Wire and arc additive manufacturing (WAAM) produces thin-walled parts superior to other additive manufacturing methods, because of its high efficiency, good compactability, and low cost. However, the WAAM accuracy is limited by its large heat input. Here, 0.8 mm 316L stainless steel welding wire is deposited via speed cold welding to form 30-layered thin-walled samples, with 2 mm thickness, and up to 65 mm height. The effects of three process parameters (the bottom current mode, scanning speed, and cooling time) on the deposition process stability, macro morphology, structure, and mechanical properties are studied. In the experiment, the probability density curves of electrical parameters of sample #GRBC-30 cm/min-10 s on the third and tenth layers were narrower than other samples, which implied a more stable process. The three process parameters mainly affect the deposition morphology and have a minor performance effect. The hardness and tensile properties mainly depend on the deposition direction. Gradual, layer-by-layer current reduction improves the bottom molding and performance, and the deposition efficiency, and stabilizes the process. Scanning speed enhancement or cooling time reduction destabilizes the end formation, reduces the effective deposition rate, and slightly degrades the performance. All deposited samples are distinctly anisotropic, but satisfy the industrial standard. Overall, deposition in speed cold welding mode, with 10 s cooling time, 30 cm/min scanning speed, and gradually reduced bottom current exhibits good stability, and the molding efficiency and mechanical properties are optimal.

Three thermal frequencies that were designated as 3, 4, and 5 Hz were used to reveal the microstructural evolution, mechanical, and corrosion properties of AA6061-T6 sheets that were welded by double pulsed gas metal arc welding (DP-GMAW) using microstructural observations, microhardness tests, tensile tests, bend tests, impact tests, and corrosion tests. A self-developed welding system consisted of a digital multifunctional welder and an arc dynamic waveform wavelet analyzer was chosen to perform welding operations and to record electrical signal waveforms during the welding process. The results show that the process of DP-GMAW was stable, and the weld beads exhibited aesthetic ripples. The ripple distance, the grain size, and porosity of the fusion zone decreased with an increasing thermal frequency. Microhardness and corrosion properties of the weld joint improved when the thermal frequency increased. In addition, all of the joints exhibited excellent bending properties. However, the impact property of the weld joint was lower than that of the BM. It indicated that the low impact energy results of the welded AA6061-T6 sheet were mainly due to the formation of micro-cracks in the fusion zone.

For diminishing welding defects such as incomplete penetration, which may easily occur during the twin wire Double Pulsed Metal Inert Gas (DP-MIG) high-speed welding, a novel method using additional shielding gas is introduced in this paper. A branch for the additional shielding gas was specially set near the back end of the protection hood for the DP-MIG nozzle. The constructed gas branch was used for enabling manual intervention in the formation of a high-temperature solid–liquid weld seam just emerging from the nozzle and also for secondary gas protection on the surface of the weld seam. The butt welding test was carried out in the 2205 duplex stainless steel plate and the weld seam was then characterized by a tensile test, metallographic analysis, X-ray non-destructive testing (NDT), hardness analysis, and impact test. The results showed that the introduction of an appropriate amount of additional shielding gas can effectively improve and diminish the unfused weld seam and also improve the mechanical properties such as the tensile properties of the weld joint, the hardness and toughness of the weld joints. Therefore, the introduction of additional shielding gas has further research potential in theory and process practice.

To avoid humping bead defects in high-speed welding, this paper proposes the method of an additional and compensatory gas jet blow molten pool. A pulsed metal inert gas high-speed welding test platform was constructed for compensatory gas jet blow molten pool. A total of 304 stainless steel sheets were used as the welding workpieces under equal heat inputs. Two high-speed butt welding processes were conducted and compared, in which the workpieces were welded with and without compensatory gas jets at 154 cm/min and 167 cm/min, respectively. After high-speed welding with compensatory gas jet blow, the weld appearance was straight, uniform, and high-quality, with no humping bead or undercut defects. The macroscopic morphologies and microstructures of cross-sections of the weld at the toe, near the surface, the middle, and the bottom portion all showed the stirring effect of the gas jet on the molten pool and improved grain refinement degrees. Hardness was enhanced in the weld center and the heat-affected zone. At welding speeds of 154 cm/min and 167 cm/min, the fracture load capacities of the welds were increased by 24.9 and 10.4%, respectively.

In order to reveal the mechanism and influencing factors of high pressure jet cavitation of nozzle in submerged environment, this study focused on the evolutionary process of cavitation bubbles and combined finite volume method and mixed multi-phase flow model to analyze the cavitation, velocity distribution and experimental cavitation intensity of fishing net cleaning equipment. Results show that the cavitation inception, growth and collapse mainly occurred in the peripheral region of the flow field. Ring-shaped cavitation erosion zone appeared on the test sample target. A lot of small dense erosion pits were densely distributed in the ring-shaped erosion zone, erosion marks were observed in the center. The cavitation erosion intensity was greatly affected by the nozzle structure. As the diameter of nozzle increased from 0.6 mm to 1 mm, the maximum gas volume fraction increased by 8.53%. The nozzle outlet enlargement angle greatly increased the cavitation intensity. The nozzle with an outlet angle of 30° exhibited the optimal cavitation erosion performance . The cavitation volume fraction of the nozzle with short necking structure was slightly larger than that of the nozzle with long necking structure at the same level in the necking length rang of 3mm to 7mm. In terms of the influence of nozzle structure on the cavitation erosion effect, the nozzle diameter D ranked the first, followed by the outlet angle α, and the necking length L was at the last.