Basic Information

What is Magnesium?

Magnesium with atomic number 12 is the lightest metal in practical use. In addition to its lightness, it has other features such as rigidity, electromagnetic wave shielding, vibration absorption, and dimensional stability, and is attracting attention as an alternative material to plastic, aluminum, and steel.

It has been nearly 100 years since magnesium production began, but until now it has not been used in large quantities because it is easily corroded and the price is high due to the small number of manufacturers that handle it. However, magnesium is now being reviewed due to the development of surface treatment technology that complements corrosion resistance, the reduction in raw material prices due to the increase in magnesium production areas such as China, and the increase in the number of processing manufacturers that handle magnesium.

Above all, with the advent of the thixomolding method, which is a safe and highly productive molding method, it has become possible to mold magnesium with the same feeling as plastic and it has led to an increase in magnesium products these days.

Composition of Major Alloys (ASTM)

Alloy name Mg Al Zn Mn Si Cu Ni Fe
AZ91D bal. 8.5-9.5 0.45-0.90 0.17-0.4 ≦0.05 ≦0.025 ≦0.001 ≦0.004
AM60B bal. 5.6-6.4 ≦0.20 0.26-0.5 ≦0.05 ≦0.008 ≦0.001 ≦0.004
AM50A bal. 4.5-5.3 ≦0.20 0.28-0.5 ≦0.01 ≦0.008 ≦0.001 ≦0.004

Features and Applications of Magnesium Alloys

Alloy name Feature Application
AZ91D High strength and good corrosion resistance Housings of electric appliances
AM60B Shock absorption with high ductility Steering wheel, seat frame
AM50A
AS41B Good creep characteristic Transmission housings

Features of Magnesium

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

Strength and Toughness

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

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

EMI Shielding

Magnesium casing has a higher EMI shielding ability than plastic one with metallic plating.

 

Application: Mobile phone casings

Machinability

Magnesium alloy has a lower resistance against cutting than almost all other metallic materials, which enables rapid machining.

Resistance of metallic materials against cutting

(Relative value compared to magnesium alloy)

NAME RESISTANCE
Magnesium alloy 1.0
Aluminum alloy 1.8
Brass 2.3
Cast iron 3.5

Resistance against Dent

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

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

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

Dimensional Stability

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

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.

Comparison of Characteristics of Each Material

NAME Density
(g/cm³)
Melting
temperature
(℃)
Thermal
conductivity
(W/Mk)
Tensile
strength
(MPa)
Yield
strength
(MPa)
Elongation
(%)
Specific
strength
Young's
modulus
(GPa)
Magnesium
alloy
(Thixomolding™)
AZ91 1.82 596 72 280 160 8 154 45
AM60 1.79 615 62 270 140 15 151 45
Aluminum
alloy
(die casting)
380 2.7 595 100 315 160 3 117 71
Steel Carbon steel 7.86 1,520 42 517 400 22 66 200
Plastics ABS 1.03 90(Tg) 0.2 35 * 40 34 2.1
PC 1.23 160(Tg) 0.2 104 * 3 85 6.7

Tensile test results of various magnesium

Material Process Barrel
temperature
(K)
Injection
velocity
(m/s)
Y.S.(MPa) T.S(MPa) El.
(MPa)
AZ91D Thixomolding™ 878 1.4 180 299 10
(Die casting) * 160 240 3
AM60B Thixomolding™ 893 1.4 148 278 19
Die casting 963 2.9 115 239 12
(Die casting) * 130 225 8
AM50A Thixomolding™ 898 1.4 140 269 20
Die casting 963 2.9 112 232 13
(Die casting) * 125 210 10

The values for die casting are taken from the catalogue

The result of tensile test obtained from thixomolded bar-type specimens at room temperature.

As a reference, catalogue values and actual data obtained with a cold chamber type die-cast process are included. As can be seen clearly, thixomolded specimens show properties superior to die cast ones. As the most of actual parts have rather complicated shapes, it is advisable to choose an adequate safety factors at the designing.

Physical properties of Mg alloy(AZ91D)

Density

1810kg/m³ (at 20℃)

Solidus temp.

470℃

Liquidus temp.

595℃

Kinematic viscosity

1.0×10⁻⁶m²/s (at 20℃)

Specific heat

1014J/kg・K(at 20℃)

Thermal conductivity

54W/m・K(at 20℃)

Latent heat

3.73x105J/kg

Difference between Thixomolding™ and Die-Casting

Item Thixomolding™ Die-Casting
Hot-Chamber Cold-Chamber
Molding temp.(℃) 590~610 630~650 680~700
Injection speed (m/s) 1~4 1~4 1~10
Injection pressure (MPa) 30~100 7~35 20~120
Material Chip Ingot Ingot
Max. machine size 1300t 900t 9000t
Blow hole Few Small Many
Surface defect Few Small Many
Shrinkage crack Few Small A few
Fluidity Excellent Good Good
Surface roughness Excellent Good Good
Flash Small Few Much
Shrinkage Few Small Many
Mold shrinkage Dimension accuracy 3.8~4.5/1000 Excellent 5~5.5/1000 Good 7~8/1000 Poor
Warp Few Small Much
Mechanical properties Excellent Good Good
Corrosion resistance Good Good Poor
Shot cycle 1(Standard) 0.8 0.9
Material cost 1(Standard) 0.85 0.9
Material yield 1(Standard) 1 1.2
Die's life 1(Standard) 0.9 0.8
Safe operation Excellent Good Poor
Protection gas Do not use Use Use
Dross/Sludge Nothing Much Much
The way into material Screw method gooseneck Ladle pour