Wrought Aluminum Technologies for Automobiles: Part One

요약:

Aluminum offers a wide range of properties that can be engineered precisely to the demands of specific automotive applications through the choice of alloy, temper and fabrication process. To name a few of its advantages, aluminum offers strength, light weight, high strength-to-weight ratio, resilience, forming and joining.

Automobiles are using more aluminum products in response to ever increasing requirements for fuel efficient and environmentally-friendly vehicles. A variety of new technologies are being developed for aluminum products. This article introduces the current status and trends of aluminum product technologies, with the main focus on sheets, extrusions, forgings and their peripheral technologies.

The Kyoto Protocol was ratified in 1997 and led to increased global attention to environmental issues. Approximately 20% of the total carbon dioxide emissions in Japan are due to automobile exhausts and improvements of automobile fuel economy are strongly desired. A target has been set of a 22.8% improvement of fuel economy for gasoline-fueled vehicles by 2010 compared to the 1995 level. On the other hand, weights of vehicles are increasing year by year due to improvements in collision safety and mounting of various information devices.

Automotive aluminum use has been growing for years (from an average of 87 pounds per car in 1976 to 248 pounds in 1999), mainly to reduce weight and improve fuel economy. Each pound of aluminum used can reduce vehicle weight as much as 1.5 pounds. Automotive frames and bodies can make even further use of aluminum’s unique combination of strength, light weight, crash-energy absorption, corrosion resistance, and thermal and electrical conductivity.

As new car prices increase (they roughly quadrupled between 1978 and 1999), durability and corrosion resistance take on new importance. Buyers want vehicles that will retain their appearance and keep a high resale value. That is something that aluminum can provide, as automakers offer longer warranties against component failure and body rust-out.

Aluminum — even unpainted and uncoated — resists corrosion by water and road salt and, in noncosmetically critical parts, its use can avoid the substantial extra costs of galvanizing, coating and painting required for steel. Aluminum does not rust like steel if the paint is scratched or chipped. Nor is it weakened or embrittled, as some plastics may be, by desert heat, northern cold, or the ultraviolet radiation in sunlight. For its new delivery vans, the U.S. Postal Service specified aluminum bodies designed to last 24 years!

Since the mid-1970s, the percentage of aluminum use in automobiles has increased almost three-fold. Today, more than a hundred different auto parts are made of engineered alloy aluminum and the list is still growing. While lighter weight and efficient function were the primary reasons for selecting aluminum, extended life through better corrosion resistance provided an added benefit that is highly important in achieving the desired useful life of the vehicle.

Aluminum offers a wide range of properties that can be engineered precisely to the demands of specific automotive applications through the choice of alloy, temper and fabrication process. To name a few of its advantages, aluminum offers:

Strength — Some aluminum alloys and tempers approach or surpass the strength of commonly used automotive steels. Automotive aluminum alloys achieve tensile strengths of 310 MPa (45 ksi) for alloy 6061-T6; 290 MPa (42 ksi) for 6111-T4; and 430 MPa (62 ksi) for alloy 7029-T6. Some aluminum alloys are heat treated to strengths approaching 700 MPa (100 ksi), although these are primarily used in the aircraft industry.

Light weight — Aluminum weights about 35 percent as much as steel by volume. Aluminum auto parts save weight directly as well as indirectly through redesign of other parts.

High strength-to-weight ratio — Aluminum’s strength-to-weight ratio is much greater than that of steel: often double, or more.

Resilience — Aluminum alloys will deflect under load and spring back, providing flexible strength and shape retention. Aluminum alloys can also be used to meet the stiffness and crash energy absorption requirements for automotive vehicle structures, while providing up to 50 percent weight savings compared with other materials.

Forming and fabricating —Aluminum can be formed and fabricated by all common metalworking methods including casting, stamping, forging, bending, extruding, cutting, drilling, punching, machining and finishing.

Joining — Aluminum can be joined by all common methods, including: welding, soldering, brazing, bolting, riveting, adhesive bonding, weld bonding, clipping, clinching, and slide-on, snap together or interlocking joints.

Generally, aluminum alloys have lower elongations, r-values and Young's modulus compared to steels. Shapes which can be formed by steel sheets can cause aluminum sheets to crack, wrinkle and to spring-back, restricting the shapes and designs of automobile bodies.

In order to improve this, it is essential to develop forming and fabrication technologies suited for aluminum alloys. One of the methods of improving the formability of aluminum alloys is a method of using their temperature dependence of plastic deformation, which utilizes higher elongations at temperatures either higher or lower than ambient temperature.

The high-temperature blow-forming, which utilizes high elongations of aluminum alloys at elevated temperatures, has been used for small scale production of panels and has achieved improved productivity recently. This method is applied to the front fender and trunk lid of Honda Legend (2004). The high-temperature blow forming gives better formability than steel sheets and provides a good example of a combination of material and forming technologies of aluminum.

The aluminum alloys for panels are required to have various properties including strength, formability, bendability, surface characteristics and weldability. The 2000 series (Al-Cu-Mg) and 7000 series (Al-Mg-Zn) alloys were used in early days and 5000 series (Al-Mg) and 6000 series (Al-Mg-Si) alloys are mostly used these days. The 5000 series are non heat treatable alloys and special 5000 series alloys, AA5022 (KS5J30) and AA5023 (KS5J32), are used for panels. The 6000 series alloys are heat treatable and have bake-hardening (BH) features which increase strength during paint baking. They are also free from stretcher-strain (SS) marks after forming. Because of those features, the 6000 series alloys are used for outer panels which require dent resistance and surface qualities.

Aluminum extrusions can have complex cross sectional shapes having a variety of thickness distributions, which are not obtainable for steels, and are gaining attention as effective products to reduce automobile weights. Aluminum extrusions for automobiles, first applied to heat-exchangers of air-conditioners, have expanded their applications to underbody and engine parts. More recently, applications to structural members such as bumper-beams are progressing.

In Japan, the production of aluminum hot forgings is approximately 30 thousand tons/year and is smaller than other formed aluminum products. About 80% of the volume is occupied by automobile applications, which have increased by 60% in the last three years (a statistics by Ministry of Economy of Japan, Trade and Industry of Japan). The increase is mainly attributed to the increase in the applications of forgings to passenger vehicle suspensions. The underbody parts improve not only fuel economy through weight reduction but also ride quality, handleability and performance, and have been used increasingly, especially in luxury vehicles.

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