There is some misinformation or a lack of information in the welding community regarding aluminum. Aluminum fabrications are not necessarily more difficult to design or weld than steel fabrications, they are just different.
And too often, these differences are not taken into account. This article presents just some of the common mistakes welders should avoid to achieve better aluminum welding, whether it’s done with TIG or MIG welding.
When welding aluminum, the designer and welder cannot assume that the properties of the parent material and the properties of the weld are equal. With steels, a weld can be made as strong as the parent material, but this is not the case with aluminum. In most cases, a weld in an aluminum alloy is weaker than the alloy being welded.
“The weld isn’t as strong as the parent material, which a lot of people don’t realize,” says Frank G. Armao, director of aluminum technology at The Lincoln Electric Co. “The heat from the welding affects the properties of the parent material. Rarely can you make a weld that is as strong as the parent material when you weld aluminum.”
Welding affects the two classifications of aluminum alloys, heat treatable and non-heat treatable, differently. The heat-treatable series aluminum alloys are 2000, 6000 and 7000, and the non-heat-treatable alloys are 1000, 3000, 4000 and 5000.
With heat-treatable aluminum alloys, the last heat-treatment step heats the material to somewhere between 325 F and 400 F. But when welding, the material around the weld becomes much hotter than 400 F so the material tends to lose some of its mechanical properties. If the welder performs post-weld heat treatments, the properties of a heat-treatable aluminum alloy can be improved, but if not, the area around the weld becomes significantly weaker than the rest of the aluminum.
Non-heat-treatable aluminum alloys are made stronger by workhardening them, which causes physical changes in the material. “But, when you weld an alloy that has been cold worked, the heat-affected zone (HAZ) of the weld goes back to the weak annealed properties,” Armao says. Therefore, the only time in the non-heat-treatable alloys that the welder can make a weld as strong as the parent material is when starting with annealed material. But in most cases, the reduced properties are acceptable.
“I was doing some work for the Navy two weeks ago,” Armao says. “Many marine aluminum alloys are non-heat treatable, and we were working with 5083-H16, which has a strength of about 50 ksi from the cold working. But the annealed properties are only about 40 ksi, so when you weld it, you get a HAZ that is 40 ksi. But a lot of times, designers don’t understand that. They design to 50 ksi and the welds fail. This is very common actually.”
Heavy duty is needed
Aluminum has a low melting point – 1,200 F compared to 2,600 F to 2,700 F for steel. “People tend to think that because of this low melting point, they can use light-duty equipment to weld the aluminum, which is not correct,” Armao says.
The thermal conductivity of aluminum is five times that of steel, which means that the heat dissipates very quickly. When welding aluminum, as fast as the heat is added, the material is trying to conduct it away. Therefore, the required welding currents and voltages for welding aluminum are higher than they are for steel so welders actually need heavy-duty equipment for aluminum.
“Typically, if you want to weld aluminum 1/2 in. thick or more, you need a 300-amp machine, whether it is TIG or MIG welding,” Armao notes.
Also, because of the low melting point, water-cooled torches are commonly used when welding aluminum. “If you are TIG or MIG welding more than 200 amps in a production environment, you need to use water-cooled torches and guns,” Armao says. Water-cooled torches allow the welder to run higher amperage for higher duty cycle times. Air-cooled torches overheat when used at the higher amperage and duty cycles needed for production work.
Spray transfer is the answer
Not all MIG welding equipment that is suitable for steel is going to be suitable for aluminum. While most people use short arc transfer when they MIG weld steel, spray arc transfer is always recommended for aluminum.
“Short arc transfer is never recommended for aluminum,” Armao says. “The currents are too low to generate enough heat in the material to guarantee good fusion, and you tend to get welding defects.”
The spray arc transfer is a smooth transfer of molten metal droplets from the end of the electrode to the molten pool. In the spray arc transfer, however, a large amount of heat is involved, which creates a large weld pool with good penetration. This can be difficult to control and cannot be used on aluminum thinner than 3/16 in.
“But pulse MIG welding allows you to get spray arc transfer at a much lower average current so now you can MIG weld pretty much any thickness aluminum you want,” Armao notes.
No preheat required
“A lot of people make the mistake of preheating every piece of aluminum they weld, which is not necessary if you have adequate equipment,” Armao says. “People get carried away with preheating, especially with heat-treatable alloys. Preheat in the absolute sense isn’t bad as long as you control it. Because the last heat-treatment step heats to between 325 F and 400 F, 200 F is as much as you want to preheat. If you preheat to 600 F, you don’t know what properties you have because the mechanical properties of the aluminum are changed.”
Preheating non-heat-treatable aluminum alloys is not as detrimental, as it does not affect the mechanical properties as significantly.
“The problem I’ve run into in a production environment,” he adds, “is that if you have 100 welders in your shop and tell them to preheat but don’t go higher than 200 F, there always a couple of guys that figure if 200 F is good, 600 F is better. So it’s difficult to control in a production environment.”
The exception would be welding a thick piece of aluminum to a thin piece. With two dissimilar thicknesses, the standard technique is to preheat the thick piece.
When welding anodized aluminum, TIG welding is most commonly used. Aluminum oxide is an electrical insulator and because the anodized coating is fairly thick, it can be difficult to strike an arc. If people try to weld over the anodized coating, the weld becomes gummy and porous. So for most welders, Armao recommends sanding or grinding the anodized coating off in that area. Then, the welder can weld that area as he would any aluminum, although the weld looks different than the rest of the piece because it’s not anodized.
There is a procedure used to weld over the anodized coating that keeps the anodized look. “But it is specialized and it is not easy to learn,” Armao says. This procedure is used most often in the marine industry.
Other considerations for welders include always TIG welding aluminum with AC polarity. Welding with DC makes it difficult to eliminate the aluminum oxide layer. The exception is when doing heavy aluminum TIG welding, such as engine blocks, where DC allows deeper penetration.