The effect of scandium on the microstructure, mechanical properties and weldability of a cast Al–Mg alloy
Introduction
Aluminium alloys with magnesium as the major alloying element constitute a group of non-heat treatable alloys with medium strength, high ductility, excellent corrosion resistance and weldability [1], [2]. Wrought Al–Mg alloys are used as structural materials in marine, automotive, aircraft and cryogenic applications while the cast forms are used mainly for their corrosion resistance in dairy, food handling and chemical processing applications. Unlike the heat treatable alloys, these materials derive their strength primarily from solid solution strengthening by Mg, which has a substantial solid solubility in aluminium. However, in order to obtain strength levels approaching the regime of the precipitation hardening alloys, high Mg levels are required. Such high levels of Mg pose processing challenges and can increase the susceptibility of the alloys to stress corrosion cracking [1], [2].
An effective alternative method to increase the strength of aluminium alloys (including Al–Mg alloys), first proposed by Willey in 1971 [3], involves the addition of scandium as an alloying element. Research carried out over the last 20 years has established that Al–Sc solid solution decomposes to form a fine dispersion of homogeneously nucleated, equilibrium precipitates of a stable L12 phase, Al3Sc, with a lattice parameter mismatch with Al of only 1.6%, which can produce a significant ageing response, despite the relatively low solubility of Sc and hence, a low volume fraction [4], [5], [6], [7], [8], [9]. The fine dispersion of Al3Sc precipitates is known to inhibit recrystallization even at large levels of plastic deformation and high annealing temperatures, enabling the use of strain hardening to push the strength of Al–Mg alloys to the levels achieved by traditional precipitation hardening [5], [7], [8], [9], [10], [11], [12]. Sc serves as a potent grain refiner in castings, particularly when used in combination with Zr [7], [8], [12], [13], [14], [15]. Sc additions to the base alloy as well as to welding filler alloys have been shown to have a beneficial influence on weldability and hot cracking resistance of aluminium alloys [16], [17], [18], [19], [20].
Much of the work on Al–Mg–Sc alloys reported in the literature has been carried out on wrought materials and there is little information available on the properties of cast alloys. In this paper, we present the results of a study of the effect of Sc addition on the microstructure, mechanical properties and the weld hot cracking behaviour of an Al–Mg alloy in the as-cast condition.
Section snippets
Experimental program
The starting materials used in the preparation of the experimental Al–Mg–Sc alloy were AA5083 alloy and two master alloys—Al–2 wt.% Sc and Al–50 wt.% Mg. The latter master alloy was used to make up slight losses of Mg that occurred during the melting. Appropriate amounts of the starting materials were melted in a clay–graphite crucible by heating to 720 °C in an induction furnace to produce an Al–Mg–Sc alloy with a nominal composition similar to that of AA 5083 with 0.2 wt.% Sc addition. The
Microstructures of cast Al–Mg and Al–Mg–Sc alloys
Fig. 2(a) is a low magnification back scattered electron (BSE) SEM micrograph of the cast Al–Mg alloy, etched using Keller’s reagent.1 The crystallographic (electron channelling) contrast present in the BSE image highlights the equiaxed dendritic grain structure of the alloy. The optical micrograph, Fig. 2(b), shows that the etching has revealed, albeit faintly, grain boundaries as well as the cellular
Effect of Sc on grain size of cast Al–Mg
Sc has been shown to be a potent grain-refining agent in aluminium alloys [7], [13]. However, Fig. 2, Fig. 3 show that addition of 0.17 wt.% Sc has not resulted in grain refinement in the cast Al–Mg–Sc alloy. Indeed, it has a coarser grain size than the Sc-free alloy, although the cell substructure within its equiaxed dendritic grains is finer. Similar results have been reported in the literature for Sc additions of ~0.2 wt.% to Al–Mg alloys of comparable nominal compositions[11], [12].
The
Conclusions
Based on this comparative study of a cast Al–Mg–Sc alloy containing 0.17 wt.% Sc and a Sc-free Al–Mg alloy of similar chemical composition, prepared under identical conditions, we make the following conclusions:
- 1.
Addition of 0.17 wt.% Sc, which is less than the binary Al–Sc eutectic composition of 0.55 wt.%, did not refine the grain size of the casting relative to that of the Sc-free alloy, although there was some refinement of the dendritic substructure.
- 2.
In both cast alloys, there was
Acknowledgements
It is a pleasure to acknowledge the partial financial support for this project provided by D. Pursell and Jervois Mining, N.L. The authors are grateful to N. Ahmed, R. Calcraft, G. Cavallaro, T. Heijkoop, W. Mazur and O. Schumann for many stimulating discussions during the course of this work. They thank J. Goss, E. Murray, P. Pedersen, M. Pullen, J. Stewart and D. Viano for the valuable assistance they provided with various aspects of the experimental program.
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