Grain refinement mechanisms during ultrasonic solidification of metals

Abstract

Solidification processes during casting that promote equiaxed, fine and non-dendritic grains tend to combat the propensity for porosity formation and hot tearing by introducing enhanced feeding characteristics, and avoiding chemical segregation. These equiaxed, fine grain size structured castings are often associated with improvements in the soundness of the cast components resulting in better mechanical properties and enhanced metal formability. The conventional method of adding a grain refining master alloy works effectively for certain alloys, however, the refinement efficiency depends on alloy composition (usually < 2% of the added particles are active). Application of external fields produces grain refinement regardless of the alloy composition and without the need for heterogeneous particles. Among various external fields, ultrasonic treatment (UST) has been found to produce excellent grain refinement by cavitation (formation, growth and collapse of bubbles) and acoustic streaming (fluid flow) effects.Although UST promotes refinement through cavitation and streaming effects, the dominant grain refinement mechanisms are still debated between nucleation and/or fragmentation of dendrites. Since the alloy composition and casting conditions could greatly influence the grain refinement mechanisms, this thesis seeks to generate a comprehensive understanding by a systematic study of (i) pure metals, (ii) eutectic and (iii) peritectic alloys during UST. The effects of temperature range, solute, potent and impotent particles have been investigated for Mg and Zn based alloys. In addition, this work also investigates the formation of equiaxed grains generated by cold surfaces of the casting (mould wall, melt surface and cold sonotrode) to establish an overall understanding on the origin of equiaxed grains.In pure metals (Mg and Zn), UST has been applied from different superheat temperature (refers to the starting temperature of UST above the melting temperature) and time duration until complete solidification. A low superheat (40°C for Mg and 20°C for Zn) produces excellent refinement consisting of fine, non-dendritic and uniform grains. The equiaxed grain area in the macrostructure increases as a function of UST time. Terminating UST earlier before complete solidification or a high superheat temperature (100°C for Mg and 30°C for Zn) causes remelting of the fine grains or coarsening through grain growth because of the re-establishment of steep temperature gradients.Though UST of pure metals produces refinement, the exact role of nucleant particles or the mechanism of grain formation is unknown. Therefore, UST has been investigated in a well-known Mg-Zr alloy system that contains potent Zr particles and solute Zr. UST was applied at two temperature ranges, UST-S and UST-L referring to UST applied during solidification and terminated above the liquidus temperature respectively. In general, UST-L and UST-S increase the efficiency of the grain refinement process by reducing the settling, narrowing the size distribution of Zr particles and increasing the number of active particles. Compared to UST-L, UST-S produces excellent refinement at minor Zr concentrations indicating that more particles are activated through flat temperature gradients imposed by acoustic streaming. The Interdependence Nucleation Model has been used to explain the UST-S refinement by incorporating a temperature gradient variable into the model. To clarify the observations regarding potent particles and solute, an Al-5 wt.% Be master alloy was also added to Mg-1 wt.% Zr alloy. Be prevents the oxidation of Mg during melting, but grain coarsening occurs due to nucleant poisoning. This investigation has shown that a drastic reduction in the amount of solute Zr content and the formation of Zr-Al intermetallic particles is responsible for the grain coarsening. Interestingly, UST-L and UST-S results in refinement, however, the efficiency is lowered by 10 to 14% compared to an unaffected Mg-1wt.% Zr alloy. This further confirms that potent particles play a significant role in achieving the best refinement during UST. The role of solute without potent particles is investigated in Mg-(0.5, 3.0 and 6.0) Zn alloys. Unlike pure metals, UST in eutectic alloys is unaffected by superheat temperature. The refinement tendency increases with incremental additions of solute.A comparison study has been performed investigating the effects of solute, potent particles and the origin of fine grains in pure metals (Mg and Zn) and eutectic (Mg-Zn) and peritectic (Mg-Zr alloys) systems. Experiments were undertaken using an electric current pulse solidification (ECP) system and melt stirring in order to study the origin of equiaxed grains from the cold surfaces during solidification. Through a systematic analysis of the fundamental mechanisms associated with nucleation, fragmentation of dendrites and wall crystals, a grain refinement mechanism has been proposed for UST solidification. A relatively cold sonotrode (~200°C) provides significant thermal undercooling causing local solidification of the metal layer at the sonotrode-melt interface, which was then separated by the instabilities created at the sonotrode surface accompanied with or without the occurrence of cavitation. Overall, this work provides an improved understanding of the mechanisms of UST and other external treatments in order to achieve the best refinement.