Gas metal arc welding (GMAW) is defined in the American Welding Society Standard Welding Terms and Definitions as “an arc welding process using an arc between a continuous filler metal electrode and the weld pool”. The GMAW process is commonly referred to as Metal Inert Gas (MIG) welding. The process uses an external shielding gas to protect the molten puddle. The equipment used is typically a constant voltage (CV) power supply. The GMAW process has four main methods to transfer the metal from the wire to the molten puddle. Understanding each transfer method is fundamental to eliminating spatter. These four modes of transfer methods will be discussed in depth below.
Transfer Methods
Short-Circuit Transfer
Short-circuit transfer is when the filler metal continuously short-circuits to transfer metal to the base material. The continuous short-circuiting causes the arc to switch on and off. Short-circuit transfer is a low-energy process and is excellent for thin gauge material. Optimum wire feed speeds and voltages will provide nearly spatter-free welds. The below videos are from the Canadian Centre for Welding and Joining. The video below shows how the wire is continuously fed into the puddle, eventually touching the puddle extinguishing the arc, and then short-circuiting to establish the arc again.
Globular Transfer
Globular transfer is when the metal transfers from the wire to the base metal in large drops. The arc does not extinguish like short-circuit transfer. Instead, the droplets are continuously transferred across the arc. The globular transfer usually occurs with shielding gases that contain large percentages of carbon dioxide (CO2). The globular transfer is undesirable and can cause significant amounts of spatter. The bead appearance is typically poor.
Spray Transfer
Spray transfer is when the small molten metal droplets are propelled across the arc to the molten puddle. Typically the droplet size is smaller than the size of the wire. Spray transfer is a high-energy process with a fluid molten puddle therefore; it is suitable for flat and horizontal welding. Optimum wire feed speeds and voltages will provide nearly spatter-free welds. To achieve spray transfer, the weld current must be above the spray transition current. Once a certain current is achieved the transfer method will switch from globular to spray.
Pulsed Transfer
Pulsed spray transfer is a modified spray transfer. The welding power supply provides pulses of high and low energy. The high pulses force the transfer into a controlled spray. Small molten droplets will be propelled from the pulses. Pulsed spray transfer can be used in all positions. Optimum wire feed speeds and voltages will provide nearly spatter free welds.
Getting the best results
Understanding the fundamentals of how the metal is transferred to the weld puddle, will help you optimize your welding process. Depending on your application, short-circuit, spray, or pulsed transfer may be appropriate. Typically globular transfer is avoided as it is an erratic and unstable welding arc. When using short-circuit, spray, or pulsed transfer appropriately, almost no spatter will stick to your parts. If you are experiencing a lot of spatter then you likely have to tweak the process variables to obtain a clean transfer method.