Applicability of pore boundary tessellation for quantitative characterization of pore evolution of binder jet 3D printed and sintered metal microstructures: A Ni-Mn-Ga case study and literature validation.

In the study of sintered microstructures, pore size and spatial distribution are relevant for the fabrication of porous structures for filtration, lightweight, and functional applications. Traditional stereology from optical micrographs is used to calculate mean pore size and separation but may result in large systematic error because of complex-shaped porosity. Pore boundary tessellation (PBT) can be used to advance microscopy image analysis by quantifying local spatial homogeneity and minimum 2D pore separation. This study focused on applying and comparing PBT of highly porous metal structures fabricated through Binder Jet Printing and partial sintering, with previous stereological measurements for a Ni-Mn-Ga magnetic shape memory alloy case study. PBT analysis of the tessellated cell pore area fractions indicates that the sintering parameters result in pore section pinch-off, shrinkage, and elimination. At 1070 °C the pore network breaks up without significant pore shrinking. At 1080 °C, the pores shrink but are not systematically eliminated. At 1090 °C the smaller pore sections in the population are preferentially eliminated, after significant shrinkage observed at 0.5 h. The distance in the solid phase between adjacent pore sections (effective cell wall thickness) was also examined. This result was similar to stereological grain size values (±30 %) for intermediate magnifications with pixel-to-μm ratios of ∼2.7, suggesting this method can be used to improve quantitative microscopy for sintered grain size estimation of this alloy, as well as other sintered metals containing grain boundary porosity (confirmed for sintered SS316L literature micrographs).