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Overview of aluminum alloy mechanical properties during and after fires
Aluminum alloys are increasingly being used in a broad spectrum of load-bearing applications such as lightweight structures, light rail, bridge decks, marine crafts, and off-shore platforms. A major concern in the design of land-based and marine aluminum structures is fire safety, at least in part due to mechanical property reduction at temperatures significantly lower than that for steel. A substantial concern also exists regarding the integrity and stability of an aluminum structure following a fire; however, little research has been reported on this topic. This paper provides a broad overview of the mechanical behavior of aluminum plates both during and following fire. The two aluminum alloys discussed in this work, 5083-H116 and 6061-T651, were selected due to their prevalence as lightweight structural alloys and their differing strengthening mechanisms (5083 – strain hardened, 6061 – precipitation hardened). The high temperature quasi-static mechanical and creep behavior are discussed. A creep model is presented to predict the secondary and tertiary creep strains followed by creep rupture. The residual mechanical behavior following fire (with and without applied stress) is elucidated in terms of the governing kinetically-dependent microstructural mechanisms. A review is provided on modeling techniques for residual mechanical behavior following fire including empirical relations, physically-based constitutive models, and finite element implementations. The principal objective is to provide a comprehensive description of select aluminum alloys, 5083-H116 and 6061-T651, to aid design and analysis of aluminum structures during and after fire.
The materials included in this study are 5083-H116 and 6061-T651. These alloys were investigated due to their prevalence as common structural alloys, especially in lightweight transportation and structural applications, and their different strengthening mechanisms. 5083 is strengthened by strain hardening (cold work). It is a weldable, moderate strength alloy which exhibits good corrosion resistance in the H116 condition. 6061 is strengthened by precipitation hardening (heat treatment). It is a weldable, high strength alloy which also exhibits good corrosion resistance. The chemical composition of the alloys are shown in Table 1.
A review of the literature devoted to the problem of efficiency of the use of aluminum alloys in automotive structures is presented. Requirements are formulated on the structure and properties of alloys for cold rolling of car parts. The results of a study of sheets from AV alloy with a fine-grained recrystallized structure and adaptability to manufacture, which makes the sheets suitable for automotive panels, and mechanical properties at the level of steel sheets after aging in the process of drying of the lacquer coating, are presented.
Aluminum is a very light metal with a specific weight of 2.7 g/cm3, about a third of that of steel. This cuts the costs of manufacturing with aluminum. Again, its use in vehicles reduces dead-weight and energy consumption while increasing load capacity. This also reduces noise and improves comfort levels.
Its strength can be adapted to the application required by modifying the composition of its alloys. Aluminum-magnesium-manganese alloys are an optimum mix of formability with strength, while aluminum-magnesium-silicon alloys are ideal for automobile body sheets, which show good age-hardening when subjected to the bake-on painting process.
Aluminum naturally generates a protective thin oxide coating which keeps the metal from making further contact with the environment. It is particularly useful for applications where it is exposed to corroding agents, as in kitchen cabinets and in vehicles. In general, aluminum alloys are less corrosion-resistant than pure aluminum, except for marine magnesium-aluminum alloys. Different types of surface treatment such as anodising, painting or lacquering can further improve this property.
Aluminum is an excellent heat and electricity conductor and in relation to its weight is almost twice as good a conductor as copper. This has made aluminum the first choice for major power transmission lines. It is also a superb heat sink for many applications that require heat to be drained away rapidly, such as in computer motherboards and LED lights.
Aluminum is ductile and has a low melting point and density. It can be processed in several ways in a molten condition. Its ductility allows aluminum material to be formed close to the end of the product’s design. Whether sheets, foil, geometrical configurations, tubes, rods or wires, aluminum is up to them all.
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