Deformation and stress evolution during laser powder bed fusion of semi-crystalline polyamide-12

Laser powder bed fusion (L-PBF) of semi-crystalline polymers such as polyamide-12 (PA12) has found increasing use in various industrial applications. However, achieving high dimensional accuracy remains a significant challenge. Despite the seemingly straightforward layer-by-layer manufacturing concept, the L-PBF process involves complex thermal histories and strongly coupled multiphysics, making the evolution of stress and deformation mechanisms still not fully understood. To address this, a comprehensive three-dimensional thermo-mechanical modeling framework is developed to simulate the L-PBF process of PA12. The model for the first time incorporates transient heat transfer, phase transformation induced volumetric shrinkage, thermo-viscoelasticity, and a modified non-isothermal crystallization kinetics. To alleviate the computational burden of part-scale simulations, a dual-mesh strategy is employed to efficiently couple thermal and mechanical fields without compromising numerical accuracy, which also enables the framework to handle L-PBF simulations of arbitrarily complex three-dimensional geometries. Particular attention is paid to the role of mechanical and thermal boundary conditions. Specifically, the underlying powder bed is modeled as a fictitious viscous medium, providing support while permitting upward displacement. Additionally, a radiative heat loss boundary condition, which more closely approximates the actual physical process, is applied to the top powder surface. The incorporation of this radiation effect significantly enhances the crystallization rate and improves the agreement with experimentally measured warpage. The model is validated against experimental warpage data under various preheating temperatures. Furthermore, strain decoupling analysis for the first time reveals that displacement induced by phase transformation is approximately 10 times greater than that caused by thermal expansion, highlighting the dominant role of crystallization-induced shrinkage in warpage formation. Numerical tests also indicate that warpage is highly sensitive to the preheating target temperature of the PA12 powder bed, while the temperature of the newly recoated powder within the tested range has a limited effect. This work provides a predictive modeling foundation for future optimization of polymer L-PBF processes at part-scale, particularly in controlling deformation and improving dimensional accuracy.