Welding Digest

Aluminum, once the most expensive metal in the world, is now commonplace. Finding aluminum was easy but learning to refine it challenged some of the greatest minds of the 1800s. Just as it was tricky to refine, it is also tricky to weld, but once you learn, it becomes a regular part of welding life. 

Aluminum has many properties that make it desirable in manufacturing. At approximately one-third the weight of steel, it is lightweight but has tensile strengths up to 90,000 lb/in.² (90 ksi). It is corrosion resistant and conducts both heat and electricity extremely well. These properties make it an excellent choice for many end products across multiple industries. However, they also make it difficult to weld. 

Due to the high thermal conductivity, it takes longer to establish an aluminum weld pool. The parameters also need to be reduced toward the end of the weld to compensate for the increased temperature. Common advice says to weld aluminum hot and fast due to its high thermal conductivity. Using a foot pedal with gas tungsten arc welding (GTAW) makes it easy to adjust the welding current rapidly from start to end. However, GTAW generally requires a lot of practice and skill to master. Gas metal arc welding (GMAW) is usually learned faster, and most modern machines provide hot start and crater fill functions for easy control as well as controls on the torch to adjust wire feed speed quickly during welding. 

What to Know before Striking an Arc 

Understanding your base metal is the first key to making great aluminum welds. Aluminum is separated into series based on the alloy and application. Each of the seven series or alloy groups uses a four-digit number code. One example is alloy group 1xxx, which is unalloyed aluminum used in foil and other products where the properties of pure aluminum are required. The other six groups are alloyed with materials such as magnesium, silicon, zinc, and copper, among others. Due to extreme risks of corrosion and cracking, some alloys, such as 2024 and 7075, shouldn’t be welded at all, which is why knowing your base is important.  

Once you know your base alloy, choosing the filler metal is often a simple matter of reading the charts provided by the supplier. Not using the correct filler metal can lead to catastrophic failure in the weld due to weld defects or the part being placed into service in an environment unsuitable to the filler metal. 

Only inert gases such as argon and helium are used in aluminum welding. In most cases, 100% argon is used because it costs less than helium, has a good cleaning effect, and, being heavier than air, effectively shields the weld pool. Helium can help achieve deeper and wider joint penetration profiles and reduce porosity, particularly when welding thick aluminum plates.

A clean welding surface is another key to welding aluminum. The natural oxide on aluminum acts as an insulator, making starting an arc difficult. It also traps water vapor, which is released when welding over the oxide layer and causes porosity. It is important to clean the material before welding with an organic solvent like acetone and then use a clean stainless steel wire brush to remove the oxide from the surface. Be careful the brush hasn’t been used on steel or other harder surfaces, as it can further contaminate the aluminum rather than clean it. 

Conductivity Affects Weld Parameters 

Aluminum has a fast freeze weld pool, so any mistakes immediately become apparent. Getting fusion at the arc start can be troublesome due to aluminum’s high thermal conductivity. These two things alone give rise to the idea that aluminum is difficult to weld. Many modern welding machines use a hot start function that uses higher voltage and welding current at the arc start to flash melt the surface. Likewise, for the end of the weld, the machines offer a crater fill function that reduces the voltage and current. This produces a convex crater fill, which avoids the crater cracks that can form from concave crater fills.

Some process variants that take advantage of aluminum’s fast freeze characteristics are available. These variants can alternate the arc power to achieve a rippled cosmetic weld. Alternating the arc power has a secondary effect of reducing heat input, which helps minimize distortion. This also can give the welder a visual cue that can aid in maintaining a consistent travel speed.

In GTAW, a rule of thumb is to use 1 A for every 0.001 in. of thickness. However, for GMAW, this isn’t always the case, depending on the process and travel speed being used. Most modern synergic power sources offer a material thickness parameter where the wire feed speed and voltage are set by simply adjusting the material thickness. There are also apps that guide which parameters to set depending on the material thickness, wire diameter, and weld size. 

Conclusion  

New uses for aluminum are being developed all the time due to its availability and recyclability, making aluminum welding a solid skill for every welder. Less experienced welders can take advantage of the automatic parameter adjustments and simple touchscreen displays of modern machines, helping to ease the skills gap prevalent in the industry.

This article was written for the American Welding Society by Shaun Relyea, head of tech support national at Fronius, USA, Portage, Ind.