Welding aluminum presents some unique challenges compared to welding steel or other common materials, particularly in terms of the chemistry and crack sensitivity.
In many cases, welding aluminum requires following some special procedures. Important factors when welding the material include: selecting the right filler metal; proper storage and thorough cleaning of the base material; and proper welding techniques.
Here are some common challenges when welding aluminum and key best practices for addressing them.
Characteristics of Aluminum
A sound weld is all about chemistry, reduction of hydrocarbons, and proper technique. The characteristics of aluminum differ from steel in several ways. The melting point of aluminum is much lower than that of steel — 1,221 degrees Fahrenheit for aluminum, compared to 2,500 degrees for steel. Aluminum also has an oxide layer that melts at approximately 3,700 degrees. This oxide layer is much harder than the aluminum and helps the material resist corrosion and abrasion. However, it also acts as an insulator that can create issues during welding.
Because the strength of aluminum tends to increase as service temperature decreases — unlike steel, which becomes more brittle as service temperatures decrease — aluminum is commonly used in cold temperature applications, such as cryogenics and liquid natural gas transportation.
While iron is a main alloy in steel, aluminum materials are mainly aluminum with the addition of a variety of elements.
Wrought alloys, like 1xxx series aluminum, are pure aluminum with no alloying elements intentionally added. The principal alloying elements in other types of aluminum are copper in 2xxx series, manganese in 3xxx series, silicon in 4xxx series, magnesium in 5xxx series, magnesium and silicon in 6xxx, zinc in 7xxx series, and other elements in 8xxx series.
Challenges of Welding Aluminum
The different characteristics of aluminum become apparent during the welding process. Thermal conductivity and issues with porosity are the two biggest differences in welding aluminum compared to steel.
Hydrogen is very soluble in liquid aluminum. As the filler material and aluminum base metal become liquid during the welding process, they absorb hydrogen and can hold it in solution. Once the molten material starts to solidify, it can’t hold the hydrogen in a homogenous mixture anymore. The hydrogen forms bubbles that become trapped in the metal, leading to porosity.
A helium/argon shielding gas mixture can be used to combat porosity issues if all other options have been tried. Be aware that with a helium mixture, voltage must be increased to overcome the higher ionization potential of this gas compared to argon. The increased voltage will cause higher heat input and additional penetration, which is why this mixture is used on thicker aluminum base materials.
Unlike with steel, the presence of hydrogen does not cause cracking in aluminum welds. However, hot cracking, which can occur as the weld solidifies, is a threat with aluminum. The solution for this comes back to chemistry. If hot cracking is an issue, refer to an aluminum filler metal selection chart to find the filler metal that best addresses this problem.
A 6061-aluminum base metal is an example of a material that is in the peak crack susceptibility at its current chemistry, making it very difficult to weld autogenously or with a similar chemistry filler material. Using a filler metal with elements such as magnesium (ER5XXX) or silicon (ER4XXX) can help push the material outside the crack-susceptible range.
Another challenge with aluminum is that it is five times more thermally conductive than steel. The cool areas of the base metal try to pull heat away from the weld pool, which can cause a lack of penetration in the weld. Because of this difference in thermal conductivity, aluminum requires much higher heat inputs than steel during welding.
Choosing a Filler Metal
It’s critical to use a selection chart when choosing a filler metal for aluminum. Each combination of aluminum designations has recommended filler metal options, depending on the weld characteristics required by the application.
A selection chart includes eight characteristics that are important in various welding applications: crack sensitivity, strength, ductility, corrosion resistance, elevated temperature service, color match after anodizing, post-weld heat treatment, and toughness. By analyzing the needs of the end component, it is possible to determine which properties are most important in the specific application and to select a filler metal that best matches the characteristics required. It is important to note that elevated service temperature with aluminum is 150 degrees to 350 degrees Fahrenheit. This and other definitions of the characteristics can be found on the aluminum selection chart.
Another option is to use a filler metal selector app, such as this one. The same information that is on the full selection chart can be found in the app, but it shows filler metal for the selected base materials only.
Proper filler metal selection is always key. For example, if the base material being welded is 6061 aluminum, good filler metal options include 4043, 4943, and 5356. A 4043/4943 gas metal arc welding (GMAW) wire or gas tungsten arc welding rod can reduce porosity and provide better weldability and increased puddle fluidity, while a 5356 product provides greater toughness and strength.
Along with choosing the filler metal best suited for the application, following some key best practices also can help achieve success when welding aluminum.
- Don’t weave. While a weave technique commonly is used to weld steel, it should not be used with aluminum. Instead, use a stringer bead, which helps ensure proper penetration and fusion. For aluminum GMAW, be sure to use higher heat inputs and a fast travel speed to stay in front of the puddle.
- Clean the metal. Thoroughly clean the base material before welding to remove oil, dirt, residue, and moisture. This helps ensure the best results and reduces chances of porosity. An acetone or aluminum cleaner works well for removing hydrocarbons that may be on the material surface. When prepping the weld joints, don’t blow them off with compressed air, as this can introduce contamination from moisture and oils if shop tools are used.
- Remove the oxide layer. After cleaning, use a stainless-steel brush — one that is new or has been used only on aluminum — to remove the aluminum oxide layer before welding. As mentioned previously, aluminum oxide has a much higher melting point than aluminum. It acts as an insulator that can cause arc start problems and very high heat is required to weld through the oxide layer. This can cause burn-through on the base material and porosity, since the oxide layer tends to hold moisture.
- Store it properly. Storage practices for base materials and filler metals also play a role in preventing porosity. Store sheets of aluminum inside when possible. If they must be stored outdoors, stand the sheets vertically rather than on top of each other to prevent trapping water, which will contribute to the formation of a thicker hydrated aluminum-oxide layer. For materials and filler metals stored outside or in an air-conditioned part of the building, bring them inside to the shop before welding—preferably the day before—to let the metal temperature stabilize and help prevent moisture in the air from creating condensation on the aluminum.
- Check the consumables. Some issues with aluminum welding—especially when using GMAW— can stem from the consumables. To help reduce the risk of porosity, use gas lines and hoses that are new or in good condition and make sure all hose connects are tight, so they’re not pulling air into the line.It’s also important to use the proper liners and drive rolls. Plastic liners and inlet guides can provide benefits over steel liners in aluminum welding because the metal or brass inlet guides and steel liners can abrade the softer aluminum wire as it travels through the drive system and liner. This can result in wire shavings that clog the liner and cause feeding issues. Similarly, U-groove drive rolls are standard for aluminum applications because other types of drive rolls can smash or distort the wire.
- Monitor the temperatures. Consult an aluminum filler metal guide or code to determine the appropriate preheating and interpass temperature ranges. Preheating can be used to reduce the thermal effects of section size when welding thick base metals or dissimilar thicknesses, but keep preheating to a minimum for aluminum applications.Heat-treatable base metals and 5xxx series base metals containing more than 3 percent magnesium should not be subjected to preheating or interpass temperatures above 250 degrees Fahrenheit for more than 15 minutes. Time spent at elevated temperatures can reduce the strength of the material and contribute to cracking.
Addressing the challenges of welding aluminum often comes down to chemistry. Making the right base material and filler metal match goes a long way toward achieving success and minimizing issues. It’s also important to follow recommended best practices for welding aluminum. And remember, the techniques and best practices for welding aluminum are very different than those for welding steel.