|I’m working on a project where the welding inspector and the fabricator are recommending that for complete penetration welds, the root pass be 3/32 inch stick and the rest of the passes will be pulsed arc wire. Can you tell me something about pulsed arc wire?
— Structural Engineer from Oakland, CA.
|1/17/07 – Response prepared by Greg Ruf, Managing Engineer for Krazan & Associates’ San Francisco Bay Area Operations
The two most common types of Gas Metal Arc Welding (GMAW) metal transfer are short-circuit and spray transfer. Both processes use constant voltage and direct current. In the short-circuit process, which uses both a constant voltage and constant current, the filler wire contacts the base metal causing a short-circuit. The short circuit processes sufficient heat to melt the filler wire where the wire is in contact with the base metal.
Spray transfer is a process where the filler metal wire melts above the base metal and is projected across the arc as globules or as fine droplets of molten metal. Spray transfer can be accomplished using conventional constant voltage constant current or pulse current techniques and equipment.
Pulsed arc welding, also known as pulsed spray welding, is a spray-transfer form of GMAW. Pulsed arc welding process is also a constant voltage direct-current process where the current is not held constant but is pulsed. Melting of the filler wire occurs at the higher current associated with the electrical pulse wave, with the droplets of molten filler metal projected across the arc from the wire to the weld puddle. Thus the spray-transfer of the filler metal.
The spray transfer process has the ability to make high-deposition welds on thick carbon steels when using larger diameter filler wire. The current AWS Welding Code D1.1 precludes the use of short-circuiting for welding of structural steel and stipulates that the spray transfer method be used for GMAW.
An advantage of the pulse method of GMAW versus that of conventional spray transfer GMAW, as cited by suppliers of the equipment, is that the average current of pulse arc is equal to and often less than that of conventional spray transfer. The pulse method of welding can result in increased penetration with less heat buildup in the joint. Spray transfer, and in particular the pulsed arc method, is also identified with better root fusion than the short circuit method of GMAW. Another advantage of the pulse method of GMAW is the reduction in spatter over that of the steady current short-circuit method.
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