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Aviation History
1972
1972 - 1969.PDF
154 SUPERPLASTICITY actual metal components by superplastic forming methods. Prestal, a 78Zn/22Al alloy with an m value of 0-5 and superplastic in the temperature range of 240°CL270°C, is one. In the aircraft field, some consideration has been given to Prestal, but in this area the greatest interest centres on the possible application of the titanium alloy IMI 318 (6A1/4V). This material is superplastic over the temperature range 925°C-950°C and is in its normal, manufactured, form a fine-grain alloy. This material is being increasingly used in the aircraft industry and, although difficult to work by normal or conventional methods, is simpler to process in the superplastic condition, where problems such as springback in forming do not arise. Considerable effort is being devoted to the development of materials with superplastic processing in view, including alloys in the stainless steel as well as zinc-aluminium ranges. One characteristic of superplastic forming is that the maximum effect is obtained at rather slow strain rates, of the order of 1 per cent/sec. For some commercial appli cations, this characteristic could be an obstacle to applica tion of the process and, here again, studies are in progress to achieve higher strain rates by further refinement of grain structure. Processing Direct application of tensile force is comparatively little used in conventional metal-forming processes—stretch- forming, as practised in the aircraft industry, is one of the exceptions, though hardly applicable to superplastic form ing. It is interesting to note, however, that the forming of aircraft spars superplastically has been considered, using a controlled stretch or strain to produce the inboard to outboard taper. Because of the extreme ductility of materials in the superplastic condition, however—in which they are capable, literally, of draping themselves over a die—it is possible for such materials to be manipulated into complex shapes, with sharp definition, by the applica tion of quite low forces. Superplastic deformation is closely similar to the form ing of thermoplastic sheet and it is possible to adopt similar techniques. Vacuum forming and blow forming of materials in the superplastic state are both practicable techniques and, as in plastic forming, the surface finish obtained on the formed part is that of the mould or die. Several circumstances related to superplasticity indicate the possibility of using materials of this kind in airframe manufacture. One is the (quite fortuitous) fact that IMI 318 titanium, as already mentioned, is by its normal constitu- R/GHT International, 3 August 1972 Main tool Top, a trial window shroud and blind box for Concorde, blow-formed in Prestal alloy, 0-030in thick. Above, a semi-diagrammatic arrangement of a die assembly for blow-forming sheet metal in the superplastic condition tion a fine-grain alloy particularly suited to superplastic processing. Also, the (at present) relatively slow strain rates characteristic of superplastic forming do not, as they could in a mass-production industry, constitute a barrier to its use as a practical production process—provided that it can offer benefits in terms of simplicity, weight-saving or cost by comparison with more traditional methods. Applications The Filton factory of BAC's Commercial Aircraft Division has carried out a considerable amount of experi mental work on superplastic forming, using both Prestal and IMI 318 with a view to both structural and non structural applications. One non-structural part, the passenger-amenities panel for Concorde, shown in an accompanying illustration, was produced as a result of the FAA embargo on smoke- emissive plastic materials. These panels are located over the seats in the passenger cabin and carry the oxygen A trial air-distributor duct for Concorde, blow-formed in Prestal alloy, 0-030in thick
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