Markus Hofele

In laser powder bed fusion of metals small melt pools with extremely short solidification times and steep temperature gradients to the surrounding material exist due to the layer‐wise and selective melting process with small areas of energy input compared to the cooler, solid ambient material. This causes high thermally induced residual stresses (RS), which can lead to the immediate rejection of the component due to critical part deformation and inhomogeneous mechanical properties under load. To prevent this, furnace‐based stress relief heat treatments are commonly applied before cutting‐off the part from the built platform. In this study a novel closed‐loop controlled laser‐based heat treatment using temperature feedback through inline pyrometer measurement is investigated, enabling a fast and highly efficient postprocess stress relief. Therefore, a laser beam is guided by a scanner optics in a meandering pattern over the top surface of AlSi10Mg cantilever specimens. It enables a decrease of near‐surface RS from 158 to 5 N mm⁻², resulting in a reduction of displacement by over 90% at material affecting depths up to 3 mm and area rates of 8 to 163 mm² s⁻¹.

In the context of the fourth industrial revolution, condition monitoring and predictive maintenance of machines are becoming increasingly important, as unplanned machine failures can lead to high costs for the operator. Many of these machine failures can be traced back to bearing damage, which must be prevented. In order to be able to make reliable statements about the remaining service life of the bearings, a condition monitoring system is required. For the development of these systems, bearings with a defined level of damage are needed. Since there are not enough reproducible methods for bearing damage so far, the defined generation of reproducible, local bearing damage is researched by using laser technology. The damage caused is checked and evaluated by means of vibration analysis and white light interferometry. The results confirm a reproducibility and scalability of the bearing damages by means of laser technology. The damaged bearings can be used for research-based development as well as in industrial implementations of condition monitoring systems.

In this study, the influence of laser polishing on the microstructural and mechanical properties of additively manufactured aluminium AlSi10Mg Laser Powder Bed Fusion (L-PBF) parts is analysed. The investigation is carried out on a 5-axis laser cell equipped with 1D Scanner optics driven by a solid-state disc laser at a wavelength of 1030 nm. Laser polishing is performed with pulsed or continuous laser radiation on samples in the initial L-PBF state or after stress relief treatment in a furnace. The metallurgical investigation of the remelting zone with a depth of 101–237 µm revealed an unchanged and homogeneous chemical composition, with a coarsened α-phase and a changed grain structure. The hardness within the remelting zone is reduced to 102–104 HV 0.1 compared to 146 HV 0.1 at the L-PBF initial state. Below the remelting zone, within the heat affected zone, a reduced microhardness, which can reach a thickness up to 1.5 mm, occurs. Laser polishing results in a reduction in residual stresses and resulting distortions compared to the L-PBF initial state. Nevertheless, the re-solidification shrinkage of the polished surface layer introduces additional tensions, resulting in sample distortions well above ones remaining after a stress relieve heat treatment of the initial state. The mechanical properties, analysed on laser polished flat tensile specimens, revealed an increase in the ultimate elongation from 4.5% to 5.4–10.7% and a reduction in the tensile strength from 346 N/mm² to 247–271 N/mm² through laser polishing. Hence, the strength resulting from this is comparable to the initial L-PBF specimens after stress relieve heat treatment.

In this study a new approach to laser polishing with periodic modulated laser power in the kilohertz regime is introduced. By varying the modulation frequency and modulation time, different periodic laser power curves with varying minimum, peak and average laser power can be created. The feasibility of the method is shown by polishing of vertical built AlSi10Mg L-PBF parts with an initial roughness of Ra = 12.22 µm. One polishing pass revealed a decreasing surface roughness with increasing energy density on the surface up to Ra = 0.145 µm. An increasing energy density results in a rising remelting depth between 50 and 255 µm and a rising relative porosity of 0.3% to 4.6%. Furthermore, the thermal process stability, analysed by the melt pool length in scanning direction, reveals a steadily increasing melt pool dimension due to component heating. Multiple laser polishing passes offers a further reduced surface roughness, especially at higher modulation frequencies and provides an improved orientation independent roughness homogeneity. The process stability regarding varying initial surface roughness revealed an almost constant relative roughness reduction rate with an achievable roughness variation after two polishing passes between Ra = 0.13–0.26 µm from an initial state of Ra = 8.0–19.2 µm.

Laser powder bed fusion is a well‐established 3D printing technique for metal alloys, but exhibits a poor surface quality. Laser polishing provides the possibility of a fast contact‐free and fully‐automatable surface treatment. This paper deals with the experimental investigation of laser polishing of laser powder bed fusion parts made of aluminium AlSi10Mg. Laser polishing is done with a 4 kW solid state disc laser in combination with a multi‐axis system and a one dimensional scanner optic. The laser is operated at continuous and pulsed operation mode. The parameter study reveals a high dependency of the achievable roughness on the laser beam intensity, the track and pulse overlap, the energy density and the number of polishing passes and polishing directions. Pulsed laser polishing mode with up to four passes from different directions revealed the lowest surface roughness of 0.14 μm Ra. With respect to the initial average surface roughness of Ra = 8.03 μm a reduction of the surface roughness of greater than 98 % could be achieved. Polishing with continuous laser radiation at one polishing pass resulted in Ra = 0.23 μm at an area rate of 20 cm²/min. Laser polishing using four passes achieved a further improvement up to Ra = 0.14 μm.