core) under the influence of vibrations with a resonance frequency >10 kHz. Vibrations in the sonotrode are excited by an ultrasonic transducer and transferred to the sonotrode from the cold end via a waveguide. Due to limitations in the transducer control system and the risk of its damage or overheating, the transducer-waveguide-sonotrode system is able to operate at frequencies deviating from the frequency for which it was designed by no more than +/- 5%. If any part of these items is detuned (e.g. as a result of smelting or erosion of the material) out of this range, what affects system vibration modes, the entire sonotrode must be replaced as tuning is usually not possible.
• In most materials, high temperature causes a decrease in Young's modulus (elasticity), what ends up in decrease of the system natural frequency. This natural phenomenon reduces said frequency for new sonotrode by 3 to 9% when melting metals at> 600 deg C. Therefore sonotrodes for use as above require cooling to minimize this effect, using a water bath or other cooling medium.
• The sonotrode body, due to its thermal conductivity > 150 W/m*K, acts as a radiator, vigorously transferring heat from the core to the cooling liquid by which the sonotrode surface is cooled
• Most of the elastic energy in the vibrating sonotrode and waveguide, which is supplied by the ultrasonic transducer, is dissipated (lost) to the coolant (cooling liquid) rather than to the capillary waves on the molten metal pool. The sum of this mechanical energy and the heat stream coming from the hot end of the sonotrode (transferred by the thermal conduction in the sonotrode) causes the sonotrode surface in the water bath to operate under the cavitation conditions. Such phenomena results in a rapid destruction (erosion) of the sonotrode outer surface and then detuning the sonotrode from the permissible frequency range of the transducer.
• The core, which is made of material having high thermal resistance, should be firmly fixed into the sonotrode body, in order for the ultrasonic vibrations to be efficiently transferred from the sonotrode towards the molten and atomized raw material. The method of joining elements is critical, because the body made of materials having high thermal conductivity (e.g. 17E-6 1/K for copper alloys) presents also high thermal expansion, while the core made of material having a high melting point and very low thermal expansion (e.g. respectively 2600-3300°C and 5E-6 1/K for tungsten or molybdenum alloys). Therefore, the interface between the body and core is prone to open at high temperature due to the thermal gradient between the molted raw material and cooled surface of the sonotrode.

[0009] The invention is illustrated in an embodiment with reference to the drawings, in which:

Fig. 1 schematically shows a sonotrode according to the invention,

Fig. 2 shows diagrams of a sonotrode with an example of liquid cooling,

Fig 3 shows a diagram of an example sonotrode texturing on its hot end.

[0010] Fig. 1 schematically shows a sonotrode assembled with a core (2) constituting the hot end of the sonotrode (H) and with a body (1). On the cold end side (C) the sonotrode is connected to the waveguide (3) and via the waveguide to the ultrasonic transducer (not shown). Part of the sonotrode and waveguide is placed in the liquid cooling chamber (6) e.g. in form of water jacket.

[0011] The sonotrode body (housing) (1) is made of an alloy or sinter of a conductivity greater than 150 W / m * K, e.g. copper alloy. The melting or thermal decomposition temperature of such a material can be up to 1000 K lower than of an atomized material.

[0012] The core (2) is the high-temperature (H) sonotrode tip, on which the atomization of molten metal takes place. The core is made of a material of high thermal resistance (melting or thermal decomposition temperature at least 200 K higher than of an atomized material) e.g. alloy of tungsten, molybdenum, niobium, sintered carbides. The core volume is less than 30% of the sonotrode volume, which is the body (1), core (2) and coating (5). The body (1) and core (2) are mechanically joined, e.g. with hydraulic press, thread, Morse taper, heat shrink fit, at a section not less than half the diameter of the core at the hot end of the sonotrode (H). Alternatively, the contact between (1) and (2) is covered with high temperature solder to reduce the contact stress as well as increase cohesion and heat exchange between elements under thermal load conditions.

[0013] The waveguide (3) preferred material should have low material damping e.g. titanium alloy, aluminum, steel. The waveguide (3) is connected to the sonotrode body (1) by means of a mechanical connector e.g. pin or screw (4).

[0014] On the surface of the body (1), in the part of the sonotrode intended to be placed in the liquid cooling chamber (6), a coating (5) of microhardness> 250 HV is applied, in the form of e.g. nickel plating, chromium plating, nitridation. Also Fe, Ni, Co, Cr or V -based hard alloys such as Stellite or high alloy steels, chromium / tungsten carbides applied by any method, e.g. arc-plasma, galvanic, HVOF, hard-facing. The coating (5) function is to protect against cavitation and corrosion of body material (1) in contact with cooling liquid like a water.

[0015] In the liquid cooling chamber (6) cooling by a flowing liquid takes place, the liquid having the following properties: specific heat > 1200 J / kg * K and vapor pressure not exceeding 70 kPa at a temperature of 90 deg C at a pressure of 100 kPa. The cooling liquid can preferably be a water, ethylene or propylene glycol, mineral oil, silicone fluids or mixtures thereof.

[0016] The liquid cooling chamber (6) is sealed with gaskets made of material resistant to a temperature higher than 300 °C. The gaskets are placed at a distance not closer than 10 mm from any sonotrode ends.

[0017] The sonotrode has a rotationally symmetrical shape, usually cylindrical, but can be also conical or of an umbrella. Its hot end (H) can be textured particularly profiled, fluted, cut, or drilled to increase area of contact between the core (2) and molten metal in the pool on its top, in order to improve atomization by augmented wettability.

[0018] The sonotrode of the present invention can be installed in an ultrasonic atomizer adapted to produce powders from materials such as steels of various grades and titanium alloys. The installation method is shown in Fig. 2. The sonotrode body is made of copper alloy, the core is made of niobium and has been fitted in the body by means of a hydraulic press. The sonotrode body is equipped with the external cooling system based on a non-cavitating liquid.

Claims

1. A sonotrode for high-temperature application in ultrasonic atomization of metals and their alloys, characterized in that it includes:

a body (1) made of a material having a thermal conductivity greater than 150 W / m * K, and

a core (2), constituting a high-temperature tip of the sonotrode, made of a material having a melting point or thermal decomposition temperature of at least 1200°C,

wherein the body (2) and the core (1) are connected mechanically or by diffusion or by both methods combined.

2. The sonotrode according to claim 1, characterized in that the connection of the body (1) and the core (2) includes: press-fit connection or a thread or Morse taper or thermo-shrink fitting or connection by brazing.

3. The sonotrode according to claim 1 or 2, characterized in that a part of or whole body (1) side surface is coated with material of hardness > 250 HV and of a thickness of up to 3 mm.

4. The sonotrode according to claim 1 or 2 or 3, characterized in that the sonotrode body (1) is equipped with a system for cooling it from the outside, preferably on at least 30% of its side surface, by means of cooling liquid having the following properties: specific heat > 1200 J/kg*K and vapor pressure not more than 70 kPa at a temperature of 90 °C at a pressure of 100 kPa.

5. The sonotrode according to claim 4, characterized in that the cooling liquid is under pressure up to 500 kPa.

6. The sonotrode according to any of claims from 1 to 5, characterized in that the top surface of its hot end, at least in the area of the core (2), is textured, preferably profiled, fluted, cut or holed up to a depth of 3 mm.

Drawing