To address the issue of adsorption instability encountered by magnetic wheel adsorption welding robots during underwater operation, this paper proposed a critical adsorption force calculation method for magnetic wheels based on centroid offset and vector superposition. This method comprehensively considers multiple failure modes, including traditional sliding failure, detachment failure, overturning failure, and the rarely studied skidding failure, effectively addressing the issue of adsorption instability caused by low accuracy in traditional adsorption force calculations. Firstly, based on the robot chassis structure, static models corresponding to four non-instability adsorption states were established, and a vector superposition method was proposed based on static coupling relationships. This method fully accounts for the influence of centroid offset on adsorption stability during actuator motion, providing a theoretical basis for the accurate calculation of the critical adsorption force of magnetic wheels. Then, a case study was conducted based on the permanent magnetic adsorption chassis of the existing underwater welding robots. The static analysis results were solved using Matlab and the variation law of the critical adsorption force of the chassis with maximum centroid offset at different spatial angles was summarized. Finally, an experimental setup was constructed to test the adsorption stability of the robot under various operational conditions. The experimental results demonstrate that the vector superposition method based on centroid offset can effectively improve the adsorption stability of underwater welding robots, providing novel theoretical support for the design and magnetic force optimization of subsequent magnetic adsorption chassis.

As China’s nuclear industry enters the third 30 years of development, the maintenance need for nuclear equipment is becoming increasingly urgent. The internal structure of the nuclear steam generator is complex, and the key structure, namely the heat transfer tube, has the restrictions like small diameter, long pipeline and difficult disassembly and installation, which make traditional repair methods extremely difficult in implementation. In order to solve the problem of corrosion damage of small-diameter nuclear heat transfer tube due to long-term high temperature and high pressure condition, an all-position automatic TIG welding gun for liner repair was designed in this paper, and the gun reliability verification and welding repair test were carried out. Firstly, the overall structure design of the welding gun was presented, and the advantages of the designed welding gun comparing with the traditional tungsten electrode TIG welding gun were described. Next, the design and verification of the welding gun transmission system with small size and high space utilization were completed. Then, the stiffness of the key part of the welding gun, namely the conductive shaft, was modelled, analyzed and calculated, and the reliability of the theoretical model was verified by finite element solution. On this basis, an optimization method of the welding gun structure was proposed. Moreover, the field test of deflection and the welding test were carried out, finding that the stiffness of the conductive shaft satisfies the field welding conditions. Finally, linear regression equations and deflection correction formulas were used to quantitatively predict the deflection of the welding gun, and the results verify the rationality of the designed structure. Welding tests results show that the rotation speed of the transmission system is stable and controllable in working condition of welding gun, and the welding seam is well formed. The developed liner welding gun can satisfy the requirements of liner welding repair of the stainless-steel heat transfer tube inside the nuclear steam generator.

The trade-off between strength and ductility has long been a challenge for Mg alloy. To address this issue, bimodal-structured AZ80 Mg alloys with varying heterogeneity levels were fabricated via low-temperature extrusion in this work. The results reveal the microstructure comprising second-phase particle (SP_p, β-Mg₁₇Al₁₂ and Mg₃Mn₂Al₁₈)-reinforced fine grains (FGs) FGs and SP_p-free coarse grains (CGs), with the heterogeneity level decreasing as extrusion temperature increases. As the heterogeneity level decreases, the synergistic deformation capacity initially improves, reaching a maximum at the moderate heterogeneity level of 0.31 GPa and 0.238, and then declines. This exceptional capacity is attributed to the hetero-deformation induced (HDI) stress, which effectively alleviates the strain gradients by activating 〈c + a〉 dislocations and non-basal 〈a〉 dislocations during deformation. An optimal combination of 287 MPa in yield strength, 393 MPa in ultimate tensile strength, and 14.96% in elongation is achieved in the alloy with a moderate heterogeneity level. The excellent strength-ductility synergy originates from the enhanced capacity of dislocations accumulation driven by remarkable capacity of synergistic deformation and the synergistic strengthening mechanisms. This work provides a new insight into the design of bimodal structure to produce high-performance Mg alloys.