Welding Aluminum

Article Reprinted from HIWT.

Reprinted with permission of
Hobart Institute of Welding Technology.
The 6th edition of Modern Welding Technology is now available from
http://www.welding.org.

WELDING ALUMINUM

By Howard B. Cary

Condensed from Modern Welding Technology,
4th edition, 1998,
published by Prentice-Hall, Inc.

Aluminum possesses a number of properties that make welding different than welding steels. These are:

1. Aluminum oxide surface coating
2. High thermal conductivity
3. High thermal expansion coefficient
4. Low melting temperature
5. The absence of color change as temperature approaches the melting point

Aluminum is an active metal and it reacts with oxygen in the air to produce a thin hard film of aluminum oxide on the surface. The melting point of aluminum oxide is approximately 3600 degrees F (1926 degrees C), which is almost three times the melting point of pure aluminum, 1220 degrees F (660 degrees C).

This aluminum oxide film, particularly as it becomes thicker, will absorb moisture from the air. Moisture is a source of hydrogen which is the cause of porosity in aluminum welds. Hydrogen may also come from oil, paint, and dirt in the weld area. It also comes from the oxide and foreign materials on the electrode or filler wire, as well as from the base metal.

Hydrogen will enter the weld pool and is soluble in molten aluminum. As the aluminum solidifies, it will retain much less hydrogen and the hydrogen is rejected during solidification. With a rapid cooling rate, free hydrogen is trapped in the weld and will cause porosity.

The aluminum oxide film must be removed prior to welding. If it is not all removed, small particles of unmelted oxide will be entrapped in the weld and will cause a reduction in ductility, lack of fusion, and may cause weld cracking. Anodized coatings must be removed before welding.

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The aluminum oxide can be removed by mechanical or chemical or electrical means.

Mechanical removal involves scraping with a sharp tool, sandpaper, wire (stainless steel) brush, filing, or any other mechanical method.

Chemical removal can be done in two ways. One is by use of cleaning solutions, either the etching types or the non-etching types. The nonetching types should be used only when starting with relatively clean parts. They are used in conjunction with other solvent cleaners.

For better cleaning, the etching type solutions are recommended but must be used with care. When dipping is employed, hot and cold rinsing is recommended. The etching type solutions are alkaline solutions. The time in the solution must be controlled so that too much etching does not occur.

Chemical cleaning includes the use of welding fluxes. Fluxes are used for gas welding, brazing and soldering. The coating on covered aluminum electrodes also contains fluxes for cleaning the base metal. Whenever etch cleaning or flux cleaning is used, the flux and alkaline etching materials must be completely removed from the weld area to avoid future corrosion.

The electrical oxide removal system uses cathodic bombardment. Cathodic bombardment occurs during the half cycle of alternating current gas tungsten arc welding when the electrode is positive (reverse polarity).

This is an electrical phenomenon that actually blasts away the oxide coating to produce a clean surface. This is one of the reasons why alternating current gas tungsten arc welding is so popular for welding aluminum.

The oxide film will immediately start to reform. The time of buildup is not extremely fast, but welds should be made within at least 8 hours after aluminum is cleaned for good quality welding.

Aluminum conducts heat from three to five times as fast as steel depending on the specific alloy. This means that more heat must be put into the aluminum even though the melting temperature of aluminum is less than half that of steel. Because of the high thermal conductivity, preheat is often used for welding thicker sections.

If the temperature is too high or the period of time is too long, it can be detrimental to weld joint strength in both heat-treated and work-hardened alloys. The preheat for aluminum should not exceed 400 degrees F (204 degrees C) and the parts should not be held at that temperature longer than necessary. Because of the high heat conductivity, procedures should utilize higher-speed welding processes using high heat input.

The high heat conductivity of aluminum can also be helpful since if heat is conducted away from the weld extremely fast the weld will solidify very quickly. This with surface tension helps hold the weld metal in position and makes all-position welding practical.

The thermal expansion of aluminum is twice that of steel. In addition, aluminum welds decrease about 6% in volume when solidifying from the molten state. This change in dimension or attempt to change in dimension may cause distortion and cracking.

Aluminum does not exhibit color as it approaches its melting temperature. Aluminum will show color above the melting point, at which time it will glow a dull red.

When soldering or brazing aluminum with a torch, flux is used and the flux will melt at the temperature of the base metal approaches the temperature required. The flux first dries out and then melts as the base metal reaches the correct working temperature.

When torch welding with oxyacetylene or oxy-hydrogen the surface of the flux will melt first and assume a characteristic wet and shiny appearance. (This aids in knowing when welding temperatures are reached.)

When welding with gas tungsten arc or gas metal arc, color is not too important because the weld is quickly completed before the adjoining area would melt.

When the factors above are taken into consideration it will allow making welded joints in aluminum with little or no more trouble than when welding steels.

With either gas metal arc or gas tungsten arc welding, the selection of filler metal is the same. The base metal composition or alloy must be known. Refer to American Welding Society specifications A5.3 and A5.10 for details. These provide for bare, solid, straightened electrode wires, coiled wires and covered electrodes. It may not be necessary to make the comparison or selection of filler metal to weld the different aluminum alloys since this has been standardized.