Contactless Ultrasonic Cavitation in Alloy Melts

Abstract

A high frequency tuned electromagnetic induction coil is used to induce ultrasonic pressure waves leading to cavitation in alloy melts. This presents an alternative ‘contactless’ approach to conventional immersed probe techniques. The method can potentially offer the same benefits of traditional ultrasonic treatment (UST) such as degassing, microstructure refinement and dispersion of particles, but avoids melt contamination due to probe erosion prevalent in immersed sonotrodes, and it can be used on higher temperature and reactive alloys. An added benefit is that the induction stirring produced by the coil, enables a larger melt treatment volume. Model simulations of the process are conducted using purpose-built software, coupling flow, heat transfer, sound and electromagnetic fields. Modelling results are compared against experiments carried out in a prototype installation. Results indicate strong melt stirring and evidence of cavitation accompanying acoustic resonance. Up to 63% of grain refinement was obtained in commercial purity (CP-Al) aluminium and a further 46% in CP-Al with added Al–5Ti–1B grain refiner.

Contactless Ultrasonic Treatment in Direct Chill Casting

“…The model predicts an intermittent pulsed resonance, similar to the behavior experimentally observed in the authors' previous work. 20 Figure 10 shows the dominant resonant mode obtained at 31 kHz, which compares favorably with the resonant model results in Fig. 9.…”

“…The primary disadvantage of an immersed sonotrode is that limited volumes can be treated, and consequently multiple sonotrodes were required for large castings, as shown by Eskin and Eskin. 1 The use of an electromagnetic coil as a contactless source of ultrasound has been demonstrated to work in aluminum crucibles 19,20 containing up to 10.5 kg of metal. This process, using a high-frequency induction coil suspended close to the liquid-free surface 21 resulted in grain refinement and degassing after 2-4 min of treatment.…”

Multiphysics Modelling of Ultrasonic Melt Treatment in the Hot-Top and Launder during Direct-Chill Casting: Path to Indirect Microstructure Simulation

“…In the casting of aluminium alloys, the improvement of microstructure by the reduction of grain size is important due to multiple benefits, ranging from improved mechanical properties, to increased resistance to hot tearing [1]. A number of methods have been tested, including the addition of a grain refiner [2], the application of an ultrasonic field through the use of a mechanical sonotrode [3], an alternating current (AC) induction coil [4], or the combination of an AC induction coil with a background DC field [5]. The efficiency of ultrasonic melt treatment (UST) is attributed to the onset of the formation and explosive collapse of bubbles in the melt (cavitation), due to large pressure oscillations [6].…”

The Influencing Factor of MgAl2O4 on Heterogeneous Nucleation and Grain Refinement in Al Alloy Melts

“…Unfortunately, endogenous MgAl 2 O 4 oxides that naturally form in the Al-Mg alloys always adsorb onto the surface of the alloy melt in the form of liquid-like films or clusters consisting of numerous individual micron-sized particles, which make it difficult to initiate heterogeneous nucleation [1]. As a result, physical fields and mechanical forces (including mechanical stirring using impeller-intensive shearing in a special unit with a 500-rpm screw rotation speed) and ultrasonic cavitation have been introduced into the alloy melt in order to break films or clusters into fine pieces, which can improve the heterogeneous nucleation potency of MgAl 2 O 4 [8,9]. Li et al [10] investigated the heterogeneous nucleation effect of MgAl 2 O 4 particles in Al-Mg alloys with different Mg contents through intensive melt-shearing treatment and provided MgAl 2 O 4 , which acted as a potent site for the nucleation of α-Al grains from their cube-on-cube orientation relationship (OR).…”