Ultrasonic Melt Treatment Research Articles

Improving mechanical properties of AlSi10Mg aluminum alloy using ultrasonic melt treatment combined with T6 heat treatment

Improving mechanical properties of AlSi10Mg aluminum alloy using ultrasonic melt treatment combined with T6 heat treatment

Numerical modelling of acoustic streaming during the ultrasonic melt treatment of direct-chill (DC) casting

Acoustic streaming and its attendant effects in the sump of a direct-chill (DC) casting process are successfully predicted under ultrasonic treatment for the first time. The proposed numerical model couples acoustic cavitation, fluid flow, heat and species transfer, and solidification to predict the flow pattern, acoustic pressure, and temperature fields in the sump. The model is numerically stable with time steps of the order of 0.01 s and therefore computationally attractive for optimization studies necessitating simulation times of the order of a minute. The sump profile is altered by acoustic streaming, with the slurry region depressed along the centreline of the billet by a strong central jet. The temperature gradient in the transition zone is increased, potentially interfering with grain refinement. The cooling rate in the sump is also altered, thereby modifying the dendrite arm spacing of the as-cast billet. The relative position of the sonotrode affects the sump profile, with the sump depth decreased by around 5 mm when the sonotrode is moved above the graphite ring level by 100 mm. The acoustic streaming jet penetrates into the slurry zone and, as a result, the growth direction of dendritic grains in the off-centre position is altered.

Joint Effect of Ultrasonic Vibrations and Solid Metal Addition on the Grain Refinement of an Aluminium Alloy

It is well known that it takes some time for the solid phase to completely dissolve upon melting, especially inside the defects of insoluble particles, e.g. oxides. Until then the oxides remain active solidification substrates in the case of subsequent solidification. It is also known that ultrasonic melt treatment causes grain refinement through activation and dispersion of solidification substrates (one of the mechanisms) and also accelerates the dissolution of solid metal in the melt. In this study we combine these effects and demonstrate that the introduction of an alloy rod into the matrix melt of the same composition results in significant grain refinement, this effect is increased by the ultrasonic vibration of the rod. The achieved grain size is comparable to that obtained by a standard Al–Ti–B grain refiner. All samples were cast using a standard TP-1 mould to enable correct comparison. The effects of the temperature range of the rod introduction as well as the application of ultrasonic vibrations are discussed.

Analysis of the 355 Aluminium Alloy Microstructure for Application in Thixoforming

The 355 aluminium alloy is known to have excellent thermodynamic characteristics that render it suitable as a raw material for rheocasting and thixoforming. However, besides the controllable transition from solid to liquid phase, the refined microstructure required in the semisolid range is one of the key factors with a strong influence on the rheology of the material. This paper intends to analyse the in situ behaviour of the microstructure, in terms of morphological change, using high-temperature laser scanning confocal microscopy. The 355 alloy was prepared via conventional casting, and refined with a 30-s exposition via ultrasonic melt treatment (UST - 20 Hz, 2 kW). The material was reheated up to the thixoforming target temperature of 595 °C at which it was maintained for 0, 30, 60, 90, and 120 s, after which all the samples were cooled in water. The samples subjected to UST prior to the heat treatment were more refined in terms of microstructural evolution; they exhibited reduction in grain size (~107 ± 16 μm), smallest primary phase particle size (~81 ± 7 μm), and high circularity shape factor (~0.59 ± 0.19 μm). In situ observation methods were employed to analyse evolution mechanisms such as Ostwald ripening and coalescence.

Combined Effects of Ultrasonic Melt Treatment and Cu/Mg Solute on the Microstructure and Mechanical Properties of a Hypoeutectic Al-7Si Alloy

The effect of Cu/Mg solute combined with ultrasonic melt treatment (UST) on the microstructure and mechanical properties of a hypoeutectic Al-7Si alloy was investigated. Cu up to 4 wt pct and Mg to 1 wt pct were added to the alloy and UST was performed at about 100 °C above the liquidus temperature. UST at such a high temperature was ineffective in refining grain sizes but enhanced the microstructural homogeneity of the alloys with the refinement of α-Al secondary dendrite arm spacing (SDAS). It was found that both the grain size and the SDAS were solute sensitive: In the Al-7Si alloy with UST they insignificantly changed with Cu but decreased with an increase in Mg content. Along with the structural refinement, UST was likely to affect the solute redistribution and the subsequent formation of secondary phases (e.g., eutectic Si and Cu/Mg-rich intermetallic compounds (IMCs)). Quantitative analysis indicated that UST had significantly minimized microsegregation of Cu and Mg solutes, decreasing the amount of the IMCs finely and uniformly distributed. The underlying reason was attributed to the reduction in the size of the SDAS, which enabled the solid-state diffusion to occur more efficiently upon solidification. The most significant increase in the tensile properties was achieved for the Al-7Si-2Cu-1Mg alloy with UST where the SDAS was well refined and the grain size was the smallest among the alloys investigated. The important role of UST on the casting structure refinement and the solute distribution is discussed and the resulting mechanical properties are further explored.

Ultrasonic Melt Treatment of Light Alloys

The application of ultrasonic vibrations when casting aluminium alloys can improve the final quality of castings. Moreover, ultrasound has no direct environmental impact as compared to traditional melt treatment routes. Ultrasound has been used with different purposes in aluminium casting: (1) degassing of aluminium alloys leading to high density and virtually gas porosity free castings; (2) promoting nucleation, thus leading to highly refined microstructures, including refinement and dispersion of intermetallic compounds; (3) improving castings mechanical properties either by promoting heterogeneous nucleation and development of equiaxed globular structures or dendrite fragmentation. This article presents the experimental work carried out so far at the University of Minho, Portugal, in this field and focuses on the results and benefits of ultrasonic melt processing when compared with traditional alternatives like the chemical routes.

Microstructure-Strengthening Interrelationship of an Ultrasonically Treated Hypereutectic Al–Si (A390) Alloy

Ultrasonic melt treatment (UST) was applied to an A390 hypereutectic Al–Si alloy in a temperature range of 750–800 °C and its influence on the solidification structure and the consequent increase in strength was investigated. UST at such a high temperature, which is about 100 °C above the liquidus temperature, had little effect on the grain refinement but enhanced the homogeneity of the microstructure with the uniform distribution of constituent phases (e.g. primary Si, α-Al and intermetallics) significantly refined. With the microstructural homogeneity, quantitative analysis confirmed that UST was found to suppress the formation of Cu-bearing phases, i.e., Q-Al5Cu2Mg8Si6, Al2Cu phases that form in the final stage of solidification while notably increasing the average Cu contents in the matrix from 1.29 to 2.06 wt%. A tensile test exhibits an increase in the yield strength of the as-cast alloy from 185 to 208 MPa, which is mainly associated with the solute increment within the matrix. The important role of UST in the microstructure evolution during solidification is discussed and the mechanism covering the microstructure-strengthening interrelationship of the ultrasonically treated A390 alloy is proposed.

Prediction of Cavitation Depth in an Al-Cu Alloy Melt with Bubble Characteristics Based on Synchrotron X-ray Radiography

The size of cavitation region is a key parameter to estimate the metallurgical effect of ultrasonic melt treatment (UST) on preferential structure refinement. We present a simple numerical model to predict the characteristic length of the cavitation region, termed cavitation depth, in a metal melt. The model is based on wave propagation with acoustic attenuation caused by cavitation bubbles which are dependent on bubble characteristics and ultrasonic intensity. In situ synchrotron X-ray imaging of cavitation bubbles has been made to quantitatively measure the size of cavitation region and volume fraction and size distribution of cavitation bubbles in an Al-Cu melt. The results show that cavitation bubbles maintain a log-normal size distribution, and the volume fraction of cavitation bubbles obeys a tanh function with the applied ultrasonic intensity. Using the experimental values of bubble characteristics as input, the predicted cavitation depth agrees well with observations except for a slight deviation at higher acoustic intensities. Further analysis shows that the increase of bubble volume and bubble size both leads to higher attenuation by cavitation bubbles, and hence, smaller cavitation depth. The current model offers a guideline to implement UST, especially for structural refinement.

Effect of Ultrasonic Melt Treatment on Microstructure and Mechanical Properties of 35CrMo Steel Casting

Effects of different power ultrasonic on microstructure and mechanical properties of 35CrMo steel casting were investigated using optical microscopy (OM), scanning electron microscopy (SEM) and hardness testing. A self-developed experiment apparatus was used for the propagation of ultrasonic vibration into the 35CrMo steel melt to carry out ultrasonic treatment. The experimental results showed that compared to the traditional casting, ultrasonic treatment can obviously change the solidification microstructure of 35CrMo steel, which is changed from coarse dendrites to fined dendrites or equiaxed grains. With the increase of ultrasonic power, equiaxed crystal is remarkably refined and its area is broadened. The micro porosity percentage of ingot casting decreases significantly and the porosity defects can be suppressed under ultrasonic treatment. The mechanical properties of 35CrMo steel ingot after heat treatment were enhanced by ultrasonic treatment: the maximum tensile strength is improved by 8.4% and the maximum elongation increased by 1.5 times.

Experimental and numerical investigation of acoustic pressures in different liquids

In an attempt to quantify the instantaneous pressure field in cavitating liquids at large forcing signals, pressures were measured in four different liquids contained in vessels with a frequency mode in resonance with the forcing signal. The pressure field in liquid metal was quantified for the first time, with maximum pressures of the order of 10–15 MPa measured in liquid aluminium. These high pressures are presumed to be responsible for deagglomeration and fragmentation of dendritic intermetallics and other inclusions. Numerical modelling showed that acoustic shielding attenuates pressure far from the sonotrode and it is prominent in the transparent liquids studied but less so in aluminium, suggesting that aluminium behaviour is different. Due to acoustic shielding, the numerical model presented cannot adequately capture the pressure field away from the intense cavitation zone, but gives a good qualitative description of the cavitation activity. The results obtained contribute to understanding the process of ultrasonic melt treatment (UST) of metal alloys, while facilitating further the guidelines formulation and reproducible protocols for controlling UST at industrial levels.

Synergistic effect of ultrasonic melt treatment and fast cooling on the refinement of primary Si in a hypereutectic Al–Si alloy

A mechanism describing the synergistic effect of ultrasonic melt treatment (UST) and subsequent fast cooling on the refinement of primary Si particles in a hypereutectic Al−Si alloy was investigated by examining inoculant particles via high-resolution transmission electron microscopy, serial sectioning, and melt filtration. The application of UST activated non-wetting MgAl2O4 particles with diameters of ∼0.5 μm to nucleate the primary Si phase. The cavitation-enhanced wetting of MgAl2O4 particles caused the formation of the AlP phase at the MgAl2O4 interface, further improving the nucleation potential. The cavitation-enhanced wetting and dispersion of inclusions (such as MgAl2O4) also resulted in the refinement and de-agglomeration of AlP particles. The UST-induced changes to the inoculant particles ultimately increased their number density, and the observed effects became more pronounced after increasing the degree of undercooling up to 20 K, leading to enhanced refinement of primary Si particles at higher cooling rates (up to 102 K s−1).

The Effect of Ultrasonic Melt Treatment on Macro-Segregation and Peritectic Transformation in an Al-19Si-4Fe Alloy

It is well documented that ultrasonic melt treatment (USMT) can refine dendritic and eutectic microstructures during solidification, but much less attention has been paid to the effect of USMT on macro-segregation and intermetallic transformations. In this research, macro-segregation and primary Fe-containing intermetallic peritectic transformations in an Al-19 wt pct Si-4 wt pct Fe alloy were investigated without and with USMT. Macrostructural examination showed that in the absence of USMT the ingot revealed considerable non-uniform distribution of both the primary Fe-containing intermetallic and primary Si particles, whereas the ingot with USMT exhibited near homogeneous distribution of both primary phases, i.e., reduced macro-segregation. The beneficial effect of USMT on relieving macro-segregation was further examined using quantitative microstructural metallography and the results indicated that the area fraction, number density, and size distribution of both primary phases became essentially uniform across the ingot after USMT. USMT further exerted a significant impact on the constitution of the primary Fe-containing intermetallics, where complex particles of δ-Al3FeSi2/β-Al5FeSi were prominent without USMT, while few δ-Al3FeSi2 particles were observed after USMT and the primary Fe-containing intermetallics existed mostly as the single-phase β-Al5FeSi. The underlying reason was attributed to the reduction in the size of the primary δ-Al3FeSi2 particles which ensures the complete transformation of most primary δ-Al3FeSi2 particles to the peritectic β-Al5FeSi phase.

Ultrasonic effect on the penetration of the metallic melt into submicron particles and their agglomerates

Deagglomeration and wetting of submicron particles in a metal melt under ultrasonic exposure are considered based on the theory of acoustic cavitation and capillary phenomena. Basic dependences linking the exposure time with physicochemical properties of the particles and the melt, as well as with acoustic radiation characteristics, are found. The experimental and calculated times of ultrasonic treatment of the aluminum melt containing submicron aluminum oxide particles are compared, and a satisfactory agreement of results is found.

초음파 용탕 처리 및 냉각속도가 Al-18Si-4Cu-3Ni 피스톤용 합금의 미세조직과 고온인장특성에 미치는 영향

초음파 용탕 처리 및 냉각속도가 Al-18Si-4Cu-3Ni 피스톤용 합금의 미세조직과 고온인장특성에 미치는 영향

Influence of ultrasonic treatment on formation of primary Al3Zr in Al–0.4Zr alloy

The effect of ultrasonic treatment on the formation of primary Al3Zr was investigated by applying ultrasound to an Al–0.4Zr alloy. Three temperature ranges were selected, i.e., 830 to 790 °C (above liquidus), 790 to 750 °C (cross liquidus) and 750 to 710 °C (below liquidus) for ultrasonication. Using the scanning electron microscopy, both the size and morphology of the primary Al3Zr particles were examined. It was found that the size was significantly reduced and the morphology changed from large throwing-star shape to small compact tablet shape. The mechanisms for refinement of primary Al3Zr were discussed. It is suggested that sonocrystallization theory via activation of aluminium oxide particles is responsible for the refinement of primary Al3Zr when ultrasonic melt treatment (UST) is applied within the fully liquid state. The refinement of primary Al3Zr particles when UST is applied in the slurry (growth stage) is due to the sonofragmentation.

Novel casting TiAl alloy with fine microstructure and excellent performance assisted by ultrasonic melt treatment

A novel casting TiAl alloy with fine homogeneous microstructure and excellent performances is obtained assisted by the ultrasonic melt treatment. The coarse columnar crystal is modified into non-dendritic structure and the large grain is refined from 3098μm to 88μm; the micro-segregation, micro-porosities and precipitates are eliminated as well. After ultrasonic melt treatment for 90s, the highest compressive strength is improved up to 1705MPa, about 12% higher than the “quasi-oriented” TiAl alloy.

Effect of transition elements on the microstructure and tensile properties of Al–12Si alloy cast under ultrasonic melt treatment

This paper shows that the addition of transition elements significantly influences the microstructure and tensile properties of Al–12Si alloy cast under ultrasonic melt treatment (UST). The alloys contained primary Si, eutectic Si and a variety of intermetallic compounds (IMCs); the addition of Mn and Ni changed the solidification sequence by forming primary α-Al15(Fe,Mn)4Si2 and ε-Al3Ni prior to primary Si formation. The primary Si and primary IMCs competitively consumed the nucleation sites introduced by UST, thereby influencing their refining efficiencies; the efficiency of primary Si refinement by UST increased with increasing the fraction of primary Si formed prior to IMC formation, whereas it deteriorated with increasing primary IMC fraction. The refining efficiency of eutectic Si and eutectic IMCs was not affected by the type and fraction of the primary phase. Using a Ti sonotrode for UST caused Ti contamination, resulting in grain refinement by forming Ti(Al,Si)3 particles and increasing the amount of Ti solutes. Regardless of transition elements content, the application of UST improved the tensile strength at 25 °C and 350 °C. The refinement of IMCs caused by UST allowed the alloy to contain more IMCs without deterioration of room-temperature tensile strength. This enhanced the increment of high-temperature tensile strength by IMCs. Ductility of heavily alloyed systems was also improved by UST.

The Effect of Ultrasonic Melt Treatment on the Microstructure and Mechanical Properties of Al-7Si-0.35Mg Casting Alloys

The Effect of Ultrasonic Melt Treatment on the Microstructure and Mechanical Properties of Al-7Si-0.35Mg Casting Alloys

Effects of ultrasonic melt treatment and solution treatment on the microstructure and mechanical properties of low-density multicomponent Al70Mg10Si10Cu5Zn5 alloy

The effects of ultrasonic melt treatment (UST) and solution treatment on the microstructure and mechanical properties of a low-density multicomponent Al70Mg10Si10Cu5Zn5 alloy at room and elevated temperatures were investigated. The as-cast Al70Mg10Si10Cu5Zn5 alloy was composed of an fcc Al solid-solution, coarse secondary phases (5–30 μm), fine Cu-containing precipitates (<1 μm), and Zn clusters (<20 nm). The UST significantly reduced the porosity and size of primary Mg2Si particles and grains, resulting in an improvement in the strength and ductility of the as-cast alloy at 25 and 350 °C. The solution treatment at 440 °C caused the dissolution of hcp Zn and fine Cu-containing precipitates. It also resulted in the formation of Zn and Cu clusters in the supersaturated Al solid-solution at 25 °C, which led to an improved room-temperature strength. A phase transformation from Mg2Si to Q-Al5Cu2Mg8Si6 occurred during the solution treatment along with the spheroidization of coarse secondary phases and grain growth, which deteriorated the ultimate compressive strengths at 25 and 350 °C and improved the ductility. Unlike the commercial Al alloys, the Al70Mg10Si10Cu5Zn5 alloy showed excellent strengths at temperatures below 200 °C. This is attributed to the combined strengthening by coarse secondary phases, fine precipitates, and clusters.

Microstructural Evolution of the Magnesium Alloy AZ31 with and without SiC Particles during Ultrasonic Melt Treatment

The aim of this study is to reveal the influence of ultrasonic treatment in the molten state on microstructural evolution of the magnesium alloy AZ31 with and without the addition of SiC particles. Therefore, a heatable wedge-shaped mold for holding the magnesium melt on the required temperature was constructed with the possibility to insert an ultrasonic horn into the melt. Previously ingots of AZ31 were molten in an electrical furnace under a protective gas atmosphere. SiC particles with an average diameter of 2 μm were added to the melt. Mechanical stirring was conducted to ensure homogeneous distribution of the particles. The molten Mg was subjected to ultrasonic sound with constant frequency of 20,000 Hz and amplitude of 12.4 μm. The time of ultrasonic treatment was defined on 60 seconds. The material solidified quickly due to the cooling with water nozzles after withdrawing of the ultrasonic horn. AZ31 without SiC particles was treated under the same conditions. The resulting microstructure was observed by optical and scanning electron microscopy. Depending on resulting intensity and the effective area a refinement in grain size and also more homogeneous distribution of precipitations in the material is achieved. The resulting microstructures of AZ31 with and without SiC addition were compared and discussed.