Aluminum alloys

Details Aluminum alloys Aluminum alloys

1984-06-04

1987-01-13

Dean, R.

Alloys or metallic compositions

Aluminum base

Copper containing

148439, 148440, 420541, C22C 2110

Patent

active

046363572

DESCRIPTION:

BRIEF SUMMARY
This invention relates to aluminium alloys having improved properties and reduced densities and being particularly suitable for use in aerospace airframe applications.
It is known that the addition of lithium to aluminium alloys reduces their density and increases their elastic moduli producing significant improvements in specific stiffnesses. Furthermore the rapid increase in solid solubility of lithium in aluminium over the temperature range 0.degree. to 500.degree. C. results in an alloy system which is amenable to precipitation hardening to achieve strength levels comparable with some of the existing commercially produced aluminium alloys.
Up to the present time the demonstrable advantages of lithium containing alloys have been offset by difficulties inherent in the actual alloy compositions hitherto developed. Only two lithium containing alloys have achieved significant usage in the aerospace field. These are an American alloy, X2020 having a composition Al-4.5Cu-1.1Li-0.5Mn-0.2Cd (all figures relating to composition now and hereinafter are in wt%) and a Russian alloy, 01420, described in UKP No. 1,172,736 by Fridlyander et al and containing Al-4 to 7mg-1.5 to 2.6Li-0.2 to 1.0Mn-0.5 to 0.3Zr (either or both of Mn and Zr being present.
The reduction in density associated with the 1.1% lithium addition to X2020 was 3% and although the alloy developed very high strengths it also possessed very low levels of fracture toughness making its efficient use at high stresses inadvisable. Further ductility related problems were also discovered during forming operations.
The Russian alloy 01420 possesses specific moduli better than those of conventional alloys but its specific strength levels are only comparable with the commonly used 2000 series aluminium alloys so that weight savings can only be achieved in stiffness critical applications.
Both of the above alloys were developed during the 1950's and 1960's.
For some years after these alloys the focus of attention of workers in the field centred upon the aluminium-lithium-magnesium system. Similar problems were again encountered in achieving adequate fracture toughness at the strength levels required.
A more recent alloy published in the technical press has the composition Al-2Mg-1.5Cu-3Li-0.18Zr. Whilst this alloy possesses high strength and stiffness the fracture toughness is still too low for many aerospace applications. In attempts to overcome problems associated with high solute contents such as, for example, cracking of the ingot during casting or subsequent rolling, many workers in the field have turned their attention to powder metallurgy techniques. These techniques whilst solving some of the problems of a casting route have themselves many inherent disadvantages and thus the problems of one technique have been exchanged for the problems of another. Problems of a powder route include those of removal of residual porosity, contamination of powder particles by oxides, practical limitations on size of material which can be produced and the inevitably higher cost.
Further work has been carried out on the aluminium-lithium-magnesium-copper system. This work has shown that by reducing the amount of solute content and optimising the composition at a more dilute level an acceptable balance of properties including fracture toughness may be achieved. This work is described in copending UK patent application No. 8304923.
Continuing work has shown that other useful alloys may be produced based on the aluminium-lithium system but having different additional alloying elements.
According to the present invention an aluminium based alloy comprises the following composition expressed in weight percent:


______________________________________ Lithium 2.0 to 3.0 Magnesium 0 to 4.0 Zinc 0.4 to 5.0 Copper 0 to 2.0 Zirconium 0 to 0.2 Manganese 0 to 0.5 Nickel 0 to 0.5 Chromium 0 to 0.4 Aluminium balance ______________________________________
Additions of zinc have been found to give improved properties without significant reduction of ductility. Zinc additions con

REFERENCES:
patent: 2381219 (1945-08-01), LeBaron
Jones et al., Journal of the Institute of Metals, vol. 88, 1959-1960, pp. 435-443.
Starke, Jr. et al., Journal of the Metals, Aug. 1981, pp. 24-32.

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