3. FILLER METAL OF MAGNESIUM BASED ALLOYS

Table 3.1 - Chemical Composition Requirments for Magnesium Electrodes

3.1 - Filler metal selection

3.1.1 - General Weldng Considerations

3.1.2 - Weldability and filler metal selection

Table 3.2 - All-weld metal properties of various Magnesium alloys

3.2 - Storage and care of filler metal

Table 3.2 - Guide to the choice of filler metal for gas shielded arc welding

Table 3.1 - Chemical Composition Requirments for Magnesium Electrodes ñ Top

3.1 Filler metal selection.ñ Top

3.1.1 General welding considerationsñ Top

Magnesium is well known for its extreme lightness, excellent machineability and weldability and the high strength-to-weight ratio of its alloys. Its specific gravity is 1.74. On an equal value basis, it weighs roughly one-forth as much as steel, and two-thirds as much as aluminum.

When heated in air to its melting point, magnesium tends to oxidize rapidly. For this reason, welding with magnesium requires the use of a protective shield, such as an inert gas or flux.

Because of its comparatively low melting point, latent heat of fusion and specific heat per unit volume, magnesium requires a relatively small amount of heat to melt. The total heat of fusion is approximately two-thirds that for aluminum and one-fifth that for steel. The high coefficients of thermal expansion (from 65 to 750 ºF about 0.000016 per º F ) and conductivity tend to cause considerable distortion during welding. In this respect, the welding of magnesium is more critical than the welding of steel and similar to the welding of aluminum.

3.1.2 Weldability and filler metal selection. ñ Top

The weldability of most magnesim alloys can be considered good to excellent if proper filler metal is employed. The use of a filler metal with a lower melting point and a wider freezing range than the base metal is advantageous as it improves weldability and minimizes weld cracking. A list of recommended filler metal used with the variuos base metal is given in table 3.2.

TABLE 3.2 - All-weld metal properties of various Magnesium alloys ñ Top

3.2 Storage and care of filler metal. ñ Top

Filler metal cleanliness and freedom from surface oxidation are important in gas shielding arc welding. Oil or other organic material as well as heavy oxide coatings on the surface of the filler metal, will interfere with coalescence of the molten weld pool and cause porosity or other weld defects, or both. For this reason, filler metal is manufactured and packaged in order to prevent contamination.

Proper storage of welding rods and electrodes in the user's plant is essential if the filler metal is to remain free of contamination until used. Packages of filler metal should not be left outdoors or in unheated buildings because the greater variation in temperature and humidity increase the possibility of moisture condensation. Properly protected filler metal can be stored for long periods of time without adverse affect on its performance.

Packages of filler metal should remain sealed until ready for use. Once removed from the container, spooled electrodes should be kept covered, even during use, to prevent surface contamination by dust, moisture, or other airborne foreign material. After welding is completed, the electrode should be returned to its original container for storage. Welding rods should also be kept covered until ready for use. Storage conditions here need not be as rigorous as for spooled electrodes because good welding practice always includes a stainless steel wool rub of the rod just prior to use to remove surface oxidation or other contamination.

TABLE 3.2 - Guide to the choice of filler metal for gas shielded arc welding. ñ Top

ΠCast alloys are generally welded with filler metal having the same or similar composition as the base metal in order to achieve maximum strength and proper response to post weld heat treatment schedules. Lacking the availability of suitable rods of such alloys, the commercially available filler metal listed will provide equivalent weldability but with the possibility of some reduction in strength.
� Welding not recommended
Ž No data available for welding this combination.

Note: When more than one filler metal is listed, they are listed in order of preference.