Managing aluminum welds
- August 12, 2013
Assessing when and how to repair aluminum MIG weld defects
by Mike Vandenberg
Due to its low melting point and the presence of an oxide layer on its surface, aluminum can be relatively tricky to weld. It is particularly prone to burn-through and distortion, not to mention it can have a difficult-to-control weld pool.
Add in the factors of welding an aluminum part that has already been in service and it complicates the process even further. These parts often have dirty surface conditions, feature potentially hard-to-reach weld joints or, in some cases, present a conflict as to whether the part is weldable and able to be repaired at all.
Before approaching any aluminum welding repair, it's important to assess the condition of the part, determine which welding process is most appropriate to use and employ the proper cleaning and welding techniques.
Can it be repaired? And how?
The first question to ask when assessing an aluminum-welding repair is: "Can it be repaired?"
Sometimes a part is simply beyond repair. It may have multiple fractures, cracks or holes that make it impractical to try to salvage. In other cases, the proper tools may not be available to make the repair or it may just be too time-consuming to fix. Before proceeding, assess whether it will cost more in labor to make the repair than to replace it with a new part.
For parts that have only small cracks or ones that can be quickly and easily repaired, it makes sense to continue with the repair process. Expensive parts are also good candidates for repair as opposed to replacement due to the cost factor. After deciding whether the part can be repaired, it's important to determine which welding process to use to complete the job.
If the part in question is under 1/8-in. thick, the TIG process will be the best
choice to help minimize heat input, and with it, the opportunity for burn-through or distortion. TIG welding is also a good choice for making repairs on cast aluminum parts or repairing parts with limited access, as a TIG torch is smaller and easier to maneuver into a constricted area than a MIG gun.
TIG welding, however, is a slow process. To expedite the repair process, MIG welding - especially using a Pulsed program - is an excellent means to make aluminum welding repairs. It offers the advantage of speed, precision and ease of training, not to mention low spatter levels.
Making repairs with Pulsed MIG
Like TIG welding, Pulsed MIG welding on aluminum repairs offers the advantage of lower heat input and greater control over the weld pool, compared to a conventional constant voltage (CV) MIG welding process.
Pulsed MIG welding is a modified spray transfer process, in which the power source switches between a high peak current and a low background current from 30 to 400 times per second. During this switch, the peak current pinches off a droplet of wire and propels it to the weld joint. At the same time, the background current maintains the arc but has such a low heat input that metal transfer can't occur.
Pulsed MIG welding offers distinct advantages during the aluminum repair process, namely:
- It can be used for welding thick and thin aluminum parts.
- It can be used in all positions, making it especially useful for repairing parts that cannot be removed from their service position.
- It minimizes burn-through and distortion on thinner parts.
- It offers high deposition rates and faster travel speeds to increase productivity.
- It offers good control over arc starts and stops, as well as over the weld pool.
Each of these characteristics can help complete repairs faster and more accurately.
Preparing aluminum parts for welding repairs
The most important steps in repairing aluminum are proper pre-cleaning and joint preparation. First, remove the part from service.
Next, implement the appropriate cleaning procedures. Aluminum is very sensitive to dirt, oil, grease and other contaminants, all of which can lead to defects like porosity. The opportunity for this defect is especially prevalent when an aluminum part has been in service and becomes damaged - contaminants can easily become embedded in cracks in the material.
Push-pull guns feature a motor inside that pulls the wire through the liner, while the feeder steadily feeds it. The combination offers precision wire feed speed and control, again lessening the chance for wire feeding issues. These guns are also more comfortable to use, since they do not carry the weight of a spool of wire on them.
Use an aluminum cleaner (ZeroTriÂ® cleaner /degreaser by LPS, for example) to remove any contaminants from the joint. Thoroughly wipe the joint with the cleaner using a soft, clean cloth.
Next, if you're working on a thicker section grind an angle (or bevel) through the damaged part. A V-groove will work well. If possible, grind out an identical V-groove on the opposite side, as welding on both sides adds strength and reduces the opportunity for the part to break further. If it is not possible to access both sides of a damaged piece, simply bevel a bit deeper into the part
Finally, use a stainless steel wire brush to remove any remaining dirt on the surface. The scrubbing action will also remove the oxide layer, making it easier to weld. A word of caution: make sure to designate the brush for this sole purpose. Using it on other applications or materials can introduce contaminants to the aluminum.
Equipment, technique and more
At a minimum, consider using a spool gun. These guns hold a small spool or wire - usually 1 pound or less and approximately 4 inches in diameter - directly on the MIG gun, minimizing the distance the wire has to travel to reach the weld pool and reducing the potential for tangles. A push-pull gun paired with a push-pull wire feeder is an even better option for making aluminum weld repairs. Push-pull guns feature a motor inside that pulls the wire through the liner, while the feeder steadily feeds it. The combination offers precision wire feed speed and control, again lessening the chance for wire feeding issues. These guns are also more comfortable to use, since they do not carry the weight of a spool of wire on them.In addition to a MIG welding power source that offers Pulsing capabilities, it's important to have the correct style of MIG gun for making aluminum welding repairs. Because aluminum wire is very soft, it cannot be fed through a conventional MIG gun; the wire can become easily tangled in the drive rolls of the wire feeder, a problem often referred to as bird-nesting.
Spool guns are one option for making aluminum welding repairs. These guns minimize the distance the wire has to travel to reach the weld pool, and therefore, reduce the potential for tangles.
Whichever gun is chosen, it's important to employ a "push" technique during the welding process instead of dragging the gun away from the weld pool. Too, when making aluminum weld repairs, flat is the ideal welding position, when possible - even though Pulsed MIG welding allows for all-position welding when necessary. It is easier to control the weld pool in the flat position and it offers greater comfort to the welding operator.
The amperage range, wire selection and wire feed speed, and shielding gas usage are also key to consider when making aluminum welding repairs.
Typically, aluminum welding repairs can take place in the range of 100 to 150 amps. Wire feed speed, however, can vary quite significantly - from 120 to 450 inches per minute (ipm) - depending on the part thickness.
To select the appropriate filler metal for the repair, it is necessary first to identify the type of aluminum being welded and determine the operating conditions the part will encounter. A filler metal selection chart is a good way to assess the characteristics of available wires, including their crack sensitivity, strength, ductility and corrosion resistance. It is also important to consider the service temperature, color match (after anodizing) and toughness requirements for the repair, along with any post-weld heat treatment that may be needed. All of these factors will contribute to the selection.
Most often American Welding Society (AWS) ER4043 or ER5356 wires will be the best choice. These wires typically operate with 100 percent argon shielding gas at an average gas flow rate of 30 to 35 cubic feet per hour (cfh).
Final thoughts on aluminum repairs
As with any welding process, following proper cleaning procedures, equipment usage and techniques are important parts of making successful aluminum repairs. Always employ best welding practices during the process and consult with a trusted welding distributor or an appropriate maintenance professional when in doubt about the course of action to take. Consider practicing on scrap aluminum to gain a stronger understanding of how the metal reacts to heat and various techniques before tackling the actual repair. Doing so can save time, money and a lot of frustration.
Mike Vandenber is product manager, industrial systems group, Miller Electric Mfg. Co.