-
-
-
-
Criteria for selection of magnesium:
-
The basis for selection of any material for constructing a specific part is that the part will serve with the best compromise between function and cost when made of that material. Magnesium is superior to other materials for a number of properties and inferior for others. Knowledge of what these properties are, and the full implication of each for the service of the part is required for proper selection of material. After selection, design must maximize exploitation of the favorable properties and minimize the unfavorable.
-
- The density of pure magnesium is 1.74 gr./cc, less than for any other structural metal. For a specific part, a lower density can be translated into a lower weight, lower cost, improved ruggedness, or all three. Whether low density will result in lower weight for a part depends upon the mechanical properties of competing materials, upon ability to manufacture the part with dimensions that will utilize mechanical properties fully, and on the design of the part.
-
- For some forms, for example high- pressure died casting; the properties of magnesium are equal to those of competing materials. In this case, of course, the part will be lighter if made of magnesium than of other metals.
-
- For other forms, for example most wrought products, some properties of magnesium are less than those of some competing materials. If the elastic modulus such as in buckling is critical, the part will be lighter if made of magnesium. If yield and tensile strengths are critical, the part will be heavier when made of magnesium. However, if the design is such that the mechanical properties of the competing material are not fully utilized, then the part maybe, even in this case, lighter when made of magnesium.
-
- Lower density can lead to lower cost for the finish part or structure in several ways. If the part is lighter when made of magnesium, less material is used to make the part and this is a cost saving. If the use all magnesium permits making other components of the structure lighter, a cost savings in those materials is realized. Because walls can be thicker without weight penalty when using a material of low density, it is sometimes possible to use monocoque construction, eliminating the need for stiffeners with their accompanying fasteners, reducing costs considerably.
-
- Because thicker walls can be used without weight penalty, other properties improve. Thus, a lawn mower housing made of magnesium resists stones thrown by the rotor much more than will a housing made of steel simply because the magnesium housing can be so much thicker and still be lighter than the steel housing. A part, even though of lightweight when they of magnesium will often be much more rugged and serviceable against abuse than the same part made of another, more dense, material. There is an extra margin of safety because of the low density.
-
- Damping
- Damping capacity is a measure of the rate at which unforced vibration disappear due to internal mechanisms in the material. The high damaging capacity of magnesium means that vibrations due to sound or mechanical stress diminish rapidly and are not transmitted readily through the structure. A high damping capacity is directly useful, for example, when the material is used as a platform for objects sensitive to vibration damage or when sound absorption is important.
-
- Magnesium is used as a platform for electronic devices in missiles because of its good damping capacity. The indirect benefits of high damping capacity are often even more important. Since vibrations due to stress are no transmitted efficiently, alternating stress applied to one part of a structure lessens as it reaches an adjacent part of the same structure, resulting in a greater fatigue life, which would be difficult to achieve trough the use of aluminum alloys as an example.
-
- The energy required to form a crack in magnesium is usually less than is required for other metals, the energy to create a permanent dense crack is more than that for other metals. There are many applications where failure is better defined by denting than by destruction. A high resistance to denting is important to luggage or to shipping containers, for example, to prevent a battered look.
-
- As with cracking, the galling tendency of magnesium is quite low, when compared to Aluminum and other metals. Thus, sliding components such as high speed looms, sliders and shuttles will often operate more smoothly and with fewer problems.
-
- Magnesium will not "crock" when handled. There are many applications where bare metal surfaces are subject to discoloration due handling and this may require that the metal be given a protective coating before use. If the environment is such that corrosion is not a problem, surface finishing of magnesium components is not required. Thus, hospital racks for bedding and clothing, paper spools, drafting tools and printing equipment can be fabricated from unfinished magnesium, resulting in lower production costs.
-
- Magnesium is the most machineable of all metals; in the sense of less power requirements for metal removal and less tool wear. A fine surface is obtained with relatively deep cuts, eliminating the need for a final, low speed pass in order to get a fine finish. Deep and small-diameter holes can be drilled rather easily in Magnesium, which is not always possible in other metals.
-
- Fewer steps, higher tool speeds, less power demand, and less down time for tool re-sharpening, translates into less cost for any machining steps required for producing a part. The metallurgy of magnesium is such that the stability of both properties and dimensions are excellent and well suited for tooling jigs which retain their dimensions over prolonged usage.
-
- Corrosion
- The essentials of the corrosion of magnesium can be summarized by three statements. It is stable in alkaline and fluoride environments. It is unstable in most acidic or chloride environments. The solution potential is higher than that for any other structural metal.
-
- Since the Earth we live on is generally considered acidic chloride is far more prevalent than chloride, the corrosion resistance of magnesium is less than that of aluminum (although better than that of mild steel). Occasionally there are specialty applications that make good use of resistance to alkali. For example, bulk- shipping containers for paint were made of magnesium because of its good resistance to alkaline cleaning after shipping.
-
- In general, magnesium finishing is required if salt water will be contacted or if there will be standing fresh water in contact with the part that will take a carbon- dioxide from the atmosphere and thus become acidic. If the part will be exposed only to dry conditions, finishing will not be required except for decorative purposes.
-
- Because of its position in the galvanic series, magnesium is subject to galvanic corrosion if it is electrically connected to any other structural metal and there is an electrolyte presents. This is the most serious corrosion problem for magnesium, and must be guarded against in any design. Removal of either the metallic connection or the electrolyte eliminates galvanic corrosion.
-
- Thus, when considering magnesium for an application, the major points to be considered are:
- Is lightweight important in the use of the part? Are the mechanical properties of magnesium adequate from a structural and a manufacturing stand point?
- Can the design benefit from a lighter and stiffer structure. Is fatigue life and reduced stress trough better damping important design considerations?
- Will the part need to be finished for protective or decorative reasons? Is denting resistance a necessity (Laptops and suit cases)?
- Are there sliding surfaces present during the use of the part so that a non-galling tendency will be useful?
- Will the service environment be such that corrosion is a problem? If so, can magnesium be finished satisfactorily at a reasonable cost?
- Does making the part require extensive or particularly difficult machining? Will ease of machining, or low cost for machining, be important?
- Can the part be made using methods that are especially suited to magnesium?
-
- Taking all factors into account, is the cost of the part, including the costs during service, competitive when produced in magnesium
|
 |
