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Aviation History
1952
1952 - 1771.PDF
27 June 1952 769 The overall effect of this technique is very complex and lengthy to describe, but it can be said that in the U.S.A. it has revolu tionized the whole picture of machining all hard alloys. In the case of titanium, the hardest alloys can be cut, through the scale, at over 30oft/min to a depth of ^in with a 0.015m feed. Again, the swarf is non-oxidized and free from any contamination; thus, complete recovery can be effected. Many further advantages in the fields of temperature-control, cleanliness, tool-life and tool- sharpening also accrue from CO2 cooling. It is worth stressing that this technique can ensure that all parts of the cutting zone are maintained at room temperature—by varying the coolant flow and direction—so eliminating burnishing, stabilizing dimensions and metallic micro-structure and greatly prolonging tool-life (one U.S. titanium-machining plant has reported an extension in tool life of 450 per cent). Sawing.—Using CO2, speeds of 4ooft/min are typical, but with cutting oils this value should be set at some isoft/min, using a high-speed steel blade of about ten teeth per inch. Drilling and tapping.—Again, this type of work is hampered by the tendency of the metal to work-harden and chip-weld, and by the tough non-curling swarf and poor thermal conductivity. Work-hardening is minimized by drilling continuously, without pilot holes, and by using high-cobalt drills. Off-centre points have been experimented with, in order to form an eccentric hole only touched by the drill along one side at any time, notched lips being used to break the swarf. Carbon-dioxide appears to be the best coolant and a good included point-angle is 135 deg. This arrangment can be used to drill a i7/64thsin hole at 450 r.p.m. (3ift/min) at o.oo6in/rev; up to 9oin should be possible between re-grinds. Reaming can be carried out with straight- or spiral-flute tools at normal speeds; a taper reamer, or one with excessive lead-angle, is preferable if chatter is to be avoided. Tapping at 12 to i5ft/min can be performed on 60 per cent thread using interrupted-tooth taps. Grinding.—Probably the most difficult operation of all, grinding is at present considered "good" if wheel-wear is less than one-third of the metal-removal rate. Aluminium-oxide wheels of about size-60 grit are suitable, grinding being carried out wet at low speed—say, 2,5O0ft/min. Press-forming.—Most shapes capable of deep-drawing in low- carbon mild steel can be repeated in titanium; but press speeds need to be lower and pressures higher. The vexed cold-welding question appears to be beaten by a new lubricant which is now under development for application to the die surface; it is hoped to make this solution adherent up to 600 deg F. Welding and brazing.—If gas-absorption is avoided, good titanium-titanium joints present little difficulty. Argon-arc and Heli-arc equipment typically produces welds with parent- metal strength, albeit with slightly reduced ductility. Spot and seam resistance welding is an everyday operation in many American factories, and the metal-surface condition appears to have little bearing on the result. Very good results have been obtained from flash-butt welding high-strength alloys, although not from similar welds in unalloyed titanium. Cooling from welding temperature still results in some "quench- hardening," but hot-rolling can be employed to improve post-weld ductility. The best brazing appears to result from silver or aluminium alloy spelter using fluoride fluxes. Tube-drawing.—By the end of this year seamless tubing should be in production by normal methods. At present, only short lengths of seamless tube can be made, welded-and-drawn tubing being more common. This is available up to about 2in o.d., and can be bent and flared nearly as well as can similar stock in stain less steel. Riveting.—Unalloyed titanium wire has been successfully cold- headed down to $in shank diameter, a flash-coating of copper sometimes being used to overcome the galling tendency. Riveting appears to be preferable in the hot state; a phosphate coating is necessary where the metal is to be attached to aluminium or cadmium. Annealing.—Most titanium alloys require heating to about 1,300 deg F for 30-50 minutes per inch of thickness. Unless thin sections are involved, an inert atmosphere is unnecessary. Light blasting and pickling should suffice to remove any scale formed. Titanium for Aircraft Structures Kinetic heating is becoming of increasing importance as a problem to be faced in the structural design of fast aircraft. The indications are that titanium alloys will be of great benefit for such structures, since they maintain excellent mechanical properties up to the level of 500 deg F—although it may be stated that the strength-temperatute properties of such alloys do not yet appear to be uniformly consistent. This may be due to the very limited quantity of metal which has so far become available for experi ment, coupled with the present variation between test-pieces. Nevertheless, supersonic aircraft—and missiles—appear to call for a structure almost totally of titanium. At present, if temperature-demands eliminate the use of light alloys, the designer has no choice but to use steel—with consequent weight penalties. It is quite conceivable that, were sufficient material forthcoming, the ideal wing for the fastest aircraft would be of solid titanium alloy. It also appears that, provided titanium production is sufficient, nearly all the steel currently used in airframes can be replaced by titanium alloy. These new alloys have excellent corrosion- resistance—comparable with that of platinum—and can thus replace stainless steel in a number of applications where non- corrodibility is required. Similar changes to titanium may be made where high-tensile steel is now used purely from the point of view of strength; many titanium alloys can approximate to H.T. steel strength up to about 900 deg F. The most inviting aircraft applications of the material appear to be in the gas turbine power unit. Up to rather over 900 deg F no other range of alloys can offer so good a strength/weight ratio; 800 6OO 0500 2 0 ? Z z S S200 IOO 0 cA \° t J V B .-A ?> Fig. 2. (Left) Strength/weight ratio against temperature for: (A) titanium alloy RC1306 (4% Al. 4% Mn); (8) aluminium alloy 14ST6; (C) magnesium alloy AZBOX; (D) commercially pure titanium RC70; (£) a hardened and tempered stainless steel, type 410; (F) type 347 stainless steel. (American alloy designations. Ordinate units in inches (Ib/sq in divided by Ibjcu in), fig. 3. (Centre) Mechanical properties of titanium alloy TM50A (2.7% Cr, 1.3% Fe, 0.25% 0, 0.02% C, 0.02% N, balance Ti). Fig. 4. (Right) Mechanical properties of titanium alloy RC130B (4% Mn, 4% Al). 200 400 6OO . 8OO TEMPERATURE (dtgF) 200 400 6CO 8OO 1pOO TEMPERATURE (dcq F) ZOO 400 600 BOO I.OOO TEMPERATURE (ocq P)
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