Thin Titanium Foils Research Articles

An efficient method for processing high-speed infrared images of nucleate boiling on thin heaters at low heat flux

In recent years, there have been many studies on boiling with high-speed infrared cameras. The infrared image contains not only the information of temperature field, but also the information of bubble dynamics. The identification of different heat transfer modes is fundamental to the identification of bubbles. Although many studies have proposed their own methods, they differ greatly from each other and require laborious and time-consuming manual trial and error processes. The research community is in dire need of an efficient method for infrared image processing. This article proposes a self-adaptive method based on statistics to identify different heat transfer modes and nucleation sites which can be executed by computer automatically and rapidly. A series of pool nucleate boiling experiments using thin titanium foil heaters are carried out, and the temperature and heat flux distributions are analyzed. Based on distinguishing outliers from heat flux data, this method can identify different heat transfer modes (evaporation, convection and transient conduction), and then nucleation sites and bubble dynamics parameters. The experimental data and other open-access data from literatures are used to for validation. A number of evaluation parameters are considered and relatively satisfactory performance are achieved.

Surface characteristics and in vitro biocompatibility of surface-modified titanium foils as a regenerative barrier membrane for guided bone regeneration.

This study evaluated surface characteristics and biocompatibility of surface-modified thin titanium (Ti) foils as a regenerative barrier membrane for future application in guided bone regeneration (GBR) surgery to augment atrophic alveolar bone. Anodic oxidation and post-heat treatment were performed to prepare various Ti foil samples. Then, the in vitro soft and hard tissue compatibility of the samples was evaluated by examining the cell responses using primary human gingival fibroblasts (HGFs) and MG63 human osteoblast-like cells. Investigated Ti foil samples showed marked differences in physicochemical surface properties. Additional 400°C heat treatment applied to the anodized Ti surface led to formation of an anatase titanium dioxide structure and well-organized nanoscale protrusions, and significantly increased surface wettability. Anodization and heat treatment enhanced the growth of HGFs and MG63 cells in Ti foil samples. Additional heat treatment for 10 and 30min further significantly improved the response of HGFs including spreading and proliferation, and upregulated the mRNA expression of cell adhesion- and maturation-related genes as well as the osteoblast differentiation of MG63 cells. Ti foil sample with thin oxide coating obtained by a 30min heat treatment exhibited poor clinical plasticity as a regenerative barrier membrane, which showed complete coating failure in the bending test. Our results indicate that anatase Ti oxide coating of a specific film thickness with nanoscale surface protrusion morphology and hydrophilic characteristics obtained by anodization and post-heat treatment would be an effective approach as a biocompatible Ti regenerative membrane for inducing better regeneration of both gingival tissue and bone.

Effective photoelectrocatalytic reduction of CO2 to formic acid using controllably annealed TiO2 nanoparticles derived from porous structured Ti foil

The rate of global warming and unfavorable climate changes caused by the drastic upsurge of carbon dioxide (CO2) emission has necessitated the development of approaches to limit the significantly high concentration of CO2 in the atmosphere. The photoelectrochemical reduction of CO2 results in a reduction of the energy required to transform this greenhouse gas into valuable end products. In this study, we fabricated cost-effective and novel 3D nanoporous structured (3DNS) TiO2 nanoparticles (T-NPs) on the surface of a thin titanium foil (T-foil) by chemical treatment with hydrogen peroxide (H2O2) followed by calcination at high temperatures in the range of 400–800 °C. The as-proposed samples were analyzed by several characterizations such as XRD, XPS, TEM, and Raman spectroscopy. At 600 °C, the anatase-dominated mixed phases of calcinated T-foil (TO600) were seen, and a maximum photocurrent density of 71.5 µA/cm2 was obtained, in comparison to the T-foils treated at other temperatures (TO400, TO500, TO700, and TO800). Because of the better photocurrent density, TO600 was selected as the photocathode material for photoelectrochemical CO2 reduction performed with or without the presence of solar light. The lowest CO2 reduction onset potential (−1.191 V) was observed on the TO600 sample in the presence of light with Ag/AgCl as the reference electrode. 1H NMR analysis of the product solution revealed the formation of formic acid as the major product of the CO2 reduction reaction after the chronoamperometric electrolysis was performed for more than 25 h. The maximum faradaic efficiency (64%) and formic acid yield (165 µmol cm−2 h−1) were obtained at an applied potential of − 1.3 V (vs. Ag/AgCl reference electrode) for TO600.

Mass transport limitations in polymer electrolyte water electrolyzers using spatially-resolved current measurement

This work utilizes spatially-resolved current measurements to provide insight into mass transport limitations in electrolyzers that are not observable from traditional polarization measurement. In this study, two types of flow-fields (parallel and triple-serpentine) and two types of diffusion media (patterned porous thin titanium foil LGDLs and Ir-coated titanium felt PTLs) were examined. A non-uniform current distribution dominated by mass transport limitations was observed to be instigated by the restriction of liquid water transport to catalyst sites. Additionally, conditions are revealed which yield similar polarization performance but dissimilar current distributions. In such cases, the transport limitations for different architectures and porous media affect polarization in different regions of the active area. Furthermore, the triple-serpentine flow-field used in this study performs better than the parallel flow-field under mass transport limited operating conditions. This indicates that the parallel flow-field used in this study is more susceptible to starvation than the triple-serpentine flow-field for the electrolyzer studied.

Efficient photoelectrocatalytic conversion of CO2 to formic acid using Ag-TiO2 nanoparticles formed on the surface of nanoporous structured Ti foil

Global warming and adverse climate change, which have been intensified by a strident increase in carbon dioxide (CO2) emissions, have necessitated the development of alternative techniques to reduce the disproportionate concentration of CO2 in the atmosphere. The photoelectrochemical reduction of CO2 is a technique of lowering the energy required to convert greenhouse gases into useful end products. Herein, we have manufactured an innovative, cost-effective silver (Ag) decorated anatase TiO2 (TO-Agx; ‘x’ stands for different concentration of Ag) nanoparticles which created on the 3D nanoporous structured surface of a thin titanium foil (Ti-foil) by the assist of chemical treatment with hydrogen peroxide (H2O2) and different concentrations (1, 5, 10, 20 mM) of silver nitrate (AgNO3) solution and followed by calcination at 500 °C. As-prepared samples were analyzed by several characterization techniques such as XRD, XPS, TEM and Raman spectroscopy. Among various samples (TO, TO-Ag1, TO-Ag5, TO-Ag20), the TO-Ag10 sample were exposed a supreme photocurrent density of 83.2 µA/cm−2 (86.1% higher than TO sample which is untreated with AgNO3 solution). Because of its high photocurrent density, the sample TO-Ag10 were selected as the electrode material for photoelectrochemical CO2 reduction reaction and a lowest reduction onset potential (−1.018 V) was observed on linear sweep voltammetry analysis in the presence of light with Ag/AgCl reference electrode. 1H NMR analysis of the product solution exposed the production of formic acid as a single product of CO2 reduction reaction after the chronoamperometric electrolysis were carried out more than 6 h. The maximum faradaic efficiency (73%) and formic acid yield (193 µmol cm−2 h−1) were found at an applied potential of −1.2 V (vs. Ag/AgCl reference electrode) for TO-Ag10 photocathode.

IR measurements of heat transfer coefficients and nucleation parameters during saturated nucleate boiling of water–glycerin mixtures

Heat transfer coefficients and nucleation parameters were experimentally measured during saturated nucleate boiling of water–glycerin mixtures on a thin smooth titanium foil. Infrared thermometry was employed to study boiling of water–glycerin mixtures at atmospheric pressure in water mass fraction range from 100 % down to 60 % and heat flux range from 0 to 200 kW m−2. Heat transfer coefficients were found to deteriorate with an increase of concentration of glycerin in the boiling mixture. The deterioration was more pronounced for mixtures with lower water mass fraction and for higher heat fluxes. Bubble departure diameters were observed to be weakly dependent on heat flux and independent of composition of the boiling mixture. Nucleation frequencies were found to increase with heat flux and to be inversely proportional to water mass fraction of the boiling mixture. Thermal energy transferred during nucleation of individual bubbles was independent of both heat flux and mixture composition. Values and trends of evaluated parameters suggest that for investigated mixtures and boiling conditions, heat transfer into the liquid phase is more important relative to the transport of latent heat by nucleating bubbles.

Experimental investigation of single-bubble growth during the saturated pool boiling of water and self-rewetting aqueous n-butanol mixtures

Self-rewetting fluids are commonly used in heat pipes, but their performance is not well understood in pool boiling. Here, we performed boiling of n-butanol-water mixtures on thin titanium foil and utilized synchronous high-speed IR and video cameras to capture transient temperature fields and bubble dynamics. At the onset of nucleate boiling, self-rewetting fluids offered higher nucleation site density and smaller bubble diameters compared to water. However, with increasing heat flux, a significant depletion of n-butanol was created in the vicinity of nucleation sites, which caused periodic temporal fluctuations in nucleation temperature and bubble evaporation energy. Average surface temperature and bubble departure diameter for self-rewetting fluids even exceeded the values of pure water. This shows that Marangoni effects cannot overcome limited mass diffusion of more volatile component towards nucleation sites in pool boiling on flat horizontal heaters. Based on our observations, the use of a self-rewetting fluid will not increase the heat transfer coefficient when compared to pure water, which is also known for conventional mixtures with negative temperature-dependent surface tension. Our experiments offer a step towards better understanding nucleate boiling of complex mixtures and a possibility for indirect evaluation of concentration variations based on synchronous high-speed video and IR thermography.

On the seam welding of ultra-thin commercially pure (CP) grade-2 titanium foils

ABSTRACT This communication reports a comparative study on the seam welding of CP grade-2 Ti foils with thicknesses of 10 µm, 50 µm and 100 µm using a repetitive nanosecond pulsed fibre laser and a lamp-pumped pulsed Nd-YAG laser. The welding was accomplished in a lap joint configuration. A fine acicular microstructure was observed in the welds made by the nano second fibre laser, whilst a coarse basket weave microstructure was observed in the welds made by the Nd-YAG laser. The tensile properties of the welds carried out by the nano-second fibre laser was found to be superior with respect to the welds carried out by the pulsed Nd-YAG laser. Welds made by the nano-second fibre laser could withstand ~17% more load, whereas elongation before fracture was almost double to that of the welds made by the Nd-YAG laser. This variation is comprehensible on the basis of the finer weld microstructure evolved in nanosecond laser welding due to the very low heat input and high cooling rate associated with this process. This study established with certitude that nanosecond duration pulsed fibre lasers are a better choice for welding thin titanium foils in comparison to long pulsed Nd-YAG lasers.

Weak Ionization Induced Interfacial Deposition and Transformation towards Fast‐Charging NaTi2(PO4)3 Nanowire Bundles for Advanced Aqueous Sodium‐Ion Capacitors

AbstractAqueous sodium‐ion capacitors (ASICs) offer great promise for inexpensive and safe energy storage. However, their development is plagued by a kinetics imbalance at high rates between battery and capacitive electrodes and a narrow voltage window due to water electrolysis. Here a unique nanowire bundles anode is designed that simultaneously affords ultrahigh rate capability and manifests robust Na+ insertion to suppress hydrogen evolution, enabling an advanced ASIC. The NaTi2(PO4)3 (NTP) is grown on thin titanium foil by elaborately utilizing the weak ionization chemistry of NaH2PO4 (NHP), where single‐agent NHP not only partially etches titanium to release TiO2+, but also induces the interfacial phase‐transformation of pre‐deposited orthomorphic Na4Ti(PO4)2(OH)2 cubes to hexagonal NTP nanowires. This anode has hierarchical architectures to facilitate charge and mass transport, thus working stably at considerably high rates of 15–150 C with high capacities. The first 2.4 V flexible solid‐state NTP‐based ASIC is designed with high energy densities (5.8–12.8 mWh cm−3; 57.9–62.1 Wh kg−1; total mass loading up to 8.1 mg cm−2) comparable to NASICON‐based devices using organic electrolytes, demonstrating outstanding stability of 10 000 cycles and no performance decay even after continuous bending at 180o. This work presents a versatile strategy to construct NASICON phosphate electrodes for advanced energy storage.

2D monochromatic x-ray imaging for beam monitoring of an x-ray free electron laser and a high-power femtosecond laser.

In pump-probe experiments with an X-ray Free Electron Laser (XFEL) and a high-power optical laser, spatial overlap of the two beams must be ensured to probe a pumped area with the x-ray beam. A beam monitoring diagnostic is particularly important in short-pulse laser experiments where a tightly focused beam is required to achieve a relativistic laser intensity for generation of energetic particles. Here, we report the demonstration of on-shot beam pointing measurements of an XFEL and a terawatt class femtosecond laser using 2D monochromatic Kα imaging at the Matter in Extreme Conditions end-station of the Linac Coherent Light Source. A thin solid titanium foil was irradiated by a 25-TW laser for fast electron isochoric heating, while a 7.0 keV XFEL beam was used to probe the laser-heated region. Using a spherical crystal imager (SCI), the beam overlap was examined by measuring 4.51 keV Kα x rays produced by laser-accelerated fast electrons and the x-ray beam. Measurements were made for XFEL-only at various focus lens positions, laser-only, and two-beam shots. Successful beam overlapping was observed on ∼58% of all two-beam shots for 10 μm thick samples. It is found that large spatial offsets of laser-induced Kα spots are attributed to imprecise target positioning rather than shot-to-shot laser pointing variations. By applying the Kα measurements to x-ray Thomson scattering measurements, we found an optimum x-ray beam spot size that maximizes scattering signals. Monochromatic x-ray imaging with the SCI could be used as an on-shot beam pointing monitor for XFEL-laser or multiple short-pulse laser experiments.

Target normal sheath acceleration with a large laser focal diameter

The dependence of the laser-driven ion acceleration from thin titanium foils in the Target Normal Sheath Acceleration (TNSA) regime on target and laser parameters is explored using two dimensional particle-in-cell simulations. The oblique incidence (θL=45°) and large focal spot size (w0=40μm) are chosen to take an advantage of quasi one-dimensional geometry of sheath fields and effective electron heating. This interaction setup also reveals low and achromatic angular divergence of a proton beam. It is shown that the hot electron temperature deviates from the ponderomotive scaling for short laser pulses and small pre-plasmas. This deviation is mainly due to the laser sweeping, as the short duration laser pulse each moment in time effectively heats only a fraction of a focal spot on the foil. This instantaneous partial heating results in an electron temperature deviation from the ponderomotive scaling and, thus, lower maximum proton energies than it could have been expected from the TNSA theory.

Spatially-Resolved Current Distribution Measurements in Polymer Electrolyte Water Electrolyzers

Operational cost, primarily power consumption, is one of the main cost obstacles for hydrogen produced by polymer electrolyte water electrolyzers (PEWE). Roughly 70% of the hydrogen production cost stems from electricity [2]. Thus, overall process efficiency has a direct influence on the hydrogen price. Dominating sources of electrochemical loss in PEWE cells are the kinetic, electric, and mass transport overpotentials. Ex-situ and modeling work have yielded insights into the physical processes occurring within an operating PEWE, but there is still need for in-situ measurements for verification [3]. Work carried out by Zhang et al. [4, 5] has successfully shown the bubble formation of water electrolysis inside a PEWE by means of an in-situ, high-speed micro-scale visualization system (HMVS). However the HMVS does not explicitly capture the local electrochemical behavior of the cell. Therefore, complex CFD models have been implemented to shed light on the inner workings of PEWE; such models need to be validated with experimental data. This work aims to provide a reliable source of experimental data by developing a technique that allows for in-situ spatial and temporal current distribution measurements, which can be used to validate existing modeling efforts.Current distribution measurements are described extensively in the works of Bender et al. [6, 7] and Mench et al. [1, 8]; a major innovation was the inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a (4.5 mm x 4.5 mm segments in this work). The measured voltage drop across each shunt resistor is used to calculate the current in each segment. Passive data acquisition on the distributed current board is done simultaneously to electrochemical control of the electrolyzer, without interrupting its operation. This apparatus is capable of using flow fields of both 5cm2 (16 segments) and 25cm2 active area (100 segments). The distributed current is then interpolated to obtain smoothed contours as shown in Figure 1b. Preliminary current distribution experiments were carried out at 80⁰C with a water flow rate of 0.5 to 15 mL/min/cm2. Experiments were also carried out with two types of porous transport layers from Zhang and co-workers [4, 5]: thin titanium foil LGDLs with 80 nm pore size and Ir-coated titanium felt. Two types of flow field where also used – parallel and triple serpentine. Current distributions were found to respond to transport layer porosity and thickness, flow field design, flow rate, and current density. A non-uniform current distribution is suspected to be ultimately dominated by mass transport characteristics and these experiments support that. Additionally, conditions that yielded near-identical polarization performance were found to have disparate current distributions. This work demonstrates the benefit of a spatially-resolved current distribution measurement technique and the insights it offers which would otherwise be impossible to probe. This work is part of DOE project #DE-EE0008426 "Developing novel electrodes with ultralow catalyst loading for high-efficiency hydrogen production in proton exchange membrane electrolyzer cells." References Clement, J.T., D.S. Aaron, and M.M. Mench, In Situ Localized Current Distribution Measurements in All-Vanadium Redox Flow Batteries. Journal of The Electrochemical Society, 2015. 163(1): p. A5220-A5228.Grigoriev, S., V. Porembsky, and V. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy. International Journal of Hydrogen Energy, 2006. 31(2): p. 171-175.Schuler, T., T.J. Schmidt, and F.N. Büchi, Polymer Electrolyte Water Electrolysis: Correlating Performance and Porous Transport Layer Structure: Part II. Electrochemical Performance Analysis. Journal of The Electrochemical Society, 2019. 166(10): p. F555-F565.Mo, J., et al., Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting. Science Advances, 2016. 2(11): p. e1600690.Li, Y., et al., Wettability effects of thin titanium liquid/gas diffusion layers in proton exchange membrane electrolyzer cells. Electrochimica Acta, 2019. 298: p. 704-708.Bender, G., M.S. Wilson, and T.A. Zawodzinski, Further refinements in the segmented cell approach to diagnosing performance in polymer electrolyte fuel cells. Journal of Power Sources, 2003. 123(2): p. 163-171.Reshetenko, T.V., et al., A segmented cell approach for studying the effects of serpentine flow field parameters on PEMFC current distribution. Electrochimica Acta, 2013. 88: p. 571-579.Ertugrul, T.Y., et al., In-situ current distribution and mass transport analysis via strip cell architecture for a vanadium redox flow battery. Journal of Power Sources, 2019. 437: p. 226920. Figure 1

Trade-off analysis of C12A7:e− deposition techniques applied to Low Work Function Tethers

Due to its extraordinary properties, the C12A7:e− electride has been suggested in the past as one of the most promising materials for coating Low Work-function Tethers (LWTs). Such subclass of electrodynamic tether, made of a conductive substrate coated with materials that enhance the electron emission through the thermionic and photoelectric effects, would constitute a fully passive and consumable-free device for deorbiting space debris from Low Earth Orbit. This work presents experimental results on the manufacturing and testing of small tape-like LWT samples, which is the relevant geometry for electrodynamic tether applications. After preparing C12A7:e− targets under specific and controlled conditions, thin titanium foils have been coated with the C12A7:e− electride by using two different physical vapor deposition techniques: magnetron sputtering (MS) and pulsed laser deposition (PLD). In the case of MS, important difficulties and defects were found, including target damage, poor growth rate, and oxidation and changes in the composition of the coating. However, the performance on the PLD coating was radically different. First, Rutherford backscattering spectrometry confirmed that the composition of the thin film with the PLD coincides with that of the target. Second, X-ray photoelectron spectroscopy and thermionic emission experiments showed that the work function of the LWT sample is 2.6 ± 0.1 eV. Therefore, the trade-off analysis indicates that PLD can be used to manufacture LWT samples with work function close to the one of the bulk C12A7:e− (2.4 eV). Nevertheless, its application as passive cathode in electrodynamic tethers still requires further research activities.

Monochromatic 2DKαEmission Images Revealing Short-Pulse Laser Isochoric Heating Mechanism

The rapid heating of a thin titanium foil by a high intensity, subpicosecond laser is studied by using a 2D narrow-band x-ray imaging and x-ray spectroscopy. A novel monochromatic imaging diagnostic tuned to 4.51keV Ti Kα was used to successfully visualize a significantly ionized area (⟨Z⟩>17±1) of the solid density plasma to be within a ∼35 μm diameter spot in the transverse direction and 2 μm in depth. The measurements and a 2D collisional particle-in-cell simulation reveal that, in the fast isochoric heating of solid foil by an intense laser light, such a high ionization state in solid titanium is achieved by thermal diffusion from the hot preplasma in a few picoseconds after the pulse ends. The shift of Kα and formation of a missing Kα cannot be explained with the present atomic physics model. The measured Kα image is reproduced only when a phenomenological model for the Kα shift with a threshold ionization of ⟨Z⟩=17 is included. This work reveals how the ionization state and electron temperature of the isochorically heated nonequilibrium plasma are independently increased.

Properties of Polyimide Film after Electron Beam Irradiation with a Dose of 1 GGy

The results of the performance tests of a three-layer foil separator for a high-current electron gun of an ELA excimer laser are presented. The separator consisted of two layers of thin titanium foil with a polyimide film between them. Such a composition withstood 5630 electron beam pulses with duration of 80 ns and an energy density per pulse of about 9 J/cm2. In this case, the polyimide film (Dupont Kapton polyimide film) with a thickness of 38 μm was irradiated with a dose of 1 GGy. The initial and induced absorption of this film was measured in the range from 20 to 0.4 μm. The mechanical tensile strength of the irradiated film decreased by 17 times.

Свойства полиимидной пленки после облучения электронным пучком с дозой 1 GGy

AbstractThe results of the performance tests of a three-layer foil separator for a high-current electron gun of an ELA excimer laser are presented. The separator consisted of two layers of thin titanium foil with a polyimide film between them. Such a composition withstood 5630 electron beam pulses with duration of 80 ns and an energy density per pulse of about 9 J/cm^2. In this case, the polyimide film (Dupont Kapton polyimide film) with a thickness of 38 μm was irradiated with a dose of 1 GGy. The initial and induced absorption of this film was measured in the range from 20 to 0.4 μm. The mechanical tensile strength of the irradiated film decreased by 17 times.

Modeling, simulation and experimental validation of fatigue behavior of thin-film titanium membranes

The next generation fully implantable pressure sensors are valuable for intracranial pressure (ICP) monitoring, particularly in the chronic conditions of hydrocephalus. However, the accuracy, particularly in terms of the sensor drift over long duration, is a key concern. An implantable pressure monitoring system will rely on a flexible thin-film membrane as part of the pressure sensor and will interfere with a corrosive fluid (saline/blood) at a temperature of approximately 37 \(^\circ \hbox {C}\). The physics of the underlying thin film (material aging, mechanical fatigue), independent of the surrounding medium, triggers drift in the long-term monitoring of ICP. Therefore, finite element modeling (FEM) of thin-film deflection and fatigue life are essential. Although the FEM provides a theoretical view of the underlying issue, it is also necessary to validate the accuracy of the model. In this paper, we present both the numerical modeling and experimental validation of thin-film deflection and fatigue life of thin film titanium foils. The salient feature of this work is the approach of the thin-film deflection and fatigue testing (as part of the entire sensor) in contrast to the standard dog-bone based fatigue testing.

Measurement of excitation functions of residual radionuclides from natTi(p,x) reactions up to 44 MeV

We studied the production of medically applicable scandium radioisotopes through the natTi(p,x) reactions with in the energy range of their threshold to 44 MeV. The excitation functions of natTi(p,x) reactions and thick target yield of scandium radioisotopes (43,44m,44g,46,47,48Sc) and by-product (48V) were also measured. For this purpose, pure thin titanium foils were stacked and irradiated with different proton energies by using the MC-50 cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS), Korea. The data acquisition and activation analysis was performed by using an off-line γ-ray spectrometry technique. The measured results were compared with literature data as well as the data from the TENDL-2017 library based on TALYS-1.9 code. The integral yields for the thick target using the measured cross section were also presented. The results reveal that natTi(p,x) reaction is one of the best route for the production of medical radioisotopes of 43,44m,44g,46,47,48Sc with controllable impurity of 48V.

Grain size effect on mechanical behavior of thin pure titanium foils at elevated temperatures

To contribute to process design of microforming at elevated temperatures, the effect of grain size on mechanical behavior of pure titanium foils was studied. Uniaxial tensile tests and microbending tests were performed at different temperatures of 298, 433, 573, and 723 K and at different strain rates of 10−3, 10−2, and 10−1 s−1. Pure titanium (Ti) foils with various grain sizes of 2.7–42.4 µm were used. As results, flow stress of Ti foils with a larger grain size showed stronger dependency on the temperature and strain rate conditions. In particular, both the reduction rate of the yield stress and that of the ultimate tensile strength relative to that at 298 K increased with increasing grain size at higher strain rates of 10−2 and 10−1 s−1. However, only at a higher temperature and at a lower strain rate of 10−3 s−1, the flow stress increased for the foils with larger grain sizes. This is attributed to the dynamic strain aging, which is known as hardening behavior due to the diffusion of additional inclusion under higher temperatures. Furthermore, in microbending tests, the reduction rate of springback angle by increasing temperature also increased with increasing grain size. This is attributed to the increased reduction of strain gradient of the foils with a larger grain size. To clarify the mechanism of grain size effect on the mechanical behavior of pure Ti foils deformed at high strain rates, a composite constitutive model involving statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) was built with the capability of being employed at elevated temperatures. The effect of strain gradient on the material behavior was discussed in terms of its relationship with the density of GNDs.

Investigation on a Novel Laser Impact Spot Welding

In this paper a novel laser impact spot welding (LISW) method is described, in which a hump was formed on the flyer plate on the intended welding spot location by local pre-forming. When the flyer and base plates were placed together to perform welding, the two plates kept in contact over their entire surfaces except at the hump, where a local air gap was enough to guarantee the impact velocity and collision angle to achieve spot welding using laser pulse energy. The presented approach was implemented to join thin titanium foils to copper foils under low laser energy system. Joints with regular shapes were obtained. The microstructure in the weld interface was studied with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was found that the jetting occurred at the central region of the weld spots due to oblique impact. Wave features were observed in the weld interfaces. The impact energy was found to have significant influence on the wave’s characteristics. Moreover, SEM images and EDS analysis did not show apparent element diffusion across the weld interface. Besides, the lap shearing test was used to characterize the mechanical properties of the spot welded joints.