As the lightest structural metal available, magnesium's combination of low density and good mechanical strength results in a high strength-to-weight ratio and very easy for hot work metal. Magnesium alloys can absorb energy elastically because of their low modulus of elasticity. Combined with moderate strength, this provides excellent dent resistance and high damping capacity. The experiments show that magnesium is almost used from room temperature to about 200°F - 350°F. Some alloys can be used in service environments to 700°F for brief exposures. Magnesium is deeply sodden misconceptions often prevent designers from specifying it as a die-cast material. But there are some differences between magnesium as a generic material and modern's die-casting alloy. They are definitely more complicated and intricate. High-purity alloy is used in fluxless, hot-chamber die-casting processing, has altered the traditional guidelines for evaluating the cost and performance of magnesium die castings.
Magnesium alloys have many advantages than the other alloys. They are the easiest of the structural metals to machine, can be shaped and fabricated by most metalworking processes. There aren't any problems for magnesium to become hardens rapidly. That cold forming is limited to mild deformation or roll bending around large radii. The qualities of pure magnesium give many advantages to alloyed with other elements for structural purposes. Cast magnesium alloys are dimensionally stable to about 200°F. Permanent-mold castings are as strong as sand castings, and they generally provide closer dimensional tolerances and better surface finish. They find some application at aircraft engine components and car wheels. Design of die-cast magnesium parts follows the same principles established for other die-casting metals.
Specifications of magnesium alloys are designated by a system established by the ASTM that covers both chemical compositions and tempers.
There are some 'letters' that describe some variation in composition of minor alloying constituents or impurities. The letters that designate the more common magnesium alloying elements are:
A - Aluminum
E - Rare earths
H - Thorium
K - Zirconium
L - Lithium
M - Manganese
Q - Silver
S - Silicon
Z - Zinc
For example magnesium alloy AZ31B contains 3% aluminum (code letter A) and 1% zinc (code letter Z).
The main problem with using of magnesium metal is its corrosion, particularly in the alloys used for die and sand casting. Nowadays scientist and engineers try to hold this problem. Resisting corrosion is solved by the two major supplies, Dow and AMAX. Both find and show that high-purity AZ91 alloys for die casting and a sand-casting grade must be prefer. The die-casting grade is now designated by ASTM as AZ91D and will, for all practical purposes, replace AZ91B. The sand-casting grade received the designation AZ91E from ASTM. The high-purity alloys are said to be as much as 100 times more corrosion resistant than standard magnesium alloys, and more resistant to saltwater than die-cast 380 aluminum alloy or cold-rolled steel, tested according to ASTM B117. The commercially important Mg-Al-Zn alloys used for die-casting and sand casting have received intensive study, resulting in the development of alloys with outstanding saltwater corrosion resistance. Research in magnesium metallurgy has shown that the ability of magnesium to resist corrosion in a service environment of salt-laden air or spray depends heavily on keeping contaminants (iron, nickel, copper) below their maximum tolerance limits during all production operations.
Factors that affect the corrosion properties of magnesium are alloy composition, heavy-metal impurities, casting variables, metallographic structure, environment, surface condition, and contact with other materials. Thermodynamically, magnesium should react completely with oxygen and as well as water. The fact that it reacts with neither of those is caused by passive behavior in many environments. In a corrosive environment, pitting or other forms of local corrosion occur as a result of film breakdown. Magnesium alloys of suitable composition and purity are corrosion resistant. The corrosion of magnesium alloys is commonly measured in a sodium chloride solution, by using either immersion or salt spray tests. These tests relate to important practical uses of magnesium alloys in automotive, aircraft, and military applications. Other important notes for and to avoid corrosion effects is metal structure -the size and distribution of the cathodic phases play an important role in corrosion and are influenced by process parameters and heat treatment. Producers of magnesium have demonstrated the importance of high-purity alloys for structural applications. However, surface contamination from handling and mechanical treatment can greatly degrade the corrosion resistance of high-purity alloys. This helps explain why ceramic blasting media containing iron oxide can be just as harmful to the corrosion properties of magnesium as steel grit. Atmosphere also play important role for future of magnesium. A magnesium alloy surface exposed to a salt-free atmosphere develops a gray film consisting mainly of magnesium hydroxide that protects the metal from corrosion. The surface film on alloys gives limited protection from further attack. Unprotected magnesium and magnesium-alloy parts are resistant to rural atmospheres and moderately resistance to industrial and mild marine atmospheres, provided they do not contain joints or recesses that entrap water in association with an active galvanic couple. In marine atmospheres heavily loaded with salt spray, magnesium alloys require protection for prolonged survival.
When we talk about corrosion we can't forget about water effect. When magnesium is immersed in distilled water without the possibility of carbon dioxide absorption, the initial corrosion rate decreases rapidly to a very low value. A protective film of magnesium hydroxide forms on the surface. Acids destroy magnesium, too. Magnesium is attacked by all acids except hydrofluoric or chromic acid. Passive films are formed in most concentrations of these acids, accounting for their use in many conversion-coating processes. Pure H2CrO4 attacks magnesium and its alloys at a very low rate. However, traces of chloride ion in the acid will markedly increase this rate.
Gases like iodine, bromine, fluorine, and dry chlorine cause little or no corrosion of magnesium at room or slightly elevated temperature. Aliphatic and aromatic hydrocarbons, ketones, and ethers are not corrosive to magnesium and its alloys. Ethanol and higher alcohols are not corrosive at ordinary temperatures, but they may react destructively at high temperature (150o C, or 300o F).
Gasoline-methanol fuel blends, in which the water content equals or exceeds about 0.25 wt% of the methanol content, do not attack magnesium. Dry fluorinated hydrocarbons, such as the freon refrigerants, do not attack magnesium alloys at room temperature, but when water is present they may stimulate significant attack. At elevated temperatures, fluorinated hydrocarbons may react violently with magnesium alloys.
Magnesium galvanic corrosion can be sort in two conditions: (1) dissimilar metal-to-metal contact and (2) bridging of the bimetal junction by a conductive solution (electrolyte). The electrochemical process, which normally occurs at the galvanic couple containing magnesium is:
Anode: Mg (metal) -> Mg2+ + 2e-
Cathode: 2H2O + 2e- -> H2 + 2 (OH)-
In salt environments, the high solubility and acidic nature of the magnesium chloride formed at the anode can result in rapid penetration of magnesium alloys. Proper protection against galvanic corrosion begins with good design. This includes good drainage to prevent entrapment of electrolyte, selection of the most compatible metals, sealing of faying surfaces, small ratios of cathode to anode area, and use of alkali-resistant barrier coatings.
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