Solidification of Aluminium Alloys Under Ultrasonication: An Overview
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An overview of our investigations on solidification microstructure formation under ultrasonication in various Al alloys and comparison against unrefined or chemically modified microstructures under identical cooling conditions is presented. Primary α-Al grains show significant refinement under ultrasonication, even better than established chemical inoculation, in the small ingots investigated. Increased solute content appears to promote grain refining efficiency under ultrasonication. Regular lamellar eutectic in Al–33 wt%Cu was observed to degenerate into rounded particle morphology and the irregular eutectic of long Si plates in Al–11 wt%Si were spheroidised into compact form near the ultrasound radiator. Grain refinement under ultrasonication appears to originate from enhanced heterogeneous nucleation under cavitation showing distinct reduction in nucleation undercooling. Eutectic modification, on the other hand, appears to originate from coarsening as the strong fluid flow created under cavitation disturbs the thin diffusion boundary layer ahead of the eutectic growth front.
KeywordsUltrasound Cavitation Solidification Grain refinement Microstructure Al alloys
Aluminium (Al) alloys are the second most used metallic structural material behind steel. However, Al alloys have certain advantages over steel where high specific strength, ductility and corrosion resistance are required . Among all lightweight structural material, Al alloys are most promising for automotive and aerospace application from the perspective of high volume manufacturing and lower cost.
Despite their good specific strength, the absolute strength of Al alloys is generally poor in as-cast condition and requires further strengthening. Cast microstructure of Al alloys also suffers from anisotropic columnar grain structure and uneven distribution of brittle eutectics. Microstructure control and refinement during solidification is necessary not only to improve performance, but also to improve further thermomechanical processability. Three different approaches to refining microstructure have been explored during solidification: (1) addition of chemical inoculants such as TiB2; (2) application of external physical fields such as ultrasonic or electromagnetic force; (3) controlling solidification parameters such as cooling rate and/or pouring temperature . Amongst all these methods, grain refining in wrought aluminium alloys through chemical inoculation has become the standard industrial practice due to its simplicity. Most commonly, Al–5Ti–1B master alloy (containing TiB2 particles) is added at a level of 1 g per kg of metal during casting of wrought Al alloys . However, this grain refiner is mostly ineffective in cast Al–Si alloys [4, 5, 6]. Eutectic Si modification is commonly achieved by adding Sr (up to 300 ppm)  or Na (up to 100 ppm)  promoting a transition from coarse flake to fine fibrous morphology leading to improved ductility. These eutectic modifiers, however, have been linked to increase porosity, hot tearing and poor surface quality in castings .
The main advantage of physically induced grain refinement over chemical means (inoculation or eutectic modification) comes from their universal applicability rather than being alloy specific. In the last 30 years, significant amount of research has been conducted exploring physical methods such as high-intensity shear [9, 10], low-frequency mechanical mould vibration , electromagnetic stirring  or ultrasonic irradiation  with varying degrees of success. Amongst them, application of ultrasound has shown most promising grain refinement potential for both cast and wrought Al alloys. Microstructure refining under ultrasound irradiation is still not well understood, especially the mechanism of grain refinement has been debated between dendrite fragmentation and enhanced nucleation. On the other hand, the effect on eutectic morphology has been debated between possible refinement and coarsening.
In this paper, we present experimental evidence on the microstructure modification potential of ultrasound in different Al alloys covering grain refinement, eutectic modification, effect of solute and comparison against established chemical refinement and suggest likely mechanism behind the microstructure modification.
2 Experimental Procedure
Five alloys were selected for the experiments; (1) commercially purity Al (CP–Al), (2) Al–5Cu (all compositions expressed in wt%), (3) Al–10Cu, (4) Al–33Cu and (5) Al–11Si (Al–10.8Si–0.3Fe–0.3Mn). The alloys were melted and homogenised in an electric resistance furnace and taken out in preheated crucibles for ultrasonication. Ultrasound (at 20 kHz) was transmitted to the solidifying melts from 750 °C for ~ 420 s till near the end of solidification (~ 545–565 °C melt temperature) through a 25 mm-diameter Ti–6Al–4V radiator introduced below the surface of the melt. The radiator was preheated (~ 400 °C) by ultrasonicating a batch of discarded melt first to avoid any chill effect. A thermocouple, connected to a multichannel data logger, was placed below the submerged radiator, and cooling curves were recorded during solidification. Identical experiments were conducted for comparison without ultrasonication for all alloys and with chemical inoculation (using Al–5Ti–1B) in CP–Al. Solidified ingots (height 65–70 mm, diameter 50 mm) were sectioned along the central vertical plane and ground and polished using standard metallographic techniques. Al–Si samples were unetched, while Al–Cu samples were anodized using Barker’s reagent (7 ml 48% HBF4 in 200 ml distilled water) at 20 V DC for 70 s using a stainless-steel cathode for microstructural analysis using a Zeiss Axioscope microscope. Grain size was measured using linear intercept method over a range of micrographs and considering over 250 grains for each sample.
3.1 Grain Refinement in CP–Al
3.2 Effect of Solute on Grain Refinement
3.3 Modification of Eutectic Microstructure
4.1 Origin of Grain Refinement Under Ultrasonication
4.2 Origin of Eutectic Modification Under Ultrasonication
Figure 3 clearly demonstrates that eutectic microstructure is noticeably altered in the cavitation zone (near the ultrasonic radiator). This alteration in morphology is observed for both regular and irregular eutectics. Unlike in primary grain refinement, eutectic modification does not appear to result from any enhanced nucleation. Eutectic in commercial Al–Si alloys forms with high nucleation rate at low undercooling . Therefore, contribution to eutectic nucleation by any AlP particle dislodged under cavitation is probably negligible. Cooling curves presented for the Al–11Si alloy (Fig. 4b) show nominal change in the eutectic nucleation undercooling under ultrasonication (unlike for α-Al nucleation), indicating negligible difference in nucleation behaviour. Modification of eutectic morphology appears to originate from coarsening and spheroidisation effects from solute homogenisation at the eutectic growth front under extremely strong convection in the region of cavitation [4, 5]. It should be noted that the diffusion boundary layer formed ahead of the eutectic growth front persists only over a very short distance. Coarsening and/or decoupling of the lamellar growth requires altering this extremely thin diffusion layer. This is possible under the shockwave created through cavitation. Accordingly, degeneration and spheroidisation of eutectic are only prominent over a short distance (about 15 mm) around the radiator. Although fluid flow persists in the bulk melt from acoustic streaming effects, it is not strong enough to alter the thin eutectic diffusion layer and the effect of ultrasonication on the eutectic morphology rapidly diminishes beyond 15 mm from the radiator. This contrasts with grain refinement where acoustic streaming distributes nuclei in the bulk melt leading to overall grain refinement in the entire ingots. The thin short Si platelets observed between α-Al grains in Fig. 4d are not directly influenced by ultrasonication as they are formed in the last eutectic to freeze after the ultrasonic withdrawal. These intergranular liquid pockets are small with large contact area with the existing solid resulting in high cooling rate and limited growth for the Si plates, thereby influencing the morphology.
5 Concluding Remark
Ultrasonication of melt during solidification till the semi-solid stage has shown significant grain refinement in commercial purity Al as well as in various Al–Cu alloys. The extent of refinement is superior to chemical inoculation in a small volume of melt.
Eutectic microstructure in both regular and irregular eutectic shows drastic decoupling of lamellar structures and coarsening to compact polygonal microstructures instead of lamellar or plate-type morphology in the area of cavitation within 15 mm from the radiator.
Grain refinement under ultrasonication originates from enhanced nucleation showing a noticeable reduction in the nucleation undercooling presumably from pressure pulse effect on the freezing point. On the other hand, modification of eutectic microstructure is caused by coarsening and spheroidisation under the strong fluid flow from cavitation.
Dr Hiren Kotadia thanks the support of WMG—High Value Manufacturing Catapult.
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