Influence of scanning speed and number of passes on femtosecond laser welding of polycarbonate

Femtosecond laser welding enables high-quality joining of transparent polymers through multiphoton absorption, offering exceptional spatial resolution and a minimal heat-affected zone. Yet, identifying process conditions that balance weld strength and throughput remains challenging. This paper examines the impact of several process parameters, such as scanning speed and the number of passes, on the shear strength of polycarbonate joints produced using a 1030 nm femtosecond laser. Scanning speeds of 10, 20, 30, 50, and 100 mm/s are investigated, each with two distinct levels of passes. The fluence is set to the maximum level that avoids offset between the optical focus and the weld seam, ensuring stable energy deposition at the interface. The pulse repetition rate is constant at 1 MHz, and the spot diameter at 1/e² is 21 μm. Weld quality is assessed through shear strength measurements. Within the studied range, increasing the number of passes generally reduces shear strength because the weld seam widens while the load-bearing capacity does not increase proportionally. At low numbers of passes, all welds exhibit shear strengths above 22 N/mm². The maximum shear strength exceeds 30 N/mm² at scanning speeds of 30 and 50 mm/s. To compare different conditions, an effective speed, defined as scanning speed divided by the number of passes, is introduced. The highest effective speed of 7.7 mm/s (corresponding to 100 mm/s with 13 passes) yields a shear strength of 26 N/mm², balancing welding strength and throughput. The results clarify how scanning speed and number of passes govern joint strength, advancing the understanding of femtosecond laser welding of transparent polymers.