102 Science and Engineering of Droplets
silicide have been arc melted prior to atomization. The original RSR system had a melt capacity of 45 kg. It is increased to an intermediate size with 135 kg melt capacity and a spray chamber of about 2.3 m in diameter. The largest RSR facilities can handle batches of up to 900 kg with a spray chamber of about 5 m in diameter and a closed-loop helium recirculation system. The production rate varies from 3 kg/min to 18 kg/min for Ni-base superalloys.
In addition to mass flow rate and melt superheat, disk rotation speed is also one of the important process parameters in RSR. For the atomization of Ni-base superalloys, the process parameters typically take the following values: melt superheat 100 °C, melt flow rate 12 kg/min, helium gas velocity 170 m/s, helium gas flow rate 60 kg/min, helium back fill pressure 33 kPa, disk rotating speed 2500 radians/s.[4] Under these operation conditions, the droplet diameter is typically smaller than 200 µm and the mass median diameter can be as small as 60 µm, corresponding to a cooling rate of about 105–107 °C/s. The RSR-atomized droplets are normally spherical and have smooth surfaces without satellites. The ejection velocities of the droplets may range from 40 to 110 m/s, depending on the diameter of the rotating disk. As the centrifugal forces at the periphery of the rotating disk increase, the size of the ejected droplets decreases and correspondingly the cooling rates of the droplets are enhanced. The cooling rates are also significantly influenced by the melt superheat since it largely determines the droplet size. The melt superheat, in turn, may be changed either in the melting stage or by varying the heat flow to the rotating disk.
2.2.7Electron Beam Rotating Disk Atomization
Electron beam rotating disk (EBRD) process[187] is a twostep centrifugal atomization process. A consumable metal ingot is melted at its bottom tip by an electron beam in high vacuum environment with some attendant refining effects. The molten metal drips onto a rapidly rotating container, either a disk or a cup, below the ingot. The liquid is ejected from the rim of the container under the action of centrifugal force and breaks up in flight into droplets. The
Droplet Generation 103
ingot is either stationary or rotating at a low speed to develop conical tip geometry. Hence, the dimensional precision of consumables is not as critical as in REP, and rough and inhomogeneous ingots may be used as consumables. The droplets ejected off the rim are deflected downwards by a water-cooled copper shield. Thus, atomization chamber can be small, and cost and space requirements can be relatively low.
The details of the development of the EBRD process have been described by Pietsch et al.[187] There are two alternative operation modes in addition to the above continuous non-contact mode. The first one may be referred to as continuous contact atomization. In this mode, liquid metal contacts the bottom surface of the container instead of melt dripping, and then flows continuously from the center to the rim of the container. The second one may be termed discontinuous non-contact atomization. In this mode, the container is first filled by dripping melt while it is rotating at a very low speed of about 3 × 10-3 radians/s. The rotating speed of the container is then enhanced to about 14 radians/s while the metal or alloy is remelted and atomized. More than one focused electron beam may be used to provide energy for melting metal.
The EBRD process has been demonstrated applicable to the atomization of a variety of reactive and refractory metals and alloys in addition to Ti alloys. The process is being further developed to utilize highly contaminated raw materials and to produce high quality powders for hot isostatic pressing application without additional treatment.
As described in detail by Pietsch et al.,[187] in the largest production plant planned, the atomization chamber will have a diameter of 16 m which can accommodate consumables of 6 m in length and 0.165 m in diameter, corresponding to ~0.5 ton for Ti. The production rate may reach 5.5 kg/min at a rotating speed of 300 radians/s and <5 radians/s for the cup and ingot, respectively. Energy efficiency is relatively high, roughly 90%, and the energy requirement is about 2.5 MJ/kg of atomized particles.[4]
Droplet Generation 105
the rolls to meet the heat transfer requirements for temperatures below about 300 °C, and epoxy-alumina powder mixture bonded to the surface of the rolls for higher temperatures. As an alternative, fluid barrier films such as light oils and releasing agents have been applied continuously to the roll surface to provide an effective thermal insulating layer between the liquid metal and rolls. Thus, the roller quenching effects required in producing rapidly solidified ribbons and splats can not be achieved in the roller atomization process.
Depending on operation conditions and metal properties, the shapes of the atomized particles may be spheroidal, flaky, acicular, or irregular, but spherical shape is predominant. The spheroidal particles are coarse. For example, roller-atomized Sn particles exhibited a mass median diameter of 220 to 680 µm. The large particle sizes and highly irregular particle shapes suggested that the disintegration process may be arrested either by the premature solidification or by the formation of a thick, viscous oxide layer on the liquid surface. The particle size distributions were found to closely follow a log-normal pattern even for non-uniform particle shapes.
Process parameters of primary importance include roll speed, differential roll speed, roll gap, metal flow rate, metal stream velocity, and melt superheat. The mass median diameter of particles diminishes exponentially as the roll speed increases. It is possible to obtain a smaller mass median diameter when one of the rolls is kept stationary rather than rotating the two rolls at the same speed. Metal flow rate seems to have a negligible effect on the mass median diameter. However, the mass median diameter increases with increasing metal stream velocity, suggesting that the relative velocity of the metal stream to the periphery of the rolls may be a fundamental variable controlling the mass median diameter. The size distribution is approximately constant for the conditions studied.
The roller atomization process has been applied to the atomization of many metals and alloys, such as lead, tin, aluminum, copper and steel. The production rate is potentially high, and the energy requirement is much lower than in commercial gas and water