A commonly observed welding defect that characteristically occurs at high welding speeds is the periodic undulation of the weld bead profile, also known as humping. The occurrence of humping limits the range of usable welding speeds in most fusion welding processes and prevents further increases in productivity in a welding operation. At the present time, the physical mechanisms responsible for humping are not well understood. Thus, it is difficult to know how to suppress humping in order to achieve higher welding speeds. The objectives of this study were to identify and experimentally validate the physical mechanisms responsible for the humping phenomenon during high speed gas metal arc (GMA) welding of plain carbon steel. A LaserStrobe video imaging system was used to obtain video images of typical sequences of events during the formation of a hump. Based on these recorded video images, the strong momentum of the backward flow of molten metal in the weld pool that typically occurred during high speed welding was identified as the major factor responsible for the initiation of humping. Experiments with different process variables affecting the backward flow of molten weld metal were used to validate this hypothesis. These process variables included welding speed, welding position and shielding gas composition. The use of downhill welding positions and reactive shielding gases was found to suppress humping and to allow higher welding speeds by reducing the momentum of the backward flow of molten metal in the weld pool. This would suggest that any process variables or welding techniques that can dissipate or reduce the momentum of the backward flow of molten metal in the weld pool will facilitate higher welding speeds and productivity.

A comprehensive survey of high speed weld bead defects is presented with strong emphasis on the formation of humping and undercutting in autogenous and non-autogenous fusion welding processes. Blowhole and overlap weld defects are also discussed. Although experimental results from previous studies are informative, they do not always reveal the physical mechanisms responsible for the formation of these high speed weld bead defects. In addition, these experimental results do not reveal the complex relationships between welding process parameters and the onset of high speed weld bead defects. Various phenomenological models of humping and undercutting have been proposed that were based on observations of events in different regions within the weld pool or the final weld bead profile. The ability of these models to predict the onset of humping or undercutting has not been satisfactorily demonstrated. Furthermore, the proposed formation mechanisms of these high speed weld bead defects are still being questioned. Recent welding techniques and processes have, however, been shown to be very effective in suppressing humping and undercutting by slowing the backward flow of molten metal in the weld pool. This backward flow of molten weld metal may be the principal physical phenomenon responsible for the formation of humping and undercutting during high speed fusion welding.

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Purpose The purpose of this paper is to add to an emerging literature on educational leadership and management in Vietnam by addressing several goals. First, the study sought to translate, adapt, and validate an existing measurement instrument, the Principal Instructional Management Rating Scale (PIMRS) Teacher Form, for use in Vietnam. Next, it aimed to describe patterns of instructional leadership evidenced among a sample of urban and rural primary school principals. Then, the researchers examined if these patterns of principal instructional leadership could be linked to one or more “antecedent variables”: school size, school location (urban/rural), principal’s gender and prior teaching experience. Finally, the paper sought insights from principals and teachers on how instructional leadership could be strengthened in the Vietnamese education context. Design/methodology/approach Both quantitative survey and qualitative methods were used in this study. The sample consisted of 569 teachers and 117 principals working in 117 primary schools located in Ho Chi Minh province of Vietnam. Data collection employed a translated and adapted Vietnamese language form of the PIMRS Teacher Form. An open-ended question posed to both teachers and principals was included in the survey instrument to gather recommendations for strengthening instructional leadership in Vietnam. The research used Rasch analysis, Cronbach’s test of internal consistency, confirmatory factor analysis, t-tests, and analysis of variance in data analysis. Findings The research achieved a preliminary validation of a Vietnamese language Teacher Form of the PIMRS. The analysis of PIMRS data gathered from teachers found that the primary school principals were perceived to be exercising instructional leadership at a surprisingly high level. Consistent with international research findings, selected evidence indicated stronger instructional leadership from the female principals, though the pattern was not strong. None of the other antecedents evidenced a significant relationship with patterns of principal instructional leadership. A number of overlapping recommendations were made by teachers and principals for strengthening instructional leadership in Vietnam. Research limitations/implications Although a Teacher Form of the PIMRS Vietnam was successfully validated, follow-up studies should be conducted with both the Teacher Form and Principal Form of this instrument. More broadly, the high scores on the PIMRS accorded to the principals in this study were deemed “surprising” in light of the lack of prior policy focus and training on this role of the principal in Vietnam. Thus, the authors recommend that this research be extended to a larger cross-level study of schools from different parts of Vietnam in order to provide additional confirmation of these preliminary findings. Practical implications Feedback from principals and teachers indicated a need for system leaders to articulate instructional leadership more explicitly as part of the principal’s role set in Vietnam. Only then will it become part of the formal expectations that shape principals’ practice and the preparation and professional development programs in which they participate. The principals also suggested that instructional leadership could be strengthened by enabling model principals to share instructional leadership practices with colleagues. Finally, teachers and principals highlighted the need to broaden, legitimate, and strengthen sources of instructional leadership within the school beyond the principal. These suggestions are not only consistent with policy actions taken in other societies, but are also grounded in the context of education in Vietnam. Originality/value The first internationally published study of educational leadership and management in Vietnam only appeared in 2012. In the succeeding years, several qualitative studies have emerged describing principal leadership practices in a handful of schools. The current study represents the first published quantitative study of school leadership from Vietnam. Although the results are preliminary in nature, the study provides both baseline data on principal instructional leadership and a validated instrument that can be used in future research.

The onset of geometric defects such as humping or discontinuous weld beads during gas metal arc welding (GMAW) frequently limits the use of higher welding speeds and increased productivity. In the present study, a dimensional analysis of the GMAW process was performed in order to identify a number of dimensionless groups formulated based on various GMAW process parameters and material properties that could be used to predict when humping or discontinuous weld beads would occur. Experimental data from bead on plate GMA welds in plain carbon steel plate made using argon and two different reactive shielding gases, welding powers between 5 and 12 kW and a range of welding speeds were then used to create dimensionless process maps. These maps showed the limiting welding speed above which the high speed weld defects occurred as a function of all influential process parameters. It was shown that all experimental data for limiting welding speeds could be collapsed onto two collinear dimensionless curves. Also, the transition from spray to rotational metal transfer was found to occur at a well defined value of one of these dimensionless parameters. The effects of workpiece preheat temperature on humping were correctly predicted and there was a good correlation between the dimensionless GMAW process map and experimental data from other independent studies. These results suggest that the occurrence of high speed weld bead defects such as humping and discontinuous weld beads as well as the transition from spray to rotational metal transfer can be predicted using these new dimensionless GMAW process maps.

In this study, the effect of oxygen in the shielding gas on the material flow behavior of the weld pool surface was discussed to clarify the dominant driving weld pool force in keyhole plasma arc welding (KPAW). To address this issue, the convection flow on the top surface of weld pool was observed using a high-speed video camera. The temperature distribution on the surface along keyhole wall was measured using the two-color pyrometry method to confirm the Marangoni force activity on the weld pool. The results show that the inclination angle of the keyhole wall (keyhole shape) increased especially near the top surface due to the decrease in the surface tension of weld pool through surface oxidation when a shielding gas of Ar + 0.5% O2 was used. Due to the change in the keyhole shape, the upward and backward shear force compositions created a large inclination angle at the top surface of the keyhole. From the temperature measurement results, the Marangoni force was found to alter the direction when 0.5% O2 was mixed with the shielding gas. The shear force was found to be the strongest force among the four driving forces. The buoyant force and Lorentz force were very weak. The Marangoni force was stronger than the Lorentz force but was weaker than shear force. The interaction of shear force and Marangoni force controlled the behavior and speed of material flow on the weld pool surface. A strong upward and backward flow was observed in the case of mixture shielding gas, whereas a weak upward flow was observed for pure Ar. The heat transportation due to the weld pool convection significantly changed when only a small amount of oxygen was admixed in the shielding gas. The results can be applied to control the penetration ratio in KPAW.

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We present controllable and reliable complementary organic transistor circuits on a PET substrate using a photoactive dielectric layer of 6-[4'-(N,N-diphenylamino)phenyl]-3-ethoxycarbonylcoumarin (DPA-CM) doped into poly(methyl methacrylate) (PMMA) and an electron-trapping layer of poly(perfluoroalkenyl vinyl ether) (Cytop). Cu was used for a source/drain electrode in both the p-channel and n-channel transistors. The threshold voltage of the transistors and the inverting voltage of the circuits were reversibly controlled over a wide range under a program voltage of less than 10 V and under UV light irradiation. At a program voltage of -2 V, the inverting voltage of the circuits was tuned to be at nearly half of the supply voltage of the circuit. Consequently, an excellent balance between the high and low noise margins (NM) was produced (64% of NMH and 68% of NML), resulting in maximum noise immunity. Furthermore, the programmed circuits showed high stability, such as a retention time of over 10(5) s for the inverter switching voltage. Our findings bring about a flexible, simple way to obtain robust, high-performance organic circuits using a controllable complementary transistor inverter.