The crater volume produced in the wire is calculated when the temperature exceeds the melting point of the wire material. Thus, wire strength is estimated based on the amount of material removed from the galvanized brass wire.
Development of a three-dimensional thermo-mechanical model of a molybdenum wire during WEDM process
Estimation of wire strength based on residual stresses induced and parametric studies
Experimental measurement and analysis of molybdenum wire erosion and deformation during WEDM of Ti-6Al-4V alloy
Methodology to determine a wire surface quality index for wire surface erosion using image processing technique
Development of a three-dimensional thermo-mechanical model of a zinc-coated brass wire during WEDM process
Experimental analysis on the erosion and deformation of zinc- coated brass wire during WEDM process
Conclusions
8.3 (b) localized ablation of material, (c) adhesion of debris, (d) formation of microholes and (e), (f) formation of cracks on the surface of the wire.
List of abbreviations
Fa, Fc, Fd Fraction of energy absorbed by the anode, cathode and dielectric respectively h Convective heat transfer coefficient (W/m2K). L Distance traveled by the wire during a single pulse cycle Lon Distance traveled by the wire during a single pulse on-time [N] Shape function matrix.
Introduction
- Wire electric discharge machining
- Importance of wire health during WEDM cutting
- Motivation for the present research work
- Scope of the present research
- Organization of the thesis
The wire electrode also plays an important role in determining the surface quality and geometric accuracy of the processed products. Erosion of the wire electrode degrades the strength of the wire and drastically reduces the life of the wire.
Literature review
Scope
Causes of wire breakage
- Thermal loads generated during spark plasma
- Thermal residual stresses generated in the wire electrode
- Wire vibration and bowing
- Forces acting on the wire electrode
The geometry of the interaction between the discharge channel and the wire tool is shown in Figure 2.4. Electrostatic forces act on the wire during the discharge delay time and an electromagnetic force acts during the time.
Influence of wire electrode on machining precision and product quality
The influence of hydrodynamic force on the wire deflection due to jet flushing was investigated by Okada et al. A particle swarm optimization algorithm was developed to demonstrate the role of wire tension and wire feed speed on the wire wear ratio (Nain et al.
Methods to monitor wire health and prevent wire breakage
- Investigation of wire morphology
- Selection of optimum process conditions
- Online wire monitoring and control systems
- Suitable selection of wire material and improvement in wire properties
Geometric properties of the wire (shape and size of the wire), tensile strength, density, elongation of the wire. However, the addition of zinc to copper significantly reduces the conductivity of the wire electrode.
Discussion
It was noted that advanced monitoring and control systems help efficiently monitor wire health and reduce wire failures. The damage that the wire sustains also has a noticeable effect on the surface quality that the workpiece achieves during machining.
Research objectives
Correlate the wire damage to the surface quality achieved by the WEDMed workpiece. Various methods have been explored to monitor wire health and preventative measures to limit the occurrence of wire failures.
A three-dimensional thermo-mechanical finite element model of the molybdenum wire electrode is developed to determine the strength of the wire during the machining
To evaluate the intensity of damage undergone by the wire electrode by determining a wire surface quality index for the allowable limit of surface erosion using image processing technique. A comprehensive analysis of the wire eroded specimens is crucial to limit wire breakage and improve the overall efficiency of the operation.
This section reports an extensive experimental investigation on the measurement and analysis of molybdenum wire erosion and deformation after WEDM of Ti-6Al-4V alloy
Three-dimensional thermo-mechanical analysis of molybdenum wire electrode during wire EDM process
Scope
The need
The strength of the wire electrode depends on the temperature and thermal residual stresses that occur in the wire during the occurrence of the discharge. Thus, the need to predict the thermomechanical characteristics of the wire during the cutting process was identified in order to predict and limit the occurrence of wire failure. With this in mind, a three-dimensional nonlinear transient thermomechanical model for molybdenum wire used during WEDM was developed in this work to calculate the temperature and induced stresses in the wire.
Generation of spark plasma on the wire surface
- Assumptions
- Governing equation and boundary conditions A. Thermal analysis
A portion of the total heat dissipated by the spark is absorbed only by the wire electrode. The initial temperature of the wire at time t = 0 is assumed to be at room temperature of 300 K. The discharge energy is distributed between the electrode of the wire and the workpiece, while the rest of the heat is lost in the dielectric.
Mechanical analysis
- Solution methodology
- Finite element formulation
- Continuum discretization
- Thermal analysis
- Mechanical analysis
- Computation of temperature and stresses generated in the wire using the developed model
- Material properties
- Process parameters used for the model
- Overview of the process model development
- Finite element meshing of the wire geometry
- A case study on the temperature and residual stresses generated in the wire
- Validation of the developed numerical model
- Validation of temperature using published data
- Experimental validation of residual stresses using X-ray diffraction technique To establish the validity of the proposed numerical model, the model results were validated
- Summary
The effects of various input parameters on the temperature and stresses induced in the wire have. The d spacing in the wire will shift slightly due to the residual stresses present in the specimen. The temperature and stresses created in the wire during the discharge phenomenon are critical factors responsible for wire breakage.
Estimation of wire strength based on residual stresses induced and parametric studies
Scope
The need
In the present work, a wire safety index is determined based on residual thermal stresses induced in the molybdenum wire during machining. These stresses are calculated using the three-dimensional thermomechanical finite element model of the wire electrode presented in Chapter 3. The evaluation of a wire safety index can provide useful insights into sustainable and economical wire-break free machining in modern industrial applications. excercise.
Overall approach for the estimation of wire strength during wire EDM process Figure 4.1 shows the overall approach of the estimation of wire strength based on thermal
Computation of peak residual stresses induced in the wire electrode
The values of peak stresses shown in Table 4.1 are peak values of equivalent residual stresses or von-Mises stresses induced in the wire. It was observed that for some process conditions the residual stresses exceeded the yield stress of the molybdenum wire material (600 MPa). The wire strength begins to decrease as the induced stresses exceed the yield stress of the wire material, i.e.
Estimation of wire safety index based on residual stresses
It has been observed that the maximum value of residual stress induced in the wire during certain machining conditions exceeds the yield point of the molybdenum wire (600 MPa). But eventually, when nodes over a large area exceed the yield point of the wire material, wire breakage can occur. High tensile residual stresses retained in the wire electrode due to rapid heating and cooling of wire surface initiate micro cracks.
Parametric studies using the developed numerical model
- Effects of process parameters on temperature generated in the wire
- Effects of process parameters on residual stresses induced in the wire
The peak temperature obtained in the wire electrode after a single discharge increases with an increase in voltage and current due to the increase in input power. The peak temperature obtained in the wire electrode after a single discharge decreases with an increase in pulse-on time. It has been observed that residual voltage on the wire electrode increases with an increase in voltage and current due to an increase in input power.
Summary
Residual stresses also occur below the surface of the wire and in some cases reach their maximum value below the surface. Sequential and random multiple discharges during WEDM operation cause wire erosion due to excessive material ablation from the wire surface. It is imperative to gain a deeper understanding of the phenomenon of wire erosion and breakage in order to predict and prevent wire failure.
The need
Experimental measurement and analysis of molybdenum wire erosion and deformation during wire EDM of Ti-6Al-4V alloy. This chapter presents a detailed experimental investigation of wire erosion and deformation during WEDM experiments on Ti-6Al-4V alloy. Thus, this chapter reports a comprehensive experimental investigation of the erosion and damage of a molybdenum wire electrode after WEDM of the industrially important Ti-6Al-4V alloy.
Overall experimental methodology
An in-depth understanding of the thread erosion mechanism must provide a proper insight into the thread breakage phenomenon; thus predicting and preventing untimely wiring failure. Although monitoring systems and numerical models presented in Chapter 2 are quite useful for predicting wire behavior, limited studies have been carried out so far explaining the erosion and failure mechanism of the wire electrode during machining (Pramanik and Basak (2016) , Pramanik and Basak (2018) ). The results presented in this chapter expand the existing knowledge in this area and help to achieve stable machining without wire breakage.
Experimental setup and methods
- Wire electric discharge machining (WEDM) setup
- Analysis of wire wear using field emission scanning electron microscope (FESEM) The surface damages of eroded wires were examined with the help of field emission scanning
- Analysis of wire deformation using optical microscope
- Measurement of workpiece surface roughness using optical profilometer
For this purpose, the wire samples were coated with a thin layer of gold in the range of 1.5 – 3.0 nm. Wire samples were collected after each experimental set and wire cross-sectional samples were prepared using a standard method. Etching of wire cross-section samples was performed to observe the grain structure of the molybdenum wire material.
Measurement and analysis of experimental results .1 Study of erosion on the molybdenum wire surface
- Measurement of deformed wire cross section diameters
- Measurement and analysis of surface roughness of machined workpieces
Instead, the molten material forms a pool of resolidified material and precipitates on the wire surface. Slight deformation of the wire periphery begins at the initiation of sparks due to sudden temperature rise and thermal shock. This reduces the overall cross-sectional area of the wire and therefore increases the stress in the wire cross-section.
- Parametric studies
- Effects of process parameters on wire surface wear
- Effects of process parameters on workpiece surface roughness
- Summary
Damage to the wire surface is small at low current levels due to the low discharge energy (Figure 5.10a). At higher current levels, craters about 20 µm in size are formed on the surface of the wire (Figure 5.10b). At lower pulse-off times, microholes or micropits were observed on the surface of the wire due to an inefficient debris removal mechanism.
Wire surface quality index for wire surface erosion using image processing technique
- The need
- Overall methodology of image processing technique
- Estimation of a wire surface quality index for the intensity of wire surface erosion using image processing technique
- Relationship between wire erosion and workpiece surface quality
- Wire breakage and its relationship with wire wear
- Summary
A wire surface quality index was determined from the FESEM images of the wire electrode by plotting an image histogram using the image processing technique. Lower mean values of the wire image histogram indicate lower surface quality of the wire samples. The wire surface quality index for the molybdenum wire decreases as the intensity of the erosion of the wire surface increases.
Scope
The need
Numerical models have proven to be a useful tool for understanding the characteristics of wire tools during spark discharges. The developed model was intended to predict the temperature and induced voltages in the wire. The amount of material eroded from the surface of the wire was also calculated when the induced temperature exceeded the melting point of the wire material.
Development of numerical model for computation of temperature and stresses generated in the wire
- Governing equations and boundary conditions
- Material properties
- Process parameters used for the model
- Overview of the process model development
- Finite element meshing of the wire geometry
Based on the temperature distribution, the stress distribution in the wire tool at the end of the pulse is predicted. Heat flux is applied at the spark location and the rest of the wire surface is applied with convection boundary condition. The rest of the wire geometry is discretized with coarse mesh to reduce the computation time.
Computation of temperature and stresses generated in the wire
After the cooling effect in a single pulse, the voltages held in the wire are the residual voltages induced. An enlarged cross-section of the wire shows that the peak value of equivalent residual stress is obtained at a certain depth below the wire surface. It is observed that the peak value of residual stresses is obtained at a certain depth below the surface and not at the top surface of the wire.
Computation of crater volume generated in the wire
The temperature generated during the discharge is plotted along the longitudinal axis of the wire. To calculate the crater depth, the temperature is plotted along the radius of the wire and the distance is calculated where the temperature exceeds the melting point of the bronze. The crater volume Vc (µm3) created on the wire electrode after a single discharge is calculated as (Chen et al. 7.8) where Rc is the radius of the discharge crater (µm) and Dc is the depth of the discharge crater (µm).
Experimental validation of the developed numerical model .1 Validation of crater volume generated in the wire
The experimental crater volume for this corresponding process set is 34517.28 µm3, which closely matches the numerically calculated value µm3) with an absolute deviation of 5.67.
Estimation of a wire safety factor for the zinc coated brass wire
Parametric studies
- Effects of process parameters on temperature generated in the wire
- Effects of process parameters on residual stresses generated in the wire
- Effects of process parameters on material removed from the wire
The influence of the process parameters on the residual stress distribution in the wire was investigated. As a result of this, the crater volume (Vc) in the wire first increases with increasing pulse duration and then decreases at further higher values of the pulse on time (Figure 7.30). Furthermore, at higher pulse duration values, the molten wire material can resolidify within the discharge time and redeposit on the wire surface, which further reduces the crater depth.
Comparison between molybdenum wire and zinc coated brass wire
Thus, the strength of molybdenum wire was estimated based on the amount of stress induced in the wire. The strength of galvanized brass wire was thus estimated based on the amount of material removed from the wire. Thus, we can conclude that the strength of the wire electrode during processing depends on the material of the wire and its thermomechanical properties.
Summary
For galvanized brass wire, the residual stresses developed in the wire were found to be below the yield strength of the brass from the wire core material. However, the temperature produced in the wire was found to exceed the melting point of brass, which is the core material (1203 K). Thus, this value of the crater volume can be considered as a threshold value above which the wire is very prone to breakage.
Scope
The need
Experimental setup and methods
Measurement and analysis of experimental results .1 Study of erosion on the zinc coated brass wire surface
- Measurement of deformed wire cross section diameters
- Measurement and analysis of surface roughness of machined workpieces
These trapped bubbles cause the formation of microholes or microholes on the wire surface (Figure 8.3d). Sharp temperature peaks during the discharge phenomenon cause residual thermal stresses on the wire surface. The total cross-sectional area of the wire is reduced, which increases the tension in the wire cross-section.
Estimation of a wire surface quality index for zinc-coated brass wire
This is because the light intensity decreases in the region where the damage occurs, so the pixel intensity decreases and the histogram shifts to the darker side. It was noted in chapter 6 that an image with a significant amount of damage on the wire surface will produce a lower average value of histogram.
Relationship between wire erosion and workpiece surface quality
This trend was also observed in the case of the molybdenum wire as shown in Chapter 6. As machining continues further for the second and third cuts, the wire erodes significantly due to the violent and erosive nature of successive random discharges during the spark cycles. The eroded and uneven wire surface further diminishes the surface quality and integrity of the WEDMed product.
Parametric studies
- Effects of process parameters on wire surface wear
- Effects of process parameters on workpiece surface roughness
The damage to the wire surface is minor at low current levels due to low discharge energy (Figure 8.8a). At higher current levels, deep and wide craters are formed on the wire surface (Figure 8.8c). At higher wire speeds, the wire moves quickly through the workpiece and has less time to form deep craters on the wire surface (Figure 8.10b).
Summary
