Welding aluminum is very different from welding steel
As those in the welding industry know, welding aluminum is very different from welding steel. Aluminum presents several unique challenges in welding applications — whether manual or robotic — due to the material’s high thermal and electrical conductivity and the low melting point relative to its oxide layer. These characteristics can lead to burn-through, warping or lack of fusion that ultimately compromise the final weld’s integrity or slow production due to the need for rework.
Since aluminum is such an exceptional thermal conductor, it pulls heat away from the weld during the welding process. This means the energy from the electrical arc that would normally create the weld is drawn elsewhere, potentially resulting in a cold, lumpy weld. Those in the industry often call this phenomenon a “heat sink.”
Welding through the aluminum oxide layer that forms on the outside of the base metal is also a challenge. It requires more heat to melt, but this oxide layer also has a much higher melting point than the base metal below, and can lead to frustrating burn-through.
For these reasons, adjustments must be made when welding aluminum versus steel. For example, it is important to reduce the weld joint gaps in an aluminum compared to steel. When joining two pieces of steel, the weld joint gap should be no more than half the thickness of the thinner member. However, when joining two pieces of aluminum, the rule of thumb is that the weld joint gap should be no more than a quarter (25 percent) of the thickness of the thinner piece.
There are even more adjustments and special considerations when welding aluminum with an automated welding system. For instance, seam tracking the welding joint with a thru-the-arc seam tracking system is not possible. Due to aluminum’s high electrical conductivity, the amperage output does not change along with a change in electrical stickout. However, it may be possible to achieve the same goal by using touch sensing technology.
A word of caution: some touch sensing systems perform the touch sense at voltages ranging from 10 to 80 volts, which may not be high enough to penetrate the electrically-insulated oxide layer. There are, however, touch sensing systems commercially available that can touch sense in the range of 460 volts.
Remember, all touch sensing applications are not the same. Consult with a welding expert to understand what voltage level is recommended for the give application.
In addition to these considerations, automated welding system manufacturers have introduced advanced technologies that address the challenges of welding aluminum robotically. Four of these advancements, discussed in more detail below, are AC welding, slope command, synchro-weave and active wire.
An AC automated welding system is comprised of a normal DC+ welding power supply coupled with a DC- welding power supply to provide square wave AC output. The amount of time spent on the DC-side of the wave is adjustable, which allows for full control of the heat input.
With AC welding, the automated welding system can reduce the amount of heat input, and thus penetration, at any given amperage or wire feed speed, offering increased control over the weld pool and completed weld. This differs from the control offered with DC welding, in which there is a fixed penetration profile with a given amperage.
AC welding helps address gap conditions and burn-through when welding aluminum with an automated welding system, which is optimal for welds that must be leak-free, such as radiators and tanks or other containers. In these applications, it is important that there is a lot of wire to fill the joint, but it must occur without burn-through on the backside, which can result in the molten material entering the container and affecting its integrity.
Slope command allows the automated welding system to use more heat input during the beginning of the weld to combat the heat draining effects of aluminum. Normally, the aluminum base metal pulls heat away from the arc start, causing lack of penetration at the beginning of the weld. The slope command allows the automated welding system to start the arc at a higher amperage than the rest of the weld. This adjustment can be made based on time, distance or to and from programmed points. It is also good for gaining sufficient strength on an application without causing burn-through.
Slope command offers the automated welding system much more control than if the welding operator could only select one heat level. This is because an amperage setting that is hot enough to compensate for the initial “heat sink” may then become too hot a certain distance along the weld joint, when the aluminum is no longer drawing away so much heat from the weld. Similarly, if the amperage is low enough to ensure it will not ultimately lead to bubbling and burn-through, it may not offer enough heat to provide sufficient penetration at the beginning of the weld.
This function also allows the automated welding system to automatically slope the amperage control, as the name implies. The welding operator or robot programmer could, for example, tell the robot to start at 200 amps and slope down to 150 amps based on a pre-determined distance, the time it will weld or to the next point that is programmed. Then the robot will geometrically reduce the amperage from 200 down to 199, 198, 197, etc. until it fulfills the slope command as programmed.
Typically, it is best to slope the amperage along the first inch of the weld, depending on the thickness of the material and how much heat sink is occurring. After a small amount of welding, thermal conductivity is no longer a major concern since the heated aluminum doesn’t draw as much heat away from the weld.
Synchro-weave technology allows the robot in an automated welding system to weave across a joint and vary the amperage in sync with the weave. It was originally developed to weld materials of dissimilar thickness since the thicker piece would need more heat to penetrate the joint, while the thinner piece would call for less amperage and penetration.
However, this function is also ideal for welding together two pieces of aluminum in a simple fillet weld configuration. In this situation, the synchro-weave technology allows the automated welding system to put more heat into the side that displays a bi-directional heat sink and less heat into the side that displays a single-directional heat sink. The approach ensures proper penetration on both sides.
The synchro-weave function can also be used in conjunction with the slope command to offer even more precise control over the weld.
Traditionally, automated welding of aluminum has been done in the pulse or spray transfer mode to gain the best quality. When using the standard short-arc mode, the wire touches the puddle, creating too much spatter. Thus, thinner aluminum weldments were typically welded with the gas tungsten arc welding (GTAW) process.
Active wire or “reciprocating” wire feed technology is an alternative for automated welding of aluminum. In this process, the wire feeder reverses the wire after touching the weld puddle, allowing for welding in short-arc mode without the high level of spatter.
The main benefit is improved travel speeds, compared to manual and robotic GTAW. Active wire also offers low spatter arc starts, due to the nature of the wire reversing action, and can handle gaps better.
Conventionally, it would be necessary to use the electrical arc to burn off a certain amount of wire length and then keep feeding the wire into the puddle to let the cycle repeat. But active wire uses hardware to pull the wire back onto the puddle instead of using electricity to burn off the wire. Note, however, this technology requires a combination of a special robotic torch, wire feed-assist motor and software.
Automated welding systems, when employed properly and with the right advanced technologies, improve the efficiency of welding aluminum and can also improve safety.
Aluminum oxide fumes can be a serious hazard, as can arc flash — especially given the material’s high reflectivity. Using an automated welding system eliminates the direct exposure of weld fume from the welding operator and it is much easier to conceal the arc from the people in the surrounding area. It is still important take safety precautions. Use a fume hood over the robot so the fumes do not pollute the shop and be sure to have the proper screens around the robot to shield the arc.
Every application is different. Some aluminum parts are better suited for AC welding and synchro-weave, while others may benefit from active wire technology and slope command. For the best results, consult with a local automated welding expert to discuss the application and learn which technologies will provide the best chance for welding success.