Development and optimization of carbide particle-reinforced titanium alloy matrix nanocomposites by selective laser melting

Nanocomposites are composites that consist of at least one nanosized reinforcement phase. By reducing the reinforcement particle size from the micrometre to nanometre level, the mechanical properties of the nanocomposite can be significantly improved beyond that of conventional composites. However, nanocomposites are not widely used in industries due to the lack of adequate processing methods. In conventional processing methods, the nanosized reinforcement particles in the matrix experience large van der Waals forces due to their high specific surface area, causing uncontrolled agglomeration of the nanosized particles and resulting in an non-uniform distribution of reinforcement nanoparticles in the matrix material. Selective Laser Melting (SLM) is a recently developed additive manufacturing (AM) technology that can produce complex, fully dense and functional parts using digital files from a computer-aided software. SLM has shown success in printing nanocomposites with a homogeneous mixture of reinforcement nanoparticles in the matrix material. However, more research is required to better understand and control the microstructural development and mechanical properties of nanocomposites printed using SLM. In this project, tungsten carbide (WC) reinforced and molybdenum carbide (Mo2C) reinforced Ti-6Al-4V (Ti64) nanocomposites with nearly full density were successfully printed via selective laser melting starting from the mixture of microsized Ti64 and nanosized carbides. Different weight fractions of WC and Mo2C reinforcements ranging from 1 wt.% to 3wt.% were designed and optimized based on the microstructure and mechanical properties of the printed nanocomposite parts. Results show that as the addition of WC and Mo2C reinforcement particles increases, the amount of needle-like α’-Ti phase martensite decreases. The boundary of the melt pool also becomes more distinct as the reinforcement content increases. Precipitates ii of unreacted reinforcement material can be observed at the melt pool boundaries for samples of each reinforcement material. WC/Ti64 samples show up to 20.88% increase in tensile strength, while Mo2C/Ti64 samples show up to 16.42% increase in tensile strength. However, there is a significant decrease in strain as the weight fraction of reinforcement increases. Mo2C/Ti64 samples show a significant increase in microhardness of up to 22.64% (3 wt%), while WC/Ti64 samples show up to 12.39% improvement in microhardness (3 wt%). Samples of both reinforcement materials show a similar rate of decrease in fracture toughness with increased addition of reinforcement material. All samples show similar wettability properties, with water contact angle ranging from 70 ° to 90 °.Bachelor of Engineering (Mechanical Engineering

A Comprehensive Investigation on 3D Printing of Polyamide 11 and Thermoplastic Polyurethane via Multi Jet Fusion

Multi Jet Fusion (MJF) is a recently developed polymeric powder bed fusion (PBF) additive manufacturing technique that has received considerable attention in the industrial and scientific community due to its ability to fabricate functional and complex polymeric parts efficiently. In this work, a systematic characterization of the physicochemical properties of MJF-certified polyamide 11 (PA11) and thermoplastic polyurethane (TPU) powder was conducted. The mechanical performance and print quality of the specimens printed using both powders were then evaluated. Both PA11 and TPU powders showed irregular morphology with sharp features and had broad particle size distribution, but such features did not impair their printability significantly. According to the DSC scans, the PA11 specimen exhibited two endothermic peaks, while the TPU specimen exhibited a broad endothermic peak (116–150 °C). The PA11 specimens possessed the highest tensile strength in the Z orientation, as opposed to the TPU specimens which possessed the lowest tensile strength along the same orientation. The flexural properties of the PA11 and TPU specimens displayed a similar anisotropy where the flexural strength was highest in the Z orientation and lowest in the X orientation. The porosity values of both the PA11 and the TPU specimens were observed to be the lowest in the Z orientation and highest in the X orientation, which was the opposite of the trend observed for the flexural strength of the specimens. The PA11 specimen possessed a low coefficient of friction (COF) of 0.13 and wear rate of 8.68 × 10−5 mm3/Nm as compared to the TPU specimen, which had a COF of 0.55 and wear rate of 0.012 mm3/Nm. The PA11 specimens generally had lower roughness values on their surfaces (Ra < 25 μm), while the TPU specimens had much rougher surfaces (Ra > 40 μm). This investigation aims to uncover and explain phenomena that are unique to the MJF process of PA11 and TPU while also serving as a benchmark against similar polymeric parts printed using other PBF processes

A numerical study on the packing quality of fibre/polymer composite powder for powder bed fusion additive manufacturing

A discrete element model has been developed to simulate the packing process of fibre/polymer composite powder for powder bed fusion additive manufacturing. The geometric shapes of polymer powder particles and fibres are represented by multi-sphere particles and individual cylinders with round ends, respectively. The numerical model can help to understand the flow dynamics of composite powder particles and the formation mechanisms of voids in powder packing processes. The numerical model has been utilised to analyse the effects of packing parameters on the packing quality of the powder bed. The simulation results suggest that the increase of the powder layer thickness is beneficial to the increase in the packing density and the decrease in the surface roughness of the powder bed. A high roller spreading velocity degrades the packing quality of the powder bed. Furthermore, a small number of fibres in the composite powder particles are in favour of the packing quality, but a further increase in the fibre number reduces it

Numerical investigation of fiber orientations and homogeneity in powder bed fusion of fiber/polymer composites

The packing characteristics of fiber/polymer powder in powder bed fusion additive manufacturing exhibit a high correlation with the mechanical behaviours of printed composite parts such as homogeneity and anisotropy. A discrete element model has been developed to investigate the packing characteristics of glass fiber/polyamide 12 (PA12) powder, which include fiber orientations, fiber homogeneity, and packing density. The predicted probability distributions of fiber orientations in the powder bed are comparable with those measured in glass fiber–reinforced PA12 composites printed via multi jet fusion. Three types of fibers with different length distributions are adopted to study the effects of the fiber length distribution on their packing characteristics. The simulation results reveal that a large average fiber length is beneficial to fiber alignment in the powder spreading direction but lowers the fiber homogeneity and packing density of the powder bed. Furthermore, varying the fiber length can provide an effective way to regulate fiber orientations in the powder packing process, which would help achieve satisfactory anisotropic mechanical properties for composite parts

In Situ Filler Addition for Homogeneous Dispersion of Carbon Nanotubes in Multi Jet Fusion–Printed Elastomer Composites

Abstract The dispersibility of fillers determines their effect on the mechanical properties and anisotropy of the 3D‐printed polymeric composites. Nanoscale fillers have the tendency to aggregate, resulting in the deterioration of part performance. An in situ filler addition method using the newly developed dual‐functional toughness agents (TAs) is proposed in this work for the homogeneous dispersion of carbon nanotubes (CNTs) in elastomer composites printed via multi jet fusion. The CNTs added in the TAs serve as an infrared absorbing colorant for selective powder fusion, as well as the strengthening and toughening fillers. The printability of the TA is theoretically deduced based on the measured physical properties, which are subsequently verified experimentally. The printing parameters and agent formulation are optimized to maximize the mechanical performance of the printed parts. The printed elastomer parts show significant improvement in strength and toughness for all printing orientations and alleviation of the mechanical anisotropy originating from the layer‐wise fabrication manner. This in situ filler addition method using tailorable TAs is applicable for fabricating parts with site‐specific mechanical properties and is promising in assisting the scalable manufacturing of 3D‐printed elastomers

Comparative study on 3D printing of polyamide 12 by selective laser sintering and multi jet fusion

Selective laser sintering (SLS) and Multi Jet Fusion (MJF) are two of the most developed powder bed fusion additive manufacturing techniques for the manufacture of polymeric components. In this work, a systematic benchmark and comparison of polyamide 12 (PA12) parts printed by SLS and MJF was conducted on the physicochemical characterization of raw powder materials (EOS PA2200 and HP 3D HR PA12) and their printed specimens, as well as the mechanical performance and printing characteristics of printed objects. Both designated-supply PA12 powders for each technique possessed almost identical thermal features, phase constitutions, functional groups, and chemical states. The mechanical strength of the MJF-printed specimens was slightly stronger than that of SLS-printed counterparts due to the synergistic effect of an area fusion mode and carbon black additive in the MJF process. The SLS-printed specimens had a better surface finish on the top surface, but the MJF-printed specimens showed much smoother front and side surfaces. Scaled-down merlions were printed by both processes for the printing accuracy assessment. The results show that the SLS-printed merlion presented higher profile deviations than those of the MJF-printed counterpart, especially in areas with sharp contours. These fundamental experimental results can provide a comprehensive understanding of SLS and MJF processes and serve as a valuable guideline for their industrial applications.This research was conducted in collaboration with HP Inc. and supported by Nanyang Technological University and the Singapore Government through the Industry Alignment Fund-Industry Collaboration Projects Grant

High-strength light-weight aramid fibre/polyamide 12 composites printed by Multi Jet Fusion

Multi Jet Fusion (MJF) is a fast-growing powder bed fusion(PBF) additive manufacturing technique which features low production costs and high production speeds. However, MJF currently suffers from limited choice of commercially available composite powders. Here, a new type of composite powder, aramid fibre (AF)–filled polyamide 12 (PA12), was developed for MJF to enhance the mechanical properties of the printed parts. The process–structure–property relationship was established by analysing the fibre arrangement in the composite and systematically investigating the effects of the fibre fraction, fibre length, layer thickness, build orientation, and post-annealing process on the structures and mechanical properties of the printed parts. The results showed significant enhancement in the mechanical performance of the AF/PA12 composites parts in the roller recoating direction along which the fibres preferred to align. The ultimate tensile strength and Young’s modulus of the optimised composite parts were increased by 27% and 179% and further improved by 40% and 216% through a post-annealing process, respectively, compared with those of the neat PA12 part. The manufacturing methodology of these high-strength light-weight composites can be further extended to other PBF techniques for applications in a broad spectrum of fields

Tensile strength and wear resistance of glass-reinforced PA1212 fabricated by selective laser sintering

Glass fibre (GF) and glass bead (GB)–reinforced polyamide1212 (PA1212) was additively manufactured by selective laser sintering. The effects of laser power and GF content on the tensile and tribological properties of the printed specimens with a base GB weight fraction of 40 wt.% were investigated. The strengthening mechanism of GFs/GBs was illustrated by analyzing the interfacial adhesion between the fillers and the PA1212 matrix. The specimens with 40 wt.% GBs and 10 wt.% GFs fabricated at a laser power of 30 W exhibited a strength of 52 MPa, a friction coefficient of 0.23, and a wear rate of 0.0011 mm3/N·m. The selected optimal laser power and GF addition contributed to the strong interfacial adhesion, which realised flat surface morphology and an adequate encapsulation of fillers in the specimen. The reinforcement of GBs/GFs in PA1212 can serve as a reference for a deeper understanding of the strengthening mechanisms for other additively manufactured engineering plastics

Surface modification of oriented glass fibers for improving the mechanical properties and flame retardancy of polyamide 12 composites printed by powder bed fusion

The orientation of glass fibers (GF) introduced by powder bed fusion (PBF) imparts enhanced mechanical properties to polyamide 12 (PA12). However, there is still much room for reinforcement of PBF-printed GF/PA12 composites. In addition, no studies have addressed the flame retardancy of PBF-printed GF/PA12 composites impaired by the candlewick-like effect of GF. This work presents a feasible and practical approach for addressing these two issues, by surface modification of GF with layered double hydroxide (LDH) to synthesize LDH@GF hybrids. Compared with the ultimate tensile strength, Young's modulus, flexural strength, and flexural modulus of the GF/PA12 composites, those of the LDH@GF/PA12 composites increased by 21.3%, 54.3%, 31.8%, and 36.7%, respectively. Meanwhile, LDH weakened the candlewick-like effect of GF and thus improved the flame retardancy of the PA12 composites. Compared with the peak heat release rate and total heat release of the GF/PA12 composites, those of LDH@GF/PA12 composites were reduced by 17.7% and 12.7%, respectively. The mechanisms for mechanical reinforcement and flame retardancy of LDH@GF hybrids were investigated and proposed. This work paves the way for PBF to prepare flame-retardant high-strength PA12 composites and provides a new solution to boost the performance of additively manufactured products.This study was supported by the RIE2020 Industry Alignment Fund – Industry Collaboration Projects (IAF-ICP) Funding Initiative, Singapore and the cash and in-kind contributions from our industry partner, HP Inc

Investigation of the mechanical properties of polyimide fiber/polyamide 12 composites printed by Multi Jet Fusion

Multi Jet Fusion (MJF) has attracted extensive attention because of its ability to print support-free complex structures. However, the mechanical properties of MJF-printed polymer parts are still unsatisfactory for certain industrial requirements. Herein, by leveraging the fibre reinforcement effect and high specific strength of polyimide (PI) fibres, this work developed PI/polyamide 12 (PA12) composites with largely enhanced mechanical performance via MJF. Specifically, the tensile strength and modulus were increased by 43% and 42%, and the flexural strength and modulus were improved by 39% and 46%, respectively, compared to those of the neat PA12 parts. Furthermore, the incorporation of lightweight PI fibres endowed the composites with high specific tensile strength (67.60 kN·m/kg) and specific flexural strength (93.70 kN·m/kg), which are superior to those of MJF-printed PA12 composites reinforced with other fibres. This work provides new insights into enhancing the mechanical performance of lightweight parts printed by MJF and other powder-based techniques