Influence of focal spot size < 25 μm on weld seam geometry and process stability in high-precision laser micro welding

Further minimization of the width of weld seams produced by laser micro welding enables advancements in applications such as medical and fuel cell technology. This study is focused on quantifying the influence of sub-25 μm focal spot diameter on weld seam geometry (aspect ratio and surface seam width). For this purpose, two focal spot diameters, 16 μm and 24 μm, are investigated on stainless steel (1.4404, thickness: 2 mm) and demonstrated on copper Cu-ETP (2.0060, thickness: 2 mm) using an IPG YLR-1000-WC single-mode fiber laser (λ = 1070 nm). Rayleigh length being proportional to the square of the spot diameter makes maintaining the optimal focus increasingly challenging for smaller spots. Despite the higher intensity at 16 μm focal spot size, the aspect ratio (depth/width) did not increase and was, on average, 11% lower than with the 24 μm spot. It was shown that a reduction of weld seam width is possible with this approach. However, due to heat transfer and weld pool dynamics, a reduction of spot size by 33% did not translate proportionally into seam width reduction. An increased standard deviation highlights the need for precise z-position control to industrialize welding with a 16 μm spot. Additionally, a humping‑free process is demonstrated at 293 W laser power and a feed rate of 977 mm/s on both stainless steel and copper Cu‑ETP (2.0060). On copper a mean weld seam width of 63 μm was achieved.