3D study of the structure of primary crystals in a rheocast Al–Cu alloy

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Abstract

An Al-10.25 wt.% Cu alloy was mechanically stirred at 1000 rpm while being cooled to and isothermally held at 619°C for various times before casting. The microstructure of as-cast materials consists of pseudo-particles and pseudo-clusters in two-dimensional (2D) views. Serial sectioning was performed and it was found that pseudo-particles belonging to a pseudo-cluster were connected in three dimensions and a pseudo-cluster was actually a single primary particle. A three dimensional (3-D) model of such a primary particle was constructed from 2D sections which showed the complexity of its structure. It is suggested that the theories based on agglomeration and disagglomeration of particles during rheocasting be re-examined in light of the experimental results.

Introduction

It is commonly believed that stirring an alloy melt in its semisolid temperature range ‘breaks’ the dendrites that have formed during solidification to small pieces. However, it has been frequently shown that the microstructure of a stirred, slowly cooled alloy consists of large primary features which have very complex shapes [1], [2], [3], [4], [5], [6]. Many researchers believe that these large primary features are formed by a process of agglomeration of small, globular particles [1], [3], [4], [6] although others are less definite in their descriptions [2], [7]. Special boundaries have also been observed and their formation has been attributed, in one possibility, to the sintering of individual primary particles [8]. Agglomeration and disagglomeration of the fine, globular particles have been the base for some studies on the rheological behaviour of semisolid melt [9], [10]. Despite the importance of knowing the true morphology of the primary particles, only a limited number of attempts have been made to study the three dimensional (3D) structure of the semisolid metals [11], [12], [13]. These studies showed that some primary particles, which often appeared separate in two dimensional (2D) sections, were connected in 3Ds, but they still interpreted the results based on the agglomeration of individual particles. In the present investigation, microstructures of a rheocast Al-10.25 wt.% Cu alloy processed under different conditions were characterized by serial sectioning and 3D imaging to shed new light on the relationships between various features as well as to reveal the morphology of the primary crystals. A 3D-computer model was also constructed.

Section snippets

Experimental materials and procedures

The alloy used was an Al-10.25 wt.% Cu binary alloy prepared from pure Al and Cu. The experimental device used to carry out the stirring and casting consisted of a vertical muffle furnace controlled by a programmable temperature controller, a stirrer driven by a DC motor, a tacho-generator by which the initial set speed could be kept constant, a crucible with a bottom pouring mechanism, a steel mould, and thermocouples placed in the melt and connected to a computer for data logging.

The alloy

Experimental results

Fig. 1 shows the microstructures of nine consecutive parallel sections from a material, which had been continuously cooled to 619°C and then isothermally held for 15 min before casting. The average distance between the sections is 83 μm. The primary features (coarse) that formed during cooling and stirring are easily distinguished from the secondary ones (fine) that formed upon casting in the mould by their size. The control points (the four black dots) can be seen near the four corners on the

Discussion

It is clear from the above observations that the primary features in a Al-10.25 wt.% Cu alloy which has been stirred during cooling to and isothermal holding at 619°C include the so-called pseudo-particles which can be identified either by a visible boundary or by a colour difference between them, and pseudo-clusters which are agglomerates of pseudo-particles. They are pseudo because the particles appear separate and clustered together in 2D views but are connected if viewed in 3D. Each

Summary

A three dimensional study of the rheocast microstructure of an Al-10.25 wt.% Cu alloy shows that the primary features consist of pseudo-particles and pseudo-clusters. The pseudo-particles in a pseudo-cluster are connected to each other in three dimensions and therefore are not true primary particles. Consequently, a pseudo-cluster is not an agglomerate of primary particles, but rather a single primary grain. The structure of such a primary grain is very complicated and may have been disfigured

Acknowledgements

We are grateful to Prof. N.J. Kim of POSTECH in Pohang, Korea for providing the facilities and assistance in carrying out the misorientation measurements. BN would like to thank Jae Joong Kim in particular for his help in SACP analysis. A special travel grant from the University of Melbourne is also acknowledged.

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1

Present address: Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran 84156.

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