In welding aluminum more than 0.188 of an inch thick, bevel the edges and notch them, as shown in view C of figure 5-10. The included angle of bevel maybe from 90 to 120 degrees.

After you have prepared the edges of the pieces properly, you should then clean the surfaces to be welded. If heavy oxide is present on the metal surface, you may have to use a stainless-steel wire brush to remove it. Dirt, grease, or oil can be removed by wiping the weld area with a solvent-soaked rag.

Aluminum plate 1/4 of an inch thick or greater should be preheated to a temperature ranging between 500°F to 700°F. This aids in avoiding heat stresses. Preheating also reduces fuel and oxygen requirements for the actual welding. It is important that the preheating temperature does exceed 700°F. If the temperature does go above 700°F, the alloy maybe severely weakened. High temperatures also could cause large aluminum parts to collapse under their own weight. Thin material should be warmed with the torch before welding. This slight preheat helps to prevent cracks.

WELDING TECHNIQUES.-After preparing and fluxing the pieces for welding, you should pass the flame, in small circles, over the starting point until the flux melts. Keep the inner cone of the flame off the flux to avoid burning it. If the inner cone of the flame should burn the flux, it will be necessary to clean the joint and apply new flux. Next, scrape the rod over the surface at about 3- or 4-second intervals, permitting the rod to come clear of the flame each time. If you leave the rod in the flame too long, it melts before the parent metal does. The scraping action indicates when you can start welding without overheating the metal. Maintain this cycle throughout the course of welding except for allowing the rod to remain under the flame long enough to melt the amount of metal needed. With practice, the movement of the rod can be easily mastered.

Forehand welding is usually preferred for welding aluminum alloys because the flame points away from the completed weld, and this preheats the edges to be welded that prevents too rapid melting. Hold the torch at a low angle when you are welding thin material. For thicknesses 0.188 of an inch and above, you should increase the angle of the torch to a near vertical position. Changing the angle of the torch according to the thickness of the metal minimizes the possibility of burning through the sheet during welding.

When welding aluminum alloys up to 0.188 of an inch thick, you have little need to impart any motion to the torch other than moving it forward. On flanged material, care must be taken to break the oxide film as the flange melts down. This may be done by stirring the melted flange with a puddling rod. A puddling rod is essentially a paddle flattened and shaped from a 1/4- inch stainless steel welding rod.

With aluminum alloys above 0.188 of an inch in thickness, you should give the torch a more uniform lateral motion to distribute the weld metal over the entire width of the weld. A slight back-and-forth motion assists the flux in its removal of oxides. Dip the filler rod in the weld puddle with a forward motion.

The angle of the torch is directly related to the welding speed. Instead of lifting the flame from time to time to avoid melting holes in the metal, you will find it advantageous to hold the torch at a flatter angle to the work The welding speed should be increased as the edge of the sheet is approached. The inner cone of the flame should never be permitted to come in contact with the molten metal, but should beheld about 1/8 of an inch away from the metal.

In the vertical position, the torch is given an up-and- down motion, rather than a rotating one. In the overhead position, alight back-and-forth motion is used the same as in flat welding.

Heat-treatable alloys should be held in a jig for welding, whenever possible. This helps to eliminate the possibility of cracking. The likelihood of cracking can also be reduced by the use of a 4043 filler rod. This rod has a lower melting range than the alloy being joined which permits the base metal to solidify before the weld puddle freezes. As the weld is the last area to solidify, all of the contraction strains are in the weld bead, rather than throughout the base metal. You may reduce weld cracking by tack welding the parts while they are in the jig and then loosening the clamps before completing the seam.

As soon as the weld is completed and the work has had time to cool, you should thoroughly wash the weld. This can be done by vigorously scrubbing it with a stiff brush while hot water runs over it until all traces of the flux are removed. This is important, because if any flux is left on the weld, it can corrode the metal. If hot water is not available, you may use a diluted solution of 10 percent sulfuric acid. The acid solution should then be washed off with cold, fresh water after using.