Aluminium - √دنیـــــــای متـــــــــــالورژی فقـــــــط از ایـــــــنجا
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تاريخ : دوشنبه ۱٢ دی ۱۳۸٤

Aluminum

Aluminium has five characteristic properties of interest to engineer:

lightness : It′s density is only 2.7 gr/cm³ as compared with 7.8 gr/cm³ for steel and 8.8 gr/cm³ for copper . For this reason the metal is used extensively , in the form of alloys for aircraft and automobile parts.

 

Electrical conductivity is about 60 percent of that of copper .

 High thermal conductivity , ex; heat exchanger components .

 

It′s resistance to corrosion is made use of it in chemical plant for handling nitric acid , and food in dustries ; while aluminum foil is used to cover chocolates , cigarettes , etc . alloys are used for marine and building purposes .

 

Aluminum is made by the electrolysis of alumina dissolved in molten cryolite . Major purifying treatment has to be carried out on the ore , but metal of high purity (99.99 percent pure) can be obtained by a special electrolytic refining process . The main impurities are silicon and iron .

Aluminum can be worked either hot or cold , and can be heat treated . (recrystallisation)

Aluminum has its resistance to corrosion to the natural film of oxide which covers its surface.

Aluminum is a relatively weak material and for engineering purposes numerous alloys have been developed which have improved properties .

The addition of small quantities of various elements to aluminum, improves the mechanical properties and foundry characteristics .

In aeroplanes and other structures the extreme lightness of the alloys enables a much greater volume to be used for a given mass .

Aluminum alloys have a coefficient of linear expansion which is about twice that of cast iron or steel and allowance must be made for this when Aluminum alloys readily absorb gases and the presence of water vapour in the furance atmosphere is particularly harmful .

When aluminum is cast , the gases are liberated and form pinholes in the casting , to overcome this trouble , various treatments of the molten metal have been done and used for degasification .

These include the bubbling of nitrogen and chloride or patented fluxes such as sodium chloride or sodium fluoride in the ratio 2:1.

The corrosion resistance of aluminum and its alloys is high the various grades of pure aluminum are most resistant , and then followed by the aluminum magnesium (Al-Mg) and aluminum manganese alloys (Al-Mg).

The alloys containing copper are the least resistant to corrosion ; but this can be improved by coating each side of the alloy with a thin layer of high purity aluminum .

Contact between aluminum alloys and other metals in the presence of moisture may makes electrolytic corrosion .

Aluminum alloys correde rapidly when in moist contact with copper and its alloys , less rapidly in contact with ferrous materials ; but it is protected by zinc (Zn), cadmium(Cd)and magnesium (Mg).

In recent years the size of aluminum alloy castings has increased as their application has improved .

Developments in the processes of die casting alloys and of forging , high strength alloys have produced during recent years and have resulted in improved mechanical properties.

                                            

It has a great affinity for oxygen , a property which enables the metal to be used as a deoxidant in steels.


ارسال توسط محسن حیدری
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