查看更多>>摘要：Wire-arc directed energy deposition (DED-arc) is a promising method to efficiently build large complex parts. However, columnar-grained structures commonly exist in DED-arc parts which exhibit anisotropic properties. In this study, the morphologies, microstructures, and mechanical properties of Inconel 718 plus superalloys fabricated by hybrid DED-arc with micro-rolling (HDMR) are investigated. Simulation and experimental results show that increasing the rolling force flattens the weld bead and significantly enhances the aspect ratio. Additionally, our simulations reveal that as the rolling force increases, there is a corresponding rise in equivalent plastic strain while longitudinal residual tensile stress decreases; intriguingly, residual compressive stress may even emerge. The HDMR process alters the morphology of the brittle Laves phase with a reduced content. Notably, a heterostructure characterized by alternating fragmented columnar (average size of 64.4 μm) and equiaxed fine grains (average size of 49.9 μm) is observed at a 30 kN rolling force, which can weaken the texture strength and enhanced dislocation density of the sample. In summary, in-situ rolling shows great potential in enhancing both tensile and yield strength while significantly minimizing the anisotropy of built samples. To validate these findings, an Inconel 718 plus superalloy combustion chamber is built utilizing the HDMR method.
查看更多>>摘要：Medium Mn steel (MMS) is attractive for lightweight design and a variety of structural applications due to its high strength and high plasticity, but it often face the "strength-plasticity trade-off'. The cooperative strengthening of transformation-induced plasticity (TRIP) effect and precipitation strengthening has the potential to overcome this problem in Medium Mn steel, however, it is usually not easy to introduce high-density nano-precipitation in soft and metastable austenite. Here, we report a nanoprecipitation migration strategy for the design of Cu bearing MMS by combing pre-precipitation, dynamic reversed transformation and re-precipitation, which achieves precipitation of high density of Cu-rich nanoparticles in the austenite and simultaneous enhancement of strength and plasticity. In warm-rolling and tempering MMS (1.63 wt% Cu addition), a large number of Cu-rich nanoprecipitates formed near the interface between austenite and ferrite during the annealing before warm-rolling. Cu-rich nanoprecipitates dissolved into clusters and heterogeneous Mn distribution occurred in the austenite during warm-rolling, moreover, phase interface migration caused the Cu-rich nano-clusters to remain in the austenite due to dynamic reversed transformation. Re-precipitation of high density of Cu-rich nanoparticles in the austenite based on these nanoclusters and heterogeneous Mn distribution during the tempering after warm-rolling. In contrast to the unrolling steels, the number density of Cu-rich nanoprecipitates in austenite increases from 4.43 × 10~(21) m~(-3) to 2.68 × 10~(22) m~(-3) in warm-rolling and tempering steels. These Cu-rich nanoprecipitates remarkably strengthened the austenite, improved the austenite stability and reduced the strain incompatibility between austenite and ferrite, which contributed to progressive transformation of austenite and spectral TRIP effect. Consequently, an increase in uniform elongation of 9 % along with an improvement in yield strength of 120 MPa and without drop of tensile strength was achieved in Cu bearing MMS. This work demonstrates how synergistic strengthening mechanisms can be activated by tailoring the number density of nanoprecipitates in metastable phase, along with the chemical/mechanical driving force for metastable phase transformation, to optimize strength and plasticity.
Alexander WesselEmin Semih PerdahciogluTon van den BoogaardAlexander Butz...
147714.1-147714.17页
查看更多>>摘要：Crystal plasticity finite element simulations are frequently employed to predict the plastic anisotropy of poly-crystalline metals based on their crystallographic texture. In age-hardenable aluminium alloys, however, the texture-induced plastic anisotropy is known to affect by precipitation. This paper presents a new modelling approach to incorporate this effect into crystal plasticity constitutive models. The approach focuses on the overall effect of precipitation, which is assumed to result in an additional directional dependency with respect to a global material orientation, superimposed with the texture-induced plastic anisotropy. This additional directional dependency is implemented into a conventional crystal plasticity constitutive model via a modified hardening law that introduces two new parameters, of which only one is treated as a free parameter. To demonstrate the applicability of the new modelling approach, it is applied to an age-hardenable AA6014-T4 aluminium alloy and compared against a state-of-the-art crystal plasticity constitutive model that considers only crystallographic texture. The results demonstrate that the new modelling approach significantly improves the prediction accuracy of the plastic anisotropy for the AA6014-T4 aluminium alloy studied.
Kali PrasadChintada UmasankarEun Seong KimRashi Rajanna...
147717.1-147717.13页
查看更多>>摘要：Advanced high strength steels (AHSS) present a promising strength-to-weight ratio for sheet metal forming applications; however, their poor stretch-flangeability, particularly concerning edge cracking, poses a significant challenge. In recent years, interrupted loading through servo press has emerged as a promising approach to enhance formability. While the past literature has unveiled the mechanism of time-dependent plastic deformation and its contribution to interrupted loading, the role of interface friction and its transient effect on stretch-flangeability is not understood. This study investigates non-conventional forming techniques involving interrupted loading during deformation. Two distinct punch motion modes, namely attach-hold (AH) and attach-detach (AD), are systematically examined to evaluate stretch-flangeability. The findings reveal that implementing interrupted loading modes (AH, AD) effectively delays failure, consequently improving stretch-flangeability. Comprehensive failure analysis and subsequent metallurgical characterization elucidate the mechanisms underpinning the enhanced stretch-flangeability. The microstructural mechanisms contributing to improved stretch-flangeability are thoroughly discussed.
查看更多>>摘要：The impact responses of the metastable refractory body-centered cubic (BCC) high-entropy alloy (HEA) HfZrTiTa_(0.53) with two different grain sizes (450μm or 140 μm) are investigated via plate impact experiments. Free surface velocity histories at different peak shock stresses are measured. Both as-received and postmortem samples are characterized with x-ray diffraction, electron back-scatter diffraction, scanning electron microscope and transmission electron microscopy. Multiple deformation mechanisms are identified, including dislocation slip, kink band formation and {332}(113) deformation twinning, and the BCC to the hexagonal close-packed (HCP) phase transformation in the BCC matrix, along with dislocation slip and {1011}(1012) deformation twinning in the HCP phase. Both the large- and small-grain samples display ductile damage. In contrast with the intergranular voids in the small-grain sample, intragranular voids are predominant in the large-grain sample, leading to its higher spall strength. Quantitative analysis of voids/cracks reveals similar damage characteristics for both grain sizes.
查看更多>>摘要：The influence of grain boundary serration treatments on the microstructure and creep behavior of Udimet-720Li superalloy was investigated. The cooling rate of super γ' solvus heat treatment has a profound impact on γ' precipitation and its interactions with grain boundaries. While air cooling yielded straight grain boundaries, slower cooling processes altered the morphologies of grain boundaries into Type-Ⅰ with moderate undulation due to continuous γ' precipitation and Type-Ⅱ with pronounced undulation due to discontinuous cellular γ/γ' formation. Creep tests at 700 ℃/700 MPa were conducted on samples with straight boundaries (STB), Type-Ⅰ serration (SRB-Ⅰ), and Type-II serration (SRB-2). Compared to the STB sample, the SRB-Ⅰ sample showed an improved creep resistance, exhibiting a minimum creep rate of 2.66 × 10~(-7) s~(-1) and a 17 % extended rupture life. This improvement is attributed to the higher fractions of secondary and tertiary γ', which effectively hinder dislocation motion, and the presence of Type-Ⅰ grain boundary serration, which deflects crack propagation. By contrast, the SRB-2 sample exhibited a higher minimum creep rate of 1.74 × 10~(-6) s~(-1) and a 42 % shorter rupture life than the STB sample. Although Type-Ⅱ serration exhibited more pronounced boundary undulation than Type-I serration, the incoherent flowery γ' particles and cellular structure reduced its impedance to dislocation motion, leading to a higher creep strain rate. The less cohesive grain boundaries caused by cellular γ/γ' precipitates also facilitated void nucleation and microcrack formation, accelerating the onset of tertiary creep. This study demonstrates that Type-Ⅰ grain boundary serration treatment can significantly improve the creep resistance of Udimet-720Li superalloy.
查看更多>>摘要：The present study compares the fatigue properties of IN939 superalloy manufactured conventionally by casting and additively by laser powder bed fusion (L-PBF). To investigate the influence of specimen orientation, i.e. load-to-build direction, and heat treatment on fatigue properties, 4 batches of additively manufactured specimens were prepared: vertically and horizontally oriented batches with either as-built microstructure (non-treated) or aged γ/γ' microstructure. Cylindrical specimens were machined from the cast rod and printed blocks, and subjected to low-cycle fatigue loading at room temperature. Fatigue testing was conducted in total strain amplitude control within ε_a = 0.27-1% with a constant strain rate of 2·10~(-3) s~(-1). Scanning and transmission electron microscopy techniques were used to analyse the specimens before and after testing. Out of all tested batches, horizontal aged L-PBF IN939 exhibited the best stress-life fatigue properties, while vertical non-treated L-PBF specimens showed the largest resistance to cyclic plastic straining. Plastic strain localization occurred in the form of persistent slip bands, which led to fatigue crack initiation at the specimen surface. The high stresses associated with the L-PBF batches favour interdendritic and intergranular cracking, increasingly so with higher batch strength. However, the cracking mechanism does not significantly affect the low-cycle fatigue life, which is primarily determined by the crack initiation phase.
查看更多>>摘要：This study examines the effects of various heat treatments on the mechanical properties and corrosion resistance of Inconel 625 alloy produced via laser wire melting. The as-deposited alloy exhibits residual stresses due to multiple thermal cycles, characterized by columnar dendrites with sub-crystalline segregation. Stress relief treatment at 750 ℃ enhances strength through diffusion strengthening and stress relief, while treatment at 1150 ℃ improves elongation by transforming dendrites and removing subgrain structures. Both treatments slightly and substantially enhance corrosion resistance, respectively, highlighting the importance of developing effective heat treatment strategies for additive manufacturing components.
查看更多>>摘要：The macrozone with sharp local texture is known as a potential cause of the reduced fatigue resistance and lifetime of titanium alloys. In this work, the microtexture evolution and slip activation of α macrozones during tensile deformation in Ti-6Al-4V alloy were analyzed by in situ EBSD. The results indicate that the α macrozones rotate with the orientation change under tensile deformation. The destruction of macrozones occurs when the c axis of the macrozone nearly overlaps with the stress direction. In such a case, the activation of the basal slip system is insufficient, which leads to severe stress concentration and local dislocation accumulation. Consequently, the α laths within the macrozone partially rotates to other directions for pyramidal <c+a> slip system activation. The large macrozone is thus segmented by the rotated a phase. In addition, a small rotation of the macrozone occurs when the angle between the tensile direction and the c-axis (θ) increases. In such a case, both basal and pyramidal <c+a> slip systems activate and contribute to the c-axis deformation. Thus, the deformation is homogeneous and the dislocation density is low within the macrozone. If only the pyramidal <c+a> slip system is activated, the macrozone nearly remains unchanged during deformation over a wide range of θ values. This work provides an available route to control macrozones in titanium alloys by pre-deformation.
查看更多>>摘要：The microstructure of additively manufactured(AM)Ti-6Al-4V significantly differs from that of traditional casting processes, and this microstructural variation directly impacts the material's mechanical properties. To ensure the safety and stability of additively manufactured Ti-6Al-4V in practical applications, this study achieves controlled dynamic loading by optimizing the cross-sectional surface density of the structural gradient flyer(SGFs). It further explores material response characteristics under varying stress and strain rates by adjusting the loading velocity in spallation experiments. The initial microstructure of Ti-6Al-4V produced via selective laser melting (SLM) was investigated and analyzed, and quasi-isentropic loading experiments were conducted on 6 mm thick samples at strain rates ranging from 5.07 × 10~4 s~(-1) to 1.24 × 10~5 s~(-1). The results demonstrate that, in comparison to as-cast alloys, the SLM-processed Ti-6A1-4V alloys exhibit reduced plasticity, primarily attributed to the higher cooling rates of SLM. The alloys predominantly display brittle fracture characteristics, with distinct transgranular fracture patterns. As the loading strain rate increases, the pulse width of the loading waveform progressively narrows, spall strength markedly rises, and cracks continue to propagate and grow. Grain refinement was evident in both the crack extension region and along the edges, accompanied by localized stress concentration, yet no signs of recrystallization were detected. The spall strength of the material exhibited heightened sensitivity to impact stresses relative to plate impact conditions. These findings offer critical theoretical insights into the potential applications of additively manufactured titanium alloys, particularly in aerospace and defense sectors.
NETL
NSTL
Elsevier