Selective laser melting of CM247LC

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

Contactless surface flattening of additive manufactured nickel-base alloy parts by ultra-short pulsed laser ablation

Nickel-base alloy samples produced by selective laser melting (SLM) exhibit high surface roughness of 14.2 μm ±4.8 μm standard deviation at the sidewalls. Ultra-short pulsed (USP) laser ablation at a pulse duration of 10 ps is applied to reduce the surface roughness. This flattening method does not require a melting phase. The influence of the tilt angle and the orientation of the SLM layers on the surface topography of the sample is investigated. Orthogonal laser ablation of Ni-base alloy samples leads to irregular surface topography and exposure of sharp spikes. Surface flattening by laser ablation is demonstrated applying a tilt angle of 80° in combination with a projected pulse fluence of 0.21 J/cm². A significant influence of the tilt angle and SLM layer orientation on the surface roughness is observed.ISSN:2212-827

Surface quality improvement and adjustment of SLM-processed CM247LC samples by modulated laser parameters

Nickel-base alloys, such as “CM247LC”, are usually used in the aviation field to manufacture high-pressure turbine blades with cooling channels, to increase the turbine entry temperature and the efficiency. Since there are many different cooling strategies for turbine blades, the requirements for the surface parameters (e.g. surface roughness) vary highly. In this study, five different surface parameters, such as root mean square height, skewness, kurtosis, developed interfacial area ratio and texture aspect ratio with different modulated laser parameters, have been measured, analyzed, plotted as diagrams and compared with reference continuous wave SLM-produced samples. Process maps with an overall surface parameter (including all five parameters) are also presented. Results show a wide range of surface parameter values, as needed for the varying surface requirements of different blade cooling systems.ISSN:2212-827

Residual stress reduction of LPBF-processed CM247LC samples via multi laser beam strategies

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.ISSN:0268-3768ISSN:1433-301

Focus shift analysis, to manufacture dense and crack-free SLM-processed CM247LC samples

Nickel-base alloy CM247LC is particularly sensitive for weld-cracking during powder-bed fusion, caused by an inappropriate laser spot diameter due to focus shifting. In this paper, the influence of a 4 mm focus shift on the melt pool geometry, intensity, relative Archimedean density, crack density, microstructure and texture is investigated, by using two different calibration procedures for the laser system. The hot calibration method is a procedure to overcome the thermally induced focus shift of 4 mm by setting the focus plane 4 mm below the build plane. After heating up to the operating temperature of the laser system by scanning two times an area of 10 x 10 mm², a stable focus diameter in the build plane is guaranteed. This procedure enables fully dense and crack-free CM247LC samples. Due to deeper melt pools, realized by higher intensity and a reduced laser beam diameter in the focus plane, the cracks in the layers below are re-molten and crack healing occurs. Between the cold and hot calibration procedure, the crack-density has been reduced relatively more than 98-times. Therefore, the laser beam diameter has been identified as an essential SLM parameter for healing hot cracks. Samples, which are manufactured with the hot calibrated system, indicate a stronger texture parallel to the build direction, compared to the cold calibrated one. Furthermore, Archimedean density, the lack of bonding and the porosity have been analyzed with varying laser scan speeds for the thermal steady state system. The increased Archimedean density correlates well with the decreased lack of bonding and porosity.ISSN:0924-013

CA single track simulation of laser conduction welding with stainless steel 316L (1.4404)

The study presents a coupled cellular automata (CA) approach for a single track microstructure simulation used for laser conduction welding. A high-power CO2 laser beam (1000W) traverses the substrate, with the beam shaped by conventional optics, which produces a Gaussian profile. The process relies on a shallow melt phase to maintain a conduction limited weld. Laser conduction welding does not require a filler material. The stainless steel material 1.4404 was used as substrate material with an initial grain size of 10µm and 20µm. The melt pool geometry, temperature history, cooling rates and diffusivities define the grain morphology. Temperature-dependent diffusivity coefficients and atomic spacing parameters are suggested. The simulation outputs of the grain morphology are qualitatively and quantitatively compared to experimental results. Initial results have shown that due to the individual melt pool conditions complex microstructures are developed. These fine, complicated microstructures cannot be satisfactorily resolved and quantified using standard optical microscopy. Electron backscatter diffraction (EBSD) has to be used for validation.ISSN:2212-827

CA single track microstructure simulation of nickel base alloy CM247LC and stainless steel S316L, including experimental validation of S316L

Based on powder-bed fusion, a coupled cellular automata (CA) approach to simulate the microstructure and concentration distribution of two different materials, stainless steel 316 L and nickel base alloy CM247LC, in the middle and high scan speed range is presented. Local non-equilibrium models for rapid solidification are considered in this study and described. The simulation outputs from S316L and CM247LC are qualitatively and quantitatively compared with each other and validated with experiments in case of S316L. The melt pool geometry defines the grain morphology. Thin columnar grains and therefore a larger number can be found in the case of CM247LC compared to S316L. The temperature history, cooling rates and diffusivities have a tremendous impact on the grain morphology, based on SLM microstructure cross-section and single crystal simulations. Furthermore, concentration maps are analysed for both materials. In case of CM247LC, concentration maps are suggested as a possibility to predict hot cracks. Temperature dependent diffusivity coefficients and atomic spacing parameters are suggested. Simulations and experimental results are in good agreement.ISSN:0264-1275ISSN:1873-419

Analysis of two parameter identification methods for original and modified Johnson-Cook fracture strains, including numerical comparison and validation of a new blue-brittle dependent fracture model for free-cutting Steel 50SiB8

A non-linear relation between fracture strain and temperature is observed in Split Hopkinson Tension Bar (SHTB) tests for free-cutting steel 50SiB8 at high strains and high temperatures, due to blue-brittleness. A fourth-degree polynomial function for the thermal parts of the original and modified Johnson-Cook (JC) fracture strain constitutive equation is suggested and the corresponding parameters are identified. Fracture modelling of 50SiB8 is not reported in literature. 50SiB8 is characterized under quasi-static, isothermal, high temperature and high strain rate conditions using smooth and notched tension specimens. For the calculation of the original and modified JC fracture parameters, two different parameter identification procedures, so-called forward and backward parameter identification methods are performed, compared with each other and explained in detail. For the stress triaxiality factor of the original and modified JC fracture model, the forward parameter identification method is suggested for the original JC fracture model, while backward parameter identification method is proposed for the modified JC fracture model. The effect of blue-brittleness in the temperature factor and the corresponding parameters are presented. Furthermore, FEM simulations of the original and the blue-brittleness including JC fracture model are compared with experiments. In the blue-brittle temperature range, the new adjusted JC model shows more accurate results, regarding fracture diameter and strain. Additionally, this work introduces a fracture model combining the Bai-Wierzbicki stress triaxiality dependent model, JC fracture models and the nonlinear temperature factor with blue-brittleness effect, while further experiments are required in order to determine its parameters