The role of ultrasonic treatment in refining the as-cast grain structure during the solidification of an Al–2Cu alloy

Abstract

Abstract. The effect of Ultrasonic Treatment (UT) over selected temperature ranges during cooling and solidification of an Al-2Cu alloy melt on the grain structure and cooling behaviour of the alloy has been investigated using a molybdenum sonotrode introduced without preheating. UT was applied over various temperature ranges before, during and after the nucleation of primary aluminium grains. It was found that ultrasonic grain refinement was achieved only when UT was applied from more than 20 o C above the liquidus temperature until below the liquidus temperature after nucleation has occurred. Applying UT from 40 o C or 60 o C above the liquidus to just above the liquidus brings the melt to a condition that favours nucleation, survival of the nucleated grains and their subsequent transport throughout the melt. Continuing to apply UT beyond the liquidus for a short time enhances both nucleation and convection thereby ensuring the formation of a fine, uniform equiaxed grain size throughout the casting. The lack of grain refinement when UT was applied from 20 o C above the liquidus temperature or from temperatures below the liquidus temperature is attributed to the formation of a strong solidified layer on the sonotrode which hinders the effective transmission of ultrasonic irradiation into the liquid metal. The application of a preheated sonotrode showed that formation of a solid layer can be prevented by preheating the sonotrode to 285 o C. Thus, an appropriate amount of superheat of the liquid metal or sufficient preheating of the sonotrode is necessary for ultrasonic grain refinement when a sonotrode is introduced into the melt.

Potential of an Al-Ti-MgAl2O4 Master Alloy and Ultrasonic Cavitation in the Grain Refinement of a Cast Aluminum Alloy

“…The influence of ultrasonication on the grain refinement of alloys treated above the liquidus was studied extensively. [27] A systematic study by Wang et al [41] in a noninoculated alloy made clear that ultrasonication in a complete liquid state had a negligible effect on grain refinement, which was substantiated in the present study. Nevertheless, the additional grain refinement was observed when ultrasonication was applied to the liquid metal containing MgAl 2 O 4 or TiB 2 particles.…”

Identifying the Stages during Ultrasonic Processing that Reduce the Grain Size of Aluminum with Added Al3Ti1B Master Alloy

“…[16] From Figure 3 is increasing towards the bulk melt temperature [14] indicating that the application of UST at any temperature above the liquidus temperature may improve the efficiency of the master alloy.…”

“…[11] Other studies have shown that UST induces significant refinement when applied during cooling from just above to below the liquidus temperature. [13,14] This study will determine precisely when, during UST, nucleation is promoted and the relative contribution these mechanisms make to the final ascast grain size. Additions of an Al3Ti1B master alloy at concentrations of 50 or 200 ppm Ti were introduced into the molten melt at 740±3°C, which was then stirred and held for 5 minutes before being transferred to the UST platform (details reported in [14] ).…”

“…[13,14] This study will determine precisely when, during UST, nucleation is promoted and the relative contribution these mechanisms make to the final ascast grain size. Additions of an Al3Ti1B master alloy at concentrations of 50 or 200 ppm Ti were introduced into the molten melt at 740±3°C, which was then stirred and held for 5 minutes before being transferred to the UST platform (details reported in [14] ). When UST was not applied the alloy was allowed to cool and solidify in the crucible without the insertion of the sonotrode.…”

“…When UST was applied, the sonotrode was switched-on without preheating and then immersed 10 mm below the top surface of the melt in the graphite-clay crucible. Based on previous research, [14] the sonotrode was heated by the melt to near the melt temperature in about 20 s. One K-type thermocouple was inserted into the melt beside the sonotrode. The UST experiments were conducted with a fixed power input of 0.75 kW at a peak-to-peak amplitude of 20 μm applied during cooling from 720 °C, which is 60°C above the liquidus temperature of CP aluminium, and UST was stopped at various temperatures above and below the liquidus temperature.…”