By Galen White, senior welding engineer, Hobart Brothers Company
Aluminum offers numerous benefits high strength-to-weight ratio, corrosion resistance, and high thermal and electrical conductivity that make it one of the most specified materials in the welding industry today. However, welding with aluminum presents some unique challenges, including a tenacious oxide layer; vastly different chemistries between alloys; the need to use a less rigid filler metal; and the need for very clean base metal. Selecting the right type of filler metal, consumables and equipment, as well as quick troubleshooting, can help to mitigate these issues. To help minimize downtime and enhance productivity, this article looks at some of the most common issues welding operators face when MIG welding aluminum and discusses tips to prevent those problems or address them quickly if they occur.
Aluminum is one of the most specified materials in the welding industry today, but welding with aluminum presents some unique challenges. Selecting the right type of filler metal, consumables and equipment, as well as quick troubleshooting, can help to mitigate these issues.
How to Avoid Hot Cracking
Cracking is one of the most critical issues that can occur in aluminum weldments. Even small cracks can prevent welds from meeting code requirements and can eventually lead to weld failure. The majority of aluminum base metals, however, can be successfully welded without cracking related problems. Using the most appropriate filler metal and completing the weld with the appropriate procedure is important to success.
While there are two types of cracking, hot cracking and stress cracking, hot cracking is the more common of the two in aluminum weldments. Hot cracking is mainly a function of chemistry, as in, how the filler metal solidifies.
There are three factors that can significantly influence the probability for hot cracking in aluminum welding:
- The susceptibility of the base material chemistry to cracking.
- The selection of the most appropriate filler metal to help prevent the formation of a crack-sensitive chemistry.
- Choosing the most appropriate joint design to dilute the base metal to help avoid a crack-sensitive chemistry in the weld.
Because particular aluminum alloys are more susceptible to cracking, it’s important to choose a filler metal that will result in a weld metal chemistry with lower crack sensitivity. When welding aluminum that has low crack sensitivity, always use a filler metal of similar chemistry. When welding aluminum that has high crack sensitivity, use a filler metal with a different chemistry to create a weld metal chemistry with low crack sensitivity.
Aluminum filler metals are identified by a numerical American Welding Society (AWS) classification that corresponds to the Aluminum Association registration number to identify the particular alloy chemistry. Not all filler metals are suitable for welding all aluminum base alloys. It’s a good idea to reference a reputable filler metal selection guide to make the best choice for welding different aluminum alloys.
Weld joint design is another important consideration that affects the chemistry of the weld and therefore can help prevent hot cracking. A weld joint with no bevel can result in inadequate base metal dilution, while a weld joint with an appropriate bevel results in adequate base metal dilution. This means a beveled edge can increase the amount of filler metal in the weld, producing a chemistry that is less likely to crack. With some aluminum base alloys, such as the 6xxx series, this is a very important factor that can strongly influence the probability for hot cracking.
Stress Cracking Issues
While not as common as hot cracking, stress cracking is a secondary cracking problem that can affect aluminum welds.
Excessive shrinkage rates during weld solidification and cooling is one cause of stress cracking. Choosing filler metal containing silicon, when appropriate, can lower shrinkage stresses to help avoid this type of cracking.
Probably the most common location of cracks in aluminum welds is in the crater, which often goes unnoticed. While they start off small, crater cracks can propagate throughout the weld and cause major failures if not addressed. The best practice is to completely fill the crater at the time the weld is produced using an automated crater fill function on the welding equipment or other approved methods of filling the crater.
Increased travel speed also can help reduce the probability of stress cracking, because it narrows the heat affected zone (HAZ) and reduces the amount the base metal melts.
Preheating may further help reduce residual stress levels of the base material during and after welding, which in turn will lower the probability of stress cracking. Avoid welding aluminum that is very cold, and avoid overheating during the preheating operation (150 degrees Fahrenheit is safe for all aluminum alloys). Overheating some base alloys, such as the 6xxx series, can result in lowering the base material tensile strength to unacceptable levels.
Cracking is one of the most critical issues that can occur in aluminum weldments, however, the majority of aluminum base metals can be successfully welded without cracking related problems. Using the most appropriate filler metal and completing the weld with the appropriate procedure is important to success
Weld Joint Porosity
While not as critical of an issue as cracking, porosity is perhaps the most common complaint when MIG welding aluminum. Porosity refers to the cavity-like discontinuities in the weld that are formed by gas entrapment during solidification.
Weld porosity primarily results from the absorption of hydrogen during melting and the expulsion of hydrogen during solidification of the weld pool. The sources of hydrogen that create porosity are:
- Hydrocarbons in the form of paint, oil, grease and other lubricants and contaminants.
- Hydrated aluminum oxide, aluminum oxide that has absorbed moisture can release hydrogen when subjected to heat during the welding operation.
- Moisture, which can come from the atmosphere (humidity) or from other sources such as compressed air, small leaks in water cooled torches, contaminated shielding gas or pre-cleaning operations.
The first step in solving this issue is to identify the source of hydrogen that is responsible for producing the porosity. Purchasing low dew point shielding gases (argon or argon/helium mixtures) helps reduce porosity, as does following the shielding gas flow rates and purge cycles recommended for the welding procedure and position being used.
Welding operators also should make sure to thoroughly clean base metals with a solvent and clean cloth or paper towel followed by stainless steel wire brushing prior to assembling the weld joint. (Typical shop rags are not clean enough for use on aluminum, as they contain residual hydrocarbons and can contribute to porosity).
It’s also important to ensure the base metal and filler metal are not wet with condensation. Bring the aluminum in from a cooler location (such as outside, for example) and allow it to sit in the welding area for 24 hours before welding. Storing unpackaged filler metals in a heated cabinet or room also helps prevent them from cycling through dew points, which avoids creating hydrated oxide on their surface.
Another key to avoiding porosity is to purchase high quality filler metals from reputable manufacturers. Such filler metals typically have been diamond shaved to eliminate harmful oxides, manufactured with procedures to produce low residual hydrogen containing compounds and weld tested to stringent AWS standards.
Preventing Erratic Wire Feeding and Arc Issues
There are a number of things welding operators can do to avoid erratic wire feeding and arc issues, starting with using a welding system and consumables designed specifically for aluminum.
It’s important to use the right kind of contact tips, with the size matched to the wire diameter being used. Also, make sure to use a contact tip designed for aluminum wire rather than a tip for steel wire, which can cause excessive burnbacks when used with aluminum.
The contact tips also should be recessed in the gas cup 1/8 to 1/4 inch for proper gas cooling of the tip and for spatter control. As the contact tip wears out, arc flaring can become a problem. To prevent this, replace contact tips as needed.
Selecting the correct type of drive roll is another important way to prevent wire-feeding issues. The majority of these problems are caused by aluminum wire shavings that originate from poor fitting and incorrectly designed drive rolls. The shavings can build up and start to clog the liners, restricting the free flow of the wire.
Welding operators should use a U-groove drive roll designed for aluminum, because a V-groove drive roll will compress the wire and deform it, causing erratic arc conditions.
Also, ensure both drive rolls are aligned and always use the lowest drive roll pressure capable of consistently feeding the wire. Performing regular maintenance and periodically replacing items that wear, including drive rolls, liners and inlet guides, also helps prevent wire-feeding issues.
Use a push-pull wire feeder or spool gun to promote optimum wire feedability. Most push-pull guns have two liners: a conduit liner, which typically lasts longer, and a head tube liner in the gun portion, which can wear out quicker. Periodic replacement of both liners is helpful, especially once clogging becomes an issue.
Minimizing Weld Discoloration and Smut
The introduction of oxygen into the shielding gas envelope via air, moisture and contaminants can increase the burning (oxidation) of the filler metal, which produces discoloration and smut. The use of certain filler metals also can contribute to this problem. While weld discoloration and smut look bad, they are easy issues to fix.
The 4xxx series filler metals produce less weld discoloration and smut than do the 5xxx series filler metals. That’s because the magnesium in 5xxx series alloys vaporizes in the arc and condenses as a black soot next to the weld bead. Most 4xxx series alloys have little or no magnesium, which reduces this problem.
Also, welding operators can minimize the air in the shielding gas by decreasing the gun angle, increasing the gas cup size, holding the gas cup closer to the base metal, cleaning spatter build-up from the gas cup and shielding the arc from drafts.
Welding operators who are more used to working with steel may be dragging their weld. Use a push angle instead, to put the arc cleaning action in front of the weld. This angle continually cleans off the weld and reduces smut.
Selecting quality filler metals is key. It’s a good idea to reference a reputable filler metal selection guide to make the best choice for welding different aluminum alloys.
Follow the Tips to Address Aluminum Issue
Using the correct equipment and filler metals and performing necessary maintenance on equipment and consumables are all keys to reducing the problems that can arise when welding aluminum.
Addressing these common concerns makes the job easier for welding operators, so they can reap the benefits that MIG welding aluminum provides: higher deposition rates, less operator training and higher productivity.