AIRCRAFT MATERIALS AND PROCESSES BY GEORGE F. TITTERTON.pdf

AIRCRAFT MAIBRIALS AND PROCESSES

Aircraft Materials and Processes FIFTH EDITION

GEORGE F. TITTERTON Assistant Chief Engineer, Grumman Aircraft Engineering Corporation Fomzerly Faculty Lecturer, Graduate Division, College of Eng ineering, New York University

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HlMALA YAN 800l(S

Price Rs. 225 Published by HIMALAYAN BOOKS New Delhi- 110 013 (India) Distributed by THE ENGLISH BOOK STORE 17-L, Connaught circus New Delhi- 110 001 Tel. : 2341 -7126, 2341-5031 Fax: 2341-7731 E-mail: [email protected]

© George F. Titterton, 1968 Indian Reprint 2013 2015 By arrangement with Pitman Publishing Corporation All rights reserved; no part of this publication may be reproduced, stored in my retrieval system, or transmitted in any form or by any means, ectronic, mechanical, photocopying, recording or otherwise, without the written permission ofthe publishers. Printed at Thakur Enterprises, Delhi

PREFACE TO THE FIRST EDITION The author's purpose in writing this book was to present in one coordinated volume the essential information on materials and processes used in the construction of aircraft. Unimportant details have been purposely omitted m the interest of brevity and readability. Within the aircraft field this volyme is rather general in scope and should meet the needs of students, engineers, a\1

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AIRCR/\Fl. MATER IALS AND PROCESSES

phosphorus and sulfur content 10 less than that lister!, but in other respects the specificati ons are about the same.

CARBON STEELS

S.A.E. 1015. A ga lvanized (zinc-coated) steel wire is made from thi s material. It is used as a loc king wire on nuts and turn-buckles and for serving nonflexible cable splices. This wire has a maximum tensile strength of 75,000 p.s.i. and a minimum elongati on of 8 to I0%. S.A.E. 1020. This steel is used for casehardened parts. In this form it is often used for bushings that must resist abrasion . It is also employed in the fabrication of stampin g dies that require a hard, wear-resisting surface. When casehardened, this steel has a core strength of 60,000 p.s.i. and good ductility. In its normal state it has an ultimate tensile strength of 55,000 p.s.i. , a yield strength of 36,000 p.s.i., and an e longati on of 22%. This steel machines well. It can be brazed or welded. S.A.E. 1025. This steel is commonly referred to as mild carbon steel or cold-rolled stock. For aircraft purposes the sheet is always purchased cold - rolled to accurate dimensions. Bar stock is either cold rolled or cold draw n. For most purposes this steel has been superseded by chrome-molybdenum steel, S.A.E. 4130. It is still used for aircraft nuts and similar standard parts, however, and also for nonstructural clamps requiring a lot of bending. In all its forms this steel has an ultimate tensile strength of 55,000 p.s.i., a yield strength of 36,000 p.s.i., and an elongation of 22%. When used for aircraft nuts, it is heat-treated and develops a minimum strength of70,000 p.s.i. In sheet form this material can be bent through 180° without cracking over a diameter equal to the thickness of the test section. The same thing can be done with bar stock over a diameter equal to twice the thickness of the test s~ction. I l,;t /

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f1.~uR~ 26: Hi&!1-.t~~P~-the electrolysis of a nhydrous magnesium chloride. The rirst procluct-i(in ·'OJ: mag nesium in this country on a commercial basis heg,rn in I9 l4. fl 1crc arc three basic methods used in this country at the present time 'l.'nr th..: ,:ed uction of magnes ium fro m its source. These are the e lectrol ytic r rO.Je:,,s~.fhe lcrrosil icon process (Pidgeon); and the carbothermic •\ k process (Hang..-:'1prgJ'. The e lecfr~ti-yti

AM-C57S AM-C58S AM-C58S-T5

QQ-M-40

AM3S AM65S AM-C52S

16.000 Press 18.000 "Press 16.000 Press 16.000, Press Hammer or press 10.000 Hammer Hammer or press

Letter A after alloy means forged and aged: letters HTA mean heat-treated and aged after forging: -T5 after alloy means forged and aged.

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TABLE 19. Magnesium-alloy Sheet, Plate, Strip-Mechanical PropcrLies Federal

Snecification American Magnesium

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32,000 39,000 28,000 32.000

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Brinell hardness (500 kg./ 10 mm.)

16,000 26,000 12,000 20,000

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Fatigue. 500 X 101' cycles (p.s. i.)

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QQ-M-44 QQ-M-44 QQ-M-54 QQ-M-54

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Letter a or O after alloy means annealed ; letter h or 1-1 means hard rolled.

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MAGNESIUM ALLOYS

FIGURE 53.

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Assembly of Magnesium SNJ-2 Wings

Sheet, Plate, Strip. Three magnesium alloys are available in the form of sheet, plate, or strip stock. Each alloy is available in the annealed, as-rolled, or hard-rolled condition. The as-rolled condition is seldom specified. Sheet is material unde r 0.25 inch thick; plate is 0.25 inch or thicker; strip is material up to 8 inches in width and up to 0.125 inch thick. Strip may be coiled or as-sheared from sheet. Sheet is available in thickness from 0.016 inch up. It can be obtained in lengths up to 144 inches and widths up to 48 inches. Strip is available in thickness from 0.016 to 0.051 inch in coils up to 125 feet long. Due to the poor cold-working properties of magnesium alloys, sheets cannot be flattened by stretcher leveling. Rupture occurs in this process before the sheets are sufficiently stretched to lie flat. Sheet stock is flatlened by placing it on a flat cast-iron surface and then superimposing additional castiron sheets to attain 300-450 p.s.i. pressure on the magnesium-alloy sheets. This assembly is then placed in a furnace. Annealed sheets require heating to 700°F. and cooling to 300°F., all under pressure; hard-rolled sheets require heating to 400°F. for QQ-M-54 alloy, and to 275°F. for QQ-M-44 alloys. Magnesium alloy sheet can be drawn, spun, formed, and welded-either arc, gas or spot. Many of these operations have to be done at elevated temperatures because of the poor cold-forming characteristics of these alloys. These operations are described in detail later in this chapter.

216

AIRCRAFT MATERIALS AND PROCESSES

T he mec hanical properti es of magnesium-all oy sheet, plate, and s trip arc given in T able 19. Other properties are as fo llows: QQ-M-44. Annealed sheet has the best cold formability but limited ga~ and arc weldability. I-lard-rolled sheet has the best combination 01· fatigue and shear strengt h as well as toughness and low notch sensiti vity. QQ-M-54. Annealed sheet has the best gas weldability and hot formabil ity. I! is a low-cost alloy of moderate strength. Hard rolled sheet has the best resistance to creep al elevated temperatures but is seldom used.

Magnesium-alloy s heet is used in the construction of oil and fue l tanks, ducts, fairi ngs, wing tips, flaps, ailerons, s tabilizers, rudde rs, e xperimental wings, and other structural applications.

SHOP FABRICATION PROCESSES The fabrication of magnesium alloys into finis hed articles may involve any number of the standard shop processes. Magnesium alloys can be machined, sheared, blanked, punched, routed, and formed by bending, drawing, s pinning, pressing, or s tretching. When these processes are applied to mag nesium alloys the technique required differs somewhat from that used with other materials. The application of these processes to m agnesium alloys will be described in the fo llowing pages. Machining. Magnesium alloys have excellent machi nin g characteristics. A smooth finish is obtained at -extremely low cost. Surface grinding is seldom necessary. Machining can usually be done at the attainable speed of the machine. Light, med ium, or heavy feeds can be used and the free cutting action of the material will produce well-broken chips which will not obstruct the cutting tool or machine. The power required for a given machining operation on magnesium all oys is approximately one-half t hat required for aluminum alloys and one-sixth that required for steel. To take full advantage of the excellent machining qualities of magnesium, the mac hine equipment mus t permit operation at high speeds and feeds ; sharp cutting tools of the correct design are necessary, and the part being machined must be rigidly supported. Due to lower culling resistance, lower specific heat; lower modulus of elasticity, and the chemical properties of magnesium alloys, there are some essential differences in machining practice when compared with other metals. These differences may be s ummarized as follows: 1. Cutli ng edges must be kept sharp and tool faces polished to insure free culling action and reduce the adherence of magnesium particles to the tool tip. Tools must be designed to allow for ample chip room, and tool clearances should be IO to 15°. Large feeds are advanlageous in reducing the frictional heat. ·

MAGNESIUM ALLOYS

217

2. Ir the precaution~ of paragraph I arc not taken· 1he magnesium part being machined may distort, owing 10 excessive heat. This distortion is most likely to happen on thin sections, in whid1 the heat will cause a large rise in temperature. Parts which tend 10 distort during machining can be stress-relieved by he;uing al 500°F., fo'r 2 hours. If the part is stored for 2 or 3 days prior to finish-machining. the same result is auained. 3. Magnesium cuts closer lo size than aluminum or steel. Reamers should be specified several ten-thousandths oversize compared to those used on other metals; laps· should be specified from several ten-thousandths lO two thousandths oversize depending on the diameter. 4. Because of its lower modulus of elasticity, magnesium will spring more easily than aluminum\ or steel. Cohsequently il must be firmly chucked but the clamping pressure· must not be great enough to cause distortion. Particular attention must be paid to light parts, which can easily be distorted by chucking or by heavy cuts. S. A cutting fluid* is used in reami ng and in screw-machine work or when cutting speeds exceed 550 feet per minute. The cutting fluid is primarily a coolant. In all other operations magnesium can be machined dry with good results. 6. In grinding, a liquid coolant* should be used or the grinding dust should be exhausted and precipitated in water. * Cutting fluids or coolams conta ining water should 1101 be used without special precautions. Advice on machining practices can be obtained without charge from magnesium producers and fabricators. Cutting tools designed for use with steel or brass can be used on magnesium but they must have a sharp cutting edge and good clearance. The basic principle in all cutting tools for magnesium alloys is to limit the friction to avoid the gene_ration of heat. and possible fire hazard. Carbon-s teel tool s can be used for reamers, drills, and taps, but high-speed steel is preferred and is mos t generally used. High-s peed steel is also used for other types of cutting tools for magnesium , hut cemented carbide tools have a much lo nger life and should be employed wherever possible. Turning, s haping, and planing tool s should be similar Lo those used for brass. Coarse-tooth milling cutters should be used, because the heavier cut obtained causes less frictional heat and consequent di stortion . Ordinary twist drills and spiral reamers with about 6° relief behind the cutting edge give satisfactory results . Threading is readily done by means of taps , dies, or lathe turning. Roll threading is not satisfac tory because it involves excessive cold working of the metal. Depths of tapped holes should be 2 o r 3 Limes the diame ter of the stud. Magnesium-alloy threaded parts will no t seize when mated with other common metals or even with parts made from the same composition alloy. Band or cirrnlar saws for cutting magnesium alloys should have from 4 to 7 teeth per inch and must be very sharp. Hanel hacksaw blades should have 14 teeth per inch. Single-cut liles arc preferable for use with magnes ium alloys.

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218

AIRCRAFT MATERIALS AND PROCESSES

Courtt sy ofAmuicon mag11csium corporation

FIGURE 54 . Hot Forming Magnesium Shee1-Gas Heating Dies

Precautions must be taken to reduce the fire hazard when machining mag nesium alloys. Cutting tools must be sharp, and machines and floor must be kept clean. Scrap should be kept in covered metal containers. Lubricants should be used for automati c-machine work or when fine cuts are being made at high cutting speeds, to minimize_the frictional heal. There is no serious danger from fire if care is exercised by the operator. Shearing. In shearing magnes ium sheet a rough, flaky fractu re is obtained if the proper equipment is not used. T he clearance between shearing blades should be o n the order of 0.003 inch, and the upper shear blade s hould have a rake a ngle of around 45°. The sheared edge may be improved by a double shearin g operation known as "shaving:· 111is consists of removing an additi onal 1/32 to 1/1 6 inch by a s~concl shearing. The maximum thicknesses recommended

MAGNESIUM ALLOYS

219

for cold shearing are 0.064 for hard-rolled sheet a nd 1/s inch for annealed sheet. These thicknesses can be increased if shearing is done at an elevated temperature, but in any case sawi ng should be resorted to for cutting plate. Blanking and Punching. These operations are practically the same as those used for other metals. A minimum clearance between the punch and the die is essential to obtain 'max imum edge smoothness. This clearance should not.exceed .5% of the thickness of magnesium being worked_. The punch and die are frequently made of materials of unequal hardness, so a sheared-in fit providing minimum clearance can be obtained. Magnesium alloys can be punched an9 blanked at room temperatures but better res ults are obtainable at elevated temperatw·es. Routing. Routing magnesium a lloys is a simple, straightforward operation. Dry routing can be done with little fire hazard if the router bit is sharp and the chips are thrown free. A low-viscosity mineral-oi l coolant is frequently used as insurance against fire. Router bits of the single or double-tlule type with polished flutes to provide good chip removal are used. Spiral-flute routers pull the chips from the work and have less tendency to load up. Forming Magnesium Alloys. Magnesium-alloy sheet and extru~ns, including tubing, can be processed with the same type of equipment used for other metals. One major difference is the necessity for heating the too ls and the work since many of the formini operations must be ~one at elevated temperatures because of the close-packed hexagonal c rystal structure of magnesium alloys. This crystal structure severely limits the amount of work that-can be done at room temperatures without inducing a shear failure. At around 400°F. recrystallization occurs with .a resultant decrease in tensile strength and ~ncrease in ductility. Al about 440°F. a second set of cystallographic slip planes comes into action, with marked increase in capacity for plastic flow. A's the temperature is further increased the ductility also increases and may reach a point as-much as nine.times the ductility at room temperatures. The recommended forming- or working-temperature ranges are given in Table 20:In addition, the minimum bend radii are given for room temperature and for the recommended working-temperature range. It will be noted that the working-temperature range for hard-rolled parts is lower than for annealed material. Hard-rolled parts are stronger because of the cold working th