Unstable Arc Research Articles

Dynamic behavior regulation of droplet transfer based on arc-bubble control by ultrasound-induced during underwater wet welding

In comparison to welding in air, the periodic formation of unstable arc bubbles is a significant factor contributing to the unstable mass transfer process during underwater wet welding (UWW). Up to now, there is no ideal method to control droplet transfer and improve arc stability. In this study, the ultrasound-induced method (UIM) was used to regulate the droplet transition behavior and improve the process stability. The behavior features and key mechanisms of arc-bubble and droplet transfer induced by ultrasound were revealed with the employment of highspeed camera and X-ray observation, respectively. The incident angle of ultrasound was taken as a variable to reveal its effects and mechanism of action on the behaviors of arc bubble and droplet transfer, weld formation quality, microstructure, and property of weld joint. The results show that ultrasound-induced a new rising mode of arc bubble, which can significantly reduce the repulsive resistance that come from the bubble rising on the droplet transfer process. Compared with underwater wet welding without ultrasound, the average diameter of molten droplets reduces from 3.2 mm to 1.7 mm, and the droplet transfer frequency increases from 7.8 Hz to 13 Hz as the incident angle of ultrasound increases to 60°. In addition, the weld seam without obvious defects could be produced by UWW with UIM, of which the Variation coefficient of weld reinforcement is reduced from 21.3 to 11.6 and 17.4 at an ultrasonic incident angle of 30° and 45°. With increasing of ultrasound incident angle, it showed that the microhardness of the weld joint has a slight increase. In the heat-affected zone, the microhardness for the joint increased from 290 HV to 320 HV while the incident angle increased to 60°. This research shows that an appropriate incident angle of ultrasound can obtain good-quality joints which has a stable metal transfer process in underwater wet welding.

Оценка сварочно-технологических свойств электродных покрытий основного типа различных производителей электродов для сварки трубных деталей и сборочных единиц поверхностей теплообмена котлоагрегатов

Introduction. New grades of high-strength steels, machining and repair processes are being introduced in the power industry. At the same time manual arc welding remains the main technological process for equipment repair in conditions of thermal power plants. Welding materials used in equipment repair should provide comparable to the base metal mechanical properties of the weld. The welding industry has long faced the problem of high sensitivity of basic type electrodes to moisture absorption. High susceptibility to cold cracking caused by diffusible hydrogen and hydrogen embrittlement are major obstacles to the wider use of basic-type electrodes for high-strength steels. Hydrogen production during arc welding is the result of the presence of hydrogen in the arc atmosphere, hydrogen-contaminated filler material, or local hydrogen residues on the source material. During welding, molecular hydrogen is dislocated by the arc energy and then easily absorbed by the molten material. Currently, the welding materials market produces electrodes with basic coating of well-known and proven brands, various national and foreign manufacturers. However, in practice there are cases of cold cracks in the weld seam after welding. Purpose of work is to assess the welding and technological properties of basic type electrode coatings of different manufacturers. The work investigates specimens weld overlaid with electrodes TMU-21U, TSU-5 of different manufacturers and the content of diffusion-mobile hydrogen in the weld overlaid metal is determined. The methods of research are mechanical static tensile tests, chemical composition analysis and metallographic studies. Determination of welding-induced hydrogen content can be accomplished by various quantitative elemental analysis methods. All test methods involve welding under defined conditions followed by deep freezing of the test specimens as quickly as possible. In this way, unintended diffusion processes are inhibited and the hydrogen introduced into the weld metal is retained. Subsequently, the diffusing hydrogen is desorbed from the test specimens in a controlled manner. Results and Discussion. An assessment of welding engineering properties of the electrodes revealed unstable arc burning. Mechanical properties of the welded metal of the investigated electrodes are at the minimum permissible level from the requirements of normative documents. The concentration of hydrogen present in the arc weld metal is multifactorially dependent on the welding procedure (process and parameters, consumables used, as well as environmental conditions (e.g. humidity). For qualitative assessment, hydrogen content of more than 15 cm3/100 g is considered high and hydrogen content less than 5 cm3 ml/100 g is considered very low. Presented results. The conducted evaluation of welding engineering properties of electrodes with basic coating showed satisfactory results. Mechanical properties of the welded metal in terms of impact toughness are at the lower permissible limit, relative elongation does not meet the requirements of normative documents. The content of diffusion-mobile hydrogen in the welded metal is higher than the declared indicators by the electrode manufacturers.

RESEARCH STATUS AND PROSPECTS FOR TIG-MIG HYBRID-ARC-WELDING TECHNOLOGY

TIG-MIG hybrid welding integrates the merits of tungsten insert-gas welding (TIG) and metal inert-gas welding (MIG), and achieves a new material-joining process with high quality, high efficiency and low cost. However, TIG-MIG hybrid welding also had some shortcomings, such as a complex process, unstable arc, large heat input, limiting its application. To further improve and promote TIG-MIG hybrid welding, numerous universities and research institutes have put forward a series of improvement programs that achieve relatively good results. In this study TIG-MIG hybrid welding is discussed in three aspects: the welding-process improvement, welding-parameters optimization and numerical simulation of the welding process. It was found that the stability of the hybrid arc welding process and the quality of the weld bead can be improved by improving the current polarity, wire type and arc swing. The TIG current, the distance between the wire and tungsten, and the heat input had an important impact on the weld quality and the formation of defects. A numerical simulation of the welding process analyzed the effects of torch angle, the distance between the wire and tungsten, the welding speed and the temperature fields on the arc morphology, molten-pool behavior, and droplet transfer. According to the above analysis, it summarized the current research status of TIG-MIG hybrid welding, and then proposed future research directions based on the existing shortcomings and deficiencies.

Comparative performance evaluation and economic assessment between traditional and vibration assisted EDM during machining of Inconel 718

Nickel based superalloy, Inconel 718 offers extensive applications in aerospace and gas turbine industries because of its outstanding properties, enabling high performance in various critical components. Despite the wide application of electric discharge machining (EDM) for difficult-to-cut conductive materials, the process becomes less productive due to unstable arcing and inadequate debris flushing during machining, leading to reduced operational efficiency. This research focuses on comparative performance evaluation and economic assessment between conventional-EDM and vibration assisted EDM (VA-EDM) while machining Inconel 718 with copper electrode. The workpiece was excited to vibrate with low frequency by employing unbalanced mass attached in a rotary motor with the aim of enhancing EDM performance. Material removal rate MRR, surface integrity (surface crack density SCD, surface roughness Ra, white (recast) layer thickness WLT) and tool wear rate TWR are analyzed to determine machining performance in comprehensive analysis. The results of the performance comparison show that vibration assisted is superior to the conventional-EDM process concerning enhanced material removal rate, and surface morphology with minimum surface crack density, tool wear rate and white layer thickness. The mean value ratios for Ra EDM/ Ra VA-EDM, TWR EDM/ TWR VA-EDM, WLT EDM/ WLT VA-EDM, and SCD EDM/ SCD VA-EDM are 1.35, 1.52, 1.21, and 1.74, respectively. Additionally, the surface integrity was enhanced using vibration-assisted EDM because it (a) increases the plasma flushing efficiency and the gap pressure gradient; (b) raises the energy of electrically charged plasma particles; (c) increases the effective flow of dielectric in the gap; (d) minimizes the number of ineffective pulses (short-circuiting and arching) on the surface; and (e) improves the dielectric flushing conditions in sparking gap. According to the findings, the overall machining expenditure per part under VA-EDM in Indian rupees (INR) 534 is lowered by 8% than expenditure under conventional-EDM (INR 576.60). The outcomes of this research have the potential to benefit numerous industries that deal with difficult-to-cut materials.

Influence of Ship-based Vibration on Characteristics of Arc and Droplet and Morphology in Wire Arc Additive Manufacturing

To explore the feasibility of using wire arc additive manufacturing (WAAM) for forming and repairing marine components during ship navigation, this study conducted deposition experiments of ER50-6 steel and investigated the interference of ship-based vibrations on the WAAM equipment and influence of the characteristics of the arc droplets and sample morphology. The results revealed that the WAAM equipment vibrated with the external vibrations from the surrounding environment, and the welding gun and base plate produced dissimilar vibrations, which yielded unstable arc shapes resembling a bell, trumpet, fan, broom, and other irregular shapes. The mode of the droplet transfer ranged from the stable spray transfer mode to extensive amounts of large droplet transfer and short-circuit transfer. Although the morphology of the obtained sample deteriorated, the fully dense and defect-free interior demonstrated the applicability of ship-based WAAM.

Evaluation of Arc Signals, Microstructure and Mechanical Properties in Ultrasonic-Frequency Pulse Underwater Wet Welding Process with Q345 Steel

The ultrasonic-frequency pulse underwater wet welding (UFP-UWW) process was achieved through a constant-voltage-mode power source connecting an ultrasonic-frequency pulse power source in parallel. The arc voltage and welding current waveforms, sound signal variations, microstructural characteristics and mechanical properties at different parameters were investigated. The results showed that the ultrasonic-frequency pulse voltage and current of the UFP-UWW process displayed a periodic high-frequency oscillation on the based values of the conventional UWW process. The arc stability of the UFP-UWW process improved owing to the fact that the proportions of the unstable arc burning region could be reduced to 1.56% after the introduction of the ultrasonic-frequency pulse current. No significant changes in weld width and penetration were observed while the weld dilution rate increased to 54.2% for the combination of 40 V–30 kHz, compared with the results of the conventional UWW process. The flux-cored arc (FCA) welding arc in the air had the same frequency response to the ultrasonic excitation signal, which verified the existence of the ultrasonic-frequency vibration induced by the periodic high-frequency electromagnetic forces. The application of the ultrasonic-frequency pulse produced finer columnar grains in the welds with an average length of 315 μm, although the amount of pro-eutectoid ferrite and acicular ferrite varied little. The mechanical properties of the welded joints were also noticeably enhanced with the application of different ultrasonic excitation frequencies. The optimum tensile strength and impact toughness of the welded joint were improved by 6.7% and 21.7% when the applied ultrasonic excitation voltage was 40 V for a pulsed frequency of 30 kHz. These results facilitate the application of ultrasonic arc welding technology in the marine field.

Structure, phase evolution and properties of Ta films deposited using hybrid high-power pulsed and DC magnetron co-sputtering

Crystalline phase and microstructure control are critical for obtaining desired properties of Ta films deposited by magnetron sputtering. Structure, phase evolution and properties of Ta films deposited by using hybrid high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS) under different fractions of DCMS power were investigated, where Ta ion to Ta neutral ratios of the deposition flux were changed. The results revealed that the number of Ta ions arriving on the substrate/growing film plays an important role in structure and phase evolution of Ta films. It can effectively avoid the unstable arc discharge under low pressure and show a higher deposition rate by combining HiPIMS and DCMS compared with only HiPIMS. Meanwhile, the high hardness α-Ta films can be directly deposited by hybrid co-sputtering compared to those prepared by DCMS. In the co-sputtering technology, pure α-Ta phase films with extremely fine, dense and uniform crystal grains were obtained, which showed smooth surface roughness (3.22 nm), low resistivity (38.98 μΩ⋅cm) and abnormal high hardness (17.64 GPa).

Analysis of transient recovery voltage and secondary arc current in transposed extra-high voltage lines in a two-phase auto-reclosing

Extra-high voltage (EHV) lines of 500–750 kV, providing transmission of electricity over long distances and at the same time performing the functions of intersystem communication at the level of the national power system, play an important role not only in normal modes, but also in emergency modes, ensuring the dynamic stability of the power system as a whole. In these lines, the overwhelming proportion of power cuts are caused by single-phase short circuits (90%), a significant part of which, being unstable arc faults, are successfully eliminated in the single-phase auto-reclosing cycle. Also, about 5%–10% of failures can be constituted by two-phase short circuits, which can be eliminated in a two-phase auto-reclosing cycle (TPhAR). The purpose of this paper is to study two-phase auto-reclosing in transposed EHV lines equipped with four-radial shunt reactors (ShR). The paper analyzes the efficiency of using a two-phase auto-reclosing to eliminate two-phase short-circuits in the lines connecting the power systems of Kyrgyzstan and Tajikistan. An algorithm is proposed for calculating the transient recovering voltages (TRV) and secondary arc currents (SAC) in the real transposed line Datka–Khujand–Dushanbe. The obtained results of TRV and SAC, which are within the permissible limits for the Dushanbe–Khujand line section, make it possible to have a dead time of TPhAR of no more than 0.6 s, in order to maintain the dynamic stability of the power system. For lines with a length of about 500 km (Datka–Khujand), equipped with three reactors, a successful TPhAR is impossible due to the appearance of resonant TRV in the circuit. The paper proposes the use of banks of capacitors connected in series in the phases of the ShR for the implementation of a successful TPhAR with the duration of the required pause of about 0.6 s.

Single-pass full-penetration welding for thick stainless steel using high-current GMAW

ABSTRACT This paper describes a high-current Gas Metal Arc Welding (GMAW) process using a constant voltage power source developed for high-efficiency welding of thick stainless steel plates. A short arc length as a welding condition is preferable to concentrate the arc and obtain a deep penetration for thick plate welding. However, it is easily found that an unstable arc derived from a short circuit happens irregularly by using a general purpose GMAW with constant voltage characteristics. A ‘low-frequency modulated voltage control’ was applied for stabilizing a high-current arc that can maintain a short arc length without short circuiting. The welded joint using the developed welding process showed the similar corrosion resistance as that using the submerged arc welding; however, porosity was observed in the welded joints using the developed welding process in the current region of 400 A or above. Consequently, a new current waveform control that changes the welding current dynamically and periodically has been developed, which realized reducing porosity and single-pass full-penetration welding of 9-mm-thickness type 304 stainless steel plates with square groove.

RANCANG BANGUN KONSTRUKSI BODI PADA AUTOMATIC WELDING CARRIER MACHINE

Manually system welding process still widely used and have a deficiency in it. Which work is accident often occur in it, the unstable arc of fire and quality of welding product still depends on the welder ability. The purpose of this study is to design a portable robotic welding tool equipped MIG(Metal Inert Gas) welding tool to support automation in the welding process. Mobility and work range of tools is supported by light and strong material and designed with relatively small dimensions and equipped leg robotics which is useful to adding work range of the tool in every single welding process. This study using experimentally and simulations methods in body construction and empirical calculation to know relevantly the result and simulation process. And the data obtained after calculating with imposition 98 N, pressure stress on the body is 3334 Pa, which is smaller than the Modulus of Elasticity Aluminum alloy type 5052, moreover, materials used are capable and safe. The riveted joint of the extensions of the body has fracture resistance 2,8 kN and used efficiency of 70% moreover this kind of extensions are efficient to apply on Automatic Welding Carrier Machine’s body.

The Mechanism of Effect of Flux Bands on The Arc Behavior in Flux Bands Constricting Arc Welding Process.

A new welding method, flux bands constricting arc (FBCA) welding, is proposed to compensate for the shortage of insufficient weld width of laser welding T-joints in high steel sandwich panels. The arc behavior (arc burning position, arc shape, arc heat, and arc stability) before and after sticking the flux bands (GMAW and FBCA welding) to the ultra-narrow gap groove was tested. Results indicate that flux bands have solid-wall constricting effect (SWCE) and thermo-compression effect (TCE) on the arc and self-producing slag and gas function in FBCA welding. In ultra-narrow gap groove, the arc burning position climbing up phenomenon (APCP) occurs without flux bands. The SWCE of flux bands on the arc effectively suppresses the APCP because of the insulation of flux bands. In the FBCA welding process, the effective heating area of the arc is increased by at least 5 mm2 compared with that in GMAW. When the groove gap decreases, flux bands not only compress the arc from an inverted bell shape to a rectangular shape, but also make the 660 °C isotherm on the core-plate to increase from 3 mm to 8 mm. In the end, the proportion of unstable arc burning time is reduced by 86.85%, the fluctuation of arc voltage and welding current are also significantly reduced by the flux bands because of their SWCE on the arc.

Simultaneous Voltage and Heat Transfer Measurements Along an Arc-Plasma Wind-Tunnel Constrictor

Experiments were conducted to simultaneously quantify the voltage distribution and heat losses along the length of an arc-plasma wind-tunnel constrictor employing air and argon gas. The objective was to generate a detailed data set and make it available electronically to the scientific community. High-spatial-resolution voltage measurements allowed the identification of a wavelike structure in the upstream part of the constrictor. It was postulated that electrode erosion produced an unstable arc geometry that was further distorted by the imposed magnetic fields. Alternatively, the wavelike structure could be related to vortex precession commonly observed in swirl-stabilized flows. Measurements of heat transfer to the wall, based on differences in cooling water temperature across each segment, revealed a mainly uniform heat transfer distribution along the constrictor, with elevated heat transfer near the cathode and anode due to the strong electric field strengths in the near-cathode and near-anode regions. By varying either the mass flow rate or current, these measurements also allowed, for the first time, the estimation of differences in radiation and convection heat transfer.

Fatigue life of underwater wet welded low carbon steel SS400

Underwater welding is widely used for maintenance and repairs of underwater structures such as undersea pipes, offshore structures and nuclear power plants. In practice, underwater welding has the disadvantage related to high cooling rate and unstable welding arc due to the water hydrostatic pressure. This affects the microstructure and mechanical properties of underwater welded joints. Many of previous research works on underwater welding have been carried out only on a laboratory scale in shallow water depth, whereas underwater welding was used to weld in the depth of the water with a metre scale. Undersea structures experience fatigue load due to the fluctuation force of water flow. Therefore, this study aims to determine the effect of water depth on the fatigue life of underwater welded joints. Low carbon steel SS400 specimens were welded underwater with depths of 2.5 m, 5 m and 10 m. The air welded joint was also evaluated for comparison purposes. Fatigue life was evaluated according to the ASTM E466 standard by using a rotary bending machine. Furthermore, tensile test, micro hardness measurement and microstructure evaluation were also conducted for gathering supporting data. The fatigue and tensile strength of the air welded joints were higher than those of the underwater welded joints. The porosities caused by the dissolved hydrogen gas, carbon (monoxide and dioxide) gases and water vapor in weld metal of the underwater welded joints decreased the fatigue and tensile strength. An interesting phenomenon on the underwater welded joints was that the deeper the water level, the higher became the fatigue, tensile strength as well as hardness. Based on the microstructure analysis, the number of acicular ferrite structures in weld metal increased as the water level depth increased.

Effect of Mo-Cu cathode composition on process stability, macroparticle formation, plasma generation, and thin-film deposition in DC arc synthesis

In this work, we present the correlation between cathode composition and features of the arcing process for Mo1 − xCux [x = 0.0, 0.07 (0.05), 0.14 (0.10), 0.21 (0.15), 0.40 (0.3), 0.73 (0.63), 0.97 (0.95), and 1.00, atomic fraction (weight fraction)] cathodes used in a DC vacuum-arc deposition system. It is found that the stability of the arcing process crucially depends on the cathode composition. The most stable arc spot and the lowest cathode potential (∼19 V) are detected for the Mo0.27Cu0.73 cathode, while the Mo0.93Cu0.07 cathode shows the most unstable arcing process with the highest cathode potential (∼28 V). The properties of the generated plasma are also strongly dependent on the relative ratio of the cathode elements. The metal ions from the Mo and Cu cathodes have peak kinetic energies around 136 and 62 eV, respectively, while for a Mo0.79Cu0.21 cathode, the corresponding energies are only 45 and 28 eV. The average charge states decreased from 2.1 to 1.6 for Mo ions and from 2 to 1.2 for Cu ions. The intensity of macroparticle generation and the size of the droplets correlate with the relative fraction of Cu. However, it is shown that, typically for the cathodes with a low amount of Cu, an increased abundance of visually observed macroparticles leads to droplet-free films. The film thicknesses and their compositions also demonstrate dependencies on the elemental composition of the cathode. These results are discussed in the light of no solubility between Mo and Cu and the high temperature of the cathode surface during the arcing process.

Fault Detection Based on the Generalized S-Transform With a Variable Factor for Resonant Grounding Distribution Networks

Fault detection in resonant grounding (RG) distribution networks remains a challenge due to weak fault signals, extremely complex fault conditions, and unstable intermittent arc grounding faults. This paper addresses this issue by applying generalized S-transform (GST) with a variable factor to conduct denoising of transient zero-sequence currents based on threshold filtering followed by time-frequency distribution filtering in sequence. Meanwhile, this paper proposes a comprehensive multi-criteria faulty feeder detection method based on the transient zero-sequence current polarity (criterion 1), the energy relative entropy (criterion 2), and the total transient current energy (criterion 3). Here, criteria 2 and 3 are based on the time-frequency representation of the GST. The performances of the proposed denoising and faulty feeder detection methods are evaluated under single line to ground faults based on simulations conducted using a modeled 10 kV RG networks with overhead and cable mixed lines in addition to reasonably sophisticated permanent and intermittent arc discharge models to ensure that the simulations faithfully represent actual complex working conditions. Compared with existing method, simulation experiments and field test show that the method proposed in this paper provide a better denoising effect with stronger self-adaptability, higher detection accuracy, and a faster calculation speed.

Research on fault feeder detection with adaptive notch filter in non-effective neutral point to ground system’ single phase to ground fault

It is difficult to detect single-phase grounding fault in non-effective neutral point to ground system because of its weak current and unstable arc. In this paper, a kind of self-adaptive notch filter is used to extract the fundamental wave of current signals. This method is verified by Matlab/Simulink simulation. The result confirms that the method can realize fault line detection timely and accurately.

Fundamental plasma diagnostic study for guiding TIG arc phenomenon by laser beam irradiation

ABSTRACT Laser-arc hybrid welding is divided into laser-based and arc-based welding. In the arc-based welding, the laser beam irradiation provides the guiding effect to the arc plasma. Some studies have reported that an unstable arc plasma becomes stable due to this guiding effect. This is considered as the influence of metal vapour generation by laser beam irradiation; however, there are few reports on the measurement of metal vapour and arc plasma state during the guiding process. Therefore, the mechanism has been still not fully understood. In this study, the fundamental study of the guiding arc phenomenon was conducted in TIG arc welding (60 A) with laser beam irradiation (170 W) under relatively low-power condition. When the base metal was stainless steel, the guiding arc by the laser beam irradiation was observed which did not happen with mild steel. Spectroscopic measurement for the guiding arc phenomenon in stainless steel showed that the metal vapour was composed of manganese and chromium, and was widely distributed near the irradiation point, but not concentrated. We also observed a reduction of plasma temperature, thought to be caused by distributed metal vapour and the resultant reduction of the current density.

Underwater Electrical Discharges: Temperature, Density and Basic Instability Features with Different Anode Materials

Plasma temperature, density and arc dynamics have been investigated in underwater arc discharges under different anodic environments. Distinct differences in the behaviour with different anode material and rich variety of interesting dynamics have been directly revealed through fast photography and emission spectroscopic analysis. Four different anode materials of high technological relevance, namely SS304, copper, zircaloy, and P91 [ASTM387-Gr91] have been used as anode material. Instabilities in the plasma channel with alternate expansion and contraction zones, highly prominent magnetic pumping near the anode arc root, formation of strong anode jet, dynamic branching of arcs, formation of distinct shock boundaries, and coexistence of multiple anode and cathode arc roots are some of the interesting features observed. Observation of Hα line and absence of Hβ line in the emission spectra is an interesting observation. Emission lines, characteristic to chosen anode material, have been used in determining the axial plasma temperature using a modified Boltzmann plot technique, suitable for spatially unstable arc discharges and pulsating arcs. Electron density inside the plasma has been determined from the width of Stark broadened Lorentzian profile of Hα line. Typical temperatures for discharges with P91, Zr, SS304 and Cu anodes are estimated around 8159 K, 14525 K, 6151 K, and 10534 K respectively under similar operating conditions. Respective electron densities per m3 in the discharge zone are estimated to be 2.97 × 1023, 3.76 × 1023, 0.41 × 1023 and 6.21 × 1023. Estimated plasma coupling parameter finds the discharge in the regime of weakly non-ideal plasma.

Arc Behavior Study Using Welding Current Module and its Impact on Residual Stress and Weld Bead in Anti-Phase Synchronized Twin-Wire Gas Metal Arc Welding

The importance of twin-wire welding in increasing the deposition rate is known for a long, but its application in gas metal arc welding is limited due to the arc-stability related issues. An unstable welding arc causes irregular weld bead and material loss in the form of spatters. Previous investigations indicate that the problem can be addressed through arc-stability induced by dissimilar twin-arcs as the electromagnetic field concentrates around the arc with higher current. The present study is intended to reduce the twin-arcs' interactions and to improve the arc stability in twin-wire gas metal arc welding by evaluating different conditions at lead and trail wires. Further, their influence on the residual stresses and weld bead geometry is studied. Bead-on-plate-welds are carried out. A data acquisition system is used to capture the electrical signals during welding. The results indicate that the unequal currents at trail and lead wires provide stability to the arc that also results in a shift in residual stresses from compressive to tensile along the weld transverse direction. In addition, the maximum residual stress is located at the weld toe. When the current difference between the trail and lead wire is more, the arc produces stable metal transfer with uniform heating and cooling that results in reduction in stresses and improvement in weld quality.

Propagation of a stable gaseous detonation in a circular arc configuration

We examine numerically the effect that detonation wavefront curvature has on gaseous detonation cellular structure in a circular arc geometry with rigid walls. For the model considered, a planar detonation is weakly unstable. The detonation curvature is induced by diffraction and flow divergence as the wave sweeps around the arc. We examine the effect of arc thickness, inner arc radius and activation energy on the cellular pattern. A combination of pseudo-Schlieren, pseudo-soot foil records and detonation shock and sonic flow loci are used to explore the observed detonation structures. We find that for sufficiently wide arcs, the detonation wavefront curvature renders the detonation hydrodynamically stable, i.e. laminar flow with no cells. The stable configuration consists of a broadly curved wave originating from the inner arc surface, which is typically connected to the outer arc boundary by a stable Mach stem. The stable detonation is driven by a small region of subsonic flow attached to the inner arc surface, outside of which the flow is supersonic. The initiation dynamics of a stable propagating wave case are explored. The evolution dynamics between an unstable (cellular) and stable arc thickness are also examined. Smaller inner arc radii and lower activation energies increase the range of stable solutions.