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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.

Aluminum foil is only 0.007 mm in thickness, but is still durable and completely impermeable, keeping any food wrapped in it free of external tastes or smells. It keeps out ultraviolet rays as well.

Moreover, the metal itself is non-toxic and odorless, which makes it ideal for packaging sensitive products such as food or pharmaceuticals. The fact that recycled aluminum can be used reduces the carbon footprint for this stage of food and beverage manufacturers as well.

Aluminum is 100% recyclable and recycled aluminum is identical to the virgin product. This makes it a much more cost-effective source material for production runs. The re-melting of aluminum requires little energy: only about 5% of the energy required to produce the primary metal initially is needed in the recycling process.

Since the last decades of the 20th century, aluminum sheet has proven to be one of the most versatile metallic materials in those applications where weight reduction plays a fundamental role. The possibility of recycling aluminum alloys an indefinite number of times is another of its great attractions. Currently, the development of new alloys that improve mechanical properties and corrosion resistance while maintaining a light weight is one of the important lines of research and development work. At the same time, new processes are being developed to manufacture better-performing aluminum-based components, overcoming difficulties in casting, the poor ductility of aluminum alloys at room temperature, and its challenging weldability. Among these processes, solid phase processing, semi-solid processing, the liquid die forging process, powder metallurgy, sheet hydroforming, incremental forming, additive manufacturing and friction stir welding and its variants allow for dissimilar joints.

Many of the advances produced in the design and processing of alloys have been obtained thanks to modeling and simulation techniques. These techniques make it possible to describe everything from phase diagrams of new compositions based on thermodynamic calculations to the flow of material during the deformation and forming processes. To face the future challenges in the aluminum bar, it is necessary to improve knowledge of the micro- and mesoscopic mechanisms that explain the mechanical behavior of aluminum alloys. A deeper understanding of these mechanisms is necessary both in components in real use, and during the manufacturing processes. Additionally, the correlation between aluminum alloy properties and their microstructure must be considered in a unified way to explain the mechanical behavior in volume and surface and against corrosion.

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