4. Mg 와 Mg 합금

4. Mg 와 _Mg 합금

Basic Information of Mg

Atomic Structure (Mg)

Number of Energy Levels: 3 First Energy Level: 2 Second Energy Level: 8

Third Energy Level: 2

• Name: Magnesium

• Symbol: Mg

• Atomic Number: 12

• Atomic Mass: 24.305 amu

• Melting Point: 650.0 °C (923.15 K )

• Boiling Point: 1107.0 °C (1380.15K )

• Number of Protons/Electrons: 12

• Number of Neutrons: 12

• Classification: Alkaline Earth

• Crystal Structure: Hexagonal

• Density @ 293 K: 1.738 g/cm3

• Color: grayish

• Compounds of magnesium have long been known. It was recognized as an element in 1755, was isolated in 1808 and was prepared in coherent form in 1831. Magnesium is the eighth most abundant element in the earth’s crust.

• The main mining areas are in Austria, China, Poland, Russia, USA, INDIA, Greece & Canada.

• Magnesium is produced by the electrolysis of seawater, dolomite or brines. It is usually extracted from sea water, which contains about 0.13% Mg in unlimited supply.( the third most plentiful element dissolved in seawater) Lime is added from the roasted sea shells and treated chemically to precipitate magnesium hydroxide. The hydroxide is filtered and treated with hydrochloric acid,

resulting in magnesium chloride, which is dried and placed in an electrolytic

cell. Magnesium results and is then formed into ingots.

Magnesium is:

the lightest of all structural metals a quarter the weight of steel

one third lighter than aluminium

has the best strength to weight ratio of common structural metals has exceptional die-casting characteristics

Excellent ability to absorb energy vibration, acts as a damper highly dent resistant

Magnesium is the ideal material for applications where weight saving is a priority, having the lowest density of all structural metals. Almost as light in weight as plastic, magnesium has the advantage of greater strength and rigidity along with inherent EMI shielding, durability, heat-dissipation and full recyclability.

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MAGNESIUM OR?

•

Magnesium or plastics?

• In comparison to plastics, magnesium often wins due to its good strength properties, higher stiffness and better breaking and elongation behavior.

Basically, it is possible to reach

comparably good values with plastics, however, very high-quality glass fiber reinforced plastics must be used. In this case, magnesium often reaches a more favorable part price.

• Additionally, these plastics cannot be recycled and are therefore very

problematic to the environment. On the contrary, magnesium can be recycled without any problems, and its resources are virtually unlimited worldwide.

Magnesium or aluminum?

Besides the lower density of magnesium in comparison with aluminum alloys (AZ91:

density 1,81 g/cm3 - AC 46000: density 2,68 g/cm3),

Further advantages are:

• better castability of magnesium

• higher production performance with casting because aluminum alloys can only be processed in a cold chamber process which shows longer cycle times

• in the hot chamber process, thinner wall thickness can be produced.

• longer tool life.

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1) The lightest metallic material

• The density of the magnesium is about 2/3 of that of aluminum and a quarter of that of steel. Magnesium is the lightest among metallic materials which are being used

practically. Application: Automobile parts and portable electronic appliance parts 2) Strong and tough

• Although magnesium alloy has a higher density than plastics, its tensile strength and Young’s modulus per unit weight are higher than plastics. This enables to make a lighter part by using magnesium alloy than plastics. Application: The frame and backside casing of liquid crystal panel for notebook PCs and mobile phones 3) Heat dissipation

• The thermal conductivity of magnesium alloy is much higher than that of plastics.

Magnesium casings of electronic appliances can dissipate heat, which is generated in the electronic circuit, much more effectively than plastic casings.

Application: Casings for PCs, LCD projectors, and TV 4) EMI(Electromagnetic Interference) shielding

• Magnesium casing has a higher EMI shielding ability than plastic one with metallic plating. Application: Mobile phone casings

5) Resistance against dent

• Magnesium alloy has a high resistance against deformation and is less easily made dents than other metal.

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Magnesium alloy characteristics:

6) Machinability

• Magnesium alloy has a lower resistance against cutting than almost all other metallic materials. This makes it possible to do rapid machining.

• Table: Resistance of metallic materials against cutting (Relative value compared to magnesium alloy)

7) Vibration absorption

• Magnesium alloy can absorb vibration energy effectively. This is a desirable property for applications which any mechanical movement is expected.

Application: Slide base for optical pick-up, and electric fan 8) Ductility

• A certain kind of magnesium alloy has a higher ductility than aluminum alloys and can absorb impact energy without brittle fracture.

Application: Steering wheels and seat flames for automobiles 9) Dimensional stability

• Dimensions of magnesium parts are very stable against time and temperature.

10) Easy recycling

• Magnesium can be recycled without any degradation in physical properties, which is often experienced in plastics recycling. Also, the energy necessary for recycling process of

magnesium alloys is smaller than that for other metals. Only 4% of energy for virgin metal production is enough for recycle process of used magnesium alloy.

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• Major alloying additions made to commercial magnesium alloys are:

Aluminum: increases strength and gives precipitation hardening properties.

Manganese: improves corrosion resistance with little effect on strength.

Zinc: Increases strength and improves workability of wrought alloys. But limits weldability.

• Limitations:

-Not suitable for engineering applications in unalloyed form.

-All materials have poor corrosion resistance and are suitable for un-

contaminated atmospheres only. Epoxide coating commonly used for corrosion resistance.

-Magnesium has closed packed hexagonal atomic structure which seriously limits its suitability for cold working.

-Low modulus of elasticity of about 4.5GPa

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A) Structural applications

• Automotive applications

• Material handling:dock boards, grain shovels, gravity conveyors.

• Commercial applications:hand held tools, luggage, computer housings and ladders.

• Aerospace applications

• Industrial machinary

B) Non-structural applications

• Alloying elements

• An oxygen scavenger and a desulfurizerin the manufacture of nickel and copper alloys; as a desulfurizer in the iron and steel industry

• A reducing agentin the production of beryllium, titanium, zirconium, hafnium and uranium.

• Pyrotechnic, both as pure magnesium and alloyed with 30% or more aluminum.

• Cathodic protection of other metalsfrom corrosion and in construction of dry-cell, sea water and reserve-cell batteries

Application

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Processing:

Magnesium alloys can be fabricated by all the usual forming methods except cold forming. Alloys are available in bar, sheet, tube and extruded forms. Welding is possible using oxy-acetylene and argon-arc processes but alloys containing zinc are not weldable due to their propensity to weld cracking. Soldering is not recommended. Other joining methods include riveting and adhesive bonding.

Certain alloys have very good casting properties and are particularly suited to die casting

at high production rates; magnesium alloys have the advantage over aluminum alloys in

that they do not attack the die steel and so die wear sticking are reduced.

Thixomolding machine has a structure similar to an injection molding machine for plastics. The chipped magnesium alloy is used as the raw material and the size of chip is about 4mm. Thixomolding machine consists of a special shot system to get a high injection speed, and a barrel and screw in which alloy feed stock is heated to semi-solid or fully melting state by electric heaters. The magnesium alloy slurry in the barrel is injected into the mold just as the same manner as the injection molding of plastics.

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Thixomolding is characterized by the thixotropic flow behavior of semi-solid slurry in the mold cavity. In this process, the semi-solid slurry shows very low viscosity, which is almost the same as that of fully melted alloy. This is because the semi-solid slurry contains spherical solid particles in the melt, which is comming from the action of shear force provided by the screw rotation.

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Recycling of Mg for Thixomolding

Mg 야금개요

 Dow Chemical(U.S.A) Co.의 Dow Process

Dolomite rock (CaMg(Co

)

)

Kiln

Precipitation to Mg(OH)₂ Filters

Neutralizer MgCl₂ Purifier Dryer Dehydration from 35 % to 73 % MgCl₂

Electrolytic

Cells

Magnesium Ingots

Seawater (해수) HCl from Organic Chlorination

Hydrochloric Aced Furnace

Liqaefaction

Chlorine Chlorine

Mg(OH)₂ 로 침전

건식제련법(Pidgeon 法)

Mixing 團 鑛

Retort Reduction Condensation

of Mg Vapor Remelting Ingot of Mg

소성 Dolomite + 분말규소철 , 6 : 1

 1150 ~ 1200 ºC

 10

torr

2(MgO • CaO) + Si = 2Mg + Ca

SiO

(vapor)

Retort 끝 부분에서 냉각

99.95 ~ 99.97 % 의 Mg