Keyhole formation is a prerequisite for deep penetration laser welding. Understanding of the keyhole dynamics is essential to improve the stability of the keyhole. Direct observation of the keyhole during deep penetration laser welding of a modified "sandwich" specimen with a 10 kW fiber laser is presented. A distinct keyhole wall and liquid motion along the wall are observed directly for the first time. The moving liquid "shelf" on the front keyhole wall and the accompanying hydrodynamic and vapor phenomena are observed simultaneously. Micro-droplets torn off the keyhole wall and the resultant bursts of vapor are also visualized. The hydrodynamics on the keyhole wall has a dominant effect on the weld defects. The emission spectrum inside the keyhole is captured accurately using a spectrometer to calculate the characteristics of the keyhole plasma plume.

Direct observation of keyhole characteristics in deep penetration laser welding with a 10 kW fiber laser

On the basis of a keyhole photograph obtained experimentally, the keyhole profiles are determined by the method of polynomial fitting. Then the behaviour of the laser beam in the keyhole is analysed by tracing a ray of light using geometrical optics theory; the Fresnel absorption and reflections in the keyhole are systematically studied, and the laser intensities absorbed on the keyhole walls are calculated. By comparing the laser intensity absorbed on the keyhole walls with the heat flux lost there, the mechanism of energy balance on the keyhole walls is discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/35/18/312/pdf

A study on fresnel absorption and reflections in the keyhole in deep penetration laser welding

The present investigation was mainly concerned with characteristics of autogeneous laser butt welding of 2 mm thickness nonheat treatableAA5052-H12,AA5083-H12 and 2 mm, 3 mm
thickness heat treatableAA6061-T6aluminum alloys. The effect of laser welding parameters,
surface cleaning, filler wire addition, and backing strip on quality of laser welded joints was clarified
using 5 kW C O 2 laser machine. 
It was found that all the investigated alloys showed tendencies for porosity and solidification cracking,
particularly, at high welding speed ( ≥ 4 m/min). Porosity was prevented by accurate cleaning of the
base metal prior to welding and optimizing the flow rate of argon shielding gas. Solidification cracking
was avoided through two different approaches. The first one is based on the addition of filler metal
as reported in other research works. The other new approach is concerned with autogeneous
welding using a backing strip from the same base metal, and this could be applicable in production. 
Preventing solidification cracking in both cases was related mainly to a considerable decrease in the
stress concentration at the weld metal center as a result of improving the fusion zone profile. 
The implementation of the new approach could help in producing weldments with a better quality due
to the absence of the filler metal, which is known as a source for hydrogen-related porosity. It can
also have a positive economic aspect concerning the manufacturing cost since welding is done
without the addition of filler metal. Not only quality and economic positive aspects could be
achieved, but also high productivity is another feature since high quality autogeneous weldments were
produced with high welding speed, 6 m/min. Hardness measurements and tensile test of AA6061 alloy welds indicated a remarkable softening of the fusion zone due to dissolution of the strengthening precipitates, and this was recovered by aging treatment after welding. For alloys AA5052 and AA5083, softening of the fusion zone due to the
loss of its work-hardened condition was much less in comparison with AA6061 alloy.

/pdf/laser-beam-welding-of-aa5052-aa5083-and-aa6061-aluminum-3mcmi01g60.pdf

Laser Beam Welding of AA5052, AA5083, and AA6061 Aluminum Alloys

Melt behavior on the keyhole front during high speed laser welding

This paper presents the results of a high speed video survey of melt flow on the front face of a keyhole created during fibre laser welding. Using fast Fourier transform techniques, quantitative values of fluid flow velocities down the keyhole front have been established. The results have led to a phenomenological understanding of some of the quality problems which arise at excess welding speeds. The downward flow velocity on the keyhole front is found to be generally independent of welding speed, and proportional to laser power.

/pdf/measurements-of-fluid-flow-on-keyhole-front-during-laser-1oinioxw1f.pdf

Measurements of fluid flow on keyhole front during laser welding

/pdf/fundamental-phenomena-in-high-power-co-2-laser-report-ii-3bwep9ucff.pdf

Fundamental Phenomena in High Power CO_2 Laser (Report II) : Vacuum Laser Welding(Welding Physics, Process & Instrument)

https://ir.library.osaka-u.ac.jp/repo/ouka/all/11395/jwri14_01_005.pdf

Fundamental Phenomena in High Power CO_2 Laser Welding (Report I) : Atmospheric Laser Welding(Welding Physics, Process & Instrument)

Fundamental phenomena during laser beam welding in steel and glass, including beam hole shape, molten metal flow and peculiar plasma behaviour, were observed dynamically using a transmission X-ray system and high speed camera. The effect of altering the flow rate of helium assist gas were also studied. It was found that the gas flow rate had a strong effect on the beam hole shape, molten metal flow and plasma production. In order to avoid the interference effect of laser plasma, a new laser welding process, which we have named “Laser Spike Seam Welding”, was developed. This process allows considerably deeper penetration than conventional continuous welding. The reasons for this superiority were analyzed by the above high-speed film method.Fundamental phenomena during laser beam welding in steel and glass, including beam hole shape, molten metal flow and peculiar plasma behaviour, were observed dynamically using a transmission X-ray system and high speed camera. The effect of altering the flow rate of helium assist gas were also studied. It was found that the gas flow rate had a strong effect on the beam hole shape, molten metal flow and plasma production. In order to avoid the interference effect of laser plasma, a new laser welding process, which we have named “Laser Spike Seam Welding”, was developed. This process allows considerably deeper penetration than conventional continuous welding. The reasons for this superiority were analyzed by the above high-speed film method.

Beam hole behaviour during laser beam welding

PURPOSE:To obtain a large-output excimer laser beam by a method wherein a plurality of sets of existing excimer laser oscillators are used and they are combined appropriately. CONSTITUTION:N-laser oscillators 1 are used; laser beams 2 generated by them are composed, and a large-output complex converged beam is formed. During this process, a beam characteristic can be changed in the following manner: to obtain a large output of a laser while an energy pulse is made a plurality of times, e.g., n times, as high as the pulse energy P by controlling the generation timing of the laser pulses 3 from the individual oscillators 1 and by making the generation time of the pulses 3 having an identical frequency (f) identical; alternatively, to obtain the large output of the laser by expanding a pulse width to a plurality of times, e.g., n times, while the generation timing of the pulses from the two or more sets of the oscillators 1 is delayed one after another by a pulse width W; alternatively, to obtain the large output by making the repetition number of the pulses of a composed beam a plurality of times, e.g., two times, as many as one in the case of one laser oscillator.

Generation of large-output excimer laser beam

/pdf/das-laser-schweissen-von-aluminium-und-alu-legierungen-4uw0nkdvxj.pdf

Tatsuharu Oda

Papers

Fundamental Phenomena in High Power CO_2 Laser (Report II) : Vacuum Laser Welding(Welding Physics, Process & Instrument)

Fundamental Phenomena in High Power CO_2 Laser Welding (Report I) : Atmospheric Laser Welding(Welding Physics, Process & Instrument)

Beam hole behaviour during laser beam welding

Generation of large-output excimer laser beam

Das Laser-Schweissen von Aluminium und Alu-Legierungen