3. Results and Discussion
3.1 Non-Metallic Inclusions
The result of carrying out laser welding in an air atmosphere
is a significant number of non-metallic inclusions formed in the
fusion zone. Figure 1(a) presents a clear boundary between the
fusion zone (FZ) and the heat-affected zone (HAZ). The
distribution of inclusions is visible in an etched metallographic
specimen (Fig. 1b), where the transitional zone (TZ) and base
metal (BM) are also revealed. In the BM, the TZ and HAZ nonmetallic
inclusions are arranged statistically randomly. A
similar situation is in the fusion zone, where it is possible to
observe uniformly arranged non-metallic inclusions, mainly of
the globular form.
A detailed identification of non-metallic inclusions present
in the base metal, HAZ, and fusion zone using EDS mapping
was carried out (Ref 16). The base metal revealed the presence
of sulfide or oxysulfide inclusions—globular or slightly
elongated in the rolling direction. Due to the mischmetal
addition during melting, the non-metallic inclusions are characterized
by a modification of the chemical composition of the
inclusions with rare-earth elements. Another type of nonmetallic
inclusions can be observed in the fusion zone. Due to
the high chemical affinity of Al and Si for oxygen, the
inclusions are mainly of the oxide type. Al has higher affinity
for oxygen than silicon. For this reason most inclusions are
globular aluminum oxides, which is confirmed by an analysis
of the chemical composition presented in Fig. 1(c) and carried
out along line 1, passing through the fusion zone. In the area
where the inclusions are present (marked with circles in
Fig. 1b) it is possible to observe clearly visible peaks coming
from aluminum and oxygen. In addition, in Fig. 1(c) it is
possible to observe a slightly lower Mn concentration in the
fusion zone, which can probably be ascribed to partial
manganese evaporation during the formation of a liquid metal
pool. Similar oxide inclusions of random arrangement in the
fusion zone were revealed by Amirthalingam et al. (Ref 15) in
high-Al and high-Si TRIP steels. They also found that
inclusions containing aluminum are greater in size than
inclusions containing silicon, which is due to the higher
temperature of Al2O2 formation.