Page 35 - MetalForming-Sep-2018-issue

in the form of high heat generation.
As noted earlier, threaded contact tips inherently increase resistance and therefore require higher voltage and amperage (wire-feed speed) settings. When switching to nonthreaded con- sumables (or even a new GMAW gun), Hassan and Johnson conducted tests that proved operators need to lower parameters to achieve similar results. With standard consumables, they pro- duced a weld bead using 0.052-in. wire, welding at 29.2 V, 300 A/500 in./min. of wire-feed speed and a 3⁄4-in. con- tact-tip-to-work distance. To produce the same amperage and weld-bead width with nonthreaded consumables, they must lower wire-feed speed by almost 23 percent (to 384 in./min.) and
voltage by about 6 percent (to 27.5). “Different consumable designs also require different shielding-gas-flow rates,” adds Hassan. “Standard con- sumables place the shielding-gas ports farther from the end of the nozzle and require flow rates of 35 to 40 ft.3/hr.
A single gun handle can offer multiple consumable choices. From L-R: slip adjustable nozzle, two-piece nozzle, heavy-duty nozzle and tip, medium-duty nozzle and tip, extended nozzle and tip and flux-vored tip retainer with heavy-duty extended tip.
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Consumable designs that place the shielding-gas outlet closer to the end of the nozzle require a flow rate of 27 to 35 ft.3/hr. cfh, with 30 ft.3/hr. being optimum.”
The proper flow rate produces a smooth, laminar flow pattern that yields optimum coverage of the molten weld puddle. Unfortunately, some welders have a more-is-better mindset and crank up flow rates, which may lead to turbulent, irregular gas flows
that inadequately cover the molten puddle and may also pull in atmos- pheric contaminants.
“The bottom line is that the perform- ance of GMAW consumables and guns varies based on design,” says Hassan. “The differences affect parameter set- tings and welding results. When selecting new options, test guns and consumables at a few welding stations to determine the right choice and parameter settings for your specific applications.” M F
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