Key Engineering Materials Vol. 897

Abstract: The typical manufacturing process of tubular metallic cardiovascular stents includes laser cutting, sand blasting, acid pickling, electropolishing, surface passivation, and cleaning. The most commonly used material for cardiovascular stents is stainless steel, such as SUS 304 and SUS 316. After the laser cutting process, substantial improvement of the stent surface morphology is required to obtain acceptable surface roughness, edge roundness, and reduction of surface defects. This study focuses on a novel post-treatment method of fluid abrasive machining to replace the conventional sand blasting and acid pickling processes, resulting in the surface smoothness and edge roundness that are suitable for cardiovascular stent fabrication. The dross deposition and striations retained after laser cutting can be significantly removed with fluid abrasive machining. Both DC current and pulse current electropolishing techniques were performed to attain the final surface and structural quality after the fluid abrasive machining process. The experimental results show that an extremely fine surface roughness and a satisfactory edge roundness can be achieved for stents through both DC current and pulse current electropolishing. The pulse electropolishing process is more effective than the DC current electropolishing process to achieve edge roundness with less weight removal.

Abstract: This paper presents the laser surface polishing of titanium alloy (Ti6Al4V) by using a nanosecond pulse laser. Air, nitrogen and argon were employed as a shielding gas in this study, where the areal roughness (Sa) of laser-polished surface was measured and compared. The results showed that argon was the suitable assist gas for improving the metal surface without causing the oxidation. The effect of laser pulse repetition rate and scan speed on the surface roughness was also investigated in this study. The use of high repetition rate together with slow scan speed was able to reduce the surface roughness of titanium alloy. The roughness was found to be reduced by 47% when the pulse repetition rate of 500 kHz and scan speed of 50 mm/s were applied.

Abstract: The properties of individual grains affect the mechanical behaviors and response of materials in micro-scaled deformation, viz., microforming, and there are unknown phenomena and deformation behaviors existing and limiting the wide application of microforming due to size effect. In this paper, a composite model combining crystal plasticity and grain boundary strengthening theories was developed for numerical investigation into the effect of grain boundaries on the plastic deformation of copper micro-upsetting. By comparing the results with and without grain-boundary structure, it is revealed that grain boundaries, which act as the barriers of crystal slip, result in the enhanced flow stress and the discontinuous distribution of stress and strain. The grain size effect is also considered in this research, and the results show the coarse-grained material reduces the flow stress and enhances the inhomogeneous deformation.

Abstract: Hot rolled steel is a material made by heating at high temperature. It has strong plasticity and is used in shipping industry, automobile industry, manufacturing industry, etc. Tensile strength refers to the maximum resistance to uniform plastic deformation of the material. It is an index of the mechanical properties of steel and determines the quality of steel to a certain extent. The influencing factors of tensile strength include steel processing parameters and chemical composition. As an improved model of RBF neural network, the generalized RBF neural network reduces the complexity of the model, improves the generalization ability of the model, and makes its application more extensive. In this paper, a generalized RBF neural network quantile regression model (QR-GRBFNN) is established to predict the mechanical properties of hot rolled strip, the mean percentage error (MAPE) and root mean square error (RMSE) are used as evaluation indexes. Experiments show that the model has better predictive performance.

Abstract: Poly (N-isopropylacrylamide) (PNIPAm) has been one of the most widely studied thermal responsive polymer in tissue engineering owing to its reversible hydrophilic-hydrophobic phase transition across its lower critical solution temperature (~32°C) that is close to human physiological temperatures. Among tissue engineering constructs, nanofibrous scaffolds offer an added advantage in mimicking the morphology of the native extracellular matrix (ECM). Electrospinning has been reported as one of the most facile method to produce PNIPAm nanofibres and neat electrospun nanofibres scaffold is known to possess poor aqueous stability, limiting its use in tissue engineering applications. In contrast, numerous studies on PNIPAm hydrogels have shown relatively good aqueous stability owing to the hydrophilic 3D crosslinked structure of the hydrogel which resist instant dissolution but rather swell to a greater or lesser extent. However, the presence of crosslinkages in PNIPAm hydrogels causes it to be hardly electrospinnable into nanofibres. In the present work, crosslinker free PNIPAm was radical polymerized to a high molecular weight of 385 kDa. To produce nanofibers, electrospinning was carried out on a dedicated %wt of PNIPAm solution containing octaglycidyl polyhedral oligomeric silsesquioxane (OpePOSS) and 2-ethyl-4-methylimidazole (EMI). Resulting PNIPAm nanofibrous network was found to strongly resemble the ECM morphology with fiber diameter of 436.35 ± 187.04 nm, pore size 1.24 ± 1.27 μm and 63.6% total porosity. Aqueous stability was studied in cell culture media over the course of 28 days. The current result shows significant improvement with a gradual mass loss up to a maximum of 35% instead of the near immediate dissolution observed in the case of electrospun neat PNIPAm scaffold without crosslinks.

Abstract: Development of polylactic acid (PLA) composites using various filler have extensively being been in focus. One of the possible natural filler is eggshell (ES) which are abundantly available derived from food industry waste. This work attempted to investigate the effect of eggshell (ES) filler on the degradation of virgin-PLA and recycled-PLA. The virgin-PLA/ES composites and recycled-PLA/ES composites were prepared using solvent casting method. The content of eggshell filler varied in the range of 0 – 20 wt%. Degradation of virgin-PLA/ES composites and recycled-PLA/ES composites were evaluated by soil buried test. After soil buried for 10 weeks, the maximum weight loss for virgin-PLA/ES composites was 14 wt% which noted at the composition of 95 wt% virgin-PLA/5 wt% ES. Nevertheless, for recycled-PLA/ES composites, the maximum weight loss observed at the composition of 90 wt% recycled-PLA/ 10 wt% ES at the value of 21 wt%. However, further addition of eggshell filler content in PLA/ES composites led to lower weight loss. Hence, the degradation of recycled-PLA/ES composites were more accelerated as compared to virgin-PLA/ES composites. The results revealed the potential of eggshell waste as a bio-filler in PLA matrix.

Abstract: Polypropylene resins have been enfolded with the automotive industry and suppliers to produce several spare parts. This is aimed at achieving zero emissions, kenaf plant which in Latin is Hibiscus Cannabinus is a natural fiber replacement resin. Natural fiber composites come in many different types, but kenaf has been exploited extensively over the last few years. The pre-board flow process of kenaf-polypropylene starts from mixing kenaf about 40% with 60% polypropylene, forming a pre-mat as the first output, entering the main treatment with the hot press into pre-board as the final output. Kenaf-polypropylene door trim is very absorbent of CO₂, which is related to natural fiber base material. Door trim with kenaf-polypropylene as the base material reduces the weight by about 30% of the previous polypropylene resin and still provides high rigidity even at a reduced weight. The entire process is requiring 48382.4 kWh / month per cycle of total power consumption.

Abstract: UiO-66 is a zirconium-based metal organic framework (MOF). It was synthesized and used by researchers due to its high water, chemical and thermal stability. The mentioned reasons in addition to other excellent properties made them a highly competitive materials for a variety of industrial problems. This study investigates the effect of the reaction time on the characteristics of the prepared UiO-66 nanoparticles. UiO-66 was synthesized by the solvothermal method and the reaction was left to take place for 18, 21 and 24 hours. UiO-66 was characterized using X-ray Diffraction (XRD) and Fourier-Transform Infrared (FTIR), the results showed that it has the same patterns and functional groups of the previously reported UiO-66. In addition, Scanning Electron Microscopy (SEM) was used to confirm the morphology of UiO-66. The smallest particle size around 200 nm was obtained at 18 hours. To investigate the thermal stability of the prepared UiO-66, Thermogravimetry analysis (TGA) was conducted. The results matched well with the literature and confirm that UiO-66 is thermally stable up to 500°C.

Abstract: Graphene nanoplatelets (GNPs) was used as a nanofiller in Poly(methyl methacrylate) (PMMA) synthesized by the Atom Transfer Radical Polymerization (ATRP) method. The first step in the synthesis of the PMMA/GNPs was the dispersion of GNPs in the PMMA liquid monomers by combining the solutions so that GNPs had superior mechanical properties, thermal stability, and electrical conductivity also lower density of mass. Then the crosslinked PMMA/GNPs nanocomposite samples were synthesized by using the in-situ polymerization method. However, there was a challenging technical problem in the application of GNPs (at a large amount) in the polymer. For the purpose of benefiting from the advantageous properties of GNPs (especially in bulk quantities) at PMMA, the major problem at the synthesis of PMMA/GNPs nanocomposite was the GNPs dispersion in the polymer matrix. This research has focused on solving that dispersion problem with the aim of enhancing the mechanical properties of the nanocomposite by utilizing the ATRP method as the effective production technic. The structural characterization of PMMA/GNPs nanocomposite was performed for the examination of the integration of GNPs in PMMA. The surface morphology of the nanocomposite was analyzed using SEM images. X-Ray Diffraction (XRD) as a non-destructive test method was used to examine the changes in the crystalline properties of the nanocomposite structure with the rise of the GNPs amount in PMMA. The bonding interactions with each other were investigated by using Raman analysis.