As many of you are aware, we are in the transitioning phase of this website as we close out TeamUV.org and transition to EngineeringAFuture.com over the next two months, so this will be Abraham’s last Presentation post here at Team UV, but not to fear, there are still two months of posts left here and the same types of articles will be carried over onto EAF (Engineering A Future), so without further ado, please enjoy the following:
Have you ever heard of Ultrasonic Melt Treatment? Me neither but I did some research and found some pretty interesting things. Ultrasonic Melt Treatment (UST) is essentially the addition of high frequency acoustic waves to a liquid melt of a metal (typically aluminum) to induce acoustic cavitation. Please note that acoustic cavitation is essential. The acoustic waves introduced into the melt have to be at a high enough amplitude and frequency to alternate pressure above the cavitation threshold to create cavities in the melt. Essentially what is happening is the rapid formation and collapse of bubbles in the melt which can have some beneficial effects before, after, and during solidification. The acoustic cavitation is said to activate the melt which means that it will accelerate diffusion, wetting, dissolution, and dispersion which will directly affect degassing, solidification, and refining of metallic alloys. Now if you’re into metallurgy this all sounds incredibly interesting, but if you’re not I will try to explain what each of these mean.
Degassing is one of the primary uses of UST in which the concentration of a certain gas will be lowered in the melt. In light aluminum alloys, acoustic cavitation will cause the growth of fine bubbles in the melt on the surface of non-wettable oxides. As a result, cavitation will cause direct diffusion of hydrogen into the bubbles from the melt. Acoustic flow will assist in floating these bubbles to the surface and out of the melt. The benefits of degassing are that lower porosities can be achieved and thus higher densities in the final material. It has also been found that lowering the concentration of hydrogen in aluminum alloys will raise the ultimate tensile strength and ductility of the material. It is noteworthy that one study concluded that UST is the best amongst other commercial degassing techniques in terms of effectiveness in degassing and in time of treatment.
Degassing of oil using UST is similar to the UST process in liquid metal melts. Photo Credit: hielscher.com
Melts are usually cleaned of inclusions before final treatment. Aluminum melts usually use mesh filters but in order to filter out very fine particulates, multiple layers of successive filters need to be used. This is often not allowed because of the restriction of capillary action of the liquid melt through the filters, however with the use of UST during filtration, a sonocapillary effect is produced. This sonocapillary effect is caused by the cavitation field that is formed which allows for the melt to freely pass through the multiple layers and disperse unwanted oxides.
Ceramic filters. Photo Credit: induceramic.com
One of the most important benefits of UST is that it produces non-dendritic structures during solidification. A dendrite is a branched tree-like structure that grows during solidification of liquid metals (fun fact: dendritic solidification is actually what is behind the unique shape of snowflakes!). As mentioned earlier, acoustic cavitation allows for the wetting of non-metallic impurities in the melt and these become sites for nucleation (aka the start of solidification). Because these nucleation sites are ahead of the solidification front, there is no growth of dendritic structures.
As non-dendritic solidification occurs, the grain becomes only a fraction of the matrix structure so there is refinement in grain size. It is important to note that grain size is only a function of cooling rate in these conditions, therefore this method of refinement is applicable to all metallic materials. One of the major benefits of materials with non-dendritic structures in Al-Si-Mg systems is that they are able to deform very easily at a semi-solid temperature ranges. Essentially what this does is allows the grains to rotate and slide relative to each other without the interference of the dendritic braches. This can dramatically improve mold filling by lowering the viscosity. The apparent viscosity of non-dendritic melts at the semi-solid temperature range is lower than that of the regular melts and is comparable to that of olive oil, at least for aluminum.
Comparison of dendritic and non-dendritic strutures. Photo Credit: scielo.org.za
Overall there are many benefits to Ultrasonic Melt Treatment in aluminum alloys such as degassing, refinement of grain size, and filtration. Although UST has only been extensively studied in non-ferrous alloys, recent research has been done on its effects in low-carbon steels and even epoxy based nanocomposites. In all cases it is the acoustic cavitation, duration of the treatment, and force of frequency that dictate the effectiveness of UST. Hopefully this taught you a little bit about UST. For more info, please refer to the paper by Eskin, G.I. “Broad prospects for commercial application of the ultrasonic (cavitation) melt treatment of light alloys”.