Dimensional assessment and process optimization of additively manufactured structured 3D porosity via primitive triply periodic minimal surface lattice structure and laser powder bed fusion technique
Creators: Mulhi, Ali
Publisher: name: University of Alberta Library
Year: 2023

<p>This thesis reports the challenges that need to be addressed before any heat transfer analysis of a proposed novel cellular-walled pipe heat sink system manufactured by LPBF technique. The proper cellular structure type selection for enhanced heat transfer performance, as well as providing a detailed analysis of its dimensional trends and CAD to manufactured deviations, are investigated. Triply Periodic Minimal Surface (TPMS) lattices have been heavily investigated lately due to their superior thermo-mechanical performance compared with their lattice counterparts. The advancement of additive manufacturing, i.e., laser powder bed fusion (LPBF), has easily enabled the manufacturing of such complex lattices. Recent studies have investigated the heat transfer performance of multiple TPMS lattice types such as Gyroid, Dimond, IWP, and Primitive structures. The Primitive TPMS (PTPMS) showed enhanced heat transfer performance mainly due to its cell shape and thickness (i.e., lattice topology). Hence, it was selected for the proposed cellular-walled pipe heat sink. Micro X-ray computed tomography (μCT) and optical microscopy (OM) were utilized to conduct the lattice dimensional analysis. Increasing the PTPMS lattice cell size from 2.9 to 10 mm showed an increase in the lattice wall thickness and pore size but a decrease in the SA:Vol ratio. However, increasing the lattice porosity from 45 to 90% resulted in a decrease in the lattice wall thickness but an increase in both the SA:Vol ratio and pore size. Comparing CAD to manufactured PTPMS lattices, the resulting lattice samples showed lower wall thicknesses and higher surface area to volume (SA:Vol) ratios than designed, which is attributed to shrinkage during the building process. The printed lattice pore size and porosity values were observed to be higher than the CAD values. Moreover, the minimum PTPMS lattice wall thickness and pore size that can successfully be printed were investigated and found to be 152 μm and 317 μm, respectively.<br>
The type of powder material used in manufacturing the cellular-walled pipe heat sink is another challenge. In the LPBF printing process, the printing parameters for any selected material need to be optimized to manufacture fully dense parts. 2507 super duplex stainless steel (2507 SDSS) is a promising material that was selected for manufacturing the proposed heat sink system. The printing parameters for 2507 SDSS, namely: laser power, scan speed, and hatch distance, were optimized. The response surface methodology was used in generating a detailed design of experiment to investigate the different pore formation types over a wide energy density range (22.22 - 428.87 J/mm3), examine the effects of each process parameter and their interactions on the resulting porosity, and identify an optimized parameter set for producing highly dense parts. Different process parameters showed different pore formation mechanisms, with lack-of-fusion, metallurgical or gas, and keyhole regimes being the most prevalent pore types identified. The lack-of-fusion pores are observed to decrease significantly with increasing the energy density at low values. However, a gradual increase in the keyhole pores was observed at higher energy densities. An optimal energy density process window from 68.24 J/mm3 to 126.67 J/mm3 is identified for manufacturing highly dense (≥99.6%) 2507 SDSS parts. Furthermore, an optimized printing parameter set at a laser power of 217.4 W, a scan speed of 1735.7 mm/s, and a hatch distance of 51.3 µm was identified, which was able to produce samples with 99.961% relative density. Using the optimized parameter set, the as-built 2507 SDSS sample had a ferrite phase fraction of 89.3% with a yield and ultimate tensile strength of 1115.4 ± 120.7 MPa and 1256.7 ± 181.9 MPa, respectively.</p>