Magnesium alloy
For extrusions, a wide range of shapes, bars, and tubes are made from M1 alloy where low strength suffices or where welding to M1 castings is planned.
Magnesium alloys are referred to by short codes (defined in ASTM B275) which denote approximate chemical compositions by weight.
The numerals correspond to the rounded-off percentage of the two main alloy elements, proceeding alphabetically as compositions become standard.
As these temperatures are easily attained and generally do not require a protective atmosphere, many formed and drawn magnesium products are manufactured.
Magnesium alloys can be spot-welded nearly as easily as aluminium, but scratch brushing or chemical cleaning is necessary before the weld is formed.
[2] Thorium-containing alloys are not usually used, since a thorium content of more than 2% requires that a component be handled as a radioactive material, although thoriated magnesium known as Mag-Thor was used in military and aerospace applications in the 1950s.
After forming, wrought magnesium alloys have a stringy texture in the deformation direction, which increases the tensile proof strength.
Magnesium alloys, however, have a lower density, stand greater column loading per unit weight and have a higher specific modulus.
Such applications can induce cyclic crystal twinning and detwinning that lowers yield strength under loading direction change.
[16] Magnesium alloys show strong anisotropy and poor formability at room temperature stemming from their hexagonal close-packed crystal structure, limiting practical processing modes.
[18] The high-temperature properties of magnesium alloys are relevant for automotive and aerospace applications, where slowing creep plays an important role in material lifetime.
[19] Addition of small amounts of zinc in Mg-RE alloys has been shown to increase creep life by 600% by stabilizing precipitates on both basal and prismatic planes through localized bond stiffening.
[citation needed] Immersion in salt water is problematic, but a great improvement in resistance to salt-water corrosion has been achieved, especially for wrought materials, by reducing some impurities particularly nickel and copper to very low proportions[20] or using appropriate coatings.
The oxide forms blackened areas called burns on the surface of the casting, and the liberated hydrogen may cause porosity.
Inhibitors such as sulfur, boric acid, ethylene glycol, or ammonium fluoride are mixed with the damp sand to prevent the reaction.
The rapid solidification caused by contact of the fluid metal with the cold die produces a casting of dense structure with excellent physical properties.
Many standard magnesium alloys are easily welded by gas or resistance-welding equipment, but cannot be cut with an oxygen torch.
A particular attraction of magnesium alloys lies in their extraordinarily good machining properties, in which respect they are superior even to screwing brass.
Actually, it is much more difficult to ignite magnesium chips and dust than is usually supposed, and for that reason they do not present great machining difficulties.
[2] There is, perhaps, no group of alloys where extrusion is more important than it is to these, since the comparatively coarse-grained structure of the cast material makes most of them too susceptible to cracking to work by other means until sufficient deformation has been imparted to refine the grain.
In contrast to the previous practice of using bored billets, mandrel piercing is now used in the extrusion of large diameter tubes in ZW3 alloy.
The stiffness of the alloys towards extrusion is increased in proportion to the amount of hardening elements they contain, and the temperature employed is generally higher the greater the quantity of these.
In coarse material, larger particles of the compounds are present that are less readily dissolved, and tend to cause a solution gradient.
In magnesium alloys, this causes internal stress, since solution is accompanied by a small contraction, and it can also influence the evenness of response to later heat treatment.
Under conditions approaching equilibrium magnesium is capable of dissolving about 12 per cent aluminium, but in cast billets 4-5 wt.% usually represents the limit of solubility.
Continuous casting improves the homogeneity of these alloys and water cooling of the dies or taper heating of the billets further facilities their extrusion.
Increasing zinc content to 5 or 6 wt.%, as in the American alloy ZK60 and ZK61, reduces sensitivity to extrusion speed in respect of mechanical properties.
Dominion Magnesium Limited in Canada have developed a method adding in the conventional manner through a master alloy.
Explanation for the low extrusion rates necessary to successfully extrude some magnesium alloys does not lie outside reasons put forward for other metals.
Mg ranks as the fourth most plentiful cation in the human body, is an essential element for metabolism, and is primarily stored in bone tissue.
