4-76. Stress corrosion occurs in welding some alloys, especially those of the magnesium, aluminum zinc types, from
heat induced residual stresses in the weld area. If not relieved the stresses will nearly always result in cracking of the
weld. To properly eliminate this condition the part should be placed in its welding jig or fixture and heated at the
temperatures reflected in Tables 4-19 and 4-20 for the particular alloy. Cool in still air after relieving. Jigs or fixtures
should be used to prevent possible warpage during relieving.
4-77. WELDING METHODS.
4-78. MANUAL WELDING, ARC TYPE . Table 4-22 lists manual welding data. In both AC and DC welding, gas flow
should start prior to starting the arc. Continue this flow for a few seconds after the arc is stopped to prevent excessive
oxidation of the tungsten electrodes. Start the DC arc by brushing the electrode over the material surface and maintain it
to about 1/8" length. Starting and stopping the arc in AC welding should be done by an operator controlled switch. The
electrode should be held nearly flush or a little below the surface of the work. In both types hold the torch almost
perpendicular to the work and the welding rod nearly parallel to the work. Move the torch and rod along the seam at an
even speed in a straight line, using a fanning or rotary motion only if large corner or fillet welding is being done. Control
the rate of movement to give an even secure bead. Work from one edge of the material to the opposite edge with as few
stops as possible. Whenever a stop is made in a bead, resume the weld by starting back in the bead about ½ " from the
stopping place. Welds in material thicker than 3/8" should be made with multiple beads. The material should be V-
grooved in accordance with the pattern of Figure 4-2. Run a small bead at the base of the groove, the length of the joint.
Wire brush this bead then fill the groove with additional beading. When possible, use a double V-groove (one on each
side) will reduce warping and improve weld soundness. The beads should be flat or concave, not convex. Maintain
medium size beads, which give maximum strength and less finishing work.
4-79. CONSUMABLE ELECTRODE ARC WELDING. This type welding is accomplished using one of three constant
potential power sources:
A motor generator with a rising volt-ampere characteristic.
A motor generator with a flat or slightly rising characteristic.
Rectified direct current with a drooping characteristic. Argon gas provides the shield. High welding speeds
and deposition rates are available with this method resulting in a weld as strong as tungsten arc welds. Table 4-23 lists
service data for this type welding. The use of this process produces three types of metal transfer. Higher density current
results in a forceful spray effect, which is especially effective when welding material over 3/16" thick. The metal from the
rod actually sprays to form a bead. Decreasing the current density and voltage results in the metal dripping from the rod
in the form of small globules. Welding of material less than 1/8" can be effectively done with this type.
4-80. Finishing of material after it is arc welded is a fairly simple process since no flux is used. Usually only a wire
brushing or cleaning as described for preweld surface treatment is required. If arc weld "smoke" is present, the material
can be cleaned in a 180° 212F bath composed of 16 ounces tetrasodium pyrophosphate (Na4P2O7); 12 ounces sodium
metaborate (NaBO2) with enough water to make one gallon of solution. Qualities required may be less or greater but
proportions should not vary. One half to 2 minutes immersion is sufficient. Excess bead may be removed by filing,
grinding, routing or some similar method.
4-81. Inspection of joints. Visual, radiographic, ultrasonic or fluorescent oil penetrant types of inspection are used on
magnesium welds to check for cracks, porosity, and other detrimental defects. A separate section of this technical
manual details inspection and test criteria and procedures. General inspection specifications are:
- MIL-I-6865, MIL-R-11471,
4-82. SPOT WELDING. This type of resistance welding is the most widely used, and provides the means for rapidly
obtaining statically strong seams in properly prepared metal. A minimum of finishing of the seams is required after
welding. Usually some stress relieving and possibly joint sealing or painting is sufficient to complete the job. Spot welds
are not as strong as riveted or adhesive bonded joints. Parts with spot weld type of joints are generally used in lower
stress applications although past use in high vibration free high stress locations have been successful. The quality of the
weld is dependent on properly prepared metal surfaces and closely controlled welding techniques. The exactness of
machine control is mandatory since the conduction properties of magnesium are such, that high current and short dwell
or weld time must be used to effect a good joint without damaging the metal.