Search

Laser Powder Bed Fusion (LPBF) 304L stainless steel has garnered significant attention due to its unique blend of strength and ductility at room temperature. However, there is limited understanding of its high-temperature tensile properties. In this study, we conducted tensile testing on LPBF 304L at temperatures ranging from 623K to 1023K in its as-built state. Additionally, we performed a comparative tensile test on traditionally hot-rolled (at 1433K) 304L stainless steel under equivalent test conditions. The results revealed that LPBF 304L steel exhibited high ductility at 973K (ultimate tensile stress (UTS) is 257.3 ± 3.6 MPa)-1023K (UTS is 218.7 ± 3.8 MPa), but its strength (yield strength (YS) decreased from 346.0 ± 4.5 MPa to 182.7 ± 3.8 MPa) and plasticity (UTS decreased from 405.7 ± 5.3 MPa to 218.7 ± 6.4 MPa) decreased slowly as the testing temperature increases. We also delved into the microstructure and its impact on high-temperature tensile deformation. The fine grains and ensuing dynamic recovery were found to be responsible for the reduced strength and ductility of LPBFed 304L samples at elevated temperatures. Furthermore, a sawtooth fluctuation emerged in the stress-strain curve at 873K, which could be attributed to dynamic strain aging (DSA) and the interaction between solute elements and dislocations. [ABSTRACT FROM AUTHOR]

For SLMed Ti–6Al–4V- x O (x = 0.11, 0.16, 0.21, and 0.25 wt%) alloys, both microstructure and mechanical properties are sensitive to oxygen content. The α′ martensites in as-built state transform to a 'bimodal' microstructure containing α lamellae interspersed with β transformed regions after 2 h annealing at 800 °C, 850 °C, 900 °C, and 950 °C followed by air cooling. The α lamellar width decreases and the β phase volume fraction increases with increasing oxygen content, due to reduced diffusivity of Al and V. The solid solution hardening effect of oxygen was quantified as: σ 0 = 472 MPa + 2159 c 0.5 , where σ 0 is the lattice frictional stress and c is oxygen atomic fraction of the alloy. Increased oxygen content causes reduced fracture toughness and higher fatigue crack growth rate in the unstable growth regime, and the appearance of bilinear growth rate of the Paris regime at high annealing temperatures. The absence of the bilinear behaviour in the low oxygen content alloy and for annealing at low temperatures was interpreted in terms of changes in microstructural characteristics caused by oxygen impeding the diffusion of Al and V. [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]

Quasi-static tensile and stress-controlled high cycle fatigue tests of solution heat-treated (SHT) Hastelloy X manufactured by electron beam powder bed fusion (PBF-EB) and laser-based power bed fusion (PBF-LB) process were performed at room temperature and 750 °C. Post-fabrication SHT was ineffective in overcoming the microstructural anisotropy observed within as-built specimens, with the grains still maintaining its columnar architecture along the build direction. A significant drop in ductility was observed in tensile specimens tested at 750 °C, which was attributed to the carbide precipitation and grain boundary sliding. Upon investigating the influence of microstructural evolution as a function of test duration, a significant increase in precipitation was observed with an increase in test duration. A notable decrease in the fatigue strength was observed at elevated temperature. The long columnar grain structure within vertically build PBF-EB specimens was found to offer higher resistance against fatigue at 750 °C, owing to its reduced grain boundary area perpendicular to the loading direction. The corresponding fatigue damage mechanisms were investigated via fractographic analysis of the fracture surfaces and longitudinal cross-sections of the fractured specimens. Irrespective of the build orientation and test conditions, the fatigue cracks that resulted in final failure were found to initiate from the specimen surface. Also, the grain boundary precipitates were found to result in intergranular cracking during elevated temperature fatigue tests. • Tensile and fatigue behaviour of PBF-EB and PBF-LB HX were compared at RT and 750 °C. • PBF-LB HX showed higher tensile strength and fatigue behaviour at room temperature. • Vertical PBF-EB HX exhibited significant resistance to fatigue degradation at 750 °C. • Cleavage facets and intergranular fracture observed after tensile test at 750 °C. • Intergranular cracks observed in PBF-EB and PBF-LB HX after fatigue tests at 750 °C. [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]

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]

Numerous studies have been conducted in the recent past to assess different nickel-based superalloys fabricated using laser-based powder bed fusion (PBF-LB). Most of these works focused on the processability, microstructural evolution, post-fabrication treatments and the mechanical behaviour of components manufactured by PBF-LB under static loading. However, limited efforts have been made to investigate the fatigue crack growth (FCG) characteristics of PBF-LB built alloys, thereby imposing a barrier to their use as critical components. As such, the aim of the present study was to evaluate and understand the effect of build orientations (0° and 90°) and post-fabrication treatments (solution heat treatment and hot isostatic pressing) on the FCG behaviour of Hastelloy X (HX) fabricated by PBF-LB process. The results from the FCG tests showed that the build orientations had no discernible influence on the fatigue threshold and FCG rate within near-threshold and Paris regions. Nevertheless, the post-fabrication treatments led to an increase in the fatigue threshold and crack growth resistance within the near-threshold region because of the alleviation of detrimental tensile residual stress alongside the crack growth direction. In conjunction with FCG tests, scanning electron microscopy and electron backscattering diffraction were used to investigate the crack path and fracture surfaces within near-threshold and Paris regions. • FCG tests were performed on as-built, SHT and HIP compact tension specimens with crack growth parallel and perpendicular to build direction. • The effect of anisotropic microstructure and residual stress on fatigue crack threshold and fatigue crack growth rate is investigated. • Transgranular mode of crack propagation was observed in near-threshold and Paris regions of crack growth in as-built and post-treated PBF-LB HX. [ABSTRACT FROM AUTHOR]

Additive manufacturing of 25Cr super duplex stainless steel (SDSS) has been successfully demonstrated using laser powder bed fusion (L-PBF) in recent years. The as-built L-PBF 25Cr SDSS exhibits a microstructure that is close to 100% ferritic (due to the fast cooling in L-PBF) and requires a high temperature thermal treatment to obtain the desired duplex structure. Directed laser deposition (DLD), as a variant of the additive manufacturing methods, is characterized by a much slower cooling rate than L-PBF and is potentially beneficial to achieve the duplex phase fraction in the as-deposited condition. DLD of 25Cr SDSS is new, the microstructure development and the mechanical properties remain unclear. The microstructure and mechanical properties of additively manufactured 25Cr SDSS produced by metal powder based DLD are reported in this work. The as-deposited microstructure revealed a balanced duplex structure, with a near 50/50 ferrite-austenite phase ratio, differences to the previously reported microstructure of SDSS produced by L-PBF are discussed. The mechanical properties of DLD 25Cr SDSS were investigated using uniaxial tensile testing and compared with the commercial wrought alloy. DLD 25Cr SDSS showed superior tensile strength to the commercial wrought alloy, the origin of the strength and strain hardening is discussed, and the flow strength for DLD and wrought 25Cr SDSS are described using a simple rule of mixtures model. The modelling supports the notion that the main difference between the mechanical response of the DLD 25Cr SDSS and the commercial alloy arises from the finer scale of the phases in the DLD SDSS. The findings demonstrate the feasibility of additive manufacturing using DLD as applied to 25Cr SDSS. • Ferrite/austenite duplex structure is formed in-situ using directed laser deposition. • Dislocations and oxide particles are beneficial to the austenite nucleation. • The high tensile strength is mainly contributed by dislocations and the fine austenite phase. • The strain hardening behaviour is understood by a flow strength modelling based on the dislocation storage. [ABSTRACT FROM AUTHOR]