ranging up to approximately 1 percent, the product is vastly superior to iron alone and is classified as carbon steel. Carbon steel forms the base of those alloy steels produced by combining carbon steel with other elements known to improve the properties of steel. A base metal (such as iron) to which small quantities of other metals have been added is called an alloy. The addition of other metals changes or improves the chemical or physical properties of the base metal for a particular use.
Nomenclature and Chemical Compositions of Steels In order to facilitate the discussion of steels, some familiarity with their nomenclature is desirable. A numerical index, sponsored by the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI), is used to identify the chemical compositions of the structural steels.
In this system, a four-numeral series is used to designate the plain carbon and alloy steels; five numerals are used to designate certain types of alloy steels. The first two digits indicate the type of steel, the second digit also generally (but not always) gives the approximate amount of the major alloying element and the last two (or three) digits are intended to indicate the approximate middle of the carbon range. However, a deviation from the rule of indicating the carbon range is sometimes necessary.
Small quantities of certain elements are present in alloy steels that are not specified as required.
These elements are considered as incidental and may be present to the maximum amounts as follows: copper, 0.35 percent; nickel, 0.25 percent;
chromium, 0.20 percent; molybdenum, 0.06 per- The list of standard steels is altered from time to time to accommodate steels of proven merit and to provide for changes in the metallurgical and engineering requirements of industry. Refer to the numerical index table in figure 6-62.
Metal stock is manufactured in several forms and shapes, including sheets, bars, rods, tubings, extrusions, forgings, and castings. Sheet metal is of an inch. Bars and rods are supplied in a variety of shapes, such as round, square, rectangular, hexagonal, and octagonal. Tubing can be obtained in round, oval, rectangular, or streamlined shapes.
The size of tubing is generally specified by outside diameter and wall thickness.
The sheet metal is usually formed cold in such machines as presses, bending brakes, drawbenches, Series designation Types 100xx - Nonsulphurized carbon stects Resulphurized carbon steels (free machining) 12xx Rephosphorized and resulphurized carbon steels (free machining) 13xx Manganese 1.75% #23xx Nickel 3.50% *25xx Nickel 5.00% 31xx Nickel 1,25%, chromium 0.65% 33xx Nickel 3.50%, chromium 1.55% 40xx Molybdenum 0.20 or 0.25% 41xx Chromium 0.50 or 0.95%, molybdenum 0.12 or 0.20% 43xx Nickel 1.80%, chromium 0.50 or 0.80%, molybdenum 0.25% 44xx - Molybdenum 0.40% 45xx - Molyhdenum 0.52% 46xx - Nickel 1.80%, molybdenum 0.25% 47xx - Nickel 1.05%, chromium 0.45%, molybdemim 0.20 or 0.35% 48xx - Nickel 3.50%, molybdenum 0.25% 50xx - Chromium 0.25, or 0.40 or 0.50% 50xxx- Carbon 1.00%, chromium 0.50% 51xx - Chromium 0.80, 0.90, 0.95, or 1.00% 51xxx - Carban 1.00%, chromium 1.05% 52xxx - Carbon 1.00%, chromium 1.45% 6lxx - Chromium 0.60, 0.80, or 0.95%, Vitalium 0.12';, 0.10% min., or 0.15% min.
81xx - Nickel 0.30%, chromium 0.40%, molybelenum 0.12% 86xx - Nickel 0.55%, chromium 0.50%, molybdenum 0.20% 87xx - Nickel 0.55%, chromium 0.05%, molybdenum 0,25% 88xx - Nickel 0.55%, chromium 0.05°%, molyilenum 0.35% 92xx - Manganese 0.85%, silicon 2.00%, chrominn 0 or 0.35% 93xx - Nickel 3.25%, chromium 1.20%, molybdenum 0.12% 94xx ~ Nickel 0.45%, chromium 0.40, molybdenum 0.12% 98xx - Nickel 1.00%, chromium 0.80%, molybdenum 0.25% *Not included in the current list of standard steels.
FIGURE 6-62. SAE numerical index.
or rolls. Forgings are shaped or formed by pressing or hammering heated metal in dies. Castings are produced by pouring molten metal into molds. The casting is finished by machining.
Spark testing is a common means of identifying various ferrous metals. In this test the piece of iron or steel is held against a revolving grinding stone and the metal is identified by the sparks thrown off. Each ferrous metal has its own peculiar spark characteristics. The spark streams vary from a few tiny shafts to a shower of sparks several feet in length. (Few nonferrous metals give off sparks when touched to a grinding stone. Therefore, these metals cannot be successfully identified by the spark test.)
Identification by spark testing is often inexact unless performed by an experienced person, or the 195
test pieces differ greatly in their carbon content and alloying constituents.
Wrought iron produces long shafts that are straw colored as they leave the stone and white at the end. Cast iron sparks are red as they leave the stone and turn to a straw color. Low-carbon steels give off long, straight shafts having a few white sprigs. As the carbon content of the steel increases, the number of sprigs along each shaft increases and the stream becomes whiter in color. Nickel steel causes the spark stream to contain small white blocks of light within the main burst.
Types, Characteristics, and Uses of Alloyed Steels Steel containing carbon in percentages ranging from 0.10 to 0.30 percent is classed as low-carbon steel. The equivalent SAE numbers range from 1010 to 1030. Steels of this grade are used for making such items as safety wire, certain nuts, cable bushings, or threaded rod ends. This steel in sheet form is used for secondary structural parts and clamps, and in tubular form for moderately Steel containing carbon in percentages ranging from 0.30 to 0.50 percent is classed as mediumcarbon steel. This steel is especially adaptable for machining or forging, and where surface hardness is desirable. Certain rod ends and light forgings are made from SAE 1035 steel.
Steel containing carbon in percentages ranging from 0.50 to 1.05 percent is classed as high-carbon steel. The addition of other elements in varying quantities adds to the hardness of this steel. In the fully heat-treated condition it is very hard, will withstand high shear and wear, and will have little 16f5 a shon deformation. It has limited use in aircraft. SAE 1095 in sheet form is used for making flat springs and in wire form for making coil springs.
The various nickel steels are produced by combining nickel with carbon steel. Steels containing from 3 to 3.75 percent nickel are commonly used. Nickel increases the hardness, tensile strength, and elastic limit of steel without appreciably decreasing the ductility. It also intensifies the hardening effect of heat treatment.
SAE 2330 steel is used extensively for aircraft parts, such as bolts, terminals, keys, clevises, and pins.
Chromium steel is high in hardness, strength, and corrosion-resistant properties, and is particularly adaptable for heat-treated forgings which require greater toughness and strength than may be obtained in plain carbon steel. It can be used for such articles as the balls and rollers of antifriction bearings.
Chrome-nickel or stainless steels are the corrosion-resistant metals. The anticorrosive degree of this steel is determined by the surface condition of the metal as well as by the composition, temperature, and concentration of the corrosive agent.
The principal alloy of stainless steel is chromium.
The corrosion-resistant steel most often used in aircraft construction is known as 18-8 steel because of its content of 18 percent chromium and 8 percent nickel. One of the distinctive features of 18-8 steel is that its strength may be increased by coldworking.
Stainless steel may be rolled, drawn, bent, or formed to any shape. Because these steels expand about 50 percent more than mild steel and conduct heat only about 40 percent as rapidly, they are more difficult to weld. Stainless steel can be used for almost any part of an aircraft. Some of its common applications are in the fabrication of exhaust collectors, stacks and manifolds, structural and machined parts, springs, castings, tie rods, and control cables.
The chrome-vanadium steels are made of approximately 18 percent vanadium and about 1 percent chromium. When heat treated, they have strength, toughness, and resistance to wear and fatigue. A special grade of this steel in sheet form can be cold-formed into intricate shapes. It can be folded and flattened without signs of breaking or failure. SAE 6150 is used for making springs; and chrome-vanadium with high-carbon content, SAE 6195, is used for ball and roller bearings.
Molybdenum in small percentages is used in combination with chromium to form chromemolybdenum steel, which has various uses in aircraft. Molybdenum is a strong alloying element.
It raises the ultimate strength of steel without affecting ductility or workability. Molybdenum steels are tough and wear resistant, and they harden throughout when heat treated. They are especially adaptable for welding and, for this reason, are used principally for welded structural parts and assemblies. This type steel has practically replaced carbon steel in the fabrication of fuselage tubing, engine mounts, landing gears, and other structural parts. For example, a heat-treated SAE X4130 tube is approximately four times as strong as an SAE 1025 tube of the same weight and size.
A series of chrome-molybdenum steel most used in aircraft construction is that series containing
0.25 to 0.55 percent carbon, 0.15 to 0.25 percent 196
3/31/99 molybdenum, and 0.50 to 1.10 percent chromium. These steels, when suitably heat treated, are deep hardening, easily machined, readily welded by either gas or electric methods, and are especially adapted to high-temperature service.
Inconel is a nickel-chromium-iron alloy closely resembling stainless steel (corrosion resistant steel, CRES) in appearance. Aircraft exhaust systems use both alloys interchangeably. Because the two alloys look very much alike, a distinguishing test is often necessary. One method of identification is to use an electrochemical technique to identify the nickel (Ni) content of the alloy.
Inconel has a nickel content greater than 50 percent, and the electrochemical test detects Ni.
Prepare a wiring assembly as shown in Figure 6-2A, and prepare the two reagents (ammonium fluoride and dimethylglyoxime solutions) placing them in separate dedicated dropper solution bottles. Before testing, the metal must be thoroughly cleaned in order for the electrolytic deposition to take place. You may use nonmetallic hand scrubbing pads or 320 to 600 grit "crocus cloth" to remove deposits and corrosion products (thermal oxide).
Connect the alligator clip of the wiring assembly to the bare metal being tested. Place one drop of a
0.05 percent reagent grade ammonium fluoride solution in deionized water on the center of a (1 by 1 inch) sheet of filter paper. Lay the moistened filter paper over the bare metal alloy being tested. Firmly press the end of the aluminum rod over the center of the moist paper.
Maintain connection for 10 seconds while rocking the aluminum rod on the filter paper. Ensure that the light emitting diode (LED) remains lit (indicating good electrical contact and current flow) during this period.
AC 65-9A CHG 1 Disconnect the wiring assembly, and set it aside.
Remove the filter paper and examine it to determine that a light spot appears where the connection was made.
9v battery LED Aluminum rod Alligator Clip stock FIGURE 6-2A. Wiring assembly schematic.
Deposit one drop of 1.0 percent solution of reagent grade dimethylglyoxime in ethyl alcohol on the filter paper (same side that was in contact with the test metal). A bright distinctly pink spot will appear within seconds on the filter paper if the metal being tested is Inconel. A brown spot will appear if the test metal is stainless steel. Some stainless steel alloys may leave a very light pink color. However, the shade and depth of color will be far less than would appear for Inconel. For flat surfaces the test spot will be circular while for curved surfaces, such as the outside of a tube or pipe, the test spot may appear as a streak. (See Figure 6-2B for sample test results.) This procedure should not be used in the heat affected zone of weldments and on nickel coated surfaces.
In 600 In 718 17-7 SS 304 SS 316 SS FIGURE 6-2B. Electrochemical test results, Inconel and stainless steel alloys.
The tensile strength of Inconel is 100,000 p.s.i., annealed, and 125,000 p.s.i., when hard rolled. It is highly resistant to sait water and is able to withstand temperatures as high as 1,600° F. Inconel welds readily and has working qualities quite similar to those of corrosion-resistant steels.
NONFERROUS AIRCRAFT METALS The term "nonferrous" refers to all metals which have elements other than iron as their base or principal constituent. This group includes such metals as aluminum, titanium, copper, and magnesium, as well as such alloyed metals as Monel and babbit.
Aluminum and Aluminum Alloys Commercially pure aluminum is a white lustrous metal which stands second in the scale of malleability, sixth in ductility, and ranks high in its resistance to corrosion. Aluminum combined with various percentages of other metals forms alloys which are used in aircraft construction.
Aluminum alloys in which the principal alloying ingredients are either manganese, chromium, or magnesium and silicon show little attack in corrosive environments. Alloys in which substantial percentages of copper are used are more susceptible to corrosive action.
The total percentage of alloying elements is seldom more than 6 or 7 percent in the wrought alloys.
Aluminum is one of the most widely used metals in modern aircraft construction. It is vial to the aviation industry because of its high strength-to-weight ratio and its comparative ease of fabrication. The outstanding characteristic of aluminum is its light weight. Aluminum metals at the comparatively low temperature of 1,250° F.
It is nonmagnetic and is an excellent conductor.
Commercially pure aluminum has a tensile strength of about 13,000 p.s.i., but by rolling or other cold-working processes its strength may be approximately doubled. By alloying with other metals, or by using heat-treating processes, the tensile strength may be raised to as high as 65,000 p.s.i. or to within the strength range of structural steel.
Aluminum alloys, although strong, are easily worked because they are malleable and ductile. They may be rolled into sheets as thin as .0017 inch or drawn into wire .004 inch in diameter. Most aluminum alloy sheet stock used in aircraft construction ranges from .016 to .096 inch in thickness; however, some of the larger aircraft use sheet stock which may be as thick as .356 inch.
The various types of aluminum may be divided into two general classes: (1) The casting alloys (those suitable for casting in sand, permanent mold, or die castings), and (2) the wrought alloys (those which may be shaped by rolling, drawing, or forging). Of these two, the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections.
Aluminum casting alloys are divided into two basic groups. In one, the physical properties of the alloys are determined by the alloying elements and cannot be 198
3/31/99 are determined by the alloying elements and cannot be changed after the metal is cast. In the other, the alloying elements make it possible to heat treat the casting to produce the desired physical properties.
The casting alloys are identified by a letter preceding the alloy number. When a letter precedes a number, it indicates a slight variation in the composition of the original alloy. This variation in composition is simply to impart some desirable quality. In casting alloy 214, for example, the addition of zinc to improve its pouring qualities is indicated by the letter A in front of the number, thus creating the designation A214.
When castings have been heat treated, the heat treatment and the composition of the casting is indicated by the letter T, followed by an alloying number. An example of this is the sand casting alloy 355, which has several different compositions and tempers and is designated by 355-T6, 355-T51, or C355-T51.
Aluminum alloy castings are produced by one of three basic methods: (1) Sand mold, (2) permanent mold, or (3) die cast. In casting aluminum, it must be remembered that in most cases different types of alloys must be used for different types of castings. Sand castings and die castings require different types of alloys than those used in permanent molds.
Sand and permanent mold castings are parts produced by pouring molten metal into a previously prepared mold, allowing the metal to solidify or freeze, and then removing the part. If the mold is made of sand, the part is a sand casting; if it is a metallic mold (usually cast iron) the part is a permanent mold casting. Sand and permanent castings are produced by pouring liquid metal into the mold, the metal flowing under the force of gravity alone.
The two principal types of sand casting alloys are 112 and 212. Little difference exists between the two metals from a mechanical properties standpoint, since both are adaptable to a wide range of products.
The permanent mold process is a later development of the sand casting process, the major difference being in the material from which the molds are made. The advantage of this process is that there are fewer openings (called porosity) than in sand castings.
The sand and the binder, which is mixed with the sand to hold it together, give off a certain amount of gas which causes porosity in a sand casting.
Permanent mold castings are used to obtain higher mechanical properties, better surfaces, or more accurate dimensions. There are two specific types of permanent mold castings: (1) The permanent metal mold with metal cores, and (2) the semipermanent types containing sand cores. Because finer grain structure is produced in alloys subjected to the rapid cooling of metal molds, they are far superior to the sand type castings.
Alloys 122, A132, and 142 are commonly used in permanent mold castings, the principal uses of which are in internal combustion engines.
AC 65-9A CHG 1 Die castings used in aircraft are usually aluminum or magnesium alloy. If weight is of primary importance, magnesium alloy is used because it is lighter than aluminum alloy. Aluminum alloy is frequently used since it is stronger than most magnesium alloys.
A die casting is produced by forcing molten metal under pressure into a metallic die and allowing it to solidify; then the die is opened and the part removed.
The basic difference between permanent mold casting and die casting is that in the permanent mold process the metal flows into the die under gravity. In the die casting operation, the metal is forced under great pressure.
Die castings are used where relatively large production of a given part is involved. Remember, any shape which can be forged can be cast.
Wrought aluminum and wrought aluminum alloys are divided into two general classes, nonheat-treatable alloys and heat-treatable alloys.
Nonheat-treatable alloys are those in which the mechanical properties are determined by the amount of cold-work introduced after the final annealing operation.
The mechanical properties obtained by cold working are destroyed by any subsequent heating and cannot be restored except by additional cold working, which is not always possible. The "full hard" temper is produced by the maximum amount of cold-work that is commercially practicable. Metal in the "as fabricated" condition is produced from the ingot without any subsequent controlled amount of cold working or thermal treatment.
There is, consequently, a variable amount of strain hardening, depending upon the thickness of the section.
For heat-treatable aluminum alloys the mechanical properties are obtained by heat treating to a suitable temperature, holding at that temperature long enough to allow the alloying constituent to enter into solid solution, and then quenching to hold the constituent in solution. The metal is left in a supersaturated, unstable state and is then age hardened either by natural aging at room temperature or by artificial aging at some elevated Aluminum Alloy Designations Wrought aluminum and wrought aluminum alloys are designated by a four-digit index system. The system is broken into three distinct groups: 1xxx group, 2xxx through 8xxx group, and 9xxx group (which is at present unused).
The first digit of a designation identifies the alloy type. The second digit indicates specitic alloy modifications. Should the second number be zero, it would indicate no special control over individual impurities. Digits 1 through 9, however, when assigned consecutively as needed for the second number in this group, indicate the number of controls over individual impurities in the metal.
198-1 (and 198-2)
The last two digits of the Ixxx group are used to indicate the hundredths of 1 percent above the original 99 percent designated by the first digit.
Thus, it the last two digits were 30, the alloy would contain 99 percent plus 0.30 percent of pure aluminum, or a total of 99.30 percent pure aluminum.
Examples of alloys in this group are:
1100-99.00 percent pure aluminum with one control over individual impurities.
1130-99.30 percent pure aluminum with one control over individual impurities.
1275-99.75 percent pure aluminum with two controls over individual impurities.
In the 2xxx through 8xxx groups, the first digit indicates the major alloying element used in the formation of the alloy as follows:
2xxx-copper.
3xxx-manganese.
4xxx--silicon.
5xxx-magnesium.
6xxx-magnesium and silicon.
7xxx-zinc.
8xxx-other elements.
In the 2xxx through 8xxx alloy groups, the second digit in the alloy designation indicates alloy modifications. If the second digit is zero, it indicates the original alloy, while digits 1 through 9 indicate alloy modifications.
The last two of the four digits in the designation identify the different alloys in the group (figure Effect of Alloying Element 1000 series. 99% or higher, excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties, excellent workability.
Iron and silicon are major impurities.
2000 series. Copper is the principle alloying element. Solution heat treatment, optimum properties equal to mild steel, poor corrosion resistance It is usually clad with 6000 or high purity alloy. Its best known alloy is 2024.
3000 series. Manganese is the principle alloying element of this group which is generally non-heat- The percentage of manganese which will be alloy effective is 1.5%. The most popular is 3003 which is of moderate strength, and has 4000 series. Silicon is the principle alloying element. This lowers the melting temperature, Its primary use is in welding and brazing.
used in welding heat-treatable alloys, this group will respond to a limited amount of heat treatment.
5000 series. Magnesium is the principle alloying element. It has good welding and corrosion resistant characteristics. High temperatures (over 150° F.) or excessive cold working will increase susceptibility to corrosion.
6000 series. Silicon and magnesium form mag nesium silicide which makes alloys heat-treatable It is of medium strength, good forming and has corrosion resistant characteristics.
Per cent of alloying elements-aluminum and normal impurities Alloy constitute remainder Cop- Sili- Man- Mag- Zine Nickel Chro- Lead Bisper con ganese nesium mium muth 1100 ....
3003 1.2 ... ... ...
• 2011 5.5 0.5 0.5 2014 4.4 0.8 0.8 0,4 ...
2017 4.0 0.5 0.5 .... ...
2117 2.5 0.3 2018 4.0 0.5 2.0 2024 4.5 0.6 1.5 2025 4.5 0.8 0.8 .....
4032 0.9 12.5 1.O 0.9 6151 1.0 0.6 ... 0.25 5052 2.5 0,25 6053 0.7 1.3 ...... 0.25 8061 0.25 0.6 1.0 0.25 ...
7075 1.6 2.5 5.6 0.3 FIGURE 6-63. Nominal composition of wrought aluminum alloys.
7000 series. Zinc is the principle alloying element. The most popular alloy of the series is 6061.
When coupled with magnesium, it results in heattreatable alloys of very high strength. It usually has copper and chromium added. The principle alloy of this is 7075.
Hardness Identification Where used, the temper designation follows the alloy designation and is separated from it by a dash: i.e., 7075-T6, 2024-T4, etc.
designation consists of a letter indicating the basic temper which may be more specifically defined by the addition of one or more digits. These designations are as follows:
-F As fabricated.
-O Annealed, recrystallized (wrought products only).
-H Strain hardened.
-H1 (plus one or more digits) strain hardened only.
-H2 (plus one or more digits) strain hardened and partially annealed.
-H3 (plus one or more digits) strain hardened and stabilized.
The digit following the designations H1, H2, and H3 indicate the degree of strain hardening, number 8 representing the ultimate tensile strength equal to that achieved by a cold reduction of approximately 75% following a full anneal, 0 representing the annealed state.
Heat Treatment Identification In the wrought form, commercially pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength and does not have the properties required for structural aircraft parts. High strengths are generally obtained by the process of alloying. The resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum.
Alloying is not the only method of increasing the strength of aluminum. Like other materials, aluminum becomes stronger and harder as it is rolled, formed, or otherwise cold-worked.
the hardness depends on the amount of cold working done, 1100 and some wrought aluminum alloys are available in several strain-hardened tempers.
The soft or annealed condition is designated O.
If the material is strain hardened, it is said to be in the H condition.
The most widely used alloys in aircraft construction are hardened by heat treatment rather than by cold-work. These alloys are designated by a somewhat different set of symbols: -T4 and W indicate solution heat treated and quenched but not aged, and T6 indicates an alloy in the heat treated hardened condition.
-W Solution heat treated, unstable temper.
-T Treated to produce stable tempers other than -F, -O, or -H.
-T2 Annealed (cast products only).
-T3 Solution heat treated and then cold worked.
-T4 Solution heat treated.
-T5 Artificially aged only.
-T6 Solution heat treated and then artificially aged.
-T7 Solution heat treated and then stabilized.
-T8 Solution heat treated, cold worked, and then artificially aged.
-T9 Solution heat treated, artificially aged, and then cold worked.
-T10 Artificially aged and then cold worked.
Additional digits may be added to TI through T10 to indicate a variation in treatment which significantly alters the characteristics of the prod- In the wrought form, commercially pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength, however, and does not have the strength required for structural aircraft parts. Higher strengths are generally obtained by the process of alloying. The 200
resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum.
Alloying is not the only method of increasing the condition.
the heat became the first commercial method for producing titanium. The United States Bureau of Mines began making titanium sponge in 1946, and 4 years later the melting process began.
The use of titanium is widespread. It is used in many commercial enterprises and is in constant demand for such items as pumps, screens, and other tools and fixtures where corrosion attack is prevalent. In aircraft construction and repair, titanium is used for fuselage skins, engine shrouds, firewalls, longerons, frames, fittings, air ducts, and fasteners, Titanium is used for making compressor disks, spacer rings, compressor blades and vanes, through bolts, turbine housings and liners, and miscellaneous hardward for turbine engines.
Titanium, in appearance, is similar to stainless steel. One quick method used to identify titanium is the spark test. Titanium gives off a brilliant white trace ending in a brilliant white burst. Also, identification can be accomplished by moistening the titanium and using it to draw a line on a piece of glass. This will leave a dark line similar in from the edge.
Alclad Aluminum The terms "Alclad and Pureclad" are used to designate sheets that consist of an aluminum alloy core coated with a layer of pure aluminum to a depth of approximately 5½ percent on each side.
The pure aluminum coating affords a dual protection for the core, preventing contact with any corrosive agents, and protecting the core electrolytically by preventing any attack caused by scratching or from other abrasions.
Titanium and Titanium Alloys Titanium was discovered by an English priest named Gregot. A crude separation of titanium ore was accomplished in 1825. In 1906 a sufficient amount of pure titanium was isolated in metallic form to permit a study. Following this study, in 1932, an extraction process was developed which Titanium falls between aluminum and stainless steel in terms of elasticity, density, and elevated temperature strength. It has a melting point of from 2,730° F. to 3,155° F., low thermal conductivity, and a low coefficient of expansion. It is light, strong, and resistant to stress-corrosion cracking. Titanium is approximately 60 percent heavier than aluminum and about 50 percent lighter than stainless steel.
Because of the high melting point of titanium, high-temperature properties are disappointing.
The ultimate yield strength of titanium drops rapidly above 800° F. The absorption of oxygen and nitrogen from the air at temperatures above 1,000° F. makes the metal so brittle on long exposure that it soon becomes worthless. However, titanium does have some merit for short-time exposure up to 3,000° F. where strength is not important. Aircraft firewalls demand this requirement.
Titanium is nonmagnetic and has an electrical resistance comparable to that of stainless steel.
Some of the base alloys of titanium are quite hard.
Heat treating and alloying do not develop the hardness of titanium to the high levels of some of the heat-treated alloys of steel. It was only recently that a heat-treatable titanium alloy was developed. Prior to the development of this alloy, heating and rolling was the only method of forming 201
that could be accomplished. However, it is possible to form the new alloy in the soft condition and heat treat it for hardness.
Iron, molybdenum, and chromium are used to stabilize titanium and produce alloys that will quench harden and age harden. The addition of these metals also adds ductility. The fatigue resistance of titanium is greater than that of structural members and aircraft skin, has 110,000
p.s.i. minimum yield strength, and contains 8 percent manganese.
Type A-110AT is a titanium alloy which contains 5 percent aluminum and 2.5 percent tin.
It also has a high minimum yield strength at elevated temperatures with the excellent welding characteristics inherent in alpha-type titanium alloys.
or unalloyed titanium by chemical analysis;
therefore, the grades are determined by mechanical properties.
Titanium Designations The A-B-C classification of titanium alloys was established to provide a convenient and simple means of describing all titanium alloys. Titanium and titanium alloys possess three basic types of crystals: A (alpha), B (beta), and C (combined alpha and beta). Their characteristics are:
weldability; tough and strong both cold and hot, and resistant to oxidation.
B (beta)-Bendability; excellent bend ductility; strong both cold and hot, but vulnerable to contamination.
C (combined alpha and beta for compromise performances-Strong when cold and warm, but weak when hot; good bendability; moderate contamination resistance; excellent forgeability.
Titanium is manufactured for commercial use in two basic compositions; commercially pure titanium and alloyed titanium. A-55 is an example of a commercially pure titanium. It has a yield strength of 55,000 to 80,000 p.s.i. and is a generalpurpose grade for moderate to severe forming. It is sometimes used for nonstructural aircraft parts and for all types of corrosion-resistant applications, such as tubing.
Type A-70 titanium is closely related to type A-55 but has a yield strength of 70,000 to 95,000
p.s.i. It is used where higher strength is required, and it is specified for many moderately stressed aircraft parts. For many corrosion applications, it is used interchangeably with type A-55. Both type A-55 and type A-70 are weldable.
One of the widely used titanium-base alloys is designated as C-110M. It is used for primary Corrosion Characteristics The corrosion resistance of titanium deserves special mention. The resistance of the metal to corrosion is caused by the formation of a protective surface film of stable oxide or chemi-absorbed oxygen. Film is often produced by the presence of oxygen and oxidizing agents.
Corrosion of titanium is uniform. There is little evidence of pitting or other serious forms of localized attack. Normally, it is not subject to stress corrosion, corrosion fatigue, intergranular corrosion, or galvanic corrosion. Its corrosion resistance is equal or superior to 18-8 stainless steel.
Laboratory tests with acid and saline solutions show titanium polarizes readily. The net effect, in general, is to decrease current flow in galvanic and corrosion cells. Corrosion currents on the surface of titanium and metallic couples are naturally restricted. This partly accounts for good resistance to many chemicals; also, the material may be used with some dissimilar metals with no harmful galvanic effect on either.
Copper and Copper Alloys Copper is one of the most widely distributed metals. It is the only reddish-colored metal and is second only to silver in electrical conductivity. Its use as a structural material is limited because of its great weight. However, some of its outstanding characteristics, such as its high electrical and heat conductivity, in many cases overbalance the weight factor.
Because it is very malleable and ductile, copper is ideal for making wire. It is corroded by salt water but is not affected by fresh water. The ultimate tensile strength of copper varies greatly.
For cast copper, the tensile strength is about 25,000 p.s.i., and when cold rolled or cold drawn its tensile strength increases to a range of 40,000 to 67,000 p.s.).
In aircraft, copper is used primarily in the 202
and resistance to both shock and fatigue. Because lockwire. of these properties, they are used for diaphragms, gears, and pumps. Aluminum bronzes are available in rods, bars, plates, sheets, strips, and forgings.
Cast aluminum bronzes, using about 89 percent copper, 9 percent aluminum, and 2 percent of other elements, have high strength combined with ductility, and are resistant to corrosion, shock, and fatigue. Because of these properties, cast aluminum bronze is used in bearings and pump parts. These alloys are useful in areas exposed to salt water and corrosive gases.
Manganese bronze is an exceptionally high strength, tough, corrosion-resistant copper zinc alloy containing aluminum, manganese, iron and, occasionally, nickel or tin. This metal can be formed, extruded, drawn, or rolled to any desired shape. In rod form, it is generally used for machined parts, for aircraft landing gears and brackets.
Silicon bronze is a more recent development composed of about 95 percent copper, 3 percent silicon, and 2 percent manganese, zinc, iron, tin, and aluminum. Although not a bronze in the true sense because of its small tin content, silicon bronze has high strength and great corrosion resistance.
Monel Monel, the leading high-nickel alloy, combines the properties of high strength and excellent corrosion resistance. This metal consists of 68 percent nickel, 29 percent copper, 0.2 percent iron, 1 percent manganese, and 1.8 percent of other elements. It cannot be hardened by heat treatment.
Monel, adaptable to casting and hot- or cold-working, can be successfully welded. It has working properties similar to those of steel. When forged and annealed, it has a tensile strength of 80,000 p.s.i. This can be increased by cold-working to 125,000 p.s.i., sufficient for classification among the tough alloys.
Monel has been successfully used for gears and chains to operate retractable landing gears, and for structural parts subject to corrosion. In aircraft, Monel is used for parts demanding both strength and high resistance to corrosion (such as exhaust manifolds and carburetor needle valves and sleeves).
K-Monel K-Monel is a nonferrous alloy containing mainly nickel, copper, and aluminum. It is produced by adding a small amount of aluminum to the Monel formula. It is corrosion resistant and capable of being hardened by heat treatment.
K-Monel has been successfully used for gears, and structural members in aircraft which are subjected to corrosive attacks. This alloy is nonmagnetic at all temperatures. K-Monel sheet has been successfully welded by both oxyacetylene and electrie-are welding.
treatment is used to put as much of the alloying ingredients as possible into solid solution, which results in high tensile strength and maximum ductility. Aging is applied to castings following heat treatment where maximum hardness and yield strength are desired.
Magnesium embodies fire hazards of an unpredictable nature. When in large sections, its high thermal conductivity makes it difficult to ignite and prevents it from burning. It will not burn until the melting point is reached, which is 1,204° F.
However, magnesium dust and fine chips are ignited easily. Precautions must be taken to avoid this if possible. Should a fire occur, it can be extinguished with an extinguishing powder, such as powdered soapstone, or graphite powder. Water or any standard liquid or foam fire extinguishers cause magnesium to burn more rapidly and can cause explosions.
Magnesium alloys produced in the United States consist of magnesium alloyed with varying proportions of aluminum, manganese, and zinc.
These alloys are designated by a letter of the alphabet, with the number 1 indicating high purity and maximum corrosion resistance.
Many of the magnesium alloys manufactured in this country are produced by the Dow Chemical ds Company and have been given the trade name of is Dowmetal alloys. To distinguish between these alloys, each is assigned a letter. Thus, we have of Dowmetal J, Dowmetal M, and so forth.
of Another manufacturer of magnesium alloys is ed the American Magnesium Corporation, a subnt sidiary of the Aluminun Company of America.
ve This company uses an identification system similar he to that used for aluminum alloys, with the 1g- exception that magnesium alloy numbers are re preceded with the letters AM. Thus, AM240C is a in., cast alloy, and AM240C4 is the same alloy in the ne heat-treated state. AM3SO is an annealed wrought alloy, and AM3SRT is the same alloy rolled after nd heat treatment.
seats.
casting charac-
SUBSTITUTION OF AIRCRAFT METALS In selecting substitute metals for the repair and maintenance of aircraft, it is very important to check the appropriate structural repair manual.
Aircraft manufacturers design structural members such treatments to meet a specific load requirement for a particular aircraft. The methods of repairing these members, apparently similar in construction, will thus vary with different aircraft.
Four requirements must be kept in mind when selecting substitute metals. The first and most important of these is maintaining the original strength of the structure. The other three are:
(1) Maintaining contour or aerodynamic smoothness, (2) maintaining original weight, if possible, or keeping added weight to a minimum, and (3) maintaining the original corrosion-resistant properties of the metal.
PRINCIPLES OF HEAT TREATMENT Heat treatment is a series of operations involving the heating and cooling of metals in the solid state. Its purpose is to change a mechanical property or combination of mechanical properties so that the metal will be more useful, serviceable, and safe for a definite purpose. By heat treating, a metal can be made harder, stronger, and more resistant to impact, Heat treating can also make a metal softer and more ductile. No one heat-treating operation can produce all of these characteristics.
In fact, some properties are often improved at the expense of others. In being hardened, for example.
a metal may become brittle.
The various heat-treating processes are similar in that they all involve the heating and cooling of metals. They differ, however, in the temperatures to which the metal is heated, the rate at which it is cooled, and, of course, in the final result.
The most common forms of heat treatment for ferrous metals are hardening, tempering, normalizing, annealing, and casehardening. Most nonferrous metals can be annealed and many of them can be hardened by heat treatment. However, there is only one nonferrous metal, titanium, that can be casehardened, and none can be tempered or normalized.
Internal Structure of Metals The results obtained by heat treatment depend to a great extent on the structure of the metal and on the manner in which the structure changes when the metal is heated and cooled. A pure metal cannot be hardened by heat treatment because there is little change in its structure when heated.
On the other hand, most alloys respond to heat treatment since their structures change with heating and cooling.
An alloy may be in the form of a solid solution, a mechanical mixture, or a combination of a solid solution and a mechanical mixture. When an alloy is in the form of a solid solution, the elements and compounds which form the alloy are absorbed, one into the other, in much the same way that salt is dissolved in a glass of water, and the constituents cannot be identified even under a microscope.
When two or more elements or compounds are mixed but can be identified by microscopic examination, a mechanical mixture is formed.
A mechanical mixture can be compared to the mixture of sand and gravel in concrete. The sand and gravel are both visible. Just as the sand and gravel are held together and kept in place by the matrix of cement, the other constituents of an alloy are embedded in the matrix formed by the An alloy in the form of a mechanical mixture at ordinary temperatures may change to a solid solution when heated. When cooled back to normal temperature, the alloy may return to its original structure. On the other hand, it may remain a solid solution or form a combination of a solid solution and mechanical mixture. An alloy which consists of a combination of solid solution and mechanical mixture at normal temperatures may change to a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original structure, or form a complex solution.
HEAT-TREATING EQUIPMENT Successful heat treating requires close control over all factors affecting the heating and cooling of metals. Such control is possible only when the proper equipment is available and the equipment is selected to fit the particular job. Thus, the furnace must be of the proper size and type and must be so controlled that temperatures are kept within the limits prescribed for each operation.
Even the atmosphere within the furnace affects the condition of the part being heat treated.
Further, the quenching equipment and the quenching medium must be selected to fit the metal and the heat-treating operation. Finally, there must be equipment for handling parts and materials, for cleaning metals, and for straightening parts.
Furnaces and Salt Baths There are many different types and sizes of furnaces used in heat treatment. As a general rule, furnaces are designed to operate in certain specific temperature ranges and attempted use in other ranges frequently results in work of inferior quality. In addition, using a furnace beyond its rated 205