Learning to make a sound metal-inert-gas (MIG) weld in aluminum can be almost as satisfying as winning a footrace. As with training for a race, it requires studying proper techniques and a lot of practice. Although you may never win a race, with a little dedication and knowledge, you can learn to weld aluminum just as easily as steel.
In its pure form, aluminum is a relatively soft metal that has many uses, but it requires the addition of an alloy metal to increase its strength and to add other qualities suitable for different applications. Common alloys include copper, magnesium, silicon, manganese and zinc (see chart, above right).
Aluminum’s inherent properties create welding challenges. First, it conducts heat about six times better than steel, so it requires higher amperages and faster wire-feed speeds than steel. Yet aluminum has a relatively low melting point (1,220 F), which means the wire can burn back to the contact tip quite easily under less-than-optimum arc-starting conditions. Good thermal conductivity coupled with a low melting point also makes thinner aluminum sections more susceptible to warping and burn-through.
Equipment requirements
Aluminum MIG welding uses a process called “spray transfer” to deliver the material from the filler wire to the workpiece. Spray transfer provides excellent fusion and bead appearance with little or no spatter. However, it requires more welding output (power) than a 115-volt MIG welder can produce. For home fabrication, you’ll need a MIG unit that uses 200- to 230-volt input and has an output of at least 200 to 250 amps.
With the MIG process, you’re limited to working on 1/8-in. or thicker aluminum stock — for thinner material, you’re better served by the tungsten inert gas (TIG) welding process (also referred to as gas tungsten arc welding, or GTAW). The TIG process uses a nonconsumable tungsten electrode and an argon shielding gas to create temperatures as high as 35,000 F for highly directed, precisely controlled welds. (Expect to see more about the TIG process in a future issue of HANDY.)
In addition to the higher power requirements, you’ll need these components to get the best results:
• Shielding gas — When working with steel, an argon blend is usually used as a shielding gas. But because aluminum reacts readily to air to create aluminum oxide, use only pure argon as a shielding gas.
• Welding wire — Like aluminum stock, aluminum filler wire is rated according to the alloys used in its composition. Although no aluminum filler wire fits all needs, 4043 and 5356 are the most common choices. The most widely used wire is probably 5356; 4043 is another favorite because its silicon alloy increases ease of welding and enhances puddle control and appearance.
• Guns — Achieving good results with aluminum largely depends on the gun. Using a MIG welder’s traditional arrangement, where the wire is pushed about 12 ft. before it reaches the end of the gun, can create problems. Because of aluminum wire’s low columnar strength, any significant bend in the gun cable causes the soft wire to buckle, which may lead to a tangle of wire at the drive rolls. Fortunately, a spool gun (see photo, p. 50) can address the special needs of feeding aluminum wire.
Spool guns have an empty weight of about 1 pound and accommodate a 4-in.-dia. spool of wire. A spool gun’s drive rolls feed the wire about 8 in. rather than forcing it through the 8 to 15 ft. of cable required with a regular MIG gun.
Spool guns typically come in three classes that match power sources rated at either 150 amps/60 percent duty cycle, 200 amps/60 percent duty cycle or 200 amps/100 percent duty cycle. Lighter-duty spool guns run 0.030- to 0.047-in. wire, whereas heavier-duty guns can run wire as thick as 1/16 in.
If you have no other choice than to use a regular MIG gun, you’ll need to install a Teflon liner that minimizes abrasion to the filler wire as it is pushed through the hose and out of the gun.
• Drive wheel — A standard MIG-wire drive wheel is milled with a V-shape groove that, though very good at delivering steel wire, tends to deform the softer aluminum wire. If you’re forced to use a standard gun, switch to a drive wheel with a U-shape profile that will cradle the wire rather than crush it.
Understanding gun angles
For MIG welding aluminum, you must use the push technique (also called the forehand or leading technique), which involves pushing the gun away from (ahead of) the weld puddle. The push technique produces lower penetration that results in less burn-through, creates a wider, flatter bead because the arc force is directed away from the weld puddle and ensures good gas coverage of the weld puddle.
In addition to using the push technique, it’s important to hold the gun at a 5- to 15-degree angle. The tip of the gun should be slightly canted backward as you push the weld puddle forward (see photo, above). This angle, also known as the travel angle, should never exceed 25 degrees — doing so will result in more spatter and less penetration.
Making the weld
When exposed to air, aluminum almost immediately forms an oxide layer on its surface. Although this oxide layer protects the base metal from corrosion, it also impedes arc starts. And because aluminum oxide melts at a temperature about 2,400 degrees hotter than the actual aluminum, the arc has to work harder to drive through the oxides. To get the best welds, prepare the workpiece by using a stainless steel brush to vigorously brush away the oxides.
Make sure the MIG unit is set to the correct polarity. For working with aluminum, configure the polarity as electrode positive (also known as DC reverse polarity); then set the gas flow to a rate of 25 to 30 cubic feet per hour. Higher gas-flow rates will not result in better welds, as the sheer force of the gas being dispelled from the gun tip is strong enough to create a venturi effect that pulls surrounding air into the weld, resulting in poor weld quality.
Set the voltage to match the thickness of the aluminum. (Many models feature charts that indicate approximate starting volts and amps.) If you’re unsure of the correct setting, set the voltage high; you can always lower it. Low voltage may create a great-looking weld, but it will pile up filler wire rather than create good fusion.
Next, adjust the wire-feed speed. A harsh sound with a lot of pops indicates that the voltage is too high and is melting the wire faster than it comes out. In this case, try increasing the wire-feed speed, but not to the point where you have too much wire protruding from the end of the gun (referred as “stickout”). Try to maintain a stickout of 3/8 in., and in between welds, snip off the ball that forms on the end of the wire.
Travel speed (the rate at which you move the gun along the joint) is very important when working with aluminum, and in many cases, you’ll need to work quickly. As you work, keep the arc on the leading edge of the puddle; don’t let the molten metal get ahead of you. A good weld will have a shiny “stacked dime” look, and you’ll notice a slight discoloration on the edges, which indicates that you’ve achieved proper penetration (see photo, left).
Welding aluminum using the MIG process is challenging, but it does not have to be a headache. By using the right equipment and practicing your technique, working with aluminum can become second nature.
Troubleshooting
Even when you use the proper settings and equipment, achieving good welds with aluminum can be tricky. To avoid the most common welding problems, try these pointers:
• The biggest cause of spatter is a wire-feed speed that doesn’t match the voltage. A properly tuned MIG welder should produce minimal spatter. If spatter occurs, adjust the
wire-feed speed.
• A contact tip should be replaced after about 8 hours of welding, as the tip hole becomes oblong rather than round. Failure to do so will result in poor power delivery.
• Change or clean the gun liner every time you’re done with a roll of wire. Remove the gun from the machine, take off the contact tip and blow out the liner with compressed air.
• Burn-back is when the wire melts back into the contact tip. It can be caused by a drive roll system that’s not tight enough, a wire that’s not properly seated in a drive roll (which causes slippage) or a piece of spatter in the contact tip. Most often, the voltage is set too high for the wire-feed speed, and the voltage burns the wire away faster than it’s being fed.
• High weld crowns often indicate insufficient penetration caused by too slow a travel speed, excess wire-feed speed or insufficient voltage (the usual culprit).
Club member Chris Roehl is product manager for Miller Electric Manufacturing Co. in Appleton, Wisconson.
Sources
Hobart Welders
800-332-3281
Lincoln Electric
888-355-3213
Miller Electric
800-426-4553