Aluminum’s high strength-to-weight ratio has long made it an obvious choice in automotive and aerospace sectors, primarily because it is lightweight and corrosion-resistant. Although aluminum is not the strongest of metals, alloying it with other metals, such as copper, magnesium, tin, and zinc, helps increase its strength, durability, and mass.
Aluminum alloys are easily workable but, as with any fabrication material, there are advantages and disadvantages. On one hand, cast aluminum products are relatively low-cost because of aluminum’s low melting point. On the other hand, they have lower tensile strengths. Additionally, aluminum alloys will warp at high temperatures. They have lower fatigue limits than steel and weaken with repeated stress, which is why aluminum alloys are rarely used in high fatigue-tolerance applications such as girders in building construction and railways.
While the industrial benefits of aluminum are impressive (beyond the benefits named above, it is recyclable, military-grade durable, and energy-efficient), it can pose unique challenges for metalworkers.
In terms of pre- or post-welding problems, aluminum shares some common ground with steel, but not always. Aluminum conducts heat six times faster than steel and has a low melting point, making it very susceptible to warping and burn-through. Aluminum wire has relatively low tensile strength, which can pose wire feeding issues and lead to weld defects if the correct equipment is not used. Common aluminum weld defects are spatter, porosity, cracking, and lack of fusion.
Porosity occurs when hydrogen enters the weld pool during melting and then gets trapped in the weld during solidification. Shielding gas will protect a molten weld pool from the surrounding atmosphere – which can contaminate the weld – but other best practices must be followed, such as correct gas flow rates and purge cycles. Use of low-dew-point gases should also be a consideration.
Weld spatter, or slag, is droplets of molten metal or non-metallic materials that splash during the welding process. These tiny bits of hot material can stick to the base material and any surrounding metallic material. The main causes of these imperfections are typically poor surface preparation and incorrect equipment settings. For metalworkers, spatter – which is generally caused by a disturbance in the weld pool – is an unnecessary and costly nuisance.
If an array of best practices are not followed, the presence of smut, especially during gas tungsten arc welding (GTAW), is all but inevitable when welding aluminum. All welds, whether produced by GTAW or gas metal arc welding (GMAW), should be bright and shiny. Smut is black, which is why many welders assume it is carbon or a sooty contaminant. In fact, X-ray analyses have proven weld smut to be a combination of aluminum and magnesium.
Discolouration and smut occur when aluminum or magnesium oxides collect on the base material and weld. As the boiling points of aluminum and magnesium are lower than the temperatures of a welding arc, aluminum and magnesium in weld filler metal actually evaporate during welding and condense on the cooler base metal if not adequately protected by shielding gas.
Where aluminum cleaning is concerned, there’s been a move away from traditional wire brushes and harsh chemical cleaning solutions. Today it is possible to use efficient, environmentally friendly electrochemical technologies. These devices won’t damage aluminum or stainless steel surfaces, and some use a pH-neutral electrolyte solution, pumped directly to the surface being cleaned, and the dynamic electrical current control prevents micropitting on weld surfaces.
“Hot cracking” is a matter of chemistry. Stress or “cold cracking” is the result of mechanical stresses. Yet small or big, a crack is a defect that could lead to a failed weld inspection because over time a crack can result in weld failure. The prevention of hot cracking is possible by using high-quality filler metals with lower crack sensitivity. Cold cracking (during weld cooling) can occur within a day of welding, usually due to trapped hydrogen in the weld via the weld pool. If excessive shrinkage stresses are present during solidification caused by a concave bead profile, a too-slow travel speed, or depression in the end of the weld (crater crack), stress cracks will emerge.
Burn-through is caused by applying too much heat to aluminum and burning a gap in it. Since welding requires enough heat to fuse the metals properly, burn-through occurs when a welder fails to balance heat and speed. To prevent burn-through in aluminum GTAW, the operator should weld at a low amperage with a long point on the electrode. With GMAW, the welder should use a gun that pulses (this is a good practice for 1/8-inch or thinner aluminum). Electrical pulses provide enough heating and cooling at proper intervals to prevent burn-through. If thick aluminum is being welded, the amperage should be set high enough to penetrate the weld joint adequately: for example, 250 amps to weld ¼-in. thick material and 350 amps to weld ½-in.-thick material. Helium could be added to the shielding gas mixture as it provides a hotter, more penetrating arc on thicker sections.
Lack of fusion, or “cold lapping,” is a common defect in aluminum welds and is often caused by the presence of aluminum oxide (which is insoluble in molten aluminum) on weld surfaces. Poor welding technique also can prevent fusion. This occurs when the voltage or wire feed speed is too low or if the welder’s travel speed is too fast. Because aluminum conducts heat faster than steel, it is prone to lack of fusion at the start of a weld until enough energy is put into the weld. Some welding equipment addresses this automatically by increasing the current at the start of a weld and then lowering it to avoid heat buildup. An excessively wide weld joint can also prevent fusion but can be resolved by narrowing the joint or directing the weld arc toward the side wall of the base plate.
Ceramic coating technologies are among the latest tools in the arsenal of aluminum metalworkers. As with steel, weld spatter can be problematic in aluminum welds. If hot spatter fuses to welding nozzles and tips, the resulting clog inhibits shielding gas from flowing freely. Poor gas flow can cause porosity, inconsistent welds, or welds that require complete reworking.
Spray applicators are now available that coat welding nozzle surfaces and prevent spatter adherence and nozzle obstructions. This allows shielding gases to flow freely and the wire to feed evenly. Compared to common gel-based products, these applications have been shown to last longer and reduce nozzle replacements, treatment costs, and spatter removal labour costs.
Beyond torch nozzles, workpieces also can be protected from spatter to ensure not only clean welds but eliminate costly reworks. Anti-spatter emulsions protect workpieces from spatter. Compatible with aluminum, stainless steel, and steel, one such emulsion is designed to help welders achieve porosity-free welds and help prevent weld cracking. It is VOC-free, solvent-free, silicone-free, and biodegradable. Crucially, the emulsion retracts in the presence of heat, leaving welding areas free of liquid.
Although ceramic coating technologies and anti-spatter emulsions are very effective, there are other measures to optimize aluminum metalworking conditions. The value of pre-weld aluminum cleaning cannot be overstated. Bad shielding gas or bad wire can cause porosity, but so can lack of pre-weld cleaning. Two steps come into play: First, it is crucial to remove all oils, greases, lubricants, solvents, and other hydrocarbons from the base material in the aluminum weld area. These contaminants contain hydrogen. If they get into the welding arc, they cause weld porosity.
Premium degreasers remove contaminants from the weld area, and some of these are specially designed for aluminum and other sensitive alloys. Some are biodegradable, which means there is no added cost for disposal.
Second, it’s important to remove oxides from any weldable surfaces. This can be done with a stainless steel power brush with fine bristles, but a light touch should be used – excessive pressure will actually burnish oxides and drive them into the aluminum surface.
Post-weld problems such as discolouration, weld smut, and heat tint on heat-affected zones also have to be addressed.
Where aluminum cleaning is concerned, there’s been a move away from traditional wire brushes and harsh chemical cleaning solutions. Wire brushes are fast but can scratch aluminum and alter finishes. Strong chemicals (pickling pastes) can clean welds but, depending on their type, cause surface damage. Health hazards and expensive disposal issues also come into play.
Today it is possible to use efficient, environmentally friendly electrochemical technologies. These devices won’t damage aluminum or stainless steel surfaces, and some use a pH-neutral electrolyte solution, pumped directly to the surface being cleaned, and the dynamic electrical current control prevents micropitting on weld surfaces. It can process metals at around 3 to 5 feet per minute, depending on the application.
In comparison, pickling involves strong acidic solutions of nitric and hydrofluoric acids. Welders have to apply the paste, wait an hour for it to work, and then rinse it off the metal. Costly environmental compliance measures come into play if special wastewater disposal techniques are required, which usually are with toxic acids. Some fabricators pay as much as $8 per litre to dispose of pickling paste and associated liquids.
When it comes to blending or finishing aluminum alloys, shops need to keep productivity and safety in mind.
During grinding or cutting of aluminum, it is important to use high-performance equipment that won’t add unnecessary steps to the job. Shops will want to use grinding and cutting wheels that won’t clog or glaze when working with aluminum and other non-ferrous metals. They also want to ensure that the wheels will last through repeated applications.
Working with aluminum alloys can throw up all manner of shop floor challenges – be they pre-weld or post-weld. For fabricators looking to prevent weld defects, innovative tools are available. For metalworkers seeking safe and efficient methods to remove heat tint and discolouration from GMAW, GTAW, and spot welds, electrochemical cleaning and polishing technologies offer outstanding approaches to correcting weld defects in an environmentally friendly manner.
In terms of cutting, blending, and polishing aluminum alloys, the technologies continue to advance more every year. A fabricator should consult its supplier about how the shop can be made more efficient with the right tools.
Jonathan Douville, ing., Eng., PMP, is senior product manager, R&D International, at Walter Surface Technologies, 5977 TransCanada Highway West, Pointe-Claire, Que., H9R 1C1, 514-630-2800, www.walter.com/en. Images courtesy of Walter Surface Technologies.
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