Page 56 - Welding Robots Technology, System Issues, and Applications

2.2.3.1 Current Welding Technology 41
Direct current electrode positive (DCEP) is the most used current in GMAW
because it gives stable electric arc, low spatter, good weld bead geometry and the
greatest penetration depth.

For low currents and voltages in combination with active shielding gases or
mixtures containing active gases, dip or short-circuiting transfer is obtained. Metal
is transferred to the work-piece by bridging at frequencies usually above 100 Hz.
This metal transfer mode gives low heat input, being suited for welding thin
sections and for positional welding.

Globular transfer is obtained for currents and voltages somewhat above those of
the dip transfer, if inert shielding gases are used. When carbon dioxide shielding
gas is used this metal transfer mode is obtained only for high currents and voltages.
Globular transfer is characterized by large drops, with size identical to the
electrode diameter or higher, transferred at low frequency. This mode of transfer
can be used in a downward direction, due to the predominance of gravitational
forces during metal transfer.

The utilization of relatively low current can give insufficient penetration and
excessive weld reinforcement, occasioned by poor wetting action of the weld
metal. Globular repelled transfer can be found when electrode negative polarity is
used with solid wire, but this mode of transfer has no industrial application due to
poor stability and high spatter levels which result.

For currents and voltages higher than for globular transfer, projected spray transfer
occurs when argon-rich shielding is used. It arises for currents above spray
transition current, which depends on the electrode material, shielding gas and
electrode diameter. It is approximately 240 A for 1.2 mm diameter carbon steel
electrodes with argon/5% CO 2 shielding [3]. This mode of transfer is characterized
by very small drops projected onto the work-piece at a very high frequency, up to
350 drops per second, presenting low spatter level. As high currents are used high
heat inputs to the work-pieces are reached, producing large weld pools with deep
penetration. This type of metal transfer is attractive when high deposition rate
welds in thick materials in a downward direction are to be performed. However it
presents limited capacity in positional welds, due to the effect of gravity forces.
For even higher currents and voltages, streaming spray transfer is obtained, but it
has no industrial application due to high weld pool turbulence caused by the
increase of the electromagnetic forces.

Drop spray transfer mode can occur in the transition between globular and
projected spray transfer, in a restricted operating range. This metal transfer mode is
characterized by a very efficient detachment of small drops from the electrode,
which are projected onto the work-piece at high velocity and with low spatter level.
This type of transfer is difficult to regulate in conventional DC power sources but
can be achieved using pulsed transfer techniques.