FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1953
1953 - 0177.PDF
6 February 1953 TABLE IV: RELATIVE COST ANALYSIS—TANK SUPPORT FITTING* Item Weight of rough forging Material cost Forging Machining Total cost of finished part Weight saved Cost/lb weight saved 8630 steel 23.5 lb £2.13 £0.67 £20.60 £23.40 — Titanium alloy RC130B 13.8 1b £87.0 £88.2 £355.0 £530.2 5.9 1b £80.0 * Conversion of dollar costs at rate of $2.82 to £1 sterling. stock was deformed with more difficulty than would be experienced in forging alloy steel, therefore heavy, and consequently expensive, plant was desirable. Finally, subsequent to forging a prolonged anneal at 650-700 deg C was beneficial. A warning should, however, be given that some of the two-phase alloys, even after such a treatment, were unstable and subsequently underwent embrittlement if used at tempera tures in the range 250-450 deg C. While these facts indicated the use of the lowest possible practical temperature for forging, there was an additional reason for avoiding high temperatures. With the titanium alloys now used for forgings (mainly 2.7 Cr, 1.3 Fe, 0.25 O2, etc.) grain growth took place rapidly at high temperatures. This would influence adversely the subsequent room temperature properties of the alloy as regards ductility, impact resistance and fatigue. Finally, above 1,150 deg C the oxygen content of those alloys whose mechanical properties depended on the presence of this gas within the material tended to decrease, with consequent subsequent decrease in static mechanical properties at room temperature. Table V emphasized how important, yet varying, was the influence of machining on the cost of an item. With a simple form such as a rotor disc, the price differential between steel and titanium alloy was rela tively low, but with a part of greater complexity, such as a turbine support, the gap broadened markedly. Two observations had to be made about the significance of Tables IV and V. The figures recorded were based, not on theory or on—what was often even more misleading —salesmen's talk; they were the result of actual experiment by intelligent people well versed in the manufacturing techniques employed for the making of such parts in the conventional materials. The cost figures for these processes in relation to the titanium alloy arose from the use of established techniques applied to a material with which experience was all bu non-existent. Almost certainly, both forging and machining costs would be greatly reduced but, at the present state of the art—at least in relation to the machining costs—there were wide differences of opinion as to the best ways of doing so. The machining of titanium and of titanium alloy could be compared with that of an 18:8 austenitic steel, but it was much more costly. A sales organization not unusually addicted to the saying of acceptable things had stated that, on the average, the machining of titanium alloy cost about three and a half times more than that of austenitic steel. At present no machine-shop foreman would fail to imply that this was a considerable understatement. Maj. Teed said that he had now made a somewhat superficial examin ation of the titanium alloys. On the basis of the mechanical properties in Table III, possibilities of weight saving were suggested. In those applications in which the design criterion was specific proof stress (as could be the case with some massive forgings), reductions in weight would seem to be obtainable, provided that an alloy having a proof stress in excess of 45 tons/sq in was selected. In view of this possibility, the airframe and engine designer would wish to have further particulars as to such mechanical properties as fatigue, notch fatigue, impact, creep, damping, etc. Before dealing with these aspects, two general observations of significance had to be made. Titanium and the present titanium alloys had quite deplorable rub bing properties: the metal was inclined to pick up any other on which it was rubbing. At least for the present, this cold-welding tendency debarred the use of the alloy for shafts unless they had a sleeve to provide a rubbing surface in the journals or ball or roller races were fitted. An internal-combustion engine piston in titanium, attractive though it seemed from several points of view, was extremely unlikely to be successful for the present. It was even doubtful if a titanium- alloy nut could be satisfactorily used in connection with a bolt of similar composition. At this state the engineer might feel that the shadowy case for the use of titanium alloys which had been disclosed had been destroyed by the last admission. There might be grounds for pessimism, but the author looked upon this difficulty as likely to be of a temporary nature. Titanium and its alloys could be given a hard surface by means of anodizing. If such a coating would not stop galling, or had too ephemeral an existence, then gas case-hardening (by heating either in oxygen or nitrogen) could be employed. Failing this, a hard surface could be produced by carburizing. These were matters for experiment, and the experiments were in hand. Under conditions in which questions of contact corrosion were unlikely to arise, a flash coat of copper might be obtained by dipping in a copper cyanide bath, and this might prevent "pick-up." Finally, a chemical treatment, giving rise to an adherent film, could be used satisfactorily in conjunction with molybdenum disulphide, subsequently used as a solid lubricant. Measured as fatigue generally had been (and still often was) measured —namely, by the application of cyclic stresses to highly polished speci mens in the form of fixed rotating beams, or as ties or columns—titanium and its alloys made by fusion methods had excellent fatigue properties. Unlike practically all the metals of the non-ferrous group, titanium had a true fatigue limit. Results obtained with the types of test-pieces enumerated showed considerable scatter, as was always the case in fatigue determinations, but it was undoubtedly true to say that the ratio of the fatigue resistance of the metal and of its alloys to their 175 respective tensile strengths exceeded, and sometimes greatly exceeded, one half. Thus the fatigue resistance of this group, in the form of the very unrealistic test-pieces already mentioned, was better than that of alloy steels. When, however, notched fatigue test-pieces were used, this was no longer the case. Using test-pieces with and without a severe notch, Hanink found that while the notched to unnotched endurance ratio for his alloy steel specimens, tested under the same conditions, was 15 per cent, for identical titanium ones made of the 2.7 Cr, 1.3 Fe, 0.25 O2 alloy the figure was only one-fifth this amount. An S/N curve of the Remington Arms Company, in tests with a less severe notch (stress concentration factor 2.7) and employing the commercially pure alloy, showed the to7 cycle endurance to be only 50 per cent of that of the unhotched specimens. These disturbing results had received confirmation from other experiments, but the position was not entirely without hope. It did seem possible that, by control of initial grain size and by close attention to metallographic structure, better resistance might be obtainable when titanium alloys were subjected to tri-axial cyclic stresses. Nevertheless, ct present it had to be admitted that the notched fatigue endurance of such strong titanium alloys as had been tested was poor to very poor. One further point should be mentioned about the fatigue character istics of the alloys. Hanink stated that the application to them of the well-known Almen shot-peening technique did not improve their fatigue limit; it considerably reduced it. While his careful experiments supported this contention, it was possible that an improvement might be obtained by a modification of the pcening technique. This view was based on the behaviour of magnesium (also a hexagonal metal) for, when shot-peening was first applied to its alloys, their fatigue resistance was definitely lowered thereby. Now, however, by a modification of the method, Found had demonstrated that his system of peening did in fact raise their fatigue resistance. Because of the variety of methods which had so far been used for the production of titanium, considerable differences had existed in the properties of alloys of the same chemical composition. These differences were probably more marked in relation to notched bar impact-resistance than with any other quality. So far, the influence of trace elements, grain size and heat treatment —all very potent in relation to the impact resistance of the ordinary engineering materials—had received insufficient study in the case of the titanium alloys. It was known that, like the ferritic steels, they were extremely sensitive to temperature. At 300-400 deg C, an Izod impact value of ioo ft lb or more could be obtained. At such a temperature the fracture would be ductile, but at a lower one, which would certainly be above room temperature, it would be wholly brittle. This change in the type of fracture was accompanied by a sharp decrease in impact resis tance, but at room temperature it was probable that the strong titanium alloys, when their chemical compositions and methods of manufacture had become standardized, would possess notched bar impact-values better than those of the current strong wrought aluminium alloys used in airframe structures. These values were likely to be comparable with those of the stronger alloy steels. TABLE V : COST AND WEIGHT ANALYSIS OF STEEL AND TITANIUM ALLOY GAS-TURBINE PARTS Item Compressor rotor disc Compressor stator blade Turbine support... Compressor rotor blade Material and approx. weight of finished part in steel and in titanium alloy (lb) Steel AMS 6342 25.0 Titanium alloy 14.0 Steel AMS 5613 0,06 Titanium alloy 0.04 Steel AMS 6322 3.0 Titanium alloy 2.0 Steel AMS 5613 0.10 Titanium alloy 0.06 Approx. cost of weight-saving, per lb £21 £62 £69 £84 It had been mentioned that commercially pure titanium could be readily argon- or hcli-arc welded, but nothing had yet been said on the use of this method for joining the strong alloys. Before doing this, one point must be made as to welding the commercial-grade metal, for it had a practical significance and might, on occasion, limit the use of the method. From time to time, emphasis had been placed on the absorption of oxygen and nitrogen when the metal was above about 800 deg C. When its gas-contentTose, so did its strength, but on the other hand its ductility decreased. It was because of this that the inert-gas type of welding gave the best results. If, however, a weld as strong and as ductile as the parent metal was required, contact of oxygen and nitrogen with the molten and neighbouring hot titanium must be prevented. The standard welding torch did this on the working face, but in the welding of sheet or plate an inert-gas atmosphere must be provided over the hot area on the reverse side of the job. The position from the metallurgical point of view in the welding of the strong alloys was too complicated to explain in the course of a review of this type, but a lucid description of the present state of the art could be obtained from a recent paper by Voldrich. Roughly speaking, all titanium alloys could be argon- or heli-arc welded, but while with those which had what the metallurgist termed a two-phase structure, the tensile strength would be satisfactory, ductility would be generally extremely low. This might be increased in some cases by subsequent heat-treatment. Brown, at present exceptionally, contended that welds made in two- phase titanium alloys by flash welding possessed good ductility. This technique was particularly adapted to repetition work and might well find aircraft engine applications. Should there be a demand for a weldable sheet with a 0.2 per cent
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
