Agradecimentos: The authors would like to thank the Brazilian research funding agencies FAPESP (Project 2015/22143-3), CNPq (PQ 304921/2017-3), and CAPES for providing this study with financial support. The authors are also indebted to the Department of Reliability Evaluation & Simulation, Joining and Welding Research Institute, at the Osaka University for their kind support Ver menos

Abstract: 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 Ver menos