The Research Centre for Metallic Glasses at the Institute of Materials Research, Tohoku University, in conjunction with the Department of Mechanical Engineering at Iwate University and Hard Industry Ltd which is based in Hachinohe, Aomori Pref., Japan, have announced the development of new atomisation technology aimed at the volume production of fine iron-based amorphous alloy powders having particle size in the single micron range. The new amorphous powders are expected to find applications in soft magnetic materials, thermal sprayed coatings, and because of their fine size and spherical particle shape, also powder injection moulding and powder metallurgy applications.
Hard Industry Ltd, which produces equipment and materials for hardfacing and thermal spraying as well as a range of machine tools, stated that the new atomisation technology, for which a patent has been applied, involves the use of a high-velocity combustion flame with kerosene combining with air to create the combustion flame. This compares with conventional gas and water atomisation processes with the former using high-pressure gas as the atomising medium, and the latter using high-pressure water. Each is said to have its drawbacks in the production of ultra-fine, spherical shaped metal powders such as safety regulations in the use of high-pressure gas (e.g. argon), and water atomisation requiring expensive high pressure pumps. In contrast, the high-velocity combustion flame atomisation process overcomes these safety requirements, and is said to require only equipment to produce the high-speed flame, thereby cutting equipment and operation costs compared with conventional atomisation processes. The combustion flame is said to have a speed of 1600 m/s and temperature of 1600°C.
The developers of the new technology state that in the first stage of the simplified atomisation mechanism, molten metal is spun at high relative velocity. In the second stage, the surface tension divides the spinning molten metal into small particles. It is, therefore, essential that both the surface tension and viscosity of the molten metal be maintained at a high temperature in order to produce the fine powder, and this is achieved by using the high-velocity combustion flame. This compares with conventional atomisation processes where conventional gas and water atomisation methods use lower-temperature media, which causes the atomisation temperature to decrease during atomisation and this suppresses the atomisation mechanism for fine powders. If the temperature of the feeding molten alloy should be increased excessively in conventional atomisation then the lifetime of the crucible and nozzle would be considerably reduced.
Hard Industry states that the newly developed atomisation equipment can be categorised into two groups, which use multiple high-velocity combustion flame burners and ring-slit-shaped high-velocity combustion flame burners, respectively. The entire process is referred to as the counter-flame jet atomisation (CFJA) method. A schematic diagram of the CFJA method using four high-velocity combustion flame burners is shown in Fig. 1. The four L-shaped burners create a cross-shaped intersection of the combustion flame by setting the four flames at a vertex angle of about 50°. Fig. 2(a) shows the combined flame after the four flames intersect. Furthermore, the combined flames at the intersection exhibit a feature called ‘shock diamonds’, which are usually observed in high-velocity flames with velocities greater than that of sonic waves. The combustion conditions of the four individual flames and the spinning speed are automatically controlled by computer. Fig. 2(b) shows the atomisation process using the four counter-crossed flames shown in Fig. 2(a). The colour of the molten alloy darkens in the area outside of the combined combustion flames.
Adding a developed specific rapid cooling system to the new atomisation process allowed the atomised molten alloy powder to be quenched in a dry state in the CFJA process, to produce an amorphous Fe73.2Cr2.2Si11.1B10.8C2.7 (Fe-2.5Cr-6.7Si-2.5B-0.7C in wt.%) alloy powder. The phases were characterised by X ray diffraction, as shown in Fig. 3. The spectra exhibit only a broad halo pattern of amorphous structure without any distinct Bragg peaks.
A scanning electron microscopy image of the amorphous atomised iron-based powder is shown in Fig. 4. Because it is difficult to classify such fine powder particle sizes, a wide dispersion in powder size can be seen. However, the smallest particle sizes are identified as being in the single micron range. The atomised powders are spherical in shape, have good flowability with good densification expected for sintered powder injection moulded and powder metallurgy processed components.