The machining experiments were performed to investigate the effects of different parameters more accurately and cost-effectively. Minitab and Expert Design software have been used to perform RSM test design method. The quality measures for all trials are shown in Table 4.
Table 4
Various parameters considered using the RSM method in Minitab software and results.
Number | Current | Pluse on time | Concentration of powder | Material removal rate (mm3/min) | Tool wear rate (mm3/min) | Machining time (min) | Surface roughness (µm) |
1 | 15 | 5 | 0 | 0.122 | 0.060 | 6.2 | 5.98 |
2 | 25 | 5 | 0 | 1.528 | 0.111 | 35.45 | 8.7 |
3 | 15 | 7 | 0 | 0.416 | 0.030 | 29.27 | 6.93 |
4 | 25 | 7 | 0 | 0.460 | 0.027 | 10.26 | 8.3 |
5 | 15 | 5 | 5 | 0.355 | 0.081 | 19.7 | 4.25 |
6 | 25 | 5 | 5 | 0.797 | 0.159 | 6.02 | 7.32 |
7 | 15 | 7 | 5 | 0.460 | 0.092 | 10.43 | 4.68 |
8 | 25 | 7 | 5 | 0.633 | 0.145 | 11.05 | 7.5 |
9 | 15 | 6 | 2.5 | 0.491 | 0.065 | 8.55 | 5.26 |
10 | 25 | 6 | 2.5 | 2.066 | 0.275 | 2.033 | 7 |
11 | 20 | 5 | 2.5 | 1.416 | 0.189 | 2.966 | 8.1 |
12 | 20 | 7 | 2.5 | 0.832 | 0.082 | 6.85 | 4.76 |
13 | 20 | 6 | 0 | 0.964 | 0.100 | 16.8 | 7.12 |
14 | 20 | 6 | 5 | 1.200 | 0.118 | 4.75 | 5 |
15 | 20 | 6 | 2.5 | 0.298 | 0.092 | 16.13 | 6.2 |
The contour plot with tolerance level of 0.05 for the performance measures under the influence of different input factors on machining specimens have been shown in Fig. 4–5. The higher flow rate could increase the amount of MRR, Fig. 4. While the micron powders was included with dielectric insulating medium, the electrical conductivity of insulating medium was also improved. Hence the MRR could be increased significantly. However the powders was mixed with insulating medium further, the dielectric nature was also affected. If dielectric nature of the insulating medium was affected, it could affect the discharge mechanism. This would result in reducing th MRR. Hence the higher MRR has been observed with concentration of 2.5 g/l. However the MRR has been reduced with concentration of 5 g/l. Hence it has been found that 2.5 g/l would be better optimal value of powder concentration. In the mechanism of EDM machining process with powder mixed dielectric medium, the larger and wider electric discharge channel could lead to lower density of the electric power at the electric discharge position. This could reduce the amount of impact forces on the workpiece surface. This has resulted on producing the smaller holes on the machined surface of titanium specimens. The higher thermal conductivity of the particles, was developed more thermal energy outside the machining gap. It has reduced the amount of thermal energy at the electric discharge position and it leads to a reduction in the material erosion from the specimen. The aluminum oxide particles are lighter and more susceptible in the machining gap. The particles could directly produce more heat energy in the gap. consequently, the amount of thermal energy in the electrical discharge channel could be reduced after the optimal value. The aluminum oxide powder due to higher thermal conductivity and lower density creates more drop in the amount of material erosion compared to EDM mode without powder. In order to investigate more deeply and confirm the stated interpretation above more clearly, the interaction diagram of different factors on the erosion rate with constant consideration of parameters, Fig. 5. This diagram comprehensively shows the effects of different parameters in different modes.
The material removal rate during electric discharge depends on the pulse current and duration. The higher electrical discharge energy could produce more erosion in the machining zone owing to higher melting and evaporating temperature. The impact driving force resulting from the evaporation of insulating medium depends on the electric discharge energy. As electric energy increases, the driving force to remove the melting material from the machining cavities is also be increased. It could create larger size of the holes obtained by the spark in the sample. The larger amounts are separated from the workpiece material for each electrical discharge since the current is increased, Fig. 6.
The tool electrode in EDM process should have high melting point and low resistance to electric current. The diagrams related to TWR with combined effects of different initial parameters, Fig. 7–9. The larger current with high pulse duration can generally increase TWR. There is an optimal amount regarding powder concentration according to the diagrams. The more powder particles can reduce the tool erosion by reducing pulse current across the machining zone. The particles combined in the insulating medium can expand the plasma channel to generate the higher thermal energy in EDM process. It could reduce the electric power density which results in the tiny tool erosion. The inclusion of particles in the fluid can transfer more heat across the machining gap. This could reduce the electrical discharge capacity on the tool electrode surface to reduce tool erosion. Hence the best concentration to reduce the tool erosion was found as 2.5 g/l of aluminum oxide powder. The diagram of the interaction of different parameters on TWR with constant consideration of the parameters, Fig. 10. It also indicate the selection of optimal concentration in order to achieve lower TWR with the combined effects of other parameters.
The higher current with pulse time less than 6 units has increased the machining time, Fig. 11. During the higher pulse time, the machining time was reduced. The merged effects of current and lower particles concentration could increase the machining time. However, the current was increased to reduce the machining time at higher concentrations 2.5 g/l. Hence the optimal value for the pulse on time should be lower for the effective machining time, Fig. 12–13. The interaction of different parameters on the tool wear with constant consideration of parameters, Fig. 14. It can be stated that the machining time in constant powder concentration was independent and constant owing to the prodcution of intended sparks.
In the different layer recasted on the machined area, there are numwerous changes like phase hanges, thiickness of white layer, material percentage, Mechanical, piysical and chemical properties, and etc. are contaned into the subcategory,Fig. 15. To investigate of fault remanied on the surface, it is imporatn to select what reasons and purpose should be considered
The surface measures of the specimen was described by spark current, electrical discharge time length, gap voltage, electrode polarity, material and workpiece properties, characteristics of dielectric fluid, concentration of chips in fluid and the dimensions of the electrode. The higher pulse current has increased the crack length and the width of the surface cracks, Fig. 16. The cracks formations are owing to the tensile stress developed by the shrinkage of the material during cooling of the workpiece surface after ignition. This higher tensile stress of the workpiece could lead to superficial cracks.The higher current has increased the electrical discharge energy to remove more molten material. The larger and deeper surface cavities could develop higher surface roughness under the larger pulse on time, Fig. 17. As a result of successive electrical discharges on the workpiece surface, cavities are created on the machining surface. These cavities are created by the eruption of molten material on the surface at the end of the pulse time. As the higher pulse duration leads to an increase in pulse energy. The dimensions of the machining cavities on the workpiece surface can also be increased. There are generally prominent edges around the cavities owing to the machining process. The edges surrounding the cavities resulting from machining have beem increased by increasing pulse on time. The quick melting of the workpiece surface in electrical erosion process and the quick freezing of the workpiece during washing with the help of dielectric fluid, surface and heat-affected areas are created on workpiece samples. These surface and subsurface defects could lead to reduce hardness, wear and corrosion resistance of the workpiece surface. The aluminium particles are used in order to maintain the surface measures in power machining mode. The powderless process produces more random and roughness of the machined surface.
The smooth and fine machining surface with uniform surface roughness under aluminum oxide powder mixed dielectric medium, Fig. 18–19. The lower width of the surface cracks was observed as compared with powder-free modes. The aluminum oxide particles led to produce of fine surface quality compared to the machining mode without powder. The higher aluminum oxide powder concentration with the dielectric fluid can increase the particles in the gap between the tool and the workpiece. It produces the instability in electric discharge machining due to production of the higher short-circuit or arc pulses. Hence the powder concentration more than 2.5 g/l, the machining surface could develop more rough and random surface by increasing the powder concentration powder. The instability in the process could also produce more random spark energy distribution owing to density of the electric power. Consequently, the impact force from the electric discharge on the workpiece surface could be generated as more heterogeneous. It was viewed that the process of topographic changes of machining surface under different concentrations was quite similar to the process of machining time under different concentrations.
Topography of machined surface without and with powder mode, Fig. 20. It can be seen that the powder mode has produced the better quality level compared to the powder-free mode. The more rough and uneven surface was observed by increasing the powder particles concentration due to the instability in the erosion process. It was also observed that the process of topographic changes of machining surface for different concentrations is quite similar to the production mechanism of surface roughness under different concentrations in PMEDM process.
The significance of different parameters on surface measures have also been investigated in order to investigate performance measures and tabulated in Table 4. In PMEDM process, the chips are almost spherical that separated from the surface of workpiece due to uniform sparking during electrical discharge.
The larger current increase the surface roughness owing to the higher discharge energy. The higher impact forces on the machining surface has caused more molten material to produce deeper and larger cavities. After the molten material erupts from the cavities, the remaining molten material around the cavities freezes and produces a rough surface during cooling due to the flow of dielectric fluid. The lower current could produce lower surface roughness due to the lower depth of the cavities. Figure 21–22 show the main effects of the input parameters on the roughness values of the machining surface. The main reason for larger craters are owing to energy from electric discharge as mentioned earlier. The surface roughness of the specimens under different concentration in the dielectric fluid, Fig. 23. The better smoothness was observed with aluminium particles mixed dielectric medium due to the higher conductivity and low density of aluminum oxide. The lower electrical resistance can increase the spark gap whereas high thermal conductivity causes more heat to be transferred out of the discharge position. Both of these factors could lead to a decrease electric power density and impact force from the electrical discharge on the workpiece surface. It has resulted in producing lower deep cavities from machining on the workpiece surface. The addition of conductive or semiconductor powder particles into the plasma channel, the resistivity of the fluid fracture decreases. Hence the EDM feed mechanism could increase the gap between the two electrodes compared to the conventional EDM machining mode to create more stable electrical discharge conditions. The production of larger and wider electric discharge channel which leads to decrease the electric power density at the electric discharge position. It has created less deeper cavities on the machining surface and resulted in the lower surface roughness. The higher thermal conductivity of the powder particles could lead to a lower thermal energy at the electric discharge position.
The optimization of the electrical discharge process when adding aluminum oxide powder with values that are considered as the ultimate justifiable goal. The range of changes considered, the weight to determine the value of the parameter and the degree of importance, Table 5.
The overlay plot diagram can decide the most optimum range of the input factors within determined range. The material removal rate should attain the maximum value whereas tool wear rate, surface roughness and machining time should reach the minimum level. The shaded area is known as the area outside the determined specifications. The yellow region area is known as the safe area for achieving optimization goals called the "sweet spot", Fig. 24. The overlay plot diagram while adding aluminium powder under different concentrations in the electrical discharge process to reach optimal range of input and output parameters. The results have shown that the optimal process parameters including I = 22 A, Ton = 7 µs and C = 5 g/l, and quality indicators such as MRRopt=0,867 mm3/min, TWRopt = 0.126 mm3/min, SRopt = 5.86 µm and Topt = 12.542 min.
Table 5
Constraints of process parameters in EDM
Constraints |
| | Lower | Upper | Lower | Upper | |
Name | Goal | Limit | Limit | Weight | Weight | Importance |
A: Peak current | is in range | 15 | 25 | 1 | 1 | 3 |
B: Pulse on Time | is in range | 5 | 7 | 1 | 1 | 3 |
C: Concentration of powder | is in range | 0 | 5 | 1 | 1 | 3 |
MRR | maximize | 0.121678 | 2.06591 | 3 | 1 | 5 |
TWR | minimize | 0.0268531 | 0.275456 | 1 | 2 | 3 |
SR | minimize | 4.25 | 8.7 | 1 | 3 | 5 |
Machining time | minimize | 2.033 | 35.45 | 1 | 1 | 3 |
Machining Time Mangament is known as a criteria for assessing of the performance of a process. To be more precie, it is a good option to take place a process in a high or poor efficiency. Making a surface with high quality at least Cost of Goods Manufactured (COGM), simultaneously, it is investifated in this study. Figure 25 shows that my investigation has been done in three stage of concentration of powders (Cp = 0–5 g/l). the descreasing procedure of machning time is visble while the without powder is compared with added powder conditions. It is the sign of improving machining time in PMEDM process.
In short view, by considering the achiviements obtained of adding Al2O3 that is shown on Fig. 26, it could be visible that the PM-EDMed Ti-6Al-4V workpeice is usable in sensitive industried like aerospace and medicine processes. The surface that has a predictable charactereistics is needed in any industries. Indeed, analyzing of time and cost management shows that the surface could be considerable in different stage of production and manufacturing when it has had a priority of quantity and quality.