What Is Aluminum Gas Welding?- A Complete Guide

Oxyacetylene and oxyhydrogen welding processes are the most commonly used gas welding processes for aluminum and aluminum alloys. Hydrogen can be burned with oxygen, using the same tips as acetylene, but the temperature is lower, and larger tip sizes are needed.

When acetylene and oxygen are brought together in proper proportions and lit, the resulting flame reaches a temperature of 6300 degrees F (3482 degrees C).

This temperature is more than sufficient to melt all commercial metals so completely that the metals being joined physically flow together to form a complete bond with mechanical pressure or hammering.

When making a weld on anything but a very thin material, a piece of a wire rod, or extra metal, is usually added to the molten metal, slightly strengthening the seam.

If the weld is good, or correct, the section where the bond is made between the two separate metals will be as strong as the base metal itself.

Basically, metals which can be welded with an oxy-acetylene flame include iron, steel, cast iron, copper, brass, aluminum, and bronze, as well as many alloys. Oxy-acetylene flames are also used for cutting metal, case hardening, and annealing.

Weld Preparation & Treatments

Cleaning The Base Metal Before Welding

Good cleaning practices are vital when welding aluminum, especially when weld quality to a specific standard is desired. In general, proper metal preparation is most important to be accomplished when the fabrications will meet the requirements of AWS D1.2, or other welding-related manufacturing code, as it relates to weld quality.

Welds of high-quality will be considerably harder to make in aluminum than steel. Aluminum has considerably more potential to create quality concerns, such as lack of fusion, lack of penetration, and porosity issues.

The tough oxide surface film on aluminum may need to be controlled to prevent a lack of fusion. The high thermal conductivity of aluminum can contribute to a lack of penetration.

The high solubility of hydrogen in molten aluminum can cause porosity to be a concern and means moisture and hydrocarbons must be eliminated prior to the welding process.

The oxide film thickness on aluminum should be controlled, while preventing the film from hydrating due to excess moisture.

Metal Storage and Weld Joint Preparation – Do’s and Don’ts

 Storage

• Store all welding wire and base metals in a dry location that does not have a lot of temperature fluctuation.
• Store welding wire in a heated room or cabinet that is dry.
• Store metal in a vertical position to minimize moisture condensation and water contamination absorption in between the layers.
• Allow all filler and base metal materials to come into the welding area 24 hours in advance to warm them to room temperature.
• Keep welding wire coverage all the time.

 Joint Preparation

• Do not use methods that leave a ground or smeared surface. There is no issue with weldability of a circular saw cut, however, band saw cut surfaces leave a smeared surface which can result in lack-of-fusion and should be filed to remove smeared metal prior to welding.
• A coarse disc grinder is better than a wheel grinder, but if grinder use is possible at all, do not use any grinder.
• If there is an option, do not use any lubricants in the joint preparation metal working process.
• Do not use chlorinated solvents in the welding area, as these solvents may form toxic gases in the presence of electric welding arcs.
• Do not use oxyfuel gas cutting, carbon arc cutting or gouging processes, or oxyfuel flames for preheating.
• These apply heat to the metal and damage the heat affected area, while they facilitate the growth and hydration of the oxide film present on the surface.
• Use plasma arc cutting & gouging and laser cutting.
• For 2xxx, 6xxx, and 7xxx series alloys mechanically remove the plasma arc and laser cut edges. The melted edges in specific alloy types will contain solidification cracks and heat affected zone conditions. Remove at least 1/8 inch of metal from the cut edge using a mechanical metal removal process equipped to cut and remove metal chips.
• Complete proper joint preparation and cleaning before assembly. Clean the surfaces of the joint with a solvent using the following methods.
• Use a clean cloth (cheesecloth or paper towel) to solvent clean and dry the metal surface of the welding joint.
• Do not use shop rags to clean welding joints. Do not blow off the joint with compressed air; compressed air is likely to contain moisture and oil contaminants.
• Stainless steel wire brush the joint only after solvent cleaning. If you wire brush before cleaning you are embedding hydrocarbons and other contaminates in the metal surface.
• After etching all the metal with a process, stainless steel wire brush all surfaces of metal to remove the by-product residuals from etching prior to welding.
• Clean tools and wire brushes frequently.

Weld Backing

Temporary backing strips can be made of copper, anodized aluminum, stainless steel, or ceramic. They are used to control penetration and are removed after welding.

The welding process requires care to ensure that the backing material does not melt into the weld puddle.

Permanent backing strips are metallic and are always made from the same alloy as the base metal is welded. For removal of backing strips, refer to AWS D1.2. Temporary backing material may not require a root opening, while permanent backing material typically would require a root opening.

Preheating And Inter-Pass Temperatures

Preheating can mitigate the thermal effects of section size when welding base metals of unequal thickness. Heat-treatable base metals and 5xxx base metals with over 3% Mg should not be preheated or postheated to inter-pass temperatures exceeding 250° F (121° C) for more than 15 minutes at a time. See AWS D1.2.

Post-Weld Heat Treatment and Age

Welding heat-treatable aluminum alloys results in a substantial loss in mechanical properties in the heat-affected zone (HAZ).

This is particularly true if the heat-treatable aluminum alloy is in the -T4 temper. However, with post-weld aging, much of the strength associated with the -T4 temper can be regained after welding.

If the base metal is welded in the -T6 temper, it will be possible to solution heat-treat and age the weld after welding. This reverts the alloy back to the -T6 temper.

However, what may not have been considered is that the filler metal used for welding may behave differently than the base metal when post-weld heat treatment and aging is applied.

If the filler metal does not respond to post-weld heat treatment and aging the same way as the base metal, then the joint may be left with lower mechanical properties than the base metal. This is not a desired outcome based on stress concentrations created in the joint.

Thus, if you choose the post-weld heat treatment and aging option, selection of the filler metal is critical. Consult Hobart and get some good metallurgical recommendations on filler metals that are suitable for your application.

Aluminum Gas Welding Process

Once the material to be welded is prepped, fluxed, and preheated, the flame is passed in small circles back and forth across the start until the flux melts. The filler should be scraped across the surface of the toe at intervals of three to four seconds to allow the filler rod to come out of the flame each time.

The scraping is the signal to start the weld without melting the aluminum. It is important to melt the base metal before applying the filler rod. Forehand welding is generally used for welding aluminum because the flame preheats the weld area, resulting in the best integrity of the weld. For thin aluminum there is little need for torch movement other than advancing.

For material 3/16 in. (4.8 mm) thick and thicker, the torch should have a smooth lateral motion to distribute the weld metal across the weld width. A gentle forward and backward motion helps the flux remove oxide.

Periodically, the welding rod should be dipped in the weld puddle and then pulled out of the puddle with a forward motion. This stripping process closes the puddle, prevents porosity, and helps the flux remove the oxide film.

WELDING RULES

Note: The procedure is oxyacetylene, for it is more technical and will change based on the availability of hydrogen.

The rules are simple, either follow them or fail.

• Open the oxygen bottle fully, then just crack the acetylene. Oxy-acetylene regulators should be set to equal pressures, from 2 to 5 pounds each- choose lower pressure for smaller tips.
• Choose a torch tip that is one size larger than would be used with steel. For example, if using a 00 (double ought) tip for a .040 steel sheet, this would be a good tip for a .040 aluminum sheet.
• If it is oily, then clean the material with solvent or lacquer thinner, or alcohol first. Immediately prior to welding, scrub both sides with a stainless brush.
• The flux can either be used on the rod (or wire) or part, or, in extreme cases, both. The flux will be a white powder which would be mixed 1/3 to either 2/3 water or alcohol.
• Safety considerations (eye protection, ample ventilation, and keeping one’s head out of the fumes) are recommended.
• Choose the correct filler metal for actual alloy to be welded to, based on common weldable aircraft alloy sheet metals.

A hollow, flux-infused rod appeared a while back. In addition to the dubious alloy, it had the annoying tendency to rightfully divide, applying, without soldering, the solder build-up to the edge of the joint.

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