Process comparison of laser deep penetration welding in pure nickel using blue and infrared wavelengths

Laser sources with wavelengths in the visible blue spectrum are suitable for heat conduction mode welding of materials like copper and nickel due to the significantly increased Fresnel absorption compared to infrared laser radiation. Recently, blue laser sources with 445 nm wavelength have become available with sufficient power and beam parameters to exceed the intensity threshold for laser deep penetration welding. In laser beam deep penetration welding, the total absorption is significantly increased due to the multiple reflections in the keyhole compared to heat conduction mode welding. However, since the absorbed energy per reflection inside the keyhole is wavelength-dependent, it can be hypothesized that the choice of laser wavelength causes changes in the local energy distribution inside the keyhole, changing the keyhole dynamics. To investigate this, laser beam deep penetration welding experiments were carried out on 2.4068 pure nickel using an infrared laser source and a blue laser source with comparable beam properties. The experiments were monitored and compared by a multi-sensor setup and metallographic analyses. This setup included measurements of airborne acoustic emissions and two high-speed video cameras for spatter tracking and tracking of the keyhole area. The use of a blue laser beam led to a lower spatter quantity, an increase of porosity and a significant change of acoustic emissions, proving the hypothesis for pure nickel.