Research On The Preparation And Forming Of Non-metallic Composite Materials Based On Selective Laser Sintering

Selective laser sintering（SLS）is one of powder bed fusion additive manufacturing（AM）technologies,which is characterized by short processing time,high material utilization,and capable of manufacturing components with highly complex structures without the need of any tools and fixtures.With the development and application of the SLS technology,the issue of materials becomes more and more prominent,whcih mainly contains:i)severely lack available materials for SLS,and polyamide-based thermoplastics inclusing polyamide 12（PA12）and polyamide 11（PA11）are currently the most used SLS materials;ii)low performance of the laser sintered parts,whcih are still inferior to the counterparts fabricated by the conventional methods,and thus can not meet the application requirements in different fields.For these reasons,this thesis carried out a research regarding the preparation and fabrication of novel composites materials based on SLS,in order to improve the performance of the SLS parts,increase the species of SLS materils and broaden the scope of the application of SLS technology,whcih is also helpful to enrich the technique and theory for making composite components.The main reaearch contents and results are as follows:This sdudy proposed a novel low-isotacticity polypropylene（PP）composite material and its preparation method for SLS technology.The key performance indicators closely related to the SLS processibility including particle shape,particle size distribution,sintering window,crystallization property,thermal stability were carefully and systematilly analysed.The resuts showed that the prepared PP composite powder has ideal processability for SLS process.The processing parameters were studied and optimized.When the laser energy density was 0.0458 J/mm²,the relative density and tensile strength of the laser sintered parts reached their maximum value,which were 97.8%and 19.9 MPa,respectively.Meanwhile the tensile modulus was 599.1 MPa and the elongation at break was as high as 122%.The thermal process during the SLS and traditional injection moulding process was compared,and their effects on the microstructures and performances were also evaluated.It was found that owing to maintaining at a high preheating temperature close to their melting point during the SLS process,the polymer melt had a low degree of supercooling and low cooling rates,which is beneficial to the increase of crystallinity and the growth of spherulite and even facilitate the appearance of new crystalline phase.However,the injection moulding parts had high degree of supercooling and cooling rate due to the cooling water in the moulds,which caused to lower crystallinity and smaller spherulites.The microstructure difference can lead to the performance variations.The laser sintered parts showed 20%higher of tensile strength,72%higher of tensile modulus and far less of elongation at break and toughness,compared to the injection moulding counterparts.This study proposed a novel processing method based on SLS to fabricate carbon fiber/polyamide 12/epoxy resin（CF/PA12/EP）ternary composites.The preparation of composites powder,SLS processing of carbon fiber green parts and vacuum infiltration of liquid epoxy resin were carefully studied.It was found that the carbon fiber green parts built by SLS process using the optimized parameters had appropriate strength,porosity and pore size distribution for infiltration.The polyamide 12 not only served as binder to connect the adjacent carbon fibers together,but also involved in the curing reaction of epoxy resin,thus improving the wettability between carbon fiber and epoxy resin matrix and increasing the interfacial bonding strength.The porosity in the CF/PA12/EP ternary composites measured by Micro-CT was 4.93%,and the maximum tensile strength and flexural strength were 101 MPa and 153 MPa,respectively,which were higher than the majority polymer based materials.This method were proved possessing the potential to fabricate CF/PA12/EP ternary composites parts with complex geometries,which are suitable for functional parts or lightweight low-bearing components.This study also investigated the strengthening and toughening mechanism of the CF/PA12/EP ternary composites.The effect of polyamide 12 content on the microstructure,mechanical properties and fracture behaviors of the CF/PA12/EP ternary composites were studied in details.The results showed that when the content of polyamide 12 in the starting composite powder was lower than 30 vol.%,the carbon fibers distributed uniformly in the epoxy resin matrix;while agglomeration of the carbon fiber were observed due to the relatively thicker polyamide 12 coating layers,and the polyamide 12 and epoxy resin matrix formed a dual phase co-continuous morphology.With the increase of polyamide 12content,the tensile strength of the CF/PA12/EP ternary composites increased first and then decreased,the tensile modulus gradually decreased and the fracture toughness increased continuously.The maximum tensile strength,tensile modulus and fractural toughness were improved 125%,122%,and 23.5%,respectively.When the content of polyamide 12 was increased above 30vol.%,the fracture mode of CF/PA12/EP ternary composite changed from the obvious brittle fracture to pseudoplastic fracture,and the damage tolerance was improved.It was conclude that the strength and modulus of CF/PA12/EP ternary composite were improved by the enhancement of carbon fiber;meanwhile,the fiber induced crack deflection,fiber bridge,microcrack and crack branch were the main toughening mechanisms.Besides,polyamide 12 as a themoplastic can passivate the crack tip.When higher content of polyamide 12 was used,dual phase co-continuous structures were formed,which can produce a large amount of plastic deformation,thus improve the fracture toughness of the CF/PA12/EP ternary composite.This study further proposed a method based on SLS to fabricate carbon fiber/silicon carbide（C_f/SiC）ceramic matrix composites.The SLS process was first used to make C_f/C preforms,and then the C_f/C preforms were infiltrated with liquid silicon（LSI）to in-situ form C_f/SiC composites through the reaction between carbon and silicon.The microstructure evolution and control methods of C_f/C preforms were emphatically studied,the formation mechanism of C_f/SiC composites microstructure was revealed,and the dimensional accuracy and mechanical properties of C_f/SiC composites were finally evaluated.The results showed that the linear shrinkage rate of C_f/SiC composite parts in this method was within 3%,which was deemed to be a near net forming process.The C_f/SiC composites were mainly composed ofβ-SiC,free silicon and unreacted carbon fibers.The maximum density,Vickers hardness,flexural strength and fracture toughness of C_f/SiC composites were 2.83±0.03 g/cm³,2310±140 Hv,249±17.0 MPa and 3.48±0.24 MPa·m^1/2.With the increase of unreacted carbon fiber,the fracture toughness of C_f/SiC composites was improved,and the fracture mode was changed from brittle fracture to non-catastrophic fracture.This method can be used to fabricate complex C_f/SiC composite parts with high strength.