Monotectic Al–Bi–Sn alloys directionally solidified: Effects of Bi content, growth rate and cooling rate on the microstructural evolution and hardness
Graphical abstract
Introduction
Al-based alloys have been widely used for engine bearings, in particular alloys having soft solutes that behave as solid lubricants such as Pb and Sn [1], [2]. In general, Al-alloys from monotectic binary systems (Al–In; Al–Bi; Al–Pb) have been identified as promising alternatives for the manufacture of wear-resistant components in the automotive industry, being capable to replace Cu–Sn–Pb conventional bearing materials [3], [4], [5]. Moreover, the addition of third elements, such as Si, Cu and Sn to monotectic binary alloys, can be an alternative permitting the load capacity of these alloys to be increased [1], [6].
The microstructure of metallic components depend strongly on the parameters of their manufacture route. Particularly during cooling in casting processes, the resulting microstructural phases may be substantially different from those prescribed by the corresponding equilibrium phase diagram. Hence, the effect of microstructural features such as the morphology, distribution and length scale of the phases forming the alloys microstructure, are fundamental to the final application properties. The influence of the alloy microstructure is even more important under dynamic operating conditions where the use of lubricants is marginal and intimate contact between parts of mechanical components can occur [7].
Al-based monotectic alloys such as Al–Pb, Al–Bi and Al–In alloys are characterized by microstructures formed by immiscible phases with the minority soft phase having droplet and/or fibre morphologies dispersed throughout the Al matrix [4], [8], [9], [10]. Such composite materials having particles, rods or filaments of one metal or compound dispersed uniformly within the matrix of another can give rise to interesting application properties. The dispersed component can provide appropriate tribological characteristics acting as a solid lubricant, while the matrix provides required structural integrity. In the case of Al-based monotectics, practical applications include self-lubricated bearings, electrical contact materials and the fabrication of porous materials [11], [12], [13]. Sn has also a solid lubricant role and can be added to Al with a view to obtaining alloys having improved antifrictional characteristics. An investigation by Cruz et al. [2] on directionally solidified (DS) Al–Sn alloys reported a microstructure formed by an Al-rich dendritic matrix with the Sn-rich eutectic mixture distributed along the interdendritic regions. The more extensive distribution of Sn throughout the microstructure was shown to be associated with regions in the DS casting that solidified under higher cooling rates.
The knowledge of solidification paths of Al–Bi–Sn alloys is fundamental for the comprehension of the final microstructure. However, the literature is limited on information regarding the phases formation of these alloys during cooling from the melt. Dai et al. [14], based on extrapolation of binary data, assumed that the Al–Bi phase diagram would be quite similar to the ternary one and have used it to analyse the solidification path of an alloy of nominal composition (Al0.345Bi0.655)67.8Sn32.2. In a further study Dai et al. [15] examined the phase transformation of six different compositions of Al–Sn–Bi alloys adopting a qualitative ternary phase diagram obtained by Grobner and Schmid-Fetzer [16] through thermodynamic calculations. Liu et al. [17] carried out experiments with four Al–Bi–Sn alloys compositions and analysed their phase equilibria at 400 °C and 500 °C. These experimental results have been assessed by thermodynamic descriptions.
In spite of the important roles of microstructural features such as morphology, distribution and length scale of the phases characterizing the microstructure of monotectic alloys on their corresponding application properties, detailed investigation on the microstructural evolution of multicomponent Al-based monotectic alloys are scarce in the literature due to the difficulty of the task. The aim of the present study is to analyse the effect of Sn addition to Al–Bi alloys of hypomonotectic, monotectic and hypermonotectic compositions on the resulting microstructure. Unsteady-state directional solidification experiments will be carried out allowing a large range of cooling rates to be attained, permitting a wide spectrum of microstructural scales to be examined. Experimental growth laws relating microstructural spacings to both the cooling rate and growth rate and the correlation with hardness are also intended.
Section snippets
Experimental procedure
A directional solidification apparatus (Fig. 1a), in which heat is extracted only through a water-cooled bottom, promoting vertical upward solidification was used in the experiments permitting a wide range of cooling rates to be attained along the length of the castings. A stainless steel mould was used, having an internal diameter of 56 mm, a height of 150 mm and a wall thickness of 10 mm. The inner vertical surface was covered with a layer of insulating alumina to minimize radial heat losses,
Solidification paths, chemical analysis along the DS castings and microstructure
In the present study, the solidification paths of three Al–Bi–Sn alloys (hypomonotectic, monotectic and hypermonotectic alloys) have been determined by the use of a computational thermodynamics software (Thermo-Calc). Fig. 2a shows the pseudo-binary diagram relative to a parameterized concentration of 1wt.%Sn, and the alloys examined in the present study are indicated in Fig. 2b. The solidification sequences of these alloys are quite similar, and the phases compositions are simple because the
Conclusions
The following conclusions can be drawn from the present experimental study:
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The microstructures of any Al–Bi–Sn alloy examined are mainly characterized by droplets of the eutectic mixture formed by Bi-rich and Sn-rich areas embedded into the Al-matrix, and are similar, except for the scale of the droplets that varies with the solidification cooling rate.
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Experimental growth laws are proposed for each alloy with the interphase spacing being expressed as a power function either of the cooling rate
Acknowledgements
The authors acknowledge the financial support provided by FAPESP – São Paulo Research Foundation, Brazil (grants 2012/08494-0; 2013/09267-0; 2013/23396-7; 2014/50502-5) and CNPq (The Brazilian Research Council).
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