Welding aluminum using the gas metal arc welding process
Welding aluminum using the GMAW process is not that much different than welding mild steel or other materials. The operator should always follow recommended best practices when welding all materials. The purpose of this article will be to give the welder basic information on successfully welding aluminum alloys.
Basic Process Steps
1. Aluminum Preparation: To successfully weld aluminum the welder should always take care to clean the base material and remove any aluminum oxide and hydrocarbon carbons that contaminate the surface of the material.
a. Aluminum oxide on the surface of the material melts at 3,700 F and the aluminum base metal melts at 1,200 F. Leaving any oxide on the surface of the base material will reduce penetration of the filler metal into the workpiece.
b. To remove aluminum oxides, use a stainless-steel wire brush only. Using a steel brush will contaminate the base metal by imbedding carbon into the aluminum. Another means of cleaning aluminum is with the use cleaning solutions that are available thru most welding distributors and industrial supply companies. When using chemical etching solutions, make sure to remove them from the work before welding. To minimize the risk of hydrocarbons from oils or cutting solvents entering the weld, remove them with a degreaser. Check that the degreaser does not contain any hydrocarbons. It has been said that if you think the base material is clean, clean it again.
2. Preheating Aluminum: Preheating the aluminum workpiece can help avoid weld cracking. The preheating temperature should not exceed 230 F. Typically, preheating is only required when welding on thick materials; it can also aid the welder when welding a thin section to a thicker section. Another factor to think about should be the joint design, for instance, a bevel should be reconsidered when welding plates in the flat position may be a better choice.
3. Welding wire: There are several different grades and sizes of aluminum welding wire; 4043 and 5356 grade weld wire are the most common, however there are several other grades that are application and base metal specific. Always consult the manufacturer on which grade is best for the base metal being welded. There are several wire selection guides available on the internet.
4. Shielding gases: Shielding gas is used to shield the weld puddle from outside contaminants. The most common shielding gas used is 100% argon when welding aluminum due to its good cleaning action and penetration profile. When welding thicker sections of ½” or greater, a mixture of 75% helium and 25% argon is commonly used due the fact that it creates a hotter arc and aids in penetration.
5. Weld gun angle: With aluminum, using a push angle rather than a pull angle will result in better cleaning action, reduced weld contamination, and improved shielding-gas coverage.
6. Travel speed: Welding aluminum dictates the use of hotter amperage and voltage setting compared to steel, along with higher travel speeds. If travel speed is too slow, the welder risks excessive burn through, particularly on thin-gage aluminum sheet.
7. Convex-shaped welds: In aluminum welding, crater cracking causes most failures. Cracking results from the high rate of thermal expansion of aluminum and the considerable contractions that occur as welds cool. The risk of cracking is greatest with concave craters, since the surface of the crater contracts and tears as it cools. Therefore, welders should build-up craters to form a convex or mound shape. As the weld cools, the convex shape of the crater will compensate for contraction forces.
While MIG welding aluminum may present some challenges, which require a little more skill to produce acceptable welds when compared to steel, with a little practice and the right equipment the welder will find that paying attention to details will result in quality weldments.