Fe-Co-2V is a soft ferromagnetic alloy used in electromagnetic applications due to excellent magnetic properties. However, the discontinuous yielding (Luders bands), grain-size-dependent properties (Hall-Petch behavior), and the degree of order/disorder in the Fe-Co-2V alloy makes it difficult to predict the mechanical performance, particularly in abnormal environments such as elevated strain rates and high/low temperatures. Thus, experimental characterization of the high strain rate properties of the Fe-Co-2V alloy is desired, which are used for material model development in numerical simulations. In this study, the high rate tensile response of Fe-Co-2V is investigated with a pulse-shaped Kolsky tension bar over a wide range of strain rates and temperatures. Effects of temperature and strain rate on yield stress, ultimate stress, and ductility are discussed.
Residual stress is a common result of manufacturing processes, but it is one that is often overlooked in design and qualification activities. There are many reasons for this oversight, such as lack of observable indicators and difficulty in measurement. Traditional relaxation-based measurement methods use some type of material removal to cause surface displacements, which can then be used to solve for the residual stresses relieved by the removal. While widely used, these methods may offer only individual stress components or may be limited by part or cut geometry requirements. Diffraction-based methods, such as X-ray or neutron, offer non-destructive results but require access to a radiation source. With the goal of producing a more flexible solution, this LDRD developed a generalized residual stress inversion technique that can recover residual stresses released by all traction components on a cut surface, with much greater freedom in part geometry and cut location. The developed method has been successfully demonstrated on both synthetic and experimental data. The project also investigated dislocation density quantification using nonlinear ultrasound, residual stress measurement using Electronic Speckle Pattern Interferometry Hole Drilling, and validation of residual stress predictions in Additive Manufacturing process models.
Intermetallic alloys possess exceptional soft magnetic properties, including high permeability, low coercivity, and high saturation induction, but exhibit poor mechanical properties that make them impractical to bulk process and use at ideal compositions. We used laser-based Additive Manufacturing to process traditionally brittle Fe–Co and Fe–Si alloys in bulk form without macroscopic defects and at near-ideal compositions for electromagnetic applications. The binary Fe–50Co, as a model material, demonstrated simultaneous high strength (600–700 MPa) and high ductility (35%) in tension, corresponding to a ∼300% increase in strength and an order-of-magnitude improvement in ductility relative to conventionally processed material. Atomic-scale toughening and strengthening mechanisms, based on engineered multiscale microstructures, are proposed to explain the unusual combination of mechanical properties. This work presents an instance in which metal Additive Manufacturing processes are enabling, rather than limiting, the development of higher-performance alloys.
Cylindrical dog-bone (or dumbbell) shaped samples have become a common design for dynamic tensile tests of ductile materials with a Kolsky tension bar. When a direct measurement of displacement between the bar ends is used to calculate the specimen strain, the actual strain in the specimen gage section is overestimated due to strain in the specimen shoulder and needs to be corrected. The currently available correction method works well for elastic-perfectly plastic materials but may not be applicable to materials that exhibit significant work-hardening behavior. In this study, we developed a new specimen strain correction method for materials possessing an elastic-plastic with linear work-hardening stress–strain response. A Kolsky tension bar test of a Fe-49Co-2V alloy (known by trade names Hiperco and Permendur) was used to demonstrate the new specimen strain correction method. This new correction method was also used to correct specimen strains in Kolsky tension bar experiments on two other materials: 4140 alloy, and 304L-VAR stainless steel, which had different work-hardening behavior.
Control of the atomic structure, as measured by the extent of the embrittling B2 chemically ordered phase, is demonstrated in intermetallic alloys through additive manufacturing (AM) and characterized using high fidelity neutron diffraction. As a layer-by-layer rapid solidification process, AM was employed to suppress the extent of chemically ordered B2 phases in a soft ferromagnetic Fe-Co alloy, as a model material system of interest to electromagnetic applications. The extent of atomic ordering was found to be insensitive to the spatial location within specimens and suggests that the thermal conditions within only a few AM layers were most influential in controlling the microstructure, in agreement with the predictions from a thermal model for welding. Analysis of process parameter effects on ordering found that suppression of B2 phase was the result of an increased average cooling rate during processing. AM processing parameters, namely interlayer interval time and build velocity, were used to systematically control the relative fraction of ordered B2 phase in specimens from 0.49 to 0.72. Hardness of AM specimens was more than 150% higher than conventionally processed bulk material. Implications for tailoring microstructures of intermetallic alloys are discussed.
This research applies nonlinear ultrasonic techniques for the quantitative characterization of additively manufactured materials. The characterization focuses on identifying the dislocation density produced during the additive constructive process in order to increase confidence on a part's performance and the success of the manufacturing process. Second harmonic generation techniques based on the transmission of Rayleigh surface waves are used to measure the ultrasonic nonlinearity parameter, β, which has proven a quantitative indicator of dislocations but has not been fully proven in additive manufactured materials. 316L and 304L stainless steel parts made from Powder Bed Fusion and Laser Engineered Net Shaping are compared between AM techniques and with wrought manufactured counterparts. β is consistently higher for additive manufactured parts. An annealing heat treatment is applied to each specimen to reduce dislocation density. β expectedly decreases by annealing in all specimens. A linear ultrasonic measurement is made to evaluate the effectiveness of using nonlinear techniques. The ultrasonic attenuation is higher for additive manufactured parts and increases at higher frequencies.
A collaborative testing and analysis effort investigating the effects of threaded fastener size on load-displacement behavior and failure was conducted to inform the modeling of threaded connections. A series of quasistatic tension tests were performed on #00, #02, #04, #06 and #4 (1/4”) A286 stainless steel fasteners (NAS1351N00-4, NAS1352N02-6, NAS1352N04-8, NAS1352N06-10, and NAS1352N4-24, respectively) to provide calibration and validation data for the analysis portion of the study. The data obtained from the testing series reveals that the size of the fastener may influence the characteristic stress-strain response, as the failure strains and ultimate loads varied between the smaller (#00 and #02) and larger (#04, #06, and #4) fasteners. These results motivated the construction of high-fidelity finite element models to investigate the underlying mechanics of these responses. Two threaded fastener models, one with axisymmetric threads and the other with full 3D helical threads, were calibrated to subsets of the data to compare modeling approaches, analyze fastener material properties, and assess how well these calibrated properties extend to fasteners of varying sizes and if trends exist that can inform future best modeling practices. The modeling results are complemented with a microstructural analysis to further investigate the root cause of size effects observed in the experimentally obtained load-displacement curves. These analyses are intended to inform and guide reduced-order modeling approaches that can be incorporated in system level analyses of abnormal environments where modeling fidelity is limited and each component is not always testable, but models must still capture fastener behavior up to and including failure. This complimentary testing and analysis study identifies differences in the characteristic stress-strain response of varying sized fasteners, provides microstructural evidence to support these variations, evaluates our ability to extrapolate calibrated properties to different sized fasteners, and ultimately further educates the analysis community on the robustness of fastener modeling.