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رشته مهندسی مواد، عاملی برای پیشرفت کشور عزیزمان ایران
  • امید اشکانی جمعه 26 آذر 1395 01:17 ب.ظ نظرات ()

    Aluminium or aluminum (in North American English) is a chemical element in the boron group with symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic, ductile metal. Aluminium is the third most abundant element in the Earth's crust (after oxygen and silicon) and its most abundant metal. Aluminium makes up about 8% of the crust by mass, though it is less common in the mantle below. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. The chief ore of aluminium is bauxite.

    Aluminium is remarkable for the metal's low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the aerospace industry and important in transportation and structures, such as building facades and window frames. The oxides and sulfates are the most useful compounds of aluminium.

    Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically, but aluminium is well tolerated by plants and animals. Because of their abundance, the potential for a biological role is of continuing interest and studies continue.

    Aluminium is a relatively soft, durable, lightweight, ductile, and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. A fresh film of aluminium serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded.

    Aluminium atoms are arranged in a face-centered cubic (fcc) structure. Aluminium has a stacking-fault energy of approximately 200 mJ/m2.

    Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper's density. Aluminium is capable of superconductivity, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss (10 milliteslas).

    Corrosion resistance can be excellent because a thin surface layer of aluminium oxide forms when the bare metal is exposed to air, effectively preventing further oxidation, in a process termed passivation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.This corrosion resistance is greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.

    In highly acidic solutions, aluminium reacts with water to form hydrogen, and in highly alkaline ones to form aluminates— protective passivation under these conditions is negligible. Primarily because it is corroded by dissolved chlorides, such as common sodium chloride, household plumbing is never made from aluminium.

    However, because of its general resistance to corrosion, aluminium is one of the few metals that retains silvery reflectance in finely powdered form, making it an important component of silver-colored paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3,000–10,000 nm (far IR) regions; in the 400–700 nm visible range it is slightly outperformed by tin and silver and in the 700–3000 nm (near IR) by silver, gold, and copper.

    Aluminium is oxidized by water at temperatures below 280 °C to produce hydrogen, aluminium hydroxide and heat:

    2 Al + 6 H2O → 2 Al(OH)3 + 3 H2

    This conversion is of interest for the production of hydrogen. However, commercial application of this fact has challenges in circumventing the passivating oxide layer, which inhibits the reaction, and in storing the energy required to regenerate the aluminium metal

    Aluminium has many known isotopes, with mass numbers range from 21 to 42; however, only 27Al (stable) and 26Al (radioactive, t1⁄2 = 7.2×105 years) occur naturally. 27Al has a natural abundance above 99.9%. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes are useful in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of 26Al to 10Be has been used to study transport, deposition, sediment storage, burial times, and erosion on 105 to 106 year time scales. Cosmogenic 26Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding drastically reduces 26Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.

    From : Wikipedia.com

    آخرین ویرایش: جمعه 26 آذر 1395 01:21 ب.ظ
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  • امید اشکانی جمعه 26 آذر 1395 01:12 ب.ظ نظرات ()

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    آخرین ویرایش: جمعه 26 آذر 1395 01:17 ب.ظ
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  • امید اشکانی جمعه 12 آذر 1395 10:09 ب.ظ نظرات ()

    جوشکاری زیر آب از زمان جنگ جهانی دوم هنگامی که کشتی‌های خسارت دیده باید سریعاً در آب تعمیر می‌شدند به وجود آمد. بیرون آوردن کشتی برای تعمیر کردن آن، هم اکنون هم بسیار هزینه بر است و صرفه اقتصادی ندارد.

    بسیاری از مردم جوشکاری زیر آب را بسیار عجیب می‌دانند، چون ماهیت جوشکاری را از آتش می‌دانند.

    ولی جوشکاری ماهیت قوس الکتریکی دارد و روشن شدن آن زیر آب کار عجیبی نیست. برای جوشکاری در خشکی، هوا یونیده می‌شود و در آب، بخار آب یونیزه می‌شود.

    جوشکاری زیر آب به دو صورت انجام می‌شود:

    1. جوشکاری خشک
    2. جوشکاری مرطوب.


    آثار منفی جوشکاری مرطوب عبارت‌اند از ترک خوردگی هیدروژنی، افت شدید دما که باعث تغییرات ساختاری و متالورژیکی می‌شود و همچنین اکسیژن با عناصر آلیاژی ترکیب می‌شود و اکسید این آلیاژها در آب حل می‌شوند. جوشکاری خشک در یک اتاقک در داخل آب انجام می‌گیرد و داخل اتاقک هوای فشرده وجود دارد که فشار داخل و خارج اتاقک را بالانس می‌کند. اتاقک‌ها را دو تکه می‌سازند و داخل آب، و روی قطعه مورد نظر دو تکه را به هم وصل می‌کنند. یک لوله رابط بین کشتی و اتاقک است و وسایل مورد نیاز را به وسیله این لوله به اتاقک می‌فرستند. این روش برای اولین بار در آمریکا انجام گرفت اما چون بسیار پرهزینه و وقت گیر است دانشمندان سعی می‌کنند مشکلات جوشکاری مرطوب را حل کنند چون سریعتر و ارزانتر است. وسایل ایمنی همان وسایل ایمنی جوشکاری روی خشکی است بعلاوه تجهیزات غواصی. جوشکاری زیر آب با صنعت نفت و گاز گره خورده‌است.

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