Welding Aluminum Alloys

The unique combination of light weight and relatively high strength makes aluminum the second most popular metal that is welded. Aluminum is not difficult to join but aluminum welding is different from welding steels.
Aluminum possesses a number of properties that make welding it different than the welding of steels. These are:

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

The normal metallurgical factors that apply to other metals apply to aluminum as well.
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 1926oC, which is almost three times the melting point of pure aluminum, 660oC. In addition, 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 retained within the weld and will cause porosity. Porosity will decrease weld strength and ductility depending on the amount.

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

Other reasons that aluminum welding is different are due to its high thermal conductivity and low melting temperature. 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 204oC, 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. Both the gas tungsten arc and the gas metal arc processes supply this requirement.

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 with gas tungsten arc and gas metal arc 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.

The final reason why aluminum is different to weld from steels is that it does not exhibit color as it approaches its melting temperature.