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Aluminum is the most abundant metal and the third most abundant chemical element in Earth's crust, comprising over 8% of its weight. This chapter reviews the characteristics of aluminum alloys that make them so attractive and note the variety of applications in which they are used. It discusses the wrought alloys and cast alloys. The general corrosion behavior of aluminum alloys depends basically on three factors: (1) the stability of the oxide film, (2) the environment, and (3) the alloying elements. Cutting tool materials for machining aluminum alloys include water-hardening steels, high-speed steels, hard-cast alloys, sintered carbides, and diamonds. The aluminum surface can be finished in more different ways than any other metal. The normal finishing operations fall into four categories: mechanical, chemical, electrochemical, and applied. There are at least two approaches to overviewing important applications of aluminum alloys: by alloy class, and by type of application. Keywords: aluminum alloys; cast alloys; cutting tool materials; wrought alloys

The mechanical properties of aluminum alioys have been well documented at temperatures down to -320°F for many years [1–3] and, within the past few years, a reasonable amount of information has been developed at -423°F with liquid hydrogen as the cooling agent [4–7]. Nevertheless, little has been known of the properties of aluminum alloys at -452°F, the temperature of liquid helium; only one alloy, 5083, has been thoroughly evaluated prior to 1966 [8].

At present, the 2000(Cu), 5000(Mg), 6000(Mg-Si) series provide most of the aluminum alloys used in cryogenic applications. Established alloys in the 7000(Zn) series, including 7075, 7079, and 7178, are more sensitive to notches than many of the alloys of the above types [1,2] and have not been seriously considered. Recently, several new 7000-series alloys have been introduced with appreciably greater notch toughness than other alloys of this type. It is. expected that these new alloys may be of considerable use in cryogenic applications.

The 'stress-corrosion-fatigue' performance of several high strength- aluminum alloys was investigated by tests of hydraulic cylinders and other types of specimens. Specimens were prepared from forgings and forging stock of alloys 2014-T6, 7075-T6, 7075-T73, 7079-T6, and X7080-T7 and from premium castings of alloy CH70-T7. Alloy 7075-T73 rated best in the corrosion-fatigue tests; no stress-corrosion cracking occurred in this alloy, and the lives of forged cylinders subjected to repeated loadings to 80% of design stress in a corrosive environment were at least 10 times as long for this alloy as for forged cylinders of alloys 2014-T6, 7075-T6, or 7079-T6. Fractographic examination showed that stress-corrosion cracking as well as fatigue cracking occurred in alloys 2014-T6, 7075-T6, and 7079-T6 in the stress-corrosion-fatigue tests. The investigation demonstrated that stress corrosion and fatigue can interact under certain conditions to produce failures in shorter times and fewer cycles than for either phenomenon occurring by itself.

To assist in making the wisest choice of materials for cryogenic applications where fracture toughness is important, a number of investigators have been using tensile tests of smooth and notched specimens to evaluate the notch sensitivity of materials at subzero temperatures[1–6]. There has been little agreement among the investigators as to the design of notch which leads to the most effective comparison of materials although specific choices have been made by some groups.

The tensile properties, plane-strain fracture toughness, axial-stress fatigue properties and fatigue-crack propagation rates, and the resistance to exfoliation and stress-corrosion cracking, were determined for several aluminum alloys. Two thicknesses of X7080-T7E41(T751) and 7178-T651 plate, and two thicknesses of 7075-T6510, 7075-T73510, X7080-T7E42(T7510) and 7178-T6510 extruded shapes, were evaluated.

There are some applications where aluminum alloy castings, because of the ease with which they can be made into relatively complex shapes, would be very desirable for service at cryogenic temperatures. Parts which would require extensive machining from plate or hand forgings and are too intricate for the effective use of die forgings, would be suited ideally for fabrication by casting.

Experience indicates that with the development of materials with higher strengths, the tendency toward catastrophic low-ductility fractures increases, particularly at cryogenic temperatures. Continued efforts to use these higher-strength materials in structural units and yet to ensure against catastrophic low-ductility fractures have markedly increased the efforts aimed at evaluating the resistance of materials to rapid crack propagation. Formerly, coarse qualitative measures were sufficient, but more critical criteria for quantitatively rating materials and for realistically designing with them are now needed.

An impressive amount of literature has been developed to define the tensile properties and notch toughnesses of aluminum alloys at very low cryogenic temperatures [1–3], Data and service experience are consistent in indicating the suitability of many aluminum alloys for cryogenic service, since they retain their relatively high ductility and notch toughness, in addition to having useful strengths at very low temperatures. Although a fair amount of data have been presented on the tensile properties and notch toughness of welds in aluminum alloys [4,5], none have been available for liquid-helium temperature to provide a guide as to which particular combinations of parent and filler alloys might be most desirable for extremely low-temperature service.

Among the readily weldable aluminum alloys showing great promise for cryogenic applications, 2219[1,2] appears to be outstanding. In tests made recently at NASA Lewis Research Laboratories[3], the strengths of both smooth and notched specimens from 2219-T62 sheet (plain, as-welded, and reheat-treated and aged-after-welding) were shown to increase with decrease in temperature to -423°F. The elongations generally remained relatively high. This alloy appears to offer medium-high strength and high toughness similar to that of the high-strength aluminum-magnesium series of alloys plus the advantages of response to postweld thermal treatments.

The interest in the use of aluminum alloys for cryogenic applications has stemmed from the long-recognized increases in tensile properties of these alloys at depressed temperatures [1–4]. The fact that the elongations, as well as the strengths, of most aluminum alloys increase as temperature decreases, and that the reduction of area values are about the same at -320oF as at room temperature, formed the basis for further study of the properties of these alloys at low temperature. Also of importance was the often demonstrated absence of transition in fracture behavior.

Because of the economic advantages in utilizing aluminum alloys for applications at very low temperatures, data were published in 1967 and 1968 for a variety of 2XXX and 5XXX aluminum alloys and for welds in those alloys at -452°F [1’2] showing their high strength, ductility, and toughness. This program has now been extended to include 7XXX alloys in several tempers.

It is well known that the mechanical properties of 301 stainless steel are greatly enhanced by forming at cryogenic temperatures [1,2]. This procedure is sufficiently advantageous that the technique of “cryoforming” stainless steels has become a fairly standard procedure, and such products are available from several suppliers.