Vehicle makers recommend MIG welding processes for today's aluminum-intensive vehicles, but repairers need to know how aluminum MIG welding differs from welding on steel.
The thought of welding aluminum is intimidating for many of us in the collision repair industry. Part of this may be that when we think of welding aluminum many of us think of the TIG welding process. The TIG process is one that many of us have not used, and that requires a higher degree of skill than the MIG welding process that we have used for years to repair vehicle bodies and structures.
The good news is that for welding on the aluminum-intensive vehicles that have been introduced in recent years, the vehicle makers recommend that we use the MIG welding process and actually recommend against using the TIG process.
While this means that welding on an aluminum-intensive vehicle doesn't require learning an entirely new process, it doesn't mean that we can just MIG weld on this material the same way we have been welding steel vehicles. Aluminum MIG welding is not hard, but it is different than MIG welding on steel. Think of snow skiing and water skiing. They are similar, and one is not really more difficult than the other, but if you try to use water skiing techniques while snow skiing you will not stay up on the skis for very long.
It's the same with aluminum MIG welding. It's not really harder than steel welding, but if you use some of the same techniques as when MIG welding steel and you won't be successful MIG welding aluminum.
Thermal and electrical conductivity
Aluminum as a metal differs from steel in some ways, greatly affecting how we weld it. Aluminum has about three times the thermal conductivity and four times the electrical conductivity of steel. The result of this is that even though the melting temperature of aluminum is less than half that of steel, about 600 degrees Celsius (1,220 degrees Fahrenheit), it requires much more amperage to weld aluminum than it does steel of the same thickness. Because of this, aluminum used on vehicle structures and body panels is typically MIG-welded using the spray-arc transfer method and not the short-circuit transfer method typically used on the thin steel of vehicle structures.The typical recommendation from the vehicle makers that build aluminum-intensive vehicles is to use the pulse spray-arc transfer method (see sidebar).
The high thermal conductivity also means that it is more difficult to get a weld puddle established at the beginning of a weld. The heat being put into the metal is rapidly pulled away from the weld and into the surrounding aluminum, causing what is referred to as a "cold start" condition. During the cold start the weld bead sits on top of the base metal instead of melting into it, leading to poor penetration and fusion at the beginning of the weld (see Figure 1).There are a number of techniques that can be used to overcome this. One way is to simply stay in one place or move very slowly at the beginning of the weld until the aluminum heats up enough to allow the weld puddle to establish itself. After the puddle is established the welding gun can be moved along the joint to make the weld. This technique helps restrict the weld material that has not penetrated to a blob at the beginning of the weld bead.
Some welding machines have a hot start feature to minimize the cold start. The hot start feature puts more amperage and voltage into the weld at the beginning, which translates to more heat. If a long weld is being made with a series of stitch welds, the cold start portion of the weld should be ground down before another connecting weld bead is made over the top of it.
Another technique for dealing with cold start is called tailing-in. When tailing-in, the weld bead is started off of the joint to give time for the puddle to be established (see Figure 2). The cold start is off of the weld joint where it has no effect. This technique helps ensure that the weld bead penetrates and has good fusion over the entire joint.
Another technique similar to tailing-in is the use of a run-on tab. A small tab is tack-welded to the beginning of the joint before welding. The weld bead is started on the tab, which is later cut off. This technique places the cold start on the tab that is removed and helps ensure that the weld bead has penetration along the entire length of the joint (see Figure 3).
Expansion/contraction rate
Aluminum also expands twice as much as steel when heated, and its contraction rate is one-and-a-half times that of steel when cooled. This means that when too much heat is used, the aluminum weld has a higher probability of cracking from internal stresses than a steel weld. The window between not enough heat and too much heat is much narrower with aluminum MIG welding compared to steel MIG welding. Dye penetrant can be used to test welds for cracks if suspected (see Figure 4).
The other issue created by this increased expansion rate involves the contact tip on the welding gun. As the electrode wire heats up and expands, the wrong contact tip can cause sticking and birdnesting. Aluminum contact tips are typically slightly oversized to compensate for the increase in expansion. Tips may be marked with an "A" or "AL" to indicate that they are designed for aluminum wire. If the tip is not designated as an aluminum tip, the next largest tip should be used.The high expansion/contraction rate of aluminum causes the weld bead to shrink and form a crater at the end when the wire feed and current are suddenly cut off. This crater forms a weak spot in the weld bead that can lead to crack formation. As in a piece of glass, once a crack forms in the weld bead it may run for the entire length. To avoid this, the crater at the end of the weld must be avoided or filled. One technique for avoiding craters is to increase the travel speed at the end of the weld and then backtrack onto the bead to form a buildup at the end of the weld bead.
Another way is to stop the wire feed at the end of the weld, pause while keeping the shielding gas flowing to cool the weld, then re-strike the arc to fill the crater. Some welding machines may have a crater fill feature that helps avoid the crater at the end. This feature typically works by gradually decreasing the amperage and voltage before the arc is extinguished at the end of the weld. This creates a small ball at the end of the weld where the crater would have been. Another technique that can be used, where the joint design allows, is to tack weld a run-off tab onto the aluminum. The weld is continued off the joint and on to the run-off tab where it is stopped. The crater is on the run-off tab, which will be cut off and discarded.
Oxide formation
Bare aluminum forms an oxide layer when it is exposed to the atmosphere. This oxide layer starts forming instantly and becomes thicker the longer the aluminum is exposed to the elements. The oxide layer forms as a barrier to protect the base metal. This oxide layer creates some issues when welding. One issue is that the oxide melts at about 2,050 degrees Celsius (3,725 degrees Fahrenheit). This is three times the melting temperature of the aluminum, so a thick oxide layer would use up much of the heat being put into the weld in order to burn through it. The melting oxide would also contaminate the weld bead, causing porosity that could result in a weak weld.In order to avoid this, it is very important that the oxide layer be removed from aluminum before it is welded. After cleaning off other contaminants, such as undercoat and seam sealers, and degreasing with a solvent, ways of removing the oxide layer from bare aluminum include brushing with a stainless steel wire brush, fiber pad, sandpaper or a plastic surface prep disc (see Figure 5). Make sure to clean both the front and back sides of panels being welded, as any contaminants on the backside will be sucked into the weld bead and may cause porosity.
Consumables
Most MIG welding on steel vehicles is done with one alloy of electrode wire, ER70S-6. Aluminum MIG welding may require a variety of electrode wire alloys. There are many different alloys of aluminum, each having different strengths and characteristics. The two main aluminum alloy series used for vehicle construction are the 5000 and 6000 series families, and the electrode wire must be compatible (see Figure 6). The two most common alloys of aluminum electrode wire used for collision repair are 4000 series and 5000 series alloys. Some vehicle makers may have specific recommendations in their collision repair information for the exact electrode wire alloy to use.Because of the increased amperage and voltage requirements for welding aluminum, the electrode wire is larger diameter than what is typically used when MIG welding steel vehicles. Common diameters of aluminum electrode wire used for collision repair go from 0.8 mm (.030) to 1.2 mm (.047). The larger diameter wire also helps avoid wire feeding problems due to the softer characteristic of aluminum wire compared to steel wire. Aluminum electrode wire, like all aluminum, forms an oxide layer that can cause weld porosity when exposed to the elements. Because of this, the wire should be stored in a sealed plastic bag when not in use.
There are also some differences with the shielding gas used for aluminum MIG welding. One-hundred-percent argon shielding gas is typically used instead of the 75 percent argon/25 percent CO2 mix used for steel welding. The increased amperage and voltage used for aluminum welding requires the shielding gas flow rate to be set higher than for steel welding. Because of this increased flow rate, larger straight shielding gas nozzles are used (see Figure 7). A good starting point for shielding gas flow rates for aluminum MIG welding for collision repair is 25-30 CFH. Be aware that too much gas flow can also cause porosity in the weld. If the gas flow rate is too high, the pressure of the gas stream may cause turbulence and pull atmospheric air into the weld zone.Welding variables
There are also some differences with MIG welding techniques when welding aluminum compared to welding steel. When MIG welding steel, the push or pull technique is used. When welding aluminum, however, we have to push the weld every time in order to be successful (see Figure 8). Using the push technique helps to direct the shielding gas to the front of the weld puddle, and provides an arc cleaning action to remove the oxide film that has started forming on the surface after it is cleaned.Welding gun-to-workpiece distance is another area where aluminum MIG welding differs from steel MIG welding. MIG welding aluminum requires a longer gun-to-workpiece distance than typically used when welding steel. MIG welding the thickness of aluminum used for vehicles requires an electrode stick-out distance of 10–16 mm (3/8–5/8 inches). This is because the higher amperages and voltages used to weld aluminum typically require a long arc length. The arc length is what controls how close to the workpiece surface the electrode wire burns. The arc is shaped like a triangle, so the longer it is the wider it also is (see Figure 10).
If the welding gun is positioned too close to the workpiece, the arc length being used will be shorter than what is intended, and the arc may burn back up into the electrode tip. Pulse MIG welding machines typically have independent adjustments for the arc length. The arc length can be used to fine-tune the weld. Increasing the arc length will create a wider weld bead with less penetration, while decreasing the arc length will create a narrower weld that is driven deeper into the workpiece. The other advantage of a longer gun-to-workpiece distance is that it helps make the joint more visible as the weld is being made.Travel speed may also require some changes when MIG welding aluminum compared to welding steel. Because of the cold start issue, travel speed at the beginning of the weld is typically slower. However, as the weld is being made and the aluminum gets progressively hotter ahead of the weld, travel speed may need to be increased. This is because the hotter the aluminum is before it is welded, the greater the penetration will be at a specific welding machine setting. Joint position will have an effect on this, as a weld made vertically up will preheat the aluminum ahead of the weld faster than a weld made horizontally or in the flat position.
Compared to steel welding, machine settings are also more critical when MIG welding aluminum. The window in which a good weld can be made is typically narrower. Often when welding steel the same settings can be used for different joint types and welding positions on the vehicle. This may not hold true for MIG welding aluminum. Different welding machine settings will typically be required for different joint types, even on the same thickness of material. Use the chart on the welding machine, or information in the repair manual as a starting point for settings. Then make and destructively test practice welds, and fine-tune the settings to your welding style in order to make a good weld.
It is easy to look at a steel MIG weld and tell if it is a good weld or not by appearance. This does not hold true of an aluminum MIG weld. An aluminum MIG weld may have a nice consistent weld bead on the front side and evidence of penetration on the back side and still not pass a destructive test. Therefore before MIG welding aluminum on a vehicle, it is critical to make practice welds on the same material in the same position that will be welded on the vehicle. The practice welds must then be destructively tested to ensure that the welding machine settings and welding technique being used will produce a quality weld on the vehicle (see Figures 11 and 12).Conclusion
MIG welding aluminum is not necessarily harder than MIG welding steel, it's just different. It's also less forgiving, so pay close attention to your technique and the welding machine settings. By using the right consumables and welding technique, anyone who can make a good steel MIG weld can make a good aluminum MIG weld.
Remember to keep the welding gun back the proper distance, always push the weld and eliminate the cold start at the beginning and the crater at the end. Make practice welds and destructively test the practice welds. Adjust the arc length and amperage or wire speed to tune the welding machine for a good weld if needed before welding on the vehicle.
While the thought of welding aluminum may be intimidating for some collision repairers, with the proper materials and procedures, this process can be a successful and profitable function.
