Search

Oxide dispersion strengthened (ODS) nickel (Ni)‐based superalloys are advanced materials known for their outstanding tensile and creep performance at temperatures exceeding 1000 °C. Nevertheless, their conventional synthesis presents a longstanding challenge in cost‐effectively producing intricate components for critical applications. In this work, electrostatic self‐assembly (ESA) of powders with the laser powder bed fusion (LPBF) process are successfully combined to produce yttria ODS Inconel 718 (IN718) alloy for the first time. This approach demonstrates a significant contribution of yttria to the strength of IN718 after the solid solution heat treatment, as evidenced by ≈50% improvement in room temperature yield strength with a 0.5 wt.% yttria addition. The addition of yttria by this process leads to a heterogeneous microstructure. This heterogeneous microstructure comprises two distinct grain areas with varying amounts of yttria nanoparticles and dislocation storage. The results show that the yield strength increase can be predicted by the combination of both Y2O3 dispersion strengthening and dislocation strengthening mechanisms. These findings offer an effective approach to tailor heterogeneous microstructures, unlocking new opportunities for cost‐effectively producing high‐performance ODS Ni‐based superalloy products with excellent mechanical properties. [ABSTRACT FROM AUTHOR]

Haynes 188 alloy with a combination of strength and ductility is of significance in aircraft engines and recent development in additive manufacturing significantly increases potential applications of this material. However, understanding the microstructure and tensile properties of Laser Powder Bed Fusion (LPBF) Haynes 188 alloy is limited. Here, a crack-free Haynes 188 alloy is successfully fabricated. The LPBF specimens are observed in two different directions: perpendicular and parallel to the building direction, referred to as "LPBF-H" and "LPBF-V" sections, respectively. To investigate the anisotropy of LPBF specimens, tensile tests were conducted on LPBF Haynes 188 alloy with different observed sections. The vertical sections show lower yield strength than the horizontal sections due largely to an elongated columnar structure. The effects of solute, grain size and dislocation on the strengthening mechanism are discussed thoroughly. The synergy effects of the grain size and the dislocation increase the strength of the LPBF alloys compared to wrought and casting equivalents. For LPBF specimens, the dislocation cellular structure is particularly investigated. To better understand the plasticity, the fracture morphology is also analyzed. The developed understanding on the microstructure and mechanical properties of Haynes 188 alloys in this work could also be applicable to other Co-based superalloys. • A Haynes 188 alloy with superior mechanical properties is successfully fabricated using the LPBF process. • LPBF Haynes 188 alloys show better mechanical strength compared to wrought and cast specimens. • The strengthening mechanism of LPBF Haynes 188 is disscussed thoroughly. [ABSTRACT FROM AUTHOR]

In this work, the effects of post-LPBF (laser powder bed fusion) heat treatments on microstructure evolution, tensile and Charpy properties of Ti-6Al-4V have been investigated. The results show that the original martensitic microstructure in the LPBF sample has converted to a lamellae mixture of α and β phases when post heat-treatment temperatures are below the β-transus (800°C, 850°C and 900°C) with air cooling. On this basis, a good balance of tensile strength and ductility was obtained from sample post heated at 800°C for 6 h followed by air-cooling, with an increase of ~42% in ductility and a significant toughness enhancement (almost 100% increase) compared to the as-built sample. The relationship betweenα lamellaethickness, β volume fraction and post-LPBF heat treatment has been established. Both α lamellae thickness and β volume fraction increase with the heat-treatment temperature. [ABSTRACT FROM AUTHOR]

Co-based and Ni-based superalloys have been widely used in aerospace, with much attention given to additive manufactured Ni-based superalloys. However, the equally important Co-based superalloys have received less attention. In this work, a Co-based Haynes 188 superalloy is fabricated using Laser Powder Bed Fusion (LPBF). This research shows the impressive mechanical properties of Haynes 188 superalloy before and after heat treatment at both room and high temperatures for the first time. In the as-built specimen, arrayed stacking faults, Lomer–Cottrell locks, dislocations, and M 6 C/M 23 C 6 eutectic carbides are found. The yield strength, ultimate tensile strength, and elongation of the as-built specimen are 642 MPa, 928 MPa, and 57.3 %, respectively. For the heat-treated specimen, the grain size increases, dislocation density reduces and the eutectic carbides dissolve into the matrix, which reduces the yield strength to 488 MPa. However, annealing twins are found in the heat-treated specimen, and the twin-twin interaction networks further enhance the pile-up of dislocations and work hardening ability during the deformation, which leads to an increase in the ultimate tensile strength and elongation to 1002 MPa and 68.1 %, respectively. This work provides an effective method to improve the mechanical properties of Co-based superalloys by additive manufacturing, which is essential for the development of high-performance superalloys and their manufacturing methods. [ABSTRACT FROM AUTHOR]

• The influence of the GB-α on the fatigue behavior of LPBF Ti–64 was investigated. • HCF property is improved due to low aspect ratio α lath and discontinuous GB-α. • Discontinuous GB-α could improve the crack propagation resistance. • α lath with a low aspect ratio inhibits the crack initiation at the interface. High cycle fatigue (HCF) performance of additive manufactured (AM) titanium products is of great significance for its structural and functional applications. However, tying fatigue performance to the complex microstructures in AM titanium alloys is challenging. The work here carried out a thorough investigation into the influence of microstructures (particularly the grain boundary α-phase (GB-α) with varied morphologies) on the fatigue performance of laser powder bed fusion (LPBF) Ti–6Al–4V (Ti-64). Results showed that the improvement in the high-cycle fatigue life of LPBF Ti-64 could be achieved by the formation of low aspect ratio α lath and discontinuous GB-α via optimized post-fabrication heat treatment. Discontinuous GB-α could fully accommodate the deformation, improving the fatigue crack propagation resistance. Moreover, α lath with a low aspect ratio could lead to less strain accumulation on the interface between adjacent α lath, and thereby inhibit the crack initiation at these interfaces. This study enhances the understanding of how LPBF-induced complex microstructures influence fatigue behavior, and provides a pathway for the improvement of fatigue performance of additive manufactured titanium alloy. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) has been increasingly used by industries to produce metal components. However, there remains a challenge to improve the productivity of the LPBF process, especially for high-volume production. In this study, we used Hastelloy-X (HX) as an example and demonstrated a hybrid approach of using a core-shell structured (0.5 mm thick dense shell + fast scanned porous core) part combining hot isostatic pressing (HIP) to improve the LPBF productivity. The microstructure evolution, mechanical properties, and the production rate of the parts built with and without the core-shell structure are compared and discussed in this work. The as-built core-shell HX showed a porous core enclosed by a dense shell, the density was restored through HIP. The HIP-processed core-shell HX revealed an artificially induced grain size gradient (large to small) from the shell to the core, while the HX without the core-shell structure presented uniformly distributed larger grains in the microstructure after HIP. The mechanical properties were studied using microhardness tests and uniaxial tensile tests. The core-shell HX showed an increment of the microhardness from the shell to the core, and the corresponding tensile properties were superior to the HX without the core-shell structure. The LPBF production rate using the core-shell approach was largely improved than the equivalent solid production route. The findings herein demonstrate the feasibility of using the core-shell approach to improve the productivity of the LPBF HX without sacrificing the tensile properties. [ABSTRACT FROM AUTHOR]

Post heat treatment is essential for additively manufactured alloys to eliminate residual stress and in some cases further improve mechanical performance. This study systematically investigated the hardening response of a Ti-6Al-2Sn-4Zr-6Mo alloy processed by laser powder bed fusion (LPBF) to a one-step heat treatment. The results revealed significant hardening at both 500 °C and 650 °C. Most notably, an ultra-high peak-hardness of 563VHN was achieved within 5 mins of heat treatment at 650 °C due to a unique martensite-decomposed α+β microstructure consisting of alternating long β laths that are ∼5 nm thick and α laths that are ∼35 nm thick, and within the α laths are also short β laths that are ∼2 nm thick with an ∼11 nm inter-spacing. The influence of both types of β laths on the α lath thickness was established and related to the ultra-high hardness observed, which provides new insight for the design of new metastable LPBF-processed Ti alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • A macroscale thermal modelling strategy has been developed by combining a published characteristic time-based heat input (CTI) model with an adaptive mesh re-mapping method. • The strategy is capable to accurately capture the peak temperatures of the thermal cycles induced by deposited tracks. • The predicted temperature histories agree well with in-situ measurements by thermocouples in both the printed part and powder bed. • Case studies demonstrate that our strategy can improve computational efficiency at least 6 times. Numerical modelling is an effective tool to investigate the rapid temperature cycling in laser powder bed fusion. However, the high computational cost limits the modelling of part-scale components due to the significantly different scales between the highly local energy input region and the global field response region. This study presents a macroscale finite element thermal simulation strategy with a track-scale resolution by combining the recently developed characteristic time-based heat input (CTI) model with a currently proposed adaptive mesh re-mapping method. Due to the uniformly distributed power density along the scan direction in the CTI model, the optimized mesh strategy allows one element to capture the temperature solutions over the majority of a scan track except at the two ends. The adaptive mesh re-mapping method is used to map simulated results from a previous scan track model to a current scan track model with a new mesh configuration. This strategy has been implemented for a Ti-6Al-4V part with the dimensions of 20 × 40 × 3 m m 3. The predicted temperature histories agree well with the in-situ measurements at the points in both the printed part and powder bed. Three case studies demonstrate that our strategy can improve computational efficiency between 6 and 30 times. [ABSTRACT FROM AUTHOR]

In this work, we presented an investigation into the deformation behavior of laser powder bed fusion (LPBF) Ti-6Al-4V (Ti64), particularly how microstructure characteristics determined the grain deformation behavior by using in-situ SEM tensile characterization. Results demonstrated that α laths with their elongation directions 45° to the tensile loading axis and α laths with the larger thickness could induce severe strain accumulation during the uniaxial tensile process. Crystallographic orientations and grain morphologies of α lath were considered the key factors that could influence the deformation behavior. The strain localization was observed in the α laths dominated by multiple slip systems. Furthermore, the α laths with their elongation directions 45° to the tensile loading axis had the longest effective slip length, which could make α laths favorable for deformation. The dislocations multiplication of coarse α laths was much greater than in fine α laths, which could also result in strain localization. These findings could shed light on the deformation behavior related to lamellar structure in LPBF Ti64 and could further help the mechanical property manipulation. • Strain distribution of LPBF Ti64 during the tensile process was characterized by using in-situ SEM tensile testing. • Crystallographic orientations and grain morphologies of α lath could determine the deformation behavior. • The strain localization was observed in the α laths dominated by both prismatic and pyramidal slip systems. • The α laths with elongation directions 45° to the tensile loading axis could make them favorable for deformation. • The dislocations multiplication of coarser α laths is more rapid than in fine α laths. [ABSTRACT FROM AUTHOR]

An Al-Mn-Mg-Sc-Zr alloy was additively manufactured and subsequently deformed to investigate the effect of high defect densities on the precipitation behavior, work hardening capability and ductility. For this, the LPBF-fabricated alloy was deformed by rotary swaging up to a true strain of 2.5 following a laser powder bed fusion (LPBF) process. Compared to the LPBF condition, swaging results in a refinement of the microstructure by one order of magnitude and an increased hardness and ultimate tensile strength (UTS) which is mainly attributed to the finer microstructure of the swaged alloy. By annealing, a higher peak-aging hardness of (209 ± 2) HV0.1 and UTS of (717 ± 2) MPa of the swaged alloy at a lower peak-aging temperature of 300 °C (1 h) was obtained. Significant improvement of uniform elongation by enhanced work hardening capability of the swaged and annealed alloy is obtained for annealing temperatures above 300 °C while strength is only moderately affected. The significant improvement of aging kinetics is discussed alongside a profound microstructural characterization of the heterogeneous grain structure and precipitate distribution. • Work hardening of high strength Al alloys is improved by deformation prior ageing. • Enhanced uniform ductility is obtained. • UTS of (612 ± 9) MPa at a uniform strain of (1.4 ± 0.2) % is found. • Heterogeneous microstructure by additive manufacturing resists cold work. [ABSTRACT FROM AUTHOR]

The defects remain a significant issue for additive manufactured titanium alloys, which hinders their further market penetration. In this work, we proposed a new understanding of the inhomogeneous microstructure formation affected by defects with detailed EBSD characterization, and how the defects and inhomogeneous microstructures determined the strain accumulation behavior by in-situ SEM tensile testing. Smaller prior β grains and coarser α laths were identified around the defects. In-situ tensile behavior investigations revealed that irregularly-shaped and elongated defects with high stress intensity factor ( Δ K I) could induce severe strain accumulation, showing that the defect morphology was the primary factor that could determine the strain accumulation behavior. In the meantime, the inhomogeneous microstructures around the defects could further contribute to the strain accumulation, with the coarser α laths more favorable for deformation and stronger microtexture for easier slip transmission. In addition, microcracks were found initiated from a spherical pore with small loading, and arrested due to the negligible strain accumulation. Further characterization showed that oxidation, which was found more probable in spherical pores, was the cause of this microcrack initiation. [Display omitted] [ABSTRACT FROM AUTHOR]

Heat treatment (HT) is an effective way to attain the desired microstructure and mechanical properties in laser powder bed fusion (LPBF) Ti–6.5Al–2Zr–1Mo–1V (TA15). However, previous studies only obtained the high tensile ductility of LPBF TA15 but were accompanied by a large loss of strength, which was unable to achieve balanced mechanical properties. Here, we evaluated the tensile properties of LPBF TA15 at their usage temperatures (500 °C), and a superior combination of high-temperature strength and ductility in LPBF TA15 was successfully achieved via a single-step heat treatment. After the heat treatment close to the β transus temperature (950 °C as used in this study), the tensile strength of the sample was ∼30% higher than that of as-cast TA15, accompanied by the obvious enhancement of ∼67% in elongation. The high strength originated mainly from the secondary α-phase and discontinuous grain boundary α-phase (GB-α), whereas the large ductility correlates to the complete decomposition of brittle α' martensite. Discontinuous GB-α could inhibit crack initiation at the grain boundary and the fine secondary α-phase impeded the dislocation motion, which could enhance the tensile strength. Meanwhile, the α lath directions after heat treatments were far away from the 45° of the load direction, which also avoided the localized deformation within α laths and thus improved the fracture resistance of LPBF TA15. [ABSTRACT FROM AUTHOR]

[Display omitted] • With the transverse loading direction, the strain localization was found within the continuous grain boundary α-phase. • Discontinuous grain boundary α-phase could fully accommodate uniform deformation together with the matrix microstructure. • An effective approach that eliminates the anisotropy in additive manufactured Ti-6Al-4V was provided. Grain boundary α-phase (GB-α) in titanium alloys is the vulnerable site for failure, while the correlation between deformation behavior and local dislocation motion is still ambiguous. In this study, we made a detailed investigation into the deformation behavior of GB-α with different morphologies in a laser powder bed fusion (LPBF) fabricated Ti-6Al-4V (Ti-64) by using in-situ and interrupted tensile testing, and discussed the underlying mechanisms of dislocation motion. The deformation-induced dislocations were found severely accumulated within the continuous GB-α with the transverse loading direction, which corresponds to the strain localization, while uniformly distributed between the continuous GB-α and surrounding grains with the perpendicular tensile loading. In contrast, discontinuous GB-α could fully accommodate uniform deformation with the matrix microstructure under both loading directions. The new understanding of dislocation motions in GB-α provides a novel perspective to effectively eliminate the tensile property anisotropy with the columnar microstructure present in LPBF Ti-64. [ABSTRACT FROM AUTHOR]

A major problem of additively manufactured parts using laser powder bed fusion (LPBF) is the as-built high surface roughness. The poor surface finishing is detrimental to the mechanical and electrochemical properties of the parts. In this work, we present systematic investigations on surface roughness reduction of LPBF produced nickel-based superalloy Hastelloy X (HX) using electropolishing. The electrolyte used in this study is an environmentally safe deep eutectic solvent (DES) comprising a eutectic mixture of choline chloride and ethylene glycol in a 1:2 M ratio, respectively. Two different groups of LPBF HX parts separated for the purpose of electropolishing the external and internal surface were prepared. The results show that both the external and internal surface roughness can be effectively reduced using electropolishing in DES. For the parts with the external surface electropolished for 5 min, a surface roughness reduction down to 1.2 μm (Ra) with a bright and reflective surface appearance was achieved. This is a significant improvement in comparison to the high as-built surface roughness of 10.3 μm (Ra) with a dull and non-reflective surface appearance. For the internal surface electropolishing, the surface quality of a 3 mm diameter internal channel in an angled LPBF HX cylindrical tube was successfully enhanced. The DES electropolishing efficiency on LPBF HX was investigated using weight reduction as a function of the electropolishing time, and the microstructure of the electropolished LPBF HX was studied. A further improvement of the electrochemical properties for the electropolished surface was revealed by anodic polarization tests and electrochemical impedance tests. The electropolished surface was also revealed to be more hydrophobic than the as-built surface by water contact angle tests, which is beneficial to the corrosion resistance. The findings herein demonstrate the surface quality of the LPBF produced HX can be effectively improved using electropolishing, and also showed the feasibility of using DES as a promising alternative electrolyte to the conventional hazardous acid mixtures. • The external and internal surface roughness of the LPBF HX parts were both effectively reduced using electropolishing. • An environmentally safe electrolyte has been applied to electropolish LPBF produced HX. • The electropolished surface showed an enhanced corrosion properties compared to the as-built surface. [ABSTRACT FROM AUTHOR]

• Complex intermetallic phases in the LPBF Al-Mn-Sc based alloy has been intensively studied by 3DAP. • Three different phases are identified in fine grain region in terms of their distribution and chemistry. • Phases in coarse grain region are revealed to exhibit distinct chemistry and distribution from those in fine grain region. The recently-developed Al-Mn-Sc based alloys specific for laser powder bed fusion (LPBF) have shown excellent mechanical performance. However, the complicated intermetallic particles in the microstructure remain to be identified, hindering the deep understanding of their effects on mechanical properties and further property improvement. In this work, a range of phases in a LPBF-built Al-Mn-Sc based alloy have been systematically studied by atom probe tomography. The results clarify characteristic intermetallic phases in two different grain-size regions in the microstructure. In the fine grain (FG) region, three distinct phases have been identified. A Sc-rich phase having an average composition of Al 3.3 (Sc 0.8 Zr 0.2) is observed at both the grain boundary (GB) and grain interior, but they have different morphologies. The Mn-rich phase with an average composition of Al 4.3 (Mn 0.9 Fe 0.1) is only observed along GBs. In addition, Mg-rich oxide has been observed either with a separate distribution or attached to Sc-rich particles. In the coarse grain (CG) region, the GB particles exhibit different size and composition from FG region. The Sc-rich GB particles contain Mg enrichment rather than Zr and the composition is determined to be Al 3.4 (Sc 0.75 Mg 0.25). The Mn-rich GB particles in the CG region, with higher Fe contents, are Al 4.3 (Mn 0.8 Fe 0.2) along the GBs and Al 4.5 (Mn 0.85 Fe 0.15) inside the grains. Many smaller Mg-rich oxides and Sc-rich particles are also observed in the internal grains of the CG region. [ABSTRACT FROM AUTHOR]

Grain boundary α-phase (GB-α) precipitation and coarsening behavior in Ti-6Al-4V (Ti-64) processed by laser powder bed fusion (LPBF) were investigated and compared with those in as-cast Ti-64. GB-α in LPBF processed Ti-64 tended to precipitate at the triple junctions (TJs) of the β-phase grain boundaries during the subsequent annealing process. Kinetic analysis on GB-α showed that the coarsening mechanisms varied in LPBF processed Ti-64 at different annealing temperatures (bulk diffusion at 750 °C and interface reaction at both 850 and 950 °C), while were consistent in as-cast Ti-64 (interface reaction at all the three temperatures). The inconsistent coarsening mechanisms were attributed to the GB-α curvature difference caused by the different α lamellae nucleation mechanisms. These findings could help rationalize the GB-α morphology evolution and benefit the further mechanical property manipulation in LPBF processed titanium alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The interest in application of Additive Manufacturing (AM) to nuclear industry stems not only from the benefits of design freedom and shortened lead time, but also from the possibility of enhancing the performance through microstructure control. One of the most important requirements for in-core structural material in nuclear power plants is helium resistance. The Laser Powder Bed Fusion (LPBF) processed 304L stainless steel possesses strong defect sinks such as high densities of dislocation-surrounded sub-grains and dispersed nano-inclusions. In this work the LPBF processed 304L in as-built and solution-annealed conditions along with a conventionally rolled counterpart were implanted with 350 keV He+ ion at 300 °C to 0.24 dpa (displacement per atom). Transmission Electron Microscopy (TEM) observations indicate significantly higher helium resistance of the as-built LPBF 304L compared to the other two samples. The sink strengths in the three samples are calculated based on the measurements of the microstructural features using simplified equations for the correlation between microstructural characteristics and helium tolerance. Based on the calculation, the cellular sub-grains and the dispersed nano-inclusions are the primary and secondary contributors to the helium resistance of LPBF 304L steel. [ABSTRACT FROM AUTHOR]