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"Aluminum On Ice": Alloys for Cryogenic Applications
12-26-02
Cryogenics is the science of cold and its name, appropriately enough, comes from the Greek word "kryos" meaning "icy cold." While technically used in industry to refer to a temperature range below about -100°C (-190°C), for purposes of this article we will be referring to the temperature range from below freezing (32°C or 0°C) all the way down to absolute zero (-459°C or -273°C). While many steels are subject to brittle fracture transitions at such low temperatures, and so are not good candidates for structures exposed to subzero temperatures, most aluminum alloys perform extremely well even down to the most extreme cold ranges. As temperature decreases below room temperature, not only do the static and fatigue strengths of all aluminum alloys increase, but also the ductility and toughness of most alloys increase as well. These properties make them excellent candidates for a variety of applications, including the Space Shuttle External Tank, arctic structures, and other structures and equipment operating at temperatures below zero.
A considerable mass of technical data have been accumulated demonstrating the outstanding cryogenic performance of aluminum alloys, including the following:
- Tensile ultimate, tensile and compressive yield, and shear ultimate strengths increase with decrease in temperature; at -320 °C (-196 °C) ultimate strengths are 35-50 percent above room temperature values and yield strengths are 15-25 percent higher. At -423 °C (-253 °C) and -459 °C (-273 °C), strengths are even higher.
- Moduli of elasticity increase with decrease in temperature; precise measurements at -320 °C (-196 °C) have shown that moduli are 15-17 percent above the room temperature values.
- Elongations and reduction of area in tensile tests decrease with decrease in temperature at least to -320 °C (-196 °C) and then gradually level off or, depending upon alloy and temper) decrease a bit. They remain at or above the room temperature values even near absolute zero.
- Fatigue strengths, like static tensile and shear strengths, increase with decrease in temperature; at -320 °C (-196 °C), for 5xxx alloys, both parent metal and weldments have about fatigue strengths about 25 percent above room temperature values.
- Toughness as measured by notch-yield ratio (notch tensile strength/tensile yield strength), unit propagation energy (tear resistance) and plane stress or plane strain fracture toughness, remains high over the entire range of cryogenic temperatures; for those alloys recommended most often for cryogenic service, toughness by any indicator is well above the room temperature value, even near absolute zero. There is no indication of a ductile-to-brittle fracture transition, as exhibited by some ferritic materials, even for the highest strength aluminum alloys.
- Corrosion resistance is exceptional over the whole temperature range.
- The physical properties of aluminum alloys are well documented down to near absolute zero, including coefficient of expansion, specific heat, conductivity, reflectivity, emmissivity, and a number of others.
The low density of aluminum alloys combined with the desirable combination of properties described above makes them prime candidates for cryogenic service. With about one-third the weight of steel, aluminum structures often may be pre-fabricated and moved quickly into place. In addition, their light weight provides opportunities to increase the ratio of live payload to deadload, and hence the efficiency of the structures like bridges and transport vehicles.
The design properties of aluminum alloys are well documented, and included in most design codes like ASME Boiler & Pressure Vessel Code, API Piping Codes, and ANSI storage tank and refrigeration specifications.
The major aluminum alloys used for cryogenic and arctic service are as follows:
- 1xxx (unalloyed) aluminum - Commercially pure aluminum (1100, with 99.00 percent minimum aluminum) and special versions of higher purity (e.g., 1350 conductor) are readily weldable, have high electrical conductivity, and exceptional corrosion/chemical resistance, but relatively low strength so are used for those applications where the former properties are paramount.
- 2xxx (Al-Cu) alloys - Certain members of the 2xxx series, notably 2219 and 2195, are readily welded and are stronger than the 1xxx, 3xxx, 5xxx, and 6xxx series, and so are used for critical applications like space tankage, and vehicles for transportation of cryogenic fluids.
- 3xxx (Al-Mn) alloys - Alloy 3003 has been quite widely used for heat exchangers in cryogenic service; it has higher strength than 1xxx varieties and is very readily brazed, welded, and soldered.
- 5xxx (Al-Mg) alloys - The 5xxx alloys offer an exceptional combination of strength, toughness, corrosion resistance, weldability, and economy of fabrication, and are the most widely used for critical arctic and cryogenic applications from tankage, bridges, chemical processing equipment. As a prime example, alloy 5083-O was the alloy selected for the 125-ft (45-m) spheres for shipboard transportation of liquified natural gas (LNG) around the world.
- 6xxx (Al-Mg-Si) alloys - Exceptional extrudibility combined with high strength, corrosion resistance, and weldability, has made the 6xxx alloys also among the favorites for arctic and cryogenic service, often in combination with 5xxx alloys. The ability to economically produce irregularly shaped extrusions with the metal placed where stresses are highest provides a major advantage of alloys like 6061 and 6063.
- 7xxx(Al-Zn) alloys - Familiar aircraft alloys like 7050, 7075, 7150, and 7475 have high strength, but are not weldable by commercial practices and generally exhibit a gradual decrease in toughness with decrease in temperature; the use of 7xxx alloys is generally discouraged, with weldable alloy 7005 being the principal exception.
For more specific information on the properties and characteristics of aluminum alloys recommended for arctic and cryogenic service, readers are referred to the Aluminum Association publication Aluminum Alloys for Cryogenic Applications, which provides a more extensive listing of the property data as well as a pictorial summary of significant applications.
In addition, aluminum alloys for cryogenic applications are also covered in the following more general publications:
- Aluminum Standards and Data 2000
- The Aluminum Design Manual
- D. A. Altenpohl, Aluminum: Technology, Applications, and Environment, The Aluminum Association, Inc. and TMS (Second Printing), 1998
All of these publications are available from the Aluminum Association Bookstore online at www.aluminum.org.
Article provided courtesy of The Aluminum Association (www.aluminum.org)
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