Selective laser sintering (SLS) is a promising technology in polymer additive manufacturing (AM). It enables the selective melting and solidification of powders using a laser based on computer-aided design (CAD) data and production of three-dimensional prototypes or parts in a layer-by-layer sintering manner. SLS technology is increasingly used for the fabrication of complex structural components in many manufacturing areas owing to its short manufacturing cycle time, high geometric freedom, and possibility of low-cost customization. Polyetheretherketone (PEEK) is a semi-crystalline polymer with high-temperature resistance and excellent mechanical properties; therefore, researchers have been experimenting with its use in SLS technology to meet the demands of complex structural components designed to function in harsh environments. However, the high melting point and viscosity of polyetheretherketone cause significant equipment requirements; accordingly, only low-density parts can be manufactured using this material. Recently, the developments in PEEK for SLS technology and high-temperature laser sintering equipment have solved this problem, allowing to produce reliable parts. The tendency to form defects in SLS-prepared parts is an inherent feature of this technology, which can have a negative impact on the mechanical properties of the produced part. Therefore, studying the relationship between the porosity and processing parameters is a popular topic. Despite the importance of the mechanical properties of sintered materials, one of the research gaps that remains to be investigated is the lack of quantitative characterization of defects in laser-sintered PEEK. Accordingly, previous research has only focused on analyzing the microstructure of a small number of samples to improve the processing parameters. In this study, an attempt to address this issue is made by analyzing the microcomputed tomography (Micro-CT) results of sintered PEEK components. The main causes of defect formation during SLS forming are also explored, and the impact on the mechanical properties is analyzed. Consequently, an important theoretical and experimental basis for further in-depth research and practical engineering applications of this material is provided.
This study evaluated the microstructure and mechanical properties of PEEK parts prepared using the SLS technology and compared them to those of parts produced by injection molding (IM). The test samples were manufactured by a high-temperature laser sintering system and PEEK HP3 powder from EOS, Germany. The IM parts were cut from the PEEK injection-molded sheets using the computer numerical control (CNC) process. After that, microstructural characterization was conducted using optical microscopy (OM), scanning electron microscopy (SEM), and Micro-CT to evaluate the PEEK samples. The mechanical properties were determined using a universal material-testing machine. The tensile strength, elongation, and elastic modulus of each sample were measured. Furthermore, the thermophysical properties related to the material properties were determined. Finally, the differences in the mechanical properties between the samples manufactured by the two manufacturing methods were compared, and the causes of the phenomena were determined based on the microstructural results.
The test results show that SLS PEEK has slightly lower tensile and flexural strengths and significantly poorer elongation at break but exhibits a higher modulus of elasticity and temperature resistance than the IM PEEK specimens (Figs. 5 and 6). The Micro-CT results show that defects can be classified into three categories based on their characteristics, namely, small, dense pores, band boundary defects, and large hemispherical defects (Fig. 7). Both pore and hemispherical defects contribute significantly to the overall porosity (Fig. 11), whereas only the number of hemispherical defects increases significantly when the porosity increases (Fig. 12). The defects in the SLS PEEK are mainly due to inadequate melt flow, uneven regional temperatures, and insufficient energy (Fig. 13). Accordingly, the defects that arise during SLS formation are the primary cause of low ductility. In addition, the long holding phase in the forming process causes the SLS PEEK to have a high crystallinity value (Table 1), which, while increasing the modulus of elasticity, also makes the material more sensitive to defects and accelerates its premature failure.
High-performance PEEK samples were successfully prepared using SLS technology. The internal microstructure and defects of PEEK were studied, and the reasons behind its mechanical characteristics were explained. Compared with the traditional IM PEEK sample, the PEEK formed by SLS exhibited slightly lower tensile and bending strengths compared to the IM sample but with a higher elastic modulus. However, its ductility was significantly poor, resulting in a failure occurring in the elastic deformation zone. The crystallinity of the SLS PEEK sample was higher than that of the IM PEEK sample, which exhibited better temperature resistance. The Micro-CT results showed that the PEEK formed by SLS exhibited a global distribution of porosity defects, strip boundary defects on the sides, and large hemispherical defects inside. By examining the morphology of the defects under the scanning electron microscope, it was observed that the formation of defects was mainly related to poor fluidity caused by the irregular powder, insufficient energy, and uneven temperature distribution of the powder bed. The research shows that defects have obvious influence on the mechanical properties of PEEK formed by SLS. The main reason for this is that large hemispherical defects easily become stress concentration points, resulting in a macroscopic performance such that the mechanical properties deviate significantly and premature failure occurs.
