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Structural-adhesive-assisted DeltaSpot welding was used to improve the weldability and mechanical properties of dissimilar joints between 6061 aluminum alloy and galvannealed HSLA steel. Evaluation of the spot-weld-bonded surfaces from lap shear tests after long-term exposure to chloride and a humid atmosphere (5% NaCl, 35 °C) indicated that the long-term mechanical reliability of the dissimilar weld in a corrosive environment depends strongly on the adhesive–Al6061 alloy bond strength. Corrosive electrolyte infiltrated the epoxy-based adhesive/Al alloy interface, disrupting the chemical interactions and decreasing the adhesion via anodic undercutting of the Al alloy. Due to localized electrochemical galvanic reactions, the surrounding nugget matrix suffered accelerated anodic dissolution, resulting in an Al6061-T6 alloy plate with degraded adhesive strength and mechanical properties. KrF excimer laser irradiation of the Al alloy before adhesive bonding removed the weakly bonded native oxidic overlayers and altered the substrate topography. This afforded a low electrolyte permeability and prevented adhesive delamination, thereby enhancing the long-term stability of the chemical interactions between the adhesive and Al alloy substrate. The results demonstrate the application of excimer laser irradiation as a simple and environmentally friendly processing technology for robust adhesion and reliable bonding between 6061 aluminum alloy and galvannealed steel.

Welded structures, such as offshore platforms, require robust toughness in their heat-affected zones (HAZ) to withstand low-temperature environments. The coarse-grained HAZ (CGHAZ) adjacent to the fusion boundary often exhibits reduced toughness due to grain coarsening, particularly under high heat input welding conditions aimed at enhancing productivity. To address this, high-nitrogen steels containing TiN particles were developed to suppress austenite grain growth by leveraging the thermal stability of TiN precipitates. Three high-nitrogen steels with varying carbon contents (0.09%, 0.11%, and 0.15%) were fabricated and subjected to crack tip opening displacement (CTOD) testing at −20 °C and −40 °C to evaluate low-temperature HAZ toughness. Results indicate that high-nitrogen TiN steels exhibit superior CTOD values (1.38–2.73 mm) compared to conventional 490-MPa class steels, with no significant reduction in toughness despite increased carbon content. This is attributed to the presence of stable TiN particles, which restrict austenite grain growth during welding thermal cycles, and the formation of fine ferrite–pearlite microstructures in the HAZ. These findings highlight the efficacy of high-nitrogen TiN steels in enhancing low-temperature fracture resistance for welded structures.

In order to reduce the weight of parts in the automobile and electronic industries, various research on dissimilar welding techniques of aluminum and steel is being carried out. Since dissimilar materials have different physical and electrochemical characteristics, joining through conventional fusion welding is challenging, and there is a high probability of a decrease in strength of the welded joints. To solve this problem, a mechanical fastening method is mainly applied to join dissimilar parts with different material properties, but this process has disadvantages in terms of productivity improvement and cost reduction because additional consumables, such as rivets, are required. In this research, we investigated the optimization of the weld bonding conditions of joints using epoxy-based adhesive bonding and DeltaSpot welding for Al/Fe dissimilar materials. For each experimental condition, the corrosion resistance and tensile shear strength of the welded joints were evaluated according to salt spray test times of 0 h, 640 h, 1280 h, and 1920 h. As a whole, as the salt spray test time increased, the tensile shear strength of the welded joints decreased. It was confirmed that weld bonding, after manual polishing of the aluminum side, resulted in the highest average tensile shear strength of 5.88 kN at 1920 h, which was an increase compared with other conditions.

Steel processing requires energy-efficient heat-treatment routes without compromising material performance. Traditional annealing furnaces used for low-carbon (LC) steels are energy-intensive and major contributors to CO₂ emissions, creating a need for sustainable alternatives. This study evaluates continuous electric furnace (CEF) annealing as a low-emission route to tailor the microstructure, texture, and mechanical properties of cold-rolled LC steel. Samples were annealed at 750 °C and 850 °C for 60 s, followed by comprehensive microstructural and crystallographic characterization using XRD, SEM, EBSD (IPF, GOS, KAM, ODF), hardness, and tensile testing. Annealing increased recrystallization from ~4% in the as-rolled condition to ~98% at 850 °C, reduced the mean KAM from 1.9° to 0.1°, enhanced the high-angle grain boundary fraction to 0.91, and promoted γ-fiber strengthening while suppressing detrimental θ-fiber components. The 850 °C condition achieved optimal mechanical performance (UTS × TE = 11.1 GPa%). These results demonstrate that CEF annealing enables sustainable processing with better mechanical performance in LC steels.

In the automobile industry, studies have been made on spot welds of non-ferrous metals such as aluminum, which has lightweight, high-strength, and excellent impact properties, in order to improve fuel efficiency through lightening of vehicle body weight. However, these dissimilar welds cause intermetallic compound formation and the galvanic corrosion in corrosion environments at the weld interface. On the other hand, it is known that the weld bonding process has the property of improving strength of spot welds by applying a structural adhesive between two metals and increasing the surface adhesive force. In this study, Al 6061-T6 with a thickness of 1mm and galvanized steel sheet (GA) with a grade of 440 MPa were used as target materials. As a process method, delta spot welding equipment was used for reducing the heat input using a special process tape and the weld bonding process by the D type structural adhesive was done. As a result, tensile shear strength after salt spray test for 1920 hours was more than twice that of spot welding condition and it was confirmed that the corrosion amount of the welds was lowered.