Working Magnesium Warning: Keep magnesium particles away from sources of  ignition. Small particles of magnesium burn very easily.  In sufficient concentration, these small particles can cause  an explosion. If water touches molten magnesium, a steam  explosion could occur. Extinguish magnesium fires with dry  talc, calcium carbonate, sand, or graphite. Apply the powder  on the burning metal to a depth of  1 ⁄ 2-inch or more. Do not  use foam, water, carbon tetrachloride, or carbon dioxide.  Magnesium alloys must not touch methyl alcohol. Magnesium is the world’s lightest structural metal. Like  many other metals, this silvery-white element is not used in  its pure state for stressed application. Instead, magnesium  is alloyed with certain other metals (aluminum, zinc,  zirconium, manganese, thorium, and rare earth metals) to  obtain the strong, lightweight alloys needed for structural  uses. When alloyed with these other metals, magnesium,  yields alloys with excellent properties and high strength- to-weight ratios. Proper combination of these alloying  constituents provide alloys suitable for sand, permanent  mold and die castings, forging, extrusions, rolled sheet, and  plate with good properties at room temperature, as well as at  elevated temperatures.  Lightweight is the best known characteristic of magnesium,  an important factor in aircraft design. In comparison,  aluminum weighs one and one half times more, iron and steel  weigh four times more, and copper and nickel alloys weigh  five times more. Magnesium alloys can be cut, drilled, and  reamed with the same tools that are used on steel or brass,  but the cutting edges of the tool must be sharp. Type B rivets  (5056-F aluminum alloy) are used when riveting magnesium  alloy parts. Magnesium parts are often repaired with clad  2024-T3 aluminum alloy.  While magnesium alloys can usually be fabricated by methods  similar to those used on other metals, remember that many  of the details of shop practice cannot be applied. Magnesium  alloys are difficult to fabricate at room temperature; therefore,  most operations must be performed at high temperatures. This  requires preheating of the metal or dies, or both. Magnesium  alloy sheets may be cut by blade shears, blanking dies,  routers, or saws. Hand or circular saws are usually used  for cutting extrusions to length. Conventional shears and  nibblers should never be used for cutting magnesium alloy  sheet because they produce a rough, cracked edge.  Shearing and blanking of magnesium alloys require close  tool tolerances. A maximum clearance of 3 to 5 percent of  the sheet thickness is recommended. The top blade of the  shears should be ground with an included angle of 45° to 60º.  The shear angle on a punch should be from 2° to 3°, with a  1° clearance angle on the die. For blanking, the shear angle  on the die should be from 2° to 3° with a 1° clearance angle  on the punch. Hold-down pressures should be used when  possible. Cold shearing should not be accomplished on a  hard-rolled sheet thicker than 0.064-inch or annealed sheet  thicker than  1 ⁄ 8-inch. Shaving is used to smooth the rough,  flaky edges of a magnesium sheet that has been sheared.  This operation consists of removing approximately  1 ⁄ 32-inch  by a second shearing.  Hot shearing is sometimes used to obtain an improved  sheared edge. This is necessary for heavy sheet and plate  stock. Annealed sheet may be heated to 600 °F, but hard- rolled sheet must be held under 400 °F, depending on the  alloy used. Thermal expansion makes it necessary to allow for  shrinkage after cooling, which entails adding a small amount  of material to the cold metal dimensions before fabrication. Sawing is the only method used in cutting plate stock more  than  1 ⁄ 2-inch thick. Bandsaw raker-set blades of 4- to 6-tooth  pitch are recommended for cutting plate stock or heavy  extrusions. Small and medium extrusions are more easily  cut on a circular cutoff saw having six teeth per inch. Sheet  stock can be cut on handsaws having raker-set or straight-set  teeth with an 8-tooth pitch. Bandsaws should be equipped  with nonsparking blade guides to eliminate the danger of  sparks igniting the magnesium alloy filings. Cold working most magnesium alloys at room temperature  is very limited, because they work harden rapidly and do not  lend themselves to any severe cold forming. Some simple  bending operations may be performed on sheet material, but  the radius of bend must be at least 7 times the thickness of  the sheet for soft material and 12 times the thickness of the  sheet for hard material. A radius of 2 or 3 times the thickness  of the sheet can be used if the material is heated for the  forming operation. Since wrought magnesium alloys tend to crack after they  are cold-worked, the best results are obtained if the metal is  heated to 450 °F before any forming operations are attempted.  Parts formed at the lower temperature range are stronger  because the higher temperature range has an annealing effect  on the metal. The disadvantages of hot working magnesium are: 1.  Heating the dies and the material is expensive  and troublesome.  2.  There are problems in lubricating and handling  materials at these temperatures. 4-84