Porosity defects significantly affect the performance of parts manufactured by laser powder bed fusion (LPBF). Reducing or avoiding defects is a key issue in the LPBF technology. The complex, small, and rapid characteristics of the melting and solidification reactions in the LPBF manufacturing process make monitoring and controlling the defects of the formed parts in real-time using existing methods challenging. Traditional trial-and-error methods cannot analyze the formation mechanism of porosity defects, owing to their shortcomings, such as long cycle time, high cost, and low efficiency. Recently, mesoscopic simulations using dynamic molten pools have provided a new method for studying the formation mechanisms and regulation of defects in the LPBF manufacturing process. However, existing mesoscopic simulation has not been sufficiently researched on the formation mechanism of interlayer porosity defects, which manifests as the following issues: the powder-spreading process does not conform to the actual situation and always uses a scanning strategy with the same scan direction, and the formation mechanism of multi-layer porosity defects under multiple processing strategies requires further research. Therefore, it is necessary to establish a multilayer additive mesoscopic model under a chess scan strategy with an interlayer angle, and comprehensively explore the porosity defect formation mechanism under this commonly used scan strategy.
Herein, AlSi10Mg powder was used as the research object. First, an irregular powder bed that fits the actual manufacturing process was established by simulating the movement of the blade based on discrete element method (DEM), and computational fluid dynamics (CFD) numerical simulation was performed using the volume of fluid (VOF) method. Based on the established model, the thermal dynamic behavior of the molten pool was analyzed, and the main fluid driving forces in the change in the molten pool morphology were summarized. Using the above simulation method, the evolution mechanism of porosity defects in the formed parts during single track, multi-track, and multi-layer additive manufacturing processes was studied by adjusting the laser power and hatch space. Finally, using LPBF additive manufacturing equipment, the porosity morphology under the chess scanning strategy with an interlayer angle of 67° was experimentally verified through process tests.
It is found in the single-track printing simulation that under a fixed scan speed, laser power plays a decisive role in the morphology of the tracks. At low laser powers, powder melting is incomplete, and the fluidity of the liquid droplet is poor. When the binding force of the loose powder is insufficient, the molten pool is prone to forming balling defects owing to the effect of surface tension (Fig.10), and porosity defects are formed because of the inability to fill the gaps between the powders entirely (Fig.11). In addition, the track is prone to distortion defects under the Marangoni effect [Fig.9(b)]. At a high laser power, the spatter phenomenon intensifies owing to the increase in the Marangoni effect and recoil pressure (Fig.12). For multi-track printing, a large hatch space leads to small overlapping areas between the tracks, resulting in the formation of porosity defects between tracks, as well as an increase in surface roughness as the penetration depth of the track edge is small and powder melting is incomplete (Fig.13). When the hatch space is small, the heat accumulation effect is aggravated, reducing the process efficiency (Fig.14). In multilayer printing, porosity defects are mainly affected by the nonuniform thickness of the powder bed. The height difference between the two ends of the tracks caused by the combined effects of the recoil pressure, Marangoni effect, surface tension, and gravity, as well as the central bump of the tracks formed by surface tension, are the main factors affecting the surface roughness of the formed parts (Fig.15). Interlayer porosity defects tend to form when the laser energy input is insufficient (Fig.16). Finally, the porosity evolution mechanism is verified through the LPBF process tests (Figs.19 and 20).
The results show that defects such as balling, distortion, and porosity tend to be formed under low laser power, whereas the spatter is aggravated under high laser power. A large hatch space causes porosity in the overlapping area, and a small hatch space aggravates the heat accumulation effect. Therefore, it is necessary to reasonably adjust the laser power, scan speed, and hatch space to maintain the laser energy input within suitable range when optimizing the processing parameters. For multi-layer printing with the chessboard scan strategy and an interlayer angle of 67°, the main factors affecting the roughness of a single-fraction solid surface are the different heights of the two ends of the tracks and the grooves between the tracks. An uneven solid surface affects the thickness uniformity of the powder layer, resulting in porosity between the layers, thus, affecting the bonding quality of the layers. An appropriate energy input will help form a surface with low roughness and provide sufficient penetration to reduce interlayer defects.
