The difference between GMA brazing to Gas Metal Arc Welding lies in the use of filler metals based on Copper alloys instead of steel. This process is mainly used in the short- or pulsed arc mode in virtually all positions. A separate joint preparation is usually not necessary.
Short-arc process
The short-arc process provides GMA brazing with low heat input. At low arc power the droplet transfer occurs during the short-circuit phase (short-arc process).
Pulsed arc process
The pulsed arc technology provides a well controllable droplet transfer with little short-circuits and a good gap-bridging characteristic for brazing overlap joints. Generally the pulsed arc process gives flatter seams than the short-arc process. Because the evaporation of the coating may lead to arc instabilities it is recommended to use a short arc length. With Argon as shielding gas a low spatter process can be obtained at optimized parameters. To keep the heat input as low as possible a low base current is required.
Special requirements to the brazing equipment
GMA brazing requires special precautions on the power source. In order to keep the evaporation of Zinc as low as possible, brazing is done at low arc power. Therefore, the power source needs to have a control range which goes to low levels of arc power. At the same time arc stability must be provided. A low base current together with a fast reacting arccontrol to obtain a short arc length is required.
Depending on each combination of filler metal and shielding gas GMA brazing requires a special pulse shape. Generally seen most available power sources on the market give good brazing characteristics. It night be necessary, however, to obtain an optimized arc characteristic from its manufacturer.
The filler wires used are softer than wires for steel welding, and require higher demands on the wire-feed units. Similar to Aluminium- or flux-cored wires wire-drive systems need to be equipped with semi-grooved wire feed rolls. The wire feed motor should be controlled to provide constant feeding speed. Hose packs must be equipped with plastic or Teflon liners. In case that hose pack length of more than 3 m for semiautomatic or more than 1.5 m for automatic brazing are necessary an extra wire-feed motor at the torch is required. For automatic brazing it is useful to use water cooled torches.
Technological remarks
An additional important influence on the seam quality for GMA brazing is the torch angle and torch guidance. With a pushing arc angle the forerunning arc preheats the Zinc coating as much that it may evaporate immediately before the arc down to a small rest. The heat energy of the molten metal droplet from the filler wire evaporates the, until then, remaining coating. Because the quantity of the remaining Zinc coating in the still molten brazing alloy is rather low the remaining time until solidification is long enough for degassing.