A review is presented on the development of aluminium-lithium alloys spanning several decades including both ingot metallurgy and rapid solidification. Initial interest in aluminium-lithium alloys derived from the benefits associated with lithium, both in the presence of solid solution and as the coherent Al3Li phase which result in a density reduction of about 3% for each weight per cent addition of lithium and an increase in Young's modulus of about 6%. In principle weight savings in aircraft structural parts could reach 15%. The less than optimum deformation and fracture behaviour of aluminium-lithium alloys has been attributed in part to the strain localization that results from the planar slip of shearable Al3Li precipitates. The physical metallurgy of aluminium-lithium alloys is reviewed together with various approaches that investigators have carried out, to various degrees of success, to improve the deformation behaviour inherent to these alloys. Many of the problems associated with ingot processed Al-Li alloys were alleviated to a certain extent by rapid solidification-powder metallurgy (RS-PM) processes. Although considerable progress was indeed achieved by RS-PM processing technology, the RS-PM approach introduced another problem, namely that of oxide contamination. Recently it has been demonstrated that “spray atomization and collection” processes, properly managed, are able to produce rapidly solidified, wrought, fully dense alloys which overcome the faults of RS-PM alloys. The developments of such processes are also reviewed.

Aluminium-lithium alloys

A mathematical representation has been developed, and computed results are presented describing the spreading and solidification of droplets impacting onto a solid substrate. This impingement is of major practical interest in plasma spraying and spray forming operations. Experiments in which molten metal drops were made to impinge onto a substrate were used to test the model. High-speed videography was used to record the spreading process, which typically took a few milliseconds for the experimental conditions employed. A comparison was made of the theoretical predictions with the experimental measurements; these were found to be in very good agreement, suggesting that the theoretical treatment of the model is sound. These calculations permit the prediction of the time and extent of the spreading process, the solidification rate, and the effect of process parameters, such as droplet size, droplet velocity, superheat, and material properties, provided that a value of the thermal contact coefficient is known. The most important finding of the modeling work is that for large droplets (∼5-mm diameter) with low impinging velocities (∼2 m/s), spreading and solidification appear to take place at comparable rates; in contrast, for small (∼100−µm diameter) particles impacting at a high velocity (∼100 m/s), the time scale for spreading appears to be shorter than the time scale for solidification (within the range of parameters of this study.)

Fluid flow, heat transfer, and solidification of molten metal droplets impinging on substrates: Comparison of numerical and experimental results

The status of rapid solidification is discussed in terms of recent progress in modelling methods of achieving solidification at high cooling rates and its effects on alloy constitution and microstructure. Applications currently in view are reviewed under the following headings: high-strength structural materials; tool and bearing materials; high-temperature materials, corrosion-resistant, catalytic and storage materials; and electrical and magnetic materials. It is concluded that the future of rapid solidification is not identifiable with any one process, activity or application and that many unresolved questions remain to challenge both the scientific and business communities.

The status of rapid solidification of alloys in research and application

Techniques are reviewed for describing crystal nucleation and subsequent advance of the crystallization front in metallic melts under the range of conditions generated during rapid solidification processing. The selection of appropriate boundary conditions in these cases is discussed, with particular reference to coupling between the undercooling at and the velocity of the growth front. It is emphasized that, although solute redistribution may occur, the process is normally under heat flow control and techniques for treating the associated phenomena are described. Some illustrative theoretical curves are presented for unidimensional heat flow toward a massive substrate, while the relevance to other processes, such as gas atomization, is periodically noted. Some attention is devoted to recalescence (removal of the undercooling) of the growth front as a result of latent heat evolution and the degree to which this might be counteracted by external cooling. A rationale is presented enabling the conditions under which recalescence could be arrested (thus permitting strongly enhanced crystal growth rates) to be estimated for different base metals. Mention is made of solute redistribution phenomena during recalescence of alloys. Finally, some implications of the data presented are briefly discussed in practical terms.

Numerical treatment of rapid solidification

Spray deposition of metals: A review

The results of development work on a novel particle metallurgy processing technique for highly alloyed tool and high-speed steel manufacture carried out at Aurora Steels Ltd are described. The technique, which employs metal splatting to achieve rapid rates of solidification, overcomes many of the problems and additional operations associated with more conventional powder routes. Conversion of the splatted deposit to finished product is achieved using conventional steelworks processing equipment. Improved hot workability increases the availability of some difficult product forms in highly alloyed high-speed steel grades. Products manufactured using this route have been submitted to extensive quality and performance testing with most encouraging results. Carbide segregation effects normally encountered during conventional high-speed steel ingot processing· are removed, and many other properties, e.g. heattreatment response, toughness, andgrindability, are all substantially improved. End user testing of highspeed steel tools in a number of product forms has demonstrated a consistent and high performance stanqard ..The process has been applied without difficulty to other steel qualities, i.e. austenitic stainless steel, and the product range currently under development includes square, round, and flat bar, forgings, sheet, and other specialist shapes. PM/OI95

D. R. G. Davies

Papers

CSD: A Novel Process for Particle Metallurgy Products