Nonferrous Treatment, Hardness, and Forming

7-21 Since 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 T1 through T10 to indicate a variation in treatment, which significantly alters the characteristics of the product.

Aluminum-alloy sheets are marked with the specification number on approximately every square foot of material. If for any reason this identification is not on the material, it is possible to separate the heat-treatable alloys from the non- heat-treatable alloys by immersing a sample of the material in a 10 percent solution of caustic soda (sodium hydroxide). The heat-treatable alloys turn black due to the copper content, whereas the others remain bright. In the case of clad material, the surface remains bright, but there is a dark area in the middle when viewed 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 5 1⁄2 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.

There are two types of heat-treatments applicable to aluminum alloys: solution heat-treatment and precipitation heat-treatment. Some alloys, such as 2017 and 2024, develop their full properties as a result of solution heat-treatment followed by about 4 days of aging at room temperature. Other alloys, such as 2014 and 7075, require both heat-treatments. The alloys that require precipitation heat-treatment (artificial aging) to develop their full strength also age to a limited extent at room temperature; the rate and amount of strengthening depends upon the alloy. Some reach their maximum natural or room temperature aging strength in a few days, and are designated as –T4 or –T3 temper. Others continue to age appreciably over a long period of time.

Because of this natural aging, the –W designation is specified only when the period of aging is indicated, for example, 7075–W (1⁄2 hour). Thus, there is considerable difference in the mechanical and physical properties of freshly quenched (–W) material and material that is in the –T3 or –T4 temper. The hardening of an aluminum alloy by heat-treatment consists of four distinct steps: 1. Heating to a predetermined temperature. 2. Soaking at temperature for a specified length of time. 3. Rapidly quenching to a relatively low temperature. 4. Aging or precipitation-hardening either spontaneously at room temperature, or because of a low temperature thermal treatment.

The first three steps above are known as solution heat-treatment, although it has become common practice to use the shorter term, “heat-treatment.” Room temperature hardening is known as natural aging, while hardening done at moderate temperatures is called artificial aging, or precipitation heat-treatment. Solution Heat-Treatment Temperature The temperatures used for solution heat-treating vary with different alloys and range from 825 °F to 980 °F. As a rule, they must be controlled within a very narrow range (±10 °F) to obtain specified properties. If the temperature is too low, maximum strength is not obtained. When excessive temperatures are used, there is danger of melting the low melting constituents of some alloys with consequent lowering of the physical properties of the alloy. Even if melting does not occur, the use of higher than recommended temperatures promotes discoloration and increases quenching strains.

7-22 Time at Temperature The time at temperature, referred to as soaking time, is measured from the time the coldest metal reaches the minimum limit of the desired temperature range. The soaking time varies, depending upon the alloy and thickness, from 10 minutes for thin sheets to approximately 12 hours for heavy forgings. For the heavy sections, the nominal soaking time is approximately 1 hour for each inch of cross-sectional thickness. [Figure 7-7] Choose the minimum soaking time necessary to develop the required physical properties. The effect of an abbreviated soaking time is obvious. An excessive soaking period aggravates high-temperature oxidation. With clad material, prolonged heating results in excessive diffusion of copper and other soluble constituents into the protective cladding and may defeat the purpose of cladding.

Quenching After the soluble constituents are in solid solution, the material is quenched to prevent or retard immediate re-precipitation. Three distinct quenching methods are employed. The one to be used in any instance depends upon the part, the alloy, and the properties desired. Cold Water Quenching Parts produced from sheet, extrusions, tubing, small forgings, and similar type material are generally quenched in a cold water bath. The temperature of the water before quenching should not exceed 85 °F. Using a sufficient quantity of water keeps the temperature rise under 20 °F. Such a drastic quench ensures maximum resistance to corrosion. This is particularly important when working with alloys, such as 2017, 2024, and 7075. This is the reason a drastic quench is preferred, even though a slower quench may produce the required mechanical properties.

Hot Water Quenching Large forgings and heavy sections can be quenched in hot or boiling water. This type of quench minimizes distortion and alleviates cracking, which may be produced by the unequal temperatures obtained during the quench. The use of a hot water quench is permitted with these parts, because the temperature of the quench water does not critically affect the resistance to corrosion of the forging alloys. In addition, the resistance to corrosion of heavy sections is not as critical a factor as for thin sections. Spray Quenching High-velocity water sprays are useful for parts formed from clad sheet and for large sections of almost all alloys. This type of quench also minimizes distortion and alleviates quench cracking. However, many specifications forbid the use of spray quenching for bare 2017 and 2024 sheet materials because of the effect on their resistance to corrosion.

Lag Between Soaking & Quenching The time interval between the removal of the material from the furnace and quenching is critical for some alloys and should be held to a minimum. The elapsed time must not exceed 10 seconds when solution heat-treating 2017 or 2024 sheet material. The allowable time for heavy sections may be slightly greater. Allowing the metal to cool slightly before quenching promotes re-precipitation from the solid solution. The precipitation occurs along grain boundaries and in certain slip planes causing poorer formability. In the case of 2017, 2024, and 7075 alloys, their resistance to intergranular corrosion is adversely affected.

Reheat-Treatment The treatment of material, which has been previously heat-treated, is considered a reheat-treatment. The unclad heat-treatable alloys can be solution heat-treated repeatedly without harmful effects. The number of solution heat-treatments allowed for clad sheet is limited due to increased diffusion of core and cladding with each reheating. Existing specifications allow one to three reheat-treatments of clad sheet depending upon cladding thickness. Straightening After Solution Heat-Treatment Some warping occurs during solution heat-treatment, producing kinks, buckles, waves, and twists. Straightening and flattening operations generally remove these imperfections.

Where the straightening operations produce an appreciable increase in the tensile and yield strengths and a slight decrease in the percent of elongation, the material is designated –T3 temper. When the above values are not materially affected, the material is designated –T4 temper. Precipitation Heat-Treating As previously stated, the aluminum alloys are in a comparatively soft state immediately after quenching from a solution heat-treating temperature. To obtain their maximum strengths, they must be either naturally aged or precipitation-hardened. During this hardening and strengthening operation, precipitation of the soluble constituents from the super- saturated solid solution takes place. The strength of the material increases (often by a series of peaks) until a maximum is reached, as precipitation progresses. Further aging, or over- 7-23 Thickness (inch) Time (minutes) Up to 0.032 30 0.032 to 1⁄8 30 40 Over ¼ 60 NOTE: Soaking time starts when the metal (or the molten bath) reaches a temperature within the range specified above.

aging, causes the strength to steadily decline until a somewhat stable condition is obtained. The submicroscopic particles that are precipitated provide the keys or locks within the grain structure and between the grains to resist internal slippage and distortion when a load of any type is applied. In this manner, the strength and hardness of the alloy are increased. Precipitation-hardening produces a great increase in the strength and hardness of the material with corresponding decreases in the ductile properties. The process used to obtain the desired increase in strength is therefore known as aging or precipitation-hardening.

The strengthening of the heat-treatable alloys by aging is not due merely to the presence of a precipitate. The strength is due to both the uniform distribution of a finely dispersed submicroscopic precipitate and its effects upon the crystal structure of the alloy. The aging practices used depend upon many properties other than strength. As a rule, the artificially-aged alloys are slightly over-aged to increase their resistance to corrosion. This is especially true with the artificially-aged, high-copper content alloys that are susceptible to intergranular corrosion when inadequately aged. The heat-treatable aluminum alloys are subdivided into two classes: those that obtain their full strength at room temperature and those that require artificial aging.

The alloys that obtain their full strength after 4 or 5 days at room temperature are known as natural-aging alloys. Precipitation from the supersaturated solid solution starts soon after quenching, with 90 percent of the maximum strength generally being obtained in 24 hours. Alloys 2017 and 2024 are natural-aging alloys. The alloys that require precipitation thermal treatment to develop their full strength are artificially-aged alloys. However, these alloys also age a limited amount at room temperature, the rate and extent of the strengthening depending upon the alloys. Many of the artificially-aged alloys reach their maximum natural or room temperature aging strengths after a few days. These can be stocked for fabrication in the –T4 or –T3 tempers. High zinc content alloys, such as 7075, continue to age appreciably over a long period of time. Their mechanical property changes being sufficient to reduce their formability.

The advantage of –W temper formability can be utilized; however, in the same manner as with natural-aging alloys; that is, by fabricating shortly after solution heat-treatment or retaining formability by using refrigeration. Refrigeration retards the rate of natural aging. At 32 °F, the beginning of the aging process is delayed for several hours, while dry ice (−50 °F to −100 °F) retards aging for an extended period of time. Precipitation Practices The temperatures used for precipitation-hardening depend upon the alloy and the properties desired, ranging from 250 °F to 375 °F. They should be controlled within a very narrow range (±5 °F) to obtain best results. [Figure 7-8] The time at temperature is dependent upon the temperature used, the properties desired, and the alloy. It ranges from 8 to 96 hours. Increasing the aging temperature decreases the soaking period necessary for proper aging. However, a closer control of both time and temperature is necessary when using the higher temperatures.

After receiving the thermal precipitation treatment, the material should be air cooled to room temperature. Water quenching, while not necessary, produces no ill effects. Furnace cooling tends to produce over-aging. Annealing of Aluminum Alloys The annealing procedure for aluminum alloys consists of heating the alloys to an elevated temperature, holding or soaking them at this temperature for a length of time depending upon the mass of the metal, and then cooling in still air. Annealing leaves the metal in the best condition for cold-working. However, when prolonged forming operations are involved, the metal takes on a condition known as “mechanical hardness” and resists further working. It may be necessary to anneal a part several times during the forming process to avoid cracking. Aluminum alloys should not be used in the annealed state for parts or fittings.

Clad parts should be heated as quickly and carefully as possible, since long exposure to heat tends to cause some of the constituents of the core to diffuse into the cladding. This 1⁄8 to 1/4 7-24 reduces the corrosion resistance of the cladding. Heat-Treatment of Aluminum Alloy Rivets Aluminum alloy rivets are furnished in the following compositions: alloys 1100, 5056, 2117, 2017, and 2024. Alloy 1100 rivets are used in the “as fabricated” condition for riveting aluminum alloy sheets where a low-strength rivet is suitable. Alloy 5056 rivets are used in the “as fabricated” condition for riveting magnesium alloy sheets.

Alloy 2117 rivets have moderately high strength and are suitable for riveting aluminum alloy sheets. These rivets receive only one heat-treatment, which is performed by the manufacturer, and are anodized after being heat-treated. They require no further heat-treatment before they are used. Alloy 2117 rivets retain their characteristics indefinitely after heat-treatment and can be driven anytime. Rivets made of this alloy are the most widely used in aircraft construction. Alloy 2017 and 2024 rivets are high-strength rivets suitable for use with aluminum alloy structures. They are purchased from the manufacturer in the heat-treated condition. Since the aging characteristics of these alloys at room temperatures are such that the rivets are unfit for driving, they must be reheat-treated just before they are to be used. Alloy 2017 rivets become too hard for driving in approximately 1 hour after quenching. Alloy 2024 rivets become hardened in 10 minutes after quenching. Both alloys may be reheat-treated as often as required; however, they must be anodized before the first reheat-treatment to prevent intergranular oxidation of the material. If these rivets are stored in a refrigerator at a temperature lower than 32 °F immediately after quenching, they remain soft enough to be usable for several days.

Rivets requiring heat-treatment are heated either in tubular containers in a salt bath or in small screen wire baskets in an air furnace. The heat-treatment of alloy 2017 rivets consists of subjecting the rivets to a temperature between 930 °F to 950 °F for approximately 30 minutes and immediately quenching in cold water. These rivets reach maximum strength in about 9 days after being driven. Alloy 2024 rivets should be heated to a temperature of 910 °F to 930 °F and immediately quenched in cold water. These rivets develop greater shear strength than 2017 rivets and are used in locations where extra strength is required. Alloy 2024 rivets develop their maximum shear strength in 1 day after being driven.

The 2017 rivet should be driven within approximately 1 hour and the 2024 rivet within 10 to 20 minutes after heat-treating or removal from refrigeration. If not used within these times, the rivets should be reheat-treated before being refrigerated. Heat-Treatment of Magnesium Alloys Magnesium alloy castings respond readily to heat-treatment, and about 95 percent of the magnesium used in aircraft construction is in the cast form. The heat-treatment of magnesium alloy castings is like the heat-treatment of aluminum alloys in that there are two types of heat-treatment: solution heat-treatment and precipitation (aging) heat- treatment. Magnesium, however, develops a negligible change in its properties when allowed to age naturally at room temperatures.

Solution Heat-Treatment Magnesium alloy castings are solution heat-treated to improve tensile strength, ductility, and shock resistance. This heat-treatment condition is indicated by using the symbol –T4 following the alloy designation. Solution heat-treatment plus artificial aging is designated –T6. Artificial aging is necessary to develop the full properties of the metal. Solution heat-treatment temperatures for magnesium alloy castings range from 730 °F to 780 °F, the exact range depending upon the type of alloy. The temperature range for each type of alloy is listed in Specification MIL-H-6857.

The upper limit of each range listed in the specification is the maximum temperature to which the alloy may be heated without danger of melting the metal. The soaking time ranges from 10 to 18 hours, the exact time depending upon the type of alloy as well as the thickness of the part. Soaking periods longer than 18 hours may be necessary for castings over 2 inches in thickness. Never heat magnesium alloys in a salt bath as this may result in an explosion. A serious potential fire hazard exists in the heat-treatment of magnesium alloys. If through oversight or malfunctioning of equipment the maximum temperatures are exceeded, the casting may ignite and burn freely. For this reason, the furnace used should be equipped with a safety cutoff that turns off the power to the heating elements and blowers if the regular control equipment malfunctions or fails. Some magnesium alloys require a protective atmosphere of sulfur dioxide gas during solution heat-treatment. This aids in preventing the start of a fire even if the temperature limits are slightly exceeded.

Air quenching is used after solution heat-treatment of magnesium alloys since there appears to be no advantage in liquid cooling. Precipitation Heat-Treatment After solution treatment, magnesium alloys may be given an aging treatment to increase hardness and yield strength. 7-25 Alloy Temperature (°F) Temperature DesignationQuench Temperature (°F) Temperature DesignationTime of Aging Solution Heat—Treatment Precipitation Heat—Treatment 2017 930–950 Cold water T4 T 2117 930–950 Cold water T4 T 2024 910–930 Cold water T4 T 6053 960–980 Water T4 445–455 1–2 hours T5 or 345–355 8 hours T6 6061 960–980 Water T4 315–325 18 hours T6 or 345–355 8 hours T6 7075 870 Water 250 24 hours T6 Generally, the aging treatments are used merely to relieve stress and stabilize the alloys to prevent dimensional changes later, especially during or after machining. Both yield strength and hardness are improved somewhat by this treatment at the expense of a slight amount of ductility. The corrosion resistance is also improved, making it closer to the “as cast” alloy.

Precipitation heat-treatment temperatures are considerably lower than solution heat-treatment temperatures and range from 325 °F to 500 °F. Soaking time ranges from 4 to 18 hours. Heat-Treatment of Titanium Titanium is heat-treated for the following purposes: • Relief of stresses set up during cold forming or machining. • Annealing after hot-working or cold-working, or to provide maximum ductility for subsequent cold- working. • Thermal hardening to improve strength. Stress Relieving Stress relieving is generally used to remove stress concentrations resulting from forming of titanium sheet. It is performed at temperatures ranging from 650 °F to 1,000 °F. The time at temperature varies from a few minutes for a very thin sheet to an hour or more for heavier sections. A typical stress relieving treatment is 900 °F for 30 minutes, followed by an air cool.

The discoloration or scale that forms on the surface of the metal during stress relieving is easily removed by pickling in acid solutions. The recommended solution contains 10 to 20 percent nitric acid and 1 to 3 percent hydrofluoric acid. The solution should be at room temperature or slightly above. Full Annealing The annealing of titanium and titanium alloys provides toughness, ductility at room temperature, dimensional and structural stability at elevated temperatures, and improved machinability. The full anneal is usually called for as preparation for further working. It is performed at 1,200 to 1,650 °F. The time at temperature varies from 16 minutes to several hours, depending on the thickness of the material and the amount of cold-work to be performed. The usual treatment for the commonly used alloys is 1,300 °F for 1 hour, followed by an air cool. A full anneal generally results in sufficient scale formation to require the use of caustic descaling, such as sodium hydride salt bath.

Thermal Hardening Unalloyed titanium cannot be heat-treated, but the alloys commonly used in aircraft construction can be strengthened by thermal treatment, usually at some sacrifice in ductility. For best results, a water quench from 1,450 °F, followed by reheating to 900 °F for 8 hours is recommended. Case Hardening The chemical activity of titanium and its rapid absorption of oxygen, nitrogen, and carbon at relatively low temperatures make case hardening advantageous for special applications. Nitriding, carburizing, or carbonitriding can be used to produce a wear-resistant case of 0.0001 to 0.0002 inch in depth.

Hardness Testing

Hardness testing is a method of determining the results of heat- treatment, as well as the state of a metal prior to heat-treatment. Since hardness values can be tied in with tensile strength values and, in part, with wear resistance. Hardness tests are a valuable 7-26 check of heat-treat control and of material properties. Practically all hardness testing equipment now uses the resistance to penetration as a measure of hardness. Included among the better-known hardness testers are the Brinell and Rockwell, both of which are described and illustrated in this section. Also included is the Barcol tester, a popular portable- type hardness tester currently in use.

Brinell Tester The Brinell hardness tester uses a hardened spherical ball that is forced into the surface of the metal. [Figure 7-9] This ball is 10 millimeters (0.3937 inch) in diameter. A pressure of 3,000 kilograms is used for ferrous metals and 500 kilograms for nonferrous metals. The pressure must be maintained at least 10 seconds for ferrous metals and at least 30 seconds for nonferrous metals. The load is applied by hydraulic pressure. A hand pump or an electric motor, depending on the model of tester, builds up the hydraulic pressure. A pressure gauge indicates the amount of pressure. There is a release mechanism for relieving the pressure after the test has been made, and a calibrated microscope is provided for measuring the diameter of the impression in millimeters. The machine has various shaped anvils for supporting the specimen and an elevating screw for bringing the specimen in contact with the ball penetrator. These are attachments for special tests.

To determine the Brinell hardness number for a metal, measure the diameter of the impression using the calibrated microscope furnished with the tester. Then convert the measurement into the Brinell hardness number on the conversion table furnished with the tester. Rockwell Tester The Rockwell hardness tester measures the resistance to penetration, as does the Brinell tester. [Figure 7-10] Instead of measuring the diameter of the impression, the Rockwell tester measures the depth, and the hardness is indicated directly on a dial attached to the machine. The dial numbers in the outer circle are black and the inner numbers are red.

Rockwell hardness numbers are based on the difference between the depth of penetration at major and minor loads. The greater this difference, the lower the hardness number and the softer the material. Two types of penetrators are used with the Rockwell tester: a diamond cone and a hardened steel ball. The load, which forces the penetrator into the metal, is called the major load and is measured in kilograms. The results of each penetrator and load combination are reported on separate scales designated by letters. The penetrator, the major load, and the scale vary with the kind of metal being tested.

For hardened steels, the diamond penetrator is used; the major load is 150 kilograms; and the hardness is read on the “C” scale. When this reading is recorded, the letter “C” must precede the number indicated by the pointer. The C-scale setup is used for testing metals ranging in hardness from C-20 to the hardest steel (usually about C-70). If the metal is softer than C-20, the B-scale setup is used. With this setup, the 1⁄16-inch ball is used as a penetrator; the major load is 100 kilograms; and the hardness is read on the B-scale. In addition to the C and B scales, there are other setups for special testing. The scales, penetrators, major loads, and dial numbers to be read are listed in Figure 7-11.

The Rockwell tester is equipped with a weight pan, and two weights are supplied with the machine. One weight is marked in red. The other weight is marked in black. With no weight in the weight pan, the machine applies a major load of 60 kilograms. If the scale setup calls for a 100-kilogram load, the red weight is placed in the pan. For a 150-kilogram load, the black weight is added to the red weight. The black weight is always used with the red weight; it is never used alone. Practically all testing is done with either the B-scale setup or the C-scale setup. For these scales, the colors may be used as a guide in selecting the weight (or weights) and in reading the dial. For the B-scale test, use the red weight and read the red numbers. For a C-scale test, add the black weight to the red weight and read the black numbers.

In setting up the Rockwell machine, use the diamond penetrator for testing materials known to be hard. If the hardness is unknown, try the diamond, since the steel ball may be deformed if used for testing hard materials. If the metal tests below C-22, then change to the steel ball. Use the steel ball for all soft materials, those testing less than B-100. Should an overlap occur at the top of the B-scale and the bottom of the C-scale, use the C-scale setup. Before the major load is applied, securely lock the test specimen in place to prevent slipping and to seat the anvil and penetrator properly. To do this, apply a load of 10 kilograms before the lever is tripped. This preliminary load is called the minor load.

The minor load is 10 kilograms regardless of the scale setup. The metal to be tested in the Rockwell tester must be ground smooth on two opposite sides and be free of scratches and foreign matter. The surface should be perpendicular to the axis of penetration, and the two opposite ground surfaces should be parallel. If the specimen is tapered, the amount of error depends on the taper. A curved surface also causes a slight error in the hardness test. The amount of error depends 7-27 Pressure gauge Hydraulic actuating unit Penetrator Elevating screw Hand pump Microscope 0 500 1000 1500 2000 3000 4000 4500 5000 3500 2500 ZEROMNDER FOR RAP TESTING c0 c50 90 80 70 60 10 20 30 40 cc c B30 BB B 4020 10 0 90 50 60 70 ZEROMNDER FOR RAP TESTING c0 c50 90 80 70 60 10 20 30 40 c c c B30 B B 40 20 10 0 90 50 60 70 Weights Weight pan Trip lever Zero adjuster Hand wheel Elevating screw Penetrator Specimen Anvil on the curvature (i.e., the smaller the radius of curvature, the greater the error). To eliminate such error, a small flat should be ground on the curved surface if possible.

Clad aluminum alloy sheets cannot be tested directly with any accuracy with a Rockwell hardness tester. If the hardness value of the base metal is desired, the pure aluminum coating must be removed from the area to be checked prior to testing. Barcol Tester The Barcol tester is a portable unit designed for testing aluminum alloys, copper, brass, or other relatively soft materials. [Figure 7-12] It should not be used on aircraft steels. Approximate range of the tester is 25 to 100 Brinell. The unit can be used in any position and in any space that allows for the operator’s hand. It is of great value in the hardness testing of assembled or installed parts, especially to check for proper heat-treatment. The hardness is indicated on a dial conveniently divided into 100 graduations.

The design of the Barcol tester is such that operating experience is not necessary. It is only necessary to exert a light pressure against the instrument to drive the spring- loaded indenter into the material to be tested. The hardness reading is instantly indicated on the dial. Several typical readings for aluminum alloys are listed in the Barcol number. To prevent damage to the point, avoid sliding or scraping when it is in contact with the material being tested. If the point should become damaged, it must be replaced with a new one. Do not attempt to grind the point. Each tester is supplied with a test disk for checking the condition of the point. To check the point, press the instrument down on the test disk. When the downward pressure brings the end of the lower plunger guide against the surface of the disk, the indicator reading should be within the range shown on the test disk.

Forging Forging is the process of forming a product by hammering or pressing. When the material is forged below the recrystallization temperature, it is called cold forged. When worked above the recrystallization temperature, it is referred 7-28 A Diamond 60 Black B 100 Red C Diamond 150 Black D Diamond 100 Black E 100 Red F 60 Red G 150 Red H 60 Red K 150 Red Scale Symbol Penetrator Major Load (kg) Dial Color⁄Number 0 50 10 90 20 80 30 70 40 60 to as hot forged. Drop forging is a hammering process that uses a hot ingot that is placed between a pair of formed dies in a machine called a drop hammer and a weight of several tons is dropped on the upper die. This results in the hot metal being forced to take the form of the dies. Because the process is very rapid, the grain structure of the metal is altered, resulting in a significant increase in the strength of the finished part.

Casting Melting the metal and pouring it into a mold of the desired shape forms casting. Since plastic deformation of the metal does not occur, no alteration of the grain shape or orientation is possible. The cooling rate, the alloys of the metal, and the thermal treatment can control the gain size of the metal. Castings are normally lower in strength and are more brittle than a wrought product of the same material. For intricate shapes or items with internal passages, such as turbine blades, casting may be the most economical process. Except for engine parts, most metal components found on an aircraft are wrought instead of cast.

All metal products start in the form of casting. Wrought metals are converted from cast ingots by plastic deformation. For high-strength aluminum alloys, an 80 to 90 percent reduction (dimensional change in thickness) of the material is required to obtain the high mechanical properties of a fully wrought structure. Both iron and aluminum alloys are cast for aircraft uses. Cast iron contains 6 to 8 percent carbon and silicon. Cast iron is a hard un-malleable pig iron made by casting or pouring into a mold. Cast aluminum alloy has been heated to its molten state and poured into a mold to give it the desired shape.

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. The shape of the die will be the cross section of an angle, channel, tube, or some other shape. Some metals, such as lead, tin, and aluminum, may be extruded cold; however, most metals are heated before extrusion. The main advantage of the extrusion process is its flexibility. For example, because of its workability, aluminum can be economically extruded to more intricate shapes and larger sizes than is practical with 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 750–850 °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. The extrusion process forms many structural parts, such as channels, angles, T-sections, and Z-sections.

Aluminum is the most extruded metal used in aircraft. Aluminum is extruded at a temperature of 700–900 °F (371–482 °C) and requires pressure of up to 80,000 psi (552 MPa). After extrusion, the product frequently is subjected to both thermal and mechanical processes to obtain the desired properties. Extrusion processes are limited to the more ductile materials. 1/16" ball 1/8" ball 1/16" ball 1/8" ball