3. Results
3.1. Process characterization
Since the bead surface is an important reflection of the process
stability and can be observed by naked eyes directly, it was used
to evaluate the process characterization. Regular and continuous
surface denotes a stable process, while irregular and discontinuous
surface indicates an unstable process. To discover the laser–arc
interaction during hybrid welding, the shapes of laser induced
plasma and arc column were recorded by a CCD camera. As shown
in Fig. 3, despite the fact that all possible welding parameters were
used, pure MIG welding obtains discontinuous and irregular welds,
indicating an unstable process. However, the sound weld and stable
process are obtained in the hybrid welding under appropriate
welding parameters. If the arc current is too small or the welding
speed is too fast, as shown in Fig. 4a andf, hybrid welding
shows some undesired surface morphologies with the defects of
overlap, undercut and discontinuity. Increasing arc current and
laser power, or decreasing welding speed could help stabilize the
hybrid welding and improve the quality of bead surface, as shown
in Fig. 4b–d.
In Fig. 5, the arc column is compressed by the laser beam during
hybrid welding compared to the shape in pure MIG welding.
When the laser power keeps constant, the compressed extent of
arc column reduces with increasing arc current because the selfionization
or self-stability of the arc column enhances gradually
with increasing arc current. The similar phenomenon was also
observed by Chen et al. (2006). As shown in Fig. 6, the higher the
laser power is, the bigger the compressed extent of the arc column
is. Based on above descriptions in the process stability, a strong link
would be exist between the plasma shape and the process stability
during the hybrid welding of Mg alloy, which will be discussed in
Section 4.1.
3.2. Effects of welding parameters on bead shape
As shown in Fig. 7, the penetration depth of hybrid welds
increases with both the laser power and the arc current, and
the increase caused by laser power is more obvious. The increment
in penetration depth caused by laser power is 2–3 mm
per kilowatt, while that caused by arc power is about 0.5 mm
per kilowatt. In addition, the penetration depth of hybrid weld
is bigger than that of pure laser weld as the laser power and