110Science and Engineering of Droplets
2.2.12Laser Spin Atomization
Laser spin atomization is conceptually a modification of REP. It was developed by Konitzer et al.[193] in the mid 1980’s. In this process, a high power CO2 laser beam is used to melt the top of a consumable metal/alloy ingot of ~10 mm in diameter while the consumable is spun at speeds up to 3650 radians/s by a high speed motor. Droplets are generated by spinning off the molten metal from the top of the consumable under the action of centrifugal force. Helium gas jets are introduced vertically to the streams of the droplets, giving rise to rapid quenching and solidification of the droplets. The evacuation of the unit prior to atomization minimizes the contamination of droplets.
The Laser-spin-atomized droplets are usually spherical, clean, and homogeneous in composition. A mass median diameter of ~100 µm has been obtained for a Ni-Al-Mo alloy. Cooling rates are estimated to be in the order of magnitude of 105 °C/s. Similarly to other centrifugal atomization techniques, droplet properties (shape, size, cooling rate, etc.) are dependent on the rotation speed, ingot diameter, superheat, and material properties.
2.2.13 Durarc® Process
Durarc® process is a centrifugal atomization process. In this process, the Durarc® electrode (a cylindrical metal/alloy of ~78 mm in diameter) is connected to a cylindrical field coil at its tip, and surrounded by several concentric cylinders which provide incoming and outgoing cooling water passages. The field coil is encapsulated within a removable, water-cooled copper tip. A magnetic field is generated by the coil with a large flux component parallel to the surface. When an arc is struck to the metal charge, held as a skull melt in the water-cooled copper crucible, the arc direction is to the melt. With the flux field parallel to the arc tip, it is normal to the arc current. Magnetic field interaction with the current causes the latter to rotate over the arc tip. With the liquid metal pool present, the arc is rotated rapidly. Electromagnetic forces also exist in the melt at the
Droplet Generation 111
point of arc attachment and lead to the expulsion of the liquid metal from the melt pool and eventually disintegration of the liquid metal into droplets. The droplets solidify and are collected subsequently.
Durarc® process was developed for atomizing titanium alloys at the Research and Development Center of the Westinghouse Electric Corporation in the early 1970’s.[194] The process utilizes the non-consumable arc melting technology for titanium alloys and involves melting of a material in a water-cooled copper skull crucible. The operation is carried out in a large chamber (2.6 m in height and 1.85 m in diameter in a pilot-scale facility), either under vacuum or in inert gas atmosphere (up to 0.92 MPa). This feature, along with the skull melting step, makes the Durarc® process attractive for reactive or high melting-temperature metals such as Ti, Zr, and others. In addition, the starting material can be scrap or contaminated feed stock and there is no limitation on its geometry. For Ti-6Al-2V- 2Sn alloy, the atomized particles are spherical and relatively coarse (³ 250 µm).
In summary, a variety of configurations of centrifugal atomization techniques have been examined for the atomization of metals/ alloys and are undergoing further development. In centrifugal atomization techniques, the spinner can be a dish or a cup, a crucible or a flat disk, or some variations of these. Centrifugal forces can also be generated by rotating a consumable metal ingot. Generally, energy requirements of centrifugal processes are relatively low. Droplet shape can vary from spherical, flattened to elongated, and droplet size generally exhibits a narrow distribution with a standard deviation of 1.3–1.5. Some problems associated with centrifugal atomization include the erosion and/or dissolution of the spinner-atomizer device, and the creep or deformation of the spinner during the atomization of high melting-temperature metals/alloys. In an overflow spinning arrangement, control of the lip is critical for consistent product definition. The rim speed and diameter substantially determine the droplet size distribution. In addition, control over the atmosphere is also of importance. Since centrifugal atomization does not require an atomizing fluid, spray chamber could be in
112 Science and Engineering of Droplets
vacuum. However, the cooling rate in vacuum is low. Therefore, a fluid or metallic impact substrate is provided for higher cooling rates at increased cost to the atomization system.
2.2.14 Vibrating Electrode Atomization
Vibrating electrode atomization is a process similar to the rotating electrode atomization. The difference between them is that in the former an electrode vibrates while in the latter an electrode rotates during atomization. In the vibrating electrode process (VEP),[195] a wire (consumable electrode) is continuously fed by rollers into an atomization chamber. The rollers are attached to an electrodynamic oscillator so that vibrations are transferred to the wire electrode. In the chamber, a water-cooled rotating electrode made of copper or graphite is placed opposite of the end of the wire electrode. When an arc is struck between the rotating electrode and the end of the vibrating wire electrode, the end is molten and the liquid metal is ejected away, leading to atomization.
In the VEP, currents used are between 600 and 1200 A at potentials between 30 and 60 V. The vibration frequency of the wire electrode is up to 500 Hz. The materials atomized via VEP include mild steel, Cr-Ni steel, Cu-Ni alloy and tungsten. The VEP is carried out in an inert atmosphere (typically argon) for most alloys, but the arc is struck under water for tungsten wire. Wire diameter is 1–4 mm, and its feed rate is 1.7–4.3 m/min. The feed rate and current density must be determined properly according to the relationship between these two variables. At lower current densities, the wire electrode tends to stick to the rotating electrode. At higher current densities, the wire electrode becomes overheated, causing it to bend or even rupture.
The VEP-atomized particles are spherical. The mass median diameter of the particles ranges typically from 300 to 500 µm. Both the mass median diameter and size range of the particles reduce with decreasing wire diameter for a given vibration frequency. The narrowest particle size distribution is produced at the resonant
Droplet Generation 113
frequency. The mass median diameter decreases slightly as the current density decreases, and the influence of the current density and vibration mode on the particle size distribution is little. Due to the simplicity and ease of use, the VEP is an attractive candidate for atomizing small quantities of high-purity, refractory metal powders of spherical shape.
2.2.15 Ultrasonic Atomization
Ultrasonic atomization is sometimes also termed capillarywave atomization. In its most common form,[142] a thin film of a molten metal is atomized by the vibrations of the surface on which it flows. Standing waves are induced in the thin film by an oscillator that vibrates vertically to the film surface at ultrasonic frequencies. The liquid metal film is broken up at the antinodes along the surface into fine droplets once the amplitude of the capillary wave exceeds a certain value. The most-frequent diameter of the droplets generated is approximately one fourth of the wavelength of the capillary wave,[142] and thus decreases with increasing frequency.
The Ultrasonic-atomized droplets are spherical and of high quality if the atomization is conducted in an inert atmosphere. Droplet size is typically 30–50 µm, but finer droplets (10–20 µm) have also been produced with this method. Frequencies of up to 150 kHz have been used. The dimension of the chamber for ultrasonic atomization can be small (1–2 m in height and 1.5 m in diameter) due to the low droplet ejection velocities. During atomization, the vibrating surface needs to be heated to the melting temperature of the metal being atomized in order to avoid solidification of the melt on the surface. Thermal damage to the ultrasonic transducer and materials selection for the vibrating surface become severe problems as temperature rises to above a few hundred degrees. Thus, the process is only applicable to metals and alloys of low melting temperatures (up to about 1000 °C), such as solder materials,[144] welding electrodes, silver and copper-base alloys for electrical contacts, dispersion hardened alloys, and powder materials needed in the chemical and
114 Science and Engineering of Droplets
pharmaceutical industries. As a result of the high energy efficiency, the energy requirement is less than 100 J/kg of atomized particles. However, because of the interrelationship among scale, frequency and droplet size, the production rate is very low, typically of the order of 0.02–0.8 kg/min for droplet sizes less than 50 µm. This method is therefore not yet established as a near-commercial atomization process for powder production. Nevertheless, scale-up using arrays of vibrators or vibrating ribbons or other shapes is technically feasible.
Recently, a variant of the ultrasonic atomization process has been developed and demonstrated by Bauckhage et al.[143] at University of Bremen, Germany. This process, termed ultrasonic standing wave atomization (USWA), is in concept different from the earlier one. As schematically illustrated in Fig. 2.24, a molten metal stream (~2 mm in diameter) falls between two ultrasonic transmitters (sonotrodes) or one transmitter and one reflector which are positioned and tuned to set up an intense standing wave field. When the transmitters are positioned 1.5 sound-wavelengths apart, the liquid metal stream falling through a central, low sound intensity zone is peeled off at its edges, forming a flat spray. Atomization is conducted in a chamber filled with inert gas at up to ~1.6 MPa pressure. The mass median diameter of droplets may range from ~10 to ~300 µm, depending on the amplitude of the sonotrodes and the gas pressure. Increasing the amplitude of the sonotrodes and/or the gas pressure can reduce the mass median diameter of droplets. The cooling rates of droplets are remarkably high, approximately in the order of magnitude of 106 K/s, as estimated from the dendrite spacing measurements. Since the relative gas-droplet velocity is very low (<0.5 m/s), it is hypothesized that the agitating effect of the ultrasound leads to the rapid cooling by enhancing convective heat transfer. The asatomized particles possess excellent sphericity and are free of contamination. Some challenging technical issues associated with the USWA process include, for example, how to keep the system resonating for very large temperature changes in the gas, and how to
Droplet Generation 115
Figure 2.24. Ultrasonic standing wave atomization of melt. Top: schematic.
Bottom: photograph. (Courtesy of Prof. Dr-Ing. Klaus Bauckhage at University of Bremen, Germany.)