Metal Properties and Aircraft Alloys

7-1 Aircraft Materials, Hardware, & Processes Chapter 7 Aircraft Metals Knowledge and understanding of the uses, strengths, limitations, and other characteristics of structural metals is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specification, or the substitution of inferior materials, may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsmanship. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various metals.

Properties of Metals

Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness, malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth. These terms are explained to establish a basis for further discussion of structural metals. Hardness Hardness refers to the ability of a material to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by cold-working the metal and, in the case of steel and certain aluminum alloys, by heat-treatment. Structural parts are often formed from metals in their soft state and are then heat-treated to harden them so that the finished shape is retained. Hardness and strength are closely associated properties of metals.

Strength One of the most important properties of a material is strength. Strength is the ability of a material to resist deformation. Strength is also the ability of a material to resist stress without breaking. The type of load or stress on the material affects the strength it exhibits. Density Density is the weight of a unit volume of a material. In aircraft work, the specified weight of a material per cubic inch is preferred since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part to maintain the proper weight and balance of the aircraft.

Malleability A metal that can be hammered, rolled, or pressed into various shapes without cracking, breaking, or leaving some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapes, such as cowlings, fairings, or wingtips. Copper is an example of a malleable metal. Ductility Ductility is the property of a metal that permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads. Chrome molybdenum steel is also easily formed into desired shapes.

Ductility is similar to malleability. Elasticity Elasticity is a property that enables a metal to return to its original size and shape when the force that causes the change of shape is removed. This property is extremely valuable, because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected do not stress them beyond their elastic limits. This desirable property is present in spring steel.

Toughness A material that possesses toughness withstands tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals. Brittleness Brittleness is the property of a metal that allows little bending or deformation without shattering. A brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a 7-2 very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals. Fusibility Fusibility is the ability of a metal to become liquid by the application of heat. Metals are fused in welding.

Steels fuse around 2,600 °F and aluminum alloys at approximately 1,100 °F. Conductivity Conductivity is the property that enables a metal to carry heat or electricity. The heat conductivity of a metal is especially important in welding, because it governs the amount of heat that is required for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also be considered in conjunction with bonding to eliminate radio interference. Thermal Expansion Thermal expansion refers to contraction and expansion that are reactions produced in metals as the result of heating or cooling. Heat applied to a metal causes it to expand or become larger. Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot rolled material.

Ferrous Aircraft Metals

Many different metals are required in the repair of aircraft. This is a result of the varying needs with respect to strength, weight, durability, and resistance to deterioration of specific structures or parts. In addition, the particular shape or form of the material plays an important role. In selecting materials for aircraft repair, these factors (plus many others) are considered in relation to the mechanical and physical properties. Among the common materials used are ferrous metals. The term “ferrous” applies to the group of metals having iron as their principal element. Iron If carbon is added to iron in percentages 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.

Steel and Steel Alloys 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 percent. 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. [Figure 7-1] Metal stock is manufactured in several forms and shapes, including sheets, bars, rods, tubing, extrusions, forgings, and castings. Sheet metal is made in a number of sizes and thicknesses. Specifications designate thicknesses in thousandths 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 machines, such as presses, bending brakes, draw benches, or rolls. Forgings are shaped or formed by pressing or hammering heated metal in dies. Pouring molten metal into molds produces castings. Machining finishes the casting. 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 test pieces differ greatly in their carbon content and alloying elements. 7-3 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 items, such 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 stressed structural parts. Steel containing carbon in percentages ranging from 0.30 to 0.50 percent is classed as medium carbon 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, withstands high shear and wear, and has little 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 nickels 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 articles such 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 its content is 18 percent chromium and 8 percent nickel. One of the distinctive features of 18-8 steel is that cold-working may increase its strength.

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 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; 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 chrome-molybdenum 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 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, as described in the following paragraph, to identify the nickel (Ni) content of the alloy. Inconel has nickel content greater than 50 percent, and the electrochemical test detects nickel.

The tensile strength of Inconel is 100,000 pounds per square 7-4 Series Designation Types 10xx Non-sulfurized carbon steels 11xx Resulfurized carbon steels (free machining) 12xx Rephosphorized and resulfurized 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 0.5 or 0.80%, molybdenum 0.25% 44xx Molybdenum 0.40% 45xx Molybdenum 0.52% 46xx Nickel 1.80%, molybdenum 0.25% 47xx Nickel 1.05% chromium 0.45%, molybdenum 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 Carbon 1.00%, chromium 1.05% 52xxx Carbon 1.00%, chromium 1.45% 61xx Chromium 0.60, 0.80, 0.95%, vanadium 0.12%, 0.10% min., or 0.15% min.

81xx Nickel 0.30%, chromium 0.40%, molybdenum 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%, molybdenum 0.35% 92xx Manganese 0.85%, silicon 2.00%, chromium 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 inch (psi) annealed, and 125,000 psi when hard rolled. It is highly resistant to salt water and can withstand temperatures as high as 1,600 °F. Inconel welds readily and has working qualities like those of corrosion resistant steels.

Electrochemical Test Prepare a wiring assembly as shown in Figure 7-2, and prepare the two reagents (ammonium fluoride and dimethylglyoxime solutions) placing them in separate dedicated dropper solution 7-5 Aluminum rod stock 9v battery − + LED Alligator clip bottles. Before testing, you must thoroughly clean the metal for the electrolytic deposit to take place. You may use nonmetallic hand scrubbing pads or 320–600 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 inch × 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. 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. 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. (Refer to Figure 7-3 for sample test results.) This procedure should not be used in the heat-affected zone of weldments or on nickel coated surfaces.

Nonferrous Aircraft Metals

The term “nonferrous” refers to all metals that have elements other than iron as its base or principal constituent. This group includes metals, such as aluminum, titanium, copper, and magnesium, as well as alloyed metals, such as Monel and Babbitt. Aluminum & 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 with principal alloying ingredients are manganese, chromium, or magnesium and silicon show little attack in corrosive environments. Alloys with which substantial percentages of copper 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 vital 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 lightweight. Aluminum melts 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 psi, but rolling or other cold-working processes may approximately double its strength. By alloying with other metals, or by using heat-treating processes, the tensile strength may be raised to as high as 65,000 psi 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 0.0017 inch or drawn into wire 0.004 inch in diameter. Most aluminum alloy sheet stock used in aircraft construction range from 0.016 to 0.096 inch in thickness; however, some of the larger aircraft use sheet stock that may be as thick as 0.356 inch. The various types of aluminum may be divided into two general classes: • Casting alloys (those suitable for casting in sand, permanent mold, or die castings) • 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 changed after the metal is cast. In the other, the alloying elements make it 7-6 steel (SS) alloys. possible to heat-treat the casting to produce the desired physical properties. A letter preceding the alloy number identifies the casting alloys. 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. For example, in casting alloy 214, 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: sand mold, permanent mold, or die cast. In casting aluminum, it is important to note 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 in mechanical properties, 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, that 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: permanent metal mold with metal cores, and semi-permanent 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.

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. However, aluminum alloy is frequently used because it is stronger than most magnesium alloys. Forcing molten metal under pressure into a metallic die and allowing it to solidify produces a die-casting; 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 that can be forged, can be cast. Wrought aluminum and wrought aluminum alloys are divided into two general classes: non-heat-treatable alloys and heat- treatable alloys. Non-heat-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, 7-7 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 temperature. Wrought Aluminum 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 currently unused).

The first digit of a designation identifies the alloy type. The second digit indicates specific 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. The last two digits of the 1xxx group are used to indicate the hundredths of 1 percent above the original 99 percent designated by the first digit. Thus, if 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 7-4] Effect of Alloying Element 1000 series: 99 percent aluminum 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 principal alloying element. Solution heat-treatment, optimum properties equal to mild steel, poor corrosion resistance unclad. It is usually clad with 6000 or high purity alloy. Its best-known alloy is 2024. 3000 series: Manganese is the principal alloying element of this group, which is generally non-heat-treatable. The percentage of manganese that is alloy effective is 1.5 percent. The most popular is 3003, which is of moderate strength and has good working characteristics. 4000 series: Silicon is the principal alloying element of this group and lowers melting temperature. Its primary use is in welding and brazing. When used in welding heat- treatable alloys, this group responds to a limited amount of heat-treatment.

5000 series: Magnesium is the principal alloying element. It has good welding and corrosion resistant characteristics. High temperatures (over 150 °F) or excessive cold-working increases susceptibility to corrosion. 6000 series: Silicon and magnesium form magnesium silicide, which makes alloys heat-treatable. It is of medium strength, good forming qualities, and has corrosion resistant characteristics. 7000 series: Zinc is the principal alloying element. The most popular alloy of the series is 6061. When coupled with magnesium, it results in heat-treatable alloys of very high strength. It usually has copper and chromium added. The principal alloy of this group 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, and so forth). The temper 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 7-8 Alloy Percentage of Alloying Elements Aluminum and normal impurities constitute remainder 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 — 1.0 — 0.9 — — — 6151 — 1.0 — 0.6 — — 0.25 — — 5052 — — — 2.5 — — 0.25 — — 6053 — 0.7 — 1.3 — — 0.25 — — 6061 0.25 0.6 — 1.0 — — 0.25 — — 7075 1.6 — — 2.5 5.6 — 0.3 — — Copper Silicon Manganese Magnesium Zinc Nickel Chromium Lead Bismuth • 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 indicates the degree of strain hardening, number 8 representing the ultimate tensile strength equal to that achieved by a cold reduction of approximately 75 percent following a full anneal, 0 representing the annealed state.

Magnesium & Magnesium Alloys Magnesium, the world’s lightest structural metal, is a silvery white material weighing only two-thirds as much as aluminum. Magnesium does not possess sufficient strength in its pure state for structural uses, but when alloyed with zinc, aluminum, and manganese, it produces an alloy having the highest strength- to-weight ratio of any of the commonly used metals. Magnesium is probably more widely distributed in nature than any other metal. It can be obtained from such ores as dolomite and magnesite, as well as from seawater, underground brines, and waste solutions of potash. With about 10 million pounds of magnesium in one cubic mile of seawater, there is no danger of a dwindling supply.

Some of today’s aircraft require more than one-half ton of this metal for use in hundreds of vital spots. Some wing panels are fabricated entirely from magnesium alloys, weigh 18 percent less than standard aluminum panels, and have flown hundreds of satisfactory hours. Among the aircraft parts that have been made from magnesium with a substantial savings in weight are nosewheel doors, flap cover skin, aileron cover skin, oil tanks, floorings, fuselage parts, wingtips, engine nacelles, instrument panels, radio masts, hydraulic fluid tanks, oxygen bottle cases, ducts, and seats. Magnesium alloys possess good casting characteristics. Their properties compare favorably with those of cast aluminum.

In forging, hydraulic presses are ordinarily used, although, under certain conditions, forging can be accomplished in mechanical presses or with drop hammers. Magnesium alloys are subject to such treatments as annealing, quenching, solution heat-treatment, aging, and stabilizing. Sheet and plate magnesium are annealed at the rolling mill. The solution heat-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 7-9 and liners, and miscellaneous hardware for turbine engines.

Titanium, in appearance, is like 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, moistening the titanium and using it to draw a line on a piece of glass can accomplish identification. This leaves a dark line similar in appearance to a pencil mark. Titanium falls between aluminum and stainless steel in terms of elasticity, density, and elevated temperature strength. It has a melting point 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 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 quench-harden and age- harden. The addition of these metals also adds ductility. The fatigue resistance of titanium is greater than that of aluminum or steel. Titanium becomes softer as the degree of purity is increased. It is not practical to distinguish between the various grades of commercially pure 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 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 does not burn until the melting point of 1,204 °F is reached. However, magnesium dust and fine chips are ignited easily. Precautions must be taken to avoid this if possible. Should a fire occur, it could be extinguished with an extinguishing powder, such as soapstone or graphite. Water or any standard liquid or foam fire extinguisher causes magnesium to burn more rapidly and can cause explosions. Magnesium alloys produced in the United States contain varying proportions of aluminum, manganese, and zinc. A letter of the alphabet designates these alloys, with the number 1 indicating high purity and maximum corrosion resistance.

Many of the magnesium alloys manufactured in the United States are produced by the Dow Chemical Company and have been given the trade name of Dow-metal™ alloys. To distinguish between these alloys, each is assigned a letter. Thus, we have Dow-metal™ J, Dow-metal™ M, and so forth. Another manufacturer of magnesium alloys is the American Magnesium Corporation, a subsidiary of the Aluminum Company of America. This company uses an identification system like that used for aluminum alloys, with the exception that magnesium alloy numbers are preceded with the letters AM. Thus, AM240C is a cast alloy, and AM240C4 is the same alloy in the heat-treated state. AM3S0 is an annealed wrought alloy, and AM3SRT is the same alloy rolled after heat-treatment.

Titanium and Titanium Alloys An English priest named Gregot discovered titanium. A crude separation of titanium ore was accomplished in 1825. In 1906, enough pure titanium was isolated in metallic form to permit a study. Following this study, in 1932, an extraction process was developed and 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 7-10 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 psi, and when cold rolled or cold drawn, its tensile strength increases to a range of 40,000 to 67,000 psi. In aircraft, copper is used primarily in the electrical system for bus bars, bonding, and as lock wire. Beryllium copper is one of the most successful of all the copper base alloys. It is a recently developed alloy containing about 97 percent copper, 2 percent beryllium, and sufficient nickel to increase the percentage of elongation.

The most valuable feature of this metal is that the physical properties can be greatly stepped up by heat-treatment, the tensile strength rising from 70,000 psi in the annealed state to 200,000 psi in the heat-treated state. The resistance of beryllium copper to fatigue and wear makes it suitable for diaphragms, precision bearings and bushings, ball cages, and spring washers. Brass is a copper alloy containing zinc and small amounts of aluminum, iron, lead, manganese, magnesium, nickel, phosphorous, and tin. Brass with a zinc content of 30 to 35 percent is very ductile, but that containing 45 percent has relatively high strength.

Muntz metal is a brass composed of 60 percent copper and 40 percent zinc. It has excellent corrosion-resistant qualities in salt water. Its strength can be increased by heat-treatment. As cast, this metal has an ultimate tensile strength of 50,000 psi, and it can be elongated 18 percent. It is used in making bolts and nuts, as well as parts that come in contact with salt water. Red brass, sometimes termed “bronze” because of its tin content, is used in fuel and oil line fittings. This metal has good casting and finishing properties and machines freely. Bronzes are copper alloys containing tin. The true bronzes have up to 25 percent tin, but those with less than 11 percent are basic types of crystals: A (alpha), B (beta), and C (combined alpha and beta). Their characteristics are: • A (alpha)—all-around performance; good weld ability; 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 forge ability. Titanium is manufactured for commercial use in two basic compositions: commercially-pure titanium and alloyed titanium. A-55 is an example of commercially-pure titanium. It has yield strength of 55,000 to 80,000 psi and is a general- purpose 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 yield strength of 70,000 to 95,000 psi. 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 is weldable.

One of the widely-used titanium base alloys is designated as C-110M. It is used for primary structural members and aircraft skin, has 110,000 psi minimum yield strength, and contains 8 percent manganese. Type A-110AT is a titanium alloy that contains 5 percent aluminum and 2.5 percent tin. It also has high minimum yield strength at elevated temperatures with the excellent welding characteristics inherent in alpha-type titanium alloys. 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 7-11 most useful, especially for such items as tube fittings in aircraft. Among the copper alloys are the copper aluminum alloys, of which the aluminum bronzes rank very high in aircraft usage. They would find greater usefulness in structures if it were not for their strength-to-weight ratio as compared with alloy steels. Wrought aluminum bronzes are almost as strong and ductile as medium carbon steel, and they possess a high degree of resistance to corrosion by air, salt water, and chemicals. They are readily forged, hot or cold rolled, and many react to heat-treatment.

These copper base alloys contain up to 16 percent of aluminum (usually 5 to 11 percent), to which other metals, such as iron, nickel, or manganese, may be added. Aluminum bronzes have good tearing qualities, great strength, hardness, and resistance to both shock and fatigue. Because 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 like those of steel. When forged and annealed, it has a tensile strength of 80,000 psi. This can be increased by cold-working to 125,000 psi, 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. Adding a small amount of aluminum to the Monel formula produces it. 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. Both oxyacetylene and electric arc welding have successfully welded K-Monel sheet. Nickel & Nickel Alloys There are basically two nickel alloys used in aircraft: Monel and Inconel. Monel contains about 68 percent nickel and 29 percent copper, plus small amounts of iron and manganese.

Nickel alloys can be welded or easily machined. Some of the nickel Monel, especially the nickel Monels containing small amounts of aluminum, are heat-treatable to similar tensile strengths of steel. Nickel Monel is used in gears and parts that require high strength and toughness, such as exhaust systems that require high strength and corrosion resistance at elevated temperatures. Inconel alloys of nickel produce a high strength, high temperature alloy containing approximately 80 percent nickel, 14 percent chromium, and small amounts of iron and other elements. The nickel Inconel alloys are frequently used in turbine engines because of their ability to maintain their strength and corrosion resistance under extremely high- temperature conditions.

Inconel and stainless steel are similar in appearance and are frequently found in the same areas of the engine. Sometimes it is important to identify the difference between the metal samples. A common test is to apply one drop of cupric chloride and hydrochloric acid solution to the unknown metal and allow it to remain for 2 minutes. At the end of the soak period, a shiny spot indicates the material is nickel Inconel, and a copper-colored spot indicates stainless steel. 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 to meet a specific load requirement for an