To improve the stability and controllability of narrow-gap welding (NGW) and systematically elucidate the mechanism by which pre-embedded cold wire modulates arc behavior and metal transfer, this study investigated the arc characteristics and metal transfer behaviors in thick-wire narrow-gap gas metal arc welding (GMAW) with a swing arc and pre-embedded cold wire. Two types of thick wires, single-strand welding wire (SWW) and cable-type welding wire (CWW), were compared under different numbers of pre-embedded cold-wire layers (n = 0, 1, 2). The results indicate that during arc swinging, the average arc spreading angle at the sidewalls is consistently smaller than that at the groove center for both wire types. For SWW, the sidewall arc spreading angle decreased by 13.1% and 18.4% with n = 0 and n = 2, respectively, compared with the groove center; for CWW, the reductions were 24.6% and 20.2%. Moreover, pre-embedded cold wire significantly increased the arc spreading angle (by up to 15.8% for SWW at the groove center) and induced deflection of arc cathode spots toward the cold wire, thereby improving heat distribution on the sidewalls and promoting sidewall penetration. In terms of droplet transfer, SWW exhibited stable spray transfer at the sidewalls and globular spray transfer at the groove center, while CWW showed rotating spray transfer at the sidewalls and a unique hanging droplet transfer mode at the groove center. The average droplet diameter of CWW at the groove center is 17% larger than that of SWW. Pre-embedded cold wire also significantly improved arc stability for both wire types, as indicated by a more concentrated voltage–current distribution, with CWW showing superior stability in the absence of cold wire, whereas no clear regularity was observed in its effect on droplet transfer. The findings suggest that pre-embedded cold wire is beneficial for applications requiring reliable sidewall fusion, as it improves heat distribution while maintaining droplet transfer stability. Further optimization of cold-wire layer number and positioning is recommended for enhanced weld quality.

To solve the problems of poor weld formation, difficult slag removal, and inferior joint microstructure and hardness in conventional narrow-gap submerged arc welding (NG-SAW), a swing arc NG-SAW process assisted by pre-embedding cold wires was proposed. Synergistically optimizing the welding energy parameters and additional cold wires ensured sound weld formation and enhanced slag detachability, while the efficiency of multilayer welding was improved by reducing the number of weld layers by 33.3%. The slag adhesion mechanism is clarified as follows: a high welding heat input facilitates elemental diffusion at the weld–slag interface, leading to the formation of a continuous and thick interlayer composed of (Fe,Mn)O and MgO-Al₂O₃-CaO phases. This interlayer strengthens the chemical bonding between slag and weld, thereby impeding slag removal. Microstructure evolution analysis of the multilayer welded joint revealed that the variable-angle design increases the groove volume and, combined with the heat-absorbing effect of the additional wires, accelerates molten pool cooling, thereby refining grains in both the weld metal zone and reheat-affected zone. Meanwhile, the tempering exerted by the heat-affected zone (HAZ) of the subsequent weld layer on the previous layer is attenuated. This promotes the gradual transformation of hard-brittle lath martensite in the coarse-grained heat-affected zone (CGHAZ) of the bottom layer into tougher tempered martensite/bainite in the CGHAZ of the upper layers. As a result, the hardness uniformity within the HAZ, the critical weak region of the joint, was enhanced by 54%, enabling synchronous improvement in microstructural homogeneity, hardness distribution, and overall welding efficiency.