This paper is the third of a three-part series covering the field of high-speed videography in welding in depth. This third part provides a comprehensive discussion of natural radiation lighting. The effect of optical filters is discussed in detail, with the beneficial use of long-wavelength edge filters around the near-infrared range. Building on Part 1, detailed descriptions of appropriate setups are provided with examples of published practical implementations. Welding processes addressed include gas metal arc welding, gas tungsten arc welding, flux-cored arc welding, submerged arc welding, electroslag welding, and laser cladding. Advantages and disadvantages are explored thoroughly. Natural radiation lighting is especially useful because of the simplicity of the equipment involved, information about the morphology and temperature of the surface of the molten metal, and information about the arc, such as the presence of metal vapors. Modern digital cameras typically have the necessary high dynamic range that enables the use of this technique. The implementations of natural radiation lighting reviewed in this paper, together with those for front and back lighting in Part 2, and the fundamental concepts and resulting quantitative guidelines in Part 1 provide welding researchers with a previously inexistent compilation of criteria to select proper equipment, accessories, and parameters for high-speed imaging of a vast variety of phenomena in welding, laser welding, and associated processes such as additive manufacturing or cutting.

AA7075, widely used in aviation structures due to its high strength and corrosion resistance, has emerged as a promising lightweight material for automotive body applications. This study investigates the resistance spot welding (RSW) process of AA7075. Although RSW offers cost reduction and efficiency advantages, welding AA7075 presents challenges such as high thermal cracking sensitivity and severe expulsion. In Part 1, the MPMA-RSW process was introduced, which combines multipulse (MP) and magnetic-assisted (MA) techniques to address these challenges. The microstructure of the welds was systematically characterized and analyzed in detail. Part 2 focuses on analyzing the mechanical properties and expulsion behavior of AA7075 RSW joints, including tensile-shear strength and fracture morphology. By evaluating the effects of expulsion on weld quality and mechanical performance, this research aims to provide insights for optimizing welding parameters to produce defect-free, high-quality aluminum joints. Additionally, it contributes to advancing AA7075 applications in critical industries by improving the weldability and mechanical properties of RSW joints.

The ductile-brittle transition temperature (DBTT) plays a critical role in determining the integrity of welded structures under common low-temperature service conditions. Fluxes are proven effective agents that significantly alter weld compositions and resulting microstructures. In this study, we systematically investigated the impact of programmed SiO2-containing fluxes upon EH36 shipbuilding steel welds via multi-scale microstructural characterization, instrumented Charpy impact testing, and fractographic analysis. We found that incremental SiO2 addition from 5 to 40 mass-% drastically enhanced the volume fraction of acicular ferrite from 0.386 to 0.747. As the SiO2 content increased, the DBTT decreased significantly from –58°C to –97°C but slightly rose to –85°C. Fractographic and secondary-crack analyses indicated that cracks preferentially propagated along grain boundary ferrite and ferrite side plates, whereas crack paths through acicular ferrite were more tortuous. We demonstrated that enhanced toughness is primarily governed by two complementary mechanisms: (1) microstructural refinement through enhanced acicular ferrite fraction, impeding microcrack propagation; (2) minimized presence of large inclusions within coarse ferrite grains, mitigating the risk of premature cleavage fracture initiation. Such findings highlight that superior low-temperature toughness necessitates balanced grain refinement and precise control over inclusions. Overall, we showcased a cost-effective strategy geared toward achieving optimized weld microstructures and mechanical performances merely by adjusting welding fluxes, which could also shed light on the development of high-performance welding consumables for alternative low-temperature structural steel.

Notched-bar stress relaxation tests were conducted on samples machined from the heat-affected zones of 347H and 316H V-groove weldments fabricated with 16-8-2 filler metal. Tests were performed between 500–650°C with two different notch radii to compare the performance of the alloys. Samples were loaded to target stress, held in stroke control for 500 h and then reloaded to the target stress for additional cycles until failure or the test was ended. Post-test, the samples were characterized by light optical and scanning electron microscopy. Results showed that this test method is a repeatable and sensitive method to produce intergranular cracking in austenitic stainless steels. Both alloys showed resistance to cracking at 500°C and susceptibility at 550°C but diverged in behavior at 600°C, where the 347H weldments showed increased susceptibility and the 316H weldments showed increased resistance. Failure times and temperatures in 347H correlated with the formation of intragranular Nb(C,N), and analysis indicated that precipitation-induced strains promoted cracking in 347H. The ability to control the stress state, creep damage and microstructure tested and produce intergranular cracking at temperatures prototypical of operation are key advantages of this test method, which should be adopted as a standard to facilitate mechanistic understanding and develop mitigation strategies for reheat-type cracking.

The influence of hatch spacing on tensile properties of high deposition rate gas metal arc-directed energy deposition of 316L and 316LSi was explored

Resistance rivet welding (RRW) is a method developed in recent years for the joining of dissimilar materials. In this study, RRW with solid rivets was used to join 6016 aluminum alloy and CR1180T ultrahigh strength steel. The nugget formation process was investigated in terms of dynamic resistance, microstructure analysis, and microhardness measurement. The mechanical properties and failure mode of RRW joints were also investigated. The results showed that the Al under the rivet leg gradually melted from the middle to both sides during the piercing stage. Molten Al was squeezed into the space under the rivet cap. A mushroom-shaped weld nugget was first formed inside the steel sheet and gradually grew toward the rivet. The microstructure of the weld nugget was coarse martensite. A layer of uniform intermetallic compounds was formed between the rivet leg and the Al sheet. Three failure modes — interface failure, pullout failure (PF), and Al sheet failure —were identified. The strain distribution of the joints with three failure modes in the tensile process was compared. Under the optimal parameters, the failure mode of the RRW joint was PF, and the average maximum tensile-shear force of the joints reached 11.4 kN.

In this study, the influence of focused ultrasonic vibration in the arc column with a nonconsumable electrode on the electrophysical characteristics of the arc, near-electrode potential drops, and the shape and structure of the arc column discharge was investigated. It was determined that a focused ultrasonic field in the electrode region of the welding arc leads to an increase in the arc discharge voltage. The impact of a focused ultrasonic field reduces the arc diameter in the anode region and the column. In contrast, near the cathode region, the diameter increases, indicating a change in the nature of the cathodic flow of charged particles in this area.

Flux viscosity affects weldability and alloying element transfer behaviors during submerged arc welding of grades of thick shipbuilding steel. It is widely accepted that viscosity is intrinsic to F-related bond structures. However, a validated relationship of the interplay remains at large. This investigation demonstrates the structureviscosity correlation by employing experimental measurements coupled with molecular dynamics calculations. The results showed that the maximum viscosity at 1500°C decreased from 4.53 to 0.375 Pa·s as the CaF2 content increased from 20 to 40 wt-%. Such a reduction could be partly attributed to the structures’ dilution effect and the highly polymerized units’ inhibition. Furthermore, CaF2 additions introduced Ca2+ and F– with high migration rates and promoted void formations among polyhedra, thereby significantly enhancing the overall diffusion coefficients from 0.25 to 2.47 Å2/ns. It was also found that F– tends to be incorporated into [AlOn ] polyhedra, with Ca2+ providing prominent structural compensation in Al2 O3-rich environments. Moreover, CaFn clusters were more favored than MgFn clusters across all involved compositions. The findings corroborated the relationship between structural features and fluoride content. Such insights could enrich our understanding of CaF2-induced viscosity behaviors and offer viable strategies for designing welding fluxes from an intrinsic structural perspective.

The effect of martensite-austenite (MA) constituent morphology, prior austenite grain size, fracture length, and notch placement on the Charpy toughness and micro-hardness in the heat-affected zone (HAZ) of heavy-gauge (19.1 mm thick) X70 microalloyed steel welded by cold-wire tandem submerged-arc welding (CWTSAW) was studied. In Part I, a series of single-pass CWTSAW samples were made using cold wire feed rates (effective heat input) of 0 mm/s (2.9 kJ/mm), 16.9 mm/s (2.6 kJ/mm), and 33.9 mm/s (2.3 kJ/mm). In Part II, the absorbed energy values at –10°C and –30°C ranged widely from 240 J (max) to 8 J (min) for effective heat inputs of 2.9 kJ/mm and 2.3 kJ/mm. The higher Charpy energies for the 2.9 kJ/mm and 2.3 kJ/mm samples tended to occur when the notch center was located in the fine-grained HAZ (FGHAZ), and much of the crack propagated through the FGHAZ. The lower Charpy energies for the same heat inputs occurred when the notch center was positioned in the coarse-grained HAZ (CGHAZ), which had a significant amount of coarse-stringer MA constituents. The 2.6 kJ/mm samples had consistently high absorbed energy results (> 190 J), whether the notch center was positioned in the coarse-grained HAZ or the FGHAZ. The number of coarse-stringer MA constituents in the CGHAZ of 2.6 kJ/mm sample was the lowest. A severity parameter (SP) was developed to correlate the severity of coarse-stringer MA and crack length in the CGHAZ/FGHAZ. The SP value increased as the number of coarse-stringer MA and fracture length in the CGHAZ increased, which correlated to reduced Charpy toughness.

A high-speed camera was employed to capture the melting process and investigate the impact of laser beam oscillation diameter on weld pool fluctuation and deposition morphology during laser wire additive manufacturing (LWAM) of 316L wire. The transient fluctuations of the weld pool caused by the laser beam were observed at different oscillation diameters. Additionally, 3D morphology measurements were conducted on the deposited layer to assess the effects of different oscillation diameters on its morphology. Further analysis examined the heat dispersion resulting from beam oscillation by plotting the energy distribution for various oscillation diameters. The key findings are as follows: Changes in oscillation diameter resulted in fluctuations within the weld pool that impacted the surface morphology of the deposit and refinement of pore condition as well as alterations in the energy distribution of the weld pool that influenced its contour geometry. Increasing the laser oscillation diameter can effectively make the keyhole shallower, weaken the bottom protrusion of the weld pool, and reduce the curvature of the top surface of the bead. Simultaneously, it can promote the stirring of the liquid solution, effectively refining the porosity area, reducing porosity within the weld pool, and improving the specimen’s density. However, larger oscillation diameters lead to more-significant hump arch formation on the deposited surface and larger roughness values.