How to Prepare Aluminum Alloys for Welding

Pre-weld cleaning is essential to the quality of aluminum and aluminum-alloy welds. Aluminum oxidizes readily and develops a dense, hard oxide film that absorbs moisture. This film hinders proper fusion and is a source of porosity and inclusions. Oil, rust, scale, and other contamination can also produce pores. The wire and joint must therefore be cleaned thoroughly to remove oxide and oil. The three principal methods are degreasing, chemical cleaning, and mechanical cleaning.

1. Degreasing

Wipe oil from the filler wire and weld area with gasoline, alcohol, acetone, ethyl acetate, carbon tetrachloride, or another suitable solvent. Clean the groove and a band extending 30–50 mm on each side.

2. Chemical Cleaning

Acidic and alkaline solutions react with oil, dirt, rust, scale, and oxide to form soluble products and expose a metallic surface on the workpiece and wire.

For full chemical cleaning:

1. Etch the workpiece and wire for 10–15 min in an 8%–10% NaOH solution at 40–60°C.
2. Rinse in cold water for about 2 min.
3. Neutralize in a 30% by volume dilute nitric-acid solution. No yellow or black spots may remain.
4. Rinse for 2–3 min in water at 50–60°C and scrub with a stiff-bristle brush.
5. Dry for about 30 min in an oven at 100–150°C.

If the workpiece is too large for full immersion, remove the film locally:

1. Heat the groove and a 30–50 mm band on each side to 80–100°C with an oxyacetylene flame.
2. Wipe these areas with 10%–15% NaOH solution. When they begin to turn white, neutralize them with 30% dilute nitric-acid solution.
3. Rinse thoroughly in cold water.
4. Air-dry the workpiece.

Neither temperature nor solution concentration should be excessive. An overly vigorous reaction forms a white film that impairs welding. Residual solution must be rinsed away to prevent localized pitting and reduced service life. Improvements in filler-wire manufacturing mean that chemical cleaning of modern wire is generally no longer necessary.

3. Mechanical Cleaning

Mechanical methods remove oxide films, corrosion products, and mill scale. They are commonly used for large workpieces, long production cycles, multilayer welds, or renewed contamination after chemical cleaning.

First degrease with gasoline, alcohol, acetone, or another organic solvent. Then use a stainless-steel wire brush or scraper to remove oxide within 50 mm of both sides of the groove until metallic luster appears. Do not use abrasive paper or a grinding wheel: soft aluminum can trap abrasive particles that later create weld defects. After mechanical cleaning, wipe the end face and both sides of the weld area with acetone or alcohol to remove residual oxide, oil, and other contamination.

Mechanical cleaning is simple but generally less effective than chemical cleaning and has difficulty removing oxide from filler wire. Using both methods gives the best result, although noncritical workpieces may sometimes be welded after degreasing alone.

Welding should begin promptly because a new oxide film forms during storage. The interval after cleaning must not exceed 4 h; otherwise the parts must be cleaned again. This requirement is especially important in high relative humidity. Electrochemically polished aluminum wire can remain usable for a relatively long period in air and for nearly six months when sealed in plastic.

4. Backing Bars

Aluminum has little high-temperature strength—only 10 MPa at 370°C—so a weld can collapse or burn through. A graphite, stainless-steel, or copper backing bar is often used to support molten metal while permitting full penetration. A copper backing bar must not contaminate the weld: if the arc reaches it, an aluminum-copper alloy can form, creating a brittle, corrosion-sensitive zone. Machine an arcuate groove directly beneath the seam to give the weld root a sound profile.

The backing should also chill the root, reducing the heat-affected zone and improving joint properties. A water-cooled copper bar provides excellent chilling, whereas stainless steel provides the poorest chilling effect.

5. Welding Fixtures

Suitable fixtures help produce repeatable, high-quality welds quickly. They reduce tack-welding requirements, improve fit-up, and simplify welding. The design should apply clamping force uniformly, provide good torch access, permit easy removal, and remain as simple and practical as possible.

6. Preheating

Non-heat-treatable aluminum alloys generally do not require preheating, whereas heat-treatable alloys can benefit from it. Preheating removes surface moisture, reduces temperature gradients through slow heating and cooling, lowers the tendencies toward distortion, lack of penetration, porosity, and cracking, increases travel speed, and reduces the time molten aluminum remains at high temperature, thereby limiting alloying-element loss.

Parts thicker than 8 mm normally require preheating. Use 100–300°C, because excessive temperature reduces weld strength and corrosion resistance. Heating may be performed with an oxyacetylene flame or in a furnace and should cover about 150 mm on each side of the seam. For joints between unequal thicknesses, do not heat the thinner side.

Environmental Requirements

Aluminum welding is sensitive to ambient temperature and humidity. The recommended environment is at least 8°C with relative humidity no higher than 80%.