A rapid method for parameter optimization for Ti-47Al-2Cr-2Nb powder in selective laser melting (SLM) based on melt pool characterizations is developed. The laser power and scanning speed are ...selected through the surface morphologies of single tracks to fabricate the high-density samples. The influences of the parameters on sectional geometric characteristics (width, height, depth, bulge area, penetration area and dilution) are analyzed by linear regression analysis, and the regression models are established to predict the geometric characteristics of single tracks. An inappropriate combination of process parameters can cause material evaporation and the melt pool instability, leading to generation of cracking and balling, which will reduce the linear correlation. The loss of Al is also an important reference criterion for the optimization of the process parameters, so Al content measurements are conducted to validate evaporation during the SLM processing. The Al loss from single tracks varies from 5.73 to 0.32at%. Applying optimal process parameters to the fabrication of TiAl parts result in densities ranging from 97.34% to 98.95%. However, more serious losses and an uneven distribution of Al in the fabricated samples are observed.
•A rapid method for optimization of parameters is developed.•The regression models are established to predict the geometrical characteristics.•The loss and uneven distribution of Al are found.•The evaporation of Al and the cracking cannot be completely eliminated.
Introduction
The role of bystanders in cyberbullying situations is critical, with the potential to significantly influence outcomes. Bystanders who demonstrate positive behaviors—such as reporting ...incidents or supporting victims—can help to mitigate the damaging effects of cyberbullying. Based on the Social Cognitive Theory, this study seeks to address the psychosocial mechanisms that underlie positive cyber-bystander behaviors.
Methods
A total of 1,716 students in Grades 8–12 from three secondary schools in China participated in this study. Path analysis was utilized to delineate the relationships between internet self-efficacy, empathy, teacher and parental support, and cyber-bystander behaviors.
Results
Adolescents who received greater support from teachers were more likely to demonstrate increased internet self-efficacy and empathy. In contrast, higher levels of parental warmth were associated with lower levels of internet self-efficacy. Teacher support and parental warmth exerted an indirect effect on positive bystander behaviors through empathy.
Discussion
The importance of parental warmth paired with Internet self-efficacy in preventing online interpersonal violence and motivate active bystander behaviors is considerable. We recommend adopting a nuanced approach that differentiates between empathy and internet self-efficacy in cyber-bystander research.
Objective/Hypothesis
This study aimed at investigating the correlation between a battery of diagnostic symptoms of definite Meniere's disease (MD) and the degree of endolymphatic hydrops (EH) in the ...inner ear.
Study Design
Prospective study.
Methods
Fifty‐four patients diagnosed with unilateral definite MD were enrolled in the study. The hearing levels of all patients at the low, middle, and high frequencies were evaluated. Three‐dimensional fluid‐attenuated inversion recovery (3D‐FLAIR) and three‐dimensional real inversion recovery (3D‐real IR) magnetic resonance imaging (MRI) were performed 24 hours after bilateral intratympanic injection of gadolinium to assess the presence and grading of EH.
Results
Various degrees of EH were observed in the vestibule and/or each turn of the cochlea in the affected ears of all patients. The duration of MD disease and low‐tone and middle‐tone hearing thresholds were proportional to the extent of EH in the vestibule and cochlear. However, no significant correlation was demonstrated between EH and other aspects of symptoms such as high‐tone hearing loss, tinnitus, and aural fullness. Of all subjects, 16.7% exhibited bilateral EH on MRI exam who were diagnosed with unilateral MD based on diagnostic criteria.
Conclusions
EH in the inner ear of MD patients exhibits a progressive deteriorative trend over time. Low‐tone and middle‐tone hearing thresholds can indirectly reflect the severity of EH in the cochlea. EH may not be the primary cause of tinnitus and aural fullness in patients with MD. Patients diagnosed with unilateral MD should maintain long‐term regular follow‐ups for the possibility of developing bilateral EH.
Level of Evidence
4 Laryngoscope, 126:974–979, 2016
Selective laser melting (SLM) and wire arc additive manufacturing (WAAM) are two representative metal additive manufacturing technologies that fabricate the net shaping of small complex parts and ...near-net shaping of large components. To explore the feasibility of SLM-WAAM hybrid fabricating large and complex components, the preliminary hybrid SLM-WAAM process of Ti-6Al-4V is investigated. In this study, horizontal (H-) and vertical (V-) SLM samples are used as substrate for WAAM process to analyze microstructure and tensile properties, and to be compared with that of SLM samples and WAAM samples. The results show that Ti-6Al-4V sample formed by hybrid fabrication consists of three typical zones: SLM zone, WAAM zone and interface zone. The fine and short β columnar grains consisting of primarily martensite α´ exist in SLM zone, then these fine grains grow coarse in the about 3mm-thick interface zone because of the repeated heating process, in contrast, the coarse β columnar grains awash with α lamellae lead to epitaxial growth in WAAM zone. There is a good metallurgical bonding in the interface zone of both H-SLM-WAAM and V-SLM-WAAM specimens and all the fractures are located in the WAAM zone during the tensile test. The mean yield strength and ultimate tensile strength of the H-SLM-WAAM specimens are 850MPa and 905MPa, respectively. In comparison, the V-SLM-WAAM samples exhibited slightly higher tensile strengths; the mean yield strength and ultimate tensile strength are 890MPa and 995MPa, respectively. The elongations of different directions are both more than 10%, satisfying the requirements of engineering applications. These tensile properties of SLM-WAAM samples are comparable with or even better than that of WAAM samples.
Several advantages of directed energy deposition-arc (DED-arc) have garnered considerable research attention including high deposition rates and low costs. However, defects such as discontinuity and ...pores may occur during the manufacturing process. Defect identification is the key to monitoring and quality assessments of the additive manufacturing process. This study proposes a novel acoustic signal-based defect identification method for DED-arc via wavelet time–frequency diagrams. With the continuous wavelet transform, one-dimensional (1D) acoustic signals acquired in situ during manufacturing are converted into two-dimensional (2D) time–frequency diagrams to train, validate, and test the convolutional neural network (CNN) models. In this study, several CNN models were examined and compared, including AlexNet, ResNet-18, VGG-16, and MobileNetV3. The accuracy of the models was 96.35%, 97.92%, 97.01%, and 98.31%, respectively. The findings demonstrate that the energy distribution of normal and abnormal acoustic signals has significant differences in both the time and frequency domains. The proposed method is verified to identify defects effectively in the manufacturing process and advance the identification time.
A major obstacle for the applications of single‐walled carbon nanotubes (SWNTs) in electronic devices is their structural diversity, ending in SWNTs with diverse electrical properties. Catalytic ...chemical vapor deposition has shown great promise in directly synthesizing high‐quality SWNTs with a high selectivity to specific chirality (n, m). During the growth process, the tube–catalyst interface plays crucial roles in regulating the SWNT nucleation thermodynamics and growth kinetics, ultimately governing the SWNT chirality distribution. Starting with the introduction of SWNT growth modes, this review seeks to extend the knowledge about chirality‐selective synthesis by clarifying the energetically favored SWNT cap nucleation and the threshold step for SWNT growth, which describes how the tube–catalyst interface affects both the nucleus energy and the new carbon atom incorporation. Such understandings are subsequently applied to interpret the (n, m) specific growth achieved on a variety of templates, such as SWNT segments or predefined molecular seeds, transition metal (Fe, Co and Ni)‐containing catalysts at low reaction temperatures, W‐based alloy catalysts, and metal carbides at relatively high reaction temperatures. The up to date achievements on chirality‐controlled synthesis of SWNTs is summarized and the remaining major challenges existing in the SWNT synthesis field are discussed.
Single‐walled carbon nanotubes with controlled properties are highly desirable for their practical applications. A critical overview of chirality‐selective growth of single‐walled carbon nanotubes by chemical vapor deposition is provided. The importance of the nanotube–catalyst interface in regulating nucleation thermodynamics and growth kinetics is outlined. The achievements in carbon nanotube synthesis are summarized and the prospects in controlled growth are proposed.
To increase building rate and save cost, the selective laser melting (SLM) of Ti6Al4V with a high layer thickness (200 μm) and low cost coarse powders (53 μm-106 μm) at a laser power of 400 W is ...investigated in this preliminary study. A relatively large laser beam with a diameter of 200 μm is utilized to produce a stable melt pool at high layer thickness, and the appropriate scanning track, which has a smooth surface with a shallow contact angle, can be obtained at the scanning speeds from 40 mm/s to 80 mm/s. By adjusting the hatch spacings, the density of multi-layer samples can be up to 99.99%, which is much higher than that achieved in previous studies about high layer thickness selective laser melting. Meanwhile, the building rate can be up to 7.2 mm³/s, which is about 2 times-9 times that of the commercial equipment. Besides, two kinds of defects are observed: the large un-melted defects and the small spherical micropores. The formation of the un-melted defects is mainly attributed to the inappropriate overlap rates and the unstable scanning tracks, which can be eliminated by adjusting the processing parameters. Nevertheless, the micropores cannot be completely eliminated. It is worth noting that the high layer thickness plays a key role on surface roughness rather than tensile properties during the SLM process. Although a sample with a relatively coarse surface is generated, the average values of yield strength, ultimate tensile strength, and elongation are 1050 MPa, 1140 MPa, and 7.03%, respectively, which are not obviously different than those with the thin layer thickness used in previous research; this is due to the similar metallurgical bonding and microstructure.
Wire arc additive manufacturing (WAAM) technique is a cost-competitive and efficient technology to produce large structure components in industry domains. Mechanical properties are mainly dominated ...by the microstructure of the components, which is deeply affected by the molten pool size. In this work, to investigate the effect of the molten pool size on microstructure and mechanical properties of the components, a series of Ti-6Al-4V alloy blocks with different width of molten pool (WMP) ranging from 7 mm to 22 mm were deposited by adjusting the wire feed speed (WFS) from 100 cm/min to 500 cm/min. It is interesting to find that the macrostructure changes from columnar grains to equiaxial grains, and then returns to large columnar grains with the increase of WMP, which is mainly caused by the different cooling rates and thermal gradients. Nonetheless, the tensile properties of the components have a tendency to decline with the increase of WMP.
Layered double hydroxides containing metal cations are applied for chemical vapor deposition synthesis of single-walled carbon nanotubes. Particularly, a bimetallic FeCo layered double hydroxides ...affords synthesis of subnanometer single-walled carbon nanotubes with dominant (6, 5) tubes, the abundance of which is further improved by a two-phase extraction approach.
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•Layered double hydroxides are applied for growing carbon nanotubes.•Low reaction temperature favors the synthesis of (6, 5) tube.•Synergism between Fe and Co accounts for the narrow chirality distribution.•A two-phase extraction technique is used to enrich small diameter tubes.
High-quality single-walled carbon nanotubes (SWNTs) were synthesized on layered double hydroxides (LDHs) containing Fe and FeCo by CO chemical vapor deposition (CVD). Systematic investigations were performed to study the effects of temperature and catalyst composition on the diameter and chirality distributions of SWNTs. It was revealed that both catalysts produced subnanometer SWNTs at low reaction temperatures, correlated with the catalyst performances and reaction parameters. Particularly, SWNTs with enriched (6, 5) species were synthesized at 600 °C from LDHs containing bimetallic FeCo. Besides, the SWNT chirality distribution was further narrowed by a two-phase extraction method and the extracted (6, 5) SWNTs occupy about 57% of all semiconducting species. This work extends the development of robust LDH-based catalysts for chirality-selective synthesis of SWNTs, which paves the way to large scale production of subnanometer SWNTs for potential applications in electronics and optoelectronics.
The authors reconstructed May–September precipitation over the Asian continent (5°–55°N, 60°–135°E) back to AD 1470 on the basis of tree-ring data, historical documentary records, ice core records, ...and the few long-term instrumental data series available in the region. They employed the method of Regularized Expectation Maximization (RegEM) and applied it to 44 subregions within the continent. Verification exercises demonstrate that the reconstruction is skillful over most of the study domain, with eastern China, India, and other regions of humid climate displaying the greatest skill. Lower reconstruction skill is observed in semiarid and arid regions, which was attributable at least in part to the scarcity of observations available for calibration/validation. The precipitation reconstructions agree well with previous reconstructions, where they are available. The explanatory value of the reconstruction is illustrated using five historically documented severe droughts in north-central China during the past half millennium. The reconstructions both validate and provide a larger-scale context for understanding these past climate events and their relationship with the Asian summer monsoons.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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