Aluminum Gas Welding Overview
The gas welding processes most commonly used on aluminum and aluminum alloys are oxyacetylene and oxyhydrogen. Hydrogen may be burned with oxygen using the same tips as used with acetylene. However, the temperature is lower, and larger tip sizes are necessary.
Oxyhydrogen welding permits a wider range of gas pressures than acetylene without losing the desired slightly reducing flame. Aluminum from 1/32 to 1 in. (0.8 to 25.4 mm) thick may be gas welded. Heavier material is seldom gas welded, as heat dissipation is so rapid that it is difficult to apply sufficient heat with a torch.
When compared with arc welding, the weld metal freezing rate of gas welding is very slow. The heat input in gas welding is not as concentrated as in other welding processes and unless precautions are taken greater distortion may result. Minimum distortion is obtained with an edge or corner welds.
Sheet or plate edges must be properly prepared to obtain gas welds of maximum strength. They are usually prepared the same as similar thicknesses of steel. However, on material up to 1/16 in. (1.6 mm) thick, the edges can be formed to a 90-degree flange. The flanges prevent excessive warping and buckling. They serve as filler metal during welding.
Welding without filler rods is normally limited to pure aluminum alloys since weld cracking can occur in the higher strength alloys. In gas welding, thickness over 3/16 in. (4.8 mm), the edges should be beveled to secure complete penetration. The included angle of the bevel may be 60 to 120 degrees.
Preheating of the parts is recommended for all castings and plate 1/4 in. (6.4 mm) thick or over. This will avoid severe thermal stresses and ensure good penetration and satisfactory welding speeds. The common practice is to preheat to a temperature of 700ºF (371ºC). Thin material should be warmed with the welding torch prior to welding. Even this slight preheat helps to prevent cracks.
Heat-treated alloys should not be preheated above 800ºF (427ºC) unless they are to be post-weld heat treated. Preheating above 800ºF (427ºC) will cause a “hot-short” and the metal strength will deteriorate rapidly.
Weld travel speed needs to increase as you weld to avoid too much melt-through on thin aluminum.
Preheat Temperature Checking Technique
When pyrolytic equipment (temperature gauges) is not available, the following tests can be made to determine the proper preheat temperatures.
- Char test: Using a pine stick, rub the end of the stick on the metal being preheated. At the proper temperatures, the stick will char. The darker the char, the higher the temperature.
- Carpenter’s chalk: Mark the metal with ordinary blue carpenter’s chalk. The blue line will turn white at the proper preheat temperature.
- Hammer test: Tap the metal lightly with a hand hammer. The metal loses its ring at the proper preheat temperature.
- Carburizing test: Carburize the surface of the metal, sooting the entire surface. As the heat from the torch is applied, the soot disappears. At the point of soot disappearance, the metal surface is slightly above 300ºF (149ºC). Care should be used not to coat the fluxed area with soot. Soot can be absorbed into the weld, causing porosity.
Aluminum welding flux is designed to remove the aluminum oxide film and exclude oxygen from the vicinity of the puddle. The fluxes used in gas welding are usually in powder form and are mixed with water to form a thin paste.
The flux should be applied to the seam by brushing, sprinkling, spraying, or other suitable methods. The welding rod should also be coated. The flux wil1 melt below the welding temperature of the metal and form a protective coating on the surface of the puddle. This coating breaks up the oxides, prevents oxidation, and permits slow cooling of the weld.
Aluminum Gas Welding Process
After the material to be welded has been properly prepared, fluxed, and preheated, the flame is passed in small circles over the starting point until the flux melts. The filler rod should be scraped over the toe surface at three or four-second intervals, permitting the filler rod to come clear of the flame each time.
The scraping action will reveal when welding can be started without overheating the aluminum. The base metal must be melted before the filler rod is applied. Forehand welding is generally considered best for welding on aluminum since the flame will preheat the area to be welded. In welding thin aluminum, there is little need for torch movement other than progressing forward.
On material 3/16 in. (4.8 mm) thick and over, the torch should be given a uniform lateral motion. This will distribute the weld metal over the entire width of the weld. A slight back and forth motion will assist the flux in the removal of oxide.
The filler rod should be dipped into the weld puddle periodically, and withdrawn from the puddle with a forward motion. This method of withdrawal closes the puddle, prevents porosity, and assists the flux in removing the oxide film.
Note: The following procedure is for oxy acetylene, because of the more technical nature and varied availability of hydrogen.
The rules are simple. Follow them or fail!
- Open the oxygen bottle fully to seat the upper packing, then just crack the acetylene. Set regulators to equal pressures, from 2 to 5 pounds each, with smaller tips needing lower pressures.
- Choose a torch tip one size larger than would be used on steel, i.e. If choosing a 00 (double ought) tip for .040 steel sheet, then move up to a 0 tip for .040 aluminum sheet.
- If oily, clean the material with solvent, lacquer thinner. or alcohol. Scrub with stainless brush on both sides just prior to welding.
- Flux either the rod (or wire), or the part — or, in extreme cases, both. The flux will be a white powder which will be mixed 1/3 with either 2/3 water or alcohol.
- Safety precautions such as eye protection, adequate ventilation, and keeping one’s head out of the fumes, are recommended.
- Choose the proper filler metal for the alloy to be welded. Common weldable aircraft alloy sheet metals.
Hollow, flux-filled rod, was made available years ago, but aside from the questionable alloy, it had the persistent bad habit of neatly dividing itself, building up the edges of the joint without joining them together.