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PRODUCT DATA of 01: Aluminium and its Alloys
Material01: Aluminium and its Alloys
General InformationAluminium alloys are some of the basic building materials of existing spacecraft and appear in many subsystems. Only a few specific points of special interest for the spacecraft designer are considered here, since the general aspects of aluminium alloy assemblies are already well known in the similar field of aeronautical design.
Main CategoriesA large number of commercial, wrought and cast alloys are available. A similarly large number of mechanical and thermal tempers are used to optimize certain properties, often at the expense of others (e.g. higher strength, but poorer corrosion resistance). Not all of these alloys or tempers are suitable for aerospace engineering, from the point of view of either mechanical performance or environmental resistance. Many product forms are available: foil, sheet, plate, profiles, sections and casting stock.

Many aluminium alloys exhibit excellent corrosion resistance in all standard tempers. However, the higher-strength alloys, which are of primary interest in aerospace applications, shall be used with caution. In structural applications preference should be given to alloys, heat treatments and coatings which minimize susceptibility to general corrosion, pitting, intergranular and stress corrosion cracking. Some alloys are clad with thin layers of pure aluminium to improve corrosion performance.

Processing and AssemblyAll classical methods find a use: shaping and forming processes (for example, wrought products produced by rolling, extrusion, forging and cast products) and joining by, for example, welding, brazing, riveting, bolting or adhesive bonding.

NOTE: Not all alloys are weldable. Most high-strength alloys cannot be brazed.

a. Space use does not raise special problems in this respect; except that processes shall be extremely reliable. Aircraft industry standards are normally followed.

b. Processing of metals gives rise to residual stresses that can cumulatively reach design-stress levels, particularly as regards fatigue phenomena. Such stresses shall be checked.

PrecautionsThe properties of aluminium alloys are strongly dependent on their previous thermal and mechanical history. This point should be taken into account in specifications and checked after processing. Brittle intermetallic compounds can form by diffusion during thermal operations (heat-treatment, welding). They can be avoided by correct choice of alloy, heat-treatments used and by suitable thermal conditions during joining operations. International or national aerospace specifications for the heat treatment of aluminium alloys are used. Residual stresses from processing (forming and heat-treatments), machining, assembly (improper tolerances during fit-up, over-torquing, press-fits, high-interference fasteners and welding), operational use, storage and transportation need evaluation to ensure that the as-designed stresses are not exceeded.

Cumulative residual stresses also have an important influence on stress corrosion resistance.

a) Corrosion shall be considered during the whole manufacture and prelaunch phase; electrolytic couples should be avoided and all metals should be suitably protected against external damage by the use of plating, conversion coatings, paints and strippable coatings. This is particularly important in special operating environments (fuel tanks for example).

b). The metallic components proposed for use inmost spacecraft shall be screened to prevent failures resulting from SCC.
Such metal-alloy selection shall in particular be applied during the design phases of all spacecraft making use of the Space Shuttle, items intended for long-term storage prior to launch, highly stresses structures, all parts used or associated with the fabrication of, for example, launch vehicles.

c) Stress corrosion cracking (SCC), defined as the combined action of sustained tensile stress and corrosion, can cause premature failure of aluminium alloys. Because metallurgical processing of aluminium alloys usually results in a pronounced elongation of grains, the variation of susceptibility with grain orientation is more extensive than for other metals (see ECSS-Q-ST-70-36). Also, because conventional processes are designed to optimize strength, residual stresses - especially in thick sections - are usually greater in aluminium products than inwrought forms of other metals. Both the residual stress distribution and the grain orientation shall be carefully considered in designing a part to be machined from wrought aluminium. Consequently, wrought heat-treatable aluminium products specified for use in the fabrication of hardware should be mechanically stress-relieved (TX5X or TX5XX temper designations) whenever possible. SCC ratings were attributed to the major aircraft alloys; these are based on service experience and testing programmes.

NOTE: See ECSS-Q-ST-70-36 and NASA-MSFC-STD-3029.

d) Three ratings of alloys were chosen: high-resistance, moderate-resistance and low-resistance to SCC (these are listed in Tables 1, 2 and 3 respectively of ECSS-Q-ST-70-36C). The alloys listed in Table 1 should be used for space applications. For alloys listed in Table 2 or 3 a detailed justification for space use shall be provided, demonstrating that SCC testing according to the standard method detailed in ECSS-Q-ST-70-37 has taken place. (Method incorporates constant load and alternate immersion in 3,5% NaCl solution).

NOTE: All of the Al--Li alloys known at present are very sensitive to SCC (Table 3).

e). Machining and assembly methods can leave residues of chemicals (particularly cutting oils and dye penetrants). Methods of cleaning shall be applied and design shall prevent inaccessible “contaminant traps”.

Hazardous and PrecludedCertain alloys and tempers are unsuitable for structural applications in long-term, manned structures, such as the International Space Station (ISS). Some 5000-series alloys and tempers are limited to a maximum use temperature of 66 ºC in ISS. Some 5000-series alloys with a high magnesium content need specific tempers to provide resistance to stress corrosion cracking and exfoliation.

a) Porous platings (corrosion protection) and aluminized layers shall not be used, because they fail to provide adequate protection and can act as sources for contamination film).

b) Electrolytic couples shall be avoided or corrected by a suitable insulation between the metals concerned.

c). Bare metal-to-metal contact shall be avoided in any moveable part.
Effects of Space environmentIn general, metals do not suffer from space-environment conditions. - Vacuum does not affect aluminium alloys. All metals in contact under vacuum conditions or in inert gas have a tendency to cold weld. This phenomenon is enhanced by mechanical rubbing or any other process which can remove oxide layers.

- Radiation at the level existing in space does not modify the properties of metals.

- Temperature problems are analogous to those encountered in technologies other than space, except for a complication arising from the difficulty of achieving good thermal contact in vacuum and due to the absence of any convective cooling. Aluminium alloys with magnesium contents greater than 3% should not be used for applications where temperatures can exceed 66 ºC.

- Atomic oxygen in low Earth orbit (LEO) does not degrade aluminium alloys.
Some Representative ProductsThere are many European manufacturers of conventional aluminium and its alloys. Procurement to internationally recognized specifications is preferred, such as ISO, MIL Specs, B.S., SAE, DIN or AFNOR specifications. The materials listed in Table A-1 (from ECSS-Q-ST-70-36), can be considered.