10.4 GMAW Operation and Welding Techniques
Stephanie Oostman
GMAW Modes of Transfer
At the beginning of this chapter, you read a reference to how there are “modes” in GMAW welding. But what does that mean, and what distinguishes the modes?
We have already discussed how MIG and MAG welding offer versatility. That is because the operator can easily interchange the relationship between wire and heat. This, along with the ability to change wire types, sizes, and shielding gasses, creates different behavioral changes in how the electrode wire melts and gets transferred to the base metal. These are referred to as transfer modes, and they are the key to the GMAW process’s diversity and utility. In GMAW, there are four transfer modes.
Spray Transfer Mode
Spray transfer refers to the behavior in which the wire from the welding machine melts at the tip and deposits into the joint so as to create fine droplets that are equal to or less than the diameter of the electrode wire being used. For this to occur, the machine must reach the transition current, which is the minimum current setting for the welding machine to transition from globular transfer and into spray transfer. It is important to note that the transition current is not universal but rather dependent on variables such as electrical stickout, electrode type, and electrode wire diameter.
For example, a 0.035-inch diameter wire of the ER-70S-6 classification may reach its transition current into spray transfer when the welder is set to 24V–32V and 350–580 inches per minute (IPM).
As the consumable wire electrode interacts with the shielding gas and is melted by the electrical arc it results in what’s called pinch force. Pinch force along with surface tension acts as a squeeze or pinch on the end of the consumable wire electrode, allowing a droplet to break free of the tip of the wire and be distributed onto the base metal or joint. For spray transfer, pinch force on the electrode is increased to the point that the molten metal produces a fine conical spray, depositing an even and virtually spatter-free weld with a deeper penetration than the other transfer modes.
For this transfer mode it is important to select the appropriate shielding gas and welder settings in order for a fine cone-like spray to occur. For ferrous applications, a shielding gas of at least 80% argon must be used. For non-ferrous metals such as aluminum alloys a shielding gas of 100% argon is recommended.
Spray transfer is best suited for the flat (1G/1F) and horizontal (2G/2F) welding positions.
Pulsed Spray Transfer Mode
Pulsed spray transfer mode(also called GMAW-P), offers welders the ability to weld spray transfer out of position, such as overhead, and to utilize thicker wire electrodes than traditional spray transfer. It also requires lower heat input, offers 98% electrode efficiency, and can be used to create a consistent root on open joints without the need for a backing bar.
GMAW-P is not available on all MIG/MAG wire feed