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The paper evaluates the performance of alternating current (AC) square waveform submerged arc welding (SAW) as a candidate technology for manufacturing thick welds for high-pressure vessels. A new mathematical formulation for calculating melting efficiency in square waveform arc welding is presented. The melting efficiency and the heat consumption are presented as a mathematical model of welding parameters, namely welding current, welding speed, current frequency, and electrode negativity (EN) ratio. The proposed approach is demonstrated through the welding of 2.25Cr-1Mo heat-resistant steel performed over a wide range of welding parameters. The investigation provides deeper insights into the interplay between process parameter, total heat consumption, and melting efficiency. The effect on flux consumption is also explained. The melting efficiency is inversely proportional to flux consumption. The welding heat does not necessarily promote the plate melting. Improper use of welding heat may lead to decreased melting efficiency and increased unwanted melting and consumption of welding flux. Compared to the conventional direct current (DC) power sources, the AC square waveform welding achieves almost the same order of melting efficiency with added advantages of better weld bead shape and flux consumption in a desirable range. The two additional parameters (frequency and EN ratio) of the AC square waveform power source provide more freedom to fine-tune the process and thereby efficiently use welding heat. The results of this investigation will be advantageous to the designers and fabricators of high-pressure vessels using AC square waveform welding.

Since Tungsten Inert Gas (TIG) arc enables to produce arc plasma with high energy density, it is suitable as a heat source especially for processes to require concentrating the heat input at a point. On the other hand, Tube Cathode Arc (TCA) to be a kind of TIG produces the arc plasma by introducing shielding gas through the central hole of the tube cathode. In the present paper, basic heat source property of argon TCA at the atmospheric pressure was numerically analyzed and compared with that for the conventional TIG arc. As a result, it was found that TCA is suitable for processes such as brazing or buildup, since it enables to heat a target material uniformly by controlling inner shielding gas flow rate. Furthermore, TCA is expected to realize high-speed welding because the arc pressure to a base metal reaches only 30% of TIG due to lower current density.

Laser welding with oscillation laser beams enables control of the heat input distribution. In this study, we attempted to develop a narrow-gap welding process with oscillation laser welding. This process is expected to prevent a lack of fusion because the entire bottom to the groove can be melted by the oscillation laser. As the first step of the study, bead-on-plate welding experiments with an oscillation laser beam were performed to investigate the relationship between the welding conditions and welding results. The experiments revealed that the oscillation conditions strongly affect the welding penetration shape. It was clarified that the wire melting phenomena in oscillation laser welding differed from those of straight (nonoscillation) laser welding. Additionally, narrow-gap laser welding experiments were performed to investigate the relationship between the oscillation conditions and gap width. The results confirmed the effectiveness of oscillation laser welding for narrow-gap welding.

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The complexity in weld profile caused by abrupt change in polarity in square waveform welding is investigated through the development of a model capable to accurately predict weld profile. A semi-analytical model is conceived wherein characteristic attributes of a composite parabolic–elliptic function, which represent the weld profile, are obtained through nonlinear regression (NLR). The proposed model is demonstrated for its efficacy in the prediction of weld profile over a wide range of welding parameters, vis-à-vis, welding current, frequency, electrode negative (EN) ratio, and welding velocity. The investigation suggests that the center and outer cores of welding arc remains more active during positive and negative polarity, respectively, that leads to distinct macroscopic zones in weld cross section and thus, necessitates a composite profile for representation of weld profile. The intersection of the zones forms a metallurgical notch which the investigation offers a method to estimate and thus control. Unlike the convention continuous arc welding, the waveform arc welding caters welding at higher velocity without compromising the weld penetration and almost abolishing the metallurgical notch as well.