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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Effect of Plate Hardening Behavior on the Deformation of Stainless Steel Metal Bellows

Tensile tests of original plate samples from three types of stainless steel metal bellows were performed at room temperature. The constitutive equations for the three hardening curves were obtained and fitted. The analysis results of the microstructure and fracture morphology of the tensile specimens show that the grain size of the plate with a high logarithmic–exponential hardening rate is uneven and the dimple of the shear fracture is elongated into an ellipse. By contrast, the grain size of the plate with a relatively low linear hardening rate is even and the dimple of the fracture is uniformly equiaxial. Finite element simulations of the hydraulic bulging and repeated limit bending deformation of the metal bellows of the three types of materials were also conducted. The repeated limit bending deformation process was tested experimentally. Although the effect of the hardening exponent on the residual stress of the metal bellows after hydraulic bulging is minimal, this exponent considerably influences the repeated limit bending deformation of the metal bellows after subsequent use. The trough hardening phenomenon is serious in the repeated limit bending process. Moreover, when the hardening exponent of the original plate is high, the resistance to bending fracture at the trough area is poor.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Friction Stir Welding of Dissimilar Al/Al and Al/Non-Al Alloys: A Review

Friction stir welding is a solid-state welding technique that has many advantages over traditional fusion welding, and has been widely adopted in the aerospace and automotive industries. This article reviews research developments in friction stir welding of dissimilar alloys systems, including combinations of aluminum alloys with Mg alloys, Cu, and steel. Microstructural evolution, hardness, tensile and fatigue properties, residual stresses, and corrosion behavior of dissimilar welds will be reported. The effects of processing parameters such as tool rotation and traverse speeds, tool position, material position, and tool geometry on the weld quality are also presented. Discussions on future research directions in friction stir welding will also be provided in the context of existing literature and future high-integrity applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Corrosion and Fatigue Behavior of High-Strength Steel Treated with a Zn-Alloy Thermo-diffusion Coating

High and low cycle fatigue tests were conducted on high-strength steel using four-point bending. The materials tested were ASTM A723 steel in the as-machined condition, grit-blasted condition, MIL-DTL-16232 heavy manganese phosphate-coated condition, and ASTM A1059 Zn-alloy thermo-diffusion coated (Zn-TDC). The ASTM A723 steel base material exhibits a yield strength of ~1000 MPa. The effects of the surface treatments versus uncoated steel were examined. The fatigue life of the Zn-TDC specimens was generally reduced on as-coated specimens versus uncoated or phosphate-coated specimens. Several mechanisms are examined including the role of compressive residual stress relief with the Zn-TDC process as well as fatigue crack initiation from the hardened Zn-Fe alloy surface layer produced in the gas-metal reaction. Additionally, the effects of corrosion pitting on the fatigue life of coated specimens are explored as the Zn-TDC specimens exhibit significantly improved corrosion resistance over phosphate-coated and oiled specimens.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Replacement of Nitrided 33CrMoV Steel with ESR Hot Work Tool Steels for Motorsport Applications: Microstructural and Fatigue Characterization

In this work tensile strength, fatigue resistance and fracture toughness of two electroslag remelted (ESR) tool steels and of 33CrMoV12 ESR steel (both in quenched and tempered condition, as well as nitrided condition) were evaluated. The role of hardness, residual stresses and inclusion sizes on the fatigue behavior was investigated. Tool steels have a tensile strength between about 1900 and 2300 MPa, fracture toughness between 35 and 33 MPa√m, while fatigue strength ranges between 725 and 992 MPa. The tensile strength and fracture toughness of the 33CrMoV12 ESR are, respectively, 1365 MPa and about 150 MPa√m. Nitriding induces a significant increase in fatigue strength from 560 to 980 MPa. These results highlight that appropriate ESR tool steels could replace nitrided steels.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of Laser Shock Peening on the Microstructures and Properties of Oxide-Dispersion-Strengthened Austenitic Steels

Oxide-dispersion-strengthened (ODS) austenitic steels are promising materials for next-generation fossil and nuclear energy systems. In this study, laser shock peening (LSP) has been applied to ODS 304 austenitic steels, during which a high density of dislocations, stacking faults, and deformation twins are generated in the near surface of the material due to the interaction of laser-driven shock waves and the austenitic steel matrix. The dispersion particles impede the propagation of dislocations. The compressive residual stress generated by LSP increases with successive LSP scans and decreases along the depth, with a maximum value of -369MPa. The hardness on the surface can be improved by 12% using LSP. In situ transmission electron microscopy (TEM) irradiation studies reveal that dislocations and incoherent twin boundaries induced by LSP serve as effective sinks to annihilate irradiation defects. Furthermore, these findings suggest that LSP can improve the mechanical properties and irradiation resistance of ODS austenitic steels in nuclear reactor environments.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Surface Modification of High-Carbon Steel Using Fiber Laser and Its Thermal, Microstructural and Tribological Analysis

In this study, surface modification of high-carbon steel was carried out using 2 kW fiber laser and thermal, microstructural and tribological characterizations were performed. The process parameters selected for laser surface treatment were laser power (600-1500 W) and scan speed (3000-4500 mm/min). Infrared pyrometer was used to record the surface temperature during the process. The metallographic analysis was carried out using an optical microscope to find the depth and width of the laser-treated zone. Scanning electron microscopy and energy-dispersive spectroscopy were performed to reveal the microstructure and compositional changes taking place in the laser-treated zone. It was observed that the dendritic growth took place from the top surface toward the center of the laser-treated zone. The amount of dendritic growth can be related to the maximum surface temperature generated during the process. XRD analysis indicated the formation of some intermetallic compounds and phase shifts taking place in the laser-treated zone. Residual stress analysis was performed to analyze the amount of stress accumulation in the laser-treated zone. When the temperature cycle was analyzed, it was found that the melting of the steel resulted in the formation of a surface under compressive stress while the non-melt hardened zone developed a surface under tensile stress. The maximum Vickers microhardness value achieved was about 958.8 HV{sub 0.5} for 1500 W power and 3000 mm/min scan speed. Scratch test was performed on the laser-treated track, and it was found that the hardened zone showed better tribological properties with respect to the parent material. Finally, it was observed that the surface roughness value increased with higher heat input probably due to high rate of oxide formation as well as due to the presence of martensite in the laser-treated zone.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High-Cycle Fatigue Behavior of Vacuum-Carburized 20Cr2Ni4 Steel with Different Case Depths

High-cycle fatigue behavior of vacuum-carburized 20Cr2Ni4 steel with three different case depths was studied by using rotating bending fatigue test. The results show that fatigue strength exhibits an increase–decrease pattern with increasing effective case dept, which is regarded to be mainly due to the unavoidable change in other parameters such as residual stress and grain size in case layer besides case depth. The present data and data in the available literature suggest that a dimensionless parameter of relative case depth is more appropriate to reflect the influence of case depth on fatigue strength. Further fracture surface analysis revealed that the fatigue failure was dominated by surface crack initiation mainly due to the extremely high cleanness of the tested steel with inclusion control. It is thus concluded that both relative case depth and other parameters should be comprehensively considered in obtaining optimized fatigue performance of vacuum-carburized steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Ceramic Rolling and Annealing on Mechanical Properties of AlCoCrFeNi{sub 2.1} Eutectic High-Entropy Alloys

It was firstly proved that AlCoCrFeNi{sub 2.1} could optimize the strength-ductility combination by ceramic rolling at room temperature. The technique greatly enhanced the strength of the materials at the price of slight ductility, which is different from the previous studies. When the thickness reduction was 23.06%, the yield strength and compressive strength were 1611.42 and 2212.99 MPa, respectively, with the plastic strain of 28.36%. Such an excellent combination of strength and ductility was attributed to the distribution of the residual stress, the high hydrostatic stress and the shear bands in ceramic rolling process. The plastic strain of the material increased to 38.54%, and the strength reduced to 1900.73 MPa after annealing at 800 °C for 1 h. A lot of like-flower patterns of recrystallized ultrafine grain were observed after annealing at 800 °C due to the sluggish diffusion in HEAs and the effect of ceramic rolling. In addition, after annealing at 800 °C, the fracture mode changed to shearing fracture of microvoid aggregation from cleavage/quasi-cleavage fracture of as-rolled sample due to the deposition of the second phase in the tested sample.

36 MATERIALS SCIENCE↗
NASA NTRS1986

On control of stresses in silicon web growth

It has been observed that the residual stresses and dislocations during the silicon crystal growth for photovoltaic applications are caused by thermal stresses. The temperatures along the boundaries of the silicon crystal ribbon are prescribed to meet the requirements of the crystal growth. It is shown that by allowing the temperatures to satisfy a second-order partial differential equation in the ribbon, all thermal stresses, and others induced by them, may be eliminated for the stress-free growth of the silicon crystal.

Utku, S.↗
DOE OSTI2018

EBSD characterization of boundary types after discontinuous precipitation in a grit-blasted nickel-based superalloy

Highlights: • During recrystallization, high-angle grain boundaries increased dramatically. • The reaction fronts are high-angle GBs, but are not always correspond to CSL type. • Σ3 twin boundaries have been confirmed to be positive on reducing residual stress. • No advancing reaction front corresponds to Σ3 twin boundary type. - Abstract: In the present work, direct and detailed insights with respect to the statistical information on boundary type of discontinuous precipitates and their advancing reaction fronts in a Re-containing single crystal superalloy DD6 have been obtained by EBSD. With the annealing time prolonging, the reaction fronts proceeded into the original matrix for gradual release of grit-blast induced stress. The average recrystallized grain diameter and the average depth of recrystallization were respective 0.83 μm and 3.98 μm for the specimen annealed at 1050 °C for 10 min, which, after 16 h, had grown to the average values of 3.22 μm and 31.17 μm. The reaction fronts of discontinuous precipitation have been confirmed to be high-angle grain boundaries, but do not always correspond to CSL type. Besides the role of strain energy dissipation for recrystallization, the formation of twin structure has no direct relationship with the proceeding of reaction fronts. The information of boundary type of discontinuous precipitation and its advancing reaction fronts would provide new insight into the cellular recrystallization behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tribological Performance of Austempered and Tempered Ductile Iron

Austempered ductile iron with its unique ausferritic structure is produced by an isothermal heat treatment process. Austempered ductile iron is a potential material to substitute for traditional steel castings and forgings in current industry due to its excellent mechanical properties. The tempering process is frequently used to enhance the ductility and toughness of a material and reduce residual stress. In this research, the phase transformation of austempered ductile iron was studied by applying various tempering temperatures with constant holding duration. It was found that the ausferritic structure was decomposed into dispersive cementite particles after receiving a tempering temperature of 538 °C or higher. The specific amount of retained austenite was analyzed by X-ray diffraction. The wear resistance of tempered austempered ductile iron was investigated by using a ball-on-disk sliding test configuration. The results were compared with conventional quenched and tempered ductile iron under equivalent hardness. Both austempered ductile iron and tempered austempered ductile iron samples had better wear resistance than quenched and tempered ductile iron. The results presented in this research can be utilized as a reference in the tempering treatment of austempered ductile iron material for future applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Static and Dynamic Thermomechanical Properties of Phase-Separated Epoxy Networks with Tuned Microstructures

Here, polymerization-induced phase separation is a useful method for the construction of heterogeneous epoxy networks with properties exceeding their homogeneous counterparts. In this work, we examine the static and dynamic thermomechanical properties of phase-separated epoxy networks salient to their application as encapsulants. Three heterogeneous epoxy-amine networks with nano-, meso-, and macro-phase-separated morphologies comprised of hard and soft domains are compared to a rigid, unstructured network. The glass transition profiles of the heterogeneous networks are complex, spanning many decades in the frequency domain. The nanophase-separated morphology leads to higher coefficient of thermal expansion, yet surprisingly is characterized by reduced residual stress. Under both quasi-static and dynamic compression (strain rates of order 10 –3 and 10 3 s –1 , respectively), the nanophase-separated network also exhibits higher modulus and strength. In split-Hopkinson bar experiments, the energy dissipation characteristics of the epoxy networks were nearly identical. Curiously, however, the Hugoniot response of the macro-phase-separated network determined by ballistic shockwave analysis indicates a remarkable ability of this material to mitigate shockwave propagation in comparison to many homogeneous and heterogeneous polymer materials. Collectively, this work reveals several previously unreported phenomena with respect to structure–property relationships in phase-separated epoxy networks, illustrating the potential value of systematically tuned microstructures for optimization of application-specific physical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Studies on Ti, Zn and Ti + Zn Bilayer Coatings on Interstitial Free Steel for Enhancement of Wear and Corrosion Resistance

The present study concerns a detailed investigation of the microstructure, phase identity, composition, residual stress, wear and corrosion behavior of Ti, Zn and Zn + Ti coatings developed on IF steel by sputter deposition technique. A significant improvement in average microhardness (350-700 VHN) is noticed following sputter deposition compared to that of as-received IF steel (190 VHN). The highest microhardness recorded is 500 VHN for Ti, 350 VHN for Zn and 700 VHN for Ti + Zn bilayer, respectively. Improvement in surface hardness is accompanied by a marked improvement in wear resistance of the sputter-deposited surface against hardened steel ball. The corrosion rate of coated sample in 3.56 wt.% NaCl solution is marginally decreased (2.95 × 10{sup −2} to 3.25 × 10{sup −2} mm/year) as compared to that of as-received IF steel (3.55 × 10{sup −2} mm/year). Ti-deposited IF steel registers the minimum corrosion rate (2.95 × 10{sup −2} mm/year) among all the coated samples.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Improving the Strength of the ZrC-SiC and TC4 Brazed Joint Through Fabricating Graded Double-Layered Composite Structure on TC4 Surface

In order to improve the ZrC-SiC ceramic and TC4 brazed joint property, graded double-layered SiC particles (SiCp)-reinforced TC4-based composite structure (named as GLS for convenience) was designed to relieve the residual stress in the joint. The GLS was successfully fabricated on TC4 substrate by double-layered laser deposition technology before the brazing process. The investigation of the GLS shows that the volume fraction of SiCp in the two composite layers was graded (20 and 39 vol pct, respectively). Ti{sub 5}Si{sub 3} and TiC phases formed in the GLS due to the reaction of SiCp and TC4. The laser power-II (the laser power for the second deposition layer) affected the microstructure of the GLS significantly. Increasing the laser power-II would promote the reaction between the SiCp and TC4. But the high laser power-II made the layer I remelt completely and the two layers became homogeneous rather than graded structure. In the ZrC-SiC and TC4 brazed joint, the CTE (coefficient of thermal expansion) was graded from the TC4 to the ZrC-SiC due to the GLS, and the strength of the joint with the GLS (91 MPa) was higher than that without the GLS (43 MPa).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Towards improved speed and accuracy of laser powder bed fusion simulations via multiscale spatial representations

Due to the growing popularity of laser powder bed fusion (LPBF) as a metal additive manufacturing technique, there is a strong need to be able to accurately predict build outcomes. Full fidelity simulations of this process are not feasible due to the vast range of length and time scales inherent to it. While part-scale codes for simulating residual stress and distortion have shown reasonable predictive capability, they often neglect many aspects of the process occurring over smaller length/time scales, and thus are unable to capture effects of process parameter adjustments or the behavior of fine features. One way of capturing aspects at more refined length scales is through the use of adaptive mesh refinement (AMR). AMR allows for the process to be simulated at scales approaching the physical spatial dimensions without drastically increasing the total degrees of freedom in the simulation. This manuscript describes the implementation of an AMR algorithm within a multiphysics, parallelized finite element code, and its application to the LPBF problem. In this work, part-scale examples are provided where the use of AMR has allowed for higher fidelity thermal and thermomechanical simulations, as compared to experimental measurements. Results from these higher resolution simulations show that while AMR is a necessary component for increased accuracy in a computationally efficient manner, other improvements are also necessary, including handling of the multiple time scales inherent to the problem and the need for improved AM-specific material models.

42 ENGINEERING↗
DOE OSTI2019

Computational Simulation of an Additively Manufactured Marine Component

Early work in finite-element analysis of additive manufacturing (AM) components was not able to include distortion and residual stress of the entire build, yielding uncertainty in the computational model predictions. To address this, current versions of recently developed computational tools are being used to understand their role in validation at the Naval Surface Warfare Center, Carderock Division, for laser powder-bed fusion (L-PBF) AM. A 316L stainless steel L-PBF build containing multiple parts, which exhibited the same local part failure over multiple AM fabrication runs, was selected for computational analysis. Single-part simulations analyzed optimization of computational speed and prediction accuracy. Full-build simulations exposed the smallest re-coater tolerances early in part fabrication as a result of part shrinkage and subsequent cupping. This cupping was predicted to change the powder spreading and result in the non-consolidation of the support structures, which paralleled the failures seen in the physical builds. Part redesign of the conventionally produced part to align with AM fabrication requirements was recommended to alleviate the build failures. This project is part of a larger program focused on validation of the computational tools to determine best practices for insertion of AM simulation into the greater Naval enterprise.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Correcting distortions of thin-walled machined parts by machine hammer peening

Thin-walled aerostructural components frequently get distorted after the machining process. Reworking to correct distortions or eventually rejecting parts significantly increases the cost. This paper proposes a new approach to correct distortions in thin-walled components by strategically applying hammer peening on target surfaces of a machined component. Aluminium alloy 7475-T7351 was chosen for this research. The study was divided in two stages. First, the residual stresses (RS) induced by four different pneumatic hammer peening conditions (modifying the stepover distance and initial offset) were characterised in a test coupon, and one of the conditions was selected for the next stage. In the second stage, a FEM model was used to predict distortions caused by machining in a representative workpiece. Then, the RS induced by hammer peening were included in an FEM model to define two hammer peening strategies (varying the coverage area) to analyse the capability to reduce distortions. Two workpieces were machined and then treated with the simulated hammer peening strategies for experimental validation. Results in the test coupon showed that pneumatic hammer peening can generate high compressive RS (-50 to –350 MPa) up to 800 μm depth, with their magnitude increasing with a reduced stepover distance. Application of hammer peening over 4% of the surface of the representative workpiece reduced the machining-induced distortions by 37%, and a coverage area of 100% led to and overcorrection by a factor of five. This confirms that hammer peening can be strategically applied (in target areas and changing the percentage of coverage) to correct low or severe distortions.

42 ENGINEERING↗