Selective laser melting of CM247LC

Autor: Gerstgrasser, Marcel
Přispěvatelé: Wegener, Konrad, Hass, Franz
Jazyk: angličtina
Rok vydání: 2021
Předmět:
Popis: Additive manufacturing (AM) processes, such as selective laser melting (SLM), are fast growing research fields. The processability of different materials with SLM presents an aspect of those recent fields of research. Especially the “unweldable” nickel-base alloys, such as CM247LC, are of interest, to gain a better understanding in case of the materials cracking behaviour. CM247LC is particularly sensitive for solidification cracking (hot cracking), not only during welding, but also during the SLM process. During the operating state, the beam diameter has to be around 90 μm, in case of a 200 W laser, to realize deep melt pools. Due to a reduced laser beam diameter and a higher intensity in the focus plane, deeper melt pools are generated and cracks in the layers below are re-molten, which leads to crack healing during SLM-processing. Between the two calibration procedures with different beam diameters, the crack-density can be reduced by a factor of 98. In case of Archimedean density and hot crack healing, the laser beam diameter is identified as an essential SLM parameter. A stronger texture parallel to the build direction is indicated in case of the smaller beam diameter. Furthermore, Archimedean density, the lack of bonding and the porosity have been analyzed with varying laser scan speeds, in case of the calibration procedure with the smaller laser beam diameter. Based on the SLM parameters, which lead to crack healing and crack free CM247LC samples, in-situ multi laser beam strategies are applied to reduce residual stresses and the corresponding part distortion. A second post heating and non-melting laser source with a defocused laser beam and a lateral offset is used to reduce the distortion of SLM-processed cantilevers. An optimum offset has been dentified, based on 9 different logarithmic offset parameter pre-tests. An upper power limit for the second heat laser beam with a beam diameter of 380 μm is identified at 65 W, to avoid re-melting cracks and defects. The distortion of the cantilevers are reduced more than 7.5 %, compared to the reference. To explain the residual stress behaviour and reduction during SLM-processing with the multi laser beam offset strategy, an analytical “two bar” model is presented. Furthermore, to understand re-melting cracks, defects and the corresponding microstructure more in detail, a laser power of 200 W is applied instead of 65 W for the second laser beam with a beam diameter of 380 μm. The re-melting crack analysis helps to understand the cracking behaviour of SLM-processed CM247LC, especially for further investigations, as further described below. A coupled cellular automata (CA) approach to simulate the microstructure and concentration distribution of two different materials, nickel-base alloy CM247LC and widely-used 1.4404 (stainless steel 316L), in the middle and high scan speed range is investigated, to understand the solidification behavior and microstructure more in detail. The numerical approach is based on local non-equilibrium models for rapid solidification. The simulation outputs from both materials are qualitatively and quantitatively compared with each other and validated with experiments in case of 1.4404. Simulations and experimental results are in good agreement. The grain morphology is defined by the melt pool geometry. Thin columnar and a larger number of grains are found in the nickel-base alloy CM247LC, compared to 1.4404. Furthermore, the grain morphology is strongly dependent on the temperature history, cooling rates and diffusivities, as demonstrated in detail with microstructure cross-section and single crystal simulations. Additionally, concentration maps are analysed in case of both materials. The concentration maps of CM247LC are identified as a possibility for hot crack predictions. To understand the process and laser parameters, which would lead to crack healing over the next applied layer, single melt pool tracks of the process and laser parameters are simulated in conjunction with gradually increased laser scan speeds. Similar observations are made regarding the reduced geometrical dimensions with higher scan speeds. By keeping the maximum laser intensity from the reference and the laser power to scan velocity ratio constant, the intensity approach provides an initial estimation for the laser spot size regarding the measured Archimedean density and crack density in the high power and high scan velocity range. The investigated cracks are identified as re-melting cracks. Solidification cracks are not observed, since the crack healing effect for those kind of cracks still occurs. Furthermore, a melt pool depth range from 66 μm to 81 μm is discovered, where not only solidification cracks can be avoided, but also re-melting cracks due to the higher laser power input. This theory can be proven by laser spot size investigations in case of beam diameters from 132 μm to 164 μm for 600 W, where the melt pool depth comes closer to the mentioned crack-free range from 66 μm to 81 μm. 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 with 200 W, a scan velocity of 800 mm/s and a laser spot of 90 μm. The production time is reduced nearly by 300 %. Dimensional analysis is used for generating a physical, analytical modelling approach with dimensionless parameters, which are derived from Buckingham’s Pi-theorem. Six main and two additional parameters, including the laser power and beam diameter, which are part of the intensity, are taken into relation. This enables to change parameters accordingly while keeping the quality of the AM-part unchanged. The sample density is coupled with the crack density over the melt pool depth, which is identified as a key factor for both parameters. The models based on the Buckingham`s Pi-theorem are in good agreement with the experiments.
ISBN:978-3-907234-70-9
Databáze: OpenAIRE
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