Wrought Aluminum Alloys for General and Special Applications: Part Two

Superplastic Aluminum Alloys

Super plasticity is the property of certain metallic materials that very high elongations without contraction till breakage can be achieved at suitable working conditions These elongations are from few hundred to 1000% or even more. Such a method of working occurs at low strain rates (< 1 s^-1), high working temperatures (> 0.5 T_{melting point}), and corresponding microstructure of material. Needed working stresses values are considerably lower than in working ordinary materials. Excellent work abilities enable wide range of applications of super plastic materials for various purposes.

The first aluminum alloys with super plastic properties had eutectoidal or eutectic composition, e.g. AlCu33 alloy. They were not applied in practice due to unsuitable mechanical properties, though they possessed good plasticity. In 1970s, super plastic alloys with compositions and mechanical properties similar to those of ordinary aluminum alloys, were discovered. Since then, the development of those materials has an upward trend. Some superplastic alloys are already industrially produced and used in practice. Among the most known and useful alloys are the AlCuZr, AlZnMgCu, AlMgMn, and AlLiX.

Basic properties which are needed for superplastic alloys, are the following:
(a) fine grained microstructure with an average grain size of 10 µm,
(b) stability of crystal grains against growth at working temperatures,
(c) stability against cavity formation during superplastic deformation,
(d) low working stresses (2 to 20 MPa), and
(e) high values of strain rate sensitivity exponent in the equation s = K•ε^m (m > 0.3).

Processes of making and working theses alloys are similar to the conventional methods used for standard aluminum alloys. Fine grained microstructure as the basic condition for superplastic working can be obtained by corresponding alloy compositions, temperatures of melt and casting, and thermo mechanical treatment. Application of superplastic materials highly reduces the manufacturing costs due to reduced consumption of energy and materials, by reducing unnecessary joining of single sections, and by using one single tool which can be made of undemanding, cheap material. Savings in tools represent up to 90% in comparison with manufacturing equally complex products of ordinary materials.

Aluminum superplastic alloys are used for manufacturing aircraft components, components of car bodies, for housing, components of various apparatuses and musical instruments, and components in building, like linings of buildings, and for decoration purposes. High-strength AlZnMgCu, AlCuZr, and AlLiX alloys are mainly used in aircraft industry; AlMgMn alloys are more generally used for road and rail vehicles, and in civil engineering. Further development of superplastic materials will be directed towards cheaper manufacturing of those materials and to rationalization of superplastic working. Further efforts to improve the plasticity are not needed. The plasticity, possessed by such present aluminum materials, already corresponds to the demands of superplastic working.

Al-Li-X Alloys

In the middle 1970s, intensive investigations on AlLi alloys took place. Those alloys have been supposed to substitute standard aircraft alloys of AlCuMg and AlZnMgCu types due to some special properties. The aim of investigation was the reduction of weight, and increased elastic modulus values of engineering materials for structural components of airplanes with the same or with increased plasticity, increased strength properties, mainly fracture toughness, and improved corrosion properties compared to those of extent alloys. Intensive development gave important results in a relatively short time.

Lithium is the lightest metal, having density 540 kg/m³. Each mass % of lithium added to aluminum reduces the alloy density for 3% at simultaneous increase of elastic modulus for 6%. The AlLi alloys can be age-hardened, if other alloying elements, such as copper and magnesium, are added, this property is still more pronounced. During artificial ageing of solution heat treated alloys, metastabile Al₃Li particles are precipitated in aluminum supersaturated aluminum solid solution. Presence of these precipitates in aluminum matrix increases strength values to those, comparable with standard high-strength AlCuMg alloys.

Binary AlLi alloys, which are known for a long time, are not applicable because lower plasticity and toughness values. These disadvantages were improved by adding other alloying elements and by using corresponding thermomechanical treatments. The best properties were achieved by the combination of lithium, magnesium, copper, and zirconium, thus the AlLiMgCuZr system represented basis for all applicable alloys. Some of those alloys, containing up to 3 mass % Li, 1 to 3 mass % Cu, up to 2 mass % Mg and up to 0.2 mass % Zr, are standardized, and they are produced in great quantities. Tensile strength of those alloys is up to 500 MPa, elastic modulus 81 GPa, fracture toughness up to 40 MPam^1/2, and density 2500 kg/m³. Super plastic properties are an additional advantage of those alloys.

In industrial manufacturing these alloys, melting and casting are problematic due to high reactivity of lithium towards oxygen, moisture, and furnace lining. Thus closed systems with argon protective atmosphere are used in melting and casting. Semi-continuous casting process is used to make extrusion bars and slab ingots for rolling. Working of cast ingots by rolling, extrusion and forging is possible with usual working procedures.

Suitable strength/weight ratio, high elastic modulus, and other properties caused that the AlLiX alloys have been intended for manufacturing structural aircraft parts. Such substitution would reduce aircraft weight for 15% which represent e. g. in the Airbus A 340 airplane material savings of 4500 kg. Advantage of AlLiX alloys in comparison with the composite materials is in fact that existent equipment can be used for their manufacturing. But the predicted new generation of high-strength alloys of AlLiX type still haven’t succeeded to substitute standard materials of structural aircraft parts, regardless of high investments and extensive research. The main reasons are too high production costs and the properties of alloys which did not reach satisfactory level in some essential fields. These fields are corrosion resistance, thermal stability, and weld ability which represent basis of further investigations.

Alcan Aerospace has been very active in the past in developing low density AI-Li third generation of alloys, in a collaborative effort with airframers. AA 2050 is one such alloy which has received commercial interest for its behavior in the medium gauge range, where it outperforms reference alloys like 2024 or 2027, with significantly higher static, F&DT and corrosion performance, in addition to lower density and higher modulus. For higher gauges, AA 2050 also offers an interesting low density alternative to 7050.

One of the challenges to the extensive use of AI-Li alloys by airframers has traditionally been the extra cost per kilo saved offered by such alloys. This is the reason why work has been performed to decrease their buy-to-fly ratio, among other cost reduction activities. One way of improving this buy-to-fly ratio consists in replacing integrally machined items by assembled parts, each being adjusted to the local thickness needs. When developing such concepts, one has to consider using as much as practical a low cost assembly technique, like what is offered by friction stir welding instead of standard riveting. In an attempt to answer such a cost challenge, Alcan Aerospace have performed friction stir welding trials of 17m long structural parts made of 2050 alloy, in collaboration with the Institute de Soudure of Metz, France.