Come out on top with the right material preparation, filler choice, and equipment
December 3, 2012
Aluminum is tough to weld, but its advantages of high strength-to-weight ratio, corrosion resistance, excellent mechanical properties at low temperatures, and recyclability put it at the top of the materials list for fabrications, including many with substantial bulk. Two industry experts shared tips on how to produce good welds in thick aluminum.
Aluminum is tough. It bullies its way into welded applications traditionally filled by steel. Its advantages of high strength-to-weight ratio, corrosion resistance, excellent mechanical properties at low temperatures, and recyclability put it at the top of the materials list for fabrications, including many with substantial bulk. Two industry experts share tips on how to get the upper hand when welding thick aluminum. It’s really not so tough, just a little different.
What is thick?
Karl Hoes, welding instructor at Lincoln Welding School, Cleveland, Ohio, defines thick as “when you start getting around 3⁄16 to 1⁄4 inch thick with aluminum. That is where you have to begin using heavy-duty equipment, especially for production welding.”
According to Thom Burns, technical services director at AlcoTec Wire Corp., Traverse City, Mich., thick is a relative term. “Although 1⁄2 in. and above is typically considered thick aluminum, equipment capability, weld position, alloy, manual versus auto, and number of passes required are also factors.”
Burns: Most people consider using mechanized systems and wire diameters above 1⁄16 in. when designing for or using thick aluminum. Certainly, materials that exceed 1 in. require larger wire and equipment that can handle the high amperages and duty cycles needed for this type of welding.
Hoes: Water-cooled GTAW [gas tungsten arc welding] torches and GMAW [gas metal arc welding] guns should be considered for production welding of aluminum when amperage exceeds 150. Lighter air-cooled GTAW torches, GMAW spool guns, and push-pull systems sometimes will work for intermittent welding in this range, but will overheat when working at the higher amperage and duty cycles needed for production work.
Burns: Materials need to be free of oil, grease, and moisture. Edges need to be more rounded than on thin materials to prevent premature melting, which causes lack of fusion (LOF) and lack of penetration (LOP). Often extended lands are used to get good fusion at the root, and the groove may be formed into a U shape.
Filler metals typically are from 3⁄32 to 3⁄16 in. These require special equipment and handling because of the springiness of the wire. When using large-diameter wire, note the duty cycle and be aware that most welding equipment such as torches and power supplies are rated using CO2, not argon or argon/helium mixes, which increase the heat to the gun and lower the duty cycle.
Hoes: Oxide removal is also critical. As aluminum is stored, the oxide layer thickens, absorbs hydrogen, and traps impurities that can be absorbed into the weld puddle. Hydrogen, readily absorbed into liquid aluminum weld metal, turns to porosity when the weld solidifies.
Remove oxide with a clean stainless steel brush. Be careful not to use too much pressure if using a power tool. A power brush can dig into the aluminum, roll it over, and burnish it, trapping contaminants under the surface. Use grinding wheels and sanding discs that are designed for aluminum to minimize contamination. Machining instead of grinding can result in a cleaner surface, but remove all coolants or lubricants before welding. Paint and coatings also need to be removed from the weld area. Before brushing, clean and degrease with acetone, nonchlorinated brake cleaners, or citrus degreasers to avoid spreading contaminants. There are stronger cleaning solvents on the market, but exercise caution to use them safely.
Aluminum welding consumables should be properly stored to avoid shop dirt contamination and excessive oxide formation. This is especially important with GMAW consumables because the insulating oxide layer can cause problems with electrical contact between the aluminum electrode and the copper contact tip. Also, a large amount of surface area on small-diameter wires forces more contaminants into the weld deposit. GTAW cut-to-length consumables can be cleaned, but it is impossible with GMAW wire.
Burns: Two things cause hot, or solidification, cracking. The first is stress-related. When the filler material solidifies at a temperature equal to or below that of the base material, stress caused by the metal shrinkage pulls on the molten pool or weld metal and causes cracking. The second is more of a chemistry issue. Alloys such as the 6XXX series that have a wide liquidus/solidus temperature range are more prone to cracking. The temperature difference creates a mushy range during solidification, forming coherent interlocking dendrites that result in voids between the grains. As the coherence range widens, it becomes more prone to what might resemble microcracking in the weld.
Stress corrosion cracking (SCC) is basically a chemistry issue in which some alloy elements, often accelerated by heat, have a galvanic reaction to each other. This is called metallurgical susceptibility. Environmental atmosphere and temperature contribute to this reaction that can lead to failure when stress is added.
Hoes: When the amperage is turned off quickly at the end of a weld, the puddle solidifies quickly and leaves a concave crater that pulls apart and cracks when it shrinks. Some advanced GMAW machines have crater-fill features with which the machine continues to weld at a reduced current for a period of time after releasing the trigger. This fills the crater and ends the weld with a convex bead, which is not prone to cracking. When using equipment without a crater-fill feature, double-back at the end of the weld to leave the crater on the weld. With manual GTAW, backing off the current with the foot pedal and adding a little extra filler metal at the end of the weld can also fill the crater.
TIG welds often have a tendency to crack if there is not enough filler metal, especially when working with crack-sensitive alloys like the 6XXX series. Dilute enough filler metal into the puddle to change the weld chemistry. Starving the puddle by not adding enough filler metal often leads to cracks. 6XXX alloys never should be autogenously welded (without filler). The silicon-alloyed 4XXX filler metal is less crack-sensitive than the commonly used 5XXX alloys but should not be used on alloys that contain more than 21⁄2 percent magnesium. The commonly used 5052 alloy is one of the only ones in the magnesium-alloyed family that can be welded with the 4XXX filler metals.
Burns: Sometimes a good-looking weld is not a good weld. Unfortunately, the best way to evaluate a weld is to cut it apart, look at the depth of penetration, fusion, and porosity level. While we can’t use this method without destroying every weld, using known welding codes, such as those published by CWB and AWS, and following qualifying procedures increase the probability of achieving quality results.
Hoes: Look at bead shape. A good weld will be flat to slightly convex, without concave shapes or hollows between the ripples or in the crater, and have good wash-in at the toes. Check carefully for cracks using dye-penetrant inspection. If welds are ropey or convex, it is usually an indication that not enough energy was provided to the weld and cold procedures or undersized equipment was used. This is often an indication of possible LOF.
Burns: Larger wire, fast travel speeds, and fewer passes all reduce the HAZ.
Burns: Welding thick material with large-diameter wire can cause overheating of the contact tip if the weld gun or torch is not sufficiently cooled. The result could be loss of contact to the wire as well as excessive buildup at the exit of the contact tip. Use the proper equipment.
Hoes: Larger wire is going to feed easier than smaller wire when pushing. Use aluminum contact tips that are slightly oversized to accommodate the expansion of the aluminum wire as it heats up. Be sure the guide tube is plastic and the U-groove drive rolls are polished to prevent wire damage. The brake should be loose enough for the roll to spin freely without dumping wire when welding stops. Drive roll tension should be set back compared to what is used for steel.
Burns: In thick aluminum welding, helium is often added to argon to increase heat input. This can flatten the weld bead, increase root width penetration, and reduce porosity.
Hoes: Helium/argon gas mixtures, often utilized when GTAW on material over 1⁄4 in. thick and GMAW on material over 1⁄2 in. thick, transfers more heat to the weld than argon, which aids in welding thicker sections of aluminum. The broader bead profile resulting from the helium addition is not as deep in the middle, but it solidifies slower and allows some of the hydrogen to escape, reducing porosity.
Be sure there are no drafts or breezes removing gas from the weld area, leaks in the system, loose fittings, or spatter buildup in the nozzle. Check the purity of the gas. Contaminated gas really shows up with aluminum welding. Low-pressure tanks, below 500 lbs., can cause problems. Change them.
Burns: Recognize that current must increase substantially to perform thick welds. With this increase in amperage, the heat and light given off as byproducts can result in burns from both temperature and ultraviolet light. Sufficient protection should be in place for both.
Hoes: Extra precautions should be taken to protect your eyes and skin and surrounding personnel. ANSI-approved safety glasses with side shields should be worn by surrounding personnel and as secondary protection from UV light under your hood. Higher levels of ozone result from thick aluminum welding conditions and enter your breathing zone. Adequate ventilation should be used to keep fume levels in conformance with OSHA standards while using caution not to exhaust shielding gas from the weld. Reference ANSI Z49.1.