Ultrasonic Refinement of Metal Melts
- Power ultrasound in molten metals and alloys shows various beneficial effects such as structuring, degassing, and improved filtration.
- Ultrasonication promotes the non-dendritic solidification in liquid and semi-solid metals.
- Sonication has significant benefits on the microstructural refinement of dendritic grains and primary intermetallic particles.
- Furthermore, power ultrasound can be used purposefully to reduce the metal porosity or to produce meso-porous structures.
- Last but not least, power ultrasound improves the quality of castings.
Ultrasonic Solidification of Metal Melts
The formation of non-dendritic structures during solidification of metal melts influences the material properties such as strength, ductility, toughness, and/or hardness.
Ultrasonically altered grain nucleation: Acoustic cavitation and its intense shear forces increase the nucleation sites and number of nuclei in the melt. Ultrasonic treatment of melts result in a heterogeneous nucleation and the fragmentation of dendrites, so that the final product shows a significantly higher grain refinement.
Ultrasonic cavitation causes the even wetting of non-metallic impurities in the melt. Those impurities turn into nucleation sites, which are the starting points of solidification. Because those nucleation points are ahead of the solidification front, the growth of dendritic structures does not occur.
Macrostructure of Ti alloy after ultrasonic treatment. Ultrasonication results in a significantly refined grain structure.
Dr. Andreeva-Bäumler, University of Bayreuth, is working with the ultrasonicator UIP1000hdT on nano-structuring of metals.
Ultrasonic effects on alloy Vicker hardness: Ultrasonication improves the Vickers microhardness in metal
(study and graphic: ©Ruirun et al., 2017)
Dendrite Fragmentation: The melting of dendrites usually begins at the root due to local temperature rise and segregation. Sonication generates strong convection (heat transfer by mass motion of a fluid) and shock waves in the melt, so that the dendrites are fragmented. Convection can promote dendrite fragmentation due to extreme local temperatures as well as composition variations and promotes diffusion of solute. The cavitation shock waves assist the breakage of those melting roots.
Ultrasonic Degassing of Metallic Alloys
Degassing is another important effect of power ultrasonics on liquid and semi-solid metals and alloys. The acoustic cavitation creates alternating low pressure / high pressure cycles. During the low pressure cycles, tiny vacuum bubbles occur in the liquid or slurry. These vacuum bubbles act as nuclei for the formation of hydrogen and vapor bubbles. Due to the formation of larger hydrogen bubbles, the gas bubbles rise. Acoustic flow and streaming assist the floating of these bubbles to the surface and out of the melt, so that the gas can be removed and the gas concentration in the melt is reduced.
Ultrasonic degassing reduces the porosity of the metal achieving thereby a higher material density in the final metal / alloy product.
Ultrasonic degasification of aluminum alloys raise the ultimate tensile strength and ductility of the material. Industrial power ultrasound systems count as the best amongst other commercial degassing methods regarding effectiveness and processing time. Moreover, the process of mold filling is improved due to lower viscosity of the melt.
Compressive properties of Ti44Al6Nb1Cr2V under various sonication times. Sonication improves the compressive strength significantly.
(study and graphic: ©Ruirun et al., 2017)
Ceramic sonotrode BS4D22L3C is a special sonotrode suitable to sonicate high-temperature liquids such as molten aluminium (e.g. for mixing and degassing).
Sonocapillary Effect during Filtration
The ultrasonic capillary effect in liquid metals is the driving effect to remove oxide inclusions during the ultrasonically-assisted filtration of melts. (Eskin et al. 2014: 120ff.)
Filtration is used to remove non-metallic impurities from the melt. During filtration, the melt passes various meshes (e.g. glass fibre) to separate unwanted inclusions. The smaller the mesh size, the better is the filtration result.
Under common conditions, the melt cannot pass a two-layered filter with a very narrow pore size of 0,4-0,4mm. However, under ultrasonically-assisted filtration the melt is enabled to pass the mesh pores due to the sonocapillary effect. In this case, the filter capillaries retain even nonmetallic impurities of 1–10μm. Due to the enhanced purity of the alloy, the formation of hydrogen pores at the oxides is avoided, so that the fatigue strength of the alloy is increased.
Eskin et al. (2014: 120ff.) has shown that ultrasonic filtration makes it possible to purify the aluminium alloys AA2024, AA7055, and AA7075 using multi-layered glass fibre filters (with up to 9 layers) with 0.6×0.6mm mesh pores. When the ultrasonic filtration process is combined with the addition of inoculants, a simultaneous grain refinement is achieved.
Ultrasonic Reinforcement of Metal Alloys
Ultrasonication is proven to be highly effective on dispersing nano particles uniformly into slurries. Therefore, ultrasonic dispersers are the most common equipment to produce nano-reinforced composites.
Nano particles (e.g. Al2O3/SiC, CNTs) are used as reinforcing material. The nano particles are added into the molten alloy and dispersed ultrasonically. The acoustic cavitation and streaming improves deagglomeration and wettability of the particles, resulting in an improved tensile strength, yield strength, and elongation.
Ultrasonic Equipment for Heavy-Duty Applications
The application of power ultrasound in metallurgy requires robust, reliable ultrasonic systems, which can be installed in demanding environments. Hielscher Ultrasonics supplies industrial grade ultrasonic equipment for installations in heavy-duty applications and rough environments. All our ultrasonicators are built for 24/7 operation. Hielscher high power ultrasonic systems are paired with robustness, reliability and precise controllability.
Demanding processes – such as refining of metal melts – require the capability of intense sonication. Hielscher Ultrasonics industrial ultrasonic processors deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available.
For the sonication of very high liquid and melt temperatures, Hielscher offers various sonotrodes and customized accessoires to ensure optimal processing results.
The table below gives you an indication of the approximate processing capacity of our ultrasonicators:
| Batch Volume | Flow Rate | Recommended Devices |
|---|---|---|
| 10 to 2000mL | 20 to 400mL/min | UP200Ht, UP400St |
| 0.1 to 20L | 0.2 to 4L/min | UIP2000hdT |
| 10 to 100L | 2 to 10L/min | UIP4000 |
| n.a. | 10 to 100L/min | UIP16000 |
| n.a. | larger | cluster of UIP16000 |
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Literature/References
- Eskin, Georgy I.; Eskin, Dmitry G. (2014): Ultrasonic Treatment of Light Alloy Melts. CRC Press,Technology & Engineering 2014.
- Jia, S.; Xuan, Y.; Nastac, L.; Allison, P.G.; Rushing, T.W: (2016): Microstructure, mechanical properties and fracture behavior of 6061 aluminium alloy-based nanocomposite castings fabricated by ultrasonic processing. International Journal of Cast Metals Research, Vol. 29, Iss. 5: TMS 2015 Annual Meeting and Exhibition 2016. 286-289.
- Ruirun, C. et al. (2017): Effects of ultrasonic vibration on the microstructure and mechanical properties of high alloying TiAl. Sci. Rep. 7, 2017.
- Skorb, E.V.; Andreeva, D.V. (2013): Bio-inspired ultrasound assisted construction of synthetic sponges. J. Mater. Chem. A, 2013,1. 7547-7557.
- Tzanakis,I.; Xu, W.W.; Eskin, D.G.; Lee, P.D.; Kotsovinos, N. (2015): In situ observation and analysis of ultrasonic capillary effect in molten aluminium . Ultrasonic Sonochemistry 27, 2015. 72-80.
- Wu, W.W:; Tzanakis, I.; Srirangam, P.; Mirihanage, W.U.; Eskin, D.G.; Bodey, A.J.; Lee, P.D. (2015): Synchrotron Quantification of Ultrasound Cavitation and Bubble Dynamics in Al-10Cu Melts.
Facts Worth Knowing
Power Ultrasound and Cavitation
When high intense ultrasonic waves are coupled into liquids or slurries, the phenomenon of cavitation occurs.
High power, low frequency ultrasound causes the formation of cavitation bubbles in liquids and slurries in a controlled way. Intense ultrasound waves generate alternating low pressure / high pressure cycles in the liquid. These rapid changes of pressure generate voids, the so-called cavitation bubbles. Ultrasonically induced cavitation bubbles can be considered as chemical microreactors providing high temperatures and pressures at the microscopic scale, where the formation of active species such as free radicals from dissolved molecules occur. In the context of material chemistry, ultrasonic cavitation has the unique potential of locally catalyzing high-temperature (up to 5000 K) and high-pressure (500atm) reactions, while the system remains macroscopically near room temperature and ambient pressure. (cf. Skorb, Andreeva 2013)
Ultrasonic treatments are mainly based on cavitational effects. For metallurgy, sonication is a highly advantageous technique to improve the casting of metals and alloys.
Besides the treatment of metal melts, sonication is also used to create sponge-like nanostructures and nano-patterns on solid metal surfaces such as titanium and alloys. These ultrasonically nanostructured titanium and alloy parts show great capacity as implants with enhanced osteogenic cell proliferation. Read more about ultrasonic nano-structuring of titanium implants!