Designers often choose aluminum over steel as a building material when weight savings are required. Many aluminum alloys, however, cannot be welded in the same manner as their steel counterparts. From selecting the right type of aluminum alloys to practicing the best welding techniques, here are ways you can increase arc welding aluminum strength.
Hot cracking is at the root of the majority of aluminum welds. This type of cracking occurs at high temperatures when the metal alloys begin to solidify and is a result of the chemical properties of the materials. Hot cracking is referred to by a number of different names including hot fissuring, hot shortness, liquation cracking and solidification cracking. Fortunately, there are ways to avoid this common cracking when welding aluminum alloys.
When using aluminum alloys as a building material, it’s tempting to simply select the strongest alloy for the job. A lot of the strongest aluminum alloys, however, are not suitable for welding because they are extremely prone to hot cracking. Even when adding a filler material to reduce crack sensitivity, some alloys are considered in the non-weldable category. If you need to weld aluminum, make sure you select alloys that are in the non-heat-treatable group, such as 1XXX, 3XXX, 4XXX, and 5XXX.
Non-Heat Treatable Alloys
This group of alloys are not strengthened via heat treatment and the chemical composition of the these alloys differ from one another. The alloys in the 1XXX series are almost pure aluminum and are good if you need material that is resistant to corrosion. The 3XXX series is strengthened by adding manganese (Mn), has moderate strength, and is resistant to corrosion. The alloys in 5XXX contain magnesium (Mg) and are the strongest in the group, while silicon (Si) the 4XXX series, which has the least crack sensitivity.
Many of the alloys in this category are considered non-weldable and include the families 2XXX, 6XXX, and 7XXX. The heat-treatable alloys are generally stronger than their counterparts and are very crack sensitive. To avoid hot cracking, the alloys in 2XXX and 7XXX should be riveted, with the exception of two alloys in the 2XXX family: 2219 and 2519. The 6XXX series, which are commonly used as building materials, are weldable, but only when used with an appropriate filler material.
The crack sensitivity of aluminum alloys depends on the material’s chemistry. If the sensitivity is high, you can add filler materials to help lower the chances of hot cracking. If the alloy has low sensitivity, add a filler that contains a similar chemistry as the parent material. If the sensitivity is high, use a filler with different chemical properties to help lower the crack rating.
When welding aluminum that is below half an inch in thickness, you should use pure argon as a shielding gas. The argon allows optimal cleaning action and is also less costly. For thicker materials, you can add helium to increase heat and allow better penetration into the material. Avoid shielding gas with mixtures of oxygen or carbon dioxide because these chemicals will oxidize the material.
You should use a different electrode when welding with aluminum. Aluminum alloys are welded with AC instead of DC, so the amount of energy required is greater. With that in mind, you should switch out your electrode for a pure tungsten or zirconiated tungsten. It's also recommended to use a larger diameter electrode when working with aluminum. Start out with a 1/8 inch diameter and increase as needed. Blunt the tip of the electrode to help prevent wander.
It's fine to preheat the aluminum, but avoid preheating too much as this can degrade the material. The amount of preheating will vary depending on how high the material was originally heated. If you are working with non-heat treatable alloys, only preheat to dry moisture and avoid temperatures in the 200 degrees Fahrenheit range.