Investigating the hot isostatic pressing of an additively manufactured continuous carbon fiber reinforced PEEK composite

Additive manufacturing has recently begun to explore its role in the composites industry, utilizing the capability of complex fiber placement and part geometry to further reduce the weight of composite structures. Additively manufactured (AM) thermoplastic composites suffer weak interlayer bonding and high void contents. This study investigates the use of hot isostatic pressing (HIP) to post-process AM continuous carbon fiber reinforced polyetheretherketone (PEEK) composites to improve their flexural, interlaminar shear, tensile, and compressive properties. Isostatic pressure (200 psi) and elevated temperatures were used in combination to compress internal voids, promote PEEK crystallization, and enhance inter-filament polymer diffusion.Three different HIP temperatures of 200 °C, 250 °C, and 300 °C, between the glass transition and heat deflection temperatures of the composites, were considered. The post-processing was found to improve properties, with flexural strength and interlaminar shear strength improvements of up to 46% and 30%, respectively, and tensile strength and modulus reaching values as high as 1312 MPa and 92 GPa. The accompanying changes in void content and polymer structure for various HIP processing parameters were investigated to account for the changes in mechanical properties and failure mechanisms. Specifically, increasing the treatment temperature was found to monotonically reduce the void content. The polymer degree of crystallinity (DOC), however, was found to change appreciably only for the highest treatment temperature (300 °C), which negatively impacted the flexural and interlaminar composite properties. Treatment at 300 °C is understood to embrittle the polymer matrix, thereby increasing the stress concentration factors of the internal porosity. HIP demonstrates a fast and robust method to post-process AM composite parts, resulting in a significant improvement in their mechanical performance whereas a high crystallinity with void presence could degrade the performance of AM composites. The outcome of this study can be utilized to improve both the additive manufacturing process and post-HIP treatment to enhance the performance of AM composites.