A method for identification and quantification of thermal lensing in powder bed fusion

With the increase in use of L-PBF for functional components, an increase in productivity demands higher laser power and continuous operation. However, both these factors can affect the optics involved. The principal components of a Laser Powder Bed Fusion system are the galvanometer mirrors and the focusing lens.

In the present work a method for identification and quantification of thermal lensing is proposed. Laser powder bed fusion experiments of stainless steel 316L powder was carried out at varying scan speeds and scan strategies in order to study the effects, of long exposure times, on the quality of single layer scans.

It was observed that the heating of silver-coated mirrors under continuous exposure led to a significant decrease in the beam quality as evidenced by weld track widening and loss in depth of melt pool penetration. Power measurements conducted at various locations along the beam path indicated that the mirrors were absorbing up to 42 % of the total power coming from the laser while also defocusing the beam.

Thus demonstrating the importance of including identification and quantification of thermal influence and possible instability of the optical elements as cause for irregular weld tracks.