by Nestor Gula
Welding aluminum is not difficult, just different
There is a perception that welding aluminum takes more skill, and is a more difficult metal to weld than regular mild steel. “It’s really not more difficult. It’s just different,” says Thomas Pfaller, technical services engineer for AlcoTec Wire Corp. “One of the things that welders often find more difficult is some of the finer points of welding aluminum, things that welders used to welding steel typically don’t consider.” He cites examples such as the thermal count activity of aluminum or the need to keep the bases and the filler metals clean.
Once you understand how to weld aluminum, it can seem easier to weld than mild steel. “People tend to find it more difficult because they don’t understand it,” says Tony Anderson, director of aluminum technology at Miller Electric Mfg. Co. “You need to have a different approach. You cannot apply all the same procedures that you would use for steel. For instance, cleaning and preparation of the material is far more critical with aluminum than it is with steel. Material prep is important because aluminum has aluminum oxide, which can affect the quality of the welding. And if you don’t understand it and take precautions in preparing for the welding, then you can run into problems.”
From the dawn of welding to the early 1940s, aluminum was extremely difficult to weld. “It was welded with oxy-acetylene using fluxes to break down the aluminum oxide,” according to Anderson. “They were problematic in that they were corrosive. The big breakthrough in welding aluminum was the inert gas welding process, usually MIG and TIG, now the primary method for joining aluminum. There are others, such as laser and plasma, but not stick welding.”
The primary reason that welding aluminum differs from welding steel is due to the material properties of aluminum. The two greatest differences are thermal and electrical conductivity. Thermal conductivty is higher for aluminum so it takes the alloy much longer to heat up when initiating the weld because much of the heat is quickly dissipated from the weld area. Electrical conductivity is also much higher and results in different settings of the welding electrical parameters, usually with a higher wire feed speed and travel speeds.
“Coupled with aluminum oxide, this makes the likelihood of a lack of fusion at the beginning of the weld greater,” says Mikael Carriere, the aluminum technical applications manager for Lincoln Electric. “Aluminum oxide has a much higher melting temperature than aluminum. Aluminum melts at 660°C (1,220°F), while aluminum oxide melts at 2,072°C (3,762°F). Aluminum oxide is formed on aluminum as soon as it is exposed to oxygen. It forms a thin layer and grows slowly in ambient conditions. In higher temperatures and in humidity, the oxide can grow faster, and because aluminum oxide is porous, the oxide can become hydrated. The hydrogen solubility of aluminum is much higher than steel and stainless steels when it is in its molten state. If any hydrogen is in the area of welding, the molten puddle will absorb the hydrogen and when the weld solidifies, the hydrogen will stay in the weld metal as porosity.”
One of the main defects in welds is porosity–spherical voids in the weld metal which decreases the tensile strength and ductility of the weld. “Porosity is created when you get contaminants either through the atmosphere in a gas form or contaminant on the plate itself and you get a kind of Swiss cheese consistency in the weld itself,” says Mike Dickman, key account manager for Abicor Binzel. “But it is a contamination within the weld puddle. It could be humid for example. You don’t have sufficient gas coverage. You don’t have gas at all. Maybe there’s gas coming in from the backside of the weld being sucked in. That can cause porosity. You might have some contaminant on the plate itself that can cause issues.”
Mostly, porosity is caused by hydrogen gas and when it gets trapped in the molten material, it causes voids or bubbles in the completed weld once it’s solidified. “Aluminum has a very high affinity for hydrogen and it can absorb hydrogen tremendously when it’s in its molten state,” says Anderson. “In room temperature, it doesn’t absorb hydrogen at all. But once aluminum melts, it wants to soak in hydrogen. If there’s any hydrogen in the area it is going to be absorbed into the molten material. Sources of hydrogen can come from different areas. One of them is humidity.” He cites another concern with aluminum oxide. “It’s a good thing in terms of the material being corrosive resistant. And it’s a bad thing because it can affect the weldability.” Aluminum oxide can absorb moisture and become hydrated. “Hydrated aluminum oxide is a problem which can cause porosity. And if you don’t understand it and remove it before you weld, you’re going to get porosity.”
Hydro-carbons are another source of porosity and weld defects. These can be found in oil, grease, cooking fluids, paint and many other kinds of foreign matter that might be present on the base material or even on the welding wire in the form of lubricants.
Another common defect in welding aluminum is cracking. “You’ve seen a cracking glass or a crack in concrete. The same basic thing happens when it comes to welding,” says Pfaller. “And this fissuring can occur in aluminum and steel welding and there’s not a code out there that I’m aware of that considers cracks to be acceptable. Typically, if you have a crack, you’re repairing a weld.”
The alloying elements of aluminum usually have a low melting temperature and cracking can be an issue, especially with crack sensitive alloys. “Craters or the ends of welds should be checked for cracking, especially if the crater was not filled in properly. Cracks in the length of a weld, also known as longitudinal cracking, can be seen right after welding,” says Carriere. “Heat treatable aluminum alloys can be susceptible to liquation cracking, which is micro-cracking that occurs in the heat affected zone. This happens when low melting alloying elements reach melting temperatures right outside of the fusion line while the matrix of the aluminum is still solid. The stresses from welding will cause these cracks to pop open. These cracks are typically not detected visually and other NDT techniques [such as ultrasonic testing for internal defects and dye penetrant testing for surface defects] are needed to detect these cracks.”
Aluminum is often welded with filler material that is a different chemistry than the base material. “It sounds rather odd unless you understand why,” says Anderson. “The reason for that is we have some aluminum base alloys that on their own are very crack sensitive. If we melt them and allow them to solidify, they will typically crack. The best example is the 6000 series, which have aluminum magnesium silicide materials. And a very common alloy is 6061. If we try to weld this material without adding filler materials, it will invariably crack during solidification because the amount of mag silicide in the material, which is added to give it optimum mechanical properties, also gives it its optimum crack sensitivity. To get a material that’s the right strength, and solidification crack sensitive, you use a filler alloy with a different chemistry, so we change the chemistry of the weld by adding a filler alloy of a different chemistry than the base material to prevent it from cracking.”
Cleanliness is one of the best ways to ensure a proper weld and this process is critical in avoiding cracking and porosity in aluminum. Before welding aluminum, you need to dry, degrease and remove the aluminum oxide. “If your filler metal or your base material is dirty, it’s almost like getting into a car accident without your seatbelt on,” explains Pfaller. “You’re making the mistake as soon as you started the car by not putting the seatbelt on. So, can you get away with it? Sometimes, but not typically. Cleaning is difficult because the way we clean it here for our test plates verses the way you might clean it in production are totally different. We’re doing X-ray quality welds. So, cleaning for us is paramount. Our recommended cleaning procedure is as follows: degrease, we use an industrial degreaser. Then we like to scrape the oxide layer off with like a heavy file. It will peel that oxide layer off because sometimes you’ll have contaminants stuck in that oxide layer. And then a light wire brushing can make sure that you’ve cleaned everything off well.”
Condensation from moving material from areas with different temperatures can also cause problems, adds Pfaller. “In the winter, companies bring parts in from outside where it’s really cold and you get condensation that forms on the parts. You can imagine that having water on the parts is not good for welding. Same thing in the summer where there’s really high humidity, swing differences in temperature can also cause that condensation. It’s one of the overlooked sources of water, hydrogen which results in porosity that a lot of people miss,” he says. Going over your workpiece with a blowtorch will remove this condensation.” SMT