What Is Aluminum Welding?

Effects of Alloying Elements and Crack Sensitivity

Crack Sensitivity in Aluminum Alloys

The behavior of the various alloying elements used in aluminum alloys during the cooling and solidification processes is the most important factor influencing the formation of solidification cracks. Elements that undergo a slower liquid-to-solid phase transition than aluminum combine with stress during solidification, leading to cracks. Critical regions in the weld metal have been identified, particularly for parts requiring precision welding. Therefore, the alloy ratios must be balanced with the weld bead geometry and different welding wires.

Another method to avoid solidification cracks is to reduce the stress on the weld. Solidification cracks are most commonly observed in the craters at the end of the weld. In aluminum welding, if the crater width is equal to or less than the weld width, the weld end cannot withstand the stress generated in the weld and cracks. Creating wider craters at the weld end will reduce the risk of cracking.

If a sufficiently wide weld metal is formed, the alloys from the welding filler wire will alter the chemical composition at the weld bead, thereby reducing crack sensitivity. Additionally, the expanded weld metal can carry a greater load, which means achieving a geometry capable of tolerating the stress generated during welding.

Use of Backing, Preheating, and Heat Treatment

Although the use of backing materials in the welding of aluminum alloys is generally challenging, it is technically permissible (see AWS D 1.2). Copper, anodized aluminum, stainless steel, or ceramic materials may be preferred as welding backing materials. When using backing, aluminum—which is particularly prone to forming bonds with backing materials—must be welded with care. For precision applications, it is recommended to use anodized aluminum backing made of the same material as the base metal. A root gap is generally not preferred when using backing materials.

Preheating, Interpass Temperature, and Aging Treatments for Aluminum Alloys

Preheating and Interpass Temperature
In aluminum alloys, the preheating process can be used to reduce thermal effects that may occur during the welding of parts of varying thicknesses (such as penetration issues or warping). Preheating is not recommended for precipitation-hardened or thermomechanically hardened materials, or for alloys containing more than 3% magnesium, as this can lead to a decrease in mechanical strength and the oxidation and burning of magnesium. Additionally, it is recommended that the interpass temperature for these alloys not exceed 120°C and that this temperature not be maintained for more than 15 minutes (according to AWS D 1.2).

Ageing Process (Precipitation Hardening)
Precipitation hardening, or aging treatment, in aluminum alloys is a metallurgical process aimed at increasing mechanical strength. In this process, elements such as copper—which form intermetallic phases with aluminum—are dissolved into the aluminum by heating. After cooling, the intermetallic phases formed at grain boundaries or within grains grow, blocking dislocation paths, which in turn increases the alloy’s strength. While the aging process can occur naturally over a long period, it can also be accelerated through a simple heat treatment to achieve artificial aging.

Any heat input applied during welding significantly reduces the mechanical strength in the heat-affected zone (HAZ) of these alloys. Welding precipitation-hardened alloys is generally quite difficult. For this reason, post-weld heat treatment must be performed, and the welding wire must be selected appropriately for the alloy. For welding aluminum alloys with enhanced mechanical strength through precipitation hardening, ACT welding can be utilized.

Argon and helium gases are primarily used in MIG and TIG welding of aluminum alloys. These gases can be used in their pure form or as mixtures in specific ratios. Pure argon gas is preferred more often due to its lower cost. Because it is 100% heavier, it provides better protection compared to pure helium. Welds made with pure argon gas produce a characteristic weld cross-section known as the “argon finger.”

On the other hand, pure helium gas increases arc ionization, resulting in more intense heat during welding. This intense heat results in a cleaner weld pool and a wider weld bead. Additionally, since the density of pure helium gas and shielding gases containing specific proportions of helium is lower than that of air, gas consumption is higher compared to argon gas.

Standard tungsten electrodes used in TIG welding of aluminum alloys vary depending on the alloying elements they contain. The main alloying elements include thorium oxide, cerium oxide, zirconium oxide, and lanthanum oxide.

Pure Tungsten Electrodes: This is the least expensive type of electrode for welding aluminum alloys. However, their low current-carrying capacity is a significant disadvantage.

Thorium Oxide-Alloyed Electrodes: Although they have a high current-carrying capacity, they are not recommended for aluminum welding. Their popularity is also declining today due to their radioactivity.

Zirconium Oxide-Alloyed Electrodes: These are used for welding aluminum and magnesium alloys. Containing 98.6% tungsten and 0.7–0.9% zirconium, these electrodes easily form a round tip in AC applications. Compared to other types of electrodes, zirconium-alloyed tungsten electrodes have the highest current-carrying capacity without breaking.

Lanthanum Oxide-Alloyed Electrodes: Tungsten electrodes containing 1.5% and 2% lanthanum provide excellent arc initiation at low current levels. They offer advantages such as long service life, stability, and reduced tip wear. Tungsten electrodes with 2% lanthanum alloy are used in critical sectors such as the aerospace industry and may also be preferred as an alternative to tungsten electrodes with 2% thorium alloy.