Manufacturing 7

Alloy 905 is a bronze casting alloy containing 10% tin and 2% zinc. In the as-cast condition, the tensile strength is about 310 MPa (45 ksi), with an elongation of 45%. It has very good resistance to seawater corrosion and is used on ships for pipe fittings, gears, pump parts, bushings, and bearings. Bronzes can also be made by mixing powders of copper and tin, followed by lowdensity powder metallurgy processing (described in Chapter 19).The porous product can be used as a filter for high-temperature or corrosive media, or it can be infiltrated with oil to produce self-lubricating bearings. COPPER-NICKEL ALLOYS Copper and nickel exhibit complete solubility (as shown previously in Figure 4-6), and a wide range of useful alloys have been developed. Key features include high thermal conductivity, high-temperature strength, and corrosion resistance to a range of materials, including seawater. These properties, coupled with a high resistance to stress- corrosion cracking, make the copper-nickel alloys a good choice for heat exchangers, cookware, desalination apparatus, and a wide variety of coinage. Cupronickels contain 2 to 30% nickel. Nickel silvers contain no silver, but 10 to 30% nickel and at least 5% zinc.The bright silvery luster makes them attractive for ornamental applications, and they are also used for musical instruments.An alloy with 45% nickel is known as constantan, and the 67%-nickel material is called Monel. Monel will be discussed later in the chapter as a nickel alloy. OTHER COPPER-BASED ALLOYS The copper alloys discussed previously acquire their strength primarily through solidsolution strengthening and cold work.Within the copper-alloy family, alloys containing aluminum, silicon, or beryllium can be strengthened by precipitation hardening. SECTION 7.2 Copper and Copper Alloys 143 TABLE 7-3 Composition, Properties, and Uses of Some Common Copper-Zinc Alloys Composition(%) Tensile Strength CDA Elongation in Number Common Name Cu Zn Sn Pb Mn Condition ksi MPa 2 in. (%) Typical Uses 220 Commercial 90 10 Soft sheet 38 262 45 Screen wire, bronze Hard sheet 64 441 4 hardware, screws. jewelry Spring 73 503 3 Drawing, 240 Low brass 80 20 Annealed sheet 47 324 47 architectural work, ornamental Hard 75 517 7 Spring 91 627 3 Munitions, 260 Cartridge 70 30 Annealed sheet 53 365 54 hardware, musical brass instruments, Hard 76 524 7 tubing Spring 92 634 3 Cold forming, 270 Yellow brass 65 35 Annealed sheet 46 317 64 radiator cores, springs, screws Hard 76 524 7 280 Muntz metal 60 40 Hot-rolled 54 372 45 Architectural work; Cold-rolled 80 551 5 condenser tube 443-445 Admiralty metal 71 28 1 Soft 45 310 60 Condenser tube Hard 95 655 5 (salt water), heat exchangers 360 Free-cutting brass 61.5 35.3 3 Soft 47 324 60 Screw-machine Hard 62 427 20 parts 675 Manganese 58.5 39 1 0.1 Soft 65 448 33 Clutch disks, bronze Bars, half hard 84 579 19 pump rods, valve stems, highstrength propellers dega-c07_139-161-hr 1/9/07 3:31 PM Page 143 144 CHAPTER 7 Nonferrous Metals and Alloys Aluminum-bronze alloys are best known for their combination of high strength and excellent corrosion resistance, and they are often considered to be cost-effective alternatives to stainless steel and nickel-based alloys. The wrought alloys can be strengthened by solid-solution strengthening, cold work, and the precipitation of iron- or nickel-rich phases. With less than 8% aluminum, the alloys are very ductile. When aluminum exceeds 9%, however, the ductility drops and the hardness approaches that of steel. Still higher aluminum contents result in brittle, but wear-resistant, materials. By varying the aluminum content and heat treatment, the tensile strength can range from about 415 to 1000 MPa (60 to 145 ksi). Typical applications include marine hardware, power shafts, sleeve bearings, and pump and valve components for handling seawater, sour mine water, and various industrial fluids. Cast alloys are available for applications where casting is the preferred means of manufacture. Since aluminum bronze exhibits large amounts of solidification shrinkage, castings made of this material should be designed with this in mind. Silicon-bronzes contain up to 4% silicon and 1.5% zinc (higher zinc contents may be used when the material is to be cast). Strength, formability, machinability, and corrosion resistance are all quite good. Tensile strengths range from a soft condition of about 380 MPa (55 ksi) through a maximum that approaches 900 MPa (130 ksi). Uses include boiler, tank, and stove applications, which require a combination of weldability, high strength, and corrosion resistance. Copper-beryllium alloys, which ordinarily contain less than 2% beryllium, can be age hardened to produce the highest strengths of the copper-based metals but are quite expensive to use.When annealed, the material has a yield strength of 170 MPa (25 ksi), tensile strength of 480 MPa (70 ksi), and an elongation of 50%. After heat treatment, these properties can rise to 1100 MPa (160 ksi), 1250 MPa (180 ksi), and 5%, respectively. Cold work coupled with age hardening can produce even stronger material.The modulus of elasticity is about , and the endurance limit is around 275 MPa (40 ksi).These properties make the material an excellent choice for electrical contact springs, but cost limits application to small components requiring long life and high reliability. Other applications, such as spark-resistant safety tools and spot-welding electrodes, utilize the unique combination of properties: (1) the material has the strength of heat-treated steel, but is also (2) nonsparking, nonmagnetic, and electrically and thermally conductive. Concerns over the toxicity of beryllium have created a demand for substitute alloys with similar properties, but no clear alternative has emerged. LEAD-FREE CASTING ALLOYS For many years, lead has been a common alloy additive to cast copper alloys. It helped to fill and seal the microporosity that forms during solidification, thereby providing the pressure tightness required for use with pressurized gases and fluids.The lead also acted as a lubricant and chip-breaker, enhancing the machinability and machined surface finish. Many plumbing components have been made from leaded red and semi-red brass casting alloys. With increased concern about lead in drinking water and the introduction of environmental regulations, efforts were made to develop lead-free copper-based casting alloys.Among the most common are the EnviroBrass alloys, which use bismuth and selenium as substitutes for lead. Bismuth is not known to be toxic for humans and has been used in a popular remedy for an upset stomach. Selenium is an essential nutrient for humans.While somewhat lower in ductility, the new alloys have been shown to have mechanical properties, machinability and platability that are quite similar to the traditional leaded materials. ■ 7.3 ALUMINUM AND ALUMINUM ALLOYS GENERAL PROPERTIES AND CHARACTERISTICS Although aluminum has only been a commercial metal for about 120 years, it now ranks second to steel in both worldwide quantity and expenditure, and it is clearly the most important of the nonferrous metals. It has achieved importance in virtually all segments of the economy, with principal uses in transportation, containers and packaging, building

CORROSION RESISTANCE OF ALUMINUM AND ITS ALLOYS Pure aluminum is very reactive and forms a tight, adherent oxide coating on the surface as soon as it is exposed to air.This oxide is resistant to many corrosive media and serves as a corrosion-resistant barrier to protect the underlying metal. Like stainless steels, the corrosion resistance of aluminum is actually a property of the oxide, not the metal itself. Since the oxide formation is somewhat retarded when alloys are added, aluminum alloys do not have quite the corrosion resistance of pure aluminum. The oxide coating also causes difficulty when welding. To produce consistentquality resistance welds, it is usually necessary to remove the tenacious oxide immediately before welding. For fusion welding, special fluxes or protective inert gas atmospheres must be used to prevent material oxidation.While welding aluminum may be more difficult than steel, suitable techniques have been developed to permit the production of high-quality, cost-effective welds with most of the welding processes. CLASSIFICATION SYSTEM Aluminum alloys can be divided into two major groups based on the method of fabrication. Wrought alloys are those that are shaped as solids and are therefore designed to have attractive forming characteristics, such as low yield strength, high ductility, good fracture resistance, and good strain hardening. Casting alloys achieve their shape as they solidify in molds or dies. Attractive features for the casting alloys include low melting point, high fluidity, and attractive as-solidified structures and properties. Clearly, these properties are distinctly different, and the alloys that have been designed to meet them are also different. As a result, separate classification systems exist for the wrought and cast aluminum alloys. WROUGHT ALUMINUM ALLOYS The wrought aluminum alloys are generally identified using the standard four-digit designation system for aluminums. The first digit indicates the major alloy element or elements as described below: SECTION 7.3 Aluminum and Aluminum Alloys 147 Major Alloying Element Aluminum, 99.00% and greater 1xxx Copper 2xxx Manganese 3xxx Silicon 4xxx Magnesium 5xxx Magnesium and silicon 6xxx Zinc 7xxx Other element 8xxx The second digit is usually zero. Nonzero numbers are used to indicate some form of modification or improvement to the original alloy.The last two digits simply indicate the particular alloy within the family. For example, 2024 simply means alloy number 24 within the 2xxx, or aluminum-copper, system. For the 1xxx series, the last three digits are used to denote the purity of the aluminum. The four digits of a wrought aluminum designation identify the chemistry of the alloy.Additional information about the alloy condition is then provided through a temper designation, in the form of a letter or letter-number suffix using the following system: -F: as fabricated -H: strain-hardened -H1: strain-hardened by working to desired dimensions; a second digit, 1 through 9, indicates the degree of hardening, 8 being commercially full-hard and 9 extra-hard -H2: strain-hardened by cold working, followed by partial annealing -H3: strain-hardened and stabilized

MAGNESIUM ALLOYS AND THEIR FABRICATION A designation system for magnesium alloys has been developed by the ASTM, identifying both chemical composition and temper, and is presented in specification B93.Two prefix letters designate the two largest alloying metals in order of decreasing amount, using the following format: A aluminum F iron M manganese R chromium B bismuth H thorium N nickel S silicon C copper K zirconium P lead T tin D cadmium L beryllium Q silver Z zinc E rare earth Aluminum is the most common alloying element and, along with zinc, zirconium, and thorium, promotes precipitation hardening. Manganese improves corrosion resistance, and tin improves castability. The two letters are then followed by two or three numbers and a possible suffix letter. The numbers correspond to the rounded-off whole-number percentages of the two main alloy elements and are arranged in the same order as the letters. Thus the AZ91 alloy would contain approximately 9% aluminum and 1% zinc. A suffix letter is used to denote variations of the same base alloy, such as AZ91A. The temper-designation suffix is quite similar to that used with the aluminum alloys.Table 7-6 lists some of the more common magnesium alloys together with their properties and uses. Sand, permanent-mold, die, semisolid, and investment casting are all well developed for magnesium alloys and take advantage of the low melting points and high fluidity. Die casting is clearly the most popular manufacturing process for magnesium, accounting for 70% of all castings.Although the magnesium alloys typically cost about twice as much as aluminum, the hot-chamber die-casting process used with magnesium is easier, more economical, and 40 to 50% faster than the cold-chamber process generally required for aluminum. Wall thickness, draft angle, and dimensional tolerances are all lower than for both aluminum die castings and thermoplastic moldings. Die life is significantly greater than that observed with aluminum.As a result, magnesium die castings compete well with aluminum2 and often replace plastic injection-molded components when improved stiffness or dimensional stability, or the benefits of electrical or thermal conductivity, are required. Forming behavior is poor at room temperature, but most conventional processes can be performed when the material is heated to temperatures between 250 and 500C (480 and 775F). Since these temperatures are easily attained and generally do not require a protective atmosphere, many formed and drawn magnesium products are manufactured. Magnesium extrusions and sheet metal products have properties similar to the more common wrought aluminum alloys.While slightly heavier than plastics, they offer an order of magnitude or greater improvement in stiffness or rigidity. The machinability of magnesium alloys is the best of any commercial metal and, in many applications, the savings in machining costs, achieved through deeper cuts, higher cutting speeds, and longer tool life, more than compensate for the increased cost of the material. It is necessary, however, to keep the tools sharp and provide adequate cooling for the chips. Magnesium alloys can be spot welded almost as easily as aluminum, but scratch brushing or chemical cleaning is necessary before forming the weld. Fusion welding is best performed with processes using an inert shielding atmosphere of argon or helium gas. While heat treatments can be used to increase strength, the added increment achieved by age hardening is far less than observed with aluminum. In fact, the strongest magnesium alloy is only about three times stronger than the weakest. Because of this, designs must be made to accommodate the material, rather than the material being tailored to the design. Considerable misinformation exists regarding the fire hazards when processing or using magnesium alloys. It is true that magnesium alloys are highly combustible

absorbing or sound- and vibration-damping shields. Lead-acid batteries are clearly the dominant product, and over 60% of U.S. lead consumption is generated from battery recycling. Other applications utilize the properties of good corrosion resistance, low melting point, and the ease of casting or forming. As a pure metal, tin is used primarily as a corrosion-resistant coating on steel. In the form of alloys, lead and tin are almost always used together. Bearing material and solder are the two most important uses. One of the oldest and best bearing materials is an alloy of 84% tin, 8% copper, and 8% antimony, known as genuine or tin babbitt. Because of the high cost of tin, however, lead babbitt, composed of 85% lead, 5% tin, 10% antimony, and 0.5% copper, is a more widely used bearing material.The tin and antimony combine to form hard particles within the softer lead matrix.The shaft rides on the harder particles with low friction, while the softer matrix acts as a cushion that can distort sufficiently to compensate for misalignment and assure a proper fit between the two surfaces. For slow speeds and moderate loads, the lead-based babbitts have proven to be quite adequate. Soft solders are basically lead-tin alloys with a chemical composition near the eutectic value of 61.9% tin (see Figure 4-5). While the eutectic alloy has the lowest melting temperature, the high cost of tin has forced many users to specify solders with a lower-than-optimum tin content. A variety of compositions are available, each with its own characteristic melting range. Environmental concerns and recent legislation have prompted a move toward lead-free solders for applications involving water supply and distribution.Additional information on solders and soldering is provided in Chapter 34. ■ 7.10 SOME LESSER KNOWN METALS AND ALLOYS Several of the lesser known metals have achieved importance as a result of their somewhat unique physical and mechanical properties. Beryllium combines a density less than aluminum with a stiffness greater than steel and is transparent to X-rays. Hafnium, thorium, and beryllium are used in nuclear reactors because of their low neutron-absorption characteristics. Depleted uranium, because of its very high density (19.1 g/cm3 ), is useful in special applications where maximum weight must be put into a limited space, such as counterweights or flywheels. Cobalt, in addition to its use as a base metal for superalloys, is used as a binder in various powder-based components and sintered carbides, where it provides good high-temperature strength. Zirconium is used for its outstanding corrosion resistance to most acids, chlorides, and organic acids. It offers high strength, good weldability and fatigue resistance, and attractive neutron-absorption characteristics. Rare earth metals have been incorporated into magnets that offer increased strength compared to the standard ferrite variety. Neodymium-iron-boron and samarium-cobalt are two common varieties. While the precious metals (gold,silver, and the platinum group metals—platinum, palladium, rhodium, ruthenium, iridium, and osmium) may seem unlikely as engineering materials, they offer outstanding corrosion resistance and electrical conductivity, often under extreme conditions of temperature and environment. ■ 7.11 METALLIC GLASSES Metallic glasses, or amorphous metals, have existed in the form of thin ribbons and fine powders since the 1960s. By cooling liquid metal at a rate that exceeds 105 to 106 C/ second, a rigid solid is produced that lacks crystalline structure. Since the structure also lacks the crystalline "defects" of grain boundaries and dislocations, the materials exhibit extraordinary mechanical properties (high strength, large elastic strain, good toughness, and wear resistance), unusual magnetic behavior, and high corrosion resistance. Recent developments have enabled the production of amorphous metal with cooling rates of only 1 to 100 C/second. Known as bulk metallic glass (BMG), complexshaped parts of this material with thicknesses up to several centimeters can now be produced by conventional casting methods, such as die casting. Because the material goes from liquid to glass, not liquid to crystalline solid, precision products can be made with a total shrinkage that is often less than 0.5%. Pellets or powders of bulk metallic

alloys are aluminum-copper-lithium, with about 4% copper and no more than 2% lithium. The weight benefits are still sufficient to warrant use in a number of aerospace applications, and the fact that they can be fabricated by conventional processes make them attractive alternatives to the advanced composites. Since aluminum alloys can comprise as much as 80% of the weight of commercial aircraft, even small percentage reductions can be significant. Improved strength and stiffness can further facilitate weight reduction. Fuel savings over the life of the airplane would more than compensate for any additional manufacturing expense. As an example of potential, the weight of the external liquid-hydrogen tank on the U.S. space shuttle booster rocket was reduced by approximately 3400 kg (7500 lb) by conversion to an aluminum-lithium alloy. ALUMINUM FOAM A material known as "stabilized aluminum foam" can be made by mixing ceramic particles with molten aluminum and blowing gas into the mixture. The bubbles remain through solidification, yielding a structure that resembles metallic Styrofoam. Originally developed around 2000 for automotive, aerospace, and military applications, the material has found additional uses in architecture and design. Strength-to-weight is outstanding, and the material offers excellent energy absorption. The fuel cells of race cars have been shrouded with aluminum foam, and foam fill has been inserted between the front of cars and the driver compartment.Tubular structures can be filled with foam to increase strength, absorb energy, and provide resistance to crushing. Still other applications capitalize on the excellent thermal insulation, vibration damping, and sound absorption that results from the numerous trapped air pockets. ■ 7.4 MAGNESIUM AND MAGNESIUM ALLOYS GENERAL PROPERTIES AND CHARACTERISTICS Magnesium is the lightest of the commercially important metals, having a specific gravity of about 1.74 (two-thirds that of aluminum, one-fourth that of steel, and only slightly higher than fiber-reinforced plastics). Like aluminum, magnesium is relatively weak in the pure state and for engineering purposes is almost always used as an alloy. Even in alloy form, however, the metal is characterized by poor wear, creep, and fatigue properties. It has the highest thermal expansion of all engineering metals. Strength drops rapidly when the temperature exceeds so magnesium should not be considered for elevated-temperature service. Its modulus of elasticity is even less than that of aluminum, being between one-fourth and one-fifth that of steel.Thick sections are required to provide adequate stiffness, but the alloy is so light that it is often possible to use thicker sections for the required rigidity and still have a lighter structure than can be obtained with any other metal. Cost per unit volume is low, so the use of thick sections is generally not prohibitive. Moreover, since a large portion of magnesium components are cast, the thicker sections actually become a desirable feature. Ductility is frequently low, a characteristic of the hexagonal-close-packed (HCP) crystal structure, but some alloys have values exceeding 10%. On the more positive side, magnesium alloys have a relatively high strength-toweight ratio, with some commercial alloys attaining strengths as high as 380 MPa (55 ksi). High energy absorption means good damping of noise and vibration, as well as impact and dent resistance. While many magnesium alloys require enamel or lacquer finishes to impart adequate corrosion resistance, this property has been improved markedly with the development of higher-purity alloys. In the absence of unfavorable galvanic couples, these materials have excellent corrosion resistance and are finding applications in a wide range of markets, including automotive, aerospace, power tools, sporting goods, and electronic products (where they offer a combination of electromagnetic shielding, light weight, and durability exceeding that of plastics and alternative metals).While aluminum alloys are often used for the load-bearing members of mechanical structures, magnesium alloys are best suited for those applications where lightness is the primary consideration and strength is a secondary requirement.

7.7 NICKEL-BASED ALLOYS Nickel-based alloys are most noted for their outstanding strength and corrosion resistance, particularly at high temperatures, and are available in a wide range of wrought and cast grades.Wrought alloys are generally known by tradenames, such as Monel, Hastelloy, Inconel, Incoloy, and others. Cast alloys are generally identified by Alloy Casting Institute or ASTM designations. General characteristics include good formability (face-centered-cubic crystal structure), good creep resistance, and the retention of strength and ductility at cold or even cryogenic temperatures. Monel metal, an alloy containing about 67% nickel and 30% copper, has been used for years in the chemical- and food-processing industries because of its outstanding corrosion characteristics. In fact, Monel probably has better corrosion resistance to more media than any other commercial alloy. It is particularly resistant to salt water, sulfuric acid, and even high-velocity, high-temperature steam. For the latter reason, Monel has been used for steam turbine blades. It can be polished to have an excellent appearance, similar to that of stainless steel, and is often used in ornamental trim and household ware. In its most common form, Monel has a tensile strength ranging from 500 to 1200 MPa (70 to 170 ksi), with a companion elongation ranging between 2 and 50%. Nickel-based alloys have also been used for electrical resistors and heating elements. These materials are primarily nickel-chromium alloys and are known by the trade name Nichrome. They have excellent resistance to oxidation while retaining useful strength at red heats.Invar, an alloy of nickel and 36% iron, has a near-zero thermal expansion and is used where dimensions cannot change with a change in temperature. Other nickel-based alloys have been designed to provide good mechanical properties at extremely high temperatures and are generally classified as superalloys. These alloys will be discussed along with other, similar materials in the following section. ■ 7.8 SUPERALLOYS AND OTHER METALS DESIGNED FOR HIGH-TEMPERATURE SERVICE Titanium and titanium alloys have already been cited as being useful in providing strength at elevated temperatures, but the maximum temperature for these materials is approximately 535C (1000C). Jet engine, gas-turbine, rocket, and nuclear applications often require materials that possess high strength, creep resistance, oxidation and corrosion resistance, and fatigue resistance at temperatures up to and in excess of 1100C (2000C). Other application areas include heat exchangers, chemical reaction vessels, and furnace components. One class of materials offering these properties is the superalloys, first developed in the 1940s for use in the elevated-temperature areas of turbojet aircraft. These alloys are based on nickel, iron and nickel, or cobalt and have the ability to retain most of their strength even after long exposures to extremely high temperatures. Strength comes from solid-solution strengthening, precipitation hardening, and dispersed alloy carbides or oxides.The nickel-based alloys tend to have higher strengths at room temperature, with yield strengths up to 1200 MPa (175 ksi) and ultimate tensile strengths as high as 1450 MPa (210 ksi). The 1000-hour rupture strengths of the nickel-based alloys at are also higher than those of the cobalt-based material. Unfortunately, the density of all superalloy metals is significantly greater than that of iron, so their use is often at the expense of additional weight. Most of the superalloys are difficult to form or machine, so methods such as electrodischarge, electrochemical, or ultrasonic machining are often used, or the products are made to final shape as investment castings. Powder metallurgy techniques are also used extensively. Because of their ingredients, all of the alloys are quite expensive, and this limits their use to small or critical parts where the cost is not the determining factor. A number of engineering applications require materials whose temperature limits exceed those of the superalloys.Figure 7-5 shows the high-temperature exhaust of a jet engine. One reference estimates that the exhaust of future jet engines will reach temperatures in excess of . Rocket nozzles go well beyond this point. Materials such as TDnickel (a powder metallurgy nickel alloy containing 2% dispersed thorium oxide) can operate

Color anodizing offers an inexpensive and attractive means of surface finishing. A thick aluminum oxide is produced on the surface. Colored dye is then placed on the porous surface and is sealed by immersion into hot water.The result is the colored metallic finish commonly observed on products such as bicycle frames and softball bats. ALUMINUM CASTING ALLOYS Although its low melting temperature tends to make it suitable for casting, pure aluminum is seldom cast. Its high shrinkage upon solidification (about 7%) and susceptibility to hot cracking cause considerable difficulty, and scrap is high. By adding small amounts of alloying elements, however, very suitable casting characteristics can be obtained and strength can be increased.Aluminum alloys are cast in considerable quantity by a variety of processes. Many of the most popular alloys contain enough silicon to produce the eutectic reaction, which is characterized by a low melting point and high ascast strength. Silicon also improves the fluidity of the metal, making it easier to produce complex shapes or thin sections, but high silicon also produces an abrasive, difficult-to-cut material. Copper, zinc, and magnesium are other popular alloy additions that permit the formation of age-hardening precipitates. Table 7-5 lists some of the commercial aluminum casting alloys and uses the three-digit designation system of the Aluminum Association to designate alloy chemistry. The first digit indicates the alloy group as follows: 150 CHAPTER 7 Nonferrous Metals and Alloys Major Alloying Element Aluminum, 99.00% and greater 1xx.x Copper 2xx.x Silicon with Cu and/or Mg 3xx.x Silicon 4xx.x Magnesium 5xx.x Zinc 7xx.x Tin 8xx.x Other elements 9xx.x The second and third digits identify the particular alloy or aluminum purity, and the last digit, separated by a decimal point, indicates the product form (e.g., casting or ingot). A letter before the numerical designation indicates a modification of the original alloy, such as a small variation in the amount of an alloying element or impurity. Aluminum casting alloys have been designed for both properties and process.When the strength requirements are low, as-cast properties are usually adequate. High-strength castings usually require the use of alloys that can subsequently be heat treated. Sand casting has the fewest process restrictions. The aluminum alloys used for permanent mold casting are designed to have lower coefficients of thermal expansion (or contraction) because the molds offer restraint to the dimensional changes that occur upon cooling. Die-casting alloys require high degrees of fluidity because they are often cast in thin sections. Most of the die-casting alloys are also designed to produce high "as-cast" strength without heat treatment, using the rapid cooling conditions of the die-casting process to promote a fine grain size and fine eutectic structure. Tensile strengths of the aluminum permanent-mold and die-casting alloys can be in excess of 275 MPa (40 ksi). ALUMINUM-LITHIUM ALLOYS Lithium is the lightest of all metallic elements, and in the search for aluminum alloys with higher strength, greater stiffness, and lighter weight, aluminum-lithium alloys have emerged. Each percent of lithium reduces the overall weight by 3% and increases stiffness by 6%. The initially developed alloys offered 8 to 10% lower density, 15 to 20% greater stiffness, strengths comparable to those of existing alloys, and good resistance to fatigue crack propagation. Unfortunately, fracture toughness, ductility, and stress- corrosion resistance were poorer than for conventional alloys. The current-generation

Nonferrous metals and alloys have assumed increasingly important roles in modern technology. Because of their number and the fact that their properties vary widely, they provide an almost limitless range of properties for the design engineer.While they tend to be more costly than iron or steel, these metals often possess certain properties or combinations of properties that are not available in the ferrous metals, such as: 1. Resistance to corrosion 2. Ease of fabrication 3. High electrical and thermal conductivity 4. Light weight 5. Strength at elevated temperatures 6. Color Nearly all the nonferrous alloys possess at least two of the qualities listed above, and some possess nearly all. For many applications, specific combinations of these properties are highly desirable. Each year, the average American requires about 65 pounds of aluminum, 21 pounds of copper, 12 pounds of lead, 11 pounds of zinc, and 25 pounds of various other nonferrous metals. Figure 7-1 classifies some of the nonferrous metals by advantageous engineering properties, and Table 7-1 shows the increasing role of the nonferrous metals in a typical family vehicle. As a whole, the strength of the nonferrous alloys is generally inferior to that of steel.Also, the modulus of elasticity is usually lower, a fact that places them at a distinct disadvantage when stiffness is a required characteristic. Ease of fabrication is often attractive.Those alloys with low melting points are easy to cast in sand molds, permanent molds, or dies. Many alloys have high ductility coupled with low yield points, the ideal combination for cold working. Good machinability is also characteristic of many nonferrous

O: annealed -T: thermally treated (heat treated) -T1: cooled from hot working and naturally aged -T2: cooled from hot working, cold-worked, and naturally aged -T3: solution-heat-treated, cold-worked, and naturally aged -T4: solution-heat-treated and naturally aged -T5: cooled from hot working and artificially aged -T6: solution-heat-treated and artificially aged -T7: solution-heat-treated and stabilized -T8: solution-heat-treated, cold-worked, and artificially aged -T9: solution-heat-treated, artificially aged, and cold-worked -T10: cooled from hot working, cold-worked, and artificially aged -W: solution-heat-treated only The various wrought alloys are often divided into two basic types: those that achieve strength by solid-solution strengthening and cold working, and those that can be strengthened by heat treatment (age hardening).Table 7-4 lists some of the common wrought aluminum alloys in each family. It can be noted that the work-hardenable alloys (those that cannot be age hardened) are primarily those in the 1xxx (pure aluminum), 3xxx (aluminum- manganese), and 5xxx (aluminum-magnesium) series. A comparison of the annealed (O suffix) and cold-worked (H suffix) conditions reveals the amount of strengthening achievable through strain hardening. The precipitation-hardenable alloys are found primarily in the 2xxx, 6xxx, and 7xxx series. By comparing the properties in the heat-treated condition to those of the strain-hardened alloys, we see that heat treatment offers significantly higher strength. Alloy 2017, the original duralumin, is probably the oldest age-hardenable aluminum alloy. The 2024 alloy is stronger and has seen considerable use in aircraft applications. An attractive feature of the 2xxx series is the fact that ductility does not significantly decrease during the strengthening heat treatment. Within the 7xxx series are some newer alloys with strengths that approach or exceed those of the high-strength structural steels. Ductility, however, is generally low, and fabrication is more difficult than for the 2xxxtype alloys. Nevertheless, the 7xxx series alloys have also found wide use in aircraft applications. To maintain properties, age-hardened alloys should not be used at temperatures over 175C (350F). Welding should be performed with considerable caution since the exposure to elevated temperature will significantly diminish the strengthening achieved through either cold working or age hardening. Because of their two-phase structure, the heat-treatable alloys tend to have poorer corrosion resistance than either pure aluminum or the single-phase work-hardenable alloys. When both high strength and superior corrosion resistance are desired, wrought aluminum is often produced as Alclad material. A thin layer of corrosion-resistant aluminum is bonded to one or both surfaces of a high-strength alloy during rolling, and the material is further processed as a composite. Because only moderate temperatures are required to lower the strength of aluminum alloys, extrusions and forgings are relatively easy to produce and are manufactured in large quantities. Deep drawing and other sheet-metal-forming operations can also be carried out quite easily. In general, the high ductility and low yield strength of the aluminum alloys make them appropriate for almost all forming operations. Good dimensional tolerances and fairly intricate shapes can be produced with relative ease. The machinability of aluminum-based alloys, however, can vary greatly, and special tools and techniques may be desirable if large amounts of machining are required. Free-machining alloys, such as 2011, have been developed for screw-machine work. These special alloys can be machined at very high speeds and have replaced brass screwmachine stock in many applications.

alloys. The savings obtained through ease of fabrication can often overcome the higher cost of the nonferrous material and justify its use in place of steel.Weldability is the one fabrication area where the nonferrous alloys tend to be somewhat inferior to steel.With modern joining techniques, however, it is generally possible to produce satisfactory weldments in all of the nonferrous metals. ■ 7.2 COPPER AND COPPER ALLOYS GENERAL PROPERTIES AND CHARACTERISTICS Copper has been an important engineering metal for over 6000 years. As a pure metal, it has been the backbone of the electrical industry. It is also the base metal of a number of alloys, generically known as brasses and bronzes. Compared to other engineering materials, copper and copper alloys offer three important properties: (1) high electrical and thermal conductivity, (2) useful strength with high ductility, and (3) corrosion resistance to a wide range of media. Because of its excellent conductivity, about one-third of all copper produced is used in some form of electrical application, such as the commutators shown in Figure 7-2. Other large areas of use include plumbing, heating, and air conditioning. Pure copper in its annealed state has a tensile strength of only about 200 MPa (30 ksi), with an elongation of nearly 60%. Through cold working, the tensile strength can be more than doubled to over 450 MPa (65 ksi), with a decrease in elongation to about 5%. Because of its relatively low strength and high ductility, copper is a very desirable metal for applications where extensive forming is required. Since the recrystallization temperature for copper is less than (500F), the hardening effects of cold working can also be easily removed. Copper and copper alloys lend themselves nicely to the whole spectrum of fabrication processes, including casting, machining, joining, and surface finishing by either plating or polishing. Unfortunately, copper is heavier than iron. While strength can be quite high, the strength-to-weight ratio for copper alloys is usually less than that for the weaker aluminum and magnesium materials. In addition, problems can occur when copper is used at elevated temperature. Copper alloys tend to soften when heated above , and if copper is stressed for a long period of time at high temperature, intercrystalline failure can occur at about half of its normal room-temperature strength. While offering good resistance to adhesive wear, copper and copper alloys have poor abrasive wear characteristics. The low-temperature properties of copper are quite attractive, however. Strength tends to increase as temperatures drop, and the material does not embrittle, retaining attractive ductility even under cryogenic conditions. Conductivity also tends to increase with a drop in temperature. Copper and copper alloys respond well to strengthening methods, with the strongest alloy being 15 to 20 times stronger than the weakest. Because of the wide range of properties, the material can often be tailored to the specific needs of a design. Elastic stiffness is between 50 and 60% of steel. Additional features include being

and high thermal or electrical conductivity may be sufficient to justify the added cost of aluminum.With the drive for lighter, more fuel-efficient vehicles, the fraction of aluminum targeted for transportation applications rose from 19.4% in 1992 to 31.8% in 2002.The use of aluminum doubled in cars and tripled in sport-utility vehicles (SUVs) and light trucks. Aluminum is being used in body panels, engine blocks, manifolds, transmission housings, and wheels.An aluminum space frame, such as the ones shown in Figure 7-3 for the 2005 Ford GT and the 2006 Corvette Z06,can reduce the overall weight of the structure,enhance recyclability, and reduce the number of parts required for the primary body structure.The all-aluminum space frame of the 2006 Z06 Corvette resulted in a 30% reduction in weight from the all-steel design of the previous model. In 2001, aluminum passed plastics as a percentage of automotive material content and is now second only to steel and iron.The average North American automobile now contains over 125 kg (280 pounds) of aluminum.

at service temperatures somewhat above . Going to higher temperatures, we look to the refractory metals, which include niobium, molybdenum, tantalum,rhenium, and tungsten. All have melting points near or in excess of . They retain a significant fraction of their strength at elevated temperature and can be used at temperatures as high as ( ) provided that protective ceramic coatings effectively isolate them from gases in their operating environment. Coating technology is quite challenging, however, since the ceramic coatings must (1) have a high melting point, (2) not react with the metal they are protecting, (3) provide a diffusion barrier to oxygen and other gases, and (4) have thermal-expansion characteristics that match the underlying metal. While the refractory metals could be used at higher temperatures, the uppermost temperature is currently being set by limitations and restrictions imposed by the coating. Table 7-8 presents key properties for several refractory metals. Unfortunately, all are heavier than steel, and several are significantly heavier. In fact, tungsten, with a density about 1.7 times that of lead,is often used in counterbalances,compact flywheels,and weights, with applications as diverse as military projectiles, gyratory compasses, and golf clubs. Other materials and technologies that offer promise for high-temperature service include intermetallic compounds, engineered ceramics, and advanced coating systems. The intermetallic compounds provide properties that are between those of metals and ceramics, and they are excellent candidates for high-temperature applications.They are hard, stiff, creep resistant, and oxidation resistant, with good high-temperature strength that often increases with temperature. The titanium and nickel aluminides offer the additional benefit of being significantly lighter than the superalloys. Unfortunately, the intermetallics are also characterized by poor ductility, poor fracture toughness, and poor fatigue resistance. They are difficult to fabricate using traditional techniques, such as forming and welding. On a positive note, research and development efforts have begun to overcome some of these limitations, and the intermetallics are now appearing in commercial products. Figure 7-6 compares the upper limit for useful mechanical properties for a variety of engineering metals. ■ 7.9 LEAD AND TIN, AND THEIR ALLOYS The dominant properties of lead and lead alloys are high density coupled with strength and stiffness values that are among the lowest of the engineering metals. The principal uses of lead as a pure metal include storage batteries, cable cladding, and radiation

construction, electrical applications, consumer durables, and mechanical equipment.We are all familiar with uses such as aluminum cookware, window frames, aluminum siding, and the ever-present aluminum beverage can. A number of unique and attractive properties account for the engineering significance of aluminum. These include its workability, light weight, corrosion resistance, good electrical and thermal conductivity, optical reflectivity, and a nearly limitless array of available finishes. Aluminum has a specific gravity of 2.7 compared to 7.85 for steel, making aluminum about one-third the weight of steel for an equivalent volume. Cost comparisons are often made on the basis of cost per pound, where aluminum is at a distinct disadvantage, being four to five times more expensive than carbon steel.There are a number of applications, however, where a more appropriate comparison would be based on cost per unit volume. A pound of aluminum produces three times as many same-size parts as a pound of steel, so the cost difference becomes markedly less. Aluminum can be recycled repeatedly with no loss in quality, and recycling saves 95% of the energy required to produce aluminum from ore. Since the 1980s, the overall reclamation rate for aluminum has been over 50%. The aluminum can is the most recycled beverage container in North America, and over 85% of all aluminum used in cars is recovered at the end of their useful life. A serious weakness of aluminum from an engineering viewpoint is its relatively low modulus of elasticity, which is also about one-third that of steel. Under identical loadings, an aluminum component will deflect three times as much as a steel component of the same design. Since the modulus of elasticity cannot be significantly altered by alloying or heat treatment, it is usually necessary to provide stiffness and buckling resistance through design features such as ribs or corrugations.These can be incorporated with relative ease, however, because aluminum adapts easily to the full spectrum of fabrication processes. COMMERCIALLY PURE ALUMINUM In its pure state, aluminum is soft, ductile, and not very strong. In the annealed condition, pure aluminum has only about one-fifth the strength of hot-rolled structural steel. Commercially pure aluminum, therefore, is used primarily for its physical rather than its mechanical properties. Electrical-conductor-grade aluminum is used in large quantities and has replaced copper in many applications, such as electrical transmission lines. Commonly designated by the letters EC, this grade contains a minimum of 99.45% aluminum and has an electrical conductivity that is 62% that of copper for the same-size wire and 200% that of copper on an equal-weight basis. ALUMINUMS FOR MECHANICAL APPLICATIONS For nonelectrical applications, most aluminum is used in the form of alloys. These have much greater strength than pure aluminum yet retain the advantages of light weight, good conductivity, and corrosion resistance.While usually weaker than steel, some alloys are now available that have tensile properties (except for ductility) that are comparable to those of the high-strength low-alloy (HSLA) structural grades. Since alloys can be as much as 30 times stronger than pure aluminum, designers can frequently optimize their design and then tailor the material to their specific requirements. Some alloys are specifically designed for casting, while others are intended for the manufacture of wrought products. On a strength-to-weight basis, most of the aluminum alloys are superior to steel and other structural metals, but wear, creep, and fatigue properties are generally rather poor. Aluminum alloys have a finite fatigue life at all reasonable values of applied stress. In addition, aluminum alloys rapidly lose their strength and dimensions change by creep when temperature is increased. As a result, most aluminum alloys should not be considered for applications involving service temperatures much above At subzero temperatures, however, aluminum is actually stronger than at room temperature with no loss in ductility. Both the adhesive and the abrasive varieties of wear can be extremely damaging to aluminum alloys. The selection of steel or aluminum for any given component is often a matter of cost, but considerations of light weight, corrosion resistance, low maintenance expense

glass can also be produced, and since many of the alloys have low melting temperatures, products can be made by reheating to a soft condition and forming by processes that are conventionally used to shape thermoplastic polymers (compression molding, extrusion, blow molding, and injection molding). Applications have just begun to emerge in areas as diverse as load-bearing structures, electronic casings, replacement joints, and sporting goods. In addition, metallic glasses have also been developed that retain their glassy structure at temperatures as high as . ■ 7.12 GRAPHITE While technically not a metal, graphite is an engineering material with considerable potential. It offers properties of both a metal and nonmetal, including good thermal and electrical conductivity, inertness, the ability to withstand high temperature, and lubricity. In addition, it possesses the unique property of increasing in strength as the temperature is elevated. Polycrystralline graphites can have mechanical strengths up to 70 MPa (10 ksi) at room temperature, which double when the temperature reaches 2500C (4500F). Large quantities of graphite are used as electrodes in arc furnaces, but other uses are developing rapidly.The addition of small amounts of borides, carbides, nitrides, and silicides greatly lowers the oxidation rate at elevated temperatures and improves the mechanical strength.This makes the material highly suitable for use as rocket-nozzle inserts and as permanent molds for casting various metals, where it costs less than tool steel, requires no heat treating, and has a lower coefficient of thermal expansion. It can be machined quite readily to excellent surface finishes. Graphite fibers have also found extensive use in composite materials. This application will be discussed in Chapter 8

nonmagnetic, nonpyrophoric (slivers or particles do not burn in air — i.e., nonsparking), and nonbiofouling (inhibits marine organism growth), as well as offering a wide spectrum of colors, including yellow, red, brown, and silver. COMMERCIALLY PURE COPPER Refined copper containing between 0.02 and 0.05% oxygen is called electrolytic tough-pitch (ETP) copper. It is often used as a base for copper alloys and may be used for electrical applications, such as wire and cable, when the highest conductivity is not required. For superior conductivity, additional refining can reduce the oxygen content and produce oxygen-free high-conductivity (OFHC) copper. The better grades of conductor copper now have a conductivity rating of about 102% IACS, reflecting metallurgical improvements made since 1913, when the International Annealed Copper Standard (IACS) was established and the conductivity of pure copper was set at 100% IACS. COPPER-BASED ALLOYS As a pure metal, copper is not used extensively in manufactured products, except in electrical applications, and even here alloy additions of silver, arsenic, cadmium, and zirconium are used to enhance various properties without significantly impairing conductivity. More often, copper is the base metal for an alloy, where it imparts its good ductility, corrosion resistance, and electrical and thermal conductivity. A full spectrum of mechanical properties is available, ranging from pure copper, which is soft and ductile, through alloys whose properties can rival those of quenched-and-tempered steel. Copper-based alloys are commonly designated using a system of numbers standardized by the Copper Development Association (CDA). Table 7-2 presents a breakdown of this system, which has been further adopted by the American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), and the U.S. government.Alloys numbered from 100 to 199 are mostly copper with less than 2% alloy addition. Numbers 200 to 799 are wrought 1 alloys, and the 800 and 900 series are casting alloys. When converted to the Unified Numbering System for metals and alloys, the three-digit numbers are converted to five digits by placing two zeros at the end, and the letter C is used as a prefix to denote the copper base. COPPER-ZINC ALLOYS Zinc is by far the most popular alloying addition, and the resulting alloys are generally known as some form of brass. If the zinc content is less than 36%, the brass is a singlephase solid solution. Since this structure is identified as the alpha phase, these alloys are

often called alpha brasses. They are quite ductile and formable, with both strength and ductility increasing with the zinc content up to about 36%. The alpha brasses can be strengthened significantly by cold working and are commercially available in various degrees of cold-worked strength and hardness. Cartridge brass, the 70% copper-30% zinc alloy, offers the best overall combination of strength and ductility. As its name implies, it has become a popular material for sheet-forming operations like deep drawing. With more than 36% zinc, the copper-zinc alloys enter a two-phase region involving a brittle, zinc-rich phase, and ductility drops markedly. While cold-working properties are rather poor for these high-zinc brasses, deformation can be performed easily at elevated temperature. Many applications of these alloys result from the high electrical and thermal conductivity coupled with useful engineering strength.The wide range of colors (red, orange, yellow, silver, and white), enhanced by further variations that can be produced through the addition of a third alloy element, account for a number of decorative uses. Since the plating characteristics are excellent, the material is also a frequently used base for decorative chrome or similar coatings.Another attractive property of alpha brass is its ability to have rubber vulcanized to it without any special treatment except thorough cleaning. As a result, brass is widely used in mechanical rubber goods. Most brasses have good corrosion resistance. In the range of 0 to 40% zinc, the addition of a small amount of tin imparts improved resistance to seawater corrosion. Cartridge brass with tin becomes admiralty brass, and the 40% zinc Muntz metal with a tin addition is called naval brass. Brasses with 20 to 36% zinc, however, are subject to a selective corrosion, known as dezincification, when exposed to acidic or salt solutions. Brasses with more than 15% zinc often experience season cracking or stress-corrosion cracking. Both stress and exposure to corrosive media are required for this failure to occur (but residual stresses and atmospheric moisture may be sufficient!). As a result, cold-worked brass is usually stress relieved (to remove the residual stresses) before being placed in service. When high machinability is required, as with automatic screw-machine stock, 2 to 3% lead can be added to the brass to ensure the formation of free-breaking chips. Brass casting alloys are quite popular for use in plumbing fixtures and fittings, low-pressure valves, and a variety of decorative hardware. They have good fluidity during pouring and attractive low melting points.An alloy containing between 50 and 55% copper and the remainder zinc is often used as a filler metal in brazing. It is an effective material for joining steel, cast iron, brasses, and copper, producing joints that are nearly as strong as those obtained by welding. Table 7-3 lists some of the more common copper-zinc alloys and their composition, properties, and typical uses. COPPER-TIN ALLOYS Since tin is more costly than zinc, alloys of copper and tin, commonly called tin bronzes, are usually specified when they offer some form of special property or characteristic.The term bronze is often confusing, however, since it can be used to designate any copper alloy where the major alloy addition is not zinc or nickel. To provide clarification, the major alloy addition is usually included in the designation name. The tin bronzes usually contain less than 12% tin. (Strength continues to increase as tin is added up to about 20%, but the high-tin alloys tend to be brittle.) Tin bronzes offer good strength, toughness, wear resistance, and corrosion resistance.They are often used for bearings, gears, and fittings that are subjected to heavy compressive loads. When the copper-tin alloys are used for bearing applications, up to 10% lead is frequently added. The most popular wrought alloy is phosphor bronze, which usually contains from 1 to 11% tin.Alloy 521 (CDA), with 8% tin, is typical of this class. Hard sheet has a tensile strength of 760 MPa (110 ksi) and an elongation of 3%. Soft sheet has a tensile strength of 380 MPa (55 ksi) and 65% elongation. The material is often specified for pump parts, gears, springs, and bearings.

property of not adversely affecting steel dies when in contact with molten metal. Unfortunately, pure zinc is almost as heavy as steel and is also rather weak and brittle. Therefore, when alloys are designed for die casting, the alloy elements are usually selected for their ability to increase strength and toughness in the as-cast condition while retaining the low melting point. The composition and properties of common zinc die-casting alloys are presented in Table 7-7. Alloy AG40A (also known as alloy 903 or Zamak 3) is widely used because of its excellent dimensional stability, and alloy AC41A (also known as alloy 925 or Zamak 5) offers higher strength and better corrosion resistance. As a whole, the zinc die-casting alloys offer a reasonably high strength and impact resistance, along with the ability to be cast to close dimensional limits with extremely thin sections. The dimensions are quite stable, and the products can be finish machined at a minimum of cost. Resistance to surface corrosion is adequate for a number of applications, and the material can be surface finished by a variety of means that include polishing, plating, painting, anodizing, or a chromate conversion coating. Energy costs are low (low melting temperature), tool life is excellent, and the zinc alloys can be efficiently recycled. While the rigidity is low compared to that of other metals, it is far superior to engineering plastics, and zinc die castings often compete with plastic injection moldings. The attractiveness of zinc die casting has been further enhanced by the zinc-aluminum casting alloys (ZA-8, ZA-12, and ZA-27, with 8, 12, and 27% aluminum, respectively). Initially developed for sand, permanent-mold, and graphite-mold casting, these alloys can also be die cast to achieve higher strength (up to 60 ksi or 415 MPa), hardness (up to 120 BHN), creep resistance and wear resistance, and lighter weight than is possible with any of the conventional alloys. Because of their lower melting and casting costs, these materials are becoming attractive alternatives to the conventional aluminum, brass, and bronze casting alloys, as well as cast iron. ■ 7.6 TITANIUM AND TITANIUM ALLOYS Titanium is a strong, lightweight, corrosion-resistant metal that has been of commercial importance since about 1950. Because its properties are generally between those of steel and aluminum, its importance has been increasing rapidly. The yield strength of commercially pure titanium is about 210 MPa (30 ksi), but this can be raised to 1300 MPa (190 ksi) or higher through alloying and heat treatment, a strength comparable to that TABLE 7-7 Composition and Properties of Some Zinc Die-Casting Alloys #3 #5 #7 ZA-8 ZA-12 ZA-27 SAE 903 SAE 925 Alloy ASTM AG40A ASTM AC41A ASTM AG408 Sa PD SPD SPD Compositionb Aluminum 3.5-4.3 3.5-4.3 3.5-4.3 8.0-8.8 10.5-11.5 25.0-28.0 Copper 0.25 max 0.75-1.25 0.25 max 0.8-1.3 0.5-1.2 2.0-2.5 Zinc balance balance balance balance balance balance Properties Density (g/cc) 6.6 6.6 6.6 6.3 6.0 5.0 Yield strength (MPa) 221 228 221 200 206 290 214 269 317 372 379 (ksi) 32 33 32 29 30 42 31 39 46 54 55 Tensile strength (MPa) 283 328 283 263 255 374 317 345 400 441 421 (ksi) 41 48 41 38 37 54 46 50 58 64 61 Elongation (% in 2 in.) 10 7 13 2 2 10 3 3 7 6 3 Impact strength (J) 58 65 58 20 42 25 29 47 5 Modulus of elasticity (GPa) 85.5 85.5 85.5 85.5 82.7 77.9 Machinabilityc E E E EVG G a S, sand-cast; P, permanent-mold cast; D, die-cast. bAlso contains small amounts of Fe, Pb, Cd, Sn, and Ni. c E, excellent; VG, very good; G, good. dega-c07_139-161-hr 1/9/07 3:31 PM Page 155 of many heat-treated alloy steels. Density, on the other hand, is only 56% that of steel (making strength-to-weight quite attractive), and the modulus of elasticity ratio is also about one-half. Good mechanical properties are retained up to temperatures of so the metal is often considered to be a high-temperature engineering material. On the negative side, titanium and its alloys suffer from high cost, fabrication difficulties, a high energy content (they require about 10 times as much energy to produce as steel), and a high reactivity at elevated temperatures (above ). Titanium alloys are designated by major alloy and amount (see ASTM specification B-265), and are generally grouped into three classes based on their microstructural features.These classes are known as alpha-, beta-, and alpha-beta-titanium alloys, the terms denoting the stable phase or phases at room temperature.Alloying elements can be used to stabilize the hexagonal-close-packed alpha phase or the body-centered-cubic beta phase, and heat treatments can be applied to manipulate structure and improve properties. Fabrication can be by casting (generally investment or graphite mold), forging, rolling, extrusion, or welding, provided that special process modifications and controls are implemented. Advanced processing methods include powder metallurgy, mechanical alloying, rapid-solidification processing (RSP), superplastic forming, diffusion bonding, and hot-isostatic pressing (HIP). While titanium is an abundant metal, it is difficult to extract from ore, difficult to process, and difficult to fabricate. These difficulties make it significantly more expensive than either steel or aluminum, so its uses relate primarily to its light weight, high strengthto-weight ratio, good stiffness, good fatigue strength and fracture toughness, excellent corrosion resistance (the result of a thin, tenacious oxide coating), and the retention of mechanical properties at elevated temperatures.Aluminum,magnesium,and beryllium are the only base metals that are lighter than titanium, and none of these come close in either mechanical performance or elevated-temperature properties.Aerospace applications tend to dominate, with titanium comprising up to 40% of the structural weight of highperformance military fighters. Titanium and titanium alloys are also used in such diverse areas as chemical- and electrochemical-processing equipment,food-processing equipment, heat exchangers, marine implements, medical implants, high-performance bicycle and automotive components, and sporting goods. They are often used in place of steel where weight savings are desired and to replace aluminums where high-temperature performance is necessary.Some bonding applications utilize the unique property that titanium wets glass and some ceramics. The titanium-6% aluminum-4% vanadium alloy is the most popular titanium alloy,accounting for nearly 50% of all titanium usage worldwide.Figure 7-4 shows the elevated temperature strength retention of several titanium alloys.

when in a finely divided form, such as powder or fine chips, and this hazard should never be ignored. In the form of sheet, bar, extruded product, or finished castings, however, magnesium alloys rarely present a fire hazard. When the metal is heated above 700C (950F), a noncombustible, oxygen-free atmosphere is recommended to suppress burning, which will initiate around 600C (1100F). Casting operations often require additional precautions due to the reactivity of magnesium with sand and water. ■ 7.5 ZINC-BASED ALLOYS Over 50% of all metallic zinc is used in the galvanizing of iron and steel. In this process the iron-based material is coated with a layer of zinc by one of a variety of processes that include direct immersion in a bath of molten metal (hot dipping) and electrolytic plating. The resultant coating provides excellent corrosion resistance, even when the surface is badly scratched or marred. Moreover, the corrosion resistance will persist until all of the sacrificial zinc has been depleted. Zinc is also used as the base metal for a variety of die-casting alloys. For this purpose, zinc offers low cost, a low melting point (only or ), and the attractive

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