^ AlumaFeed Synergic Aluminum Welding System: Part of Kemplon Engineering’s Armory (Source: http://www.millerwelds.com/products/mig/alumafeed_weld_system/)
Big doesn’t necessarily mean better. Nor does heavier mean stronger. Ever since its low-weight-yet-high strength became evident, aluminum became the metal of choice in aerospace, automobiles, trailers, and heat exchangers. Plus, it boasts of tremendous corrosion resistance and electrical conductivity.
But then, there is always a catch. Pure aluminum is soft and has to be alloyed to improve its properties. Popular additions include magnesium, manganese, copper, silicon, and zinc. Aluminum alloys are either heat-treatable or non-heat-treatable. The latter give a soft weld while you have to heat-treat the former to get strong welds.
Aluminum’s high thermal conductivity causes rapid heat loss. You have to employ currents and voltages higher than those used for welding steel. This, despite aluminum’s low melting point.
Hyper conductivity also causes low penetration i.e. weld joints do not extend to the root of the weld groove, particularly at thick sections. Its low melting point means you have to speed up the welding lest the soaring electric power cause burnthrough i.e. holes in the base material. Thin sections are particularly burnthrough-prone.
Thick sections suffer from poor fusion because aluminum forms oxides readily that inhibit proper fusion. Low columnar strength precludes smooth feeding of filler wire. The wire is prone to birdnesting i.e. tangling between the liner and drive roll. Finally, high solubility of hydrogen in molten aluminum makes the weld porous.
Issues & Solutions
GTAW/TIG is mostly used for welding aluminum but GMAW/MIG offers value through better deposition rates, greater productivity, and less skilled operators. Aluminum is welded with AC rather than DC. Zirconiated tungsten electrodes that carry greater current offer best results. Use blunt tips because arc drifts around pointed tips.
Standard manufacturers diamond shave feeder wires before selling to remove the oxide layer. Feeding mechanisms such as Spool Guns, Push-Pull Guns, and the Continuous-Feed Push System eliminate birdnesting.
Typical issues with aluminum welding:
- Cracks: Hot Cracking and Stress Cracking
- Poor Penetration
Hot Cracking is a largely chemical phenomenon while Stress Cracking is predominantly mechanical. Possibility of hot cracking depends on:
- Filler Metal
- Cracking-Vulnerability of Base Material: 6000 series alloys are more crack-sensitive
- Joint Designs that limit addition of filler material
Stress Cracking originates from excessive shrinkage stresses during solidification due to:
- Vastly Restrained Joints
- Concave Bead Profile: convex-shaped welds are better because the shape compensates for contraction forces
- Crater Cracks: i.e. end-of-the-weld depressions
- Lethargic Traverse Speed
- Crack-Resistant Filler Material: operators choose filler of equivalent strength for welding steel. Selecting aluminum-welding fillers depends on base metal composition, dilution, welding convenience, ductility, required weld strength, corrosion, joint design, cracking possibilities, and color compatibility
4043 and 5356 are the most common aluminum fillers:
- 4043 contains silicon that gives great puddle control and simplifies welding
- 5356 is perhaps the most popular with exceptional tensile and columnar strength
Dilution is the quantity of base metal that melts and forms a part of the weld joint
- Proper Joint Design such as beveled groove joints allow addition of more filler material that improves base metal dilution and lowers crack-vulnerability
- Silicon-Containing Filler Material lowers stress cracks
- Approved Crater Filling Methods
- Boosting Travel Speed slashes stress cracking by limiting the width of the heat affected zone (HAZ) and lowering the amount of molten base material
- Preheating to 2300F (TIG) or 2500F (MIG) minimizes residual stresses and stress cracking
Pulsed-MIG minimizes burnthrough of sheets of thickness 1/8-inch or below as it slashes heat input. This also reduces spatter. Pulsed power sources use high peak-current only while separating the molten filler drop from the filler wire. Otherwise, they use low current that diminishes heat input.
Pulsed Welding saves $0.9 per pound feeder wire by permitting use of thicker wires. An integrated Pulsed-MIG aluminum welding unit costs 30% less than an assembled one and provides welds with a TIG-like weld appearance.
- Ensure Base Metal and Filler Wire free from moisture, oil, lubricants, paint, or grease:
- Clean joints with special solvents and dedicated steel brushes. Don’t use shop rags. Brush only in one direction and regularly clean all cutting tools and brushes
- Cover welding wire at all times
- Store metals vertically to lower condensation and water accumulation
- Introduce filler and base material in the welding area 24 hours before welding so familiarize them to workshop conditions
- Don’t use chlorinated solvents, lubricants, or processes such as carbon arc cutting, gas cutting, or gouging for joint preparation. Such processes hydrate the oxide film on the wire and impair the heat-affected area. Employ laser cutting and plasma cutting-gouging
- Use Low Dew-Point Shielding Gases that don’t condense during the elevated-temperature welding
Poor Penetration prevention:
- Select Correct Current/Amperage: thumb rule suggests 250amp current for welding ¼-inch thick aluminum sheets and 350amps for ½-inch thick
- Add Helium to the Shielding Gas Mixture: to boost penetration and weld-root width because its thermal conductivity and ionization potential hike the heat transfer from the arc to the base metal. Never use shielding gas containing oxygen or carbon dioxide as they oxidize aluminum
MIG and TIG aluminum welding normally use 100% argon for its good arc cleaning, economy, and clean welds. For lower porosity and deeper root penetration, MIG uses 75:25 (%) helium:argon mixture while TIG uses 75:25 argon:helium
Aluminum and magnesium oxides condense on the weld and base material to produce Discoloration and Smut. MIG-welding is most prone because filler wire passes through the arc leaving it exposed to vaporization and condensation. Prevention:
- Filler Material with lowest possible magnesium content
- Push-Gun Angle that creates cleaning action ahead of the arc and washes away smut
- Lower Contact-to-Work Distance (CTWD) minimizes possibility of wire vaporization-condensation
- Clean Nozzles maintain consistent shielding gas flow
- Large Nozzles minimize oxygen-introducing drafts
Precautions as meticulous as these require close monitoring over the entire process. After all, they prevent expensive and possibly lethal mishaps.
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