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It is of great significance in the field of engineering to repair the surface defects of ZM6 cast magnesium alloy by an arc welding method. Compared with the traditional tungsten inert gas (TIG) welding repair technology, cold metal transfer (CMT) welding repair has the advantages of low heat input, small repair deformation, and high efficiency. It is of great research value to repair the surface defects of ZM6 cast magnesium alloy by CMT welding. In this paper, the effect of CMT welding repair parameters on defect repair forming is systematically studied, and a repair process window free of unfused defects is obtained. The effects of preheating temperature of base material, wire-feeding speed, welding speed, stick-out length of welding wire and shielding gas flow on the spread of magnesium alloy melt and weld formation were investigated by a surface surfacing method. During the welding process, a camera was used to capture images of the arc and droplet features. A pit defect with a depth of 11.5 mm was machined on the surface of the casting, and the effect of five different repair paths on the formation of the repair area was studied. In order to make the repair area have better fusion, reasonable repair parameters are as follows: The preheating temperature range is 310–450 °C, the wire-feeding speed range is 5–7 m/min, the welding speed range is 8–10 mm/s, the stick-out length of the welding wire is 12 mm, the shielding gas flow rate is 20 L/min, and the repair path adopts a continuous linear reciprocating welding path. This study has important significance for guiding the development of CMT repair technology of cast magnesium alloy. [ABSTRACT FROM AUTHOR]

K-TIG welding is an exceptionally efficient welding technique that enables full penetration with a single pass, without the need for groove preparation or wire filling. Comprehending the distinct arc characteristics of K-TIG welding is crucial for exploring the metallurgical process, heat, and mass transfer, and serves as a guide for design considerations. In this study, 2D numerical models of arc plasma in K-TIG welding were established. The arc characteristics of the keyhole and non-penetrated states were compared, while the influence of welding parameters on arc characteristics was further investigated. Finally, the calculated arc pressure was validated by experiments. Results show that the electric potential gradient in the arc column decreases once the keyhole is formed. Simultaneously, the temperature increases at the center of the anode surface and decreases on both sides away from the arc center. Higher welding currents and smaller tungsten tip angles will lead to an increase in the temperature and velocity of arc plasma. As the tungsten tip height increases, the arc temperature increases on both sides but decreases in the arc center, and the pressure in the inner wall of the keyhole shrinks. [ABSTRACT FROM AUTHOR]

Tungsten inert gas (TIG) is currently one of the most widely applied welding technologies. However, the arc energy divergence and high heat input of conventional TIG can lead to welding defects in heat-input-sensitive alloys (such as Inconel 718). Introducing a fast-frequency pulsed current (≥20 kHz) in TIG creates a welding process named fast-frequency pulsed tungsten inert gas (FFP-TIG). The arc behavior of FFP-TIG has advantages over conventional TIG in terms of arc contraction, arc pressure, and energy density. With an increase in the FFP current amplitude or frequency, the arc diameter and arc area are reduced, the arc pressure at the center of the arc is increased, and the arc energy divergence and arc temperature are reduced. The FFP-TIG weld method was adapted for Inconel 718 based on FFP-TIG arc behavior. This study provides fresh insight into the development and application of FFP-TIG arc physics research and industrial applications. [ABSTRACT FROM AUTHOR]

Swing arc narrow gap GMA welding experiments were carried out with a Box–Behnken response surface design. Weld metal and heat-affected sizes were measured from the joints obtained, and an ANOVA was performed to obtain well-fitting models for definition of the heat-affected length. Overlapping patterns and microstructures were analyzed and observed in zones within the heat-affected length through the thickness direction. In addition, thermal processes in typical zones of HAZs were calculated by FEM and analyzed to explain the patterns in the typical coarse grain heat-affected zones (CG-HAZs) with thermal simulated microstructures attached. It was realized that a single pass could only be confused with an austenitized process by two passes. The coarse grain heat-affected zone of a single pass could be divided into an unaltered coarse grain heat-affected zone (UACG-HAZ), a supercritically reheated coarse grain heat-affected zone (SCRCG-HAZ) and an intercritically reheated heat-affected zone (IRCG-HAZ). It is likely that there would be an intercritically reheated UACG-HAZ upon the UACG-HAZ. The microstructures in the CG-HAZs and the UACG-HAZ were mainly lath bainite and a little acicular ferrite; the microstructures in the SCRCG-HAZ were short lath bainite, granular bainite and acicular ferrite and the microstructures in the IRCG-HAZ were massive textures and secondary austenite decomposition products. The cooling times in the typical bainite transformation procedures were similar to one another in a secondary austenitized process and significantly longer than those in a single austenitized thermal cycle, which caused similar patterns in reheated CG-HAZs and an increase in acicular ferrite compared to CG-HAZs. The prior austenite grain sizes caused differences among the reheated CG-HAZs. [ABSTRACT FROM AUTHOR]

An 80%Ar-10%CO2-10%He ternary gas mixture was used as the shielding gas during the narrow-gap welding of thick Q690E high-strength steel plates. Complete and defect-free welded joints were obtained, and the microstructure and mechanical properties of the welded joint were investigated. The weld zone consists of a solidification area and interlayer zone, and the heat-affected zone consists of a coarse-grain heat-affected zone (CG-HAZ) and a fine-grain heat-affected zone (FG-HAZ). The microstructures of the weld zone are mainly lath bainite (LB), acicular ferrite (AF), and granular bainite (GB). The microstructure of the CG-HAZ is lath martensite (LM) and the microstructure of FG-HAZ is GB. Methods with different heat inputs were used to study their effects on the mechanical properties of the welded joint. It was found that the microstructure and mechanical properties of the welded joints are better with lower heat input. With tandem wire narrow-gap GMA welding, the tensile strength of the joints declined from 795.3 to 718.3 MPa and the impact toughness at −40 °C resulted in a weak position in the weld zone, which declined from 76~81 J to 55~69 J, when the welding speed reduced from 350 to 250 mm/min. With oscillating-arc narrow-gap GMA welding, the tensile strength achieved 853.4 MPa and the impact toughness at −40 °C was around 69~87 J. The results indicated that, under the appropriate heat input, the tensile strength of the joint exceeds 770 MPa and the low temperature impact toughness at −40 °C exceeds 69 J. A 155 mm-thick Q690E steel welded joint was obtained and the mechanical properties of the welded joint meets the requirements of the offshore drilling platforms. [ABSTRACT FROM AUTHOR]

Narrow gap TIG welding is a high efficiency and low-cost welding technique for heavy structures building. Due to the narrow groove's constriction, the TIG arc characteristics are different from butt welding. Understanding the unique arc characteristics of narrow gap TIG welding is the foundation for investigating the heat and mass transfer, metallurgic process, as well as process design. This research conducted numerical simulation on the TIG arc plasma in the narrow groove. The effects of welding current and arc length on the arc characteristics are investigated. Results show that, with the welding current increasing, the global velocity magnitude of plasma rises. The evolution of axial velocity and radial velocity has different responses to the current changes. The arc pressure increases drastically, and the global temperature of arc plasma also goes up. With the arc length increasing, global axial velocity rises, but the axial velocity and its gradients decrease near the anode surface. Centripetal radial velocity near the cathode increases, while centrifugal radial velocity rises at the outside of arc plasma and drops near the central axis. The maximum arc pressure on the anode surface decreases. At the lower part of the arc, arc temperature decreases near the central axis and increases at the outside of the arc. [ABSTRACT FROM AUTHOR]

For short-circuiting transfer, a smaller short-circuiting current is useful to obtain a better quality of welded joints, such as less spatter and better welding surface. By analyzing images from the high-speed camera, welding electrical signal, and current-voltage (U-I) cycle diagram, stable welding process could be obtained by adding pulsed ultrasonic into the arc. The ultrasonic radiator height, as one of the most important parameters, could affect the energy of acoustics field in pulsed ultrasonic-assisted gas metal arc welding. According to droplet transfer frequency, short-circuiting current and U–I cycle diagram, the ultrasonic radiator height was optimized and the optional ultrasonic radiator height was determined to be 12 mm. The sound radiation force was the main reason for the reducing short-circuit transfer cycle. [ABSTRACT FROM AUTHOR]

In this paper, the effect of laser on the welding process in metal inert gas (MIG) welding of aluminum alloys was studied exploiting high-speed imaging and welding electrical signals collecting. Bead-on-plate welding experiments were conducted using conventional short-circuiting transfer MIG welding and laser-short-circuiting transfer MIG hybrid welding. The obtained results point out that it took less time to strike the arc in laser-MIG hybrid welding, and the stability of arc striking was increased. When the laser was introduced into the arc atmosphere, the arc was compressed, and the welding energy was more concentrated. The arc cathode spot was fixed to the laser beam impinging spot, so the arc could burn stably. In laser-MIG hybrid welding, the droplet short-circuiting transfer stability and the weld appearance were improved significantly. Moreover, an analysis method of judging droplet transfer stability which bases on statistical techniques was proposed. By this method, it can be concluded that the short-circuiting transfer process became more stable in laser-MIG hybrid welding. [ABSTRACT FROM AUTHOR]

• Non-axial symmetric arc characteristics in narrow gap TIG welding were simulated and validated experimentally. • Both arc weaving and anode surface shape changes influence the temperature distribution and heat efficiency of TIG arc significantly. • Arc weaving makes the arc pressure deviate from Gaussian distribution while anode surface changes have limited effect. Narrow gap TIG welding is a high-quality solution for joining thick structures with reduced costs. However, the matching between numerous parameters is required to achieve satisfactory weld quality since its nature of multi-layer and single pass welding makes the repair difficult. This study aims to lay the theoretical foundation for the process design by building a deep understanding of the arc characteristics during narrow gap TIG welding. The 3D MHD simulation is carried out to investigate the influence of arc weaving and anode surface shape on characteristics of steady arc plasma, and the results are validated by experiments. Results show that as the electrode weaves from groove center to the side wall, local concentration of current density leads to the climbing of arc root on the side wall. The flow pattern of arc plasma changes from reverse circulation to inclined downward, results in the decrease of heat efficiency. As the shape of anode surface changes from concave downward to convex upward, the arc root shrinks but with a decrease of heat efficiency. [ABSTRACT FROM AUTHOR]

Swing-arc narrow gap GMA vertical-up welding is a high-efficiency technique. However, typical lack-of-fusion defect could still exist. We tried to clarify the forming mechanism of the typical defects. Experiments of EH40 grade steels under various processing parameters were carried out in this study. Processual phenomena of molten behavior and arc stability were described and analyzed by synchronously collecting and counting the high-speed graphic and electronic features of the welding procedure. Effects of traveling speed, swing amplitude (α), sidewall staying time (t s), and the peak pulsed voltage (U p) on the metal transfer, statistical characteristics of the max pulsed current (I p) and the wire tip position, and weld formation were investigated. The tendency and the forming mechanism of the lack-of-fusion defect were investigated. It is realized that a relatively fast traveling speed would increase the tendency of a convex bead and make the arc more divergent in the swinging period, which would increase the trend of interlayer lack-of-fusion defect. The change of any parameter among α , t s , and U p which makes the I p -value distribution much above the threshold globular-spray transfer I p -value range would lead to a deviation from the stable pattern of the wire-tip trajectory and a severe climbing on a certain sidewall. The sidewall lack-of-fusion defect usually occurs on the other side after the severe climbing on the previous side. The continuous arc premature climbing onto the sidewall during the swinging period causes the sidewall lack-of-fusion defect. [ABSTRACT FROM AUTHOR]