Basic Knowledge of magnesium
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 | AI | 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 | 1,810 kg/m3 (at 20℃) |
| Solidus temp. | 470℃ |
| Liquidus temp. | 595℃ |
| Kinematic viscosity | 1.0×10-6 m2/s (at 20℃) |
| Specific heat | 1,014 J/kg･K (at 20℃) |
| Thermal conductivity | 54 W/m･K (at 20℃) |
| Latent heat | 3.73×105 J/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 | 1,300t | 900t | 9,000t |
| 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 | 3.8～4.5/1,000 Excellent | 5～5.5/1,000 Good | 7～8/1,000 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 |