Comparison of Properties and Bead Geometry in MIG and CMT Single Layer Samples for WAAM Applications

The process of Wire Arc Additive Manufacturing (WAAM) utilizes arc welding technology to fabricate metallic components by depositing material in a selective layered fashion. Several welding processes exist that can achieve this layered deposition strategy. Gas Metal Arc Welding (GMAW) derived processes are commonly favored for their high deposition rates (1–4 kg/h) and minimal torch reorientation required during deposition. A range of GMAW processes are available; all of which have different material transfer modes and thermal energy input ranges and the resultant metallic structures formed from these processes can vary in their mechanical properties and morphology. This work will investigate single-layer deposition and vary the process parameters and process mode to observe responses in mechanical properties, bead geometry and deposition rate. The process modes selected for this study were GMAW derived process of Metal Inert Gas (MIG) and Cold Metal Transfer (CMT). Characterization of parameter sets revealed relationships between torch travel speeds, wire feed speeds and the specimen properties and proportions. Differences were observed in the cross-sectional bead geometry and deposition rates when comparing MIG and CMT samples though the influence of process mode on mechanical properties was less significant compared to process parameter selection.

Effect of PECVD SiNx deposition process parameters on electrical properties of SiNx/AlGaN/GaN structures

The effect of parameters of plasma enhanced chemical vapor deposition (PECVD) processes for SiNx film fabrication on the electrical parameters of dielectric/АlGaN/GaN structures has been studied. The effect of growing film composition, additional heterostructure surface treatment with nitrogen plasma before dielectric deposition and HF biasing during treatment on the parameters of the С–V and I–V curves of SiNx/АlGaN/GaN structures has been analyzed. We show that films with nitrogen to silicon concentration ratios of 60 and 40% and a high oxygen content exhibit a decrease in the positive fixed charge in the structures although the I–V curves of the structures exhibit current oscillations. Information has been reported on the effect of PECVD process mode on current oscillation parameters, e.g. period and amplitude, and length of I–V curve section in which oscillations occur. Possible explanation of these oscillations has been suggested. Additional nitrogen plasma treatment of heterostructure surface before monosilane supply to the chamber changes the magnitude and sign of fixed charge and reduces the free carrier concentration in the 2D gas channel of SiNx/АlGaN/GaN heterostructures. Experimental evidence has been provided for the effect of PECVD process parameters and surface preparation on the electrical parameters of the heterostructures grown.

Assessing Exergy-Based Economic and Sustainability Analyses of a Military Gas Turbine Engine Fueled with Various Fuels

This research put forth exergy-based economic and sustainability analyses of a (J85-GE-5H) military turbojet engine (TJE). Firstly, sustainability, conventional exergoeconomic and advanced exergoeconomic cost analyses were executed utilizing kerosene fuel according to real engine working circumstances. The engine was likewise investigated parametrically, considering H2 fuel utilization. The sustainable economic analysis assessment of the TJE was finally actualized by comparing the acquired outcomes for both fuels. The entire engine’s unit exergy cost of product (cPr) with kerosene was determined 76.45 $/GJ for the military (MIL) process mode (PM), whereas it was computed 94.97 $/GJ for the afterburner (AB) PM. Given the use of H2, the cPr increased to 179 and 288 $/GJ for the aforementioned two modes, seriatim. While the sustainability cost index (SCI) values were obtained 52.86 and 78.84 $/GJ for the MIL and AB PM, seriatim, they became 128 and 244 $/GJ when considering H2. Consequently, the higher exergy demolitions occurring in the afterburner exhaust duct (ABED) and combustion chamber (CC) sections led to higher exergy destruction costs in the TJE. However, the engine worked less cost efficient with H2 fuel rather than JP-8 fuel because of the higher cost value of fuel.

The Ubiquitous Influence of Mentors on Postgraduates

Tutors and graduate students are a force that can not be ignored in the scientific research team of colleges and universities. Understanding the construction of the ubiquitous influence of mentors on graduate students is of great significance to strengthen the ubiquitous influence among students, promote the interaction between teachers and students, and improve the quality of graduate education in colleges and universities. Based on 28 interview data, this paper collates and analyzes the ubiquitous influence by using Nvivo11.0 software, and summarizes the ubiquitous influence into three aspects, namely, the basis of ubiquitous influence-spatio-temporal dispersion, the process-mode diversity of ubiquitous influence, and the comprehensiveness of the result-result of ubiquitous influence, and constructs the concept of tutor's ubiquitous influence on graduate students.

Comparative Performance Metric Assessment of A Military Turbojet Engine Utilizing Hydrogen And Kerosene Fuels Through Advanced Exergy Analysis Method

This study dealt with evaluating the (J85-GE-5H) military turbojet engine (TJE) in terms of exergetic and advanced exergetic analyses at Military (MIL) and Afterburner (AB) process modes by utilizing kerosene (JP-8) and hydrogen (H2) fuels. First, exergy and advanced exergy analyses of the engine were performed using JP-8 fuel as per actual engine operating conditions. These analyses of the turbojet engine using hydrogen fuel were also examined parametrically. The performance evaluation of the engine was lastly executed by comparing the obtained results for both fuels. Based on the parametric studies undertaken, the entire engine’s exergetic efficiency with JP-8 was reckoned 30.85% at the MIL process mode while it was calculated as 16.98% at the AB process mode. With the usage of H2, the efficiencies of the engine decreased to 28.62% and 15.33% for the above mentioned two modes, respectively. As the supreme exergy destructions occurred in the combustion chamber (CC) and afterburner exhaust duct (ABED) segments, the new technological developments should be considered to design more efficient engines. As a result, the engine worked less efficiently with hydrogen fuel due to the enhancement in exergy destructions. Conversely, the greenhouse gas (GHG) emission parameters lessened with the utilization of H2 fuel.