The effect of ultrasonic treatment on the mechanisms of grain formation of as-cast high purity zinc

The potential for producing a large refined equiaxed zone by ultrasonic treatment (UST) of high purity zinc was investigated in order to improve the mechanical performance and formability. The macrostructure of cast ingots changed from large columnar grains without UST to three zones of fine columnar grains adjacent to the mould walls of the ingot, a refined equiaxed zone and a zone of a mixture of coarse equiaxed and columnar grains. A small zone of equiaxed grains was obtained when UST was applied during cooling from 440 °C to 419 °C for 2 min. The size of the equiaxed zone increased from about 20% of the casting's cross section to 50% when UST was applied for 3 or 4 min. In contrast, the application of UST for a longer time from a higher temperature (450–419 °C for 4 min) resulted in a smaller equiaxed zone of 18% indicating that a specific combination of UST time and temperature is required for the formation of a large equiaxed zone. The factors affecting the formation of the equiaxed zone throughout the solidification cycle are described

An analytical approach to elucidate the mechanism of grain refinement in calcium added Mg–Al alloys

The present study investigates the grain refinement of Mg-3Al, Mg-6Al and Mg-9Al alloys by calcium addition. The maximum reduction in grain size has been observed at 0.2% Ca addition in Mg-Al alloys, in which any further addition (up to 0.4%) has marginal improvement in grain refinement. The mechanism associated with the grain refinement of Mg-Al alloys by Ca addition is discussed in terms of growth restriction factor (Q) and constitutional undercooling (ΔT) using analytical model. The influence of growth restriction factor (Q) on the final grain size of Ca-added Mg-Al alloys are calculated with the help analytical model by assuming that the number of nucleant particles is not altered through Ca addition. For accurate grain size calculations, the value of Q has been estimated with reliable thermodynamic database using Scheil solidification simulation. The comparison of predicted and experimental grain size results indicate that constitutional undercooling activation of nucleation events plays dominant role in grain refinement in Mg-Al alloys by calcium addition, whereas the increase in growth restriction value has negligible effect

Discussions on the mechanism of grain refinement in ultrasonic treated Ti containing Mg–Al alloys

The present study investigates the role of titanium in grain refinement of Mg–3Al and Mg–9Al alloys. Minor amount of Ti addition

Ultrasonic-assisted synthesis of graphite-reinforced Al matrix nanocomposites

A novel approach to produce Al-2 vol.% graphite nanocomposites using micron-sized graphite particles has been reported using conventional stir casting technique combined with ultrasonic treatment. Microstructural observations indicate that the visible agglomerations and porosities are significantly reduced after ultrasonic treatment. Transmission electron microscopy studies of ultrasonic-treated composites reveal that the size of the graphite particles is reduced substantially and its morphology is transformed into flake type structures. The width of the graphite flakes is reduced markedly with the increase in ultrasonic processing time and it is found to be in the range of 100-120\ua0nm with an aspect ratio of 8.83 after 5\ua0min of ultrasonication. Added to that, considerable improvement in the hardness values are noted for ultrasonic-treated Al-2 vol.% graphite composites when compared to conventional untreated composites. The mechanism behind the significant reduction in graphite particle size and porosity, uniform distribution of graphite particles and hardness increments are discussed

Ultrasonic assisted grain refinement of Al–Mg alloy using in-situ MgAl2O4 particles

MgAl2O4 particles were in-situ synthesised in Al–Mg melts using SiO2 precursor assisted by ultrasonic treatment (UT). 7 to 8 Fold reduction in grain size is achieved in Al–1Mg–0.1SiO2 alloy subjected to 5 min UT. Reason behind the grain reduction is explained in terms of ultrasonic cavitation activated SiO2 reduction reaction, thereby increasing the number density of MgAl2O4 particles and their wettability

Effect of ultrasonic treatment on the alloying and grain refinement efficiency of a Mg – Zr master alloy added to magnesium at hypo- and hyper-peritectic compositions

Magnesium – Zirconium master alloys are the dominant grain refiner for a range of commercial magnesium alloys that exhibit good elevated temperature properties. However, the grain refining efficiency achieved by the master alloy is relatively low because Zr particles quickly settle to form sludge. In this study ultrasonic treatment (UST) is applied to investigate whether the grain refinement efficiency can be improved by increasing the Zr solute content, producing a more uniform dispersion of Zr particles and minimizing sludge formation. It was determined that, for a range of Zr contents from 0.2 to 1.0 wt% Zr, UST of the melt above the liquidus temperature decreased the grain size due to an increase in the amount of solute Zr and the number of Zr particles. For Zr contents less than 0.5 wt% the grain size decreased further when UST was applied from above to below the liquidus temperature to include the onset of nucleation of α – Mg grains. Additionally, the application of UST substantially decreased the amount of sludge which increased the number density of Zr particles and improved their likelihood of successful nucleation. The Interdependence nucleation model was successfully applied to predict the grain size of each casting condition where the parameter z relates to the temperature gradient which in turn is affected by the acoustically-induced convection. A lower value of z when UST is applied facilitates nucleation and the survival of grains. The factors affecting grain refinement under each condition are evaluated in terms of solute and particle Zr contents, cavitation and acoustic streaming and their effect on nucleation and as-cast grain size

Mechanisms of grain formation during ultrasonic solidification of commercial purity magnesium

Ultrasonic treatment (UST) during the solidification of commercial purity Mg was investigated over different time-temperature ranges: (i) UST in the liquid region only (ii) during the transition from liquid to solid including the onset of nucleation and (iii) UST initiated before the end of solidification but well after the onset of nucleation of α-Mg for the formation of equiaxed grains. If UST is terminated at the end of the liquid stage or above 650 °C there is a negligible effect on the final grain size. The solidification treatment (case ii) produces incremental grain refinement when UST is applied from 690 to 650 °C for 1 or 2 min, however, from 750 to 650 °C for 3 or 4 min shows only coarse, non-equiaxed grains. Introducing UST well after the onset of solidification produces a region with the finest grains of all treatment ranges investigated. The degree of grain refinement was found to be temperature dependent where low temperature initiation resulted in a higher number density of grains. The mechanism of grain formation during UST is discussed with respect to the processing parameters and thermal conditions driving the formation of equiaxed grains

Ultrasonic assisted grain refinement of Al–Mg alloy using in-situ MgAl2O4 particles

MgAlO particles were in-situ synthesised in Al-Mg melts using SiO precursor assisted by ultrasonic treatment (UT). 7 to 8 Fold reduction in grain size is achieved in Al-1Mg-0.1SiO alloy subjected to 5 min UT. Reason behind the grain reduction is explained in terms of ultrasonic cavitation activated SiO reduction reaction, thereby increasing the number density of MgAlO particles and their wettability