Substitution of powder metallurgy (P/M) components for steel began in North America during the 1980s. At that time, process advances in P/M technology permitted the manufacture of certain complex automotive components that could meet the strength requirements found in steel, but meet them at a fraction of the cost, because of the superior near-net-shape properties of the P/M production process and the elimination of certain machining stages.

            The largest consumer of P/M components both worldwide and in North America is the automotive industry, which accounts for 60 percent of total North American consumption and 70 percent of North American ferrous consumption. Major P/M automotive applications include engines (e.g., connecting rods, main bearing caps, and camshafts) and transmissions. The amount of P/M components in automobiles has more than doubled during the past 20 years, from an average of 17 pounds in 1980 to 40.5 pounds in 2003, an increase of 4 percent from 2002, and is expected to rise to 50 pounds during the next 10 years. P/M components fall into two principal groups:

            •Components of certain metals that are difficult to produce by methods other than P/M processes due to their high melting point, for example, tungsten, molybdenum, or tungsten carbide. Porous bearings, filters, and many types of hard and soft magnetic components of these metals are produced exclusively by P/M processes.

            •Components for which P/M processes are cost-effective alternatives to machined components, castings, and forgings due to the superior near-net-shape capabilities of the P/M process and the elimination of costly machining steps. Examples include connecting rods, planetary gear carriers, clutch plates, and camshafts. There are three principal P/M processes for producing automotive components. The particular process selected will depend largely on the desired properties and geometries, with an individual process being more suited to certain components.

Advantages and disadvantages

            The principal advantages of P/M processing and components are as follows:

            •P/M processes create components with very good surface finishes.

            •P/M is suitable for a large number of alloy combinations, permitting variations in properties such as high temperature, performance, toughness, and hardness.

            •The near-net shape of P/M parts having close dimensional tolerances reduces, or in some cases, eliminates the need for cutting, machining, and other costly secondary operations. Near-net-shape parts also reflect less scrap loss as the P/M process normally yields a metal part that retains 95 percent of the raw powder material.

            •P/M also allows small, intricate, metal parts to be produced faster than with traditional methods. In contrast, the principal disadvantages of P/M processing and components include the following:

            •High material costs are such that on a unit weight-basis, P/M parts are considerably more expensive than wrought or cast parts.

            •Due to their higher porosity, P/M parts tend to have lower resistance to corrosion than do those produced by traditional forging methods.

            •P/M components have lower plasticity properties in terms of impact strength, ductility, and elongation than do traditional forged steel components.

            The P/M industry is essentially comprised of powder producers and component and product producers.

Engine applications

            Connecting rods account for the single-largest P/M end use, representing 50,000 short tons, or nearly 12 percent, of steel powder consumed annually. P/M connecting rods were first introduced into U.S. automobile manufacture in 1986 and have steadily gained market share, due largely to economic factors; their price has dropped below that of conventional precision-forged steel rods once the cost of including the in-line machining operation to finish conventional connecting rods is factored into the overall cost of production. At the same time, advances in P/M-forging technology have increased the strength attributes of the connecting rods sufficiently to make them competitive with conventional connecting rods.

            P/M processes have increasingly gained acceptance since the 1990s in the production of main bearing caps and camshaft lobes. Casting of main bearing caps tends to be an expensive process due the amount of machining required to obtain close tolerances and the consequent loss of scrap metal. Because P/M processes tend to produce a near-net-shape part that fits the bearing cap to the connecting rod, there is less material waste and less capital investment in expensive machining operations. Near-net forming of camshaft lobes also results in reduced machining and lower production costs relative to a traditional camshaft. According to industry representatives, the application of P/M to both main bearing caps and camshafts will likely follow the same course as P/M in connecting rods, resulting in the majority of these items being produced from powder metals within the next 10 to 20 years.

Transmission applications

            The fastest-growing P/M application in automobiles is for transmission components. Automatic transmissions contain P/M planetary gear carriers and pinion gears; manual transmissions contain P/M clutch pressure plates, shift levers, and detent plates; and transfer cases contain P/M sprockets, planetary gear carriers, and pinion gears. P/M components in transmissions compete principally with welded stampings, steel castings, and grey iron castings. Advances in compacting and sintering technology have lowered the cost of P/M components below that cost of conventionally stamped and cast components.

            Because P/M enables fabricators to produce more complex components than conventional forming methods, P/M processes permit the reduction of subsequent machining steps, leading to lower costs. A typical planetary gear carrier, for example, contains a number of finely detailed lubrication channels, pinion pockets, cored pin holes, and face grooves that have become too expensive to machine after conventional stamping or casting. In contrast, the degree of complexity of P/M components is essentially limited only by the design skills of the die maker.

Advanced aluminum sheet in auto bodies

            U.S. interest in aluminum26 for auto body applications dates back to the mid-1970s when sudden rises in petroleum prices forced automakers to lower the weight of automobiles. Light-weight aluminum was viewed as an ideal substitute for steel because it enabled automakers to control the weight of the vehicle while adding required safety-related features such as air bags and extra padding. Although the use of aluminum in automobiles has been growing since the 1970s, application has been largely confined to die castings, extrusions, and forgings in the engine block, transmission, and wheels. Substitution of aluminum for steel has been largely influenced by regulatory requirements for automakers to meet fuel efficiency standards through reductions in vehicle weight and to meet certain standards for recycling of material. In order to displace large amounts of steel, aluminum would need to become a primary metal in the body of the automobile. Currently nearly 27 percent of the weight of an average automobile is accounted for by the auto body. Use of aluminum sheet has been limited to a small number of closure panels (comprised largely of lift gates, hoods, and deck lids) that are comparatively easy to form. The quantity of aluminum sheet contained in the average automobile produced in the United States was 29 pounds in 2002, up from 27 pounds in 1999. Aluminum sheet applications account for only 11 percent of the total amount of aluminum in automobiles.

Advantages and disadvantages

            The principal advantages of aluminum sheet applications in the automotive industry are as follows:

            •Aluminum has one-third the density of steel and satisfies the torsion and stiffness requirements of an automotive material. The strength-to-weight ratio of aluminum is often double that of steel. As a result of its light weight, aluminum enables automakers to better meet fuel economy standards without sacrificing many of the performance characteristics of steel.

            •Aluminum body parts are typically stamped on the same tooling used to stamp steel parts. As a result, no significant capital investment would be needed to transition from steel to aluminum body parts. However, there are also four principal obstacles to the increased use of aluminum sheet in such applications:

            •Under present manufacturing methods, aluminum sheet is five times more expensive than steel by unit weight. The higher cost of aluminum panels is related both to its higher price and its more difficult forming attributes for complex body components. Also adding to the cost disparity with steel is that approximately 50 percent of a sheet, whether of steel or higher-cost aluminum, is largely wasted in the form of scrap when stamping the final component.

            •Complex body components, including door panels and inner trunk components which contain sharp creases or deep recesses to accommodate various safety features, are more difficult to form because aluminum tears more readily when subjected to relatively low rates of strain, leading to splitting and wrinkling of the metal in tight corners. For this reason, steel sheet tends to be preferred in complex body components. Unlike steel, the metal has a tendency to spring back when the aluminum part is removed from the die, making it more difficult for aluminum to retain its dimensional tolerances.

            •The high thermal and electrical conductivities of aluminum (three times that of high strength steel) pose problems in resistance spot welding; because aluminum quickly dissipates heat, its welding requires more energy to be applied, often resulting in distortion of the aluminum panel.

            As a result of these obstacles, an auto body part of aluminum sheet is often subjected to additional stamping stages or may be divided into two parts and joined together rather than stamped as a single part. Both alternatives involve added costs, making the aluminum part more expensive than a comparable steel part. Hence, the use of aluminum sheet has largely been confined to specific segments of the auto body market, such as closure panels, which are easier to form. Aluminum has been able to gain some market share in closure panel applications37 because automakers are increasingly substituting lower-weight, high-strength, low-alloy (HSLA) steels for traditional carbon steels in body panels; and many of these HSLA steels face similar formability and cost problems as does aluminum.

            At the same time, the aluminum and automotive industries have sought to adopt new process technologies to deal with formability problems. Although such processes are not yet capable of producing significant commercial quantities of aluminum body panels at a price competitive with steel, research is continuing at a rapid pace. Two prominent examples of these technologies are superplastic forming and electromagnetic forming.

Supply infrastructure

            Alcoa Inc, with headquarters in Pittsburgh, PA, and Alcan Aluminum Corp. (Canada) are the major producers of aluminum sheet certified for use in automotive applications. Most aluminum auto body parts in North America are stamped in-house by automakers largely because the stamping of body parts has been a core business for OEMs since the inception of the automotive industry in the United States. The OEMs still possess sufficient press capacity to stamp the majority of their body components internally and when in-house capacity is insufficient to meet their production needs, they rely on Tier 1 suppliers to fill their remaining needs on a contract basis.

            Although the North American automotive industry currently uses various amounts of aluminum to form hoods, lift gates, and deck lids in 20 to 30 automobile models, significant aluminum use has been largely confined to the following models:

            •Ford Motor Co. uses aluminum sheet for the hoods of its redesigned F-series trucks. This use is considered the largest single application of aluminum sheet in the North American automobile industry, consuming more rolled aluminum annually than any other single automotive component application.

            •General Motors’ (GM’s) family of full-sized sport-utility vehicles (GMC Yukon, Chevy Tahoe and Suburban, and Cadillac Escalade) incorporate an aluminum lift gate; and the Oldsmobile Aurora V-8 model (production discontinued during 2003), which boasted the highest aluminum content of any automobile sold in the United States that has sales volumes greater than 10,000 automobiles per year, featured an aluminum hood and trunk lid.

            •The 2004 Chevy Malibu Maxx features an aluminum lift gate and the 2004 Cadillac SRX has an aluminum hood.

            In addition to these automotive components produced using conventional stamping techniques, GM has implemented a variation of superplastic forming (SPF) technology that it refers to as quick plastic forming, to produce the trunk on its now-discontinued Oldsmobile Aurora and the lift gate on its Malibu Maxx. According to GM, its quick plastic forming process is considerably faster than conventional SPF, allowing the company to produce components at the rate of one every 1 to 2 minutes.

            Among Japanese automakers, both Nissan Motor Corp. and Mazda Motor Corp. feature aluminum in auto body applications. Nissan began to use aluminum sheet for the first time in the hoods and deck lids of the 2002 Altima models. Nissan also plans to install aluminum hoods and deck lids in its Maxima sedans, expected to be launched onto the market by the 2005 model year. Decisions by Nissan and Mazda represent somewhat of a trend reversal for the Japanese transplant automakers, since transplants tend to use less aluminum sheet in car or truck applications, on average, than U.S. automakers. Japanese transplant automakers have instead concentrated on developing cast aluminum for engine applications.

Outlook

            Use of P/M and aluminum sheet components likely will increase in the future as automakers continue to seek components that are both less expensive to produce and lighter weight. The rate of this advance will depend largely on the continuation of processing improvements that will permit both reductions in the cost of P/M and aluminum sheet components, and expansion of product use beyond the narrow range currently being produced from these materials. At the same time, manufacturers of components with traditional materials and manufacturing methods can be expected to resist further penetration by P/M and aluminum sheet by improving their own products.

Reprinted from the U.S. International Trade Commission’s Industry Trade And Technology review.