研究目的
Investigating the mechanism of pore evolution during the laser post-processing of an LPBF part to improve the quality of the fabricated part.
研究成果
1) Following post-processing with a laser scanning speed of 1 m·s-1, a hatch spacing of 54 μm, and a laser power of 200 W, the void fraction of the entire sample was reduced from 2.51% to 0.77%, and the maximum diameter of the pore was reduced from 50 to 20 μm. 2) A multi-physics coupled finite element model based on the Level-set method was built to analyze the evolution mechanisms of pores. Different laser scanning strategies and defect depths produce three types of pore evolution: (1) pore eliminated, (2) surface pore formed or (3) pore retained. 3) Mass transfer resulting from the Marangoni effect on the bubble surface is the essence of pore evolution. 4) The main factor impacting a gas bubble rising in the molten pool is the mass transfer resulting from the Marangoni effect rather than buoyancy. 5) The backward Marangoni flow in the molten pool results in mass transport to the tail region, and a vortex is thus caused, which causes the pore to rise to the tail region of the molten pool.
研究不足
The study focuses on the pore evolution during laser post-processing of an LPBF part, but the mechanism of porosity reduction is still unclear. The model simplifies the simulation by neglecting the vapor phase and assuming constant laser absorptivity.
1：Experimental Design and Method Selection:
A laser beam was utilized to scan the top surface of a Ti–6Al–4V alloy part. The morphologies of the internal layers as well as the distribution of the porosity inside the Ti–6Al–4V sample were experimentally measured and compared using micro-computed tomography (micro-CT). A multi-physics coupled finite element model based on the Level-set method was built to analyze the evolution mechanism of the pore.
2：Sample Selection and Data Sources:
The powder used for fabricating the samples was provided by AP&C from Canada. All the samples investigated were manufactured by using LPBF equipment in an argon protective atmosphere.
3：List of Experimental Equipment and Materials:
SEM equipment (FEI Inspect F50, Thermo Fisher Scientific, USA), micro-CT equipment (nanoVoxel 2000, developed by Sanying Precision Instruments and Tianjin University, China), LPBF equipment (HK-M250, Wuhan Huake 3D Technology Co., Ltd., China).
4：Experimental Procedures and Operational Workflow:
The build platform was preheated to 473.15 K. The laser power was set at 275 W, the hatch spacing was set at 54 μm, the thickness of the powder layers was set at 50 μm and the laser scanning speed was fixed at 750 mm·s-1. Each sample was formed into a rectangular Ti–6Al–4V plate with dimensions of 1.0 × 0.4 × 0.6 mm
5：15 K. The laser power was set at 275 W, the hatch spacing was set at 54 μm, the thickness of the powder layers was set at 50 μm and the laser scanning speed was fixed at 750 mm·s-Each sample was formed into a rectangular Ti–6Al–4V plate with dimensions of 0 × 4 × 6 mmData Analysis Methods:
3.
5. Data Analysis Methods: The three-dimensional volume data of the sample were reconstructed by software based on the two-dimensional projection image data acquired. These image data sets and the void fraction of the whole sample were further visualized and analyzed by software.
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