This article outlines a detailed study of solution treatments and delta precipitation treatments carried out on laser-directed energy deposited (DED) alloy 718 specimens. Two different sets of DED process parameters were used in high and low energy conditions that yield different microstructural features to study the effect of process parameters on delta precipitation. These two conditions were subjected to solution treatment at 1010 °C and 1050 °C each for 1 h, which improved homogeneity and altered grain texture with introduction of annealing twins. The as-built and solution-treated specimens served as the initial reference condition for subsequent delta processing treatments (DPT) performed at three temperatures of 850 °C, 900 °C, and 950 °C to study the effect of short- and long-term exposures ranging from 1 to 48 h. When as-built specimens were subjected to DPT, interdendritic delta precipitates were observed at Nb-rich regions. In contrast, solution-treated specimens under short-term exposure to DPT resulted in intergranular delta phase precipitates whereas under long-term exposures to DPT yielded predominantly intragranular delta precipitates, which grew denser and longer with increased time of treatment. For longer exposure times of 24 and 48 h, a continuous film of intergranular delta phase was noticed. The morphology, location, and volume fraction of delta phase precipitates studied in this research are imperative for designing the performance of alloy 718 built by DED process.

Two experimental FeCrAl alloy overlay welds on tube shields were exposed in the superheater of a full-size waste fired boiler for 6 months. The tube shields were in different tube lines and positions within the superheater chamber to investigate possible heterogeneities in the exposure environment. The visual inspection of the exposed tube shields and the corrosion-erosion rates calculated from the analysis of cross-sections showed that the mid-length roof location experienced the most aggressive environment. The compositional differences between the two experimental alloys were not found to be determinant in their performance under these specific exposure conditions. It was concluded that erosion had a decisive influence on the results. The identification of local differences within the superheater chamber is important when deciding on the material selection for the different areas and locations to be protected. The output of this study is therefore interesting for further design consideration of superheaters as well as for future planning of exposures.

Additive manufacturing (AM) introduces a new domain for zero waste and cleaner production. Research for verification of materials in AM and effects of the process on the material behavior, however, demands a significant amount of materials, energy, and man-hours. The design of suitable physical simulation techniques that can duplicate complex AM thermal cycles without performing AM is therefore crucial for cleaner and more sustainable AM research. This paper aims at introducing a novel technique to reproduce AM thermal cycles in a controlled way on a small sample, thereby supporting sustainable alloy verification and cleaner research. In this technique, a stationary arc is applied to a disc-shaped sample mounted on a water-cooled chamber, where the arc and water provide rapid heating and cooling, respectively. In the present study, a super duplex stainless steel (SDSS) was used as the experimental alloy to simulate the evolution of microstructure and properties during wire-arc additive manufacturing. The experiment was performed using the stationary arc with the holding time of 5 s, applied 1, 5, or 15 times. The total processing time was only 450 s (7.5 min) for the 15 a.m. thermal cycles experiment. The SDSS showed a progressive increase in the austenite fraction at 600–1200 °C and the formation of detrimental sigma phase at 700–1000 °C, but a reduction of austenite fraction above 1300 °C. The results were in good agreement with the literature, verifying the applicability of the physical simulation technique for AM research. Calculations showed that using arc heat treatment as the initial step is 6–20 times more efficient in different respects (materials, energy, and man-hours) compared to wire arc additive manufacturing. Therefore, this methodology can be implemented to gain an understanding of materials in AM applications thereby eliminating the need for investments in additive manufacturing of a specific component. © 2020

Thermo-fluid dynamic and experimental approaches are used to investigate the influence of 20° uphill, downhill and sideway substrate orientation during metal deposition over a previously deposited bead in a V-groove. The computational fluid dynamic model with free surface deformation and metal transfer gives insight into the melt pool flow and causes of defect formation observed on the solidified beads. The experimental metallographs, high-speed images and computational results show good agreement. It is found that the deposition of a second layer on a smooth first layer cooled down to room temperature leads to large changes in melt pool flow patternat 20° substrate inclination compared to flat condition. It results in undercut and humps with the uphill orientation and undercut with the side inclination. Therefore, lower angle range is necessary for multilayer gas metal arc deposition for these two last configurations.

In this article, process parameters such as laser power, deposition speed, and powder feed rate are varied at three levels, and their effect on geometrical characteristics and microstructural features of laser-direct energy deposited single-track Alloy 718 specimens is analyzed. Furthermore, the influence of standard heat treatments recommended for wrought form of Alloy 718 is investigated on as-built deposits. The main aim of the research is to curtail the amount of secondary Nb-rich precipitates such as Laves and NbCs either during the process or by subsequent heat treatments. The volume fraction analysis of Nb-rich phases shows that processing at high laser power conditions is ideal for minimizing segregation. Upon subjecting as-built deposits to (i) solution treatment, (ii) solution treatment and aging, and (iii) direct aging, a difference in volume fraction of Nb-rich phases is noticed compared to the as-built condition. Characterization of size, morphology, phase constitution through volume fraction estimation, and elemental concentrations employing electron dispersive spectroscopy analysis indicates dissolution of Nb-rich phases when subjected to heat treatments. The delta phase precipitation preferentially occurs in the top and bottom regions and sparsely in the middle region of the specimens subjected to solution heat treatment. In case of specimens subjected to direct aging (718 °C/8 h and 621 °C/8 h), delta phase is not discernable, indicating that a higher temperature (>900 °C) treatment may be necessary for delta precipitation and growth

Complex precipitation behavior of Cu-rich particles (CRPs) was investigated and simulated in continuously cooled and quench-aged super duplex stainless steel. Atom probe tomography (APT) and scanning electron microscopy showed that slow cooling resulted in nonuniform multimodal CRP precipitation and spinodal decomposition, while in the fast cooled and quench-aged conditions, more uniform precipitation of CRPs with no visible spinodal decomposition was found. Depletion of Cu, Ni, and Mn was observed in the ferrite next to the CRPs during growth, but not during dissolution. Some evidence of Ostwald ripening was seen after slow cooling, but in the quench-aged condition, particle coalescence was observed. Large CRPs disappeared next to a ferrite–austenite phase boundary after slow cooling when Cu was depleted due to the diffusion to austenite as also predicted by moving boundary Dictra simulation. Comparing Cu depleted areas next to CRPs analyzed by APT and moving boundary Dictra simulation of CRP–ferrite showed that the effective Cu diffusion coefficient during the early-stage precipitation was about 300 times higher than the Cu diffusion coefficient in ferrite at 475 °C. Using the effective diffusion coefficient and a size-dependent interfacial energy equation, CRP size distribution was successfully predicted by the Langer–Schwartz model implemented in Thermo-Calc Prisma. Applying a short aging time and continuous cooling increased the hardness and decreased the toughness values compared to the solution annealed condition. A nonuniform distribution of Cu in ferrite, the duplex structure, and partitioning of alloying elements among different phases are factors making CRP precipitation in duplex stainless steels complex.

Avoiding low austenite fractions and nitride formation are major challenges in laser welding of duplex stainless steels (DSS). The present research aims at investigating efficient means of promoting austenite formation during autogenous laser welding of DSS without sacrificing productivity. In this study, effects of shielding gas and laser reheating were investigated in welding of 1.5-mm-thick FDX 27 (UNS S82031) DSS. Four conditions were investigated: Ar-shielded welding, N2-shielded welding, Ar-shielded welding followed by Ar-shielded laser reheating, and N2-shielded welding followed by N2-shielded laser reheating. Optical microscopy, thermodynamic calculations, and Gleeble heat treatment were performed to study the evolution of microstructure and chemical composition. The austenite fraction was 22% for Ar-shielded and 39% for N2-shielded as-welded conditions. Interestingly, laser reheating did not significantly affect the austenite fraction for Ar shielding, while the austenite fraction increased to 57% for N2-shielding. The amount of nitrides was lower in N2-shielded samples compared to in Ar-shielded samples. The same trends were also observed in the heat-affected zone. The nitrogen content of weld metals, evaluated from calculated equilibrium phase diagrams and austenite fractions after Gleeble equilibrating heat treatments at 1100 °C, was 0.16% for N2-shielded and 0.11% for Ar-shielded welds, confirming the importance of nitrogen for promoting the austenite formation during welding and especially reheating. Finally, it is recommended that combining welding with pure nitrogen as shielding gas and a laser reheating pass can significantly improve austenite formation and reduce nitride formation in DSS laser welds. © 2020, The Author(s).

In this study, Submerged Arc Welding (SAW) and Gas Metal Arc Welding (GMAW) processes were used in multi-pass welding of 33 mm thickness super duplex stainless steel plates. Recommended and higher than recommended arc energy and interpass temperatures were used. Both GMAW and SAW processes were able to produce large thickness weldments meeting the microstructural, mechanical, and corrosion resistance requirements, and also when using higher than recommended arc energy and interpass temperature. It was possible to reduce the number of welding passes by half when using higher than recommended arc energy and interpass temperature. The SAW process needed only half of the welding time required for the GMAW process to produce a weldment with nearly the same number of weld passes, when using recommended arc energy and interpass temperature. Based on the results of this investigation, the practical recommendations for welding large thicknesses should be revised and updated.

The objective of this work was the development of a methodology to parametrize wire + arc additive manufacturing (WAAM), aiming dimension repeatability, and tolerances. Parametrization of WAAM is a difficult task, because multiple parameters are involved and parameters are inter-dependent on each other, making overall process complex. An approach to study WAAM would be through operational maps. The choice of current (Im) and travel speed (TS) for the desirable layer width (LW) determines a parametrization that leads to either more material or less material to be removed in post-operations, which is case study chosen for this work. The work development had four stages. First stage, named ‘mock design’, had the objective of visualizing the expected map and reduce further number of experiments. At the second stage, ‘pre-requisite for realistic operational map’, the objective was to determine the operating limits of TS and Im with the chosen consumables and equipment. Within the ‘realistic operational map’ stage, a design for the experiments was applied to cover a parametric area (working envelope) already defined in the previous stage and long and tall walls were additively manufactured. Actual values of LW (external and effective layer width) were measured and an actual operating envelope was reached. According to the geometry-oriented case study, a surface waviness index (SWindex) was defined, determined, and overlapped in the envelope. It was observed that the walls with parameters near the travel speed limits presented higher SWindex. This operational map was further validated (fourth stage) by selecting a target LW and finding corresponding three parametric set (covering the whole range of operational map) to produce walls on which geometry characterization was carried out. After geometry characterization, obtained LW was compared with the target LW (the maximum values were very tied, with deviations from +0.3 to 0.5 mm), with a SWindex deviation at the order of 0.05. Both results evidence high reproductivity of the process, validating the proposed methodology to parametrize WAAM.

The present study aims at investigating bead geometry and the evolution of microstructure with thermal cycles in multipass shielded metal arc welding of a V-groove 13-mm type-2507 super-duplex stainless steel (SDSS) plate. The weld consisted of 4 beads produced with arc energies of 0.81-1.06 kJ/mm. The upper beads showed lower base metal (BM) dilution than the first bead. Thermal cycles were recorded with thermocouples, indicating that the cooling rate decreased in the as-deposited weld zone when adding a new bead. Ferrite fraction in the as-welded condition was lower for the upper beads. The austenite grain morphology in reheated passes varied depending on the local peak temperatures and the number of reheating passes. Sigma phase precipitated in a location reheated by the third and fourth passes that was subjected to a critical peak temperature for sigma precipitation. Ferrite content, measured using image analysis and Fisher FERITSCOPE technique, showed that the ferrite fraction moved toward 50/50% in the weld metal with an increasing number of reheating cycles. Finally, a schematic map showing an overview of the microstructure in the multipass SDSS weld was introduced.