Thick Recast Layer Research Articles

Influence of Microstructure and Alloy Composition on the Machinability of α/β Titanium Alloys.

A comparative study was conducted for the machining of two α/β titanium alloys, namely Ti-6Al-4V (Ti64) and Ti-6Al-7Nb (Ti67), using wire electric discharge machining (WEDM). The influence of cutting speed and cutting mode on the machined surfaces in terms of surface roughness (Ra), recast layer (RL), and micro-hardness have been evaluated. Rough cut (RC) mode at a cutting speed of 50 µm/s resulted in thermal damage; Ra was equal to 5.68 ± 0.44 and 4.52 ± 0.35 µm for Ti64 and Ti67, respectively. Trim-cut mode using seven cuts (TRC-VII) at the same speed decreased the Ra to 1.02 ± 0.20 µm for Ti64 and 0.92 ± 0.10 µm for Ti67. At 100 µm/s, Ra reduced from 2.34 ± 0.28 µm to 0.88 ± 0.12 µm (Ti64), and from 1.42 ± 0.15 µm to 0.90 ± 0.08µm (Ti67) upon changing from TRC-III to TRC-VII. Furthermore, a thick recast layer of 30 ± 0.93 µm for Ti64 and 14 ± 0.68 µm for Ti67 was produced using the rough mode, while TRC-III and TRC-VII modes produced layers of 12 ± 1.31 µm and 5 ± 0.72 µm for Ti64 and Ti67, respectively. Moreover, rough cut and trim cut modes of WEDM played a significant role in promoting the surface hardness of Ti64 and Ti67. By employing the Response Surface Methodology, it was found that the machining mode followed by cutting speed and the interaction between them are the most influential parameters on surface roughness. Finally, mathematical models correlating machining parameters to surface roughness were successfully developed. The results strongly promote the trim-cut mode of WEDM as a promising machining route for two-phase titanium alloys.

Effects of process parameters on fatigue behavior and surface integrity of tool steel produced by electrical discharge turning

AbstractElectrical discharge machining is used in the production of countless parts with complex geometries and micro dimensions, from many elements of industrial molds to parts of motors and pumps. Also, most of these parts are cylindrical and it is always more meaningful to study their rotational fatigue behavior to predict their response during their operation. This study concentrated on the impacts of machining parameters on the surface quality and fatigue behavior of tool steel shaped by electrical discharge turning. The results based on Taguchi methodology have shown that discharge current affects Ra and Rz more, and pulse duration more affects the mean spacing of profile irregularities, Sm. As a result of the heat affected depth in the machined region, which changes in proportional with the processing parameters, the microhardness decreased from the sample surface to the core. The maximum hardness was measured at current of 12 A, pulse duration of 3 μs and pulse interval of 7 μs. According to the fatigue tests, it was found that the fatigue life decreased with the increase in Rz and Sm values. Moreover, high microhardness and thick recast layer reduced the fatigue strength of the samples with relatively smooth surface topography.

The effect of discharge energy and nano Al2O3 on μED milling of Inconel 718

Micro EDM is a well-recognized machining process for complex and precise three-dimensional micro features. However, the disadvantages are high machining time, poor surface finish, and a thick recast layer that restricts the wide establishments. Powder mixed EDM (PMEDM) is a recent advancement in the EDM process that enhances the sparking frequency and inter-electrode gap. This paper presents the effect of Al2O3 nanopowder on micro ED milling of Inconel 718 at chosen levels of discharge energy (3.2 μJ to 7.2 μJ, 32 μJ to 72 μJ and 320 μJ to 720 μJ). To understand the dynamics of the process, comprehensive experimental research was carried out using a full factorial design with 60 experiments. Material removal rate, tool wear rate, and surface finish were evaluated. The surface topography and material migration to the machined surface was also studied. The results showed significant improvement of 157%, 20%, and 21% in MRR, TWR, and surface roughness respectively in nanopowder mixed micro-EDM when compared with conventional micro-EDM. The addition of Al2O3 nanopowder reduced crater size in all three discharge energy ranges and reduced carbon deposition on the machined surface.

Investigation of machining and modified surface features of 316L steel through novel hybrid of HA/CNT added-EDM process

Hydroxyapatite powder (HAp) mixed-EDM process causes surface defects including a high roughness, crack and a thick recast layer attributing a low wettability and amorphous alloys as well as a low material erosion rate (MER) while processing the metallic biomaterials. To mitigate these occurrences, many measures have been conducted. This research aims to investigate the feasibility of hybrid HA and carbon nanotubes (CNT) powders mixed EDM process for processing 316L steel. This study also aims to explore the impacts of CNT and the associated variables on machining and surface characteristics of 316L steel. To ameliorate the machining and surface responses, a high electrically and thermally conductive CNT particles have been added in the HAp suspended mineral oil. To address the set objectives, several characterizations techniques including profilometer, goniometer, energy dispersive X-ray (EDX), X-ray diffraction (XRD), and scanning-electron microscope (SEM) have been carried out. In this study, the multiple additives (HAp and CNT) mixed-EDM process has been successfully introduced. Addition of the CNT in the HAp mixed mineral oil improves MER to 26.47 mg/min and declines SR to 3.16 μm. A thin uniform recast layer of 12.4 μm attributing cracks free surface and a high hydrophilic in nature having contact angle of 35° has been obtained in this study. Elemental composition analysis reveals a high content of Ca, P, O and C in the recast layer formed. The oxides, carbides, and CNT reinforced Ca-P based oxides with both crystalline and amorphous phases have been achieved. In this research work, a novel and effective method for processing the biomedical devices made of 316L steel has been proposed.

Experimental Investigation on Surface Integrity and Wear Behavior of Ti-6Al-7Nb Alloy under Rough and Trim Cut Modes of Wire Electrical Discharge Machining

Surface properties of metallic material significantly affect its fatigue, tribological and other functional properties. Titanium (Ti) alloys are hard to machine material, and therefore, wire electrical discharge machining (WEDM) is preferred over the conventional turning and milling process for machining of Ti-alloys. In the present work, surface integrity and wear behavior of Ti-6Al-7Nb have been evaluated under rough cut and trim cut modes of WEDM process. Rough cut mode resulted in high thermal damage with high surface roughness (Ra 2.34 μm), thick recast layer (Avg. 13.71 μm) and high crack density with large size micro-cracks. Significant improvement in surface properties of Ti-6Al-7Nb has obtained using trim cut mode with reduced surface roughness (0.94 μm), thin recast layer (3.51 μm), and the average crack length and average crack density reduced by 30% approximately. Energy-dispersive spectroscopy analysis showed that the trim cut mode yielded a high percentage of Ti and O elements in machined surface layers and reduced the contents of Al, Cu and C, thus favors the improved biocompatibility of Ti-alloy. Rough cut and trim cut modes of WEDM played the significant role in enhancing the surface hardness of Ti-6Al-7Nb, thus affected the coefficient of friction (c.o.f.) and wear rate. Trim cut mode resulted in lowest c.o.f. and wear rate as compared to rough cut machining and polishing. The results of the present work are highly motivated to promote the trim cut mode of WEDM as a promising manufacturing route for Ti-alloy.

Surface and subsurface characteristics of laser polished Ti6Al4V titanium alloy

This paper aims at investigating the effects of laser power, laser pulse repetition rate and scan speed on the polished metal surface and subsurface characteristics. The influence of initial surface roughness of workpiece on the polishing performance was also taken into account. In this study, Ti6Al4V titanium alloy was selected as a specimen to be polished by a nanosecond pulse laser. The results revealed that the surface roughness was able to be improved by about 43% when the laser fluence of 1–3 J/cm2 was applied. In addition, the proper polishing condition was dependent on the initial surface roughness of workpiece, and a better surface improvement was subjected to a lower initial surface roughness. A thick recast layer, high degree of oxidation and large heat-affected zone with lots of micro-cracks were obtained when too high laser power and slow scan speed were used in the polishing of titanium alloy. With regard to the findings of this study, the suggested polishing conditions could be applied for smoothing the different levels of initial surface roughness of titanium alloy and possibly other similar metals with less subsurface damage.

Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5

The extensive use of Titanium alloy Ti-6Al-4V in defense, aerospace, automotive and biomedical sectors has made this material most demandable among industries. Due to low thermal conductivity it is difficult to machine by conventional machining processes. Electric discharge machining is a viable option to machine extremely harder and electrically conductive materials. There are a lot of possibilities in EDM to machine super alloys. Electrical discharge machining has also huge potential when it comes to biomaterials. Super alloy like Titanium grades 5 is most suitable for the biomedical field due to its desirable bio-compatible properties. In this study effect of cryogenic treatment of tool, current, pulse on time, pulse off time and voltage has been investigated on tool wear rate. Optimized conditions for minimum tool wear rate have been found and microstructural analysis was also carried out to observe the surface topography of machined surface with non treated and cryogenic treated tool electrode. It has been observed that tool wear rate of cryogenic copper tool is comparatively more as compared to non treated copper tool electrode. The effect of current, on-time and type of tool electrode is found to be more pronounced as compared to other machining parameters. Thick recast layer with wide surface cracks is observed on the surface machined by the non treated tool electrode. Whereas narrow micro cracks with thin recast layer is observed on the surface machined by cryogenic treated tool electrode.

Analysis of Surface Topography of Inconel 718 of Miniature Spur Gear Machined Using WEDM

WEDM is very frequently used as one of the precise non-conventional machining process for generating complex profile or difficult size and shape products. This paper mainly deals with the important aspect surface topography of the miniature spur gear on Inconel 718 generated by WEDM process with five machining inputs such as GV, Ip, Ton, Toff and feed rate. At high discharge energy discontinuous type and non-uniform nature thick white recast layer is generated over work surface. In the present content, progression appraisal to generate best quality spur gear with excellent surface finish is the prominent criteria. RSM is used for optimising the responses such as SR and MRR. To develop performance parameters, intelligent model is generated using ANFIS. From the result obtained, it is cleared that at low discharge energy such as low voltage and short pulse on time, free from voids, continuous and uniform thin white layer are achieved.

Study of surface integrity and effect of process parameters in wire electrical discharge turning of Ti-6Al-4V

Wire electrical discharge turning set-up has been developed by modifying the conventional five axes CNC WEDM machine. The main objective of this setup is to achieve cylindrical forms on hard to cut materials. This work focuses on the study of effect of input process parameters like pulse on-time, pulse off time, gap voltage, spindle rotational speed on output responses like surface roughness, material removal rate and wire wear ratio. A mathematical model of responses has been developed using response surface methodology and the optimal value of process parameters has been obtained by desirability function. Surface morphology studies of the machined surface and the worn-out wire has also been elaborated by FE-SEM images. The results show that with an increase in machining parameters value except pulse-on time, all the desired machining outcome decreases. Surface roughness, material removal rate and wire wear ratio have been found in the range of 1.99 μm – 1.37 μm, 7.55 – 13.66 mm3/min and 0.05–0.08, respectively. The formation of thick recast layer over the machined surface has also been discussed. The reduction in wire dimension has been calculated by optical microscopy and its morphology has been discussed by FE-SEM images.

Parametric study on the recast layer during EDWC of a Ni-rich NiTi shape memory alloy

Shape memory alloys (SMAs) have exceptional properties, namely, shape memory effect, superelasticity and other enriched physical and mechanical properties. The conventional machining of these alloys is associated with several severe difficulties. Electric discharge wire cutting (EDWC), an advanced machining process, is utilized to machine Ni50.89Ti49.11 SMA under this investigation. The aim of this investigation was to reveal the effect of EDWC variable parameters such as pulse on time ( TON), wire feed (WF) rate, spark gap voltage (SV), wire tension (WT) and pulse off time ( TOFF) on recast-layer thickness (RLT) and a foreign element atomic content (FEAC) using the one-factor-at-a-time approach. Scanning electron microscope equipped with energy dispersive X-ray spectroscopy facility was utilized to analyse RLT and FEAC. RLT and FEAC are varied linearly and nonlinearly respectively with TON, TOFF and SV. A thick recast layer with a high percentage of FEAC was formed on the machined surface of Ni50.89Ti49.11 SMA at a higher TON and a lower TOFF and SV. However, RLT and FEAC are slightly varied with WF and WT.

Research on an EDM-based unitized drilling process of TC4 alloy

To reduce the recast layer and cracks formed during the small hole, electrical discharge machining (EDM) of the TC4 alloy, negative polarity EDM, and electrical discharge machining/electrochemical machining combined process (EDM/ECM) were carried out. The machining efficiency, the recast layer, and the cracks were studied. The experimental results showed that when compared with common positive polarity EDM under the specific processing condition, the recast layer of the negative polarity EDM decreased by 65% in thickness but the material removal rate (MRR) decreased by 31%. In EDM/ECM, MMR was 18% higher than the positive polarity EDM, and the majority of the recast layer and the cracks were removed. However, issues including the low efficiency occur during the penetration period. The following paper analyzed the causes of these problems and proposed a novel unitized hole drilling process. In this unitized process, EDM/ECM was initially used and then converted to negative polarity EDM with external flushing water during the penetration period. The general machining efficiency of a single hole increased by 73% over conventional EDM/EDM. Furthermore, only a 7-μm thick recast layer remained at the hole’s outlet, with no obvious cracks found.

Surface modification of β-phase Ti implant by hydroaxyapatite mixed electric discharge machining to enhance the corrosion resistance and in-vitro bioactivity

The study presented an innovative method for surface modification of β-phase titanium alloy using hydroxyapatite mixed electric discharge machining (HAM-EDM). The process enables one to deposit in-situ a biomimetic nano-porous HA-containing layer while shaping the base titanium, hence modifying the surface properties of the original substrate. A series of the dedicated HAM-EDM on titanium alloys have been conducted. Surface integrity, topography, and elemental composition of the modified surface were investigated by FE-SEM, EDS, XRD, and indentation techniques, while in vitro cell study was performed to evaluate biocompatibility and cell attachment of the treated surface. The morphology characterization results revealed that a natural bone-like nano-porous surface topography has been imparted on the β-phae Ti implant surface using the HAM-EDM. The EDS and XRD examinations showed that the deposited layer comprised of Ti, Nb, Ta, Zr, O, Ca and P elements and formed biocompatible phases such as Ca3(PO4)2, CaZrO3, Nb8P5, CaO, TiP, Nb4O5, and TiO2, TiH on the β-Ti implant surface, which improved the bioactivity of the alloy and beneficial for the promotion of osseointegration. The results revealed that a 18–20μm thick recast layer containing biocompatible phases was generated, which has excellent metallurgical bonding with the base surface and offered mechanical interlocking to delamination. The HA deposited surface shows am improved hardness of 1275HV which is 3-fold higher than the untreated surfaces; predominantly owing to the deposition of hard oxides on the modified surface. The HA-deposited bioceramic layer presented an excellent and higher corrosion resistance as compared to EDMed and un-treated specimens in simulated body fluid. The in-vitro bioactivity results confirmed that the nano-porous HA-containing layer exhibited the superior bioactivity and promotes adhesion, growth, proliferation, and differentiation of human osteoblastic MG-63 cells.

Analysis and Optimization of WEDM Performance Characteristics of Inconel 706 for Aerospace Application

Wire Electrical Discharge Machining (WEDM) has established itself for manufacturing of precise and complex shape components for aerospace application due to the high quality requirement of aerospace components such as normal residual stress, no cracks, no recast layer, no porosity; still there is a need to optimize the control parameter settings and evaluate the performance characteristics of the WEDM process. The experiments have been conducted on Inconel 706 which is a newly-developed superalloy specially for aircraft application. A hybrid approach has been used to optimize the material removal rate (MRR) as well as surface roughness (SR) and significant control parameters have been identified using analysis of variance (ANOVA). Microstructure analysis revealed the formation of microglobules, melted debris and microholes on the machined surface, but no microcrack was detected due to the high toughness of the alloy. Energy dispersive X-ray spectroscopy (EDAX) has been carried out to study the metallurgical changes in the WED machined surface. The topography analysis of the curved surface revealed the best surface quality of the machined component at low pulse on time and high pulse off time. A thick recast layer of 39.6 µm was observed at high pulse on time and low servo voltage. Microhardness of the machined surface was changed up to a depth of 70 µm due to cyclic thermal loading during the WEDM process.

Study on recast layer thickness and residual stress during WEDM of SMAs

Shape memory alloys (SMAs) are difficult to machine by thermoelectric processes due to the affinity of titanium (Ti) for oxygen, and the specific properties of SMAs have been taken into account for the design of SMA actuators. In this investigation, wire electro-discharge machining of a specific SMA is investigated with specific reference to the recast layer thickness and residual stress state. The effects of processing parameters such as pulse on time, pulse off time and servo voltage on the recast layer thickness and residual stress state were evaluated. A scanning electron microscopy investigation on the recast layer showed that its thickness was a strong function of pulse on time and servo voltage, with the thinnest recast layer commensurate with high servo voltage and short pulse on time. The residual stress state was found to follow closely the recast layer thickness – that is, a thick recast layer implied a large tensile residual stress and vice versa. In conclusion, it was demonstrated that a short pulse on time combined with a high servo voltage produced the lowest residual stresses and had the least impact on the surface and subsurface properties of the material.

Effect of Cryogenic Treatment of Tool Electrode on the Machining Performance and Surface Finish during Electrical Discharge Machining of Hastelloy C-4

Advancements in machining science provided a wider scope for the development of composites and advance materials. As the researches are going on into this field newer materials are coming into existence, having complex meteorological structure and high mechanical-resistance ability. Due to the extreme strength, toughness and hardness of these materials advance machining processes are taking over the conventional machining processes in this field. Electrical discharge machining (EDM) is one of its own kind of advance machining process, which has been used during this research work. This study investigates the performance and surface finish of cryogenic treated and non-treated copper tool electrode during electrical discharge machining of Hastelloy C-4. Effect of cryogenic treatment, current, Pulse on time, Pulse off time was investigated on surface roughness. The comparative study reveals that pulse on time, tool electrode and current are the most influential parameters that affect Performance and surface finish during EDM. The study shows that cryogenic treated copper tool generates better surface finish than the conventional copper tool electrode. Microstructural studies show that thick recast layer was observed in case of cryogenic treated copper electrode and also deep shallow craters were found at higher value of current.

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.

Surface modification of AISI H13 tool steel with silicon or manganese powders mixed to the dielectric in electrical discharge machining process

In electrical discharge machining (EDM) process, one of the most important aspects is the surface quality of the workpiece. When a uniform and thick recast layer is achieved with characteristics of low roughness, high hardness, and the absence of pores and micro-cracks, it acts as a kind of coating. Such surface is required by mold-making industry, where the molds are subjected to chemical and abrasive wear, and the surface needs to present high resistance against corrosion and abrasive forces. The use of powder particles suspended in the dielectric is a way to provide such improvement and, at the same time, avoiding the need for subsequent polishing. This work investigated the influence of silicon and manganese powders with fine particle sizes, using two different concentrations, suspended in the dielectric when EDM machining AISI H13 tool steel. It evaluated the surface roughness, hardness, and the chemical composition and micro-structure of the recast layer; using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) techniques. The best results were obtained for silicon powder; presenting the surface roughness improved about five times, when compared to the conventional EDM process, as well as a thick and uniform recast layer without micro-cracks and pores. The silicon and the manganese powders also promoted an increase of the recast layer hardness of about 40 % when compared to the conventional EDM process.

Laser drilling of micro-hole arrays in tantalum

X ray collimator optics for space application require an array of high aspect ratio holes of 60:1 with a minimal tantalum (Ta) thickness of ≥2 mm and a very high open area fraction (hole versus wall fraction) of 70% to achieve high collimator efficiency. Each collimator with a drilled area of 110 mm × 70 mm contains several million holes and need a fast drilling process. Laser percussion drilling was performed using an IR pulsed disk laser in a 1 and 2 mm thick Ta plate. A tightly spaced hexagonal closed packed pattern was used to maximize open area fraction with hole-to-hole spacing as small as 80 μm. However, this resulted in a high concentration of debris and a thick recast layer on the remaining walls between the holes. Different process gases were investigated to minimize debris formation and reduce the recast layer thickness. Ramping of pulse energy during the drill cycle was investigated to minimize the adhesion between the substrate and recast layer. Chemical etching was used to remove the debris and recast from the top surface and the inside of the laser-drilled holes. Hole cross sections showed that a high aspect ratio was achieved with a hole diameter of about Ø50 μm in 2 mm thick Ta. To achieve the shortest drilling time of 200 ms per hole, the process parameters were optimized and a hybrid nozzle, with both horizontal and vertical gas flow, was developed and implemented.

Sintered diamond as a hybrid EDM and grinding tool for the micromachining of single-crystal SiC

Sintered diamond was used as a hybrid tool for micro-scale electrical discharge machining (EDM) and grinding of single-crystal SiC to achieve both high efficiency and surface integrity. Material removal behavior, surface topography, and subsurface damage for both process steps were investigated under various conditions. The results showed that SiC decomposed into Si and C during EDM, creating a very thick recast layer which had remarkably lower hardness than the bulk. Owing to the electrical dressing effect in EDM, diamond grains protrude out of tool surface and grind the recast layer in a ductile manner with low tool wear. An extremely smooth surface (Ra=1.85nm) was obtained.

A hybrid Taguchi-artificial neural network approach to predict surface roughness during electric discharge machining of titanium alloys

In the present study, electric discharge machining process was used for machining of titanium alloys. Eight process parameters were varied during the process. Experimental results showed that current and pulse-on-time significantly affected the performance characteristics. Artificial neural network coupled with Taguchi approach was applied for optimization and prediction of surface roughness. The experimental results and the predicted results showed good agreement. SEM was used to investigate the surface integrity. Analysis for migration of different chemical elements and formation of compounds on the surface was performed using EDS and XRD pattern. The results showed that high discharge energy caused surface defects such as cracks, craters, thick recast layer, micro pores, pin holes, residual stresses and debris. Also, migration of chemical elements both from electrode and dielectric media were observed during EDS analysis. Presence of carbon was seen on the machined surface. XRD results showed formation of titanium carbide compound which precipitated on the machined surface.