Recast Layer Formation Research Articles

Comparative study of tool wear rate with conventional and cryogenically treated electrode in electrical discharge drilling

Cryogenically treated electrically conductive tool-electrodes have been used for machining of super alloys. This enhanced and modifies the properties of tool-electrode, which improves machinability of EDMed alloys. Present study determines the effects of cryogenic treatment on performance of Cu-electrode for hole making on Inconel-718 super alloy. Cu-tool electrodes (having helical-threads) have being given rotational motion for effective flushing of debris, reducing formation of recast layer with less tool wear. EDM performance measure viz. tool wear rate has been evaluated against the variation of tool diameter, discharge current, pulse-on time, pulse-off time, tool rotation and depth of hole using Taguchi's method. The results reported that hole depth and discharge current are the two most significant process parameters effecting tool wear rate (TWR). It is also found that tool wear rate obtained lesser for cryogenically treated tool electrode.

Magnetic Field Assisted EDM: New Horizons for Improved Surface Properties

The current research study for hybrid machining of Al-based metal matrix composites (MMCs) employs magnetic field coupled traditional electrical discharge machining (EDM) to address the manufacturing demands in aeronautics, automobile, medical equipment, etc. The input processing parameters, for instance, magnetic field intensity, pulse-on/off duration, peak current, variant of electrodes as well as workpiece were evaluated to determine their after-effects on the responses in terms of microhardness (MH) and recast layer formation while machining of Al-SiC composites. The experimental results show 22% decrease in the surface microhardness values and thinner recast layer formation at magnetic field coupled higher spark energy. The results demonstrate the process stability and exhibit a good accord with experimental verification.

Crystallinity, surface morphology, and chemical composition of the recast layer and rutile-TiO2 formation on Ti-6Al-4V ELI by wire-EDM to enhance biocompatibility

In this study, the surface morphology, chemical composition, and crystallography of the recast layer on wire electrical discharge machining (wire-EDM) modified Ti-6Al-4V ELI were investigated through scanning electron microscopy, backscattered electron imaging, energy-dispersive x-ray spectroscopy, and x-ray diffraction. The characteristics of the two tailored cutting strategies, namely, main cut and finish trim cut, were evaluated in terms of micro-cracks, micro-pores, recast layer thickness (RLT), surface roughness (SR), phase transition, and micro-hardness. In addition, the relative changes of the SR, RLT, and kerf width were demonstrated with regard to peak current and pulse spacing. Noticeably fewer amounts of micro-cracks, micro-voids, remarkable truncation of RLT and SR, and nanoporous structure were attained at finish trim cut mode. It was also found out that an α → α′ + rutile-TiO2 phase transition occurred on the recast layer of wire-EDM modified sample at finish trim cut. Furthermore, recast layer showed higher micro-hardness compared to that of the heat-affected zone and bulk material. In essence, the wire-EDM finish trim cut treatment is a potential technology to feasibly enhance the biocompatibility for Ti-6Al-4V ELI alloy through the formation of rutile-TiO2 and amelioration of the surface morphology.

Comparison of machining characteristics of metal matrix composites using CO2 laser curve cutting process

In the present work, the effects of input variables of laser beam machining on the machining characteristics of the metal matrix composites reinforced with SiC, Al2O3, and ZrO2 particles were investigated. The comparison of the machining characteristics has been done to analyze the behavior of various reinforced particles with the variation of laser machining variables. Various output characteristics such as dross height, kerf deviation, and striations angle have been investigated and compared with each metal matrix composite material. Parameters such as cutting speed, reinforced particles, and cut profile were found to be the most significant factors influencing the various output characteristics. The morphological changes in the structure have been examined using scanning electron microscopy and X-ray diffraction technique for the agglomeration of the reinforced particles. The crack and recast layer formation has been examined in the specimens of higher quantity of the reinforced particles. It was analyzed that the metal matrix composites material reinforced with SiC particles has shown different behaviors as compared to other metal matrix composites materials.

Study of the Recast Layer of Particulate Reinforced Metal Matrix Composites machined by EDM

This study explores the relationship between the recast layer zone and the electrical discharge machining (EDM) process parameters behaviors on metal matrix composites (MMCs). The characteristics of recast layer are investigated by using a L27 Taguchi’s design of experiments. The formation of recast layer in three types of aluminum based particulate reinforced MMCs have been examined in combination with process parameters in the variant dielectric medium. The influence of each selected parameter is obtained by ANOVA analysis. It is found that the formation of recast layer is significantly affected by the reinforcement architectures in matrix phase and the spark energy of EDM. The cross-sectional micrograph of recast layer is assessed for the metal removal mechanism and EDM-induced surface alterations on the machined zone.

Modeling and optimization of process variables of wire-cut electric discharge machining of super alloy Udimet-L605

This paper presents the behavior of Udimet-L605 after wire electric discharge machining and evaluating the WEDM process using sophisticated machine learning approaches. The experimental work is depicted on the basis of Taguchi orthogonal L27 array, considering six input variables and three interactions. Three models such as support vector machine algorithms based on PUK kernel, non-linear regression and multi-linear regression have been proposed to examine the variance between experimental and predicted outcome and preferred the preeminent model based on its evaluation parameters performance and graph analysis. The grey relational analysis is the relevant approach to obtain the best grouping of input variables for maximum material removal rate and minimum surface roughness. Based on statistical analysis, it has been concluded that pulse-on time, interaction between pulse-on time x pulse-off time, spark-gap voltage and wire tension are the momentous variable for surface roughness while the pulse-on time, spark-gap voltage and pulse-off time are the momentous variables for material removal rate. The micro structural and compositional changes on the surface of work material were examined by means of SEM and EDX analysis. The thickness of the white layer and the recast layer formation increases with increases in the pulse-on time duration.

Fabrication of various shaped tungsten micro pin arrays using micro carving technology

This paper describes a state of the art in micro-structuring high strength metallic materials. Tungsten micro pin arrays in a variety of shapes are fabricated using a micro carving technology, which combines laser beam machining and electrochemical etching processes. First, micro pin arrays were rough-shaped by laser beam machining along a pre-defined scanning path to control their structural shape. The micro pin array in this stage had near-conical shape of structures due to a recast layer. Next, the genuine shape of micro pin arrays came to the surface via electrochemical etching process to elute the recast layer into electrolyte. Quantitative elemental analysis with energy-dispersive spectroscopy (EDS) was implemented to characterize the formation of recast layer on a micro pin structure after the laser beam machining process. The atomic percentage EDS maps indicated that higher percentage of tungsten was detected on the core micro pin structure, whereas relatively large percentage of oxygen was found on the recast layer (O 9%, W 91% in the center area, and O 53%, W 47% in the outer area).

Effect of wire diameter on surface integrity of wire electrical discharge machined Inconel 706 for gas turbine application

Inconel 706 superalloy has established itself in the field of gas turbine industry because of its easy fabricability combined with high mechanical strength. Due to its high stress rupture and tensile yield strength, conventional machining of this superalloy exhibits poor surface and low dimensional accuracy of the machined components. It is well known that most of the gas turbine components include complex shaped profile with high precision and hence, wire electrical discharge machining (WEDM) of Inconel 706 has been performed to achieve the feasibility in manufacturing of complex shaped components for gas turbine application. In the current investigation, the effect of wire diameter on WEDM performance characteristics such as cutting speed, surface roughness, surface topography, recast layer formation, microhardness, microstructural and metallurgical changes have been evaluated. It was investigated that smaller diameter wire is advantageous over the larger diameter wire since it improves productivity as well as surface quality of the machined components under the same settings of control parameters. In addition, smaller diameter wire has shown comparatively lower recast layer thickness, minimum hardness alteration and shorter manufacturing time. The XRD result has confirmed the presence of residual stress within WED machined component.

Effect of Wire Material on Productivity and Surface Integrity of WEDM-Processed Inconel 706 for Aircraft Application

Inconel 706 is a recently developed superalloy for aircraft application, particularly in turbine disk which is among the most critical components in the gas turbine engines. Recently, wire electrical discharge machining (WEDM) attained success in machining of gas turbine components which require complex shape profiles with high precision. To achieve the feasibility in machining of these components, the research work has been conducted on Inconel 706 superalloy using WEDM process. And, the effect of different wire materials (i.e., hard brass wire, diffused wire, and zinc-coated wire) on WEDM performance characteristics such as cutting speed, surface topography, surface roughness, recast layer formation, residual stresses, and microstructural and metallurgical alterations have been investigated. Even though, zinc-coated wire exhibits improved productivity, hard brass wire was found to be beneficial in terms of improved surface quality of the machined parts. Additionally, lower tensile residual stresses were obtained with hard brass wire. However, diffused wire has a moderate effect on productivity and surface quality. Under high discharge energy, higher elemental changes were observed and also the white layer was detected.

Experimental investigations on surface integrity issues of Inconel-690 during wire-cut electrical discharge machining process

Nickel-based alloys are finding a wide range of applications due to their superior properties of maintaining hardness at elevated temperatures, low thermal conductivity and resistance to corrosion. These materials are used in aircraft, power-generation turbines, rocket engines, automobiles, nuclear power and chemical processing plants. Machining of such alloys is difficult using conventional processes. Wire-cut electrical discharge machining is one of the advanced machining processes, which can cut any electrically conductive material irrespective of its hardness. One of the major disadvantages of this process is formation of recast layer as it affects the properties of the machined surfaces. In this study, experimental investigation has been carried out to study the effect of wire-cut electrical discharge machining process parameters on micro-hardness, surface roughness and recast layer while machining Inconel-690 material. Interestingly, hardness of the machined surface was found to be lower than that of the bulk material. The micro-hardness and recast layer thickness are inversely related to the variation of process parameters. Recast layer thickness, surface roughness and hardness of the wire-cut electrical discharge machined surfaces of Inconel-690 are found to be in the range of 10–50 µm, 0.276–3.253 µm and 122–171 HV, respectively, for different conditions. The research findings and the data generated for the first time on hardness and recast layer thickness for Inconel-690 will be useful to the industry.

A review on recent developments in machining methods based on electrical discharge phenomena

The process of electrical discharge machining (EDM) is one of the modern machining processes that machining stiff and high-strength parts such as ceramics and heat-treated steels, which traditional methods cannot be successful in their materials removal, is the most important applications of the this process. Deep drilling, machining inclined surfaces and small-scaled machining are the other applications of this process. Despite the unique applications of this method of machining, low materials removal rate (MRR), high surface roughness (SR), high tool wear rate (TWR), formation of recast layer on workpieces surfaces (that is location of defects and cracks) and environmental problems are the main problems and limitations of this process. This paper reviews the current research trends in EDM process containing dry EDM, near dry EDM, magnetic field assisted EDM, ultrasonic vibrations assisted EDM, and powder mixed EDM processes which were developed in order to overcome the limitations of EDM process.

Parametric analysis of recast layer formation in wire-cut EDM of HSLA steel

This paper presents an experimental investigation to determine the main wire electrical discharge machining (WEDM) process parameters which contribute to recast layer formation in high-strength low-alloy (HSLA) steel. Influence of process parameters, namely, pulse-on time, pulse ratio, power (discharge current), pulse (spark gap), and wire speed, has been explored systematically on hardened HSLA steel by varying machining conditions using factorial design of experiments. Analysis of variance was performed and pulse-on time and wire speed were found to be significant parameters affecting recast layer. Topographical images indicated surface irregularities like craters, globules, and micro-voids which affect the surface morphology of machined surface. Micrographs of SEM revealed existence of three distinguishable surface layers including recast layer (amorphous and columnar structure), heat-affected zone, and base material. EDS analysis was performed, and it was found that recast layer is free of alloying effects from electrode (molybdenum wire); however, surface layer is oxidized.

Multi-objective optimization of laser curve cutting of aluminium metal matrix composites using desirability function approach

The machining of aluminium metal matrix composite (MMC) material reinforced with SiC particles has been identified as typical process with traditional machining process. In the present study, curve cutting has been performed on the MMC material using CO2 laser cutting system. The investigations have been carried out to identify the thermal effects of SiC particles on surface roughness and kerf deviation. To optimize the quality characteristics, the response surface model is formulated using desirability functional approach. The optimal settings of various input variables have been identified for quality characteristics. The input parameters like cutting speed, reinforced particles and arc radius have been examined as significant factors. The adequacy of predicted response surface model was verified by performing the set of experiments and predicted error percentage method. The formation of recast layer and new phase Al4C3 was detected using SEM and XRD plot, respectively.

Development of a quantitative method for the characterization of hole quality during laser trepan drilling of high-temperature alloy

Short-pulsed lasers are of significant industrial relevance in laser drilling, with an acceptable compromise between accuracy and efficiency. However, an intensive research with regard to qualitative and quantitative characterization of the hole quality has rarely been reported. In the present study, a series of through holes were fabricated on a high-temperature alloy workpiece with a thickness of 3 mm using a LASERTEC 80 PowerDrill manufacturing system, which incorporated a Nd:YAG millisecond laser into a five-axis positioning platform. The quality of the holes manufactured under different laser powers (80–140 W) and beam expanding ratios (1–6) was characterized by a scanning electron microscope associated with an energy-dispersive X-ray analysis, focusing mainly on the formation of micro-crack and recast layer. Additionally, the conicity and circularity of the holes were quantitatively evaluated by the apparent radius, root-mean-square deviation, and maximum deviation, which were calculated based on the extraction of hole edge through programming with MATLAB. The results showed that an amount of melting and spattering contents were presented at the entrance end and the exit end of the holes, and micro-cracks and recast layer (average thickness 15–30 µm) were detected along the side wall of the holes. The elemental composition of the melting and spattering contents and the recast layer was similar, with an obvious increase in the contents of O, Nb, and Cr and a great reduction in the contents of Fe and Ni in comparison with the bulk material. Furthermore, the conicity and circularity evaluation of the holes indicated that a laser power of 100 W and a beam expanding ratio of 4 or 5 represented the typical optimal drilling parameters in this specific experimental situation. It is anticipated that the quantitative method developed in the present study can be applied for the evaluation of hole quality in laser drilling and other drilling conditions.

Study of Magnetic Field-Assisted ED Machining of Metal Matrix Composites

The present study deals with an investigation of the hybrid electric discharge (ED) machining process executed in a magnetic field for improving process performance. Previous magnetic field-assisted electric discharge machining (MFAEDM) techniques, however, are limited to use with a class of magnetic workpieces. In this particular study, the magnetic field was coupled with the conventional EDM plasma zone to test the hybrid process on Al-based metal matrix composites (MMCs). The machining parameters, for instance, peak current as well as duration of pulse-on were selected to nail down thereafter effects on the response parameters like the material removal rate (MRR) and the surface integrity. The experimental results show an improvement of 12.9% MRR and reduction in recast layer formation at higher spark energy in the magnetic field environment. As the experimental outcome implied that the MFAEDM imparted appreciable process stability, a highly efficient pertinent process of EDM with high quality of the machined surface could be accomplished to satisfy modern industrial applications.

Evaluation of WEDM performance characteristics of Inconel 706 for turbine disk application

Inconel 706 is a newly developed superalloy, which offers high mechanical strength alongwith easy fabricability thus making it suitable for turbine disk applications. Although Inconel 706 exhibits a substantial increase in stress rupture and tensile yield strength compared to other superalloys, its conventional machining yields poor surface finish and low dimensional accuracy of the machined components. Hence, wire electrical discharge machining (WEDM) of Inconel 706 has been performed and various performance attributes such as material removal rate (MRR), surface roughness (SR), recast surface, topography, microhardness, microstructural and metallurgical changes of the machined components have been evaluated. The experimental results revealed that servo voltage, pulse on time, and pulse off time greatly influence the MRR and SR. Due to high toughness of Inconel 706, no micro cracks were observed on the machined surface. Micro voids and micro globules are significantly reduced at low pulse on time and high servo voltage. But, there is a propensity of thick recast layer formation at high pulse on time and low servo voltage. EDAX analysis of recast surface exposed the existence of Cu and Zn which have migrated from the brass wire. The subsurface microhardness was changed to 80μm due to significant thermal degradation.

Correlation of surface roughness and recast layer thickness in electrical discharge machining

Electrical discharge machining (EDM) is an extensively used method in the machining of electrically conductive materials. Recast or white layer formation is undesirable, but inevitable, result of EDM and needs to be understood and accurately determined to efficiently perform post-treatment processes for removing the recast layer caused by EDM process. In this study, recast layer thickness and surface roughness data obtained from experimental study were analyzed and a correlation between these two parameters has been established. Image-processing technique has been used for obtaining of recast layer thickness data. It was observed that the correlation between recast layer thickness and surface roughness increases remarkably with the increase of working current and pulse time. The correlation obtained in this study has the potential to predict the recast layer thickness on spark-eroded surfaces from simple surface roughness values instead of using the prevailing time-consuming and tedious etching and polishing method. The possible approximation of the recast layer thickness using a thermal model is also discussed.

Surface modification induced phase transformation and structure variation on the rapidly solidified recast layer of titanium

In this study, neodymium-doped yttrium orthovanadate (Nd:YVO4) as a laser source with different scanning speeds was used on biomedical Ti surface. The microstructural and biological properties of laser-modified samples were investigated by means of optical microscope, electron microscope, X-ray diffraction, surface roughness instrument, contact angle and cell cytotoxicity assay. After laser modification, the rough volcano-like recast layer with micro-/nanoporous structure and wave-like recast layer with nanoporous structure were generated on the surfaces of laser-modified samples, respectively. It was also found out that, an α→(α+rutile-TiO2) phase transition occurred on the recast layers of laser-modified samples. The Ti surface becomes hydrophilic at a high speed laser scanning. Moreover, the cell cytotoxicity assay demonstrated that laser-modified samples did not influence the cell adhesion and proliferation behaviors of osteoblast (MG-63) cell. The laser with 50mm/s scanning speed induced formation of rough volcano-like recast layer accompanied with micro-/nanoporous structure, which can promote cell adhesion and proliferation of MG-63 cell on Ti surface. The results indicated that the laser treatment was a potential technology to enhance the biocompatibility for titanium.

An investigation on the hole quality during picosecond laser helical drilling of stainless steel 304

Precision drilling with ultra-short pulse lasers (e.g., picosecond and femtosecond) has been advocated to significantly improve the quality of the micro-holes with reduced recast layer thickness and no heat-affected zone. However, a combination of high-power picosecond laser with helical drilling strategy in laser drilling has rarely been reported in previous studies. In the present study, a series of micro-holes with circular, triangular, rectangular, and rhombic shapes (diameter 0.6 mm) were manufactured on stainless steel 304 using a newly developed laser drilling system which incorporated a picosecond laser and a high-speed laser beam rotation apparatus into a five-axis positioning platform. The quality of the helical drilled holes, e.g., recast layer, micro-crack, circularity, and conicity, were evaluated using an optical microscope, an optical interferometer, and a scanning electron microscope. In addition, the microstructure of the samples was investigated following etching treatment. It was demonstrated that the entrance ends, the exit ends, and the side walls of the micro-holes were quite smooth without accumulation of spattering material and formation of recast layer and micro-crack. No tapering phenomenon was observed, and the circularity of the holes was fairly good. There was no distinctive difference with regard to the microstructure between the edges of the holes and the bulk material. Picosecond laser helical drilling can be an effective technique for manufacturing of micro-holes with very high quality. The development of high-power picosecond laser would promote picosecond laser drilling to be more industrial relevance in the future.

Multi Process Parameter Optimization of Diesinking EDM on Titanium Alloy (Ti6Al4 V) Using Taguchi Approach

Out of the several unconventional machining systems EDM (Electric Discharge Machining) is one of the most versatile and universally accepted newer machining systems used in industries for machining of conductive materials. The inherent disadvantage of EDM is low machining rate and inferior machined surface. Titanium Alloy Grad-V (Ti6Al4V) is a robust material with wide application in aerospace and in other field. The superior thermo-mechanical properties of Titanium Alloy Grad-V (Ti6Al4V) are excellent which increased the uses of this material in many engineering fields. In the present work the L-9 Orthogonal Array is used for optimizing the process parameters in Die-Sinking EDM on Titanium Alloy Grad-V (Ti6Al4V).The SEM microscopy of the machined surface was done to know the micromechanics responsible during machining. The micro cracks on the machined surface and the formation of recast layer were observed in the SEM micrograph. The XRD of the machined surface was also carried out to know the kinetics of phase change during the thermo-mechanical process of machining. The XRD analysis indicates some trace of formation of cubic titanium carbide and some trace of cubic and hexagonal carbon were also observed.