Burnback Wire Stubbing Porosity

David Almy, Welding Engineer -- Miller Electric Mfg. Co.

Q: I have gas flow from the gun, but not a clean weld. How can I solve this?

A: Porosity and black welds indicate poor gas coverage or contamination. To ensure good coverage, check for: proper gas flow (CFH), leaks in the system (loose fittings, cuts in the gas hose, worn O-rings) and excessive spatter in the nozzle. Make sure the contact tip is recessed about 1/8" in the nozzle. To ensure 100% coverage and prevent contamination, Miller designed a unique gas delivery system for its XR-Edge™ gooseneck gun. It even sends gas through the contact tip with the wire. This enhances the cleaning action and reduces porosity and post-weld clean-up.

Q: What causes aluminum wire to burn back to the contact tip?

A: 90% of "burnbacks" result from poor arc starts caused by incorrect run-in speed, not tuning the wire feeder to the power source and poor electrical continuity between the wire and contact tip. (Note that the patent pending XR-Edge gun design helps eliminate poor electrical pick-up.)

Q: How can I prevent birdnesting?

A: Soft aluminum wire is prone to buckling, so using a larger diameter may help. Better yet, invest in a push-pull feeder. In such systems, a torque motor at the wire spool steadily feeds the wire while a drive motor located in the gun precisely controls wire speed at the arc. This maintains constant tension, so the wire feeds consistently, even when the cable is looped, and at distances up to 50 ft. The XR-Edge torque motor features a high and a low torque setting, letting you adjust performance, respectively, for larger or smaller diameter wires.

The inherent properties of aluminum make it prone to poor MIG arc starts. For example, aluminum wire requires a lot of current to initiate the arc, yet the wire melts very quickly once the arc starts. It requires a fast wire feed speed, but any oxide on the weldment delays arc initiation because it melts at a much higher temperature. Thus, the solution to arc starting problems often involves fine-tuning the speed at which the wire approaches the weldment.

Assuming you have a "push-pull" style wire feeder, start by finding the run-in speed control. This sets the wire feed speed from the time you pull the gun trigger until arc initiation (after sensing an arc, the machine switches to welding speed). Because run-in speed is generally slower than welding speed, the arc has more time to establish itself. If you experience wire stubbing, use a slower run-in speed.

Always adjust run-in speed first. For additional fine-tuning, Miller incorporated a unique feature in its XR-Edge™ push-pull feeder that adjusts how long the wire drive motors take to ramp up to full speed. This "motor ramp control" can help tune the wire feeder to better match a welding power source's arc starting characteristics. To prevent long, flaring arc starts or wire burning back to the contact tip, increase ramp control. For example, ramp control is factory-set at maximum speed to match Miller's fast-responding Invision™ 354MP inverter. Slower ramp speeds have benefited users who pair the XR-Edge with magnetic amplifier-type CV welders or CC machines for running larger wires.


Smooth Groove, Smoother Performance
The soft nature of aluminum wire leads to feeding problems. For example, when the wire slips through the drive rolls, burnbacks and arc stumbling often result. Don't over-tension the drive rolls or use knurled drive rolls as a "solution." This inevitably deforms and defaces the wire, producing shavings. These wire particles then build up in the drive roll grooves, cable liner and contact tip. Ironically, this also causes burnbacks and arc stubbing. Instead, look for a wire feeder system with 1) the new-style smooth groove drive rolls and 2) all gear driven drive rolls (no idler roll).

Both the gun and the feeder rolls on Miller's new XR-Edge have these features, and this particular system offers excellent feeding performance without deforming and defacing the welding wire.

For more information on wire feeder systems and welding power sources that give you the flexibility and control to optimize MIG welding aluminum, call 1-800-4-A-MILLER (1-800-426-4553, ext. 603) or visit our website at www.mesinlas.com

Information courtesy of Miller Electric