Metal Transfer Mode
The manner, or mode, in which the metal transfers from the filler wire to the weld pool largely determines the operating features of the process. There are three principal metal transfer modes:
- Short circuiting (dip transfer)
- Spray transfer
- Pulsed transfer
Short-circuiting and pulsed metal transfer are used for low current operation, while spray transfer is only used with high welding currents. In short-circuiting or 'dip' transfer, the molten metal forming on the tip of the wire is transferred by the wire dipping into the weld pool. This is achieved by setting a low voltage. Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimise spatter. Inductance is used to control the surge in current which occurs when the wire dips into the weld pool.
For spray transfer, a much higher voltage is necessary to ensure that the wire does not make contact, i.e. short-circuit, with the weld pool. The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (less than the diameter of the wire). However, there is a minimum current level or threshold, below which droplets are not forcibly projected across the arc. If an open arc technique is attempted much below the threshold current level, the low arc forces are insufficient to prevent large droplets forming at the tip of the wire. These droplets transfer erratically across the arc under normal gravitational force. The pulsed mode was developed as a means of stabilising the open arc at low current levels, i.e. below the threshold level, to avoid short-circuiting and spatter. Spray type metal transfer is achieved by applying pulses of current, each pulse having sufficient force to detach a droplet.
Conventional MIG/MAG welding is carried out using a constant voltage power source which provides an inherently stable 'self adjusting' arc. For pulsed welding, either a constant voltage or constant current power source with voltage feedback is used.
Advantages and Disadvantages of MIG Welding
MIG welding allows for the fast production of high quality welds and, due to a lack of flux being used, there is no chance of slag being trapped in the weld metal. The shielding gas protects the arc, meaning that there is little loss of alloying elements and only minor weld spatter. MIG welding can be operated in several ways, including semi and fully automatically, and is a versatile process which can be used to join a variety of metals and alloys.
The disadvantages of MIG welding are that it cannot be performed in a vertical or overhead position, due to the high heat and fluid nature of the weld pool. Also, the equipment used by a MIG welder can be complex.
Advantages and Disadvantages of MAG Welding
Because the weld area is protected by the shielding gas, MAG welding creates no oxidation. This is a fast welding process, which means that there is a lower heat effect on the surrounding material. MAG welding can be performed in all positions, making it one of the most widely-used welding processes.
The disadvantages include the experience needed to perform this process correctly. MAG welding cannot be performed outdoors as the welding gas needs to be protected from the wind, while all rust must be removed from the workpiece before welding commences. Flux cored arc welding is more suitable for outdoor applications or underwater welds, which can also be better performed using shielded metal arc welding or gas tungsten arc welding. As with all arc processes, proper PPI must be worn and, in particular, eye protection.
MIG and MAG Welding Expertise
TWI has a considerable amount of experience in the development and qualification of MIG/MAG welding procedures for a variety of applications across industry.