STRUCTURAL 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 Properties of Metais 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.
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 is that property which enables a metal to return to its original shape when the force which 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 will not stress them beyond their elastic limits. This Explanation of Terms Hardness refers to the ability of a metal to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by coldworking 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 will be retained. pereined, Hardness and strength are closely associated properties of metals.
Brittleness is the property of a metal which 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 very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals.
A metal which can be hammered, rolled, or pressed into various shapes without cracking, breaking, or having some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapessuch as cowlings, fairings, or wingtips. Copper is an example of a malleable metal.
Ductility is the property of a metal which A material which possesses toughness will withstand tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals.
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 in order to maintain the proper weight and balance of the 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 is the property which 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 will be 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.
Contraction and expansion are reactions pro- 191
duced in metals as the result of heating or cooling.
Heat applied to a metal will cause it to expand or become larger. Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot-rolled material.
Selection Factors Strength, weight, and reliability are three factors which determine the requirements to be met by any material used in airframe construction and repair. Airframes must be strong and yet as light in weight as possible. There are very definite limits to which increases in strength can be accompanied by increases in weight. An airframe so heavy that it could not support a few hundred pounds of additional weight would be of little use.
All metals, in addition to having a good strength/weight ratio, must be thoroughly reliable, thus minimizing the possibility of dangerous and unexpected failures. In addition to these general properties, the material selected for a definite application must possess specific qualities suitable for the purpose.
The material must possess the strength required by the dimensions, weight, and use. There are five basic stresses which metals may be required to withstand. These are tension, compression, shear, bending, and torsion.
The tensile strength of a material is its resistance to a force which tends to pull it apart. Tensile strength is measured in p.s.i. (pounds per square inch) and is calculated by dividing the load, in pounds, required to pull the material apart by its cross-sectional area, in square inches.
The compression strength of a material is its resistance to a crushing force which is the opposite of tensile strength. Compression strength is also measured in p.s.i.
When a piece of metal is cut, the material is subjected, as it comes in contact with the cutting edge, to a force known as shear. Shear is the tendency on the part of parallel members to slide in opposite directions. It is like placing a cord or thread between the blades of a pair of scissors. The shear strength is the shear force in p.s.i. at which a material fails. It is the load divided by the shear area.
Bending can be described as the deflection or curving of a member due to forces acting upon it.
The bending strength of material is the resistance it offers to deflecting forces.
Torsion is a twisting force. Such action would occur in a member fixed at one end and twisted at the other. The torsional strength of material is its resistance to twisting.
The relationship between the strength of a material and its weight per cubie inch, expressed as a ratio, is known as the strength/weight ratio.
This ratio forms the basis for comparing the desirability of various materials for use in airframe construction and repair. Neither strength nor weight alone can be used as a means of true comparison. In some applications, such as the skin of monocoque structures, thickness is more important than strength, and, in this instance, the thickness or gage is best. Thickness or bulk is necessary to prevent buckling or damage caused by careless handling.
Corrosion is the eating away or pitting of the surface or the internal structure of metals. Because of the thin sections and the safety factors used in aircraft design and construction, it would be dangerous to select a material possessing poor corrosion-resistant characteristics.
Another significant factor to consider in maintenance and repair is the ability of a material to be formed, bent, or machined to required shapes. The hardening of metals by cold-working or forming is termed work-hardening. If a piece of metal is forned (shaped or bent) while cold, it is said to be cold-worked. Practically all the work an aviation mechanic does on metal is cold-work. While this is convenient, it causes the metal to become harder and more brittle.
If the metal is cold-worked too much, that is, if it is bent back and forth or hammered at the same place too often, it will crack or break.
Usually, the more malleable and ductile a metal is, the more cold-working it can stand. Any process which involves controlled heating and cooling of metals to develop certain desirable characteristics (such as hardness, softness, ductility, tensile strength, or refined grain structure) is called heat treatment or heat treating. With steels the term "heat treating" has a broad meaning and includes such processes as annealing, normalizing, hardening, and tempering.
In the heat treatment of aluminum alloys, only two processes are included: (1) The hardening and toughening process, and (2) the softening process The hardening and toughening process is called heat treating, and the softening process is called annealing.
Aircraft metals are subjected to both shock and 192
fatigue (vibrational) stresses. Fatigue occurs in materials which are exposed to frequent reversals of loading or repeatedly applied loads, if the fatigue limit is reached or exceeded. Repeated vibration or bending will ultimately cause a minute crack to occur at the weakest point. As vibration or bending continues, the crack lengthens until the part completely fails. This is termed shock and fatigue failure. Resistance to this condition is known as shock and fatigue resistance.
It is essential that materials used for critical parts be resistant to these stresses.
METALWORKING PROCESSES There are three methods of metalworking:
(1) Hot-working, (2) cold-working, and (3) extruding. The method used will depend on the metal involved and the part required, although in some instances both hot- and cold-working methods may be used to make a single part.
Hot-working Almost all steel is hot-worked from the ingot into some form from which it is either hot- or cold-worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the interior is still molten. The ingot is then placed in a soaking pit which retards loss of heat, and the molten interior gradually solidifies. After soaking, the temperature is equalized throughout the ingot, then it is reduced to intermediate size by rolling, The rolled shape is called a bloom when its section dimensions are 6 x 6 inches or larger and approximately square. The section is called a billet when it is approximately square and less than 6 x 6 inches. Rectangular sections which have a width greater than twice their thickness are called slabs. The slab is the intermediate shape from Blooms, billets, or slabs are heated above the critical range and rolled into a variety of shapes of uniform cross section. The more common of these rolled shapes are sheet, bar, channels, angles, I-beams, and the like. As will be discussed later in this chapter, hot-rolled material is frequently finished by cold-rolling or drawing to obtain accurate finish dimensions and a bright, smooth surface.
Complicated sections which cannot be rolled, or sections of which only a small quantity is required, are usually forged. Forging of steel is a mechanical working at temperatures above the critical range to shape the metal as desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained, Pressing is used when the parts to be forged are large and heavy; this process also replaces hammering where high-grade steel is required.
Since a press is slow acting, its force is uniformly transmitted to the center of the section, thus affecting the interior grain structure as well as the exterior to give the best possible structure throughout.
Hammering can be used only on relatively small pieces. Since hammering transmits its force almost instantly, its effect is limited to a small depth.
Thus, it is necessary to use a very heavy hammer or to subject the part to repeated blows to ensure complete working of the section. If the force applied is too weak to reach the center, the finished forged surface will be concave. If the center was properly worked, the surface will be convex or bulged. The advantage of hammering is that the operator has control over both the amount of pressure applied and the finishing temperature, and is able to produce small parts of the highest grade. This type of forging is usually referred to as smith forging. It is used extensively where only a small number of parts are needed. Considerable machining time and material are saved when a part is smith forged to approximately the finished shape.
Steel is often harder than necessary and too brittle for most practical uses when put under severe internal strain. To relieve such strain and reduce brittleness, it is tempered after being hardened. This consists of heating the steel in a furnace to a specified temperature and then cooling it in air, oil, water, or a special solution. Temper condition refers to the condition of metal or metal alloys with respect to hardness or toughness.
Rolling, hammering, or bending these alloys, or heat treating and aging them, causes them to become tougher and harder. At times these alloys become too hard for forming and have to be re-heat treated or annealed.
Metals are annealed to relieve internal stresses, soften the metal, make it more ductile, and refine the grain structure. Annealing consists of heating the metal to a prescribed temperature, holding it there for a specified length of time, and then cooling the metal back to room temperature. To produce maximum softness, the metal must be cooled very slowly. Some metals must be furnace cooled;
others may be cooled in air.
Normalizing applies to iron-base metals only.
Normalizing consists of heating the part to the proper temperature, holding it at that temperature until it is uniformaly heated, and then cooling it in still air. Normalizing is used to relieve stresses in metals.
Cold-Working Cold-working applies to mechanical working performed at temperatures below the critical range. It results in a strain hardening of the metal.
In fact, the metal often becomes so hard that it is difficult to continue the forming process without softening the metal by annealing.
Since the errors attending shrinkage are eliminated in cold-working, a much more compact and better metal is obtained. The strength and hardness, as well as the elastic limit, are increased;
but the ductility decreases. Since this makes the metal more brittle, it must be heated from time to time during certain operations to remove the undesirable effects of the working.
While there are several cold-working processes, the two with which the aviation mechanic will be principally concerned are cold-rolling and colddrawing. These processes give the metals desirable qualities which cannot be obtained by hot-working.
Cold-rolling usually refers to the working of metal at room temperature. In this operation, the materials that have been rolled to approximate sizes are pickled to remove the scale, after which they are passed through chilled finishing rolls. This gives a smooth surface and also brings the pieces to accurate dimensions. The principal forms of cold-rolled stocks are sheets, bars, and rods.
Cold-drawing is used in making seamless tubing, wire, streamlined tie rods, and other forms of stock.
Wire is made from hot-rolled rods of various diameters. These rods are pickled in acid to remove scale, dipped in lime water, and then dried in a steam room where they remain until ready for drawing. The lime coating adhering to the metal serves as a lubricant during the drawing operation.
The size of the rod used for drawing depends upon the diameter wanted in the finished wire. To reduce the rod to the desired size, it is drawn cold through a die. One end of the rod is filed or hammered to a point and slipped through the die opening. Here it is gripped by the jaws of the drawing block and pulled through the die. This series of operations is done by a mechanism known as a drawbench.
In order to reduce the rod gradually to the desired size, it is necessary to draw the wire through successively smaller dies. Because each of these drawings reduces the ductility of the wire, it must be annealed from time to time before further drawings can be accomplished. Although cold-working reduces the ductility, it increases the tensile strength of the wire.
In making seamless steel aircraft tubing, the tubing is cold-drawn through a ring-shaped die with a mandrel or metal bar inside the tubing to support it while the drawing operations are being performed. This forces the metal to flow between the die and the mandrel and affords a means of controlling the wall thickness and the inside and outside diameters.
Extruding The extrusion process involves the forcing of metal through an opening in a die, thus causing the metal to take the shape of the die opening.
Some metals such as lead, tin, and aluminum may be extruded cold; but generally metals are heated before the operation is begun.
The principal advantage of the extrusion process is its flexibility. Aluminum, because of its workability and other favorable properties, can be economically extruded to more intricate shapes and larger sizes than is practicable with many other metals.
Extruded shapes are produced in very simple as well as extremely complex sections. In this process a cylinder of aluminum, for instance, is heated to 750° F. to 850° F. and is then forced through the opening of a die by a hydraulic ram. The opening is the shape desired for the cross section of the finished extrusion.
Many structural parts, such as channels, angles, T-sections, and Z-sections are formed by the extrusion process.
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 of 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 constituent.
Identification If carbon is added to iron, in percentages 194
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