Ultrasonic processing of lightweight alloys: A critical review

Ultrasonic melt processing of light alloys has enjoyed a revival in the last 15 years. Although the scientific foundation and first examples of industrial application date back to the 1950s–1970s, the technological application of ultrasound in melt and solidification processing has not been fully accomplished. In recent years, the availability of advanced reliable equipment, new basic knowledge gained through modeling and dedicated experiments, and the industrial demand for clean, environment friendly technology sparked an interest in this technology and ensuing research. This paper reports on the currently achieved level of ultrasound application in light metal processing, i.e. degassing and grain refinement of light alloys and metal-matrix composite material manufacturing, and discusses challenges that still prevent large-scale implementation, both from fundamental and applied points of view. The main mechanisms underlying the effects of ultrasonic processing such as cavitation in melts, nucleation and fragmentation of solid phases, forced convection induced by cavitation zone and acoustic streaming, and mixing and distribution of solid inclusions are explained. The paper is illustrated by examples of research done under the supervision of the author.

Piston Al-Si alloys have very complex compositions and multi-phase heterogeneous structure, so it is necessary to control the formation of primary and eutectic compounds. In this study, the ultrasonic melt processing (USP) of a eutectic Al-Si piston alloy (AA4032-type) was performed in a permanent mold and during direct-chill (DC) casting to study its effects on the structure refinement and modification. The principal difference between these two ways of casting is that in the permanent mold the solidification front progressively moves towards the ultrasound source, while in the DC casting the position of the solidification front is fixed in space. The results showed that the USP can successfully refine primary Si, Fe-containing intermetallics and aluminum grains. Refinement of primary Si was accompanied by the increase in its amount, which was attributed to both enhanced heterogeneous nucleation and fragmentation. The refinement of Fe-containing intermetallics and Al grains resulted from the fragmentation mechanism and were more pronounced when USP was applied below the liquidus temperature in the permanent mold. However, the eutectic phases coarsened upon USP, and this effect was most pronounced when USP was applied to the semi-solid material. This was related to the strong attenuation of acoustic waves, which effectively heats the semi-solid material and induces corresponding coarsening of the phases. Acoustic streaming induced by an oscillating sonotrode affected the depth of the sump while simultaneously decreasing the macrosegregation, which reflects the dominant role of the melt flow directed against natural convection. The results demonstrated the importance of the solidification stage at which the USP was applied and the specifics of the USP mechanisms acting at the different stages of solidification.

Ultrasonic melt processing (UMP) has garnered significant attention from both academic and industrial communities as a promising solution to critical challenges in the metal casting industry. This technique offers a clean, environmentally friendly, and energy-efficient approach to improving melt quality and achieving structural refinement. However, due to the opaque nature of metals, understanding the fundamental mechanisms governing the interactions among ultrasonic bubbles, acoustic streaming, and the melt remains still challenging. This review traces the evolution of UMP research, from its inception in the mid-20th century to recent advancements, with particular emphasis on the application of state-of-the-art synchrotron X-ray imaging and computational modeling. These approaches have been instrumental in unraveling the complex, multiscale dynamics occurring across both temporal and spatial scales. Key findings in various metallic alloy systems are critically reviewed, focusing on new insights into cavitation bubbles, acoustic streaming, and the interactions of growing solid phases in different alloys. Additionally, the review discusses the resulting phenomena, including grain refinement, fragmentation, and the mitigation of solidification defects, in detail. The review concludes by identifying critical research gaps and emerging trends, underscoring the indispensable role of in situ studies and robust theoretical frameworks in advancing UMP. These developments are poised to reshape the future of innovation in materials science and engineering.

Ultrasonic melt processing (USP) is gaining quite an interest in recent years due to the benefits of this technology to the melt quality and structure refinement. A number of mechanisms have been identified that govern the effects of USP at different stages of melt processing. Technologically it is advantageous to apply USP to the fluid melt rather than to a mushy solidifying alloy. In this case heterogeneous nucleation on available or activated/multiplied substrates is the main mechanism. Among these substrates, primary crystals of Al3Zr phase were shown to be potent and effective. This paper gives a review of the own research into the role of Al3Zr in structure refinement in various groups of Al alloys, from solid-solution type to hypereutectic. This overview includes the evidence of a possible eutectic reaction between Al and Al3Zr in Al-rich alloys, mechanisms of Al3Zr formation and refinement under USP (that enables these primary crystals to be active substrates for Al and some other primary phases), the role of USP in facilitating primary solidification of Al3Zr in the Al-Zr system, and the additional benefits of solute Ti presence. The paper is illustrated with the data obtained over the last 15 years of research led by the author.

Ultrasonic melt processing (USP) technique was used to study the effect of Zr addition on the structure refinement and mechanical properties of hypereutectic binary alloy in three different alloys (Al–Si, Al–Fe, and Al–Ni) as potential alternatives to the Al–Si eutectic system especially for high-temperature applications. Mechanical properties of these alloys were controlled through both structure refinement by USP and also Al3Zr nano-precipitation hardening. Significant refinement of primary intermetallics was achieved under USP during the Al3Zr formation in solidification process. The residual Zr in the aluminium solid solution enabled precipitation hardening at 450 °C. As a result, the tensile properties, especially ductility, were considerably improved at room and elevated temperatures. The mechanical properties were analyzed with respect to the volume fraction of intermetallic phases. Electrical conductivity was measured to better explore their potential applications. The effects of alloying elements and structural changes on the mechanical behaviour and electrical conductivity were discussed.

Ternary Al–Ce–Ni alloys have a potential in the manufacture of automotive and airspace components, as well as in replacing traditional aluminum alloys in high‐temperature applications, which is determined by the formation of fine and thermally stable Al11Ce3 and Al3Ni eutectic. Herein, the microstructure and mechanical properties of a hypoeutectic Al4Ce2Ni alloy using Zr and Zr + Sc additions combined with ultrasonic melt processing and dispersion hardening are improved. As a result, the grain structure of the as‐cast alloys is significantly refined and the annealing at 350 °C leads to a considerable hardening effect, especially in the alloys with Zr + Sc additions (doubling the hardness). Al3Zr and Al3(Zr,Sc) coherent particles are identified as hardening nanoprecipitates. The compressive mechanical testing at room and elevated temperatures shows that the additions of Zr and Zr + Sc improve the strength with the additional increase caused by ultrasonic melt processing.

Ultrasonic melt processing enjoys the revival of interest in the last 1015 years. Although the main fundamental works as well as lab-scale and pilot-scale demonstrations date back to the 1950-1980s, the ultrasonic melt processing of light metals has not become a major technology. Recently the deficiencies of current technological approaches brought back the interest to ultrasonic treatment for degassing, grain refinement and composite materials. The current attempts to repeat the earlier results, to gain more fundamental insight using advanced means available and to up-scale the positive effects to the industrial scale show frequent lack of understanding of the basic controlling mechanisms. This paper describes the main mechanisms of ultrasonic melt processing, shows frequent mistakes, and gives some guidelines for technology up-scaling. The paper is illustrated with the latest experimental results.

Ultrasonic melt processing attracts since the 1930 a lot of interest both from academic researchers and industry. In the last 10 years the interest to ultrasonic melt processing grew with regard to understanding the underlying mechanisms of previously established effects, developing numerical models of ultrasonic cavitation and the development of nanocomposite technology. This review paper summarises the mechanisms involved in the ultrasonic melt processing, including cavitation, flows, nucleation, activation, fragmentation and their consequences for degassing, structure refinement and particle dispersion. Some typical mistakes made by researchers in performing experiments and in interpretation of the results are discussed. New advanced methods of studying ultrasonic treatment and phenomena are considered. The paper also gives an outlook to future developments and challenges.This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.

Refining the α-Al grain size and controlling the morphology of intermetallic phases during solidification of Al alloys using ultrasonic melt processing (USMP) and Al-Ti-B have been extensively used in academic and industry. While, their synergy effect on the formation of these phases has not yet clearly demonstrated. In this paper, the influence of USMP and Al-Ti-B on the solidified microstructure of multicomponent Al-4.5Cu-0.5Mn-0.5Mg-0.2Si-xFe alloys (x = 0.7, and 1.2 wt%) has been comparatively studied. The results show that the USMP + Al-Ti-B method produce a more profound refinement effect than the individual methods. In addition, the area of single Fe-rich phases in both alloys with USMP + Al-Ti-B are also refined compared with conventional methods. A mechanism is proposed for the refinement, which are the deagglomerated TiB2 parties induced by USMP providing more effective nucleation sites for α-Al, and the refined interdendritic regions limited the growth of Fe-rich phases in the following eutectic reaction. Finally, the application of combined USMP + Al-Ti-B methods is feasible in microstructural refinement, resulting in the improving the casting soundness and mechanical properties of alloys.

3D characterization of ultrasonic melt processing on the microstructural refinement of Al[sbnd]Cu alloys using synchrotron X-ray tomography

Hypereutectic Al–Fe and Al–Ni alloys offer a potentially attractive combination of properties, e.g. high-temperature strength and stability, high elastic modulus and low coefficient of thermal expansion. This potential, however, cannot be reached unless the structure of these alloys is refined so that their processing becomes possible. In this study, we for the first time apply ultrasonic melt processing for refining the structure of hypereutectic Al-4% Fe and Al-8% Ni alloys with 0.3 wt% Zr addition. Both primary Al3Fe and Al3Ni particles as well as aluminum/eutectic grains are significantly refined. It is suggested that cavitation-induced fragmentation of primary Al3Zr crystals plays a significant role in the nucleation of intermetallics as well as aluminum. Furthermore, the hardness and tensile properties of the alloys substantially increase after ultrasonic treatment due to the refined structure, which also contributes to the considerably enhanced ductility of the alloys. As a result, the fracture mode changes from brittle fracture to ductile fracture. The increase in ductility makes the alloys suitable for hot deformation, which is demonstrated by lab-scale hot rolling. In addition, precipitation hardening of the alloys can be achieved by high-temperature annealing at 450 °C due to retained Zr in the Al solid solution upon solidification. The results are supported by the analysis of the composition of a supersaturated solid solution of Zr in Al and scanning and transmission electron microscopy that confirms the precipitation of coherent Al3Zr nanoparticles. It is demonstrated that a combination of ultrasonic melt processing and alloying with Zr makes it feasible to develop new class of hypereutectic casting and wrought alloys based on the Al–Fe and Al–Ni systems.

Effect of ultrasonic melt processing and cooling rate on microstructure evolution of Al–Cu–Mn–Mg–Fe–Si alloy

Ultrasonic processing (USP) during direct-chill (DC) casting of light metal alloys is typically applied in the sump of a billet. This approach, though successful for structure refinement and modification, has two main drawbacks: (a) mixture of mechanisms that rely heavily on dendrite fragmentation and (b) a limited volume that can be processed by a single ultrasonic source. We suggest moving the location of USP from the sump to the launder and applying it to the melt flow for continuous treatment. The apparent benefits include: (a) degassing of the melt volume, (b) grain refinement through activation of non-metallic inclusions, fragmentation of primary crystals, and deagglomeration of grain refining substrates, and (c) a possibility to use a single ultrasonic source for processing large melt volumes. To optimize this process with regard to the acoustic intensity and melt residence time in the active cavitation zone, flow modification with baffles as well as informed location of the ultrasonic source are required. In this paper, we demonstrate the results of experimental trials where the degassing degree and grain refinement have been the indicators of the USP efficiency for two aluminium alloys, i.e. LM25 and AA7050. The results are supported by acoustic measurements and computer simulations.

A Novel Melt Processing for Mg Matrix Composites Reinforced by Multiwalled Carbon Nanotubes

Numerical and experimental studies about the effect of acoustic streaming on ultrasonic processing of metal matrix nanocomposites (MMNCs)