Correlation vs. Causation: The Effects of Ultrasonic Melt Treatment on Cast Metal Grain Size

J Ferguson,Benjamin Schultz,Pradeep Rohatgi,Kyu Cho

doi:10.3390/met4040477

J Ferguson, Benjamin Schultz + Show 2 more

Open Access

https://doi.org/10.3390/met4040477

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Abstract

Interest in ultrasonic treatment of liquid metal has waxed and waned for nearly 80 years. A review of several experiments representative of ultrasonic cavitation treatment of Al and Mg alloys shows that the theoretical mechanisms thought to be responsible for grain refinement are (1) cavitation-induced increase in melting temperature predicted by the Clausius-Clapeyron equation and (2) cavitation-induced wetting of otherwise unwetted insoluble particles. Neither of these theoretical mechanisms can be directly confirmed by experiment, and though they remain speculative, the available literature generally assumes that one or the other or both mechanisms are active. However, grain size is known to depend on temperature of the liquid, temperature of the mold, and cooling rate of the entire system. From the reviewed experiments, it is difficult to isolate temperature and cooling rate effects on grain size from the theoretical effects. Ultrasonic treatments of Al-A356 were carried out to isolate such effects, and though it was found that ultrasound produced significant grain refinement, the treatments also significantly chilled the liquid and thereby reduced the pouring temperature. The grain sizes attained closely correlated with pouring temperature suggesting that ultrasonic grain refinement is predominantly a result of heat removal by the horn and ultrasonic stirring.

Highlights

There are several known methods, by which one can reduce the grain size of a casting, including: addition of inoculants/grain refining agents, increasing solidification rate, and by controlling the pouring temperature
The theoretical mechanisms generally proposed to account for ultrasonic grain refinement in Al and
Mg alloys are (1) a cavitation-induced rise in melting temperature predicted by the Clausius-Clapeyron equation and (2) cavitation-induced wetting of otherwise unwetted insoluble particles

Summary

Introduction

There are several known methods, by which one can reduce the grain size of a casting, including: addition of inoculants/grain refining agents, increasing solidification rate, and by controlling the pouring temperature. Different cooling rates were achieved by treating the specimens with ultrasound (amplitude of 40 μm, frequency, intensity, horn dimensions and depth were not reported) in a certain temperature range, pouring the melt in either a graphite mold (0.8 K/s), copper mold (2.0 K/s), through a launder to a steel mold (2.0 K/s), or by direct chill (DC) casting (6.0 K/s). While the authors did insert the water-cooled horn in the melt without applying ultrasound for the same period of time, the resulting temperature of the melt is likely not as homogenous as in the sonicated melts, where solids forming on the probe will be knocked off when the waveguide is removed and mixed into the surrounding liquid. Experiments designed to isolate thermal effects from cavitation-induced effects are described to help clarify the effects of UST on grain size in aluminum alloy A356

Results and Discussion

Experimental Section

Conclusions