To date, the possibility of obtaining, using traditional technology, new aluminium alloys with substantially higher technological and service properties than those of known alloys has been actually exhausted.
A fundamentally new approach to the formation of the structure and properties of alloys is the implementation of synthesis process in matrix alloy microvolumes. In conventional technology, the extraction of intermetallic phases is a consequence of supersaturation of the component solution with a decrease in temperature with respect to equilibrium solubility in the cooling process before complete solidification - the greater the component content relative to the equilibrium solubility, the larger the particle size of the intermetallide phases. It is this circumstance that is a factor limiting the maximum permissible content of poorly soluble components in alloys.
In the proposed technology, poorly soluble components are introduced into the matrix melt in the form of particles of pure metals heated up to a certain temperature or intermetallic nonequilibrium or metastable phases relative to the temperature of the matrix melt, while its temperature at the minimum possible level is maintained by cooling during magnetohydrodynamic mixing. One of the effective ways of heating and introducing particles with a size of not more than 50 microns into the melt is the use of a plasma torch of a special design.
Argon is used as plasma-forming gas, temperature of the plasma jet at the outlet of the nozzle is about 100,000 C, the jet rate is close to the sound speed. Powder of the injected component is fed from the dispenser to the zone to the arc, passes through the active zone of argon ionisation, while the particles are heated to about 15,000 °C, the surface of the particles is ion-etched, purified from adsorbed oxygen, the particles are accelerated to the rate of the jet exit from the nozzle, thus acquiring kinetic energy sufficient to overcome the surface tension of the melt when the particles enter the melt. The matrix melt with a temperature close to the solidus is mixed near the solidification front in two directions by the MHD inducer, which ensures a uniform distribution of the material components in volume and, which is very important for some eutectic-type matrix alloys (356, 357), a non-dendritic thixotropic structure. Particle hardeners of nanometric dimensions, preliminarily embedded in carrier particles of plastic metals, components of the matrix alloy (copper, nickel, etc.) can be introduced into the matrix melts in a plasma jet through the joint treatment of the mixture of nanoparticles with carrier particles in ball mills.
An important feature of the technology is the ability to "design" materials with the desired properties by introducing a set of additives necessary for this and forming the required structure of the material during synthesis.
The processes of plasma injection of components into a matrix melt, melt MHD mixing are well combined in casting processes, including continuous casting of ingots from aluminium-based alloys and composites.
Source: Light Metal Age, March/April 2017 • Volume 75, No 2, p 48 – 51.