Process‐ and Material‐Induced Surface Structures During Laser Polishing

Abstract

Laser polishing is a technique for smoothing the surface of metallic substrates. The roughness after polishing does not only contain remains of the initial surface roughness, it also contains new roughness due to surface structures, which are introduced by the polishing process. Since these structures greatly contribute to the roughness, it is necessary to understand the mechanisms that lead to their formation so as to prevent the structures from forming and allow higher smoothing of the surface. Hence, in this publication it is investigated experimentally and numerically why these structures occur, what the influencing parameters are and how their influence on the roughness can be reduced, for both laser macro and laser micro polishing. It could be seen that the structures are influenced by the process parameters and the material of the workpiece. For a low surface roughness, the process parameters have to be adapted antithetically in some cases which means that it is not possible to prevent all structures at the same time and that surface structures will always occur during laser polishing. The process parameters must be adapted in such a way that all structures together lead to a surface roughness as low as possible.

Influence of Overlap on Surface Quality in the Laser Polishing of 3D Printed Inconel 718 under the Effect of Air and Argon

“…This causes the formation of bulges, which, in combination with the zigzag strategy, results in the shaping of the periodic structure perpendicular to the scanning direction. However, when overlap is high enough, this effect is no longer dominant, which proves the findings presented by Nüsser et al, as shown in Figure 8 b and Figure 9 b [ 14 ]. When higher overlaps are applied, energy density is increased exponentially, which results in higher local heat accumulation on the surface.…”

“…The material’s relocation mechanism is linked with the laser thermal energy process (LP) and it might lead to the non-uniformity of the distribution of amplitudes (peaks and valleys). This is the result of the forming of material- and process-induced surface structures during melt-pool re-solidification [ 14 , 23 ]. Therefore, the Abbott–Firestone curve can be used to obtain information regarding the percentage distribution of the asperities over the analyzed surface.…”

“…Moreover, when the zigzag scanning method is applied, the surface is characterized by a periodic structure perpendicular to the scanning direction. This effect was noticed by many researchers and categorized by Nüsser et al as bulges [14]. The wavelength of the structure is equal to the hatching pitch.…”

“…The most common scanning strategy for LP is the zigzagging of parallel lines ( Figure 1 ) [ 13 ]. However, this has many drawbacks, such as the overall high heat impact (due to the required overlap between tracks), and the formation of step structures, ripples, bulges, and undercuts [ 14 , 15 ]. The distance between neighboring tracks is kept constant and it is called the hatching pitch ( h ).…”

Smoothing additive manufactured parts using ns-pulsed laser radiation

“…Mainly, continuous wave operating laser sources (cw) are used for macro-applications with remelting depths up to 200 µm. In contrast, pulsed laser sources (pw) are generally used for micro polishing with remelting depths of several micrometers [21]. Lambarri et al investigated the laser surface smoothing process of nickelbased alloys and were able to show a reduction in roughness as a function of the scanning speed and laser power using cw laser radiation [22].…”