Influence of focal length on the Laser Metal Deposition process with coaxial wire feeding

In some applications of Laser Metal Deposition (LMD) the advantages of the utilization of wire instead of powder additives can be used. The use of a wire as a solid additive material eliminates major disadvantages of powder-based LMD processes, namely contamination of the process cell with metal powder, significant material losses during the process, health and safety issues as well as the impact of insufficient powder quality. A closed annularly shaped laser beam surrounding the wire is one of the distinguishing features of the discussed LMD processing head. The laser beam and wire are arranged coaxially to each other, with the wire being fed through the inside of an annularly shaped laser beam without any shadowing. The annularly shaped intensity profile leads to a significant improvement in wire-based LMD. Furthermore, in contrast to lateral wire feeding, two substantial technological advantages, namely the independence of the feed direction and the processing of complex 3D geometries can be identified. Since the angle of incidence on the wire and base material depends on the caustic of the annularly shaped laser beam, the focal length has a significant influence on the LMD process. The results of experiments with different focal lengths are discussed. Material samples from Inconel 718 are built and evaluated metallographically. With the help of a high-speed camera the varying process behavior in dependence of the focal length and laser beam quality is analyzed. Finally, an outlook towards future research will be given.