3-122. Adequate precaution shall be taken to prevent contamination of the electrode. Cont2min2tion is indicated by the
inability to produce a clean weld without a black deposit. When this begins to occur, correction can be made by grinding
the contaminated part away. A black deposit at the edge of the weld does not necessarily indicate electrode
cont2min2tion, if the weld surface is smooth and clean.
3-123. INERT GAS SHIELDED TUNGSTEN ARC WELDING DIRECT CURRENT (STRAIGHT POLARITY). The use
of (DC) straight polarity current provides a "hotter" arc than that obtained with AC. Straight polarity DC puts the heat in
the work rather than in the electrode. Another advantage over AC is the fact that the direct current straight polarity gives
a more concentrated arc.
3-124. The hotter and more concentrated arc will achieve quicker melting of base metal, moreover, deeper-penetrated
weld beads and requires less plate-edge preparation. Welding is faster, not only because the arc is hotter, but also
because vees and grooves can be el1min2ted or reduced in size so consequently less filler metal is required.
3-125. Until the development of thoriated tungsten, high heat resistant electrodes the DC straight method was not
suitable for welding aluminum. Using the improved thoriated tungsten electrodes, this method produces joints as good
as those obtained with the AC method on flat and vertical butt joints, horizontal and vertical fillet joints and corner joints
in the thicker material. AC is preferred for welding the thinner gauge materials.
3-126. The appearance of the finished DC weld will differ in appearance from the AC weld, i.e., AC welds are bright,
shiny and clean and the DC welds will have a dull film caused by oxide. The oxide film is strictly a surface film and does
not indicate lack of fusion, porosity, or inclusions in the weld. The film can be removed by light wire brushing to obtain a
3-127. The mechanical properties of welds made by the DC method are equal to those obtained by the AC method. The
distortion of the welded material is usually less with DC than with AC. The reason being that total heat input to
accomplish the weld is required and the heat spread in the base metal is less.
3-128. Welding with the DC method is similar to the technique used for gas welding. The filler rod is held in contact with
the joint and the arc is moved in straight steady progression longitudinally over the filler wire at a rate which fuses the
filler metal into the desired shape of stringer heads. The filler wire is not moved, dipped or oscillated.
3-129. Generally using this method, one inch of filler rod should be used for each inch of weld to provide beads that are
uniform. The bead size will be controlled by the size of the filler rod used. Large area welds, fillet and butt type, can be
built up by multiple stringer heads. When welding with the DC (straight polarity) method care shall be taken to strike the
arc within the weld area to prevent undesirable marking of material.
3-130. See Table 3-15 for Typical Metal Thickness Versus Filler Wire Diameter, Electrode, Welding Speed Machine
Setting (current AMPS, ARC Voltage and Inert gas flow.)
3-131. SPOT WELDING. Spot welding is a process where two or more pieces of like metal are joined by the fusion of a
concentrated area between the pieces. The material at the spot area is heated to its melting point which changes the
material from a solid to liquid. The metals then combine while in the liquid state and upon cooling, solidify to form the
3-132. This system of welding is classified as resistance welding. The heat for welding is developed as a result of
imposing resistance to the flow of electric current. The current is supplied by the spot welding machine thru the electrode
and the resistance is imposed by the metal being welded. Essentially this is a method of converting electrical energy to
3-133. Aluminum is a low resistant metal and a high electric current flow thru the metal is required to produce enough
heat to make the spot weld. This factor necessitates the use of high current capacity spot welding machines.
3-134. Precise control of energy (heat) input to weld area is required to obtain a good spot weld. The actual heat applied
is determined by the electrical current input and the resistance of the metal. The spot welding is utilized to control the
current, in addition it controls the application time and pressure.
3-135. Spot welding can be accomplished on all the aluminum alloys. The welding characteristics of the alloys will vary
considerably. Generally the clad alloys are more difficult to spot weld than the bare alloys. The cause being that the low-
resistance, high melting point surface(cladding material)at the contacting interfaces tend to decrease weld strength