Welding and Joining of Metallic Materials: Microstructure and Mechanical Properties

The study of welding and joining technologies for metallic materials has long been fundamental to advancing numerous industries, including aerospace, automotive, and energy [1,2]. These techniques not only enable the fabrication of complex components but also serve as a critical factor in the mechanical performance and reliability of structures [3,4]. A thorough understanding of the microstructure and mechanical properties of welded joints is essential for improving joint strength, durability [5], and overall material performance, particularly in demanding environments [6,7].

In recent years, technological advancements in welding processes [8] and the development of new materials [9] have prompted an increasing need for in-depth research to optimize these techniques [10,11]. In this context, the Special Issue “Welding and Joining of Metallic Materials: Microstructure and Mechanical Properties” brings together a collection of ten innovative research articles. These contributions focus on elucidating the relationships between welding parameters, the resulting microstructures, and the mechanical properties of both similar and dissimilar material joints.

This Special Issue aims to bridge the gap between fundamental scientific research and practical industrial applications, addressing the challenges posed by modern welding processes. By exploring advanced welding techniques, investigating the microstructure of various alloys, and evaluating the performance of welded joints under different conditions, the published articles provide valuable insights into how these processes can be refined and adapted for enhanced performance.

The ten contributions can be broadly categorized into three key research themes. This Editorial discusses their scientific significance, offering readers a cohesive overview of the advancements in welding and joining technologies presented in this Special Issue.

• Process optimization and advanced welding techniques

Three papers explore the optimization of welding processes and the development of advanced techniques to enhance weld quality, energy efficiency, and mechanical properties.

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Contribution (1): Effect of Laser Heat Input on the Microstructures and Low-Cycle Fatigue Properties of Ti60 Laser Welded Joints [12]. This study investigates the effects of varying laser heat input on the microstructure and mechanical properties of Ti60 titanium alloy welded joints. The results reveal that different heat inputs significantly affect the morphology of the weld zone (WZ), reducing porosity in Y-type WZs, which enhances low-cycle fatigue resistance. The research contributes valuable insights into optimizing laser welding parameters for improved weld quality.

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Contribution (2): Effect of Exothermic Additions in Core Filler on Arc Stability and Microstructure during Self-Shielded Flux-Cored Arc Welding (FCAW) [13]. This paper explores the use of exothermic additives in flux-cored arc welding to enhance arc stability and deposition efficiency. By optimizing the CuO/Al and CuO/C ratios in the core filler, this study demonstrates how filler composition influences arc stability and welding current, providing a route to more energy-efficient welding processes. The mathematical models developed in this work further enhance the prediction of welding parameters.

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Contribution (3): Particle Swarm Method for Optimization of ATIG Welding Process to Join Mild Steel to 316L Stainless Steel [14]. This article presents the optimization of activating tungsten inert gas (ATIG) welding using particle swarm optimization (PSO) to join mild steel to 316L stainless steel. The findings show that the optimized flux composition significantly improves weld penetration and hardness, without compromising the mechanical properties of the joint. This study offers a promising solution for improving the quality of dissimilar material welds.

2.

Microstructure and mechanical properties of welded joints

Four contributions focus on the intricate relationship between welding-induced microstructural changes and their effects on the mechanical properties of welded joints, shedding light on material behavior under different conditions.

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Contribution (4): Microstructural Optimization of Sn-58Bi Low-Temperature Solder Fabricated by Intense Pulsed Light (IPL) Irradiation [15]. This paper introduces IPL soldering as a novel method for optimizing the microstructure of Sn-58Bi solder. By regulating the irradiation time, this study demonstrates how the solder microstructure evolves from immature to refined, impacting mechanical properties such as hardness. This technique offers potential for improving low-temperature soldering processes in electronics manufacturing.

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Contribution (5): Experimental and Computational Study of Microhardness Evolution in the HAZ for Al–Cu–Li Alloys [16]. This paper focuses on simulating the microhardness evolution in the heat-affected zone (HAZ) of Al-Cu-Li alloy welds. By replicating the thermal cycles experienced during welding, the research explores the dissolution and coarsening of strengthening precipitates in the HAZ, contributing to a deeper understanding of how welding affects the mechanical properties of advanced aluminum alloys.

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Contribution (6): Heat-Affected Zone Microstructural Study via Coupled Numerical/Physical Simulation in Welded Superduplex Stainless Steels [17]. This work introduces a hybrid approach, combining physical and numerical simulations to investigate the HAZ of super-duplex stainless steel (SDSS). The authors successfully simulate thermal cycles using a Gleeble^® machine and compare the results with actual welds, providing valuable insights into how thermal management during welding can preserve the microstructural balance between ferrite and austenite, essential for maintaining corrosion resistance and mechanical integrity.

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Contribution (7): Structure–Property Correlation between Friction-Welded Work-Hardenable Al-4.9Mg Alloy Joints [18]. The research evaluates the microstructure and mechanical properties of AA5083 H116 joints produced by rotary friction welding. This study reveals that grain refinement in the dynamically recrystallized zone (DRZ) enhances joint strength, although slight reductions in ductility are observed. These findings are particularly relevant for applications in aerospace and automotive sectors where high-strength aluminum alloys are frequently used.

3.

Welding of dissimilar materials

Three papers address the complexities and challenges involved in joining dissimilar materials, focusing on optimizing welding parameters and techniques to produce strong and high-performance joints.

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Contribution (8): Study of the Microstructure and Mechanical Property Relationships of Gas Metal Arc Welded Dissimilar Protection 600T, DP450, and S275JR Steel Joints [19]. This paper addresses the challenges of welding dissimilar steels through gas metal arc welding (GMAW). The findings reveal that the mechanical properties of the joints, such as tensile strength and hardness, can be optimized by selecting appropriate welding parameters. In particular, DP450/S275JR dissimilar joints exhibit the highest joint efficiency, making this technique highly applicable to industrial processes.

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Contribution (9): Effect of Microchemistry Elements in Relation to Laser Welding Parameters on the Morphology of 304 Stainless Steel Welds Using Response Surface Methodology [20]. This study investigates the effect of sulfur content on the weld bead morphology of AISI 304 stainless steel by using laser welding. The response surface methodology (RSM) analysis reveals how small variations in sulfur content and welding parameters, such as power and focus point, can significantly influence weld pool formation and mechanical performance.

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Contribution (10): Investigation of the Microstructure and Mechanical Properties in Friction Stir Welded Dissimilar Aluminium Alloy Joints via Sampling Direction [21]. This paper investigates the microstructure and mechanical properties of dissimilar aluminum alloys joined by friction stir welding (FSW), with an emphasis on the effect of sampling direction. This work shows that the longitudinal joints exhibit higher tensile strength and elongation compared to the transverse joints, providing critical insights into the optimization of FSW for dissimilar aluminum alloys.

In essence, this Special Issue on Welding and Joining of Metallic Materials: Microstructure and Mechanical Properties brings together cutting-edge research that spans advanced welding techniques, microstructural analysis, and the challenges of welding dissimilar materials.

This Special Issue reflects the incredible effort and dedication of the authors who shared their valuable findings. We are deeply grateful to each contributor for their insights, and to the peer reviewers for their hard work in upholding the quality of these papers. A big thank you also goes to everyone else involved in making this possible.

The contributions to this Special Issue are listed as follows:

• Contribution (1)—Zhang, Q.; Ren, L.; Lei, X.; Yang, J.; Zhang, K.; Zhang, J. Effect of Laser Heat Input on the Microstructures and Low-Cycle Fatigue Properties of Ti60 Laser Welded Joints. Crystals 2024, 14, 677.
• Contribution (2)—Lozynskyi, V.; Trembach, B.; Katinas, E.; Sadovyi, K.; Krbata, M.; Balenko, O.; Krasnoshapka, I.; Rebrova, O.; Knyazev, S.; Kabatskyi, O.; et al. Effect of Exothermic Additions in Core Filler on Arc Stability and Microstructure during Self-Shielded, Flux-Cored Arc Welding. Crystals 2024, 14, 335.
• Contribution (3)—Touileb, K.; Djoudjou, R.; Ouis, A.; Hedhibi, A.C.; Boubaker, S.; Ahmed, M.M.Z. Particle Swarm Method for Optimization of ATIG Welding Process to Joint Mild Steel to 316L Stainless Steel. Crystals 2023, 13, 1377.
• Contribution (4)—Go, H.; Noh, T.; Jung, S.-B.; Sohn, Y. Microstructural Optimization of Sn-58Bi Low-Temperature Solder Fabricated by Intense Pulsed Light (IPL) Irradiation. Crystals 2024, 14, 465. https://doi.org/10.3390/cryst14050465.
• Contribution (5)—Maritsa, S.; Deligiannis, S.; Tsakiridis, P.E.; Zervaki, A.D. Experimental and Computational Study of Microhardness Evolution in the HAZ for Al–Cu–Li Alloys. Crystals 2024, 14, 246.
• Contribution (6)—da Silva, L.O.P.; Lima, T.N.; Júnior, F.M.d.S.; Callegari, B.; Folle, L.F.; Coelho, R.S. Heat-Affected Zone Microstructural Study via Coupled Numerical/Physical Simulation in Welded Superduplex Stainless Steels. Crystals 2024, 14, 204.
• Contribution (7)—Mahajan, A.M.; Krishna, K.V.; Quamar, M.J.; Rehman, A.U.; Bandi, B.; Babu, N.K. Structure–Property Correlation between Friction-Welded Work and Hardened Al-4.9Mg Alloy Joints. Crystals 2023, 13, 1119.
• Contribution (8)—Elmas, M.; Koçar, O.; Anaç, N. Study of the Microstructure and Mechanical Property Relationships of Gas Metal Arc Welded Dissimilar Protection 600T, DP450 and S275JR Steel Joints. Crystals 2024, 14, 477.
• Contribution (9)—Touileb, K.; Attia, E.; Djoudjou, R.; Hedhibi, A.C.; Benselama, A.; Ibrahim, A.; Ahmed, M.M.Z. Effect of Microchemistry Elements in Relation of Laser Welding Parameters on the Morphology 304 Stainless Steel Welds Using Response Surface Methodology. Crystals 2023, 13, 1138.
• Contribution (10)—Mabuwa, S.; Msomi, V. Investigation of the Microstructure and Mechanical Properties in Friction Stir Welded Dissimilar Aluminium Alloy Joints via Sampling Direction. Crystals 2023, 13, 1108.

As welding technologies evolve and new materials are introduced, the interplay between microstructure and mechanical properties will continue to be a focal point of research. We believe this collection of articles offers a clear snapshot of the latest breakthroughs and makes a significant contribution to advancing our knowledge in this vital area. We hope the findings presented in this reprint will inspire further innovations in the field of welding and joining.