Aluminum Welding: Getting to the Basics

March 3, 2020

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Manufacturers specify aluminum alloys for their high strength-to-weight ratio and corrosion resistance, which makes them an attractive choice in industries such as transportation, automotive and aerospace, as well as for general manufacturing and fabrication. Aluminum alloys afford the opportunity to reduce the weight of completed parts without sacrificing durability, and stamped and fabricated aluminum parts can better withstand extreme service conditions compared to many other materials.

Aluminum’s high thermal conductivity adds to its appeal. It can be used for applications such as radiators and heat exchangers, and at a lower cost than copper, for example.

When welding aluminum, metal formers must understand the basics to obtain optimum results. From understanding potential pitfalls to following best practices for pre-cleaning, equipment setup and more, each contributes to higher-quality welds and supports good productivity. Knowing the basics also can help prevent costly downtime for rework.

The Challenges

Due to its high thermal conductivity, aluminum requires welding at relatively high amperage, which increases the potential for burnthrough on thinner sections. This characteristic also makes aluminum prone to incomplete fusion at the start of welds, since the rest of the base material pulls away the heat so quickly, and makes crater fill difficult at the end of welds. Welding power supplies with adaptive features will help prevent these two issues. Other suggestions: Travel slowly at the beginning of each weld, and increase the contact tip-to-work distance at the end of each weld.

Porosity also challenges welders working with aluminum, and can result from improper base-material preparation; the presence of contaminants (dirt, oil, grease or paint); filler metals exposed to moisture; loose gas connections; and shielding-gas contamination. The presence of hydrogen in the weld pool—from the filler metal and the aluminum itself—also can lead to porosity. Hydrogen is very soluble in aluminum and can easily become trapped as the weld solidifies.

Hot cracking also can arise during aluminum welding, and results from chemistry as opposed to mechanical stresses. Certain aluminum alloys, such as 6000-series grades, are more susceptible to hot cracking than others. The filler metal used also affects the likelihood of hot cracking, so take care to select a weld wire with relatively low crack sensitivity when welding crack-sensitive grades.

Lastly, restrictive joint designs can contribute to hot cracking. For that reason, design weld joints wide enough to reduce the stresses that the aluminum weld metal undergoes during the welding process.

Surface Cleaning and Preparation

The surface of aluminum sheet and plate contains an oxide layer that requires removal prior to welding. This thin, hard layer provides the material with its corrosion resistance, but it also melts at a higher temperature than the aluminum below (3700 F versus 1221 F). Because of that temperature difference, the oxide layer can cause penetration issues during welding.

To begin preparing for aluminum welding, thoroughly wipe the surface of the workpiece material with a clean cloth and a solvent, such as acetone, that cleans well and can effectively degrease the material. When using flammable liquids such as acetone, take care to work in an area away from the weld cell to avoid the risk of fire. Never use any liquid containing chlorine to clean aluminum, as chlorine becomes toxic when exposed to the heat of the welding arc.

Using a stainless-steel brush or grinding wheel dedicated to the purpose of cleaning aluminum, brush or grind the surface to remove the oxide layer. Because the oxide layer will start forming as soon as the exposed aluminum comes in contact with air, perform this step as close as possible prior to welding.

Although it isn’t necessary, after removing the oxide layer the operator can wipe down the surface with a clean cloth and acetone to remove any shavings.

Equipment, Consumables, and Technique

When gas-metal-arc welding (GMAW) of aluminum, the soft aluminum welding wire is subject to deformation and wire shaving during feeding. To prevent this, take care to use the proper equipment and consumables. In addition to a power source specifically designed for welding aluminum, U-grooved drive rolls and non-metallic inlet guides for the wire feeder help ensure good weld quality. These components provide smooth wire feeding and prevent wire shaving. Welding operators should set the lightest drive-roll tension that will effectively feed the wire.

Use a push-pull welding gun and a non-metallic Teflon liner, along with contact tips designed specifically for welding aluminum. If these contact tips are not available, use a standard copper contact tip with a bore diameter approximately 10 percent larger than the wire diameter, to reduce wire shaving while still providing the appropriate electrical conductivity to create a stable arc.

GMAW settings for aluminum will vary with the diameter of welding wire being used, as well as the thickness of the aluminum base material. Consult the filler-metal specification sheet for parameter settings--amperage, voltage and wire-feed speed.

To gain consistent arc characteristics, shield aluminum welds with argon or an argon-helium gas mix. Note that the addition of helium provides greater penetration and a wider bead profile when used on thicker sections of aluminum. However, helium, more expensive than argon, also can create a slightly less stable arc. The needs of the application will determine whether helium’s additional cost is worthwhile compared to changing the weld-joint design or making other adjustments.

Skilled welders employ specific techniques on aluminum. When welding with solid aluminum wire, they use a push angle that creates a wide weld bead and good tie-in at the toes of the weld. Compared to steel, aluminum’s greater thermal conductivity may require not only higher current or voltage, but also faster travel speeds. Because of this, the term “hot and fast” is often used to refer to aluminum welding.

Filler-Metal Considerations

Filler-metal selection for aluminum depends on the specific workpiece alloy and the desired properties of the finished weld. Consider these performance factors when selecting the filler metal:

Crack sensitivity
Weld strength
Weld ductility
Corrosion resistance
Elevated service temperatures
Color match after anodizing
Post-weld heat treatment
Toughness

Filler-metal manufacturers offer selection charts that can help match specific aluminum alloys to the proper filler metal, based on these factors. That said, welding wires designated as Type 4043 and 5356 are among the most common options. Type 4943 filler metals also offer some unique characteristics that make them a desirable substitute for Type 4043 products.

Type 4043 aluminum filler metals will develop a fluid weld pool, thanks to the addition of 5-percent silicon. This improves wetting action, or flow into the joint, helping to minimize cracking and reduce the need for extra post-weld cleaning. Type 5356 filler metals contain magnesium, which increases weld toughness and strength; however, it also creates more smut (black soot) at the toes of the weld that will require post-weld cleaning.

Using Type 4943 weld wire rather than a Type 4043 wire will result in higher weld strength (due to additional magnesium), with comparable weldability and crack resistance.

Parting Thoughts

Welding operations, when working with aluminum, can benefit from establishing some basic best practices. Start with ensuring cleanliness and proper material preparation. Then, always follow manufacturers’ recommendations for power-source and weld-gun setup, and consumable installation, to optimize weld quality.

And, be sure to follow recommended safety instructions. MF