A novel graded laser energy input strategy for fabrication of horizontal overhanging Ti6Al4V metallic structures with PBF-LB method
Description
Data will be made available on request.A graded laser energy input strategy including three fabrication steps involving the optimizations of a single molten track, single-sided cantilevered structure, and horizontal overhanging structure as a bridge-like type was investigated. Results revealed that a defocus distance of 5 mm combined with a low laser power of 20 W and scanning speed of 600 mm/s is suitable for fabricating the 1st layer, which is slightly sintered to exhibit lower surface roughness and serves as a structural support for the upper layers. For the next two layers, a slightly higher laser power of 40 W was proven to be optimal for minimizing powder adhesion on the bottom surface, forming satisfactory bonding without interlayer delamination and eliminating warpage originating from continuous accumulation of internal stress. Since the 4th layer, a higher volume energy density (VED) with a laser power of 120 W and no defocus distance aimed at additionally remelting the previous three layers was proved to be effective in further reduce the porosity formed due to incomplete melting while maintaining lower warping height. The 30-layer horizontal overhanging structure fabricated using a graded strategy reaches satisfied balance between dimensional accuracy and surface quality, displaying higher density of 99.72 ±0.21 %, lower thickness deviation of approximately 20 %, top surface roughness of 13.3±1.19 μm and bottom surface roughness of 22.2±2.28 μm.
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Higher-magnification microstructures were observed using an optical microscope (OM, MX6R) and a field emission scanning electron microscope (SEM, Hitachi-SU8220). The surface morphology and roughness were detected using optical 3D metrology and surface roughness measurement equipment (Alicona, InfiniteFocus SL) equipped with a high-precision charge-coupled device (CCD). Two randomly selected maps with areas of 1 mm2 for each sample were used to obtain the areal surface roughness denoted as Sa. All six measured Sa. on three replicate samples corresponding to one group of processing parameters were recorded in Excel software to form a data set to further calculate the average value and standard deviation. Internal defects were detected using micro-computed tomography (μ-CT, nanoVoxel-3000). The μ-CT scanning parameters were set as an X-ray source voltage of 135 kV, beam current of 55 μA, exposure time of 0.5 s, and resolution of 10 μm, with a copper sheet installed in front of the X-ray source for filtering. The horizontal overhanging structures fabricated in this study inevitably exhibited warpage, which was evaluated by measuring the variation in warping height away from the original point of the base part. The maximum warping height was determined by measuring the difference between the height of the lifted end point and the position of the horizontal virtual line of the base part in the side view. The variation in warping height at other positions was obtained based on multi-segment data stitching within a 2 mm × 2 mm field of view which was processed using MATLAB software to calculate the average warping heights at different horizontal positions. To accurately determine the thickness of the horizontal overhanging structures containing multiple-layers, three replicate samples were categorized into two groups and one half was selected for cross-sectional morphology observation using OM. Each sample was measured at fifteen points to obtain the average thickness, and the standard deviation represented as an error bar was calculated.
Institutions
- Guangdong University of Technology