4-63. Gas Weld Repair Of Castings. The procedures of paragraph 4-60 generally apply to this work. Parts so welded
should be stress relieved per Table 4-18 or heat treated according to Table 4-20. All excess flux must be removed as
soon as the part is cool enough to handle.
4-64. ARC WELDING. The advantages of arc welding over gas welding of magnesium are many. Included are: high
strength efficiency, porosity free welds, elimination of flux requirement, no restrictions on type joints possible, detrimental
heat side effects virtually eliminated, any thickness of commonly used alloys can be welded, rapid aid accurate welds
and less after weld clean up and finishing required. Inert gas, concentrated at the point of fusion, shields the metal from
the atmosphere, preventing the rapid oxidation which would be present otherwise. Methods utilizing AC or DC current
are used and either tungsten electrodes or consumable electrodes are employed. The method used depends upon the
alloy, design or part, thickness of alloy and equipment available. Joining of dissimilar magnesium alloys can be done
with the resultant joint retaining the approximate strength of the alloy with the lower weld strength. Use of welding rod
with higher alloy content than that of the material being joined, improves the weld strength. Peening of the weld at 300° -
400F heated condition also improves the weld strength. Overpeening must be guarded against to prevent weld
cracking. Stress relieving of weldments is necessary in some alloys listed in Table 4-18 and discussed in paragraph 4-
95. Table 4-25 lists some common faults in arc welding and the causes.
4-65. Equipment. Welding machines are of the stable arc DC continuous amperage control type, the AC type; having a
high frequency current superimposed on the normal current and a constant potential (reverse polarity of electrode
positive, work negative) DC type. Material under 3/16" thick may be welded with any machine; thicker material requires
an AC machine which has better penetrating power. Machines should be of 100-200 ampere capacity. Direct current
machines should have continuous amperage control in low ranges. They can be utilized in place of AC types when
smooth welds are desired or when a weld is made on material without back up support.
4-66. Electrodes begin at 3/32" diameter for 0.030" thickness material and range to ½ " for thicker stock and are of
tungsten for the non-consumable electrode welding with the stable arc DC and AC machines. The DC constant potential
type welder uses consumable electrode wire of 1/16" to 3/32" diameter.
4-67. Welding rods serve as filler metal, coming in 36" long pieces for manual welding and in wire reels for automatic
machines. Refer to paragraph 4-55 for rod information.
4-68. The inert gas used is either argon or helium and must be of high purity. Normally one-third the volume of argon is
required for any given amount of welding, compared to helium. Helium is better for use with DC reverse polarity welding,
argon for AC. The gas control should be such that it starts to flow a fraction of a second before striking an arc and
continues to flow several seconds, after stopping the arc.
4-69. Operators should be equipped with the necessary goggles, helmets, and fire resistant clothing. The gases used
are considered non-toxic and no special precautions are necessary for their use.
4-70. Suitable fixtures and jigs should be used to properly support and securely hold the part being welded to prevent
distortion and warping.
4-71. Arc Welding Procedures for Joining Sheet, Plate and Extrusions. As in gas welding, the first important procedural
step in successful arc welding is surface preparation. Cleaning of the surface is the same as for gas welding. Joining
edges should be clean and smooth. Sheared edge of sheet stock above 0.080" thickness should be double sheared,
thinner stock than this may be single sheared. Preparation of the abutting edges or surfaces of the alloy material for
typical joints are as shown in Figure 4-2. Clean welding rods should be used. If cleaning is necessary, straight rods may
be cleaned with steel wool or in a chromic acid sodium nitrate dip; coiled rod by using the: dip. All surface and rod
cleaning should be done just before welding.
4-72. In addition to cleaning and proper surface and edge conditioning, the material to be welded should be properly
supported in rigid jigs or fixtures to prevent any movement in any way during the actual welding. In magnesium with its
high thermal expansion and conductivity properties, parts being welded will move if not securely restrained. One
particular fixture of primary importance in welding flat material seams is the backing plate. The plate may be of
magnesium, steel or aluminum. A groove machined in the plate to the approximate size of the bead to be made serves
to prevent concavity of the weld when it cools. The material being welded should be securely clamped to the backing
plate with the proposed seam line centered over the groove. A seam gap no greeted than 1/16" is generally used.
Clamps or hold down devices should be placed as close to the weld area as possible to prevent metal distortion and bead
lift-up. All fixtures used must be clean and rust free, especially in the weld areas to prevent weld porosity. If materials of
different thicknesses are being welded and the thickness difference is ¼ " or more, heat the heavier section with a gas
torch to 350F. A typical backing plate is shown in Figure 4-3.