Aluminum Welding Gas welding of certain aluminum alloys can be accomplished  successfully, but it requires some practice and the appropriate  equipment to produce a successful weld. Before attempting  to weld aluminum for the first time, become familiar with  how the metal reacts under the welding flame. A good example for practice and to see how aluminum  reacts to a welding flame, heat a piece of aluminum sheet  on a welding bench. Hold a torch with a neutral flame  perpendicular to the sheet and bring the tip of the inner  cone almost in contact with the metal. Observe that the  metal suddenly melts away, almost without any indication,  and leaves a hole in the metal. Now repeat the operation,  only this time hold the torch at an angle of about 30° to the  surface. This allows for better control of the heat and allows  the surface metal to melt without forming a hole. Practice  by slowly moving the flame along the surface until the  puddle can be controlled without melting holes. Once that is  mastered, practice on flanged joints by tacking and welding  without filler rod. Then, try welding a butt joint using flux  and filler rod. Practice and experience provides the visual  indication of the melting aluminum so that a satisfactory  weld can be performed.  Aluminum gas welding is usually confined to material  between 0.031-inch and 0.125-inch in thickness. The  weldable aluminum alloys used in aircraft construction are  1100, 3003, 4043, and 5052. Alloy numbers 6053, 6061, and  6151 can also be welded, but since these alloys are in the  heat-treated condition, welding should not be done unless  the parts can be reheat treated. Proper preparation prior to welding any metal is essential to  produce a satisfactory weld. This preparation is especially  critical during oxy-acetylene welding of aluminum. Select  the proper torch tip for the thickness of metal being welded.  Tip selection for aluminum is always one size larger than  one would normally choose for the same thickness in a steel  sheet. A rule of thumb:  3 ⁄ 4 metal thickness = tip orifice.  Set the proper regulator pressure using the following method  for oxy-acetylene welding of aluminum. This method has  been used by all aircraft factories since World War II. Start  by slowly opening the valve on the oxygen cylinder all the  way until it stops to seat the upper packing. Now, barely  crack open the acetylene cylinder valve until the needle on  the gauge jumps up, then open one-quarter turn more. Check  the regulators to ensure the adjusting screws are turned  counterclockwise all the way out and loose. Now, open both  torch valves wide open, about two full turns (varies with the  torch model). Turn the acetylene regulator by adjusting the  screw until the torch blows a light puff at a two-inch distance.  Begin by tacking the pieces. The tacks should be applied  1–1 1 ⁄ 2-inches apart. Tacks are done hot and fast by melting  the edges of the metal together, if they are touching, or by  adding filler to the melting edges when there is a gap. Tacking  requires a hotter flame than welding. So, if the thickness of  the metal being welded is known, set the length of the inner  cone of the flame roughly three to four metal thicknesses in  length for tacking. (Example: .063 aluminum sheet =  3 ⁄ 16– 1 ⁄ 4  inch inner cone.) Make the best possible fit-up for joints to avoid large gaps  and select the appropriate filler metal that is compatible with  the base metal. The filler should not be a larger diameter than  the pieces to be welded.  [Figure 5-26] The material to be welded must be free of oil or grease. It  should be cleaned with a solvent; the best being denatured  isopropyl (rubbing) alcohol. A stainless toothbrush should  be used to scrub off the invisible aluminum oxide film just  prior to welding but after cleaning with alcohol. Always  clean the filler rod or filler wire prior to use with alcohol  and a clean cloth. Apply flux either to the material, the filler, or both if needed.  The aluminum welding flux is a white powder mixed one  part powder to two parts clean spring or mineral water. (Do  not use distilled water.) Mix a paste that can be brushed on  the metal. Heating the filler or the part with the torch before  applying the flux helps the flux dry quickly and not pop off  when the torch heat approaches. Proper safety precautions,  such as eye protection, adequate ventilation, and avoiding  the fumes are recommended. Special safety eyewear must also be used to protect the  welder and provide a clear view through the yellow-orange  flare given off by the incandescing flux. Special purpose  green-glass lens have been designed and patented especially  for aluminum oxy-fuel welding by TM Technologies. These  lenses cut the sodium orange flare completely and provide  the necessary protection from ultraviolet, infrared, blue light,  and impact. They meet safety standard ANSI Z87-1989 for  a special-purpose lens. Now, hold the torch away from the body and light it with the  striker, adjusting the flame to a bright yellow bushy flame  with the regulator screw. Add oxygen by slowly turning in  the oxygen regulator screw to get a loud blue flame with a  bright inner cone, perhaps a bit of the “fuel-rich” feather or  carburizing secondary cone. By alternately turning in each of  the torch valves a little bit, the flame setting can be lowered  to what is needed to either tack or weld.  5-18