Effect of self-nucleation on the crystallization kinetics and the microstructure of polypropylene parts manufactured by Laser Powder Bed Fusion

Laser Powder Bed Fusion (LPBF) of thermoplastic polymers is an additive manufacturing (AM) technique experiencing a remarkable growth in the recent years. Nowadays the most widely used materials are polyamides (PA12, PA11...) but massively available LPBF-compatible polyolefin powders recently arrived on the market. The need of understanding the crystallization process for these new materials is essential and this was addressed in the present work. Isothermal and non-isothermal crystallization kinetics was investigated by differential scanning calorimetry and parts microstructure was analyzed using polarized light optical microscopy, scanning electron microscopy, transmission electron microscopy and wide-angle X-rays diffraction experiments. It was shown that it is important to consider the significant nucleating effect of the sub-surface layers solidification during the building stage. Indeed, half-crystallization times of the PP material are consistent with experimental observations and time range in LPBF only if crystallization occurs from a self-nucleated molten state. Evidence of the presence of nuclei were given thanks to microscopic and wide-angle X-rays diffraction (WAXD) techniques. Alternate patterns of very finely microstructured, oriented transcrystalline regions and isotropic spherulitic re-gions every 100 mu m (i.e. the layer thickness during printing) were highlighted.