6.5 Conclusions
181
and then accumulation and solidification there. The side-blown gas obviously
helps liquid metal flow to the rear of the weld pool.
(3) A local high-pressure area can be formed in the vicinity of a locally depressed
gas–liquid interface in the front of the weld pool after auxiliary gas is blown to
the workpiece surface. Backwards from the area, with the increase of distance
from the front of the weld pool, pressure constantly decreases.
The following are the findings of the research on behaviors of the keyhole and
weld pool under the effects of metallic vapors.
(1) Confluence formed by side-blown gas and top-blown gas flow shows a tilted,
bell jar shaped pressure distribution in the upper area of the workpiece surface
and the axis of the “bell jar” is the flowing direction of confluence. There is
a local high-pressure area in the vicinity of the intersection point of the axis
of the “bell jar” and the workpiece surface. In addition, due to the inhibition
of auxiliary gas flow on high-speed gas flow ejected from the keyhole, a local
high-pressure area can be found in the surrounding areas above the keyhole
opening.
(2) Under the effect of metallic vapors, the changes of the keyhole and the weld
pool can be divided into three stages, i.e. initial stage, expansion stage and
refilling stage. It is assumed that the keyhole is formed instantaneously in the
welding process and the whole welding process is the constant cycles of these
three stages.
In the initial stage, liquid metal around the keyhole flows in the direction
departing the axis of the keyhole. The local uplift obviously exceeding the
initial upper surface of the weld pool is not formed close to the keyhole opening,
and the diameter of the keyhole changes slightly in the direction of whole
thickness of the workpiece. In the expansion stage, molten metal around the
wall in the middle of the keyhole constantly expands and moves to the direction
far away from the axis of the keyhole. However, molten metal in the rear of
keyhole opening on the lower and upper surfaces moves towards the axis of
the keyhole and shows the trend of closing the keyhole opening. In addition,
local uplift formed on the metal surface in the rear of the keyhole opening
can enhance the effects of side-blown gas on the part, so that a part of molten
melt in the local uplift behind the keyhole opening on the upper surface flows
to the rear of the weld pool. In the refilling stage, molten melt in the weld
pool behind the lower keyhole opening flows towards the keyhole and from
the bottom up under the effect of surface tension. Driven by surface tension
and gravity, surrounding liquid metal behind the upper keyhole opening shows
a similar flowing trend to the liquid metal behind the lower keyhole opening,
that is, liquid metal flows towards the axis of the keyhole. However, molten
melt in the rear of the weld pool around the upper surface flows to the direction
far away from the keyhole. Such a flowing behavior is formed under the effects
of auxiliary gas flow after overcoming the influences of surface tension and
gravity, which proves the control effects of auxiliary gas flow on the weld pool
during laser welding.
181
and then accumulation and solidification there. The side-blown gas obviously
helps liquid metal flow to the rear of the weld pool.
(3) A local high-pressure area can be formed in the vicinity of a locally depressed
gas–liquid interface in the front of the weld pool after auxiliary gas is blown to
the workpiece surface. Backwards from the area, with the increase of distance
from the front of the weld pool, pressure constantly decreases.
The following are the findings of the research on behaviors of the keyhole and
weld pool under the effects of metallic vapors.
(1) Confluence formed by side-blown gas and top-blown gas flow shows a tilted,
bell jar shaped pressure distribution in the upper area of the workpiece surface
and the axis of the “bell jar” is the flowing direction of confluence. There is
a local high-pressure area in the vicinity of the intersection point of the axis
of the “bell jar” and the workpiece surface. In addition, due to the inhibition
of auxiliary gas flow on high-speed gas flow ejected from the keyhole, a local
high-pressure area can be found in the surrounding areas above the keyhole
opening.
(2) Under the effect of metallic vapors, the changes of the keyhole and the weld
pool can be divided into three stages, i.e. initial stage, expansion stage and
refilling stage. It is assumed that the keyhole is formed instantaneously in the
welding process and the whole welding process is the constant cycles of these
three stages.
In the initial stage, liquid metal around the keyhole flows in the direction
departing the axis of the keyhole. The local uplift obviously exceeding the
initial upper surface of the weld pool is not formed close to the keyhole opening,
and the diameter of the keyhole changes slightly in the direction of whole
thickness of the workpiece. In the expansion stage, molten metal around the
wall in the middle of the keyhole constantly expands and moves to the direction
far away from the axis of the keyhole. However, molten metal in the rear of
keyhole opening on the lower and upper surfaces moves towards the axis of
the keyhole and shows the trend of closing the keyhole opening. In addition,
local uplift formed on the metal surface in the rear of the keyhole opening
can enhance the effects of side-blown gas on the part, so that a part of molten
melt in the local uplift behind the keyhole opening on the upper surface flows
to the rear of the weld pool. In the refilling stage, molten melt in the weld
pool behind the lower keyhole opening flows towards the keyhole and from
the bottom up under the effect of surface tension. Driven by surface tension
and gravity, surrounding liquid metal behind the upper keyhole opening shows
a similar flowing trend to the liquid metal behind the lower keyhole opening,
that is, liquid metal flows towards the axis of the keyhole. However, molten
melt in the rear of the weld pool around the upper surface flows to the direction
far away from the keyhole. Such a flowing behavior is formed under the effects
of auxiliary gas flow after overcoming the influences of surface tension and
gravity, which proves the control effects of auxiliary gas flow on the weld pool
during laser welding.