Based on SLM parameters from previous works, which guarantee fully dense and crack free CM247LC samples, multi laser beam strategies have been pursued to reduce residual stresses or rather distortion during LPBF processing. By using a second post heating and non-melting laser source with a defocused laser beam and lateral offset, cantilever distortion is reduced more than 7.5%, compared to the reference. Based on pre-tests with 9 different offset parameters, the optimum offset has been identified. Also, an upper limit for the laser power of 65 W is identified for the second heat laser beam with a spot diameter of 380 μm, to avoid re-melting and creating new defects. A theoretical “two bar model,” to explain the residual stress behavior and reduction with multi laser beam offset strategy during the LPBF process, is presented. Furthermore, re-melting cracks, defects, and microstructure are analyzed in conjunction with the second defocused offset laser, in case of a 200 W laser power, an increased scan speed of 1300 mms/s, and a reduced hatch distance. Secondary electron signal (SE) images of re-melting cracks are analyzed and compared to SE-image of hot cracks (solidification cracks). Based on electron backscatter diffraction (EBSD), the results of the microstructure from the last mentioned multi laser beam approach, which creates re-melting cracks, are presented and analyzed.
High power parameters with increased scan velocities and beam diameters are investigated, to decrease the production time for crack sensitive alloy CM247LC. Results are compared to crack-free reference LPBF-samples in the low power range. The re-scaling approach for the high power range is based on the constant maximum laser intensity from the reference parameters in the low power range. While keeping the laser power to scan velocity ratio constant, the re-scaling approach, also called “intensity approach”, provides an initial estimation for the laser spot size in the high power range. The investigated cracks from the high power range are identified as “re-melting cracks”. Solidification or hot cracks are not observed, since the crack healing effect for those kinds of cracks still occurs. Furthermore, a melt pool depth range is discovered, where not only solidification cracks can be avoided, but also re-melting cracks, which are resulting from higher laser power inputs. This theory can be proven by further laser spot size optimization, where the melt pool depth comes closer to the mentioned range. The Archimedean density and crack density results, in case of the 600 W power parameter with 2400 mm/s scan velocity and a beam diameter of 164 µm, are close to the one obtained from the reference samples, based on 200 W. With the intensity approach and laser beam diameter optimization, the production time can be reduced by 300%. Based on dimensional analysis, a model, which combines the samples density with the crack density through the melt pool depth, is presented. Six main and two additional process and laser parameters are taken into relation. The result from the model and the measured values from experiments are in agreement. Additionally, the influence of the doubled layer thickness and an increased hatch distance by 50% with varying scan velocities on the Archimedean density and crack density is analysed.
Dieser Beitrag stellt ein neues Methodenmodell für den Bereich Forschung und Entwicklung am Beispiel Beschlägeindustrie, sowie die Eingliederung dieser Methodik in den Produktinnovationsprozess vor. Es ist dabei auch die Auswahl der Fertigungstechnologien mit einer Technologiedatenbank in der Methodik abgebildet.
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