Process-Structure-Property Interdependencies in Non-Isothermal Powder Bed Fusion of Polyamide 12

Abstract

Non-isothermal laser-based powder bed fusion (LPBF) of polymers suggests the potential for significantly extending the range of materials applicable for powder-based additive manufacturing of polymers, relying on the absence of a material-specific processing window. To allow for the support-free manufacturing of polymers at a build chamber temperature of 25 °C, applied processing strategies comprise the combination of fractal exposure strategies and locally quasi-simultaneous exposure of distinct segments of a particular cross section for minimizing crystallization-induced part deflection. Based on the parameter-dependent control of emerging cooling rates, formed part morphologies and resulting mechanical properties can be modified. Thermographic in situ measurements allow for correlating thermal processing conditions and crystallization kinetics with component-specific mechanical, morphological, and microstructural properties, assessed ex situ. Part morphologies formed at crystallization temperatures below 70 °C, induced by reduced laser exposure times, are characterized by a nano-spherulitic structure, exhibiting an enhanced elongation at break. An ambient temperature of 25 °C is associated with the predominant formation of a combined (α + γ)-phase, induced by the rapid cooling and subsequent laser-induced tempering of distinct layers, yielding a periodic microstructural evolution. The presented results demonstrate a novel approach for obtaining nano-spherulitic morphologies, enabling the exposure-based targeted adaption of morphological properties. Furthermore, the thermographic inline assessment of crystallization kinetics allows for the enhanced understanding of process-morphology interdependencies in laser-based manufacturing processes of semi-crystalline polymers.

Thermal Intra-Layer Interaction of Discretized Fractal Exposure Strategies in Non-Isothermal Powder Bed Fusion of Polypropylene

“…The emergence of curling is frequently interlinked to build process interruptions, hence impairing the processability of polymer-based material systems with a small thermal processing window [16,17]. Considering an elevated extent of shrinkage of semi-crystalline polymers [18], maintaining inherent advantages of quasi-isothermal LPBF of polymers such as support-free manufacturing while extending the range of applicable polymers indicates the necessity for novel approaches for the additive manufacturing of polymers at reduced ambient temperatures [3,6].…”

“…Therefore, the influence of varied exposure strategies and emerging temperature fields is associated with the emerging consolidation phase in quasi-isothermal processing. In contrast, the occurrence of a consolidation phase in non-isothermal LPBF [3,6] is inherently restricted due to a considerable supercooling and the corresponding rapid crystallization of the polymer melt [3,6], depending on the emerging cooling rate [25]. Existing research on support-bound LPBF at reduced build chamber temperatures is predominantly based on parameter adaptions of existing linear exposure strategies [7][8][9]26,27].…”

“…To date, laser-based processing routes are characterized by the application of quasi-isothermal processing, interlinked to inherent limitations in the range of applicable materials and the recyclability [1,2] of unfused material. Considering laser-based additive manufacturing processes, thermal processing conditions are of significant relevance with regard to governing the heating and melting of the powder particles, and ultimately, the solidification kinetics and the microstructural evolution [3] of the resulting part. The interaction between the laser beam and the powder bed, as well as the thermal history of the material during the process, exhibit a significant impact on the processing stability, associated with exposure-induced residual stresses in LPBF of metal alloys [4,5].…”

“…Aforementioned considerations are of significant relevance for the stable layer formation in non-isothermal LPBF of polymers, constituting the physical foundation for the localized meso-scale compensation of material shrinkage. Given the significance of aforementioned thermal intra-layer phenomena and the sensitivity of non-isothermal LPBF of polymers towards thermal processing conditions [3], an enhanced understanding of exposure strategies on emerging thermal processing conditions is essential for enabling support-free LPBF.…”

“…In contrast to quasi-isothermal processing, recent developments indicate the possibility for considerably enhancing thermal boundary conditions [3,6], extending the range of material systems applicable for laser-based powder bed fusion (LBPF) of polymers. Previously explored approaches for the non-isothermal powder bed fusion of polymers at reduced ambient temperatures are limited by the use of support structures [7][8][9] and limited build rates [6], impairing the ecological and economic viability of existing processing routes.…”