选区激光烧结聚醚醚酮成形缺陷及对力学性能的影响研究

Forming Defects and Its Influence on Mechanical Properties of Polyetheretherketone Manufactured by Selective Laser Sintering

将选区激光烧结(SLS)技术应用于聚醚醚酮(PEEK)等高性能聚合物的制备,为严苛使用条件下复杂构件的制造提供了一种新的选择。研究了SLS制备PEEK材料的微观结构、成形缺陷与力学性能之间的关系。采用光学显微镜(OM)、扫描电子显微镜(SEM)和微计算机断层扫描(Micro-CT)对SLS制备的PEEK样品进行微观结构和成形缺陷的分析表征,包括结晶程度、孔隙、未熔合缺陷等。使用万能材料试验机进行室温力学性能测试,并与传统注塑(IM)样品对比。研究结果表明,SLS制备的PEEK样品具有较高的结晶度,其弹性模量和耐热性皆优于IM PEEK,但SLS PEEK样品的强度和塑性较IM PEEK低,拉伸强度为(86.5±3.3)MPa,弯曲强度为(161.9±23.7)MPa,断裂伸长率仅为(2.1±0.2)%。结合断口形貌观察和Micro-CT缺陷分析,对SLS制备过程中缺陷的形成机理及其对力学性能的影响作用进行了讨论,SLS成形过程中产生的层间大尺寸缺陷是导致强度较低和延展性较差的主要原因。

Objective 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 threedimensional 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 thismaterial.Recently,the developments in PEEK for SLS technology and high-temperature laser sintering equipment have solved thisproblem,allowing to produce reliable parts.The tendency to form defects in SLS-prepared parts is an inherent feature of thistechnology,which can have a negative impact on the mechanical properties of the produced part.Therefore,studying the relationshipbetween the porosity and processing parameters is a popular topic.Despite the importance of the mechanical properties of sinteredmaterials,one of the research gaps that remains to be investigated is the lack of quantitative characterization of defects in laser-sinteredPEEK.Accordingly,previous research has only focused on analyzing the microstructure of a small number of samples to improve theprocessing 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 impacton the mechanical properties is analyzed.Consequently,an important theoretical and experimental basis for further in-depth researchand practical engineering applications of this material is provided.Methods This study evaluated the microstructure and mechanical properties of PEEK parts prepared using the SLS technology andcompared them to those of parts produced by injection molding(IM).The test samples were manufactured by a high-temperature lasersintering system and PEEK HP3 powder from EOS,Germany.The IM parts were cut from the PEEK injection-molded sheets usingthe 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 weredetermined using a universal material-testing machine.The tensile strength,elongation,and elastic modulus of each sample weremeasured.Furthermore,the thermophysical properties related to the material properties were determined.Finally,the differences inthe mechanical properties between the samples manufactured by the two manufacturing methods were compared,and the causes of thephenomena were determined based on the microstructural results.Results and Discussions The test results show that SLS PEEK has slightly lower tensile and flexural strengths and significantlypoorer elongation at break but exhibits a higher modulus of elasticity and temperature resistance than the IM PEEK specimens(Figs.5and 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 contributesignificantly to the overall porosity(Fig.11),whereas only the number of hemispherical defects increases significantly when theporosity 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.Inaddition,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.Conclusions High-performance PEEK samples were successfully prepared using SLS technology.The internal microstructure anddefects of PEEK were studied,and the reasons behind its mechanical characteristics were explained.Compared with the traditional IMPEEK sample,the PEEK formed by SLS exhibited slightly lower tensile and bending strengths compared to the IM sample but with ahigher 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 temperatureresistance.The Micro-CT results showed that the PEEK formed by SLS exhibited a global distribution of porosity defects,stripboundary defects on the sides,and large hemispherical defects inside.By examining the morphology of the defects under the scanningelectron 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 onthe mechanical properties of PEEK formed by SLS.The main reason for this is that large hemispherical defects easily become stressconcentration points,resulting in a macroscopic performance such that the mechanical properties deviate significantly and prematurefailure occurs.