Multi-kW laser cladding using cylindrical collimators and square-formed fibers

TL;DR: In this article, an asymmetric collimator based on cylindrical lenses with an 1:3.3 aspect ratio of focal lengths is proposed for industrial laser cladding applications.

Abstract: In industrial laser cladding applications various new possibilities have opened up by introduction of laser sources with powers over 10 kW. Higher laser power allows higher deposition rates, which enables new applications for example in heavy engineering. However, to fully utilize the high power, beam area in focus needs to be increased significantly compared to for example welding. For high brightness lasers, this often requires complicated processing optics as the beam is usually Gaussian when defocused. In most surface treatment applications process would benefit from homogenous intensity distribution instead of a Gaussian one. In this paper we present ideas for cladding applications using a 12 kW disc laser coupled into a square-formed fiber with a 1000x1000 μm-core. The output of the fiber is collimated by a newly developed collimator based on cylindrical lenses with an 1:3.3 aspect ratio of focal lengths. The asymmetrically collimated beam is then condensed to a homogeneous rectangular spot on the work-piece using an f=500 mm focusing unit. With this setup we reach a spot size of 7.4x2.2 mm = 16.3 mm 2 , implying laser power densities up to 740 W/mm 2 . The asymmetric collimator is based on efficiently water-cooled cylindrical lenses with different focal lengths. Having interchangeable fiber connector interfaces and Optoskand's standard exit interface, the collimator can easily be implemented in optical heads. We present results on the optics performance including power transmission, image quality and focal shifts at power levels up to 12 kW. Results of preliminary cladding tests using the asymmetrical optics and offaxis tandem wire feeding will also be presented orally. Deposition rate and efficiency using high power levels will be investigated. Analyses of cladding bead geometry and microstructure will be performed.