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Source: Yangtze River Delta G60 Laser Alliance
It is reported that research teams such as the Paul Scheler Institute in Switzerland and the Federal Institute of Technology in Zurich have an impact on the laser energy density on surface roughness, powder erosion zones, and pore formation mechanisms. The relevant research is published in the journal Communications Materials under the title "Opera ndo topographical microscopy during laser based powder bed fusion of alumina".
Laser powder bed melting (LPBF) is a powder bed based additive manufacturing process that uses high-energy laser beams to scan point by point for powder metallurgy bonding, thereby printing high-performance parts. However, due to the presence of numerous structural defects, the mechanical properties of dense ceramics manufactured by LPBF are poor. The research team conducted Operando chromatography microscopy observation on the LPBF process of magnetite modified alumina to gain a deeper understanding of its potential mechanism.
The effect of laser energy density on surface roughness, powder erosion zone, and pore formation mechanism was studied. Increasing laser power will significantly increase the width of the molten pool, but it will not increase the depth of the molten pool and will not result in molten pool depression. The force generated by recoil pressure has no significant impact on the dynamic behavior of the molten pool. Increasing power can avoid molten pores, but it will enhance the formation of spherical pores, which are formed by reaching the boiling point of liquid alumina or introducing bubbles by injecting hollow powder particles into the liquid.
Opera ndo X-ray tomography during laser powder bed melting process
In order to visualize the three-dimensional microstructure evolution during LPBF process, operando X-ray tomography microscopy experiments were conducted on the TOMCAT beam line of the Swiss light source (SLS) (Figure 1).
The scanning mode consists of 5 concentric circles with an aperture of 200 μ m. Obtain a cylindrical sample with a diameter of 2mm. Powder consists of micron and submicron scales α- The spray drying particles composed of alumina particles and 5.2% magnetite nanoparticles were confirmed by XRD analysis. The specifications of the particles are summarized in Figure 2.
The Influence of Laser Power on Surface Roughness and Spheroidization Effect
3. Figure 3 shows six representative snapshots of the selected power (P=3.35, 7.4, and 14.5 W). The selected perspective for these images is shown in Figure 1c as position "A". Visualization is created as volume rendering, with contrast corresponding to the attenuation coefficient of the X-ray beam. Adjust the color range, perspective, and light and shadows used for rendering to enhance the visibility of surface roughness. The color scale corresponds to the linear attenuation coefficient, which depends on the density of the studied material. The sliced material appears red, while due to the volume effect of the surface, the surface of the material appears green. For samples of 3.35 and 7.4 W, the laser scanning pattern starts from the inner ring, and for samples of 14.5 W, the laser scanning pattern starts from the outermost ring. 
The Effect of Laser Power on Powder Erosion
Figure 4 is the perspective view shown at position "B" in Figure 1c, which selects the time frame for processing a single ring at laser power of 7.4 W and 15.5 W. The image of "before laser scanning" in Figure 4 shows the surface of the previous layer before powder deposition after solidification (7.4 W is dark green, 15.5 W is cyan).
3D visualization of molten pool
Figure 5 compares the representative 3D rendering of the molten pool at laser power of 7.4 W and 15.5 W. The melting point density of solid alumina is 3.73 g/cm3, and the melting point density of liquid alumina is 3.05 g/cm3. The sample treated at 15.5 W shows a smooth and flat layer and a stable molten pool, which is beneficial for the material segmentation process. In contrast, the surface roughness of the samples treated at 7.4 W is significantly higher, resulting in unstable melt pools, making the segmentation process more challenging.
Figure 6 shows the visualization of the evolution of the molten pool during a 100 millisecond laser scanning process. Within the given time, the average volume displayed in the liquid phase is 8.7.106 ± 0.4.106 μ M3 indicates that the size of the molten pool has high stability over time. At 7.4 W, the width of the molten pool is 240 μ m. Depth between 30 and 60 μ Changes between m, total volume 1.7.106 ± 0.2.106 μ M3, significantly reduced compared to the molten pool at 15.5 W.
Contrary to the observed situation in LPBF metal processing, the shape of the horizontal (transverse) section of the molten pool is circular rather than elongated (Figures 5 and 6). On the cross-section of the sagittal plane (perpendicular to the laser scanning trajectory), the molten pool appears shallow and wide.
Pore formation mechanism
Two types of pores can be detected: irregularly shaped non fusible pores and spherical pores. Figures 7 and 8 show the volumetric and cross-sectional views of the sample at laser power of 7.4 W and 15.5 W, respectively. When the energy density is low and the power is 7.4 W, the wettability between the solid material (previous layer) and the liquid is poor. As shown by the pink arrow in Figure 7d and e, the liquid cannot penetrate the grooves on the rough surface, leaving pores. The yellow arrow indicates insufficient melting of pores, and the process has already begun. Obviously, due to the lower temperature of the previous layer, the wetting effect is poor. Figures 9 and 10 show two possible mechanisms for generating such pores under different time frames: the formation of pores and the presence of pre existing pores within the powder. The difference between the boiling point (2980 ° C) and melting point (2072 ° C) of alumina is relatively low. Therefore, it is impossible to reach the boiling point during laser processing, leading to the formation of pores. On the other hand, the SEM image of the powder (Fig. 2) shows that some spray dried particles, especially larger particles, are hollow. Therefore, injecting such particles into the molten pool will create pores in the liquid phase.
In this study, the effect of laser power on surface roughness, powder erosion, melt pool evolution, and pore formation mechanism in LPBF processing of magnetite modified alumina was studied.
Related paper links:
Makowska, M.G., Verga, F., Pfeiffer, S. et al. Operando tomographic microscopy during laser-based powder bed fusion of alumina. Commun Mater 4, 73 (2023). https://doi.org/10.1038/s43246-023-00401-3
