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Article Citation - WoS: 13Citation - Scopus: 14The Effect of Strain Rate on the Hydrogen Embrittlement Susceptibility of Aluminum 7075(Asme, 2023) Baltacioglu, Mehmet Furkan; Cetin, Baris; Bal, BurakThe effects of changing the strain rate regime from quasi-static to medium on hydrogen susceptibility of aluminum (Al) 7075 were investigated using tensile tests. Strain rates were selected as 1 s(-1) and 10(-3) s(-1) and tensile tests were conducted on both hydrogen uncharged and hydrogen charged specimens at room temperature. Electrochemical hydrogen charging method was utilized and the diffusion length of hydrogen inside Al 7075 was modeled. Material characterizations were carried out by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) and microstructural observations of hydrogen uncharged and hydrogen charged specimens were performed by scanning electron microscope (SEM). As opposed to earlier studies, hydrogen embrittlement (HE) was more pronounced at high strain rate cases. Moreover, hydrogen enhanced localized plasticity (HELP) was the more dominant hydrogen embrittlement mechanism at slower strain rate but coexistence of hydrogen enhanced localized plasticity and hydrogen enhanced decohesion was observed at a medium strain rate. Overall, the current findings shed light on the complicated hydrogen embrittlement behavior of Al 7075 and constitute an efficient guideline for the usage of Al 7075 that can be subject to different strain rate loadings in service.Article Citation - WoS: 38Citation - Scopus: 47Focus Variation Measurement and Prediction of Surface Texture Parameters Using Machine Learning in Laser Powder Bed Fusion(Asme, 2020) Ozel, Tugrul; Altay, Ayca; Kaftanoglu, Bilgin; Leach, Richard; Senin, Nicola; Donmez, AlkanThe powder bed fusion-based additive manufacturing process uses a laser to melt and fuse powder metal material together and creates parts with intricate surface topography that are often influenced by laser path, layer-to-layer scanning strategies, and energy density. Surface topography investigations of as-built, nickel alloy (625) surfaces were performed by obtaining areal height maps using focus variation microscopy for samples produced at various energy density settings and two different scan strategies. Surface areal height maps and measured surface texture parameters revealed the highly irregular nature of surface topography created by laser powder bed fusion (LPBF). Effects of process parameters and energy density on the areal surface texture have been identified. Machine learning methods were applied to measured data to establish input and output relationships between process parameters and measured surface texture parameters with predictive capabilities. The advantages of utilizing such predictive models for process planning purposes are highlighted.Article Estimation of Discretization Uncertainty Using the Γ - Reθ Transition Model for Transitional Flows on 6:1 Spheroid(Asme, 2022) Atik, HediyeThis paper aims to estimate the surface mesh size related discretization uncertainties using the gamma-Re-theta transition model combined with the shear stress transport (SST) k-omega turbulence model. For comparison, this work employs an available experimental study performed with a 6:1 prolate spheroid. The grid convergence index (GCI) study is performed for axial force, surface skin friction, and pressure coefficients with three levels of meshes. The transition model estimates the axial force coefficients (CX), approximately half of which are obtained using fully turbulent calculations with higher GCI values. The GCI values around the axial force coefficients for the level-2 mesh are less than 1% based on fully turbulent calculations. However, with the transition model, these values for the same mesh level increase to 10%. While the GCI values of surface pressure coefficients are very small based on both fully turbulent and transition model calculations, these coefficients show differences at the trailing part of the spheroid. Significant differences are also observed in the surface friction coefficients. While the model captures drastic changes in terms of transition in the surface friction coefficients at the suction side of the spheroid, such drastic change is not observed in fully turbulent calculations. On the other hand, there is no sign of any transition phenomenon at the pressure side, contrary to the observations of experimental measurements. The transition model is not able to estimate the transition front geometry correctly. The GCI values of the surface friction coefficients increase dramatically, up to 765% around the transition regions.Article Citation - WoS: 4Citation - Scopus: 4Predicting and Measuring Surface Enlargement in Forward Rod Extrusion(Asme, 2016) Duran, Deniz; Ozdemir, IzzetSurface enlargement during bulk metal forming processes is one of the key parameters controlling the tribology at the tool-workpiece interface. Not only the surface roughness evolution but also the integrity of the lubricant layer critically reposes on surface enlargement. As an attempt to address this issue, in the first part of this work, a general, deformation gradient based surface enlargement description is implemented in a commercial finite element program. In the second part, forward rod extrusion tests with different area reductions are conducted using customized steel workpieces in which cylindrical copper rods are embedded through the depth. By sectioning the extruded parts and by identifying the position of the copper rods on the lateral surface, average surface enlargement values could be measured locally at different positions along the extrudate. Comparison of experiments and numerical predictions reveal that the deformation gradient based description performs reasonably well in capturing surface enlargement profiles both qualitatively and quantitatively.
