Thermal‐History Engineering of Aged Polyamide 12 for Powder Bed Fusion: Effects on Crystallinity, Porosity, and Tensile Performance

Repeated thermal exposure during polymer powder bed fusion progressively alters the processing behavior of polyamide 12 (PA12), limiting powder reuse and reducing the mechanical reliability of printed parts. This study investigates how thermal‐history control can be used to mitigate the effects of aging in highly reused PA12 powder. Aged powder, reused up to six times without virgin‐powder refresh, was processed through three intervention strategies: pre‐sintering powder annealing, in‐process adjustment of build conditions, and post‐sintering part annealing. The effects of these strategies were evaluated by combining differential scanning calorimetry, x‐ray computed tomography, and tensile testing, with the aim of linking thermal behavior, crystallinity, porosity formation, and mechanical performance. Thermal aging reduced the degree of crystallinity of PA12 from 49.1% in the virgin powder to 44.7% after six reuse cycles, indicating a progressive modification of the polymer's ability to reorganize during heating and cooling. Under standard processing conditions, highly aged powder produced parts with reduced tensile strength, stiffness, and elongation at break, together with porosity levels of approximately 8%. The most effective recovery was achieved through in‐process thermal adjustment, in which increasing the bed temperature to 180°C and the layer thickness to 0.175 mm reduced porosity below 2.3% and recovered 53% of the tensile‐strength loss induced by aging. In contrast, post‐sintering annealing mainly affected stiffness but did not restore tensile strength because pre‐existing porosity was not eliminated. Pre‐sintering powder annealing at 135°C had the greatest effect on ductility, increasing elongation at break by 16.7%. Overall, the results show that the mechanical recovery of aged PA12 is strongly associated with changes in crystallization behavior, consolidation quality, and defect formation. The observed trends are consistent with a possible contribution from melt‐coalescence mechanisms, although rheological measurements would be required to directly verify this interpretation. Thermal‐history control can therefore be used as an engineering route to tailor the structure–property response of reused PA12 powders in polymer powder bed fusion.