Penetration-Mode welds with the desired quality. These variables can

Penetration-Mode HLAW. To fully utilize the benefits of an
expensive laser system, HLAW is conducted primarily in penetration mode. This
is sometimes referred to as arc-assisted laser welding. In penetration-mode
HLAW, the laser generates a keyhole in the metal. Both deep penetration and
high processing speeds can be achieved with keyhole laser welding. A keyhole is
formed when a laser beam with sufficiently high-power density causes melting
and vaporization of the base metal. As the metal is vaporized, it rapidly
expands and pushes away from the substrate. This expansion exerts a reactive
force on the melted substrate called the evaporative recoil force. This recoil
force pushes the melted metal away to form a depression. The melted metal is
continually pushed out until the depression has formed into a deep keyhole.

The keyhole can be partially or
fully through the thickness of the metal. In the steady-state condition after
the keyhole is established, continual vaporization of the bottom and walls of
the keyhole holds it open against the forces of surface tension and gravity.
The relationship of laser power density and travel speed dictates the
penetration and width of the keyhole for a given base metal. Power densities on
the order of 106 to 108 W/cm2 are typical for keyhole laser welding.

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Knowledge of each variable and the
ability to precisely control is necessary to consistently produce hybrid welds
with the desired quality. These variables can produce competing effects on the
weld attributes, and balancing the performance of each variable is essential to
successful hybrid welding. Table 1 lists the general effects of each HLAW variable
on hybrid weld attributes. The effects listed are typical for welding butt joints
over 6mm (0.24in.) thick or for welding thinner sections at travel speeds above
3 m/min (120 in./min).

Hybrid laser arc welding is
applicable over a wide range of travel speeds. Generally, the determining
factor for welding speed is the productivity requirement. As travel speed
increases, hybrid weld penetration will decrease. To maintain the required weld
penetration at increasing travel speeds, more laser power and an increased rate
of filler-metal deposition is required. If the existing laser equipment is
limited in power, then a compromise must be made among travel speed, laser
power, and weld penetration. At travel speeds on the order of 4 m/min (160
in./min) or greater, joint-filling capabilities from the GMAW system can be
limited. Gas metal arc welding systems are inherently limited to a maximum
current output. For a given electrode diameter, there is a maximum wire feed
speed at the maximum current rating of the GMAW power supply. This limitation
can lead to insufficient filler-metal addition at faster travel speeds. If the
required reinforcement or fillet size cannot be met for a given travel speed
due to the limitations of the GMAW power supply, the travel speed, GMAW source,
wire diameter, joint design, or the number of passes must be re-evaluated.
Additional GMAW torches with separate power supplies and wire feeders could be
used to overcome deposition limitations.

Process Orientation. The HLAW process can be oriented in two directions: arc leading
or laser leading. The GMAW process can be positioned behind or in front of the
traveling laser keyhole. If the GMAW process travels behind the laser beam, the
HLAW process orientation is referred to as laser leading. If the GMAW process
travels ahead of the laser, the HLAW process orientation is referred to as arc
leading. Figure 1 illustrates the laser-leading and arc-leading process

The main difference between the two
orientations is the angle of the GMAW torch with respect to the direction of
travel. Torch angle can have an effect on the deposited GMAW bead. In the
laser-leading HLAW configuration, the GMAW torch is traveling behind the laser
beam, positioned at a “push angle.” In the arc leading configuration, the torch
is at a “drag angle,” traveling in front of the laser beam. This difference in
torch angle can produce different weld surface geometries. In the laser leading
orientation, the deposited weld bead is relatively wide and flat, with large
weld toe angles. With arc leading, the deposited weld bead is more narrow and
convex, with sharper weld toe angles. Torch angle can be adjusted for each
process orientation, but there is a limitation to how close the torch can be
positioned to the beam axis, due to the beam convergence angle and obstructions
from the laser-focusing optic assembly. Alternatively, the laser beam axis can
be tilted while the GMAW torch is positioned normal to the work.

Another reported difference between
the two process orientations is in penetration. If the laser beam is positioned
in the arc depression of the GMAW process, the arc-leading configuration can
provide slightly more penetration for HLAW. However, there is conflicting data
reporting that the laser-leading process provides deeper penetration. In either
case, the reported gain in penetration is generally considered insignificant
for most manufacturing applications.