3-136. The bare alloys classed as high strength, i.e. 2017, 2024, 7075, 7079, etc., are generally the easiest to spot weld
because they have less tendency for sheet separation and they develop more uniform welds strength consistency. A
disadvantage associated with these alloys is that welds develop more cracks and porosity (small cavities). The low
strength alloys (1100, 3003, 5052, 5056, 6061, etc) have opposite tendencies; normally they are free from cracks and
small cavities but are subject to more severe surface indentation and sheet separation with inconsistant welds.
3-137. Corrosion Resistance Spot Welding of unclad alloys 2017, 2024, 7075, 7079 and 7178 should be avoided if td be
used in a corrosion environment. These alloys may be selective attached under corrosive conditions. Spot welds have
adequate corrosion resistance in the above alloys if clad coated; spot welds in the low strength alloys i.e., 1100, 5052,
3003, 5056, and 6061 have satisfactory corrosion resistance.
3-138. WELD FORMATION. Aluminum will expand when heated and will contract when cooled. This fact causes
trouble in the formation of welds and also alignment of parts to which applied. When the metal expands a space larger
than the cooled metal can occupy is created. The above is overcome to a certain degree by applying pressure (high)
with the current. The pressure forces the sheets together preventing the metal from spitting or flashing. This pressure
also reduces the areas as fast as the metal shrinks which counteracts the tendency of crack formation as a result of
3-139. Welder performing the welding operation should be properly trained. The above requires that he is trained to
operate each specific spot welding machine involved in addition to other required spot welding techniques. Electrode
(tip) maintenance is one of the major requirements of the system and controlled by the welder. Electrodes shall be kept
clean and of the proper contour and shape.
3-140. The tip contour of welding electrode will depend upon (1) the thickness of the metal to be welded (2) the type of
welding process being used (3) surface appearance required (4) distribution of weld indentations required (5) the rigidity
of the welding arms (6) accuracy of alignment of top and bottom electrode. Optimum results usually are obtained with a
spherical working face on the electrodes. The spherical surface provides for easier alignment, minimum sheet
separation and easier maintenance of electrode tip for cleanliness/proper contour.
3-141. Electrode tip radius normally used in spot welding aluminum ranges from 2 inches on 0.020 inch thick to a 10
inch radius on 0.091 inch stock. Indentation can be minimized or entirely eliminated on one side by using a flat faced tip
electrode on one side and a spherical faced tip on the other. The flat surface requirement is an important factor where
supersonic aircraft is involved.
3-142. WELD STRENGTH (SPOT). The weld strength will be determined by the degree penetration and the area of the
spot. Where penetration is less than 20% into one or both sheets weld strength will be below minimum requirements. If
the weld penetration exceeds 80% the thickness of either sheet, cracked welds will usually result. Welds where the
penetration is approximately half way (50%) through each sheet are considered the most satisfactory.
3-143. The most practical way to determine the condition, penetration and etc., of a spot weld is to make sample setups
and testing the sample by peeling or pulling the sample apart. Then depending on the condition of the sample, change
the machine setup until optimum results as obtained. The above should be accomplished frequently to determine that
proper conditions are maintained.
3-144. Table 3-16 will be used only as a guide in selecting machine setting, since the values will vary with different
welding machines, materials, tips, surface conditions, etc. Minimum pressures are required to prevent cracks or porosity
in the weld, while excessive pressures deform the sheet. The dwells will be long enough to obtain complete fusion in the
weld area. The best procedure is to set pressure and dwell for the thinnest sheet in the pile-up and then adjust the
current to obtain a good spot of the desired strength. A good spot weld will be free from cracks or porosity to the extent
permitted by Specification MIL-W-6858 will have a penetration of about ½ the sheet thickness, and a diameter of about
three or four times the sheet thickness. A cross section of the spot, sanded smooth, may be given a coarse etch in hot
caustic solution to determine the shape and size of the spot. Spot welds in alloys 1100 and 3003 are acceptable when
meeting the above conditions. For alloys, 2017, 2024 and 5052, single spot shear specimen welds will be made in
addition to determine the spot strength at that machine setting. This strength will be above the minimum value given in