Evolution of Surface Topography and Microstructure in Laser Polishing of Cold Work Steel 1.2379 (AISI D2) Using Quadratic, Top-Hat Shaped Intensity Distributions.

Within the scope of this study, basic experimental research was carried out on macro-laser polishing of tool steel 1.2379 (D2) using a square intensity distribution and continuous wave laser radiation. The influence of the individual process parameters on surface topography was analyzed by a systematic investigation of a wide range of process parameters for two different, square laser beam diameters. Contrary to a typical laser polishing approach, it was shown that short interaction times (high scanning velocity and small laser beam dimensions) are required to reduce both micro-roughness and meso-roughness. A significant reduction of surface roughness of approx. 46% was achieved from Raini = 0.33 ± 0.026 µm to Ramin = 0.163 ± 0.018 µm using a focused square laser beam with an edge length of dL,E = 100 µm at a scanning velocity of vscan = 200 mm/s, a laser power PL = 60 W and n = 2 passes. However, characteristic surface features occur during laser polishing and are a direct consequence of the laser polishing process. Martensite needles in the micro-roughness region, undercuts in the meso-roughness region, and surface waviness in the macro-roughness region can dominate different regions of the resulting surface roughness spectrum. In terms of mechanical properties, average surface hardness was determined by hundreds of nano-indentation measurements and was approx. 390 ± 21 HV0.1 and particularly homogeneous over the whole laser polished surface.

Surface Structuring by Laser Remelting (WaveShape): Microstructuring of Ti6Al4V for a Small Laser Beam Diameter and High Scan Speeds.

The appearance of a surface is a crucial characteristic of a part or component. Laser-based micromachining gets increasingly important in generating tailored surface topographies. A novel structuring technique for surface engineering is surface structuring by laser remelting (WaveShape), in which surface features are created without material loss. In this study, we investigated the evolution of surface topographies on Ti6Al4V for a laser beam diameter of 50 m and scan speeds larger than 100 mm/s. Surface features with aspect ratios (ratio of height to width) of almost 1:1 were achieved using the WaveShape process. Furthermore, wavelengths smaller than 500 m could be effectively structured using scan speeds of up to 500 mm/s. The experimental results showed further that the efficiency of the WaveShape process in terms of achieved structure height per unit time significantly increases for high scan speeds.

Shape Deviation of Surface Structures Produced by WaveShape (Structuring by Laser Remelting) on Ti6Al4V and a Method for Deviation Reduction.

Laser structuring by remelting (WaveShape) is a manufacturing process for metal surfaces in which structures are generated without material removal. The structuring principle is based on the controlled motion of the three-phase line in the area of the solidification front. The contour of the solidification front is imprinted into the remelting track during the continuous solidification process. Typically, harmonic surface structures in the form of sinusoidal oscillations are generated by means of WaveShape with virtually no material loss. However, a significant shape deviation is often observed over a wide range of process parameters. In this study, it was found that much of the shape deviation is concentrated at a spatial wavelength equal to half the spatial wavelength used for structuring. Therefore, an approach to reduce the shape deviations was specifically investigated by superimposing a compensation signal on the harmonic structuring signal. In this approach, a compensation signal with half the spatial wavelength was varied in phase and amplitude and superimposed on the structuring signal. Amplitude and phase shift of the compensation signal were further investigated for selected laser beam diameters and spatial wavelengths. This demonstrated that a shape deviation of harmonic surface structures on titanium alloy Ti6Al4V could be reduced by up to 91% by means of an adapted compensation signal.

Glass processing with pulsed CO2 laser radiation.

Recent results of processing fused silica using a high-power Q-switched CO2 laser source with a maximum output power of 200 W are presented. Compared to the processing with continuous wave laser radiation, the main advantage of pulsed laser radiation is the influence of the light-matter interaction with high laser peak power at small average laser power. An application for the approach presented in this paper is the flexible manufacturing and form correction of optics. This laser-based process is nearly independent of the surface geometry and can even be enhanced by laser polishing and expanded to other glass materials. Hence, the high-power Q-switched CO2 laser source is used to ablate glass material with an ablation rate up to 2.35 mm3/s and also for ablating glass material locally in a vertical dimension down to 3 nm.

Optical set-up for dynamic superposition of three laser beams for structuring and polishing applications.

Structuring by remelting is an innovative approach for structuring metallic surfaces with laser radiation, where no material is removed but reallocated while molten. Based on this remelting principle an innovative structuring technique is investigated, where laser beams are superposed. A melt pool is generated by a cw laser beam with constant feed rate. A pulsed laser is superposed onto the cw laser and evaporates a small amount of molten material and, therefore, generates vapour pressure, which shapes the melt pool surface. The solidification follows this newly shaped surface. For this process a new optical system was designed and built up, which allows the combination of cw and pulsed laser beams.