An overview and critical assessment of the mechanisms of microstructural refinement during ultrasonic solidification of metals

A refined, equiaxed grain structure and the formation of finer primary intermetallic phases are some of the notable benefits of ultrasonic processing of liquid/solidifying melts. Ultrasonic treatment (UST) has been widely explored in Al and Mg-based alloys due to its operational versatility and scalability. During UST, the refinement of grain and primary intermetallic phases occurs via cavitation-induced fragmentation mechanisms. In addition, UST improves the efficiency (activation of particles) of the conventional grain refinement process when potent particles are added through master alloys. Though the UST’s ability to produce refined as-cast structures is well recognized, the understanding of the refinement mechanisms is still debated and unresolved. Significant efforts have been devoted to understanding these mechanisms through the use of sophisticated techniques such as in-situ/ real-time observation, lab-scale and commercial-scale casting processes. All these studies aim to demonstrate the significance of cavitation, fragmentation modes, and alloy chemistry in microstructure refinement. Although the physical effects of cavitation and acoustic streaming (fluid flow) are primary factors influencing the refinement, the dominant grain refinement mechanisms are affected by several solidification variables and casting conditions. Some of these include melt volume, solute, cooling rate, potent particles, grain growth (equiaxed, columnar or dendritic), and the cold zones of the casting where the onset of nucleation occurs. This review aims to provide a better insight into solidification variables emphasizing the importance of cold zones in generating fine structures for small- and large-volume (direct chill) castings. Another important highlight of this review is to present the relatively less explored mechanism of (acoustic) vibration-induced crystallization and discuss the role of cavitation in achieving a refined ingot structure.     

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al₃Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.     

Effect of ultrasonic melt processing and Al-Ti-B on the microstructural refinement of recycled Al alloys

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.     

Broad prospects for commercial application of the ultrasonic (cavitation) melt treatment of light alloys.

Theoretical and applied aspects of ultrasonic melt treatment are considered. Industrial applications of the cavitation treatment for degassing, filtration, and grain refinement are illustrated using data for commercial alloys. Some new fields of use such as semisolid processing, low-friction alloys, and natural composites are discussed. In general, wide prospects of commercial application for the ultrasonic melt treatment of light alloys are shown.     

Characterization of acoustic cavitation in water and molten aluminum alloy

High-intensive ultrasonic vibrations have been recognized as an attractive tool for refining the grain structure of metals in casting technology. However, the practical application of ultrasonics in this area remains rather limited. One of the reasons is a lack of data needed to optimize the ultrasonic treatment conditions, particularly those concerning characteristics of cavitation zone in molten aluminum.The main aim of the present study was to investigate the intensity and spectral characteristics of cavitation noise generated during radiation of ultrasonic waves into water and molten aluminum alloys, and to establish a measure for evaluating the cavitation intensity. The measurements were performed by using a high temperature cavitometer capable of measuring the level of cavitation noise within five frequency bands from 0.01 to 10 MHz. The effect of cavitation treatment was verified by applying high-intense ultrasonic vibrations to a DC caster to refine the primary silicon grains of a model Al–17Si alloy. It was found that the level of high frequency noise components is the most adequate parameter for evaluating the cavitation intensity. Based on this finding, it was concluded that implosions of cavitation bubbles play a decisive role in refinement of the alloy structure.     

Ultrasound cavitation induced nucleation in metal solidification: An analytical model and validation by real-time experiments

Microstructural refinement of metallic alloys via ultrasonic melt processing (USMP) is an environmentally friendly and promising method. However, so far there has been no report in open literature on how to predict the solidified microstructures and grain size based on the ultrasound processing parameters.In this paper, an analytical model is developed to calculate the cavitation enhanced undercooling and the USMP refined solidification microstructure and grain size for Al-Cu alloys. Ultrafast synchrotron X-ray imaging and tomography techniques were used to collect the real-time experimental data for validating the model and the calculated results. The comparison between modeling and experiments reveal that there exists an effective ultrasound input power intensity for maximizing the grain refinement effects for the Al-Cu alloys, which is in the range of 20-45 MW/m². In addition, a monotonous increase in temperature during USMP has negative effect on producing new nuclei, deteriorating the benefit of microstructure refinement due to the application of ultrasound.     

Cavitation erosion mechanism of titanium alloy radiation rods in aluminum melt

Ultrasound radiation rods play a key role in introducing ultrasonic to the grain refinement of large-size cast aluminum ingots (with diameter over 800 mm), but the severe cavitation corrosion of radiation rods limit the wide application of ultrasonic in the metallurgy field. In this paper, the cavitation erosion of Ti alloy radiation rod (TARR) in the semi-continuous direct-chill casting of 7050 Al alloy was investigated using a 20 kHz ultrasonic vibrator. The macro/micro characterization of Ti alloy was performed using an optical digital microscopy and a scanning electron microscopy, respectively. The results indicated that the cavitation erosion and the chemical reaction play different roles throughout different corrosion periods. Meanwhile, the relationship between mass-loss and time during cavitation erosion was measured and analyzed. According to the rate of mass-loss to time, the whole cavitation erosion process was divided into four individual periods and the mechanism in each period was studied accordingly.     

In situ observation and analysis of ultrasonic capillary effect in molten aluminium

An in situ synchrotron radiographic study of a molten Al–10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode.This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.     

A high-temperature acoustic field measurement and analysis system for determining cavitation intensity in ultrasonically solidified metallic alloys

A high-temperature acoustic field measurement and analysis system (HTAFS) was self-designed and developed to achieve real-time acoustic field analysis and quantitative cavitation characterization within high-temperature liquids. The acoustic signal was acquired by a high-temperature resistant waveguide and calibrated by separate compensation of line and continuous spectra to eliminate frequency offsets. Moreover, a new method was proposed to derive from the continuous-spectrum sound intensity and line-spectrum sound intensity in the frequency band above 1.5 times the fundamental frequency to characterize the intensity of transient cavitation and stable cavitation. The acoustic field characteristics within solidifying liquid Al-7 %Si alloy were successfully determined by this system. With the increase of ultrasound amplitude, the acoustic pressure in the alloy melt increased to be stable, the transient cavitation intensity first rose and then declined, and the stable cavitation intensity remained unchanged. Combined with the structural evolution of the primary α(Al) phase, the transient cavitation intensity was determined to be the dominant factor for the ultrasound-induced grain refinement effect.     

Interactions at the planar Ag3Sn/liquid Sn interface under ultrasonic irradiation

The interactions at the interface between planar Ag₃Sn and liquid Sn under ultrasonic irradiation were investigated. An intensive thermal grooving process occurred at Ag₃Sn grain boundaries due to ultrasonic effects. Without ultrasonic application, planar shape of Ag₃Sn layer gradually evolved into scalloped morphology after the solid-state Sn melting, due to a preferential dissolution of the intermetallic compounds from the regions at grain boundaries, which left behind the grooves embedding in the Ag₃Sn layer. Under the effect of ultrasonic, stable grooves could be rapidly generated within an extremely short time (<10 s) that was far less than the traditional soldering process (>10 min). In addition, the deepened grooves leaded to the formation of necks at the roots of Ag₃Sn grains, and further resulted in the strong detachment of intermetallic grains from the substrate. The intensive thermal grooving could promote the growth of Ag₃Sn grains in the vertical direction but restrain their coarsening in the horizontal direction, consequently, an elongated morphology was presented. All these phenomena could be attributed to the acoustic cavitation and streaming effects of ultrasonic vibration.     

Investigation on ultrasonic cavitation erosion of TiMo and TiNb alloys in sulfuric acid solution

Two kinds of Ti-alloys, i.e., TiMo and TiNb alloys are manufactured in this paper, and their ultrasonic cavitation erosion behaviors in 0.1 M H₂SO₄ solution are evaluated by the mean depth erosion (MDE), SEM and white light photograph. The results show that MDE of TiMo and TiNb alloys obviously increase with increasing the cavitation erosion time, however, they evidently decrease with the increment of Mo or Nb content at each fixed cavitation erosion time, and even some large blank areas (uneroded areas) still exist on the sample surface after ultrasonic cavitation erosion for 2 h in the case of Ti10Mo and Ti20Nb samples, implying the enhanced anti-cavitation erosion property of Ti-alloy by adding Mo or Nb element. The MDE of Ti10Mo or Ti20Nb sample is lower than that of TC4 sample in the case of each cavitation erosion time, indicating the better cavitation erosion resistance of of Ti10Mo or Ti20Nb sample. The influences of Mo and Nb on the passivity of TiMo and TiNb alloys during the ultrasonic cavitation erosion are detected by potentiodynamic curves. The results display that Ti, TC4, TixMo (x = 1, 5, 10) and TixNb (x = 5, 10, 20) samples are all almost in the passive state within the potential region from 0V_SCE to 1.5V_SCE during ultrasonic cavitation erosion, and the passive current density evidently decreases with increasing Mo or Nb content, indicating the enhanced passive characteristic by adding Mo or Nb alloys during the ultrasonic cavitation erosion.     

Ultrasonic-accelerated metallurgical reaction of Sn/Ni composite solder: Principle,kinetics, microstructure,and joint properties

The high-melting-point joints by transient-liquid-phase are increasingly playing a crucial role in the die bonding for the high temperature electronic components. In this study, three kinds of Sn/Ni composite solder pastes composed of different sizes of Ni particles were synthesized to accelerate metallurgical reaction among Sn/Ni interfaces under the ultrasonic-assisted transient liquid phase (U-TLP) soldering. The temperature evolution, microstructure and mechanical property in joints composed by these composite solder pastes with or without ultrasonic energy were systemically investigated. The intermetallic joint consisted of high-melting-point sole Ni₃Sn₄ intermetallic compound with a little residual Ni was obtained under the conditions of no pressure and lower power (200 W) in a high-temperature duration of only 10 s, its shear strength was up to 45.3 MPa. Ultrasonic effects significantly accelerated the reaction among the interfaces of liquid Sn and solid Ni, which attributed to the temperature rise caused by acoustic cavitation because of large number of liquid/solid interfaces during U-TLP, resulting in accelerated solid/liquid interfacial diffusion and growth of intermetallic compounds. This intermetallic joint formed by U-TLP soldering has a promising potential for applications in high-power device packaging.     

Microstructure evolution and grain refinement of ultrasonic-assisted soldering joint by using Ni foam reinforced Sn composite solder

In this investigation, ultrasonic-assisted soldering at 260 °C in air produced high strength and high melting point Cu connections in 60 s using Ni foam reinforced Sn composite solder. Systematically examined were the microstructure, grain morphology, and shear strength of connections made with various porosities of Ni foam composite solders. Results shown that Ni foams as strengthening phases could reinforce Sn solder effectively. The addition of Ni foam accelerated the metallurgical reaction due to great amount of liquid/solid interfaces, and refined the intermetallic compounds (IMCs) grains by ultrasonic cavitation. The joints had different IMCs by using Ni foam with different porosity. Layered (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ phases both existed in Cu/Ni60-Sn/Cu joint while only (Cu,Ni)₆Sn₅ IMCs grew in Cu/Ni98-Sn/Cu joint. As ultrasonic time increasing, Ni skeletons were dissolved and the IMCs were peeled off from substrates and broken into small particles. And then, the IMCs gradually dissociated into refined particles and distributed homogeneously in the whole soldering seam under cavitation effects. Herein, the Cu/Ni60-Sn/Cu joint ultrasonically soldered for 60 s exhibited the highest shear strength of 86.9 MPa, as well as a high melting point about 800 ℃ for the solder seam composed of Ni skeletons and Ni-Cu-Sn IMCs. The characterization indicated that the shearing failure mainly occurred in the interlayer of the soldering seam. The homogeneous distributed granular IMCs and Ni skeletons hindered the crack propagation and improved the strength of Cu alloy joints.     

Effects and mechanism of ultrasonic irradiation on solidification microstructure and mechanical properties of binary TiAl alloys

In spite of their high temperature and reactivity, the binary TiAl alloys are successfully imposed by the ultrasonic irradiation and the microstructure evolution, solidification behaviors and mechanical properties are elaborately investigated. After ultrasonic irradiation, a high quality ingot without shrinkage defects and element segregation is obtained and the coarse dendrite structure is well modified into fine non-dendrite globular grains. The coarse lamellar colony and lamellar space of Ti44Al alloy is refined from 685 μm to 52 μm and 1185 nm to 312 nm, respectively (similarly, 819 μm to 102 μm and 2085 nm to 565 nm for Ti48Al alloy). For Ti48Al alloy, the α peritectic phase is simultaneously precipitated from the melt as well as the β primary phase before the peritectic reaction and the solidification is transformed into the mixed α-solidifying and β-solidifying. Ultrasonic irradiation promotes the peritectic reaction and phase transformation completely and the phase constituent becomes more close to the equilibrium level. The compressive strength of Ti44Al and Ti48Al alloys are increased from 623 MPa to 1250 MPa and 980 MPa to 1295 MPa, respectively. The grain refinement and dendrite transformation enhance the grain boundary sliding improving the plastic deformation ability. Ultrasonic irradiation significantly accelerates the melt flow and solute redistribution and the main grain refinement mechanism is the cavitation-enhanced nucleation by inclusion activation and heightened supercooling.     

Considering agitation in ultrasonic electroplating through observation of cavitation

Ultrasonic electroplating produces various effects, including refinement of the plating film structure, by generating localized agitation through cavitation bubbles. However, details of the agitation mechanism have not been clarified because ultrasonic cavitation is very small in scale and occurs rapidly, and its reproducibility is low. Therefore, using laser-induced cavitation, which can generate cavitation similar to ultrasonic waves with high reproducibility, the author attempted to elucidate the conditions and frequency of cavitation generation that affect the agitation phenomenon in ultrasonic electroplating. By controlling the laser irradiation position, three different cavitation conditions were established, and the microstructures of the plated films produced were compared. Microstructural refinement was the most advanced under the condition of microjet generation. The frequency of cavitation generation at any position in the ultrasonic electroplating was estimated to be < 1 Hz.     

Production of natural and synthesized aluminum-based composite materials with the aid of ultrasonic (cavitation) treatment of the melt

The application of ultrasonic melt treatment to the production of natural and synthesized aluminum-based composite materials is considered in terms of underlying basic ideas and commercial implementations. It is shown that the ultrasonic cavitation treatment combined with microalloying of hypereutectic Al-Si natural composites (alloys) promotes the formation of structures suitable for further deformation. The use of highly impure starting materials becomes also possible. The combination of ultrasonic cavitation treatment with electromagnetic stirring allows one to considerably improve the size and spatial distribution of ceramic particles in metal-matrix composites.     

Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al–Si-based alloys

The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al–5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al–5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}_α-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al₅FeSi intermetallic was also observed, depending on subsequent controlled cooling.     

Numerical study of the synergistic effect of cavitation and micro-abrasive particles

In ultrasonic-assisted machining, the synergistic effect of the cavitation effect and micro-abrasive particles plays a crucial role. Studies have focused on the investigation of the micro-abrasive particles, cavitation micro-jets, and cavitation shock waves either individually or in pairs. To investigate the synergy of shock waves and micro-jets generated by cavitation with micro-abrasive particles in ultrasonic-assisted machining, the continuous control equations of a cavitation bubble, shock wave, micro-jet, and micro-abrasive particle influenced by the dimensionless amount (R/R₀), a particle size-velocity–pressure model of the micro-abrasive particle was established. The effects of ultrasonic frequency, sound pressure amplitude, and changes in particle size on micro-abrasive particle velocity and pressure were numerically simulated. At an ultrasonic frequency of 20 kHz and ultrasonic sound pressure of 0.1125 MPa, a smooth spherical SiO₂ micro-abrasive particle (size = 5 µm) was obtained, with a maximum velocity of 190.3–209.4 m/s and pressure of 79.69–89.41 MPa. The results show that in the range of 5–50 μm, smaller particle sizes of the micro-abrasive particles led to greater velocity and pressure. The shock waves, micro-jets, and micro-abrasive particles were all positively affected by the dimensionless amount (R/R₀) of cavitation bubble collapse, the larger the dimensionless quantity, the faster their velocity and the higher their pressure.     

Acoustic resonance for contactless ultrasonic cavitation in alloy melts

Contactless ultrasound is a novel, easily implemented, technique for the Ultrasonic Treatment (UST) of liquid metals. Instead of using a vibrating sonotrode probe inside the melt, which leads to contamination, we consider a high AC frequency electromagnetic coil placed close to the metal free surface. The coil induces a rapidly changing Lorentz force, which in turn excites sound waves. To reach the necessary pressure amplitude for cavitation with the minimum electrical energy use, it was found necessary to achieve acoustic resonance in the liquid volume, by finely tuning the coil AC supply frequency. The appearance of cavitation was then detected experimentally with an externally placed ultrasonic microphone and confirmed by the reduction in grain size of the solidified metal. To predict the appearance of various resonant modes numerically, the exact dimensions of the melt volume, the holding crucible, surrounding structures and their sound properties are required. As cavitation progresses the speed of sound in the melt changes, which in practice means resonance becomes intermittent. Given the complexity of the situation, two competing numerical models are used to compute the soundfield. A high order time-domain method focusing on a particular forcing frequency and a Helmholtz frequency domain method scanning the full frequency range of the power supply. A good agreement is achieved between the two methods and experiments which means the optimal setup for the process can be predicted with some accuracy.