Page 33 - MetalForming March 2014

designed specifically for aluminum. Use the right kind of contact tips, with the size matched to the wire diameter being used. And, use a con- tact tip designed for aluminum wire; using a tip designed for steel wire can cause excessive burnback when welding aluminum.
The contact tip should be recessed inthegascupby1⁄8to1⁄4in.toensure proper gas cooling of the tip, and for spatter control. As the contact tip wears, arc flaring can become a prob- lem. To prevent this, replace contact tips as needed.
Selecting the correct type of drive roll will help to prevent wire-feeding issues when delivering aluminum weld wire to the joint. The majority of these problems result from aluminum wire shavings that originate from poor fit- ting and incorrectly designed drive rolls. The shavings can build up and clog the weld-gun liners, restricting the free flow of the wire. Use a U-groove drive roll in the wire feeder, designed for aluminum; a V-groove drive roll will compress the wire and deform it, caus- ing erratic arc conditions.
Also, carefully align the drive rolls, and always use the lowest drive-roll pressure capable of consistently feed- ing the wire. Performing regular main- tenance and periodically replacing items that wear, including drive rolls, liners and inlet guides, also helps pre- vent wire-feeding issues. Use a push- pull wire feeder or spool gun to pro- mote optimum wire feedability. Most push-pull guns have two liners: a con- duit liner, which typically lasts longer, and a head-tube liner in the gun por- tion, which can wear out more quickly. Periodic replacement of both liners proves helpful, especially once clog- ging becomes an issue.
Minimizing Weld Discoloration and Smut
Introduction of oxygen into the shielding-gas envelope via air, moisture and contaminants can increase burn- ing (oxidation) of the filler metal, which produces weld discoloration and smut. The use of certain filler metals also can
contribute to this problem.
While weld discoloration and smut
look bad, they are easy issues to avoid. The 4xxx series of filler metals produce less weld discoloration and smut than do the 5xxx series—magnesium in 5xxx-series alloys vaporizes in the arc and condenses as black soot next to the weld bead. Most 4xxx-series alloys contain little or no magnesium. Also, welders can keep air out of the shield-
ing gas by decreasing welding-gun angle, increasing gas-cup size, hold- ing the gas cup closer to the base metal, removing spatter buildup from the gas cup and carefully shielding the arc from drafts. Welders used to working with steel may be dragging their weld. Use a push angle instead, to put the arc- cleaning action in front of the weld. This angle continually cleans the weld and reduces smut. MF
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