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Aluminum contaminations, particularly iron, present a serious challenge to aluminum recycling technology. This is why many studies have focused on the reduction of detrimental effects of iron-containing contaminations through addition of elements such as manganese and chromium. However, the desirable modifying effect is often difficult to achieve because it would require concentrations of added elements greater than the allowable limits for many aluminum alloys. Thus, an alternative way to obtain the modifying effect, by using a much smaller amount of the modifying elements which are added to the aluminum melt as solid particles, was proposed in this study. The main goal of the present study was to investigate the possibilities of fabricating an aluminum master alloy by adding CrSi2 particles onto the surface of the vortex formed during mechanical agitation of molten aluminum. Two kinds of CrSi2 powder were used: one was commercial powder, and the other was self-synthesized CrSi2 via mechanical alloying by planetary ball milling. The results revealed that CrSi2 particles with a larger size penetrate the melt better. Particles of three kinds were found to exist in the Al melt after the addition of CrSi2 powder: (1) inclusions of eutectic origin formed at the last stage of crystallization, (2) mixtures of Al-Cr compounds and original CrSi2 particles and (3) original CrSi2 particles. Low melt temperatures and short treatment times were found to favor the fabrication of master alloys because they impeded the dissolution of CrSi2 particles into the Al melt and, thus, allowed one to fabricate the master alloy containing the particles of the second and third types. [ABSTRACT FROM AUTHOR]

Although channel-type macro-segregations are a well-known defect formed in Direct Chill (DC) casting of aluminum alloys, their formation mechanism remains unclear up to now. To investigate the possible mechanisms, we carried out numerical simulation considering thermo-solutal flow and solidification coupled with phase diagram. The main emphasis is placed on the effect of shrinkage flow and slurry flow on the macro-segregation characteristics. The shrinkage flow causes negative segregations to form along the center axis of billet especially at high casting speeds, while channel-type segregations are not formed under these conditions. On the other hand, the slurry flow in mushy zone results in formation of the channel-type segregations in large billets at low casting speeds. In addition, formation of three types of channel-type segregations is predicted in the following locations (1) flow separation zone, (2) flow stagnant zone, (3) mushy zone where the upward solutal buoyant flow is generated. The formation condition of channel-type segregation is considered using solutal Rayleigh number for binary alloys. [ABSTRACT FROM AUTHOR]

Recent trends in the aluminum industry toward increasing the recycling rate of aluminum alloys have led to difficulties in controlling the formation of intermetallic compounds (IMCs). The up-to-date investigations have revealed that ultrasonic irradiation is a promising tool to refine and modify the intermetallic compounds formed in the melt before or during casting, and the reported mechanisms include ultrasound-assisted nucleation and fragmentation of compounds. This work presents results concerning the modification effect of ultrasound on model Al 3 Zr compounds with emphasis on the effect of vibration amplitudes. The results reveal that needle-like Al 3 Zr compounds can be significantly modified to thicker shape when amplitude increases from 38 µm to 48 µm, and then change to more rounded shape with the further increase of amplitude. Theoretical consideration and numerical simulation show that in addition to the above-mentioned mechanisms, cavitation-induced heat generation and mircrojet-driven mass transfer are also of prime importance in controlling the morphology and growth of IMCs. • A wide range of ultrasound vibration amplitude is examined. • Needle-like Al 3 Zr compounds can be significantly modified to more rounded shape when amplitude increases. • Behavior of cavitation bubble near compound particle is simulated numerically. • Cavitation-induced heat generation and microjet-driven mass transfer play a prominent role in controlling the morphology and growth of primary Al 3 Zr compounds. [ABSTRACT FROM AUTHOR]

• The phenomena occurring near or on the melt free surface during ultrasonic treatment for molten aluminum are examined. • Ultrasonic treatment can greatly promote the formation of alumina particles on the melt surface around sonotrode. • A part of formed alumina particles can be entrained from the free surface into the melt bulk. • The entrained alumina particles can serve as nucleation sites for primary Al3Zr compounds. Although the ultrasonic treatment of molten aluminum has been studied for long period, there is still much to be revealed for this process. Many studies have focused on the investigation of acoustic cavitation and streaming under the horn tip and their effects on the treatment efficiency. However, to the best of our knowledge, no attempt has been done to explain phenomena occurring near or on the melt free surface. Thus, the goal of this study is to investigate phenomena occurring at the free surface during ultrasound irradiation and clarify their possible influence on the ultrasound treatment performance. The results of high temperature and water model experiments reveal that ultrasound irradiation significantly promotes the formation of alumina particles on the melt free surface around sonotrode, and part of these particles can be entrained into aluminum melts. Furthermore, TEM observation results suggested that the entrained alumina inclusions can serve as nucleation sites for the primary Al 3 Zr compounds. Most importantly, the oxidation and entrainment of particles from free surface are likely to be controllable by the immersion depth of sonotrode into molten aluminum. [ABSTRACT FROM AUTHOR]