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The application of cemented carbides is desired in several fields due to their superb mechanical properties. The downside of the large strength and wear resistance occurs upon machining, which, especially for conventional machining, is expensive or even impossible. Hence, Electrical Discharge Machining (EDM) is used since it works independently from the mechanical properties of the workpiece. However, the ED-machining of cemented carbides is also characterised by low productivity and high tool wear when it is compared to the machining of steel. This paper investigates the material removal phenomenon of different cemented carbide grades by means of a high speed video camera. In order to allow observation, the single discharges are conducted in air. A large number of experiments with different energy levels is analysed by an algorithm to identify the particle size, particle speed and the ejection angle. The results can be used to identify the dominant material removal mechanism. This should in the future be used to create a machining technology which is dedicated to the specific cemented carbide grade. [ABSTRACT FROM AUTHOR]

In general, pulsed lasers with high peak power have been used for the micro-groove formation. However, the processing speed is limited by the pulse repetition rate. On the other hand, CW laser can be expected to perform the high-speed processing by continuous energy input. The mechanism of micro-groove formation by CW laser was investigated by high-speed observation and the thermal fluid analysis. In the perpendicular irradiation of CW laser, the molten metal flows symmetrically around the keyhole to the backward direction, and micro-grooves remain at both edges of molten region. In contrast, in the angled irradiation, the molten metal at the reflection-side scatters as spatters. The remained molten metal flows from the reflection-side to the incident-side through the bottom of keyhole, since the recoil pressure is generated from the reflection-side to the incident-side. In addition, high-speed scanning contributes to keeping the sufficient time and force to move the molten metal in the backward direction. Then, the micro-groove remains at the reflection-side, while the upheaval is formed at the incident-side by gathering the molten metal from the reflection-side and the front of keyhole. Asymmetrical behavior of molten metal flow in angled irradiation of CW laser can create micro-groove in the reflection-side. [ABSTRACT FROM AUTHOR]

Failure mechanism of sandwich panels is very complicated. The objective of this study is to demonstrate the usefulness of high-speed observation to elucidate the failure mechanism of sandwich panels. Failure processes of carbon fiber reinforced plastic composite/Nomex™ honeycomb sandwich panels were observed using two synchronized high-speed cameras with an 8 µs frame rate. The sandwich panel was subjected to a four-point bending load in accordance with ASTM-D7249. Digital image correlation was also adopted for measuring the surface strain. Results show fracture initiation: (1) local (intra-cell or wrinkling) buckling of the face sheet at the edge (Type I and III), or (2) local buckling (intra-cell or wrinkling) of the face sheet at the middle of the specimen (Type II). Following local buckling of the face sheet, debonding between the face sheet and the honeycomb core occurred within 10–20 µs. Subsequently, crack propagation of the face sheet occurred, leading to final failure of the sandwich panel. It was about 150–200 µs from the local buckling of face sheet to the final failure of sandwich panel. Results demonstrated that high-speed observations are effective to elucidate failure mechanisms of the sandwich panel subjected to a bending load. [ABSTRACT FROM AUTHOR]

Demand for high precision machining of complicated shapes has recently increased with the miniaturization of mechanical and electronic products. Therefore, further development of fine wire electrical discharge machining (EDM) technology using a thin wire electrode has been also requested. However, there might be a limit in improving the machining performance of fine wire EDM using conventional wire only by optimizing discharge pulse conditions and gap control. Therefore, it is essential to develop a new type of thin wire electrode for further improvement of wire EDM performance. In this study, high-zinc-content brass (γ-phase brass) coated steel core wire as a new type wire electrode was trially made to improve the machining stability. In wire EDM, zinc content of wire surface would contribute to rapidly cool down the gap by the evaporation due to its low boiling point. However, the surface of γ-phase brass coated wire tends to become rough in the heat treatment process of the wire. Thus, another type of the γ-phase brass coated wire with smoothed surface was also made by an extra finish drawing. The wire EDM characteristics using these wires were compared with those using conventional brass coated steel wire. Furthermore, the distribution of spark locations during process was evaluated by high-speed observation to investigate the influence of γ-phase brass and the surface roughness of wire. As a result, the cutting speed using the γ-phase brass coated steel wire with smoothed surface is faster than that using conventional wire because of uniform distribution of spark location.

This paper proposes two new methods to observe discharge phenomena without interference from the plasma in electrical discharge machining. The first method uses a bandpass filter with a bandwidth of 800–820 nm and laser illumination with a wavelength of 800–820 nm. The second method also uses a bandpass filter with a bandwidth of 800–820 nm; however, in this method, the tungsten material is used not only as the tool electrode but also as the illumination source. First, the discharge process was observed using traditional methods to investigate the influence of the plasma on the observation of the discharge process. Then, the process of removing molten material from both the tool electrode and workpiece, as well as molten pool movement, was observed using the first method proposed in this paper. The material removed from the tool electrode was scattered upward along the end profile of the tool electrode, while the material removed from the workpiece was distributed along the horizontal direction. To explain these phenomena, the flow distribution in the gap was qualitatively analyzed using a fluid simulation. Finally, the discharge process was also observed using the second method proposed in this paper. A tungsten tool electrode can emit light with a wavelength of 800–820 nm, which shines on the observed region during the discharge process. The observation results verified the phenomenon of multiple explosions of the molten pool during the discharge process. Moreover, it was found that the material removed by the explosion of the molten pool was scattered in different directions and hardly influenced by the flow distribution in the gap. • This paper successfully used a bandpass filter to observe the molten pool in EDM. • This paper successfully used the tungsten to observe the molten pool in EDM. • This paper reveal the reason of the flying direction of the removed material. [ABSTRACT FROM AUTHOR]

This paper proposes an electrical discharge machining (EDM) for use with carbon-fiber-reinforced silicon carbide (C f _SiC) composite, which are materials that are extremely difficult to machine. The preliminary machining experiment indicates that EDM is a promising method to fabricate the C f _SiC composite. However, owing to the distinct material properties of the C f _SiC composite, the machining characteristics are quite different from those of metal materials in the EDM process, which mainly reflects the difference in the material removal process. To address the above issue, this study investigated the influence of thermal stress on the removal of material during EDM of the C f _SiC composite. In the scanning electron microscopy (SEM) result, the morphology of the machined surface demonstrates the fracture removal of the carbon fiber with a discharge on the parallel surface and small-area spalling removal on the cross-section of the carbon fiber with a discharge on the vertical surface. In addition, it also exhibits a brittle fracture of the SiC coating and SiC matrix. Furthermore, a novel observation method that can avoid the interference of the discharge plasma by using a distinct band pass filter and laser illumination of a specific wavelength, is employed to investigate the discharge process of the C f _SiC composite. It further confirms the fracture removal process of the SiC coating and carbon fibers. Finally, a thermal–electric–mechanical coupling simulation is conducted to investigate the stress distribution on the discharge surface of the C f _SiC composite. It is found that the thermal stress on the machined surface exceeds the tensile strength of the SiC coating, carbon fiber, and SiC matrix. Thus, based on the above results, it is concluded that the thermal stress plays a significant role in high efficiency material removal from the C f _SiC composite during EDM. [ABSTRACT FROM AUTHOR]

The cavitation cloud generated by submerged high-speed water jet has periodicity. The periodicity is provided by alternately repeating the bubbles generation and shedding process and cavitation nucleus filling process. Therefore, the construction of the filling process allows control of the shedding frequency of the cavitation clouds and may improve the aggressive intensity of the cavitating jet. In this paper, the water flow holes were provided at the nozzle throat outlet in order to form a stable and continuous filling flow. The aggressive intensity of this nozzle was compared by the mass loss of JIS A1050P. In addition, the cavitating jet was observed with a high speed video camera to confirm the characteristics of the jet. As a result, the mass loss per unit area increased as the diameter of the water flow holes increased. It is speculated that the enhanced flow around the cavitating jet due to the action of the water flow holes caused the increase in the damming pressure on the specimen. The increase in damming pressure increased the impact density and collapse impact force of the cavitation bubbles. Moreover, the large water flow holes resulted in stabilization of the shedding frequency of the cavitation clouds and growth scale. It may be a factor that the stable supply of cavitation nuclei to the cavitation main generation field was realized. However, since the water flow holes did not change the shedding frequency significantly, it is considered that the influence of the frequency on the aggressive intensity is small in this experiment.

Laser powder bed fusion (LPBF) is a promising additive manufacturing technology for producing metal parts with complex geometric features. However, the issue concerning process stability and repeatability still hinders its future acceptance by the industry. Gaining a better understanding of the behavior and stability of the evaporation process is an important step towards further insights into the complex interaction between laser and material. In this study, we used off-axis high-speed camera to observe vapor plume evolution in single-track formation on bare Ti-6Al-4V plates; the results showed that evaporation has a strong effect on melting quality even if the keyhole is not developed. We then expanded the experiments to multi-track level and found that the melting mode can change as the result of heat accumulation. The results show the possibility that keyhole regime may be reached even if it starts with a combination of parameters below the threshold for keyhole formation in single-track-level observation.

Effects of the driving frequency on temperature in a multiphase ac arc were successfully clarified. A multiphase ac arc is one of the thermal plasmas, which is suitable for producing functional materials such as nanoparticles, in-flight glass melting, and waste treatment in terms of its large plasma volume, high-energy efficiency, and low flow velocity. The temperature and arc behavior in various driving frequencies were investigated by the observation system consisting of a high-speed video camera and bandpass filters for plasma spectroscopy. The temperature measurement was conducted on the basis of the Boltzmann plot method. The obtained results indicated that the arc behavior and its temperature field were strongly influenced by the driving frequency. An increase of the frequency led to concentrated arc region, higher arc temperature at the center of electrode region and lower arc fluctuation. The temperature and its distribution can be controlled depending on the material properties to be treated and the purpose of the process. [ABSTRACT FROM AUTHOR]

Temperature characteristics of a multiphase AC arc in various working pressures were investigated by an innovative observation system consisting of a high-speed video camera and band-pass filters. Thermal plasmas have been widely applied to many industrial fields because of their unique advantages such as high temperature, high enthalpy, and rapid quenching capability. In particular, the multiphase AC arc is advantageous in terms of large plasma volume and high energy efficiency. Therefore, this heat source has been applied to innovative material processing such as in-flight glass melting, and functional nanoparticles fabrication. However, the temperature field and its fluctuation characteristics in the multiphase AC arc have not been understood because of the difficulties of temperature measurement due to their rapid fluctuation in millisecond timescale as well as the axisymmetric spatial characteristics. To understand and control the fluctuation phenomena is important to realize this method as industrial technology. Temperature measurement system using a high-speed camera was constructed to visualize the temperature fields of the multiphase AC arc. The fluctuations in the two-dimensional intensity distributions of particular line emissions from atomic argon were successfully observed. By analyzing these images using the Boltzmann plot method, the temperature distribution was estimated. The experimental results indicated that the arc temperature fluctuated in the range from 6,000 to 12,000 K. Higher temperature, smaller arc existence area, and decrease in the diameter of the arc were observed with an increase of working pressure. The arc temperature in the multiphase AC arc is sufficiently high to treat the refractory metals and ceramics powders at high processing rate.

In order to improve cutting capability of a submerged abrasive suspension jet (ASJ), a ventilated sheathed nozzle for producing an air coated ASJ has been developed. Cutting characteristics of air coated ASJs under submerged condition are provided by cutting tests with specimens of aluminium alloy. Unsteady behaviour of the air coated layer around the jet is also investigated by high speed observations of air coated water jets. The cutting capability of air coated ASJs and the unsteady behaviour of the air coated layer of the submerged jets are arranged properly by the cavitation number based on the pressure in the ventilated cavity and Strouhal Number. [ABSTRACT FROM AUTHOR]

• Multi-physics coupling model considering practical boundary conditions was developed. • The effect of difficult forces acting on melt pool were analyzed. • The role of evaporation recoil pressure on material removal was revealed. • Melt pool dynamics and material removal were directly observed and analyzed under practical conditions. The melt pool dynamics and material removal mechanism in electrical discharge machining (EDM) is not clear yet. In this study, a thermo-hydraulic coupling simulation model considering practical boundary conditions was proposed to investigate the melt pool evolution and material removal on the anode in EDM, with the simulated results directly verified by the high-speed observation of the discharge process. Discharge currents of 5–15 A, a discharge duration of 10 µs, and workpieces of zinc and steel were used in the model, with the forces acting on the melt pool comprehensively integrated and their effects on the melt pool dynamics and material removal analyzed. The simulation results show that on the anode the evaporation recoil pressure is one of the dominant factors that drive the melt flow and cause the molten material removal. The increase of discharge current results in larger evaporation recoil pressure, and thus larger volume of the molten material is removed. Under the effect of evaporation, obvious fluctuation of the melt pool will occur. Meanwhile, the evaporation recoil pressure oscillates with the melt pool fluctuation, which may induce a wave-like motion of the molten material and promote the generation of debris particles. Novel high-speed observation methods were proposed to observe the melt pool evolution and the material removal, with the observed melt pool behavior consistent with the simulated results. The observation also shows that the intermittent removal of the molten material occurs in the case of long discharge duration, which is considered to be caused by the intermittent increase of the evaporation recoil pressure during the melt pool fluctuation. [Display omitted] [ABSTRACT FROM AUTHOR]

Precision micro-machining without cracks and with a narrow heat affected zone is highly required for the effective use of high-performance material. For this purpose, it is expected that the higher harmonics of Nd:YAG laser could perform the precision micro-machining by its high photon energy. However, even using harmonics of Nd:YAG laser, the heat affected zone is inevitable due to the plasma generation. In order to reduce the influence of plasma, it is important to understand the generation mechanism of plasma. Therefore, the laser induced plasma was observed by the high-speed shutter camera, and the influence of laser wavelength on machining characteristics was investigated.

In this study, the movements of thin tungsten wire electrode during fine wire EDM were investigated by direct observation using a high-speed camera. The results showed that the amplitude in the direction parallel to the machining direction was a little larger than that in the perpendicular one. Also, the backward wire deflection could be confirmed even in fine wire EDM. Furthermore, the wire vibration mode was found out by frequency analysis and the vibration amplitude could be reduced by adjusting the workpiece position to the 3rd order node of the vibration. In addition, the amplitude envelope of wire electrode during the process was clarified. [ABSTRACT FROM AUTHOR]

Wire movements and vibrations are one of the most important phenomena of wire electrical discharge machining (EDM) process. Unstable wire electrode behavior and large wire vibration amplitude during the process cause wire breakage, low shape accuracy and large machined surface roughness. In this study, direct observation using high-speed camera investigated the movements of tungsten wire electrode during fine wire EDM process. The results showed that the vibration amplitude and frequency depended mainly on the wire tension, and the amplitude direction. The amplitude in the direction parallel to the machining direction was a little larger than that in the perpendicular one. In addition, the backward deflection of wire could be confirmed even in fine wire EDM. Also, the wire vibration mode was analyzed, and it could be varied with the machining position of the thin workpiece, which possibly led to smaller wire amplitude. Furthermore the amplitude envelope of wire electrode during the process was clarified. [ABSTRACT FROM AUTHOR]

High-speed observation of visible and infrared radiation was performed to measure the molten pool geometry, velocity of the solid–liquid interface and temperature profile during laser spot welding of aluminum. Hot cracking occurred at a late stage of solidification for the investigated laser pulse shapes. Hot cracking could be minimized by using a pulse shape with two distinct power levels and a final cooling slope to shut down the laser power. The drop of the laser power from the first to the second power level led to a high cooling rate and high interface velocity at the beginning of solidification. This drop in temperature and molten pool diameter released strains originating from thermal contraction and solidification shrinkage at the beginning of solidification, where spot welding is expected to have a higher ductility. After this initial high solidification rate, low interface velocities were observed during solidification within the second power level. The final solidification rate increased again as a function of the cooling time of the last laser pulse section. This type of solidification process was also found in metallographic microstructures with greater dendritic structures during solidification within the second power level. The strain release at the beginning of solidification minimized residual strains for the remaining solidification, so that crack-free and full penetration bead-on-plate seam welding with overlapping spot welds was possible. [ABSTRACT FROM AUTHOR]

Highlights: [•] We observe that a bubble induce damage to tissue mimic under ultrasound exposure. [•] Violently oscillating bubble pushes and pulls surface of tissue mimic. [•] Cell viability test shows two types of damage, the detachment and dead of cells. [•] Damaged area is smaller than maximum bubble size, and larger than initial size. [•] The damage is localized even above transient cavitation threshold. [Copyright &y& Elsevier]

In order to investigate the impact fracture behavior of graphite-related materials, high-velocity steel spheres accelerated by a two-stage light gas gun were impacted onto three kinds of polycrystalline graphites and a 2D-C/C composite. The polycrystalline graphites have different maximum particle sizes and static dissipation energies. The in situ observation of impact fracture was performed by taking photographs with high-speed framing cameras of the impacted (front) surfaces of those specimens. High pressure caused by shock wave produced a gush of fine fragments in a fluidlike manner immediately after the impact. The apparent volume of fragments was dependent on the maximum particle size, but independent of the thickness of the specimen. The crack patterns of the impacted surface were closely related to the static dissipation energy. The fracture behavior of graphite having the lowest dissipation energy was classified into two cases: with and without fracturing on the rear surface. In the latter case, the time of crack appearance was a function of impact velocity. However, in the former case, both impact velocity and specimen thickness influenced the time of crack appearance. [Copyright &y& Elsevier]

Laser powder bed fusion (LPBF) is a promising additive manufacturing technology for producing metal parts with complex geometric features. However, the issue concerning process stability and repeatability still hinders its future acceptance by the industry. Gaining a better understanding of the behavior and stability of the evaporation process is an important step towards further insights into the complex interaction between laser and material. In this study, we used off-axis high-speed camera to observe vapor plume evolution in single-track formation on bare Ti-6Al-4V plates; the results showed that evaporation has a strong effect on melting quality even if the keyhole is not developed. We then expanded the experiments to multi-track level and found that the melting mode can change as the result of heat accumulation. The results show the possibility that keyhole regime may be reached even if it starts with a combination of parameters below the threshold for keyhole formation in single-track-level observation. [ABSTRACT FROM AUTHOR]

Nanoparticles (NPs) are used as smart materials in some industrial areas. The methods that utilize arc plasma are a good choice in terms of raising production rate of NPs. In this paper, alumina NPs are synthesized from alumina bulk by transferred-type dc arc plasma, and a high-speed observation of the alumina vapor in the arc plasma is carried out in order to investigate on the evaporation behavior of the alumina bulk. [ABSTRACT FROM AUTHOR]

The use of ac arc plasma involves a problem of instability of the arc plasma at current zero point. A method for stabilizing the transferred-type ac arc plasma has been developed by superimposing a pulse current on an ac current only at each current zero point. In this paper, a high-speed observation of the ac arc plasma is carried out in order to investigate on the stabilization of the ac arc plasma by superimposing a pulse current. [ABSTRACT FROM AUTHOR]