ABSTRACT The piscicide antimycin has been used as an effective fish toxicant to eradicate nuisance fishes because it is not as detectable by fish as other chemical piscicides. We evaluated the effects of antimycin and its detoxicant, potassium permanganate, on periphyton and benthic macroinvertebrates during a brook trout (Salvelinus fontinalis) restoration project in Great Smoky Mountains National Park, USA. Sites within adjacent treated and untreated streams were sampled before and after treatment to remove non‐native rainbow trout (Oncorhynchus mykiss). We observed negative effects on periphyton with a reduction in chlorophyll a up to 1‐month post‐treatment. The treatment also had significant, short‐term (< 1 month) effects on two macroinvertebrate orders (Ephemeroptera and Plecoptera) (p = 0.0003), but effects were not long‐lasting (i.e., < 1 year) (p = 0.905). These results suggest that antimycin can be used effectively to reduce non‐native fish species in sensitive coldwater streams with only short‐term effects on aquatic biodiversity and native fish food resources.
This study presents a three-dimensional numerical analysis to examine the effects of heat input (810, 900, 990, and 1080 (J/mm)) on the material flow material, heat transfer behaviors, residual stress, and distortion during multilayer deposition of Titanium-zirconium-molybdenum (TZM) alloys. Transient simulations focused on the first two and last deposited layers, predicting the shape and size of the molten pool. The results illustrate that arc force plays a critical role in influencing the material flow for each layer's deposition. Among the heat transfer modes, conduction dominates over radiation and convection. The temperature gradient, distortion, and residual stress of the WAAM-TMZ specimens deposited under various heat inputs were evaluated. The lowest heat input (810 J/mm) resulted in the minimum distortion along the height direction, attributed to the smaller temperature gradient and more uniform temperature distribution, leading to uniform dispersion of stress and reduced final distortion. The highest yield strength (195 MPa) and elongation (1.28 +/- 0.08 %) were observed in samples fabricated with the heat input of 900 J/mm. The outcome of this research is to develop an effective procedure to analyze the thermomechanical properties of wire arc additive manufacturing of refractory alloys.
Multilayer depositions with varying interface behaviors affect the mechanical properties of deposited materials, so atomic-scale deposition mechanisms provide a better understanding of material behavior under multiple diffusion conditions. In this research, molecular dynamics is applied to investigate the behavior of the interface in Ti6Al4V-NbZr1 bimetallic structure deposited by wire-arc directed energy deposition (W-DED) in various heat input conditions. In addition, the interactions between the bimetallic structure and the distribution and size of dislocation loops are studied during deformation. It was found that the nano-melting pool forms before solidification, and the crystal growth proceeds by directional solidification, which can be equiaxed or columnar. Interdiffusion of the system shows asymmetrical diffusion behavior, and Nb atoms show a greater tendency to diffuse into the matrix in higher heat input conditions. According to the cluster analysis, the cluster number decreases from 76,138 to 75,720 for the first deposited layer, whereas it increases from 88,046 to 90,309 for the final deposited layer as heat input increases. Surface roughness decreases from 1.6 to 0.9 & Aring; while the interface width increases from 30 to 50 & Aring; as the heat input increases. It was concluded that atomic-size mismatch-induced lattice distortion enhances residual stress, resulting in dislocation loops. The formation of numerous 1/6 (112) Shockley and 1/2 (111) interstitial dislocation loops, along with a low amount of (100) and mixed loops, was also observed. At the substrate-interface, the biaxial stress is compressive, whereas the deposited layers exhibit tensile behavior.
A coupled thermo-mechanical finite element framework was developed to analyze the influence of heat treatment on the thermal history, phase evolution, and mechanical responses of Inconel 625 walls fabricated by wire arc additive manufacturing (WAAM). The model captures transient heat transfer, molten pool behavior, phase transformations, and the resulting residual stress and distortion fields. Mesh sensitivity analysis was performed to ensure numerical stability and computational efficiency. The simulations demonstrate that heat treatment promotes a more homogeneous phase distribution and stabilizes the arc process, resulting in a more uniform residual stress profile. Temperature fields increase nonlinearly with build height, and the stress state transitions from tensile near the substrate to compressive in upper layers. Distortion is more sensitive to heat treatment than residual stress, with notable reductions observed in thin-wall geometries. A multi-material configuration combining stainless steel and Inconel 625 was also modeled, revealing sharp interfacial transitions and characteristic morphological features consistent with experimental observations. Overall, the results provide mechanistic insight into heat-treatment-driven improvements in structural integrity, offering guidance for optimizing WAAM process parameters for both single- and multi-material builds.
We establish finite-sample closed-loop stability guarantees for Model Predictive Path Integral (MPPI) control applied to discrete-time Linear Time-Invariant (LTI) systems with additive Gaussian process disturbances. The key observation is that, for unconstrained LTI/quadratic systems with the DARE terminal cost, the exact finite-horizon MPC law has the same first control action as the infinite-horizon LQR law for every planning horizon. Thus, finite-sample MPPI can be analyzed as a stochastic perturbation of LQR. First, we show that the MPPI control law approximates the LQR feedback with high probability. The approximation error decomposes into a Monte Carlo term that decreases with the sample count and an infinite-sample temperature bias that persists at finite temperature but vanishes as the temperature is reduced. The resulting constants are written in terms of the horizon-dependent stacked cost matrices, making explicit that the finite-sample certificate is parametrized by the selected planning horizon. Second, we use a Lyapunov perturbation argument to prove practical exponential stability in expectation. On sample paths that remain in a compact Lyapunov sublevel set over a finite operating horizon, the expected state norm decays exponentially up to three residual floors: a process-noise floor, an MPPI approximation floor, and a confidence floor from the per-step sampling failure probability. The sufficient sample threshold is explicit and computable from the DARE solution, LQR stability margin, MPPI sampling parameters, temperature, and planning horizon. In the joint limit of infinite samples and vanishing temperature bias, the result recovers the stochastic LQR stability bound.
