Showing papers by "Hamid Garmestani published in 2019"

Fabrication of core-shell structured Ni@BaTiO3 scaffolds for polymer composites with ultrahigh dielectric constant and low loss

Abstract: Dielectric composites have drawn increasing attention owing to their wide applications in electrical systems. Herein, a novel design of dielectric composites consisting of core-shell structured porous Ni@BaTiO3 scaffolds infiltrated with epoxy was developed. It is demonstrated that the dielectric constants of the composites could be as high as 6397@10 kHz, which is approximately 1777 times higher than pure epoxy matrix (er ≈ 3.6@10 kHz). Meanwhile, the dielectric loss (tanδ ≈ 0.04@10 kHz) remains comparable to that of pure epoxy (tanδ ≈ 0.01@10 kHz). It is believed that the strong charge accumulation and interfacial polarizations on the huge interfaces, especially the Ni/BaTiO3 and Ni/epoxy interfaces, give arise to the substantially enhanced er. Besides, the sintered insulating BaTiO3 coating can block the transportation of charge carriers, resulting in the low loss. The ultrahigh dielectric constants and low loss make these composites promising candidates for microstrip antennas, field-effect transistors and dielectric capacitors.

Analytical Modeling of In-Process Temperature in Powder Bed Additive Manufacturing Considering Laser Power Absorption, Latent Heat, Scanning Strategy, and Powder Packing.

Abstract: Temperature distribution gradient in metal powder bed additive manufacturing (MPBAM) directly controls the mechanical properties and dimensional accuracy of the build part. Experimental approach and numerical modeling approach for temperature in MPBAM are limited by the restricted accessibility and high computational cost, respectively. Analytical models were reported with high computational efficiency, but the developed models employed a moving coordinate and semi-infinite medium assumption, which neglected the part dimensions, and thus reduced their usefulness in real applications. This paper investigates the in-process temperature in MPBAM through analytical modeling using a stationary coordinate with an origin at the part boundary (absolute coordinate). Analytical solutions are developed for temperature prediction of single-track scan and multi-track scans considering scanning strategy. Inconel 625 is chosen to test the proposed model. Laser power absorption is inversely identified with the prediction of molten pool dimensions. Latent heat is considered using the heat integration method. The molten pool evolution is investigated with respect to scanning time. The stabilized temperatures in the single-track scan and bidirectional scans are predicted under various process conditions. Close agreements are observed upon validation to the experimental values in the literature. Furthermore, a positive relationship between molten pool dimensions and powder packing porosity was observed through sensitivity analysis. With benefits of the absolute coordinate, and high computational efficiency, the presented model can predict the temperature for a dimensional part during MPBAM, which can be used to further investigate residual stress and distortion in real applications.

Tin dioxide nanoparticles with high sensitivity and selectivity for gas sensors at sub-ppm level of hydrogen gas detection

Abstract: A wet chemical method was employed to prepare Au-loaded sensor using tin dioxide (SnO2) nanoparticles (NPs) which has excellent hydrogen (H2) gas sensing properties The structural, compositional, morphological, and electrochemical properties of these materials are characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy and electrochemical workstation, respectively The results show that the response time of Au-loaded sensor based on SnO2 NPs to 100 ppm H2 is 26 s at 350 °C, which is much shorter than that of the pristine SnO2 sensor Meanwhile, the effect of operating temperature and Au loading on ‘n’ value (factor for evaluating sensitivity response) is studied and the results demonstrated that the Au-loaded sensor based on SnO2 NPs can detect H2 gas down to 04 ppm Moreover, the Au-loaded sensor based on SnO2 NPs with an excellent selectivity to H2 gas against carbon monoxide, methane, and sulfur dioxide is illustrated in this paper, which indicates that the Au-loaded sensor using SnO2 NPs is a good candidate for practical H2 sensors and other industrial applications Statistical analysis was performed on the FESEM images of sensors based on SnO2 NPs and 05 atomic (at)% Au-loaded SnO2 NPs Roughness parameters were evaluated and a correlation was established between the morphology, topography and chemical composition of the samples

Statistical, morphological, and corrosion behavior of PECVD derived cobalt oxide thin films

Abstract: Experimental parameters have direct influences on materials properties and therefore their applications. The effect of plasma power on the properties of cobalt oxide thin films, prepared using plasma-enhanced chemical vapor deposition technique, on stainless steel substrates have been addressed in this paper. The structural, morphological, and compositional properties of these films were investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) technique. The XRD patterns demonstrated the growth of polycrystalline Co3O4 thin film with a cubic spinel structure such that the intensity of (511) and (311) peaks increase as the plasma power increases to 100 W. It is observed that crystallite size increases by increasing the plasma power and the maximum crystallite size is found to be 64.8 nm for 100 W. The AFM results illustrate that the surface roughness and grain size increase by increasing the plasma power, and the film deposited at lower plasma power has more uniform and smoother surface, mainly owing to the increase in surface diffusion that in turn causes the coalescence of the grains. The results of XPS spectra indicated the formation of Co3O4 thin films on stainless steel substrates and there were no other elements other than Co, O in the XPS spectra. Additionally, stereometric analysis and fractal dimension of the 3-D surface microtexture of the AFM micrographs were analyzed and the Kolmogorov–Smirnov test was used to assess the normal distribution of quantitative variables. The results of statistical analysis corroborated the experimental results and proved that the surface roughness increased upon an increase in plasma power. Moreover, the corrosion behavior and the surface morphology of the cobalt oxide thin films were investigated using the potentiodynamic method and scanning electron microscopy. The results of these analysis proved that as the plasma power increases the corrosion resistance improves against the H2SO4. The sample which deposited at 100 W plasma power has the minimum corrosion current and the corrosion resistance of steel substrate was improved by controlling the anodic reactions resulted from a protective Co3O4 thin film. These results are useful for building and designing stainless steel devices in corrosive environments.

Heat Source Modeling in Selective Laser Melting.

Abstract: Selective laser melting (SLM) is an emerging additive manufacturing (AM) technology for metals. Intricate three-dimensional parts can be generated from the powder bed by selectively melting the desired location of the powders. The process is repeated for each layer until the part is built. The necessary heat is provided by a laser. Temperature magnitude and history during SLM directly determine the molten pool dimensions, thermal stress, residual stress, balling effect, and dimensional accuracy. Laser-matter interaction is a crucial physical phenomenon in the SLM process. In this paper, five different heat source models are introduced to predict the three-dimensional temperature field analytically. These models are known as steady state moving point heat source, transient moving point heat source, semi-elliptical moving heat source, double elliptical moving heat source, and uniform moving heat source. The analytical temperature model for all of the heat source models is solved using three-dimensional differential equations of heat conduction with different approaches. The steady state and transient moving heat source are solved using a separation of variables approach. However, the rest of the models are solved by employing Green's functions. Due to the high temperature in the presence of the laser, the temperature gradient is usually high which has a substantial impact on thermal material properties. Consequently, the temperature field is predicted by considering the temperature sensitivity thermal material properties. Moreover, due to the repeated heating and cooling, the part usually undergoes several melting and solidification cycles, and this physical phenomenon is considered by modifying the heat capacity using latent heat of melting. Furthermore, the multi-layer aspect of the metal AM process is considered by incorporating the temperature history from the previous layer since the interaction of the layers have an impact on heat transfer mechanisms. The proposed temperature field models based on different heat source approaches are validated using experimental measurement of melt pool geometry from independent experimentations. A detailed explanation of the comparison of models is also provided. Moreover, the effect of process parameters on the balling effect is also discussed.

Analytical modeling of transient temperature in powder feed metal additive manufacturing during heating and cooling stages

Abstract: This work presents a physics-based predictive model for transient temperature during heating state and cooling state in powder feed metal additive manufacturing (PFMAM). The deposition dimension, heat transfer boundary conditions, laser absorption, and latent heat are considered in the presented model. The temperature solution is constructed from the superposition of moving point heat source solution and heat sink solution based on a stationary coordinate with respect to the part boundary. The heat source solution is activated during heating state and deactivated during cooling state. The temperature profiles and molten pool evolution were predicted with respect to the processing time in single-track deposition of PFMAM of Inconel 718. Close-agreements were observed upon validation to the experimental results in the literature. The presented model has high computational efficiency without resorting to the mesh and iterative calculation. The high prediction accuracy and high computational efficiency allow the temperature prediction for large-scale parts, and process-parameter planning through inverse analysis.

Finite element simulation of residual stress in machining of Ti-6Al-4V with a microstructural consideration:

Abstract: Machining-induced residual stress plays significant role in the corrosion resistance and fatigue life of the manufacturing end-product. The high temperature condition in the turning process could i...

Analytical Thermal Modeling of Metal Additive Manufacturing by Heat Sink Solution.

Abstract: Metal additive manufacturing can produce geometrically complex parts with effective cost. The high thermal gradients due to the repeatedly rapid heat and solidification cause defects in the produced parts, such as cracks, porosity, undesired residual stress, and part distortion. Different techniques were employed for temperature investigation. Experimental measurement and finite element method-based numerical models are limited by the restricted accessibility and expensive computational cost, respectively. The available physics-based analytical model has promising short computational efficiency without resorting to finite element method or any iteration-based simulations. However, the heat transfer boundary condition cannot be considered without the involvement of finite element method or iteration-based simulations, which significantly reduces the computational efficiency, and thus the usefulness of the developed model. This work presents an explicit and closed-form solution, namely heat sink solution, to consider the heat transfer boundary condition. The heat sink solution was developed from the moving point heat source solution based on heat transfer of convection and radiation. The part boundary is mathematically discretized into many heats sinks due to the non-uniform temperature distribution, which causes non-uniform heat loss. The temperature profiles, thermal gradients, and temperature-affected material properties are calculated and presented. Good agreements were observed upon validation against experimental molten pool measurements.

Modeling of texture development in additive manufacturing of Ni-based superalloys

Abstract: Solidification and inelastic thermal strains are the sources of development of crystallographic texture in metallic parts additively manufactured. This works provides a simulation framework to account for evolution of texture in additive manufacturing of Ni-based superalloys by considering both solidification and thermal strain contributions to formation of texture. A novel and validated physics-based approach yields the thermal strains as a result of different cooling rates in the part, and a modified crystal plasticity formalism alters a theoretical solidification texture to obtain the final orientation attributes. Results show close consistency with independent experimental reports with a better agreement, compared to current modeling approaches which only consider the contribution of solidification to texture development.

Residual stress prediction for turning of Ti-6Al-4V considering the microstructure evolution:

Abstract: An analytical model for residual stress prediction considering the effects of material dynamic recrystallization under process-induced mechanical and thermal stresses is proposed. The effect of microstructure evolution on residual stress generation during the turning process is considered. The Johnson–Mehl–Avrami–Kolmogorov model is used to calculate grain size evolution due to thermal mechanical effects in the machining process. A modified Johnson–Cook flow stress model is developed by introducing a material grain growth–induced softening term. The classic Oxley’s cutting mechanics theories are implemented for machining forces calculation. A hybrid algorithm accounting for thermal, mechanical, and microstructure evolution effects is used to predict the residual stress profile on a machined workpiece surface. The proposed method is implemented for the orthogonal turning of Ti-6Al-4V material. Comparison is conducted between the model prediction and the literature measurement residual stress data. The gene...

Residual stress prediction based on MTS model during machining of Ti-6Al-4V:

Abstract: The material microstructure attributes are largely ignored in the machining community for the machining mechanics modeling. A physical-based mechanical threshold stress (MTS) model is proposed for ...

Statistical representation of the microstructure and strength for a two-phase Ti–6Al–4V

Abstract: The strength of materials is a function of the direction of the tensile/compression test, which means, the measured strength values can be different in different directions. This discrepancy comes from the microstructure and morphological features of the material which gives it an apparent anisotropy. This study uses a layered fast spherical harmonics formulation to represent the full map of two-point statistics for two-phase materials and the statistical distribution of the strength variation. In a recent study by the authors, the strength of a two-phase microstructure of Ti–6Al–4V alloy was measured. These microstructures were used in the current study for calibration and validation of the model and the statistical variation of strength. This approach is fast and efficient due to using spherical harmonics, furthermore, the model can output strength in all directions simultaneously rather than a single direction.

Grain size sensitive–MTS model for Ti-6Al-4V machining force and residual stress prediction

Abstract: Material properties are significantly influenced by the parameters of the machining process. The accurate prediction of machining force and residual stress reduces power consumption, enhances material properties, and improves dimensional accuracy of the finished product. Traditional method using the finite element analysis (FEA) costs a significant amount of time, and the archived mechanical threshold stress (MTS) model without consideration of microstructure of the material yields inaccurate result. In this paper, a grain size–sensitive MTS model is introduced for the machining process of Ti-6Al-4V. A grain size–sensitive term is introduced to the modified MTS model to account for evolution of the grain size. The grain size–sensitive MTS model takes the microstructure of the Ti-6Al-4V into consideration for the calculation of machining force and residual stress. The grain size–sensitive term is introduced into the athermal stress component using the initial yield stress, strain hardening coefficient, and the Hall-Petch coefficient. The analytical result is compared with those of experimental studies and the traditional Johnson-Cook model to prove the validity in the prediction of machining force and residual stress. The proposed model explores a new area for calculating cutting forces and residual stress.

Through-Thickness Strain Gradient in a Hot-Rolled Al-Mg Alloy Obtained by Nanoindentation and Glancing Angle X-Ray Diffraction

Abstract: Combined nanoindentation and glancing angle X-ray diffraction (GAXD) methods were used to study the mechanical properties of near-surface microstructures (NSMs) of a hot-rolled 5xxx aluminum alloy. Nanoindentations with a sharp indenter were carried out with penetration depth ranging from 250 nm to 4500 nm at the near-surface regions as well as the bulk of the hot-rolled alloy. The primary indentation parameter presented here was the hardness, given that plastic strain obtained from GAXD experiment could be correlated with the hardness value. Analysis of nanoindentation hardness results proved that NSMs of this alloy on average were harder than the bulk due to the presence of different surface features discussed here. The GAXD experiments were performed between a minimum incident angle 0.025°, equivalent to 765 nm penetration depth, and a maximum angle 1°, equivalent to 3000 nm penetration depth. The strain-induced broadening in the diffraction peaks was calculated by the Williamson–Hall technique. Both indentation hardness and glancing angle diffraction results confirmed strain gradient as a function of depth in the NSMs of the alloy. Both techniques also showed that the thickness of the subsurface layer (i.e., several micrometers below the surface) of hot-rolled sample was approximately 2.44 μm at the tested areas. Additionally, reported results provided evidence for the ability to use glancing angle X-ray diffraction as a nondestructive tool for the measurement of subsurface layers' thickness as well as microstrain (non-uniform or plastic strain) as a function of depth.

Effects of Scan Strategy on Thermal Properties and Temperature Field in Selective Laser Melting

Abstract: Temperature field is an essential attribute of metal additive manufacturing in view of its bearings on the prediction, control, and optimization of residual stress, part distortion, fatigue, balling effect, etc. This work provides an analytical physics-based approach to investigate the effect of scan strategy parameters including time delay between two irradiations and hatching space on thermal material properties and melt pool geometry. This approach is performed through the analysis of the distribution of material properties and temperature profile in three-dimensional space. The moving point heat source approach is used to predict the temperature field. To predict the temperature field during the additive manufacturing process some important phenomena are considered. 1) Due to the high magnitude of temperature in the presence of the laser, the temperature gradient is usually high which has a crucial influence on thermal material properties. Consequently, the thermal material properties of stainless steel grade 316L are considered to be temperature-dependent. 2) Due to the repeated heating and cooling, part usually undergoes several melting and solidification cycles. This physical phenomenon is considered by modifying the heat capacity using the latent heat of melting. 3) The multi-layer aspect of metal AM process is considered by incorporating the temperature history from the previous layer since the interaction of the successive layers has an impact on heat transfer mechanisms. 4) Effect of heat affected zone on thermal material properties is considered by the superposition of material properties in regions where the temperature fields of two consecutive irradiations have an overlap since the consecutive irradiations change the behavior of the material properties. The goals are to 1) investigate the effects of temperature-sensitive material properties and constant material properties on the temperature field. 2) Study the behavior of thermal material properties under different scan strategies. 3) Study the importance of considering the effect of heat affected zone on thermal material through the prediction of melt pool geometry. 4) Investigate the effect of hatching space on melt pool geometry. This work is purely employed physics-based analytical models to predict the behavior of material properties and temperature field under different process conditions, and no finite element modeling is used.