Key Engineering Materials Vol. 1040

Abstract: Optimizing the mechanical properties of aluminum to titanium welds is crucial to establish applications for dissimilar lightweight structures in the aerospace industry. In this context, solid-state welding technologies have proven effective in terms of short joining cycles, allowing the combination of cost-effective production and structural weight optimization. However, metallurgical effects between aluminum and titanium in the joint interface are still not completely understood due to differences in physical as well as chemical characteristics. In this study, aluminum alloy 6013 was welded to Ti6Al4V by refill Friction Stir Spot Wel ding, including systematic variations of Mg and Si alloying element content in the used AA6013 sheets. In total five different Al alloys were welded to the titanium to investigate the influence of Mg and Si during processing. Apart from the material selection, the weld strength is mainly influenced by the intermetallic compound thickness at the interface, which in turn primarily depends on the exposed temperature cycle. Consequently, major interest during this study was given on the temperature evolution, interfacial features and the global mechanical properties.

Abstract: To address the long cycle, high cost, and low efficiency of traditional Aluminum Foam Sandwich preform preparation, this study employs Friction Stir Welding to fabricate preforms with uniform powder mixing. Integrated experimental-simulation approaches reveal the formation mechanism. Temperature field analysis via infrared thermography and Fluent simulation confirms a peak temperature of 522°C at 2000 r/min rotation speed, generating an 85°C/mm thermal gradient and expanding the >450°C zone to 1.8 times the shoulder diameter. Concurrently, flow field modeling demonstrates intensified vortex flow at 2000 r/min, with tracer particles verifying uniform TiH₂/Al₂O₃ dispersion and onion ring radius expansion under 50 mm/min welding speed. Microstructural characterization identifies optimal joint quality through refined nugget zone grains averaging 1.3 μm and porosity below 2% at parameters of 2000 r/min rotation speed, 50 mm/min welding speed, 3 mm spacing, and 0.1 mm reduction. These results establish a methodology for regulating preform structural uniformity.

Abstract: Welding distortion remains a significant challenge in vehicle structures incorporating aluminum components. This study investigates the application of pre-tension to mitigate welding distortion in an aluminum T-joint structure. Through finite element (FE) simulation, the mechanism by which pre-tension alters the residual stress state and reduces distortion is analyzed and comprehensively explained. Based on the findings, an optimized pre-tension condition is proposed to minimize welding distortion in the T-joint configuration.

Abstract: C-Mn steel welds of the secondary circuit of pressurised water reactors might be subjected to static strain ageing (SSA) due to the segregation of free interstitial atoms, especially nitrogen, on dislocations at temperatures of 250°C-300°C in operating conditions. Thus, several parameters such as the initial free nitrogen content and the amount of prestrain are likely to affect the sensitivity of the material to SSA, both influences being investigated in this paper. To this end, a broad experimental campaign based on internal friction (IF) measurements was conducted to follow the evolutions of the solute content. Preliminary studies emphasized the presence of nitrogen solutes in the as-welded material, whose content tends to increase from 250°C to 300°C due to the dissolution of ε carbonitrides. Regarding the parametric study of SSA, it was shown that the amount of segregated nitrogen first increases with the amount of prestrain or the initial concentration of nitrogen solutes before saturating from a critical threshold. Its value depends on the other parameter, leading to either no effect, a continuous increase or an increase before saturation of segregation. Therefore, a cross influence of both parameters was highlighted.

Abstract: The operational integrity of supercritical steam power units necessitates comprehensive understanding of welded joint behaviour under high-temperature service conditions. Advanced steam cycle technology requires meticulous periodic evaluation of pressure-bearing components to ensure structural integrity throughout extended service periods. This requirement is particularly critical for components subjected to the most severe operational parameters, including superheater tubes, main steam pipelines, and steam collectors operating above critical temperature. For pressure components not directly exposed to exhaust gases, microstructural degradation represents the primary degradation mechanism governing component lifespan.P92 (X10CrWMoVNb9-2) steel, characterized by a tempered martensitic microstructure with 9% chromium content, has been extensively utilized for high-pressure applications in supercritical steam power generation systems. This advanced creep-resistant steel demonstrates superior mechanical properties, including exceptional high-temperature strength retention, oxidation resistance, and creep rupture strength under prolonged thermal exposure.Welded joints of pressure elements in steam boilers are potentially the weakest points when assessing their service life. These joints exhibit enhanced susceptibility to microstructural degradation and mechanical properties deterioration, particularly within the heat-affected zone (HAZ), during extended high-temperature operation. Therefore, systematic material characterization of welded joints relative to base material performance is essential for understanding long-term degradation mechanisms.This investigation presents systematic creep testing methodology and experimental results for P92 welded joint specimens subjected to annealing at temperatures of 600°C and 650°C for durations up to 10,000 hours. Both abridged and long-term creep tests were performed with the results of determination of creep strength and creep speed in steady state.

Abstract: One of the main problems with the use of steels for elevated temperatures is their limited weldability. This is mainly due to the fact that these materials may contain in their chemical composition. Due to the susceptibility to cold cracking, PWHT is necessary, especially in high-stiffness welded structures. In addition, depending on the condition after heat treatment or in the absence of heat treatment, precipitates may appear in the microstructure of the steel, affecting its mechanical properties. It is important in this case to ensure the high quality of welded joints, which means that the manufacturer has to demonstrate a very high technical culture. Currently, thin-walled pipe butt joints are welded manually using a tungsten electrode with solid wire material (TIG method). One of the solutions that can significantly speed up the welding process of components for work at elevated temperatures is the use of an electron beam welding. In addition, the ability to make welded joints without the use of filler material and to achieve narrow heat-affected zones may find application in the welding of modern materials used in the power industry. This paper presents the welding experience of materials assigned for the power industry (TEMPALOY AA1 and T92) by use of electron beam. In this article authors present the results of tests gained during first steps of welding welded joints. The article also includes preliminary results on the service life of the fabricated joints.

Abstract: Non-metallic inclusions in weld metals often have a role inducing the formation of acicular ferrite, which is known to improve the toughness and other properties of the weld metal. The ability of the inclusions to promote the acicular ferrite formation depends on various factors such as chemical composition, morphology and size of the inclusions. In multipass welding, additional thermal cycles affect the inclusions in the pre-existing weld passes, potentially causing compositional and morphological changes in the inclusions. These changes may influence the inclusions’ ability to promote the formation of acicular ferrite. In the current study, the thermal cycles of multipass welding were produced on a single pass weld by physical simulation. Coarse-grained heat-affected zones (CGHAZ-W) in weld metal were simulated using three different cooling times from 800 °C to 500 °C (t_8/5). Inclusions in the heat-affected zones of the weld were analyzed using field emission scanning electron microscope equipped with energy dispersive spectroscope (FESEM-EDS), after which they were classified according to their chemical composition. The results showed that the inclusion content in the weld metal was affected by the thermal cycles. In the CGHAZ-W simulation the number of Mn-bearing inclusions increased compared to the unaffected single-pass weld metal. Increasing cooling time was observed to increase the area fraction of MnS in CGHAZ-W. The increase of these inclusions was expected to affect the microstructure by increasing the acicular ferrite fraction.

Abstract: Thermal cycling tests were conducted on solder joints of wafer-level chip scale packages with Sn-3.0Ag-0.5Cu (mass%) to investigate the effect of difference in the thermal cycle profile on the thermal fatigue properties. The tests were conducted using three different types of chambers and the temperature range was-40°C to 125°C. The result showed that the thermal fatigue life tends to be shorter when the temperature change rate in the thermal cycle profile is slow. This means that the effect of creep on thermal fatigue life is large. The analogous analysis result was obtained by finite element analysis. The microstructural analysis of damaged joints revealed that recrystallized grains form in the solder and crack progresses at their grain boundaries. Thus the thermal fatigue life was suggested to be dominated by the ease of formation of recrystallized grains in solder.

Abstract: The effect of electrolytic Ni plating on the Cu electrode on electromigration in the Sn-3.0Ag-0.5Cu (mass%) solder joint was investigated. The energization test was conducted using Ni-plated Cu/Sn-3.0Ag-0.5Cu/Ni-plated Cu joints. For comparison, the same test was done using joints without Ni plating. The scalloped Cu₆Sn₅ layer formed at the joint interface without Ni plating. In the joint with Ni plating, granular (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ formed at the joint interface on the solder side and on the Ni side, respectively. Crack propagation was observed on the cathode side in both joints by current load. Crack propagated at the Cu₆Sn₅/solder interface in the joint without Ni plating and at the (Cu,Ni)₆Sn₅/solder interface in the joint with Ni plating, respectively. The Ni plated layer inhibited the diffusion of Cu facilitated by current load so that the growth of the reaction layer on the anode side was also inhibited. Therefore, the Ni plating is expected to improve the EM life.