Its Repair is challenging and its use is spreading. Aluminum is making new demands on skills and equipment at every shop.

By Bob Jansen, Contributing Editor

Selecting A GMA (MIG) Wire Feeder System A push feeder is similar to what is used for most steel GMA (MIG) welding. A variable speed motor pushes wire through the liner to the gun. With push feeders, the liner length must usually be limited to about 3 meters (10 ft.). Aluminum electrode wire tends to buckle and bind at the drive rolls when using long liners with a push feeder. This can cause birdnesting. A push feeder may not work well for feeding the small diameter electrode wires required for some exterior body panels, especially a soft electrode like 4043. An advantage of a push feeder is the small size of the gun for access to most areas. A push-pull feeder has two motors and two sets of drive rolls. One set of each is in the power source cabinet and the welding gun. This provides uniform feeding for small diameter electrode wire. Soft electrode wire, such as 4043, can also be used with no problems. A push-pull feeder can have a liner more than 9 m (30 ft.) long. A possible disadvantage is the complexity of this type of feeder system, sometimes requiring a separate feed cabinet. A spool gun feeding system is common on GMA (MIG) welding machines set up for aluminum. A spool gun has a small spool of electrode wire, usually 100 mm (4 in.) in diameter, attached directly to the welding gun. The spool weighs about .45 kg (1 lb). There is a motor and drive roll set and a short gun liner also contained on the gun. The short gun liner allows the feeding of 4043 electrode and the use of knurled drive rolls without a problem. There is virtually no limit to the length of cable from the gun to the power source, since the electrode wire is not being fed through it. Another advantage of a spool gun is that because the feeder system is contained in the gun, a GMA (MIG) welding machine can be set up for both steel and aluminum at the same time. One possible disadvantage is that a spool gun may have limited access to welding areas due to its size. Also, the small size of the spool gun means more frequent changes.

Store GTA (TIG) filler rods and GMA (MIG) electrode wire in airtight plastic bags to prevent moisture contamination. They should also be stored at room temperature.

Except on spool guns, knurled drive rolls are not used when welding aluminum as they can shave off the electrode wire creating flakes of aluminum oxide that collect in feed areas, causing arcing and binding in the contact tip.

This contact tip, stamped “A” for aluminum, means that the stamped hole diameter is less than the actual hole diameter. Aluminum electrodes expand more than steel, the larger hole prevents binding as the electrode heats up.

This sample butt joint shows two common problems with GMA (MIG) continuous welds, an end crater (left) and a cold start (right).

These GMA (MIG) fillet welds were made during original construction on this 2002 Honda Insight to weld a casting to sheet metal.

Some vehicles with aluminum structures use slot welds, or GMA (MIG) fillet welds, on the edge of slot holes for joints away from the panel edge.

If you haven’t been challenged yet with welding aluminum alloys, you soon will be. The list of vehicles with structural and non-structural aluminum parts increases each model year. Aluminum can be welded using a variety of processes, but the most common are Gas Metal Arc (GMA) or MIG welding and Gas Tungsten Arc (GTA) or TIG welding. The most recommended welding process for collision repair among vehicle makers is the GMA (MIG) welding process, though there are exceptions. Porsche, for example, shows the GTA (TIG) welding process exclusively in its aluminum body repair manual. Still, Porsche does not recommend against the GMA (MIG) welding process. The GTA (TIG) process is used most often to weld damaged cosmetic panels, such as hoods, fenders, doors, roofs, quarter panels and deck lids. Structural part welding is generally done using a GMA (MIG) welder. This article will discuss both the GMA (MIG) and GTA (TIG) processes for welding aluminum.

Aluminum vs. Steel
Repair processes used on aluminum are often compared to how these processes are used on steel, and welding is no exception. Differences in welding process techniques, variables and equipment are primarily due to the unique properties of aluminum. Compared to steel, aluminum is a much better heat and electrical conductor. This requires higher amperage and voltage settings than when welding the same thickness of steel. For GMA (MIG) welding thick structural parts, such as 6 mm (1¼4 in.) or thicker, a 208V or 220V machine is recommended. The heat expansion and contraction of aluminum is about twice that of steel. This can lead to distortion and more weld cracks than when welding on steel. Aluminum is also softer than steel, requiring differences in equipment accessories.

Aluminum Welding Preparation
There are some preparation steps when working with aluminum that apply to both welding processes. Because moisture is a problem with aluminum, it can be prevented by making sure the aluminum is at room temperature before welding. Store aluminum GMA (MIG) electrode wire in an airtight plastic bag when not in use. Store filler rods used for GTA (TIG) welding in a similar manner.

Before welding any metal, the metal must be clean; this is especially important with aluminum. Remove oil and dirt with a degreaser and solvent using a plastic woven pad, and then use a stainless steel brush to clean off the aluminum oxide. Aluminum oxide is a natural coating that forms on bare aluminum to protect the aluminum from corrosion. It’s important to remove the aluminum oxide before welding because of the differences in melting temperature between aluminum oxide and the aluminum alloy base metal. Aluminum oxide melts at about 2,050°C (3,725°F), while the base metal melts at about 650°C (1,220°F). So, if the aluminum oxide is not removed, penetration into the workpiece will be difficult. After wire brushing, avoid handling the surfaces to be joined with bare hands. This will contaminate the clean surface and may affect weld quality.

The stainless steel brush must be dedicated for use on aluminum. This also goes for any hand tools and abrasives that are used. Tools and abrasives used for steel may contaminate an aluminum surface. Collision repair facilities that do a lot of aluminum repair may have a separate part of the facility dedicated to aluminum. At the least, identify the tools used on aluminum with labels or write on the tool “for aluminum use only.” There are some abrasives that are made especially for working on aluminum. The Metal Shop article in the April issue, “Aluminum Vehicle Repair Methods,” will discuss more about keeping aluminum repairs separate from steel.

GMA (MIG) Welding Advantages
There are several advantages to using GMA (MIG) welding for aluminum. Most technicians are already familiar with the GMA (MIG) welding process because it is also the most recommended for collision repair welding of steel. Most GMA (MIG) welding equipment already found in collision repair facilities can be converted to weld aluminum, although there are more welding machines designed exclusively for aluminum available today than ever before. GMA (MIG) welding can be used on a wide variety of aluminum alloys and thicknesses and has a good production rate.

GMA (MIG) Welding Equipment Requirements
GMA (MIG) welding machines that are designed to weld aluminum have some notable differences when compared to GMA (MIG) welding machines designed for welding steel. One example is the electrode wire feeding system. There are push feeders, push/pull feeders, and spool guns available. (See the accompanying sidebar for considerations to keep in mind when selecting a wire feeding system for GMA (MIG) welding aluminum.)
Another difference with a GMA (MIG) welding machine designed to weld aluminum is the drive rolls. Instead of a “V” groove, the drive rolls usually have a “U” groove. The sharp edges of the “V” would shave off the soft aluminum electrode wire, resulting in flakes of aluminum oxide that collect in feed areas of the system. This can cause arcing and binding in the contact tip. For this same reason, knurled drive rolls are not used unless the electrode is being driven with a spool gun, which has an extremely short gun liner.

The inner surface of the gun liner is made of nylon, Teflon, or coated steel. These materials allow easy passage of the soft electrode wire. The helically wound steel liner used for welding steel would chafe the aluminum electrode wire, allowing aluminum and steel flakes to travel through the liner. The flaking can cause the gun to clog and contaminate the weld. For similar reasons, steel wire guides are also substituted with nylon or Teflon.

The contact tip uses a larger inside hole diameter than when welding steel. This is because aluminum electrode wire expands more than steel. The larger diameter hole prevents restriction of wire flow, which would result in inconsistent wire speed and possible birdnesting. If there are scratches on the electrode wire as it’s feeding, an unusual arcing behavior, or irregular feeding, the wrong contact tip may be the culprit. Some manufacturers oversize contact tips used for aluminum, so the number stamped on the tip may not be the actual hole diameter. The tip may also be stamped “A” for aluminum.

Straight shielding gas nozzles are generally used for welding aluminum. Straight nozzles are used to envelop larger bead and joint areas, and deliver more gas flow. Tapered nozzles may be used in areas with limited access.

Electrode wire for GMA (MIG) welding aluminum is classified in one of four different series, either 1000, 2000, 4000 or 5000, with each number identifying a specific type of filler alloy. Electrode wire commonly used for collision repair includes alloys in the 4000 series, usually 4043, or in the 5000 series, usually 5356. Closely matching the electrode wire to the base metal helps ensure good weld quality. The 5356 alloy electrode is close to a general, all-purpose electrode that will work with most series alloys. Recommended electrode wire diameters vary according to metal thickness, the amount of welding current, joint type, and vehicle maker recommendations. Electrode wire diameters common for collision repair include 0.8 mm (.030 in.), 0.9 mm (.035 in.) and 1.2 mm (.047 in.). Some wire diameters used in collision repair may be as small as 0.6 mm (.025 in.).

For shielding gas for GMA (MIG) welding aluminum, 100 percent argon is used. This makes aluminum GMA (MIG) welding a truly inert gas process. Pure argon provides a good cleaning action, a stable arc, and is easy to control on thin aluminum. The flow meter is generally adjusted to 25–50 cfh, slightly higher than the 25–30 cfh recommended for collision repair welding on steel. This range depends on the type of transfer method, thickness of the aluminum and the nozzle size used.
All of these equipment requirements add up to several modifications if a collision repair facility chooses to convert an existing GMA (MIG) welding machine to weld aluminum. Even if all of these changes are made, the converted welding machine may have relatively low amperage and voltage. This will allow for only short-circuit transfer and limited choice of electrode wire diameter. The GMA (MIG) welders used for steel that are found in most collision repair facilities usually have a push feeder, which also limits the choice of electrode wire diameter and alloy.

GMA (MIG) Welding Process Variables
With the right equipment and accessories on hand, there are some more differences in machine setup and process variables that must change when GMA (MIG) welding aluminum as compared to welding steel.

The following three electrode transfer methods are most often recommended for GMA (MIG) welding aluminum: short-circuit, spray-arc and pulsed spray-arc. Short-circuit transfer uses lower voltage than spray transfer. It also produces less penetration and small weld puddles. Spray-arc transfer uses higher voltage, current and wire speed than short-circuit transfer. This transfer method sprays a tiny stream of molten drops across the arc. The molten drops are much smaller than the diameter of the electrode wire, producing little or no spatter. A characteristic sound of spray-arc transfer is a steady humming when the welder is correctly adjusted. If the machine is capable of pulsed spray-arc transfer, this is usually the best option for thin-gauge aluminum. Pulsed spray-arc transfer pulses between low and high current power. Electrode wire is transferred only during the high current pulses. The total average current is lower than with nonpulsed-spray arc transfer. This means the chances of burnthrough are lessened on thin aluminum.

There are machines available today that feature digital control of the pulse transfer for welding aluminum. Voltage, wire sped and pulse parameters for specific welds can be programmed into the machine, sometimes by the vehicle maker.

GMA (MIG) welding aluminum works best using the push technique. The push technique is when the welding gun is pointed and pushed away from the weld puddle. This technique helps direct the shielding gas at the front of the weld puddle, providing an arc-cleaning action to remove aluminum oxide from the surface. The pull technique, commonly used when welding steel, is not used for aluminum because it increases the chance of porosity and a poor weld by not providing enough shielding gas and cleaning action.

The thickness of the aluminum being welded, along with the weld position, determine how much voltage and amperage are needed for a given weld. There is usually a chart for the particular machine under the machine cover.

Cold Starts, Craters, Cracking and Suck-Back
The unique properties of aluminum also cause some unique problems when GMA (MIG) welding, including at the beginning of a weld and at the end. The problem at the beginning of a weld is poor penetration, often called cold start. If the welder has a hot-start function, this can be avoided. A hot-start function automatically provides a surge of current and wire speed when the gun is first triggered. If there’s no hot-start function on the machine, it’s necessary to compensate for cold start. One method is to start the weld off the joint and move into the joint after the arc is established, sometimes called tailing in. This can be used for any weld. Another method to compensate for a cold start is to slow down the travel speed or momentarily pause at the start of the weld. The difference in travel speed need only be slight, just enough to prevent a cold start at the beginning, and burnthrough in the middle, of the weld.

Lack of heat may be the problem at the beginning of the weld, but too much heat is a problem at the end, especially on continuous welds. Too much heat can cause craters. Some machines have a feature that automatically ramps down the current and wire speed when the trigger is released, cooling the weld puddle and avoiding crater formation. If there’s no such feature on the machine, welding technique must be used to prevent craters.

One way to prevent craters is to increase the weld speed at the end of the weld and reverse direction, filling the crater before stopping the arc. Another method is to stop the welding arc and pause to allow the weld puddle to solidify. Keep the gun in position close to the weld, and keep the gun triggered halfway to keep the gas flowing. Then, briefly trigger the gun to fill the crater.

A repair method of avoiding both cold starts and craters when aluminum GMA (MIG) welding is to tack-weld small aluminum tabs at the start and stop points. These run-on and run-off tabs may also be an extension of the insert backing when doing a butt joint with backing. These are used as areas for the weld to be started and stopped, to keep the cold start area or crater of the weld from being made on the panel. Run-on and run-off tabs are removed after welding.
Due to cold start and crater problems, stitch and skip welding techniques are usually not used when GMA (MIG) welding on aluminum, unless the welding machine has a hot-start function. Triggering the gun on and off also stops and starts the shielding gas flow. Besides creating problems with cold starts and craters, stitch and skip welding can result in weld porosity and crater cracking.

On the subject of cracking, cracks in the crater and other weld cracks are more common with aluminum than steel. This is because of the greater contraction that occurs when the weld metal solidifies. Cracks can also occur if there is contaminated aluminum, electrode wire or shielding gas, if the wrong alloy electrode wire is being used or if there is improper cooling due to low welding speeds or inadequate penetration, trapping internal stress. The use of dye penetrant is necessary to look for cracking in welds, since the cracks may not be evident. The Metal Shop article in the April issue, “Aluminum Vehicle Repair Methods,” will discuss more about the use of dye penetrant, since cracking is also an issue when straightening aluminum.

Suck-back, or draw-back, is when the weld bead shrinks back during cooling to form a crater on the backside of the weld. Suck-back is usually found on overhead welds when gravity pulls down the molten metal, and suck-back may be caused by excessive heat. Suck-back can result in cracks in the weld bead or base metal.

To avoid suck-back, increase travel speed or reduce the voltage and current settings on the machine. On butt joints, reduce the root gap during fitup.

Types of GMA (MIG) Welds
The types of aluminum GMA (MIG) welds used for collision repair are about the same as for steel: plug welds, fillet welds and butt joint with backing welds. V-grooves are usually used when making butt joints on material that is thicker than 3 mm (1¼8 in.). The angle of the V-groove is usually about 60 degrees between the two mating panel edges. There are generally more fillet welds on aluminum vehicles than steel vehicles, both during repairs and original construction. Fillet welds are used to weld extrusions to sheet metal, extrusions to castings and where sheet metal is welded to castings. GMA (MIG) fillet welds are also used on some vehicles to join panels away from a mating flange. A slotted hole is made in one of the pieces to be joined, then a fillet weld is made on the edge of the slot. Repairs are done in a similar manner, though the slots are usually pre-cut in the replacement panel.

If you currently GMA (MIG) weld aluminum on a daily basis, or plan to in the future, consider enrolling in the I-CAR aluminum welding programs that culminate in the I-CAR Automotive Aluminum GMA (MIG) Welding Qualification Test. The test is performed on 5000 and 6000 series alloy aluminum coupons in two thicknesses. A butt joint with backing weld, fillet weld, and plug weld are required to be made in both the vertical and overhead positions. (Read more about part one of the I-CAR Aluminum Welding Qualification Test in the Training section on page 54).

GTA (TIG) Welding Process
GTA (TIG) welding is certainly not a new process, but it’s gaining more attention in the collision repair industry for use on aluminum. GTA (TIG) welding uses an arc between a non-consumable tungsten electrode and the weld puddle on the workpiece, and 100 percent argon shielding gas is used. The use of a filler rod is an option, but is almost always used on aluminum due to aluminum’s unique properties. The simplest comparison is to oxyacetylene welding. Think of the tungsten electrode as the oxyacetylene flame. Besides being very similar in basic concept, the two welding processes require similar hand skills. The GTA (TIG) torch holds the tungsten electrode and directs the shielding gas into the weld site. The tungsten is the conductor that carries the amperage, establishing an arc at the workpiece, and producing a weld puddle.

GTA (TIG) has many advantages that make it desirable for repairs to specific areas of aluminum vehicles and, with proper training, allow for consistent results. The weld is spatter-free and there is precise control of the welding variables. GTA (TIG) welding also has limitations and possible problems that can occur. It is a slower process than GMA (MIG). Technician coordination and dexterity are required and must be learned. It’s difficult to shield the weld zone in drafty conditions. Also, contamination of the weld site can easily occur if the gas shield is inadequate or the tungsten electrode gets caught in the weld puddle.
GTA (TIG) Welding Variables

The main adjustment on a GTA welding machine is the amperage, which has a direct affect on penetration. Most of the machines used for collision repair welding on aluminum have a range of approximately 8 to180 amps. The peak amperage is set on the welding machine. The operator is then able to control actual weld amperage up to the peak setting with a remote control, such as a foot pedal or fingertip knob. Each machine has a specified amperage range and duty cycle rating.

These are the criteria that must be considered when purchasing a unit.

Although not usually adjustable on the welding machine, voltage can also be increased or decreased based on the amount of amperage being used, the shape of the tungsten electrode and the arc length or distance from the end of the tungsten electrode to the workpiece. Higher voltage is achieved by using a larger, more rounded electrode and by decreasing the arc length.

GTA (TIG) welding machines operate in three different polarity modes: Direct Current Electrode Negative (DCEN), Direct Current Electrode Positive (DCEP) and Alternating Current (AC). For most collision repair welding, the welding machine is set to AC. Continuous high frequency (HF) is required when AC welding aluminum to help initiate a non-touch start and help produce a stable arc. HF is very high voltage and very low amperage. HF used for GTA (TIG) welding is very close to the frequency of radio waves. As a result, HF noise can cause interference and even harm to some computers, telephone lines, communication systems and other electronic devices. It is recommended to remove parts from the vehicle before GTA (TIG) welding whenever possible. If they can’t be removed, the battery should be disconnected and any nearby computers removed.

In the mid-1970s, welders were produced that allowed the AC to change from positive to negative in a square wave pattern rather than a conventional wave pattern. With square wave AC, maximum cleaning or penetration can both occur. These machines adjust the percentage of each cycle that is positive and negative, allowing fine-tuning to favor the cleaning action or penetration of the weld puddle. Other machines are now available that allow for the AC output frequency, or number of times per second that amperage changes from positive to negative, to be adjusted. Machines that allow for this added control are called inverters.

Inverters use sophisticated electronic devices, instead of transformers, to produce welding voltage and amperage. Inverters also eliminate the need for continuous high frequency because they produce true square wave AC and change polarity so fast that no additional stabilization is needed.

GTA (TIG) Welding Techniques
With the GTA (TIG) welding process, the technician uses torch speed along with torch angle as two variables to help determine the amount of penetration. Travel speed and technique also help determine the welding bead width. The torch head is angled, but flexible heads are also available to get the proper torch-to-work angle.

Filler rod is almost always used when GTA (TIG) welding aluminum. The chances of aluminum weld cracking are great when no filler material is used in the process. The most common aluminum alloy rods for use as filler metal when welding on vehicles are 5356 and 4043, the same series that is used most often for GMA (MIG) welding aluminum. The filler rods come in different diameters and chemical makeup. Generally, the rods are 900 mm (36 in.) long and cut to convenient sizes to be able to manipulate. The rod is dipped into the leading edge of the weld puddle and removed once some material is deposited. This procedure is very similar to using a filler rod when oxyactelyene welding. Selection of the filler rod material is done by identifying the alloy of aluminum or metal being welded and using a reference guide for rod selection. The diameter of the rod is first selected according to the base metal thickness and is increased as the amperage setting increases.

The tungsten electrode type and size are selected using reference guides based on the type of material to be welded, type of current and peak current settings.

Electrodes are color coded for identification. Pure tungsten electrodes are used for aluminum welding with conventional equipment and are color-coded green. If an inverter power supply is used, thorium (red), cerium (orange), or lanthanum (gray) are used. Cerium and lanthanum are becoming more common because a radioactive element is added to thorium to improve arc stability. These electrodes have to be grinded before use, so special safety procedures have to be used when grinding thorium rods. The shape of the electrode tip is also important with GTA (TIG) welding.

Preparing for aluminum
With the increased use of aluminum in today’s vehicles, every collision repair facility should be prepared for repairing aluminum. This includes welding. The familiar GMA (MIG) process can be used, but it requires different equipment setups or new equipment as well as new techniques. GTA (TIG) welding is also gaining attention for welding external panels, but requires even more investment in equipment and practice. m

This article was submitted by I-CAR, the Inter-Industry Conference on Auto Collision Repair. I-CAR is a collision repair training organization dedicated to improving the quality and safety of collision repairs for the ultimate benefit of consumers.