A Comparison of the Use of Ultrasonic Melt Treatment and Mechanical Vibration in the Manufacture of Al5Si5Zn Alloy Feedstock for Thixoforming
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The use of physical agents when casting aluminum alloys has proven to be an effective route for grain refinement and avoids the inconvenience of residual impurities left in the material when chemical agents are used. The application of ultrasonic waves to the molten metal before casting generates acoustic cavitation, which promotes extensive heterogeneous nucleation and contributes to degassing of the metal. In addition, the application of mechanical vibration during solidification has been proven to promote dendrite fragmentation, and therefore, grain refinement. The aim of this work is to evaluate microstructural refinement due to cavitation produced by ultrasonic melt treatment (UST) of Al5Si5Zn alloy (Al-5wt pctSi-5wt pctZn) and to compare the resulting microstructure with that achieved with and without simple mechanical vibration (MV) during casting so that the best manufacturing procedure for refining aluminum silicon feedstock for subsequent thixoforming can be identified. After casting, the alloy produced under each condition was partially melted to a 0.45 solid fraction to obtain a primary phase with a spheroidized microstructure. The rheological behavior of each semisolid slurry was also evaluated. Microstructural characterization was performed using optical and scanning electron microscopy. Mechanical performance was evaluated by means of tensile tests and hardness measurements. The use of ultrasonic stirring for 30 seconds resulted in slightly better mechanical performance than the other casting conditions. However, because of the short life expectancy of the sonotrodes, mechanical vibration can be considered a simpler, superior solution for feedstock production.
The authors would like to thank the Brazilian research funding agencies FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo—Project 2015/22143-3), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico—PQ 304921/2017-3) and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001) for providing financial support for this study. The authors are also indebted to the Faculty of Mechanical Engineering at the University of Campinas and Sonitron Ultra Sônica Ltda.
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