1.3 Research on Weld Pool Behavior in Laser Welding
15
But in the welding process, due to reasons such as multiple process parameters
and complex working conditions, welding quality problems often occur. Especially
in the process of welding thick section structures and large size panels with long
weld joints, it is easy to cause poor welding forming, serious welding defects, or
unstable welding process with frequent interruptions, which affect the qualified rate
of the weldment.
As studies have shown that in any welding process, the main factor closely related
to the quality of the weld joint is the stable existence of a weld pool, and for the deep
penetration laser welding process, the main manifestation is the stable existence of
the keyhole in the weld pool. Therefore, a stable welding process for deep penetration
laser welding is a prerequisite of ensuring the quality of the weld joint, and the basis
for a stable welding process is the stable existence of a keyhole in the welding process.
By observing the deep penetration laser welding process and by dissecting the weld
joints, it is found that the defect of welding quality is often caused by the instability of
the weld pool keyhole in the welding process. Researches have demonstrated that the
main factor closely related to the quality of a weld joint during the welding process
is the stable existence of a weld pool. Based on experiments and researches on the
deep penetration laser welding process, it is revealed that, in the deep penetration
laser welding process, many parameters such as laser welding power, focus position,
welding speed, assembly gap, etc. in existence of the keyhole and in formation and
disappearance of the keyhole affect directly existence state and morphology of the
keyhole. So, the influence of different process parameters on the behavior of the
keyhole under deep penetration laser welding conditions is studied and analyzed to
identify the pattern by which different welding process parameters exert influence
on the welding quality, to create favorable conditions for further realization of the
control over the welding process and quality.
In coping with the issue that the keyhole simulation and the calculation of convective heat transfer of the weld pool in deep penetration laser welding cannot be
organically combined or that the coupling relationship is extremely complicated,
and with the view of practical application, the research team of the author employs a
method of combining experimental research with computational simulation, on the
basis of works of current mathematical simulation research on deep penetration laser
welding and the energy conservation theory (see Fig. 1.5), in an attempt to build a
heat source model suitable for the finite volume method for deep penetration laser
welding process simulation, and uses FLUENT, a commercial computational fluid
dynamics software, to simulate the basic features of the keyhole formation and the
flow of the weld pool in the laser welding process. On this basis, the interacting flow
field between the side blowing auxiliary airflow and the keyhole jet flow is calculated, which provides a basic theoretical foundation for improving protection of the
welding area and the quality of weld joints. Applying the basic process theories on
deep penetration laser welding, the hydrodynamics of the weld pool and the stability
of the keyhole are studied jointly from the perspective of weld pool hydrodynamics,
with the expectation of improving the quality of laser welding. The main research
works and achievement are as follows:
15
But in the welding process, due to reasons such as multiple process parameters
and complex working conditions, welding quality problems often occur. Especially
in the process of welding thick section structures and large size panels with long
weld joints, it is easy to cause poor welding forming, serious welding defects, or
unstable welding process with frequent interruptions, which affect the qualified rate
of the weldment.
As studies have shown that in any welding process, the main factor closely related
to the quality of the weld joint is the stable existence of a weld pool, and for the deep
penetration laser welding process, the main manifestation is the stable existence of
the keyhole in the weld pool. Therefore, a stable welding process for deep penetration
laser welding is a prerequisite of ensuring the quality of the weld joint, and the basis
for a stable welding process is the stable existence of a keyhole in the welding process.
By observing the deep penetration laser welding process and by dissecting the weld
joints, it is found that the defect of welding quality is often caused by the instability of
the weld pool keyhole in the welding process. Researches have demonstrated that the
main factor closely related to the quality of a weld joint during the welding process
is the stable existence of a weld pool. Based on experiments and researches on the
deep penetration laser welding process, it is revealed that, in the deep penetration
laser welding process, many parameters such as laser welding power, focus position,
welding speed, assembly gap, etc. in existence of the keyhole and in formation and
disappearance of the keyhole affect directly existence state and morphology of the
keyhole. So, the influence of different process parameters on the behavior of the
keyhole under deep penetration laser welding conditions is studied and analyzed to
identify the pattern by which different welding process parameters exert influence
on the welding quality, to create favorable conditions for further realization of the
control over the welding process and quality.
In coping with the issue that the keyhole simulation and the calculation of convective heat transfer of the weld pool in deep penetration laser welding cannot be
organically combined or that the coupling relationship is extremely complicated,
and with the view of practical application, the research team of the author employs a
method of combining experimental research with computational simulation, on the
basis of works of current mathematical simulation research on deep penetration laser
welding and the energy conservation theory (see Fig. 1.5), in an attempt to build a
heat source model suitable for the finite volume method for deep penetration laser
welding process simulation, and uses FLUENT, a commercial computational fluid
dynamics software, to simulate the basic features of the keyhole formation and the
flow of the weld pool in the laser welding process. On this basis, the interacting flow
field between the side blowing auxiliary airflow and the keyhole jet flow is calculated, which provides a basic theoretical foundation for improving protection of the
welding area and the quality of weld joints. Applying the basic process theories on
deep penetration laser welding, the hydrodynamics of the weld pool and the stability
of the keyhole are studied jointly from the perspective of weld pool hydrodynamics,
with the expectation of improving the quality of laser welding. The main research
works and achievement are as follows: