Resolidified Layer Research Articles

High-speed dry electrical discharge machining

A novel high-speed dry electrical discharge machining (EDM) method was proposed in this study. Using this method, the material can be rapidly melted by extremely high discharge energy and flushed out of the discharge gap by high-pressure and high-speed air flow. The material removal rate (MRR) of dry EDM was significantly improved by the proposed method. The MRR of dry EDM is usually in tens mm3/min, whereas the MRR of the proposed method can be as high as 5162mm3/min, which improves the MRR by 2nd to 3rd order of magnitude. Investigation was conducted systemically. The influences of work piece polarity, discharge current, pulse duration time, gas pressure, and electrode rotation speed on machining performance were studied. The machining mechanism of this method was thoroughly analyzed. Moreover, the re-solidified layer, surface morphology, elementary composition, and phase of AISI 304 stainless steel for high-speed dry EDM were also investigated. Theoretical and technical foundations were laid for the industry application of dry EDM.

An Investigation of the Effect of Surface Integrity and Layer Thickness Using T90Mn2W50Cr45 Tool Steel

The formation and spherical form of the resolidified layer on T90Mn2W50Cr45 tool surface processed using wire electrical discharge machine was investigated and analysis was done in detail for optimization by response surface methodology. The surface layer modification was examined by scanning electron microscope.

Tungsten recrystallization and cracking under ITER-relevant heat loads

The tungsten surface structure was analyzed after the test in the QSPA-T under heat loads relevant to those expected in the ITER during disruptions. Repeated pulses lead to the melting and the resolidification of the tungsten surface layer of ∼50μm thickness. There is ∼50μm thickness intermediate layer between the original structure and the resolidified layer. The intermediate layer is recrystallized and has a random grains’ orientation whereas the resolidified layer and basic structure have texture with preferable orientation 〈100〉 normal to the surface. The cracks which were normal to the surface were observed in the resolidified layer as well as the cracks which were parallel to the surface at the depth up to 300μm. Such cracks can result in the brittle destruction which is a hazard for the full tungsten divertor of the ITER. The theoretical analysis of the crack formation reasons and a possible consequence for the ITER are given.

A Novel Compound Machining of AISI 321 Stainless Steel Using Small Diameter Electrodes

Experiments of high-efficiently compound machining (CM) combined by EDM and arc machining of AISI 321 stainless steel at 658mm3/min material removal rate (MRR) using solid electrodes of different materials and different diameters of less than 5mm were conducted. The CM process parameters, including electrode rotation speed, peak current, electrode materials and electrode diameter, were studied to evaluate the effect on MRR, relative electrode wear rate (REWR) and width of overcut (WOC). The re-solidified layer of 321 for CM was also investigated. Faster electrode rotation speed, higher peak current, larger electrode diameter and pure tungsten electrode material are essential to achieve lower REWR and higher MRR. The processing conditions were bad using electrode diameter of 2mm. The re-solidified materials are significantly more densely than base materials.

Influence of a large pulsed electron beam (LPEB) on the corrosion resistance of Ti−6Al−7Nb alloys

Pulsed electron-beam irradiation was used as a finishing process for Ti−6Al−7Nb titanium alloy to increase the corrosion resistance and surface hardness. The corrosion characteristics were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy to obtain Nyquist plots and log(f)–|Z| plots. The surface hardness was examined using nano-indentation with a Berkovich tip. The corrosion resistance of the irradiated surface increased, exhibiting a nobler corrosion potential, a higher polarization resistance, and a lower corrosion current density than the untreated surface. The variation in the contact angle and formation of a re-solidified passivation layer indicate a stable surface. The surface hardness increased significantly following irradiation, which was attributed to an increase in the density of dislocations and a decrease in the fraction of the pre-dominant slip plane.

Corrosion inhibition and surface hardening of KP1 and KP4 mold steels using pulsed electron beam treatment

We report the use of pulsed electron beam polishing (PEBP) as a surface treatment for KP1 and KP4 steels for applications in molds for injection molding. The corrosion of the re-solidified layer was analyzed using potentiodynamic polarization tests. The corrosion potentials became nobler by 50 and 30mV vs. the saturated calomel electrode (SCE) for KP1 and KP4 steels, respectively, and the corrosion currents decreased by 50μA for both materials. Variations in the contact angle and formation of oxides in the re-solidified layer indicated that the surface became more stable following PEBP. The corrosion mechanism was pitting of the re-solidified layer. The nano-hardness of KP1 increased by 316%, and that of KP4 increased by 144%, which is attributed to the increased density of dislocations in the re-solidified layer.

Suppression of cavitation in melted tungsten by doping with lanthanum oxide

Melting and boiling behaviour of pure tungsten and 1 wt% lanthanum-oxide-doped tungsten (WL10) are investigated, focusing on the material selection with respect to material loss induced by cavitation. Melting experiments under high heat loads are carried out in the high heat flux facility GLADIS. Pulsed hydrogen neutral beams with heat flux of 10 and 23 MW m−2 are applied onto the adiabatically loaded samples for intense surface melting. Melt layer of the two tungsten grades exhibit different microstructure characteristics. Substantive voids owning to cavitation in the liquid phase are observed in pure W and lead to porous resolidified material. However, little cavitation bubbles can be found in the dense resolidified layer of WL10. In order to find out the gaseous sources, vapour collection is performed and the components are subsequently detected. Based on the observations and analyses, the microstructure evolutions corresponding to melting and vapourization behaviour of the two tungsten grades are tentatively described, and furthermore, the underlying mechanisms of cavitation in pure W and its suppression in WL10 are discussed.

Comparative Study of Microstructural and Corrosion Characteristics of Excimer Laser-Melted AA2124-T4 and AA6061-T4 Alloys

A KrF excimer laser was introduced for laser surface melting (LSM) of the aluminium alloys AA2124-T4 and AA6061-T4. The microstructural and compositional analysis was conducted using SEM, low-angle XRD, and TEM; the corrosion behaviour of as-received (AR) and laser-treated (LT) samples was evaluated by electrochemical techniques and immersion test in a 3.5% NaCl solution. A melted layer consisting of a re-solidified layer with refined microstructure and dissolution of intermetallic particles (IMPs), and a thin film of aluminium oxides at the top, was formed after LSM for both alloys. The corrosion resistance of both alloys was improved after LSM. The results of immersion test showed different corrosion behaviour for LT AA2124 and LT AA6061. The delamination of the melted layer was observed for AA2124 but was not observed for AA6061 after exposure to 3.5% NaCl solution for 24 h. This was attributed to the formation of copper-rich segregation bands in the melted layer of AA2124 due to higher content of copper in AA2124 than AA6061. A significant number of micro-pores were present in the melted layer for AA2124 treated with high number of laser pulses, leading to the decrease of the corrosion resistance.

Experimental and Morphological Investigations Into Electrical Discharge Surface Grinding (EDSG) of 6061Al/Al2O3p 10% Composite by Composite Tool Electrode

In this study, a special experimental setup of EDSG using EDM and surface grinding machine has been developed in the laboratory to investigate the effect of seven input parameters namely tool polarity, peak current, pulse on-time, pulse off-time, rotational speed, abrasive particle size, and abrasive particle concentration on material removal rate (MRR) as performance measure of the process. The novelty of the present research work is that successful efforts have been made to machine the 6061Al/Al2O3p 10% metal matrix composites (MMC) by composite tool itself. The copper-based composite tool electrodes were fabricated by powder metallurgy route with different sizes of abrasives of silicon carbide, while 6061Al/Al2O3p 10% MMC were fabricated through stir-casting process. The research outcome will identify the important parameters and their effect on MRR of 6061Al/Al2O3p 10% composite in EDSG. The experimental results reveal that tool polarity, peak current, and rotational speed are the most influential parameters that affect MRR in EDSG process. The micro-structural and morphological analysis of machined surfaces has also been carried out to analyze the surface topography. It has been concluded that the abrasive particles substantially improves the MRR after removing the resolidified layer from the machined surface.

Study of the Surface Integrity of Powder-Metallurgy High-Speed Steel (S390) Multi-Cut by Wire Electrical Discharge Machining

The surface integrity of powder metallurgical high speed steel (S390) cut by wire electrical discharge machining (WEDM) has a great influence on the its fatigue life. In this paper, a study focused on surface integrity including white layer, surface finish, retained austenite, carbide and residual stress changed with multi-cutting has been described. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrograph (EDS) analysis were performed in the examination of the evolution of surface integrity. The experimental results reveal that the surface roughness, micro-cracks, white layer thickness and surface residual stress decrease with the increase of cutting pass, but the amount of retained austenite in the resolidified layer of the surface after the second cutting pass is the highest, compared with the other cutting passes. The content of carbides increases with the cutting pass and few carbides appear in the top section of the white layer of the first two cutting passes.

Processing of TiC-CNT Hybrid Composite Coating on Low Alloy Steel Using TIG Torch Technique

In the present study, a surface layer of TiC-CNT hybrid compositecoating has been developed on low alloy steel (LAS) by using powder preplacement and tungsten inert gas (TIG) torch melting techniques.The weighed powder mixture with the content of 1.0 mg per millimeter square area and containing TiC-4wt. % CNT particles was preplaced on LAS surfaces using a suitable binder and then melted under the TIG arc with an operating current of 100 A and energy input of 1440 J/mm. The microstructural and microhardness characteristics of the processed composites coating were analyzed using SEM, EDXand Vickers microhardness testers. Under the melting condition used in this investigation, the arc completely dissolved the composite powder mixture that resolidifiedinto a melt track with radial marks and smooth surfaces. Themelt cross sections were hemispherical in shape and produced 1 mm deep hard coating layer which were free from porosity and cracks. SEM micrographs TiC-CNT hybrid coated surface glazed at 100A consists of fineprecipitated TiC in form globular and cubic dendrites and this is the reason for hardness increment to an average of three times greater than that of substrate value. The result obtained indicated that TIG arc melting process is applicable to generate resolidified layer of precipitated TiC microstructure with uniformly distribution in the coating region.

Permeability control method with laser for porous bushings of aerostatic bearings

This paper presents the experimental results of investigating the feasibility of new method to control the permeability of porous bushings used for aerostatic bearings. Sintered bronze discs are used as porous bushings, and their surfaces are irradiated with pulsed Nd:YAG laser to generate re-solidified layer which plays role as an air restrictor in the air feeding line. After above surface modification process, the change in air permeability of bushings is examined and the influence of irradiating conditions is also investigated. Then, a stability test and a stiffness test are performed to exam the effect of the re-solidified layer generated by surface modification process. The surface modified bushings have achieved high fluid resistance for air, and high stability of bearing. Furthermore, it is confirmed that the surface modification with laser has improve the static stiffness of thrust bearing with porous bushing.

Melt behavior on the keyhole front during high speed laser welding

The flow of molten metal on the front wall of a laser generated welding keyhole has been observed by high speed photography, optically measured by mapping the flow of ripples on the liquid surface and theoretically calculated. A clear downward flow can be observed and measured by a Particle Image Velocimetry algorithm. A theoretical calculation of the melt thickness on the keyhole front is also presented. Results indicate that the thickness of the liquid on the keyhole front is similar to that of the resolidified layer found in micrographs of the front if the laser is suddenly turned off. The measured surface ripple flow speeds are between two and four times as high as the theoretical average fluid flow rate.

Melt layer erosion of pure and lanthanum doped tungsten under VDE-like high heat flux loads

Heat loads expected for VDEs in ITER were applied in the neutral beam facility GLADIS at IPP Garching. Several ∼3mm thick rolled pure W and W–1wt% La2O3 plates were exposed to pulsed hydrogen beams with a central heat flux of 23MW/m2 for 1.5–1.8s. The melting thresholds are determined, and melt layer motion as well as material structure evolutions are shown. The melting thresholds of the two W grades are very close in this experimental setup. Lots of big bubbles with diameters from several μm to several 10μm in the re-solidified layer of W were observed and they spread deeper with increasing heat flux. However, for W–1wt% La2O3, no big bubbles were found in the corrugated melt layer. The underlying mechanisms referred to the melt layer motion and bubble issues are tentatively discussed based on comparison of the erosion characteristics between the two W grades.

EXPERIMENTAL INVESTIGATION OF ELECTRODE WEAR AND RAPIDLY RE-SOLIDIFIED LAYER THICKNESS IN T90Mn2W50Cr45

This paper presents the relevance of standard Response surface methodology (RSM) for studying the effect of Electrode Wear (EW) and Rapidly Re-solidified Layer Thickness (RRLT) in the tool steel of T90Mn2W50Cr45 used in electrical discharge machining process. The process parameters Pulse on time, pulse off time, and pulse current were altered in the experiment. The Response surface methodology (RSM) is used for developing a mathematical model for the Electrode Wear and Rapidly Re-solidified Layer Thickness. The data obtained for Central Composite Rotatable Design (CCRD) has been used in mathematical model. The end result of Analysis of Variance (ANOVA) has been applied to verify the lack of fit and capability of the built-up models. The predicted and experimental values were quite close, which indicates that the developed model can be effectively used to predict the Electrode Wear and Rapidly Re-solidified Layer Thickness in the machining of tool steel of T90Mn2W50Cr45.DOI: http://dx.doi.org/10.5755/j01.mech.18.6.3164

S134012 ニッケル粉末混入放電加工面の表面特性に関する研究([S134]先進特殊加工技術)

Recently, EDM finishing technology has remarkably progressed, and very small surface roughness can be readily obtained by controlling the electrical discharge conditions. Also, EDM using powder mixed working fluids can improve the surface characteristics, such as wear resistance and corrosion resistance. Therefore, the EDM finished surface is well expected to be applied as a final metal mold surface in the near future. In that case, the surface characteristics of EDMed surface would influence the molding performance and the life of metal mold. As a new surface functionalization of EDMed surface, the surface characteristics of EDMed surface using a nickel powder mixed fluid is experimentally discussed in this study, since nickel coating has been widely applied to improve the corrosion resistance and the mold releasability of metal mold surface. Experimental analysis clarified that the resolidified layer containing nickel content could be formed on the surface by EDM in nickel powder mixed fluid. Also, the water repellency and the releasability of molded resin from the EDMed surface could be improved.

Super-finished Surfaces using Meso-micro EDM

A novel stochastic orbiting strategy combined with extremely low stray capacitance sinking EDM machine offers the capability to achieve super-finished surfaces (Ra<0.1μm) without pre-polishing of the copper tool electrodes. Surface defects such as inhomogeneity, cracks, arc spots and black spots have been analysed and reduced, while improving material removal rate with lower tool wear. Super-finished surfaces thus generated have uniform white layer (re-solidified layer) with an average thickness below 1μm. Unlike mirror finishing using EDM with powder mixed dielectric, absence of the additives in dielectric results in smaller gap width during finishing, delivering sub-micron accuracy of the machined features. With the tool electrodes machined using conventional machining techniques such as milling, turning or wire-EDM having surfaces with Ra≤0.2μm, advamcements of the work offers manufacturing industry an economic and energy efficient super-finishing process, especially in meso-micro scale.

Molecular Dynamics Simulation of Residual Stress Generated in EDM

Residual stress is one of the main characteristics of the surface machined by electrical discharge machining (EDM). In this paper, the formation mechanism of the surface residual stress, and its distribution were simulated using molecular dynamics methods. It was found that a larger pressure gradient was formed in the melting area during discharge. It was also found that there existed larger stress gradient in the melting region and in the solid region in contact with the melting area. The pressure gradient and the shear stress field is the important reason for the formation of the bulge of the discharge crater. From the distribution of the residual stress, it was found that the residual stress on the electrode surface is tensile while it is compressive inside the electrode, indicating that cracks are easy to be generated in the re-solidified layer.

Cutting NiTi with Femtosecond Laser

Superelastic shape memory alloys are difficult to machine by thermal processes due to the facility for Ti oxidation and by mechanical processes due to their superelastic behavior. In this study, femtosecond lasers were tested to analyze the potential for machining NiTi since femtosecond lasers allow nonthermal processing of materials by ablation. The effect of processing parameters on machining depth was studied, and material removal rates were computed. Surfaces produced were analyzed under SEM which shows a resolidified thin layer with minimal heat affected zones. However, for high cutting speeds, that is, for short interaction times, this layer was not observed. A depletion of Ni was seen which may be beneficial in biomedical applications since Ni is known to produce human tissue reactions in biophysical environments.

A014 Formation of re-solidified layer on the surface of dental hard tissue by Er:YAG laser irradiation

A014 Formation of re-solidified layer on the surface of dental hard tissue by Er:YAG laser irradiation