White Layer Thickness Research Articles

Investigation of crack density, white layer thickness, and material characterization of biocompatible material commercially pure titanium (grade-2) through a wire electric discharge machining process using a response surface methodology

Biocompatibility is usually observed as the absence of communication of a material with living tissue. Commercially pure titanium grade-2 has become the preferred biocompatible material for various devices mainly used in orthopedic and dental implants and it is also used in aviation and aircraft. Commercially pure titanium has good ductility, higher stiffness, and fatigue resistance. The novelty of the present research work was focused on studying the effect of wire electric discharge machining factors on surface roughness, material removal rate, crack density, and white layer thickness. After machining, the wire electro-discharge machined surface was analyzed through a scanning electron microscope. Further energy dispersive analysis and X-ray diffraction spectroscopy techniques were applied to investigate the material migration on a work surface and brass wire. Each output response has been modeled through analysis of variance to analyze the adequacy. It was observed that pulse on time, pulse off time, peak current, and spark gap voltage are the most significant factors. These factors have been significantly deteriorating the microstructure of machined samples remarkably deeper, leading to wider craters, globules of debris, and micro-cracks. A white layer thickness was also observed in a discontinuous and non-uniform pattern at the cross-section of the machined sample due to rapid heating and quenching phenomena in the wire electric discharge machining process. A multi-objective optimization “desirability” function was applied to obtain the optimal solutions by numerical and graphical methods. In essence, the wire electric discharge machining is a non-traditional process to achieve the accuracy of biocompatibility parts of commercially pure titanium and also finds the improvement in surface morphology.

Investigation of machining performance in die-sinking electrical discharge machining of pentagonal micro-cavities using cylindrical electrode

The aim of the present article is to fabricate pentagonal micro-cavities and describe the influence of current on the machining performances. Machining of EN-24 alloy steel samples was performed in die-sinking electrical discharge machining (EDM) machine with varying values of current using polygon cycle approach. The machined pentagonal cavities were examined under optical microscope and scanning electron microscope (SEM) to evaluate machining performances in terms of corner error, white layer formation, surface crack distribution, and globule formation. It is found that as the value of current increases, there is more formation of white layer with non-uniform distribution of cracks and the thickness of white layer increased from 6.21 to 8.20 µm with increase in current. On the other hands, surface finish deteriorates when the current value rises. In addition to this, there is an enhancement in tool wear rate with increasing current. At the higher values of current, the spark energy increases which leads to greater melting and evaporation and production of smoke and bubbles on the dielectric surface. This study revealed that die-sinking EDM coupled with short electronic pulses and precise electrode movement is capable of producing microstructures under appropriate operating conditions.

A study on Electrical Discharge Machining of Titanium Grade2 with experimental and theoretical analysis

Titanium alloys, due to their unique properties, are utilized in numerous modern high-end applications. Electrical Discharge Machining (EDM) is a non-conventional machining process, commonly used in machining of hard-to-cut materials. The current paper, presents an experimental study regarding the machining of Titanium Grade2 with EDM, coupled with the development of a simulation model. The machining performance indexes of Material Removal Rate, Tool Wear Ratio, and Average White Layer Thickness were measured and calculated for different pulse-on currents and pulse-on times. Moreover, the developed model that integrates a heat transfer analysis with deformed geometry, allows to estimate the power distribution between the electrode and the workpiece, as well as the Plasma Flushing Efficiency, giving an insight view of the process. Finally, by employing the Response Surface Methodology, educed regression models that correlate the machining parameters with the corresponding results, while for all the aforementioned indexes, ANOVA was performed.

A comparative study on the surface integrity of single-step and multi-step sequential machining in electric discharge machining

In the past, the effect of electrical parameters, dielectric, and electrode material and shape on surface integrity in electric discharge machining (EDM) was widely studied by scholars. However, these researches are mainly based on single-step EDM that can be categorized into either roughing or finishing. The production of components by EDM is a process from roughing to finishing in industry, which is a multi-step machining process. Therefore, in order to provide a better guideline for the fabrication of parts through the single-step EDM approach (refers to the last step of multi-step machining in this study), it is necessary to investigate and quantify the difference in surface integrity between single-step and multi-step sequential machining. In this study, the surface integrity of single-step and multi-step machined samples is evaluated and compared. The results demonstrate that the surface roughness, white layer thickness, and surface residual stress of multi-step machined samples are higher than those of single-step machined samples. In the finishing stage, the difference in surface roughness and residual stress between them becomes even larger. It was indicated that the surface state of the sample obtained in the previous processing steps has an influence on the final machining quality during multi-step sequential EDM. In addition, the change of the surface morphology of the tool electrode caused by the previous machining steps would also affect the surface roughness of the workpiece processed in the current step. There is no significant difference in the crystalline phase between single-step and multi-step sequential machining.

Surface Integrity Enhancement of Incoloy 825 During Electric Discharge Machining

The aim of the current research work is to enhance the surface integrity of EDMed (electro-discharge machined) Incoloy 825 by suspending graphite powder particles in dielectric during electro-discharge machining (EDM). For this purpose, a specially designed and fabricated re-circulation dielectric tank was integrated with the existing EDM setup. The effect of three process variables, i.e., peak current, pulse-on time, and duty cycle was studied on EDM characteristics such as material removal rate, surface roughness (Ra), radial overcut, surface microhardness, microhardness in sub-surface regions, recast layer, phase changes and residual stress. Results showed that the machining rate increased up to 23% using powder-mixed EDM compared to conventional EDM. Significant reduction in surface roughness, white layer thickness, surface cracks and residual stress was observed while machining with graphite mixed dielectric. The addition of powder resulted in the formation of carbides and other alloy compounds on the machined surface which enhances the microhardness of surface and subsurface region.

Performance evaluation of AE-pulse of wire EDM process on Ti-10V-2Fe-3Al alloy by Taguchi GRA technique

Titanium alloy, Ti-10V-2Fe-3Al is one of the recently developed materials having special characteristics of deep hardenability, ductility, fracture toughness including high strength to weight ratio. But, titanium alloys have poor machinability due to the formation of built-up edges and tool wear in conventional machining processes. Wire electrical discharge machining wire EDM is found to be the most preferred machining process among other non-traditional processes for cutting of the difficult-to-cut materials. The surface integrity aspects of the machined component play a crucial role in the functional performance of the component. The performance of an advanced AE-pulse (Anti Electrolysis Equi-energy) over E-pulse on surface integrity aspects in the wire EDM process is studied in detail. Experiments were carried with various levels of pulse-on-time, pulse-off-time, peak current setting, and servo voltage through a Taguchi L9 orthogonal array. The responses such as kerf width, surface roughness, material removal rate, and white layer thickness were studied through gray relational analysis (GRA) technique. The optimized results were validated through experimental studies. It is observed that the optimized process parameter values were Ton: 12 μs Toff:40μs IP No.:10 SV:10V, which gave 28 % of the significant reduction of white layer thickness, 16% improvement of MRR, 5% reduction of surface roughness, and 4% of reduction of kerf width.

Numerical analysis of the effect of tool wear on surface integrity during hard turning

The white layer formed during hard turning deteriorates surface integrity, thereby severely affecting the fatigue life of machined parts. A 2D FEM-based simulation model has been developed to simulate cutting temperature profiles in workpieces for the prediction of white layer thickness developed below the machined surface. Machining was simulated for fresh and worn tools with predefined flank wear. The temperature profile generated in the workpiece was analyzed for the critical temperature for phase transformation leading to white layer formation. The critical temperature was found to be deeper in the subsurface with an increase in cutting speed and wear, indicating a thicker subsurface white layer. Experimental observations using fresh tools were found very close to the simulation results. The model was extended to simulate temperature profile and predict the white layer for machining with tools having predefined tool flank wear.

Prediction of white layer formation in μ-WEDM process of NiTi shape memory superalloy: FEM with experimental verification

Microscopic changes in the surface of nickel-titanium (nitinol) shape memory alloys (SMAs) in micro-wire electro-discharge machining (μ-WEDM) due to the formation of a resolidified layer on the machined surface, called white layer, are one of the main drawbacks in the processing of such alloys. Since these changes significantly affect the shape memory and elastic recovery characteristics of these alloys, reduction of the white layer thickness (WLT) based on the selection of optimum process parameters is essential to raise the quality of the machined parts. In this regard, a finite element model (FEM) has been developed to simulate the effects of μ-WEDM process parameters, including discharge current, pulse on-time, pulse off-time, and servo voltage, on the heat distributing in Ni55.8Ti SMA to predict the WLT. The flushing efficiency of electric discharges and the effect of flow regime of the dielectric fluid on the heat distribution in the workpiece and the formation of the WLT are analyzed. Experimental data are used to verify the accuracy of the FEM. The results show that the developed model can predict the WLT in μ-WEDM process of Ni55.8Ti SMA with an average error of 14%. The effects of discharge parameters on the formation of the WLT are discussed in details based on the results of the FEM.

Influence of tool characteristics on white layer produced by cutting hardened steel and prediction of white layer thickness

In the dry and hard cutting process of hardened steel, the white layer of the machined surface has a great influence on the service performance and life of the part. The cutting force and cutting heat produced in the process are among the important factors affecting the white layer characteristics. In the process of machining, in addition to the cutting parameters, the characteristics of cutting tools can also lead to significant changes in both service performance and part lifetime. Therefore, it is necessary to further study the changes in the cutting force, cutting heat, and white layer characteristics under the influence of the tool characteristics. In this paper, hard cutting tests of hardened steel were carried out by cutting tools (including PCBN tools and ceramic tools) with different thermal conductivities and flank wear under different cutting speeds. The influence of the cutting speed and tool characteristics on the cutting force, flank temperature and white layer characteristics was analyzed, and a prediction model for the white layer thickness was established. It was found that the cutting speed, tool wear degree, and tool thermal conductivity all have a significant influence on the cutting force, cutting temperature, and white layer thickness. Among the results, the thickness of the white layer first increases and then decreases with increasing flank temperature, and the critical temperature at the maximum thickness of the white layer is the actual final temperature of austenite transformation. Changes in the cutting force indirectly affect the temperatures of the austenite and martensite phase transitions, thus affecting the thickness of the white layer. The prediction results show that a fuzzy neural network based on particle swarm optimization can effectively predict the thickness of the white layer.

The Accuracy of Finishing WEDM of Inconel 718 Turbine Disc Fir Tree Slots.

Servicing aircraft engines sometimes requires manufacturing only a single piece of a given part. Manufacturing a turbine disc using traditional methods is uneconomical. It is necessary to use a different machining method recommended for small lot production. One of the proposed methods is WEDM (wire electrical discharge machining). The article presents the results of the research on finishing WEDM of Inconel 718 turbine disc fir tree slots. The influence of infeed, mean gap voltage, peak current, pulse off-time, and discharge energy on the shape accuracy, surface roughness, microcracks, and the white layer thickness were determined. Mathematical models were developed based on the DoE (Design of Experiment) analysis. The statistical significance of the models was verified with the ANOVA (Analysis of Variance) test. The machining parameters control methods that allow achieving the required shape accuracy, surface roughness, and surface layer condition were presented. The obtained surface roughness was Ra = 0.84 μm, the shape accuracy of the slot in the normal-to-feed direction was Δd = 0.009 μm, the profile shape accuracy was Δr = 0.033 μm, and the thickness of recast (white) layer was approximately 5 μm.

A Comprehensive Study on Processing Ti-6Al-4V ELI with High Power EDM.

Electrical Discharge Machining (EDM) consists of a non-conventional machining process, which is widely used in modern industry, and especially in machining hard-to-cut materials. By employing EDM, complex shapes and geometries can be produced, with high dimensional accuracy. Titanium alloys, due to their unique inherent properties, are extensively utilized in high end applications. Nevertheless, they suffer from poor machinability, and thus, EDM is commonly employed for their machining. The current study presents an experimental investigation regarding the process of Ti–6Al–4V ELI with high power EDM, using a graphite electrode. Control parameters were the pulse-on current (Ip) and time (Ton), while Machining performances were estimated in terms of Material Removal Rate (MRR), Tool Material Removal Rate (TMRR), and Tool Wear Ratio (TWR). The machined Surface Roughness was calculated according to the Ra and the Rt values, by following the ISO 25178-2 standards. Furthermore, the EDMed surfaces were observed under optical and SEM microscopy, while their cross sections were also studied in order the Average White Layer Thickness (AWLT) and the Heat Affected Zone (HAZ) to be measured. Finally, for the aforementioned indexes, Analysis Of Variance was performed, whilst for the MRR and TMRR, based on the Response Surface Method (RSM), semi-empirical correlations were presented. The scope of the current paper is, through a series of experiments and by employing statistical tools, to present how two main machining parameters, i.e., pulse-on current and time, affect major machining performance indexes and the surface roughness.

Implementation of Jaya algorithm for process parameter optimization during EDM processing of NiTi 60alloy

Electrical Discharge Machining (EDM) process is an advanced machining process used in wide range of manufacturing applications especially for hard materials. Functioning and quality of EDMed machined parts/components plays important role when put to the service. For this not only selection of optimum machining process but also selection of effective optimization technique need to be given special attention. With this intention the target of the current research is to survey the different optimization techniques (Response surface, Taguchi, Grey regression analysis, Artificial nueral network etc.), research gap indicates scope for implementation of Jaya algorithm which is a modern optimization technique. With use of Jaya algorithm technique research work was carried out to evaluate white layer thickness (WLT) and micro hardness (MH) as output parameters during EDM machining of NiTi60 alloy. These output parameters were evaluated with variations in input process parameters considering voltage (V), current (C), pulse on (Pon) and pulse off (Poff) time. The experiments were conducted using L9 Taguchi orthogonal array and mathematical predictions models were generated using regression analysis. Jaya algorithms were developed with the intention to minimize the WLT and minimize the MH as the objective functions. It is concluded that Jaya optimization technique can be effectively implemented to optimize the process parameters in EDM process which shows considerable reductions in WLT from 5.914 to 4.947 µm and MH from 208.71 to 200.3 respectively.

Micro-WEDM of Ni55.8Ti shape memory superalloy: experimental investigation and optimisation

Nickel-titanium superalloy has gained significant acceptance for engineering applications as orthotropic implants, orthodontic devices, automatic actuators, etc. Considering the unique properties of these alloys, such as high hardness, toughness, strain hardening, and development of strain-induced martensite, micro-wire electro-discharge machining (μ-WEDM) process has been accepted as one of the main options for cutting intricate shapes of these alloys in micro-scale. This paper presents the results of a comprehensive study to address the material removal rate (MRR) and surface integrity of Ni55.8Ti shape memory superalloy (SMA) in the μ-WEDM process. The effects of discharge current, pulse on-time, pulse off-time, and servo voltage on the performance of this process, including MRR, white layer thickness, surface roughness, and micro-hardness of the machined surface, were investigated by multi-regression analysis using response surface methodology (RSM). The optimisation of input parameters based on the gradient and the swarm optimisation algorithms were also conducted to maximise the MRR and minimise the white layer thickness, surface roughness, and micro-hardness of the machined samples.

Surface morphology and effect of cutting force and temperature in formation of the white layer during hard machining

In hard machining, different tribological phenomenon occur, out of which white layer formation and surface characterization are an important aspect. The white layer is found to be harder and fragile. It may cause a crack and may lead to failure in machined parts. Here, an experimental work is conducted in machining hardened AISI D2 steel. Cutting speed, feed, and depth of cut are taken as the input parameters. The level of each parameter is two. The white layer development process considering the impact of cutting force, temperature and morphology of hardened surfaces, etc. are covered in the study. The study of white layer morphology and hardness are carried out with FESEM and microvicker's tester. The parametric study reveals occurrence of white layer due to the cutting force and temperature. Temperature has an important effect in increasing the surface hardness due to which the thickness of white layer changes. ANOVA analysis showed that cutting speed is the influencing variable for white layer thickness and microhardness, whereas depth of cut is influential for temperature, flank wear and cutting force.

Investigation of surface integrity in the laser-assisted turning of AISI 4340 hardened steel

The use of laser-assisted turning (LAT) can improve different aspects of the machinability of high hardness/strength materials. The heat applied in this process reduces the strength of the material and upon a proper selection of laser heat source and machining process parameters, a significant improvement in the machining process can be achieved. This research studies the effect of machining and laser heat source parameters on the surface integrity of the LAT process of AISI 4340 hardened steel with a hardness of 560 HV using Response Surface Methodology (RSM). The effect of machining process parameters (feed rate, depth of cut and cutting speed) and laser power on the surface integrity characteristics of the machined surfaces (white layer thickness, microhardness, surface roughness, and surface chemical composition) are investigated. A detailed finite element simulation of the process has been performed to better understand the physics of the process and to interpret the experimental results. Laser power and feed rate are shown to be the most significant parameters affecting the surface integrity of the machined surfaces. It has been shown that a proper selection of machining and laser heat source parameters makes it possible to minimize the adverse effect of laser heating in the LAT process. This can pave the way for the widespread application of the LAT process by eliminating one of the most important obstacles of this process by controlling the laser heat diffusion into the workpiece.

Experimental Investigation of White Layer Formation on Machining Silicon Steel in PMEDM Process

The silicon steel can be easily machined using electrical discharge machining process due to its higher strength. Since the white layer thickness and propoerties can affect the surface performance measures in the process, it is important to investigate the effects of process parameters on white layer. In the present study, an experimental investigation was made to analyze the effects of process parameters on white layer formation while machining silicon steel using powder mixed dielectric medium in such process. It was inferred that powder mixed insulating medium could produce lower thickness of white layer with the uniform distribution over the machined surface owing to the higher thermal and electrical conductivity of the powder particles. The lower micro cracks were observed in the white layer region in electrical discharge machining process with powder mixed dielectric medium.

Cutting force based surface integrity soft-sensor when hard machining AISI 4140

AbstractWorkpiece rim zone modifications during hard machining can be explained with the high thermo-mechanical loads induced by the cutting process. The formation of White Layers with a fine-grained microstructure by dynamic recrystallization (DRX) is one of those surface modifications that can negatively affect the functionality of a machined part by changing the residual stress state and facilitating crack initiation. As a consequence, the fatigue life of the machined parts is reduced. It is therefore of great interest to understand the thermo-mechanical conditions which induce White Layers formation in order to be able to control them by in-situ measurements if necessary. For this purpose, a cutting force based Soft-Sensor is developed in this study which enables the in-process estimation of White Layer thickness. Therefore, a cutting force based analytical model is used to estimate the resulting temperature fields and correlated with validated numerical chip formation simulations. In addition, the predictions of the White Layer thickness of the analytical model are then compared using light microscopy and the results of the numerical finite element model, in which a DRX model is additionally implemented.

Modelling of Grain Size Evolution with Different Approaches via FEM When Hard Machining of AISI 4140

Thermo-mechanical loads during hard turning lead to the formation of so-called White Layers on the machined surface. Characterized by a very fine microstructure and high hardness, White Layers have a negative effect on the fatigue life of a component. The fundamental mechanism for the White Layer formation is the dynamic recrystallization (DRX). Therefore, in the current work, two different DRX models, Helmholtz free energy and Zener-Hollomon, are implemented into Abaqus/Explicit to predict the thickness of the White Layer when hard turning quenched/tempered AISI 4140 and the results are compared with each other. For the simulation of the machining process a Finite Element Method (FEM) model based on the Coupled-Eulerian-Lagrangian (CEL) method is built up. Although both DRX models achieved a very good match between predicted and measured White Layer thickness and grain size evolution on the workpiece rim zone, the Zener-Hollomon model produced more closer agreement.

Mechanism and properties of plasma nitriding AISI 420 stainless steel at low temperature and anodic (ground) potential

AISI 420 stainless steel was nitrided at low temperatures and anodic potential in a plasma atmosphere of ammonia (NH3) assisted by hollow cathode discharge. The specimens were placed on a worktable connected to the wall of vacuum chamber that was earthed (0 V) and used as anode of glow discharge. Plasma nitriding was carried out at a temperature ranging from 390 to 510 °C for 4 h in NH3 atmosphere. The thickness of white layer was in a range of 6 to 40 μm. The surface hardness reached to 1250 ± 72 HV0.05 for the specimen nitrided at 510 °C for 4 h. Both corrosion and wear resistance of AISI 420 stainless steel were simultaneously improved after plasma nitriding at 450 °C for 4 h.

Performance analysis of Cu-W-B4C composite tool during electrical discharge machining of titanium alloy

Electrical discharge machining (EDM) is broadly used in machining of parts having intricate shapes made of difficult-to-machine materials. During EDM, tool fabrication time through conventional machining processes is generally excessively high. Therefore, alternate tool material and fabrication strategy is sought to reduce machining lead time. In this work, composite tools made of copper, tungsten and boron carbide have been prepared through powder metallurgy (PM) route. The performance of these tools is studied during machining of titanium alloy work piece. The input parameter considered are tool material parameters such as weight percentage of tungsten and boron carbide in the tool and machining parameters like peak current, pulse-on-time and duty cycle. The performance of these tools are evaluated considering the performance measures such as material removal rate (MRR), tool wear rate (TWR), average surface roughness (Ra), surface crack density (SCD), white layer thickness (WLT) and micro-hardness (MH). It is observed that the tool materials have been transferred onto the machined surface forming a hard layer with increased weight percentage of oxygen and carbon during machining. The PM tools exhibit similar performance as solid copper tool with respect to MRR, TWR and surface integrity.