Microstructural evolution and thixoformability of semi-solid aluminum 319s alloy during re-melting

Significance Statement

There have been a lot of works published on the effect of heating temperature and soaking time on microstructural evolution during isothermal re-melting process [e.g. H.V. Atkinson, D. Liu, Mater. Sci. Eng. A 496 (2008) 439]. However, the heating rate, which plays an essentially important role for grain structural evolution during re-melting, has rarely been discussed. In addition, the coarsening of silicon particles, which will reduce mechanical property of subsequently made parts, has rarely been studied during re-melting of typical semi-solid alloys produced from Al-Si alloys, such as A356, 357, 319s. In this study, we aim to characterize both the microstructural evolution mechanisms during partial melting of semi-solid 319s alloy and subsequent thixoformability. The impact of heating rate on re-melting was investigated in this study using in-situ observation of microstructural evolution in re-melting experiments.

Re-melting of semi-solid billets is the critical stage in thixocasting for obtaining a good thixotropic slug, which in turn determines formability of the slug and finally quality of the parts. Parameters such as the re-heat temperature, heating rate and soaking time strongly affect the structural evolution, and thus the thixotropic behavior. Maximizing heating rate and minimizing the soaking time in the semi-solid state is always desired expected to avoid coarsening of microstructure (primary grains and also Si particles), which may cause poor mold-filling capacity for thin sections and mechanical properties of the parts. This study indicates, however, over maximizing heating rate may induce over superheating and rapid melting of eutectics, which may result in narrowing the process window of thixocasting. Usually, heating temperature is only controlled for a desired liquid fraction. As a conclusion, an appropriate heating rate and soaking time shall be determined by theoretical analysis and experiments based on the alloy and billet size.

Microstructural evolution and thixoformability of semi-solid aluminum 319s alloy during re-melting. Advances in Engineering

About the author

Xiaogang Hu is currently a Ph. D candidate jointly in General Research Institute for Nonferrous Metals (GRINM) and University of Science and Technology Beijing (USTB). He spent one year in University of Leicester, UK as a visiting academic researcher studying rheology of semi-solid alloy. His PhD project is numerical modelling of semi-solid processing of aluminium alloys, especially interested in the modelling of semi-solid die casting using both one-phase and multiphase approaches. He also has a background on producing semi-solid slurry and post-processing of semi-solid castings. 

About the author

 Dr. Qiang Zhu is a full-time professor in GRINM from 2010. He is the Director of the Beijing Advanced Metal Shaping Engineering Centre. Semi-solid metal processing including alloying making, through semi-solid billet making, extrusion, forging, rheocasting and thixoforming, to component heat treatment, surface treatment etc. is one of the R&D interests.

Dr Qiang Zhu received his BSc and MEng in Materials Science and Engineering from the University of Science and Technology Beijing and his PhD (Dr.-Ing.) in Materials Science and Engineering from the University of Erlangen-Nuremberg, Germany. He was a research fellow in University of Sheffield. He won four awards from the Royal Society in the UK for international collaboration with Chinese organisations (1999-2001, 2003). And after that, He worked in Cummins Turbo Technologies Limited (UK) as a Principal Engineer in Metallurgy and Materials. During that period, he won the Steve Powell Award of Cummins Turbo Technologies twice (2004, 2006) and 2009 International Die Casting Competition Award of North American Die Casting Association. He has organised 10s technical conferences as a chairman or as a member of the organising committee and given 10s invited presentation to academic conferences or other research organisations. He has published 101 academic papers and 27 patents.

 

Journal Reference 

Journal of Alloys and Compounds, Volume 649, 15 November 2015, Pages 204-210.

X.G. Hu1,2,  Zhu1, H.X. Lu1,  Zhang1,D.Q. Li1, S.P. Midson1

Show Affiliations
  1. General Research Institute for Nonferrous Metals, Beijing, China
  2. University of Science and Technology Beijing, Beijing, China

 

Abstract

The aim of this paper is to characterize both microstructural evolution and thixoformability during partial melting of semi-solid 319s alloy. The thixoformability criteria of 319s was initially investigated by thermodynamic analysis. In-situ observation of partial re-melting was performed by a Confocal Laser Scanning Microscope to determine the effect of heating rate on melting characteristics. Meanwhile, the microstructural evolution of 319s alloy at extremely low heating rate was also investigated in order to understand the mechanism of re-melting process. The studies demonstrated that 319s alloy is suitable for thixocasting because of the controllable liquid fraction in the operating window of 15 °C. The process window was effected by both temperature and heating time. The primary particles evolution in 319s alloy can be divided into four stages, and the coarsening rate during isothermal test is 227 μm3/s. The effective method to obtain desirable microstructure is to manage the time in the semi-solid state by controlling heating rate and soaking time.

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