Aluminum’s corrosion resistance and high strength-to-weight ratio, as well as its high electrical conductivity, make it an excellent choice for many applications from aerospace to heat exchangers, trailer fabrication and, most recently, automotive body panels and frames.
Don’t wait for defects to appear in your aluminum welds. Be proactive by understanding how to prevent them from happening in the first place.
Fixing problems in your welding operation quickly and efficiently can go a long way in minimizing downtime and unnecessary costs. But learning how to prevent the problems in the first place is even more beneficial, regardless of the material you are using for the application.
Welding aluminum poses some unique challenges. In addition to a low melting point and high thermal conductivity, aluminum is especially prone to burn-through on thin sections and may experience lack of fusion on thick ones. Weld defects like cracking, weld smut/soot, and porosity are also real concerns.
Still, aluminum’s ability to resist corrosion, its high strength-to-weight ratio, as well as its high electrical conductivity make it an excellent choice for many applications from aerospace to heat exchangers, trailer fabrication and, most recently, automotive body panels and frames.
To avoid a negative impact on productivity and quality, it is important to understand the causes of aluminum weld defects, implement steps to prevent them, and find ways to quickly rectify errors should they occur. Here are answers to some common questions to help you troubleshoot the process.
What Causes Cracking in Aluminum Welding?
Hot cracking and stress cracking can occur during aluminum gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) processes. Both types of cracks, even when small, can prevent welds from meeting code requirements and can eventually lead to weld failure. Hot cracking is predominantly a matter of chemistry, while stress cracking is the result of mechanical stresses.
Three main factors increase the probability that hot cracking will occur during aluminum welding. The first factor is how susceptible the base material is to cracking. For example, some alloys like the 6000 series are more prone to cracking than others. The second factor is which filler metal you use. Third is joint design—some joint designs restrict the addition of filler metal.
Stress cracking can occur when an aluminum weld cools and excessive shrinkage stresses are present during solidification. This could be due to a concave bead profile, a too slow travel speed, a highly restrained joint, or depression in the end of the weld (crater crack).
How Do I Stop Cracks From Happening?
In some cases, preventing hot cracking can be as simple as choosing a filler metal with a weld metal chemistry with lower crack sensitivity. Each aluminum filler metal has an American Welding Society (AWS) classification that corresponds to the Aluminum Association registration number, and together the two identify the particular alloy chemistry.
Always reference a reputable filler metal selection guide to make the best choice because not all aluminum filler metals are suitable for every aluminum base material. Some filler metal guides give recommendations specifically for several weld characteristics, such as cracking, strength, ductility, corrosion resistance, elevated-temperature service, color match after anodizing, postweld heat treatment (PWHT), and toughness. If cracking is a concern, select the filler metal that has the highest rating in the cracking category.
Also, using an appropriate joint design can help prevent hot cracking. For example, a beveled groove joint is a good option because it allows for the addition of greater amounts of filler metal, which increases the amount of base metal dilution, making it less prone to cracking.
To avoid a negative impact on productivity and quality, it is important to understand the causes of aluminum weld defects, implement steps to prevent them, and find ways to quickly rectify errors should they occur.
It is possible to prevent stress cracking by using a filler metal containing silicon. When allowable, this type of filler metal lowers shrinkage stresses, particularly in crack-sensitive areas like the beginning and end of the weld (or craters). Also, use an automated crater fill function or other approved methods of crater filling to minimize the opportunity for cracking to occur in the crater. Increasing your travel speed also can help decrease the opportunity for stress cracking on aluminum by narrowing the heat-affected zone (HAZ) and reducing how much the base metal melts.
Preheating is also an option to combat stress cracking because it minimizes the residual stress levels that are present in the base material during and after welding. Closely monitoring the heat input is key to making this work. Too much heat can lower the base material’s tensile strength in some alloys to unacceptable levels.
What’s the Best Way to Avoid Burn-through or Poor Penetration?
Using a pulsed GMAW process is a great defense against burn-through on 1⁄8-in. or thinner aluminum. Power sources with this capability operate by switching between a high peak current and low background current. In the peak current phase, a droplet from the aluminum wire pinches off and propels toward the weld joint, while during the low background current phase, the arc remains stable without metal transfer. The combination of these high-peak and low-background currents reduces heat input to prevent burn-through and offers the added benefit of creating little to no spatter.
When you are welding thick aluminum, it’s particularly important to set your amperage high enough to penetrate the weld joint adequately. A good rule of thumb is to use 250 amps to weld material that is ¼ in. thick and about 350 amps to weld material that is ½ in. thick. In some instances, consider adding helium to the shielding gas mixture for its ability to provide a hotter, more penetrating arc on thicker sections. For the GMAW process, a mixture of 75 percent helium balanced with 25 percent argon is a good option. Use a mixture of 25 percent helium and 75 percent argon when welding thick sections of aluminum with GTAW to increase penetration.
Why Are My Welds Discolored?
Discoloration and smut occur when aluminum or magnesium oxides collect on the base material and weld. This phenomenon is most common during GMAW because as the filler wire passes through the arc and melts, some of it reaches the vaporization temperature and condenses on the cooler base metal that is not adequately protected by the shielding gas.
Choosing the appropriate filler metal, for example, a 4000 series aluminum filler metal which contains little to no magnesium (as compared to 5000 series aluminum filler metals which contain about 5 percent magnesium), reduces the opportunity for this element to vaporize into the arc and condense on the weld in the form of soot.
Shortening the contact-to-work distance (CTWD) and using the appropriate gun angle and shielding gas flow rate also can minimize weld discoloration. Use a push angle, which helps drive the cleaning action from the arc in front of the weld to help remove smut. Increasing the nozzle size of the GMAW gun or GTAW torch helps protect the arc from drafts that could introduce oxygen into the process. Always keep the nozzle clean of spatter to ensure consistent shielding gas flow to protect the weld pool.
How Do I Eliminate Porosity?
Porosity is a common discontinuity that occurs primarily when hydrogen enters the weld pool during melting and then gets trapped in the weld during solidification. You can do several things to prevent this from happening. First, be certain that the base metal and the filler metal are clean and dry. Wipe down the aluminum before welding using a solvent and clean cloth to remove any paint, oil, grease, or lubricants that could introduce hydrocarbons into the weld. Then brush the weld joint with a clean stainless steel brush dedicated for the job. If the aluminum base material has been stored in a cool location, allow it to acclimate to the temperature in the shop for 24 hours before beginning the welding process. This keeps condensation from forming on the aluminum.
Storing unpackaged filler metals in a heated cabinet or room also can help reduce the risk of porosity. Doing so keeps products from cycling through dew points and minimizes the chance for hydrated oxide development on the surface of GMAW wires or GTAW cut lengths.
Purchasing filler metals from a reputable manufacturer is always a good idea, as these companies typically diamond-shave the wires and GTAW cut lengths to eliminate harmful oxides and follow procedures to produce low residual hydrogen-containing compounds.
Finally, consider purchasing low-dew-point shielding gases as protection against porosity. Follow all recommended welding procedures for shielding gas flow rates and purge cycles.
As with any welding process on any material, following some basic guidelines is critical to obtaining the best results. Aluminum’s mechanical and chemical composition can make the process a bit tricky. Always follow best practices for cleaning and storing the material and filler metals, and carefully select the right equipment. After all, having everything in order before welding is easier than trying to fix problems later.
Senior Welding Engineer
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Troy, OH 45373