Resolidified Material Research Articles

Forensic Investigation of Electrical Conduct Copper Bead Microstructure as an Effort to Identify Causes of Fire

The purpose of this study was to evaluate the characteristics of the bead formed due to short circuit, overload and direct flame treatment on NYM 3x1.5 copper power cable. Handling of short circuit and overload is carried out at a current load of 800% of the current carrying capacity (144 Amperes) and direct flame treatment is carried out at a temperature of 960 degrees Celsius. The bead specimens formed from each treatment were examined and tested in the laboratory: chemical composition examination, visual inspection, macro and micro structural examination, hardness testing, and SEM-EDS examination. The difference in the characteristics of the arc bead that is formed under short circuit conditions and overload is that in short circuit conditions the damage point is localized at a certain point, namely at the short circuit contact point, while under overload conditions the point damage is localized at one or several specific locations along the wire. The macro characteristic of arc beads formed under short-circuit and overload conditions is that they contain many cavities and a clear transition boundary between the melted/ re-solidified material and the non-melted material. While the characteristics of the granules in the form of globular formed in the direct flame treatment, do not show sharp transitions between melting/ re-solidified materials. The micro structure of NYM 3x1.5 beads of electrically conducting copper wire material under the treatment conditions: short circuit, overload and direct ignition, is an alpha (α) phase dendritic structure.

Simulation of two nanoparticle melting to understand the conductivity drop of 3D-printed silver nanowires

The future flexible sensor technology will require low-temperature, fast processing 3D printing techniques that will rely heavily on the quality of nanoparticle (NP) Ink. Electrical conductivity has been found to decrease in majority of the 3D printed electronic wires. Herein we report a fundamental understanding in drop of electrical conductivity in processed silver (Ag) line wire, generally found in Ag-nanoparticle Ink, using large-scale atomistic molecular dynamics (MD) simulations of sintering of five different sizes of Ag NPs. To preserve the high conductivity of pure silver wires, the integrity of the pristine face-centered cubic (FCC) crystalline structure must be retained in the processed line wires. Simulations show that the pristine Ag FCC structures of the nanoparticles are not recovered after melting and resolidification, instead, the resolidified material is paracrystaline. The breakdown of pristine FCC structures might be the cause of the drop in conductivity of processed Ag wires. Simulation results suggest that the intermediate size nanoparticles retain highest percentage of the pristine silver face-centered cubic (FCC) structure after pulse treatments. Our results show that the most promising Ag-Ink should contain smaller to medium size silver NPs that can retain FCC structure after 3D printing of the Ag-Ink.

Numerical simulation and experimental study of optimized jet shape in electric arc electrochemical composite machining

A favorable flow field enhances the product transport capacity and also increases the electrochemical mass transfer rate. In this paper, an end-side connected electrode suitable for electric arc electrochemical composite milling was proposed based on the existing flushing form. The aim is to simultaneously enhance the machining of the groove surface and the side. The flushing form was optimized by the flow field, and it was shown that the internal flushing increased the area of the high flow rate zone of the bottom gap. In the experiments, it was found that the internal flushing reduced the secondary discharge probability, the accumulation of re-solidified material on the groove surface was decreased, and the surface dissolution was uniform with low oxidation. However, the central jet is lost along the course and a zero velocity zone exists at the corners of the groove. Therefore, the sidewall open flow field is proposed to reduce the sidewall recast layer. It was verified that the sidewall slots of the positive flow type improved both the surface quality of the groove surface and the side wall. It was also found that the increase in the number of open grooves facilitates the generation of uniform debris removal.

Case study for the formation of a surface alloy on cemented tungsten carbide using ultrashort MHz to GHz burst-pulses

The authors of this paper report on the generation and utilization of high-frequency combined MHz- and GHz-bursts consisting of ultrashort laser pulses in order to form a surface alloy on the composite material cemented tungsten carbide. The resulting topographies, ablation depths and chemical compositions of the irradiated material surfaces were characterized with respect to the modification of the surface of a composite material whose elements tungsten carbide (WC) and cobalt (Co) permit the formation of a surface alloy only in a common meta-stable liquid phase. The results of this case study demonstrate that using high-frequency bursts of ultrashort laser pulses in the investigated parameter range, the formation of a surface alloy is possible without a tendency to change the chemical composition of WC and Co, and the depth of re-solidified material depends on the combination of MHz and GHz burst-pulses.

ELECTROCHEMICAL ARC DRILLING OF NICKEL–TITANIUM SHAPE MEMORY ALLOY USING MOLYBDENUM ELECTRODE: INVESTIGATION, MODELING AND OPTIMIZATION

In the present scenario, electrochemical arc machining (ECAM) (hybrid of electric discharge erosion and electrochemical dissolution) is an evolving procedure for difficulty in machining the materials due to constraints of existing processes. This research aims to investigate the machinability of Ni[Formula: see text]Ti alloy through electrochemical arc drilling using molybdenum electrode. Electrolyte concentration (ethanol with ethylene glycol and sodium chloride), supply voltage, and tool rotation are considered as the variable factors to evaluate the ECAM performance characteristics in drilling blind hole operation concerning overcut (OC), tool wear rate (TWR) and materials removal rate (MRR). Consequently, response surface methodology is implemented for predictive modeling of various performance characteristics. Finally, multi-objective optimization through desirability function approach (DFA) has produced a set of optimal parameters to improve the productivity along with the accuracy, which is the prime requirement for the industrial applicability of the ECAM process. Results demonstrated that supply voltage is the influential key factor for improvement of machining rate. Scanning electron microscope (SEM) photographs revealed the development of heat affected zone (HAZ), white layer, melted droplet, craters, re-solidified material, ridge-rich surface and voids as well as cavities around the end-boundary surfaces of a blind hole. Composition analysis through energy dispersive spectroscopy (EDS) indicated the oxygen content on the machined surface because electrolyte breakdown causes oxidation to take place at elevated temperatures across the machining zone. Moreover, carbide precipitation like TiC was found in the melting zone of the drilled hole, as revealed by X-ray diffraction (XRD) analyses, which has the affinity to reduce the SMA properties in HAZ.

Rapid Multicomponent Alloy Solidification with Allowance for the Local Nonequilibrium and Cross-Diffusion Effects.

Motivated by the fast development of various additive manufacturing technologies, we consider a mathematical model of re-solidification of multicomponent metal alloys, which takes place after ultrashort (femtosecond) pulse laser melting of a metal surface. The re-solidification occurs under highly nonequilibrium conditions when solutes diffusion in the bulk liquid cannot be described by the classical diffusion equation of parabolic type (Fick law) but is governed by diffusion equation of hyperbolic type. In addition, the model takes into account diffusive interaction between different solutes (nonzero off-diagonal terms of the diffusion matrix). Numerical simulations demonstrate that there are three main re-solidification regimes, namely, purely diffusion-controlled with solute partition at the interface, partly diffusion-controlled with weak partition, and purely diffusionless and partitionless. The type of the regime governs the final composition of the re-solidified material, and, hence, may serve as one of the main tools to design materials with desirable properties. This implies that the model is expected to be useful in evaluating the most effective re-solidification regime to guide the optimization of additive manufacturing processing parameters and alloys design.

A micromechanically motivated multiscale approach for residual distortion in laser powder bed fusion processes

For the broader industrial usage of metal additive manufactured parts, especially made by laser powder bed fusion, a better prediction and understanding of the warpage and of eigenstresses within the final part are necessary. Due to the diverse and sophisticated metallurgical and thermal processes during production, physically motivated simulations are rather complex and time consuming. Therefore, various simplifications concerning the process and material models are frequently made, which leads to practicable simulation times at the expense of physical accuracy. This motivates the procedure in this contribution: A multiscale approach combining various modelling levels, in particular regarding the heat source and material model. The model is based on three finite element simulations on different levels and with different specific aims, i.e. the laser scan model, the layer hatch model and the part model. For the smallest scale considered, a sophisticated thermomechanically fully coupled model based on a phase transformation model explicitly considering the powder, molten, and re-solidified material is incorporated. The goal of this detailed simulation is to extract an effective heat source for the next level of simulation, i.e. the layer hatch model. The layer hatch model is used to extract the inherent strains for the part model. With these strains, the remaining deformation and eigenstresses of arbitrary large parts with the same material and laser parameters can be efficiently and directly computed. The capabilities of the present framework are investigated by simulations on the behaviour of a twin cantilever beam, where the effect of different process parameters on the overall material and structural response is carried out.

The Machinability Characteristics of Multidirectional CFRP Composites Using High-Performance Wire EDM Electrodes

Due to the abrasive nature of the material, the conventional machining of CFRP composites is typically characterised by high mechanical forces and poor tool life, which can have a detrimental effect on workpiece surface quality, mechanical properties, dimensional accuracy, and, ultimately, functional performance. The present paper details an experimental investigation to assess the feasibility of wire electrical discharge machining (WEDM) as an alternative for cutting multidirectional CFRP composite laminates using high-performance wire electrodes. A full factorial experimental array comprising a total of 8 tests was employed to evaluate the effect of varying ignition current (3 and 5 A), pulse-off time (8 and 10 µs), and wire type (Topas Plus D and Compeed) on material removal rate (MRR), kerf width, workpiece surface roughness, and surface damage. The Compeed wire achieved a lower MRR of up to ~40% compared with the Topas wire when operating at comparable cutting parameters, despite having a higher electrical conductivity. Statistical investigation involving analysis of variance (ANOVA) showed that the pulse-off time was the only significant factor impacting the material removal rate, with a percentage contribution ratio of 67.76%. In terms of cut accuracy and surface quality, machining with the Compeed wire resulted in marginally wider kerfs (~8%) and a higher workpiece surface roughness (~11%) compared to the Topas wire, with maximum recorded values of 374.38 µm and 27.53 µm Sa, respectively. Micrographs from scanning electron microscopy revealed the presence of considerable fibre fragments, voids, and adhered re-solidified matrix material on the machined surfaces, which was likely due to the thermal nature of the WEDM process. The research demonstrated the viability of WEDM for cutting relatively thick (9 mm) multidirectional CFRP laminates without the need for employing conductive assistive electrodes. The advanced coated wire electrodes used in combination with higher ignition current and lower pulse-off time levels resulted in an increased MRR of up to ~15 mm3/min.

White Layer Thickness Variation in Die Sinking EDM

Surfaces that were machined by Electrical Discharge Machining (EDM) inevitably exhibit a thin layer of resolidified material, commonly called “white layer”. It is usually regarded as disadvantage of ED machining as it induces tensile residual stresses to the parent material and therefore facilitates the formation of cracks. It therefore was subject to a number of scientific investigations that often dealt with its (non-)crystalline structure or the stress induction. Some research groups have also focused on the thickness of the white layer. However, the variation of its thickness has not yet been investigated comprehensively and therefore constitutes the focus of this work. Experiments were conducted on a conventional sinking EDM machine. A heat-treatable steel was used as a workpiece material and graphite was used for the tool electrode. The influence of the discharge duration te as well as the discharge current ie on the white layer formation were investigated. Results show that significant thickness variations occur on every ED machined surface and that increasing discharge energy leads to an increase of both, white layer thickness as well as its variation.

Parameters optimization for microcrack width in laser trepanned hole

Abstract Ceramic composite like Zirconia Toughened Alumina (ZTA) acquire with excellent toughness, high hardness and temperature resistance. These characteristics demand applications in manufacturing of ballistic body parts, cutting tool inserts, turbine blades and combustion chambers. Conventional machining methods are unsuitable to drill/cut holes in ZTA plate due to its improved mechanical properties. Researchers have found that laser beam machining has capability to drill or cut holes in structural ceramics. Formation of microcracks is inherent in laser drilled hole due to adhered layer of resolidified material and development of thermal stresses. The chances of formation of microcracks are more in laser drilling or cutting of hard and brittle material like ZTA. Sometimes, widening of microcrack propagates underneath recast layer and demolishes the hole surface quality and changes parent material properties, as well. Therefore, selection of optimum level of control factors or input process parameters became essential during laser drilling of ZTA to overcome the intensity of microcracks. In this research paper, the microcrack width of laser trepan drilled hole has been optimized using Artificial Intelligence tool such as Genetic Algorithm. Optimum result shows that microcrack width has been reduced by 17.66%. The comparative improvement has also been confirmed in SEM image.

Investigation of the Machinability of the Inconel 718 Superalloy during the Electrical Discharge Drilling Process

The properties of the Inconel 718 superalloy are used in the manufacturing of aircraft components; its properties, including high hardness and toughness, cause machining difficulties when using the conventional method. To circumvent this, non-conventional techniques are used, among which electrical discharge machining (EDM) is a good alternative. However, the nature of removing material using the EDM process causes the thermophysical properties of Inconel 718 to hinder its machinability; thus, a more extensive analysis of the influence of these properties on the EDM process, and a machinability analysis of this material in a wider range, using more process parameters, are required. In this study, we investigated the drilling of micro-holes into the Inconel 718 superalloy using the EDM process. An experiment was conducted to evaluate the impact of five process parameters with a wide range of values (open voltage, time of the impulse, current amplitude, the inlet dielectric fluid pressure, and tube electrode rotation) on the process’s performance (drilling speed, linear tool wear, the side gap thickness, and the aspect ratio of holes). The analysis shows that the thermal conductivity of this superalloy significantly influences the effective drilling of holes. The combination of a higher current amplitude (I ≥ 3.99 A) with an extended pulse time (ton ≥ 550 µs) can provide a satisfactory hole accuracy (side gap thickness ≤ 100 µm), homogeneity of the hole entrance edge without re-solidified material, and a depth-to-diameter ratio of about 19. Obtaining a high dimensional shape accuracy of holes has an enormous effect on their usability in the structure of the components in the aviation industry.

Beryllium melting and erosion on the upper dump plates in JET during three ITER-like wall campaigns

Data on erosion and melting of beryllium upper limiter tiles, so-called dump plates (DP), are presented for all three campaigns in the JET tokamak with the ITER-like wall. High-resolution images of the upper wall of JET show clear signs of flash melting on the ridge of the roof-shaped tiles. The melt layers move in the poloidal direction from the inboard to the outboard tile, ending on the last DP tile with an upward going waterfall-like melt structure. Melting was caused mainly by unmitigated plasma disruptions. During three ILW campaigns, around 15% of all 12376 plasma pulses were catalogued as disruptions. Thermocouple data from the upper dump plates tiles showed a reduction in energy delivered by disruptions with fewer extreme events in the third campaign, ILW-3, in comparison to ILW-1 and ILW-2. The total Be erosion assessed via precision weighing of tiles retrieved from JET during shutdowns indicated the increasing mass loss across campaigns of up to 0.6 g from a single tile. The mass of splashed melted Be on the upper walls was also estimated using the high-resolution images of wall components taken after each campaign. The results agree with the total material loss estimated by tile weighing (~130 g). Morphological and structural analysis performed on Be melt layers revealed a multilayer structure of re-solidified material composed mainly of Be and BeO with some heavy metal impurities Ni, Fe, W. IBA analysis performed across the affected tile ridge in both poloidal and toroidal direction revealed a low D concentration, in the range 1–4 × 1017 D atoms cm−2.

Performance of a TR-iso-pulse generator in micro ED-drilling

This study describes a TR-iso-pulse generator which has better machining performance than an RC-pulse generator in micro-electrical discharge drilling (micro ED-drilling). Although machining with a TR-pulse generator is more efficient compared to when using a RC-pulse generator in macro-scale EDM, an RC-pulse generator is often used in micro EDM because it can generate the short discharge pulse necessary to minimize the discharge energy. However, to increase the machining efficiency, it is necessary to develop a TR-iso-pulse generator capable of generating a small amount of uniform discharge energy. In this research, a TR-iso-pulse generator was constructed for micro ED-drilling. For an accurate performance comparison between an RC-pulse generator and a TR-iso-pulse generator, each pulse generator was set up to generate the same discharge energy for a single discharge. Through micro ED-drilling, material removal rate (MRR) and relative wear ratio (RWR) were measured with respect to feed rate, and then the periphery of the machined hole, longitudinal section, lateral overcut, and surface roughness were compared for machining accuracy analyses. As a result, MRR was increased by 121% and RWR was decreased by 39% in the TR-iso-pulse generator compared with the RC-pulse generator. In addition, while lateral overcut was increased by 4.63%, less amounts of re-solidified material were deposited at the periphery of the machined hole and surface roughness was also reduced by 22% in the TR-iso-pulse generator.

Tribological behavior of nanosecond-laser surface textured Ti6Al4V

Titanium alloys are used due to their high specific strength and remarkable corrosion resistance. Their wear resistance however is poor, which in this paper is counteracted by laser surface texturing. Linear textures were created by the use of a nanosecond-pulsed laser, accompanied by melt bulges of resolidified material on the sides. Packing density, finishing procedure and atmosphere during laser texturing were varied between the experiments. Melt bulges lasered in air turned out to be tribologically beneficial in grease-lubricated sliding contact, reducing wear volume on Ti6Al4V significantly by up to a factor of 160, if a packing density of 5% or more was chosen. Further investigations of melt bulges with energy dispersive X-ray spectroscopy (EDX) and scanning transmission electron microscopy (STEM) revealed an increased content in interstitial oxygen and nitrogen and a purely martensitic α’-phase microstructure. Critical limitation of plastic deformation and a saturation of electronic bonds of the titanium atoms by interstitial elements is thought to be responsible for the reduction in adhesive tendency and therefore the pronounced decrease in wear.

The Dependence of Microstructural Evolution and Corrosion Resistance of a Sandwich Multi-Layers Brazing Sheets on the Homogenization Annealing

The influence of the microstructure on the corrosion susceptibility and mechanism of aluminum sheet was studied. The homogenization annealing treatment results in the segregation of copper on the surface of the re-solidified clad material and the increase of the diffusion depth of Si during brazing. The band of dense precipitates has a higher corrosion potential than clad and core material, and the high density particles in the diffusion zone (40 um<;depth<;70 um) are identified as α-Al(Fe-Mn)Si particles. Thus, the band of dense precipitates are catalytic sites for cathodic reduction reactions. Moreover, the effect of homogenization annealing treatment on the local electrochemical activity and corrosion properties are studied. The results indicate that the band of dense precipitates formed in the diffusion zone in the sample without homogenization annealing during brazing process, which improves the corrosion resistance of the material. Compared with sample with homogenization annealing at the same thickness, the time for the perforation during seawater acetic acid test of the sample without homogenization is longer than that of the homogenization annealed sample.

Response of fusion plasma-facing materials to nanosecond pulses of extreme ultraviolet radiation

AbstractThe experimental study of damage to tungsten (W), molybdenum (Mo), and silicon carbide (SiC) surfaces induced by focused extreme ultraviolet laser radiation (λ ~ 47 nm/~1.5 ns/21–40 µJ) is presented. It was found that W and Mo behaved similarly: during the first shot, the damaged area is covered by melted and re-solidified material, in which circular holes appear – residua of just opened pores/bubbles, from which pressurized gas/vapors escaped. Next cracks and ruptures appear and the W has a tendency to delaminate its surface layer. Contrary, single-crystalline SiC has negligible porosity and sublimates; therefore, no escape of “pressurized” gas and no accompanying effects take place. Moreover, SiC at sublimating temperature decomposes to elements; therefore, the smooth crater morphology can be related to local laser energy density above ablation threshold. When more shots are accumulated, in all three investigated materials, the crater depth increases non-linearly with number of these shots. The surface morphology was investigated by an atomic force microscope, the surface structure was imaged by a scanning electron microscope (SEM), and the structure below the surface was visualized by SEM directed into a trench that is milled by focused ion beam. Additionally, structural changes in SiC were revealed by Raman spectroscopy.

Towards the simulation of Selective Laser Melting processes via phase transformation models

Selective Laser Melting (SLM) – as one of a number of additive manufacturing techniques – is a promising method for the manufacturing of complex structures and may bring about significant improvements in the context of custom-made designs and lightweight constructions. However, the complex multiphysical processes occurring during SLM necessitates the establishment of appropriate constitutive and process models in order to quantitatively predict the properties of the final workpiece. In particular, the accurate determination of process-induced eigenstresses is a challenging yet important task. In this work, a constitutive modelling framework stemming from phase transformations in shape memory alloys is adopted to the modelling of the changes of state during SLM. This model is based on energy densities and energy minimisation in general and specifically serves as a basis for further enhancements such as the consideration of multiple solid phases of the underlying material. This is particularly considered important due to the fact that the cooling rates during SLM are heterogeneously distributed and that thus different solid phases may form out of the molten material pool. As a first step, the present overall model comprises three phases of the material, namely powder, molten, and re-solidified material. The thermomechanically fully coupled Finite-Element-based process model incorporates approaches for, e.g., the laser beam impact zone and the layer construction model.

Electric discharge hole grinding in hybrid metal matrix composite

ABSTRACTMoving toward a hybrid approach, a hybrid process, electric discharge hole grinding (EDHG) was used to machine a hybrid metal matrix composite (MMC) (Al6063/SiC/Al2O3/Gr). Here, holes were drilled and ground in a single step process (EDHG) using a novel tool electrode. The experiments were designed using response surface methodology (RSM). The objective of this study was to investigate the effect of electric discharge diamond hole grinding operation on the surface roughness (SR) of the hole. The input process parameters were current, duty factor, tool speed and flushing pressure. It was found that the process is very effective in producing a finished hole. A comparison of surface roughness was made between electric discharge drilling (EDD) and electric discharge diamond hole grinding, thereby showing the effectiveness of the electric discharge diamond hole grinding process. The grinding action of the process is clearly visible in the scanning electron microscopic (SEM) image. It was observed that the craters, globules of resolidified material and micro cracks, which are normally seen on surfaces machined by electric discharge machining (EDM), are completely ground off by electric discharge diamond hole grinding.

Laser Induced Surface Morphology of Molybdenum Correlated with Breakdown Spectroscopy

Surface modifications of laser irradiated molybdenum have been correlated with plasma parameters. Nd:YAG laser (1064 nm, 10 ns) was employed at various laser irradiances ranging from 6 to 50 GW/cm2 under argon environment. The ablation efficiency has been investigated by measuring the crater depth using surface profilometry analysis. Scanning electron microscope (SEM) analysis reveals the formation of coarse grains along with cracked boundaries, cavities and cones at the central ablated areas. Whereas, uplifted re-solidified material, cavities, ridges, droplets and cones were observed at boundary regions. Laser Induced Breakdown Spectroscopy (LIBS) analysis has been performed to evaluate electron temperature and number density of molybdenum plasma. Electron temperature and electron density varies from 6670 to 9305 K and 0.62 × 1018 to 0.72 × 1018 cm−3 respectively. Both the parameters showed similar trend in variation with laser irradiance i.e. an initial increase from 13 to 19 GW/cm2 followed by a decrease from 19 to 25 GW/cm2 and then a saturation from 25 to 50 GW/cm2. The initial increasing trend is attributed to the enhanced excited vapor content of the ablated material, confinement effects of the surrounding argon and absorption of laser energy into the molybdenum vapor plasma during the trailing part of laser pulse leading to ignition of laser supported combustion (LSW) waves. The decreasing trend is attributed to the shielding effect and saturation is explainable on the basis of the formation of a self-regulating regime. Surface modifications of laser irradiated molybdenum were correlated with the plasma parameters.

Femtosecond laser ablation of enamel.

The surface topographical, compositional, and structural modifications induced in human enamel by femtosecond laser ablation is studied. The laser treatments were performed using a Yb:KYW chirped-pulse-regenerative amplification laser system (560 fs and 1030 nm) and fluences up to 14 J/cm2. The ablation surfaces were studied by scanning electron microscopy, grazing incidence x-ray diffraction, and micro-Raman spectroscopy. Regardless of the fluence, the ablation surfaces were covered by a layer of resolidified material, indicating that ablation is accompanied by melting of hydroxyapatite. This layer presented pores and exploded gas bubbles, created by the release of gaseous decomposition products of hydroxyapatite (CO2 and H2O) within the liquid phase. In the specimen treated with 1-kHz repetition frequency and 14 J/cm2, thickness of the resolidified material is in the range of 300 to 900 nm. The micro-Raman analysis revealed that the resolidified material contains amorphous calcium phosphate, while grazing incidence x-ray diffraction analysis allowed detecting traces of a calcium phosphate other than hydroxyapatite, probably β-tricalcium phosphate Ca3(PO4)2, at the surface of this specimen. The present results show that the ablation of enamel involves melting of enamel’s hydroxyapatite, but the thickness of the altered layer is very small and thermal damage of the remaining material is negligible.