THE LASER USER
ISSUE 114 AUTUMN 2024 LASER WELDING
Figure 3: Cu -Cu welds a) Low wobble amplitude - crown b) Undercut c) Wider bead - higher wobble amplitude.
Figure 4: Cu -Al welds a) High laser power – porosity, cracks b) Low power c) controlled penetration with wider bead - higher wobble amplitude.
non-wobble welding. Laser beam wobbling promotes reduced porosity, grain refinement, and enhancement of mechanical properties. The wobble parameters influence the welding mode, namely conduction, transition, and keyhole mode, and thereby, the weld morphology. Good welds can be produced successfully using the laser wobbling technique.
Different wobble shapes, such as linear, circle, infinity, or figure-of-8, can be used in laser wobble welding. Each shape produces a weld with different geometry and aspect ratios, which is observed due to the change in heat distribution associated with it. The wobbling of laser beams in a circle shape produces welds with reduced porosity and lower melt ejections than other wobble shapes. The circular wobble shape has uniform velocity and energy absorption by the keyhole, which makes it stable during welding and produces smooth welds without spatters. Therefore, circular wobble was used in this study.
Discussion
In the Cu-Cu lap configuration, the welds produced in non-wobble conditions show deeper penetration and more defects (porosity, melt- through). On the contrary, in the welds produced with beam wobbling, the penetration was lowered and a broader weld bead was formed. Cu-Cu welds showed high back-reflection, and high heat input was required to produce the welds. The welds were fabricated using laser power varying from 4.5-8 kW, with weld speeds ranging from 75-250 mm/sec. The wobble
amplitudes and the frequencies varied from 0.5-2 mm and 50-300 Hz. The welds were analysed using 3D microscopy capabilities.
Figure 3 shows the Cu-Cu welds produced with different sets of process parameters. If the wobble amplitude is lowered, it produces crowns and higher melt ejections due to keyhole instability. However, a higher wobble amplitude produces a broader weld bead with reduced penetration. The increase in defocusing downwards increases the penetration and produces undercut defects.
The welds in the Cu-Al configuration were also produced with similar parameters. The higher laser power or increased heat input caused deeper penetration with severe cracking. However, the penetration could be controlled in this configuration, and acceptable welds as per industrial requirements were produced using optimum parameters with the right combination of heat input and wobble parameters. Figure 4 shows the Cu-Al welds produced with different sets of process parameters.
It is difficult to penetrate the bottom Cu sheet in the Al-Cu configuration with similar welding
parameters. Obtaining satisfactory penetration in copper is quite challenging when aluminium of higher thickness is positioned on top. The aluminium melt pool on top has insufficient temperature to melt the copper at the bottom. Moreover, wobbling reduces the laser power density and heat distribution, making melting copper at the bottom even more challenging. Wobbling produces wider beads at the top Al and reduces penetration in the bottom Cu. Al on top promotes centreline cracking at high laser power and welding speeds.
Conclusion
To conclude, the laser power, speed (heat input), and wobble influence the penetration and weld geometry in all three material configurations. Laser wobbling can be used to tailor the shape of the welds and control the defects. Laser wobble can also be used to minimise the back reflection of the beam from reflective surfaces.
Contact: Ragavendran Meenakshisundaram
r.meenakshisundaram@
sheffield.ac.uk https://www.amrc.co.uk/
Raga Meenakshisundaram is a Technical Lead in welding at AMRC Northwest, University of Sheffield. He works on joining technologies for EV batteries and surface engineering.
SEE OBSERVATIONS P26 19
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