Aluminum Workshop: Arc blow when welding aluminum

Q: Is it possible to encounter arc blow when welding aluminum? Since aluminum isn’t magnetic, I don’t think so, but my friend thinks it is. A cup of coffee is riding on your answer.

A: It looks like you’d better get ready to pay up. It is possible to encounter arc blow when welding aluminum. We usually think of arc blow occurring with magnetic materials such as carbon steels. In this case, the steel can be magnetized in many ways. For instance, the steel can be magnetized when you machine it with a magnetic chuck. Additionally, the flowing welding current can induce magnetic fields in the steel. To attack arc blow problems in steel, you often can change the location where the ground cable is attached to the piece and/or change the direction of welding.

Arc blow in aluminum welding is somewhat different, but the end result is the same. You’re right: aluminum alloys are nonmagnetic and can’t be made to become magnetic. So where does the arc blow come from?

Where Arc Blow Comes From

Let’s assume you’re using a steel fixture to hold the pieces of aluminum you’re welding. It’s possible that part or all of the fixture has become magnetic over time. Welding currents, especially high welding currents, can cause this to happen.

Now you load the aluminum pieces into the fixture and begin to weld. Suddenly the weld deflects sharply due to arc blow. Why? Arc welding uses a flowing current, and any flowing current produces a magnetic field. If another magnetic field is present in the area—for instance, from your magnetized fixture—the two magnetic fields will interact and deflect the arc, sometimes strongly.

It’s actually fairly common for robotic welding fixtures or other automated welding fixtures, such as a weld seamer, to become magnetic over time. So how do you know if this is your problem? First, purchase a small gaussmeter, which measures magnetic field strength. These commonly are used in magnetic particle testing. A number of sources for gaussmeters can be found on the Internet.

Scan the weld area with the meter. Any magnetic field over 20 gauss will cause some arc blow (by comparison, the Earth’s field is about 0.5 gauss). By the time the field strength reaches 50 gauss, welding will be very difficult.

Now that you know your fixture is magnetic, what can you do? Deguassing is part art, part science. It’s not impossible to do it yourself, but it can be difficult. Several companies specialize in degaussing, which you also can find on the Internet.

Q: What is the right contact tip size for using 3/64-inch filler wire to gas metal arc weld (GMAW) aluminum alloys? Some people use a tip one size larger (for example, a 1/16-in. tip on 3/64-in. wire). Others use the size the manufacturer specifies for 3/64-in. wire. To add to the confusion, I measured the diameter of the hole, in 3/64-in. contact tips from several manufacturers and found a large range of sizes.

A: First, remember that the main purpose of the contact tip is to transfer the welding current to the wire. To do that, the wire must make good electrical contact with the wire. The curve in the filler wire from the wire cast helps. It acts like a spring to keep the wire in contact with the contact tip, but if the hole in the tip is too large, you lose contact intermittently. If this happens, you’ll begin to see burnbacks and have to buy many contact tips. The same thing will happen if you use a wire straightener and remove all the cast from the wire.

On the other hand, if the hole is too small, you’ll also have feeding difficulties. You can’t feed a 3/64-in. (0.0468-in.) wire through a 3/64-in. hold. The hole diameter must be somewhat bigger than the wire diameter for good feeding. How much bigger? In my experience, the right hole diameter for feeding aluminum wire is 0.008 in. to 0.009 in. larger than the actual wire diameter. A clearance of 0.015 in. is too large.

You might want to take 10 contact tips or so and plug-gauge the hole diameter; you might be surprised at what you find. Most manufacturers do a good job of controlling the diameter, but if you see a lot of variation among tips, I recommend changing suppliers.

A popular misconception is that you need a larger contact tip for feeding 4043 than for feeding 5356 because 4043 expands more when it is heated. Actually, the coefficient of thermal expansion of 5356 is higher than that of 4043, but they’re close enough that you generally can use the same contact tip for either.