Effect of power modulation frequency on porosity formation in laser welding of SAE 1020 steels

This study aimed to assess and to understand the effect of modulation frequency on weld bead porosity. A fiber laser source with a maximum power of 10 kW was used on SAE 1020 plates. A square wave modulation with pulse power at 4 kW and base power at 0 kW was applied, which resulted in a power average of 2 kW. Welding speed was set at 1 m/min, and frequency varied from 20 to 100 Hz. To analyze the pores, longitudinal cross-sections were prepared. The results indicate that the welds produced at low frequencies (20 Hz and 25 Hz) contained a significant amount of large spherical pores. On the other hand, a reduced number of smaller pores were found for high frequencies (50 Hz and 100 Hz). This can be correlated to the penetration depth oscillation, linked to the weld pool fluid flow and the keyhole collapse, which was greater at lower frequencies. In addition, other effects could be noted, such as the widening of the weld bead for low-frequency values, which can be beneficial when the process requires low geometrical tolerances and higher penetration depths for increased degrees of overlap at higher frequencies.