Microstructural characterisation of Al―Hf and Al―Li―Hf spray deposits
Research Highlights
► The main mass in the spray was concentrated within an angle of 6° from the central axis. ► The gas jet angle downstream of the convergence region was approximately 25°. ► Spherical particles of up to 140 μm were observed near the perform centre axis. ► Al-1.6wt%Hf preform grain size decreased with distance from the spray symmetry axis. ► Filamentary L12-Al3Hf and L12-Al3(Li,Hf) precipitates were observed in both performs. ► δ′ precipitates were observed in the Al-1.6wt%Hf-3.2wt%Li preform.
Introduction
Al―Li alloys have attracted a considerable interest particularly in aerospace applications due to a combination of reduced density and increased elastic modulus – each wt.% addition of Li to Al reduces the density by 3% and increases the elastic modulus by 6% – as compared to conventional high-strength Al alloys. These advantages offer a great potential for weight savings of structural parts in aircraft and space structures, but Li can only be added up to 4 wt.% by conventional casting methods. An increase of the Li content in alloys can, nevertheless, be achieved via Rapid Solidification processing routes, such as Gas Atomization/Spray Forming.
When Al alloys containing more than 1 wt.% Li are quenched from the single-phase field and aged, decomposition of the supersaturated solid solution is widely believed to take place as a two stage process : α > δ′(Al3Li) > δ(AlLi), e.g. see [1], [2]. Because of the crystallographic similarity and the small lattice mismatch between the matrix and the δ′ phase, the latter tends to appear as spherical precipitates [3], [4]. During ageing, the δ′ precipitates begin to coarsen and growth of the equilibrium phase δ occurs on the grain boundaries. The solute necessary for the continued growth of the δ phase is supplied by the dissolution of the δ′ particles in the vicinity of the grain boundaries.
The primary strengthening precipitates of the Al―Li system, the δ′, is shearable [2], [5], [6], [7] and the heterogeneous precipitation of the equilibrium δ phase produces Precipitate Free Zones (PFZ's) [8]. To improve the toughness of these systems, many efforts have concentrated on the addition of alloying elements which form non-shearable precipitates, resulting in homogeneous slip. Ternary alloying additions, like Zr and Hf have been reported to improve strength and toughness by retarding subgrain boundary migration through the formation of small coherent Al3(Zr/Hf) precipitates [9], [10]. Because the solid solubility of transition elements like Hf in Al is low, Rapid Solidification processing is required in order to obtain a complete solid solution.
Throughout the literature, microscopic studies focusing on the positional variation of the particle (drop) size on preforms are scarce. This study presents aspects of the characterisation of spray formed Al―1.6 wt.%Hf and Al―3.2 wt.%Li―1.6 wt.%Hf alloys, from macroscopic characteristics of the preforms such as shape, microscopic porosity, particle size distribution, grain size and the presence of intermetallics in the microstructure.
Section snippets
Spray Forming
Both Al alloys were spray formed in a close coupled atomizer. The atomizing die consisted of a circular ring of 18 jets, the cross section of which is shown in Fig. 1. The diameter of each jet was 0.75 mm and its inclination from the vertical direction was 20° towards the center of the flow. The distance between two antisymmetric jets (i.e. the diameter of the atomizing die) was 22 mm.
The melt (liquid metal) issued from a boron nitride tube in the center of the ring of jets. The melt superheat
Spray Forming
The cross sections of the Al―Hf and the Al―Li―Hf spray deposits (preforms) are shown in Fig. 2 and verify that the centre of the spray cone contains more mass than the outer regions of the flow. The difference in peak height between the two preforms is attributed to the higher mass that was spray formed in the case of the Al―Li―Hf alloy. Of course, the factors affecting the shape of the preform for a given set of spray forming conditions are interrelated and include the type of atomizing gas,
Conclusions
The spray produced by the close coupled atomizing configuration used in this study had cylindrical symmetry. Most of the drop mass in the spray was within an angle of approximately 6° from the central axis. The angle of the gas jet formed below the convergence region was approximately equal to 25°.
Microscopic examination of the preforms revealed that they were dense near the centre axis while porosity was present at the edges. Spherical particles with an apparent diameter of up to 140 μm were
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