Key Engineering Materials Vol. 1052

Abstract: Additively manufactured titanium alloys such as Ti-6Al-4V have been used as functional components in industry due to their excellent mechanical properties. However, the machining of these alloys is a challenge due to their enhanced tensile/yield strength, low elastic modulus, poor thermal conductivity, and microstructural anisotropy. Thermal assisted machining (TAM), as a hybrid manufacturing technology, can improve the machinability of additively manufactured alloys. The main aim of this paper is to investigate the effect of temperature buildup on the machinability of additively manufactured Ti alloy with different build directions in the TAM process. It was found that the surface integrity was notably enhanced by preheating, and it was the best at 90° build orientation. Serrated chips were generated at room temperature, and curlier chips were formed in high-preheating machining environment. By analyzing the surface quality, the influence of the build-up orientation on the surface quality at different temperatures was evaluated.

Abstract: The successful implementation of fused filament fabrication (FFF) 3D printing using recycled plastics requires a deep understanding of the thermal behavior of the plastics throughout the printing process. This study investigated the influence of wall thickness of the printed sample, nozzle temperature, and cooling fan speed during 3D printing on the cooling rate, crystallinity, and tensile properties of recycled polyethylene terephthalate (rPET). The experimental process commenced with the collection of discarded rPET bottles, followed by thorough cleaning and washing to remove any adhesives and contaminants. Afterward, the bottles were cut and ground into flakes and then converted into filaments using a single-screw filament extrusion process. In-situ thermal analysis was conducted by integrating an infrared (IR) thermal camera into the 3D printing setup to monitor real-time temperature changes during the printing process. Results revealed that cooling rates increased markedly with reduced wall thickness, rising from 17.53 °C/min for the 3.6 mm wall thickness to 62.92 °C/min for the 1.2 mm wall thickness. Nozzle temperature exhibited a non-linear influence, with the highest cooling rate of 65.47 °C/min recorded at 240 °C, while enhanced cooling fan speed (100%) further accelerated cooling to 45.00 °C/min. Differential scanning calorimetry (DSC) and Raman spectroscopy confirmed that a slower cooling rate generally promoted crystallinity, which was observed in thick-walled and low-cooling speed prints. Tensile testing demonstrated a strong correlation between crystallinity and tensile performance, with ultimate tensile strength (UTS) reaching 55 MPa at 240 °C and 54.8 MPa at 25% cooling fan speed, outperforming previously reported rPET values. The use of rPET in FFF and the findings of this study contribute to the further exploration of rPET's potential in sustainable additive manufacturing practices.

Abstract: This study investigates the vibration characteristics of 3D-printed polylactic acid (PLA) cantilever beams using a hybrid analytical–numerical–experimental framework. Two low-cost sensing techniques—an MPU6050 accelerometer and a GoPro Hero10 vision-based system—are systematically evaluated against analytical Euler–Bernoulli and numerical ANSYS models. The analytical and numerical approaches show strong consistency for the first three natural frequencies (Mode 1: 10.22–10.31 Hz; Mode 2: 64.04–64.58 Hz; Mode 3: 179.34–180.95 Hz). Experimentally, the GoPro accurately captures the first mode (10.5 Hz), while the accelerometer successfully detects the first two modes but deviates in the third mode due to nonlinear mass-loading and sensor–structure coupling effects. The findings highlight both the capability and limitations of low-cost SHM tools and provide new insights into nonlinear behaviour in lightweight polymeric beams. The novelty of this work lies in its multi-method validation and explicit quantification of nonlinear deviations, offering a practical framework for accessible vibration-based monitoring.

Abstract: In fuel manufacturing, a catalyst is an additive added from the base material to the intermediate product, which functions to accelerate the process of forming the intermediate product. Meanwhile, an additive is a material added to the intermediate product to improve the properties of the fuel before it undergoes combustion. Keywords: additive, nanoparticle, biodiesel.

Abstract: Biodiesel is an environmentally friendly and renewable alternative fuel; however, it continues to face technical challenges related to oxidative stability, combustion efficiency, and exhaust emissions. One widely studied solution involves the use of fuel additives, particularly calcium oxide (CaO). CaO possesses strong basicity, high thermal stability, and notable catalytic activity, making it applicable in both the production and application stages of biodiesel. As a heterogeneous catalyst, CaO accelerates the transesterification process, enhancing biodiesel conversion efficiency. It also acts as an adsorbent, removing water, free fatty acids, and other impurities, thereby improving fuel purity and storage stability. Moreover, CaO contributes to more efficient combustion and has been shown to reduce emissions of carbon monoxide and particulate matter. Despite these benefits, challenges remain, including the risk of residue formation and engine deposits. Recent studies highlight the superior performance of CaO, particularly in nanoparticle form, compared to other inorganic additives. Future research should focus on surface modification strategies, dosage optimization, and long-term engine performance assessments. With proper engineering approaches, CaO holds significant potential to support the development of more efficient, stable, and sustainable biodiesel formulations for cleaner energy applications.

Abstract: Biodiesel B40, a blend of 40% biodiesel and 60% diesel, has become a focal point in Indonesia’s clean energy transition. The potential for emission reductions, sustainable resource utilization, and the opportunity for integrating nanoparticle- and oxygenate-based additive technologies make B40 a promising alternative fuel. However, the chemical nature of the unsaturated ester compounds in B40 presents new challenges in the formation of NOx emissions and oxidative stability. This review aims to summarize the latest research on the molecular structure, combustion reactivity, and engine performance optimization strategies when using B40. Emphasis is placed on additives such as CeO₂, phenolic antioxidants, and short-chain oxygenates that enhance thermal efficiency and mitigate emissions. Through analysis from molecular perspectives to energy policy, this review highlights comparative trade-offs, industrial scalability, and research gaps. The paper also recommends standardized testing, hybrid additive approaches, and techno-economic assessments to accelerate the industrial deployment of B40.

Abstract: Preparation of activated carbon from juvenile durian fruit by activation with KMnO₄ for methylene blue adsorption was studied. The juvenile durian fruit was pyrolyzed at temperatures of 400, 450, 500, 550, and 600 °C and activated with KMnO₄. The results demonstrated that the carbonization at 600 °C yielded the highest iodine number of 298.30 mg_iodine/g_biochar. Subsequently, the methylene blue adsorption was investigated using a Box-Behnken designed batch experiment. The experimental design included three variables at three levels: adsorbent dosage (g), initial methylene blue concentration (mg/L), and adsorption time (min). The optimum conditions for methylene blue adsorption efficiency reached approximately 99.9% at an adsorbent dosage of 0.55 g, an initial methylene blue concentration of 10 mg/L, and an adsorption time of 90 min.

Abstract: Industrialization has led to widespread aquatic contamination, with dyes being among the most prominent pollutants found in various water bodies. Major contributors to dye pollution include the textile, printing, leather, cosmetics, and chemical industries, with the textile industry alone being responsible for approximately 13% of the dyes released into aquatic environments. This study focuses on comparing the photocatalytic degradation performance of synthesized catalysts prepared in the presence of biopolymers. Pullulan was selected as a capping agent to aid the synthesis process and promote the formation of nanosized catalysts. Three types of catalysts, namely copper oxide, zinc oxide, and a composite of both, were synthesized, and their performance was evaluated through the photocatalytic degradation of methylene blue. Among the three, zinc oxide demonstrated the highest degradation efficiency (99%), followed by the composite (27%), while copper oxide exhibited negligible photocatalytic activity (14%). Further optimization of the best-performing catalyst (zinc oxide) was conducted by varying parameters such as catalyst dosage (0.05-0.15g) and solution pH (5-9). The results showed that zinc oxide achieved the highest degradation under acidic conditions (pH 5) with a dosage of 0.15 g, requiring only 70 minutes to reach nearly 100% degradation. Overall, this study provides valuable insights into the influence of catalyst type on the photocatalytic degradation of methylene blue.

Abstract: Diesel vehicle emissions containing nitrogen oxides, volatile organic compounds, and soot pose significant health and environmental risks. Diesel particulate filters (DPFs) reduce soot emissions by capturing particulate matter through their porous structure, but are often made from costly materials like silicon carbide and cordierite. This study addresses this gap by developing a DPF using recycled carbon fibers (CF) coupled with bentonite-supported copper-manganese (Cu/Mn) catalysts. CFs were recovered from polymer composite wastes using two distinct approaches: a two-step pyrolysis involving thermal decomposition and oxidation of resin, and a chemical treatment via acid digestion and catalyzed separation. These processes yielded clean, structurally intact fibers suitable for filter fabrication. Sintering the filter produced a composite with enhanced structural cohesion, porosity, and thermal stability, which makes it suitable for particulate entrapment. Scanning electron microscopy (SEM) images revealed that sintered filters exhibit dispersed bentonite and carbon fibers, with pyrolyzed fibers providing a more compact structure. The resulting filter exhibited an average specific surface area of 56.58 m²/g and an average pore size of 3.36 nm, while analyses confirmed the presence of Cu and Mn oxides within the bentonite matrix with synergistic interactions between catalysts. Thermogravimetric analysis (TGA) showed that Cu/Mn-bentonite catalysts reduced soot oxidation onset temperatures to approximately 245.26°C and 471.89°C, providing efficient catalytic performance at lower temperatures while maintaining stability. These results effectively demonstrate the potential of recycled carbon fibers for integration with Cu/Mn catalysts to develop DPFs.