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Van Tao Le
To cite this article: Van Tao Le (2021): The influence of additive powder on machinability and surface integrity of SKD61 steel by EDM process, Materials and Manufacturing Processes, DOI:
10.1080/10426914.2021.1885710
To link to this article: https://doi.org/10.1080/10426914.2021.1885710
Published online: 19 Feb 2021.
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The influence of additive powder on machinability and surface integrity of SKD61 steel by EDM process
Van Tao Le
ATC, Le Quy Don Technical University, Hanoi, Vietnam
ABSTRACT
The powder mixed electrical discharge machining, also called PMEDM, is gaining much attention, because it is demonstrated as a good solution for enhancing productivity and surface integrity. In this research, the influence of main process parameters, including peak current, pulse on time, and powder concentration in the PMEDM with tungsten carbide powder on the machinability of SKD61 steel – i.e., material removal rate (MRR) and tool wear rate (TWR), was firstly investigated. Subsequently, the surface integrity of the recast layer, including the chemical composition, the recast layer thickness (RLT), and the percentage of the surface micro-crack density acreage (PSCDA) was analyzed and discussed. The results show that peak current, pulse on time, and powder concentration have influence on the machinability and surface integrity. MRR and TWR have changed in an uptrend when peak current, pulse on time, and powder concentration increase. At Ip = 3A; Ton = 200 µs; Cp = 60 g/l, the largest change in MRR and TWR are 165.714% and 163.830% respectively as compared with the EDM method. The chemical composition of machined surfaces was also transformed. In comparison to the EDM method, PSCDA and RLT generated by the PMEDM method were significantly reduced, up to 52.558% and 63.366%, respectively.
ARTICLE HISTORY Received 7 November 2020 Accepted 18 January 2021 KEYWORDS
EDM; PMEDM; RLT; micro- crack; MRR; TWR;
machinability; surface integrity; PSCDA; chemical composition
Introduction
Electrical discharge machining (EDM) covers the machining of most of the high hardness conductive materials, for example SKD61, SKH54, NAK80, SUS440C. Hence, the EDM method is used widely in the industry. However, it gives low material removal capacity, the surface quality of the EDM machining method still has many limitations, such as the surface rough- ness is not good, the microstructure has many defects, the recast layer is not normally useful for the work surfaces.
Therefore, since its inception until the present time, it has always been researched for improved productivity and quality.
The EDM method provides heat at the spark discharge process from 8,000°C to 12,000°C,[1,2] this is very features. According to Marashi et al.[3] about four decades ago, researchers reported that the effect of impurities mixed in insulating liquids on the machining process and surface quality of workpieces. Thus, the conductive powder particles are mixed in the insulating solvent of the EDM method, which is called the PMEDM method, this is a good choice to enhance machining productivity and sur- face quality. The participation of conductive powder produces less the electrical insulation ability of the dielectric liquid and varies the gap of spark discharge of the inner part of the tool and the workpiece.[4–6] In addition, the electrical sparks are more uniform and the discharge channel is extended wider.[7,8]
The spark discharge is reduced in energy,[9] and the craters become wider and shallower. Therefore, the process of spark discharge becomes stable, enhances the machined efficiency and the quality of surfaces.[10–13]
Therefore, the application of the PMEDM method to improve machining properties and surface properties is
a promising solution. This is accomplished through a combination of the properties of the powder material, ther- mal energy of the EDM process, and the substrate material of workpieces, which form chemical and physical processes. The result is desirable properties of workpiece surfaces such as smooth roughness, high hardness of surfaces, few defects, and improved machining performance. Many researchers have investigated the different features of PMEDM. In 1980, Erden et al reported that,[13] the domination of the conductive sus- pended in the insulating oil solvent of the EDM process, that has enhanced the quality of surfaces, the ability to remove material, and tool wear reduction. Recently, researchers have concentrated on enhancing surface quality, material removal rate (MRR), and tool wear rate (TWR). Some of the studies are mentioned as follows:
Prakash et al.[14] studied surface integrity of Ti35Nb7Ta5Zr β alloy in the EDM process with Si powder. This study showed a significant improvement in surface modification. In addition, improvement in the MRR and the TWR were also found. The Si, Mn powder was mixed into the dielectric liquid for study.
Authors reported in,[15] the recast layer obtained when Si or Mn powder adding to the dielectric fluid was more uniform than that of the EDM process. Surface roughness (SR) of speci- mens was machined by EDM process with Si powder, it gave the best result. At the experimental technology modes in the report, the surface of the sample by the PMEDM method did not have micro-cracks and pores. According to.[16–22] SiC powder has been studied on many different aspects. Tripathy et al.[16] reported the mixing of SiC powder in the dielectric liquid to assess the impact on the MRR, the TWR, and the SR.
CONTACT Van Tao Le [email protected] ATC, Le Quy Don Technical University, Hanoi, Vietnam.
https://doi.org/10.1080/10426914.2021.1885710
© 2021 Taylor & Francis
ability of each type electrode with added powders for surface modification. SiC powders suspended in the dielectric liquid of the EDM process, which was reported in.[,2122] The results showed the improvement of surface properties of samples due to the participation of SiC powder.
Ti and TiO2 powder were studied in the report.[23–25] Ti powder was mixed in the deionized water of the EDM process to be investigated.[23] The results showed a decrease in the micro-cracks of surfaces when the powder concentration increased. There was no micro-crack on the surface at 6 g/l.
The RLT was also significantly improved when Ti powder was involved. In,[24] the study of the effects of TiO2 nanoparticles and the rotation speed of the tool electrode in the EDM process were investigated. The result reported that, there was a significant enhancement in surface roughness, MRR, and TWR. The craters were shallower. The micro-cracks were gradually reduced on the surface of workpieces as compared to the normal EDM, they were reported in.[25]
Carbon nanotube has been added to the solvent insulation of the EDM process in.[11,26] Surface integrity, MRR and TWR were investigated.[11] The results were better surface properties, MRR and TWR than conventional EDM. In,[26] authors have shown that MRR, TWR, and surface quality (surface rough- ness, micro-crack) have been significantly improved.
Some other powders including Urea, reduced graphene oxide, MoS2, CeO2, Mo, and Cr have also been studied, speci- fically as follows: Yan et al.[27] reported the effects of Urea in the pure water of the EDM process on the SR, the micro hardness of surfaces, and the resistance to wear of surfaces.
The results showed that the above indicators were better than the surface of the normal EDM process. The addition of reduced graphene oxide powder in the EDM process to machine 55NiCrMoV7 steel was investigated by Świercz et al.-
[28] Authors reported a change in surface modification. The SR and the RLT have a significant decrease as compared to the normal EDM process. Besides, there is an assessment of the formation of the RLT through the positive polarity and the negative polarity. MoS2 powder combined with ultrasonic vibration. It has been experimented by Prihandana et al.[29]
Hossain et al.[30] reported that the influence of CeO2 powder with particle size less than 5 mm mixed in the pure water.
Results for MRR, TWR, surface roughness, micro hardness, and microcrack have been improved with different concentra- tions as compared to the EDM process. Through experiments
In addition, several studies have used different powders in the same study to evaluate the role of each powder for surface modification in the EDM process. For example, Wu et al.[33]
revealed the domination of Surfactant and Al powders mixing to improve the SR. The best roughness of surfaces was less than 2 μm. Jabbaripour et al.[34] studied the domination of SiC, Al, Cr, Gr, and Fe powders in the EDM process to the SR of the TiAl alloy. In this study, it had indicated that the Al powder improved best the roughness as compared to normal EDM by 32%. Talla et al.[35] analyzed the domination with three type powders including Al powder, Graphite powder, and Si pow- der in the EDM process on Inconel 625. Authors pointed to the effect of each type of powder to improve surface quality and MRR. The graphite powder had the highest effect on the MRR.
The Si powder had the highest effect on the micro hardness of surfaces.
Up to now, there are currently very few studies about the tungsten particles domination to surface variation: The effect of the tungsten powder mixing into the dielectric liquid on the micro hardness of surfaces of three types of OHNS, D2, and H13 steels were investigated by Kumar and Batra.[36]
Resulting in the micro hardness of surfaces of three types of steel were increased by more than 100%. The increase in the micro hardness of surfaces led to the increase in wear resis- tance. This is very useful for the mold and the mechanical part, which require the high abrasion resistance. Bhattacharya et al.[37] explored the domination of the tungsten powder by mixing them into the insulating liquid to process the copper material with a W-Cu electrode. The result gave a fine surface and the improved micro hardness. Mohanty et al.[38] investi- gated for surface integrity of Ti6Al4V material during EDM machining with the participation of tungsten disulfide (WS2) powder in the deionized water. The result showed a significant enhancement in the micro hardness (HV), the RLT, the deposition rate of material (MDR), and the SR.
Wear resistance was tested. The result showed better than the that of normal EDM process. According to,[39,40] Le et al investigated initially the influence of tungsten carbide powder on the change of the SR and the micro hardness of surfaces.
The result showed that, there was a improvement in the roughness of surfaces and the micro hardness of surfaces by the PMEDM method as compared to the EDM method. The best upturn in the surface roughness and the micro hardness was 57.98% and 129.17% respectively.
In the overview mentioned above, many types of different powders were investigated to achieve the machinability and the surface quality for the specific application in the industrial manufacture. However, limited studies have reported on the effect of tungsten carbide powder on properties of the recast layer, and machinability of the SKD61 steel processed by PMEDM. Tungsten carbide is a metallic alloy with a high melting temperature (about 3422°C), a low thermal expansion, and high strengths. Its resistance to oxidation, acid and alkali corrosion of tungsten carbide is very good. SKD61 steel is largely in the hot stamping, tool, die, and machine parts.
SKD61 steel is a material with good mechanical properties.
Especially, when SKD61 steel is treated by chemical method and/or heat treatment method. Therefore, the combination of the good physical and chemical properties of tungsten carbide in the powder and that of SKD61 steel was processed by the EDM method. This measure helps to enhance the working life of the hot stamping, tool, die, and machine parts. In[3,10,41–46]
were reported that the modification of surfaces was studied in fine-finish and semi-finish process. In addition, the PMEDM process in this study was the final and near-final task in the manufacturing process of the working surface of the hot stamping, tool, die, and machine parts. Hence, this research aims to investigate the domination of the pulse on time, the peak current in fine-finish and semi-finish process, and powder concentration to the machinability and the surface integrity of SKD61 steel by the EDM process with tungsten carbide pow- der. In this study, surface integrity includes the composition of the chemical elements, the RLT, and the percentage of the surface micro-crack density acreage (PSCDA). The machin- ability includes the MRR and the TWR.
Materials and methods
The Fig. 1 shows the experimental diagram of this study.
Materials
SKD61 steel was used for experimentation. It was supplied by the manufacturer Daido corporation of Japan. The nominal chemical composition of SKD61 steel is 0.38 C-1Si-5Cr- 1.25Mo-1 V-0.4Mn- balance Fe(wt.%). The oil EDM fluid 2 was used as an insulating dielectric fluid. It was supplied by the manufacturer Shell. The powder with commercial code WC- 727-6 used in this experiment was supplied by the manufac- turer Praxair Surface Technologies. Its chemical composition is 5.56 C-11.9Co-0.02Fe-82.5 W- 0.02 other components (wt.%), and the particle size of less than 31 μm.
Methods
Samples were machined on the electrical discharge device system (CNC-460 EDM). It was supplied by Aristech Company (as Fig. 2). The dimensions before and after machining was given in Table 1. The powder was mixed into the dielectric liquid with the different concentrations (as Table 1). Before the samples and copper electrodes were put into machining by the PMEDM method. They were machined with high accuracy in terms of geometric dimen- sions (0.01 mm) and geometric errors (0.02 mm) by the fine grinding method, then the samples and electrodes were attached to the CNC-460 EDM, which was carefully mounted with high precision.
Figure 1. The experimental diagram.
Criteria and basis for selection of process parameters: For electrical parameters, based on the literature overview and the configuration of the controller settings of the electrical dis- charge machine- the CNC-460 EDM. In addition, the aim of this study was to evaluate the surface integrity and machin- ability in the fine-finish and semi-finish process. Among elec- trical parameters, peak current and pulse on time had a strong impact on machinability and surface integrity of SKD61 steel.[9,33,38,47] Hence, the main domination of electrical para- meters including the peak current, and the pulse on time was selected. Variable levels of these electrical parameters were chosen as shown in Table 1. Other parameters such as the discharge voltage, the pulse off time, and polarity of the elec- trode were fixed as shown in Table 1. For powder concentra- tion, it was through the exploratory tests on the basis of the electrical parameters selected as above, combined with the thermal and electrical properties of the tungsten carbide pow- der. Hence, variable levels of powder concentration in this experiment were selected as shown in Table 1.
The content of the chemical elements and the SEM of surface microstructure were measured by energy dispersive X-ray spec- troscopy (EDX) method on a Scanning Electron Microscope
JSM6610LA-Jeol from Japan. The content value of chemical elements in this study was used for analysis and evaluation. It was the medium value of of the results of measurements on three different measurement region of the sample, obtained after the machined process. It was set up as shown in Fig. 3.
The material removal rate (MRR) is interpreted as the amount of removal of the material from the workpiece per minute. Likewise, the tool wear rate (TWR) is interpreted as the amount of tool wear of the tool per minute. An electronic balance (TE214S-Sartorius digital balance with read-ability of 0.0001 g) was used to determine the mass of workpiece mate- rial and tool electrode before and after machining
MRR g min
� �
¼ M1 M2
Machining time (1) TWR g
min
� �
¼ m1 m2
Machining time (2) In formula (1), M1, M2 is the mass of workpiece (g) before machining, after machining, respectively. In formula (2), m1, m2 is the mass of tool (g) before machining, after machining, respectively. Machining time in formula (1),(2) is the processing time to achieve the sample size after machining as shown in Table 1.
Recast layer of samples were determined by the Murakami reagent and photographed by the optical microscope AXIO- A2M. The samples were cold cast in epoxy resin, then the samples were polished with fine paper armor with a grain size of 3000. The samples polished with felt as in Fig. 4. After that the samples were immersed in the solution for about 60s, before being placed on the optical microscope AXIO-A2M to take pictures of recast layer as Fig. 5.
Method for determining the recast layer thickness (RLT):
Figure 2. Experimental principle schematic.
Table 1. Experimental conditions.
Deposition Condition Detail
The peak current (A) (Ip) 2A; 3A
The pulse on time (μs) (Ton) 50 μs; 200 μs
The pulse off time (μs) (Toff) 50 μs
The dielectric fluid Shell EDM Fluid 2
Polarity of electrode Negative (−)
Tool electrode Cu (99%)
The current voltage (V) 120 V
The powder concentration (g/l)(Cp) 20 g/l; 40 g/l; 60 g/l
Dimension of the sample DxL = 19x50mm
Dimension after machining of the sample DxL = 19x49.7 mm
RLT μmð Þ ¼
Srecastlayer regionIð Þ
Lrecastlayer regionIð ÞþSrecastlayer regionIIð Þ
Lrecastlayer regionIIð ÞþSrecastlayer regionIIIð Þ
Lrecastlayer regionIIIð Þ
3 (3)
Srecast layer and Lrecast layer were carried out Axiovision Cam 4.82 software as Fig. 5. The measurement result of the recast layer thickness in each technology mode was the medium value of the three different measurement region on the sample. The value of the medium recast layer thickness is calculated as according to the formula (3). It was analyzed and evaluated in the following section.
Method for determining the percentage of the surface micro-crack density acreage (PSCDA) was performed as fol- lows: First of all, the acreage of micro-cracks on the surface were determined, and were carried out Axiovision Cam 4.82 software as in Fig. 6. The measurement and calculation result of the percentage of the surface micro-crack density acreage in each technology mode was the medium value of the three different measurement region on the sample. The value of the average of the percentage of the surface micro-crack density acreage was calculated as according to the formula (4). It was analyzed and evaluated in the following section.
PSCDAð Þ%
¼ P
Smicro crack regionIð Þ Smicrograph of SEMðregionIÞþ
P
Smicro crack region IIð Þ Smicrograph of SEMðregion IIÞþ
P
Smicro crack region IIIð Þ Smicrograph of SEMðregion IIIÞ
3 �100%
(4)
Results and discussions Material removal rate (MRR)
As shown in Figs. 7 and 8, they show that the MRR of the PMEDM method has been improved as compared to the MRR
Figure 3. The content of the chemical element by EDX method at Ip = 2A; Ton = 200 µs; Cp = 0 g/l.
Figure 4. The sample preparation process.
Figure 5. Method for determining Srecast layer and Lrecast layer by micrograph of the optical microscope at Ip = 2A; Ton = 200 µs; Cp = 0 g/l.
of the EDM method. The improvement of the MRR of the PMEDM method is more or less, this completely depends on the combination of the electrical parameters and the powder concentration. The cause of the improved MRR is the partici- pation of suspended powder in the EDM process. This is explained as follows: Firstly, when the conductive powder is present during the spark discharge process. This is the decisive factor for the change in the spark discharge process. It forms the spark discharge channel at many locations between the tool electrode and the workpiece electrode. The properties of the discharge channel also change. Its spark discharge structure is stratified. These make the spark discharge more stable than the spark discharge in the absence of conductive powders.
Secondly, the powder is suspended in the solvent medium.
This has reduced the insulating capacity of the dielectric,[48,49] which improves the conductivity of the solvent
medium, reduction of solvent viscosity. These improve the spark discharge more evenly. The molten material is better washed away from the surface of the sample at the end of the spark discharge process. This improves the stabilization of the spark discharge. This leads to avoiding the phenomena of arcing and short circuit. Hence, this improves the capacity of the removal of material.
Continuing to observe Figs. 7 and 8, they show that for different peak currents, pulse on times, and powder concentra- tions. The results of the MRR are also different, as follows:
When the peak current was fixed at Ip = 2A or 3A, the pulse on time was changed from 50 µs to 200 µs, the result of the MRR was increased as the powder concentration was increased from 20 g/l to 60 g/l, respectively. Also, at the peak current and the pulse on time were fixed, the MRR was increased as the powder concentration was increased from 20 g/l to 60 g/l, respectively.
Figure 6. Method for determining the acreage of micro-cracks by micrograph of SEM at Ip = 2A; Ton = 200 µs; Cp = 0 g/l.
Figure 7. The material removal rate at Ip = 2A.
These results also comply with the influence rule of the main process parameters in previous studies.[14,18,24,26,32,50]
Although previous studies have investigated other powders, which have a different electrical and thermal conductivity as compared to the tungsten carbide powder. However, at Ip = 3A;
Ton = 50 µs when the powder concentration was increased from Cp = 40 g/l to Cp = 60 g/l, the MRR was decreased from 0.0603 g/mm to 0.0564 g/mm, respectively. This has been reported[50] on nonconformity of electrical parameters when the powder concentration was increased. At Ip = 3A; Ton
= 200 µs; Cp = 60 g/l, the largest change of the MRR by the PMEDM method was 165.714% as compared with the EDM method. The cause of these phenomena is explained as follows:
Due to the peak current, and the pulse on time increases. They produce the increase of discharge energy. Hence, the thermal energy is increased, leading to the amount of molten material on the workpiece surface is increased.[51–53] The powder is suspended in the solvent medium. This has reduced the solvent viscosity. Therefore, the molten material is better washed away from the surface of the sample at the end of the spark discharge process. As a result, the MRR is increased. At Ip = 3A; Ton
= 50 µs, when the powder concentration was changed from Cp
= 40 g/l to 60 g/l then the MRR was reduced. it can be that due to the combination of the pulse on time, the pulse off time, and the peak current is inappropriate, The pressure of the gas bubble breaks at the previous spark discharge being large.
Resulting in the powder density in the next spark discharge is low. The energy of the resonance spark discharge is reduced.
The spark discharge process produces the reduction of dis- charge energy. In addition, due to the concentration powder is excessive. This is incompatibilities leading to the problems of the spark discharge such as the phenomena of arcing and short circuit. This interrupts the spark discharge process. Therefore, the heat channel is formed, it has a low source of thermal energy. Consequencly, the capacity of the removal of material is reduced.
In general, the result of the MRR was obtained by experi- ment with tungsten carbide powder, consistent with the pre- vious studies[14,18,24,26,32,35,50] including powders such as Ag, carbon nanotube, Cr, SiC, Si, TiO2, Al, Gr, and Si. However,
these studies reported the results at different process para- meters, but the rule of influence on the MRR was essentially the same as the result in this study with tungsten carbide powder. In the previous studies, the improvement in MRR was better than that of MRR in this study. Because the previous studies used powders with the low molten point, and the electrical parameters of the process were often chosen to gen- erate higher energy than that of this study. Hence, the amount of material removed from previous studies is larger than that in this study.
Tool wear rate (TWR)
According to Figs. 9 and 10, they represent the experimental results of the TWR by the EDM and the PMEDM method. At the peak current includes Ip = 2A and 3A with change of pulse on time and powder concentration from Ton = 50 µs to Ton
= 200 µs and from Cp = 0 g/l to Cp = 60 g/l, respectively. The results show that the TWR has changed in the uptrend as the process parameters were increased. The cause of the change in TWR was the participation of conductive powder in the EDM process. This is explained as follows: The conductive powder is present during the spark discharge process. The characteristic of the discharge channel is also changed. It discharges stratifi- cation, more stable than the spark discharge in the absence of conductive powders. The powder is suspended in the solvent medium. This has reduced the insulating capacity, improved the conductivity of the solvent medium, reduction of solvent viscosity. Hence, the molten material is better washed away from the surface of the sample at the end of the spark discharge process. This leads to avoiding the phenomena of arcing and short circuit. The reasons mentioned above, they have improved the heat channel, it is more stable. Hence, this changes the tool wear rate of the PMEDM method as compared to that of the EDM method.
Also following Figs. 9 and 10, with different peak currents, pulse on times, and powder concentrations, the results of the TWR are also different. At Ip = 2A or 3A, the pulse on time was changed from 50 µs to 200 µs. The result of the TWR was
Figure 8. The material removal rate at Ip = 3A.
increased as the powder concentration was increased from Cp
= 20 g/l to Cp = 60 g/l. Also, at the peak current and the pulse on time were fixed, the TWR was increased as the powder concentration was changed from Cp = 20 g/l to Cp = 60 g/l.
However, at Ip = 3A; Ton = 50 µs when the powder concentra- tion was increased from Cp = 40 g/l to Cp = 60 g/l, resulting in the TWR was decreased from 0.0115 g/mm to 0.0108 g/mm. At Ip = 3A; Ton = 200 µs; Cp = 60 g/l, the largest change of the TWR by the PMEDM method is 163.830% as compared with the EDM method. The cause of these phenomena is explained as follows: Due to the peak current, the pulse on time increased. This produces an increase in discharge energy.
Hence, the thermal energy is increased. This changes the volume of molten material on the tool surface. The TWR at Ip = 3A; Ton = 50 µs; Cp = 60 g/l reduces as compared with the TWR at Ip = 3A; Ton = 50 µs; Cp = 40 g/l. It can be that due to the combination of the pulse on time, the pulse off time, and the peak current is inappropriate, the pressure of gas bubble breaks at the previous spark discharge being large. Resulting in the powder density in the next spark discharge is low, the energy of the resonance spark discharge is reduced, the spark discharge process produces the reduction of thermal energy.
Moreover, due to the concentration powder is excessive. This is incompatibilities lead to the problems of the spark discharge such as the phenomena of arcing and short circuit. This inter- rupts the spark discharge process. The heat channel is formed, it has a low source of thermal energy. Hence, the thermal energy is transferred to the tool electrode, it is reduced.
Resulting in the capacity of the removal of material is reduced.
According to documents[14,18,24,26,32,50] researched on the TWR when the EDM process had the participation of conduc- tive powder particles. They included powders such as Si, SiC, TiO2, carbon nanotube, Cr, and Ag. These studies have reported on the different TWRs. This depends on the thermal and electrical properties of each powder. For powders with good thermal conductivity such as carbon nanotube and Ag, the TWR has greater value than the TWR of conventional EDM. For powders with thermal conductivity such as Si, SiC, TiO2, and Cr, the TWR has smaller value than TWR of con- ventional EDM. Tungsten carbide powder in this study has lower thermal conductivity than carbon nanotube and Ag, and higher than Si, SiC, TiO2, and Cr. In this study, the result of TWR by PMEDM is higher than that of EDM. When the peak current, the pulse on time, and the powder concentration were
Figure 9. The tool wear rate at Ip = 2A.
Figure 10. The tool wear rate at Ip = 3A.
changed. The resulting trends of TWR in this study are con- sistent with carbon nanotube and Ag powder in document,[26,50] which has been published.
Through the investigation of TWR, it reveals that the amount of removal material of the tool is much lower than that of the workpiece. The cause of this phenomenon: The energy generated by the spark discharge process, it is distrib- uted differently to the tool electrode and the workpiece elec- trode, is about 48% and 34%, respectively.[50,53–56] Hence, the heat absorption into the tool and the workpiece is completely different. In addition, the material of the tool electrode is copper, it has a higher conductivity of electrical and thermal than the workpiece material.[57] When the heat is absorbed into the tool electrode, it disperses into the surrounding environ- ment very quickly, which hinders the melting of the tool electrode material. Therefore, the value of TWR is much lower than that of MRR.
Chemical composition on the surface of the recast layer According to Table 2. The top surface layer of samples was processed by the EDM process, the chemical composition of them (Table 2-No.1;5;9;13) have been changed as compared to that in the substrate of SKD61 steel, as follows: The Mn element has been completely removed. The percentage of the Fe element has reduced. Some elements such as C, Si, Mo, V, and Cr remained. The percentage of C, Si, Mo, and Cr was increased as compared to the substrate of SKD61 steel. The percentage of the V element has reduced in Table 2- No.5. But in Table 2-No.1;9;13, the percentage of the V element has increased.
The element O has appeared on the surface in a number of modes such as Table 2- No.1; 9; 13. The top surface layer of samples was processed by the PMEDM process. The chemical composition of them (Table 2- No.2;3;4;6;7;8;10;11;12;14;15;16) have been changed as compared to that of the substrate of SKD61 steel, and that on the recast layer of the EDM, as follows:
The Mn element was completely absent on the top of the surface.
The percentage of the Fe element has been reduced as compared to the substrate of SKD61 steel, but it increased as compared to the surface layer of samples by the EDM method. The percentage of the C element increased as compared to that of the substrate
layer, but still lower than the percentage of the C element on the recast layer of the EDM method. The Co element has appeared on the surface at Ip = 2A; Ton = 200 µs; Cp = 20 g/l, and Cp = 40 g/
l; and at Ip = 3A; Ton = 200 µs;Cp = 20 g/l. The Cu element has appeared on the surface in a number of modes such as Ip = 3A;
Ton = 50 µs;Cp = 20 and 40 g/l, Ip = 3A; Ton = 200 µs;Cp = 40 and 60 g/l. The W element has appeared on the surface of the recast layer. The exception in mode Ip = 3A; Ton = 200 µs; Cp = 40 g/l the W element hasn’t appeared on the surface of the recast layer.
The percentage of the W element depends on the technological mode. The presence of the W element on the surface of the recast layer can improve mechanical, physical, and chemical proper- ties. The cause of these phenomena is due to the spark discharge process to form the heat channel, which transforms the percen- tage of elements and the chemical composition on the surface.
In addition, according to Table 2, the percentage of the C element tends to increase when the concentration increases from Cp = 20 g/l to Cp = 60 g/l at all the pulse on times and the peak currents. The exception in mode Ip = 3A; Ton = 50 µs, the percentage of the C element tends to reduce when the concen- tration increases from Cp = 20 g/l to Cp = 60 g/l. The percen- tage of Si and W elements tend to increase when the powder concentration increases from Cp = 20 g/l to Cp = 40 g/l in all electrical parameters. But at Cp = 60 g/l the percentage of them tends to reduce as compared to that of Cp = 40 g/l. Continuing to consider other elements such as V, Cr, Fe, and Mo, the percentage of V, Cr, Fe, and Mo elements tends to increase or reduce differently when the concentration increases from Cp = 20 g/l to Cp = 60 g/l. This depends on a combination of the pulse on time, the pulse off time, the peak current, and the powder concentration. The cause of these phenomena is due to the spark discharge process, which forms a thermal channel.
The thermal channel melts and causes chemical and physical processes on the surface. Moreover, the presence of conductive powder particles in the spark discharge process changes the discharge and the formation of the plasma channel. This makes the variation powder particle density in the next discharge process, which depends on powder concentration, peak cur- rent, pulse on time, and pulse off time. These are the causes, which change the elements and the percentage of elements on the surface.
Table 2. The chemical composition in weight percentage on the surface of the samples.
No. The different technology modes
The chemical composition in weight percentage (%wt)
C Si V Cr Fe Co Mo W Cu O
1 Ip = 2A; Ton = 50 µs;Cp = 0 g/l 15.800 1.867 1.180 6.323 67.270 0.0 5.063 0.0 0.0 2.5
2 Ip = 2A; Ton = 50 µs;Cp = 20 g/l 6.450 1.660 0.920 5.457 77.100 0.0 1.563 6.85 0.0 0.0
3 Ip = 2A; Ton = 50 µs;Cp = 40 g/l 6.503 1.993 0.833 5.203 76.093 0.0 1.680 7.693 0.0 0.0
4 Ip = 2A; Ton = 50 µs;Cp = 60 g/l 13.513 1.553 0.930 5.180 72.73 0.0 1.627 4.467 0.0 0.0
5 Ip = 2A; Ton = 200 µs;Cp = 0 g/l 19.483 1.993 0.697 5.727 63.103 0.0 3.487 0.0 0.0 0.0
6 Ip = 2A; Ton = 200 µs;
Cp = 20 g/l
2.243 0.757 1.097 6.923 83.123 2.677 2.957 0.223 0.0 0.0
7 Ip = 2A; Ton = 200 µs;Cp = 40 g/l 9.057 1.060 0.910 5.637 78.620 2.317 1.527 0.873 0.0 0.0
8 Ip = 2A; Ton = 200 µs;Cp = 60 g/l 11.987 1.323 0.773 5.503 78.350 0.0 1.403 0.660 0.0 0.0
9 Ip = 3A; Ton = 50 µs;Cp = 0 g/l 16.567 2.050 1.203 6.783 66.120 0.0 3.477 0.0 0.0 3.800
10 Ip = 3A; Ton = 50 µs;Cp = 20 g/l 7.457 1.023 0.873 5.593 80.313 0.0 1.353 1.260 2.130 0.0
11 Ip = 3A; Ton = 50 µs;Cp = 40 g/l 7.280 1.137 0.847 5.470 80.860 0.0 1.113 1.597 1.700 0.0
12 Ip = 3A; Ton = 50 µs;Cp = 60 g/l 6.610 1.077 0.757 5.503 83.683 0.0 1.480 0.890 0.0 0.0
13 Ip = 3A; Ton = 200 µs;Cp = 0 g/l 18.310 2.093 1.027 6.333 67.773 0.0 2.587 0.0 0.0 1.880
14 Ip = 3A; Ton = 200 µs;Cp = 20 g/l 2.420 0.837 0.923 6.777 84.370 2.540 2.057 0.077 0.0 0.0
15 Ip = 3A; Ton = 200 µs;Cp = 40 g/l 8.573 1.080 0.863 5.233 83.307 0.0 0.383 0.0 0.560 0.0
16 Ip = 3A; Ton = 200 µs;Cp = 60 g/l 10.540 1.163 0.697 5.243 79.000 0.0 1.637 0.357 1.36 0.0
ment and Coban element of the tungsten carbide powder, the copper element from the tool electrode, and the Carbon and
the surface of the sample in small amounts. Hence, the recast layer has a small thickness. Another reason, the addition of
Figure 11. The recast layer thickness at Ip = 2A.
Figure 12. The recast layer thickness at Ip = 3A.
powder has reduced the insulating capacity of the dielectric,[48,49] which improves the conductivity of the solvent medium, reduces solvent viscosity. These improve the spark discharge evenly. The molten material is better washed away from the surface of the sample at the end of the spark discharge process. This has supported the improvement of recast layer thickness by the PMEDM method.
Continuing to observe Figs. 11 and 12, at Ip = 2A with the variation of the pulse on time and the powder concentration from Ton = 50 µs to Ton = 200 µs and from Cp = 20 g/l to Cp
= 60 g/l, respectively. The value of RLT results by the PMEDM method have been significantly improved as compared to the RLT by the EDM method. However, it can be seen that with a concentration of Cp = 20 g/l at Ton = 50 µs or 200 µs, the RLT is improved better than the concentration Cp = 40 g/l and Cp
= 60 g/l at Ton = 50 µs or 200 µs. Continuing to consider the change in the RLT at Ip = 3A with the variation of the pulse on time and the powder concentration from Ton = 50 µs to Ton
= 200 µs and from Cp = 20 g/l to Cp = 60 g/l, respectively. The value of RLT results by the PMEDM method have been sig- nificantly improved as compared to the RLT by the EDM method. At Cp = 20 g/l and Ton = 200 µs the largest improve- ment of the RLT by the PMEDM method is 63.366% as com- pared with the EDM method. Moreover, the RLT by the PMEDM method at Ip = 3A is improved better than the RLT by the PMEDM method at Ip = 2A with all the pulse on time and the powder concentration. This results are consistent with .[17,38] The causes of the RLT changes with the presence of conductive powder particles, it is explained as follows: In addition to the reasons mentioned above, then due to the peak current, the pulse on time, and the pulse off time are combined reasonably. This creates the pressure to break the gas bubble of the previous spark discharge stage as small. This produces a high density of powder particles in the next dis- charge channel. Hence, it supports the spark discharge process in all aspects: the gap of spark discharge is increased, the spark discharge channel is expanded. As a result, the density of sparks is more even. This improves the RLT of samples by the PMEDM method.
In,[14,15,17–20,38,58–60] the recast layer thickness was studied with the participation of different powders including Ag, SiC, WS2, Cu, Si in the EDM process. The recast layer thickness
depended on the process parameters and properties of the powder and workpiece material. However, these studies only evaluated the recast layer thickness at some points on the recast layer. These works were not comprehensive. In this study, the recast layer thickness was evaluated more comprehensively on the recast layer. Therefore, the recast layer thickness results showed a better representation of the recast layer. In general, the change law of the RLT in this study with tungsten carbide powder upon the change in process parameters is completely consistent with the previous studies such as mentioned above.
The recast layer is carried out by the PMEDM method also has complete differences from the recast layer of the EDM method, Tungsten carbide phases are present in the recast layer of samples by the PMEDM method. Hence, the abrasion resistance of the recast layer was processed by the PMEDM method better than the normal EDM method, as in Fig. 13 The micro-crack of surface
Study and evaluation of surface micro-crack after PMEDM process is useful in engineering. Because, the micro-cracks in the surface greatly affect the working life of the part. According to some studies of micro-crack on the surface,[11,15,18,26,61]
which have been published, most researchers have evaluated the length of the micro-crack on the surface, while the width of micro-crack has not been considered of researchers. Hence, in this study, the length and width of the micro-crack are con- sidered for a comprehensive assessment through the percen- tage of the surface micro-crack density acreage (PSCDA).
According to Figs. 14 and 15, it is noticed that the PSCDA of surfaces were machined by the EDM method. Their value are 2.886%, 4.123%, 3.657%, and 4.325%, corresponding to Ip
= 2A; Ton = 50 µs; 200 µs and Ip = 3A; Ton = 50 µs; 200 µs.
Meanwhile, the PSCDA of surfaces were machined by the PMEDM method. Their value are 2.205%, 1.674%, 1.845%, corresponding to Ip = 2A; Ton = 50 µs; Cp = 20, 40, 60 g/l, 2.148%, 1.956%, 2.066% corresponding to Ip = 2A; Ton
= 200 µs; Cp = 20, 40, 60 g/l, 2.405%, 2.689%, 2.945% corre- sponding to Ip = 3A; Ton = 50 µs; Cp = 20, 40, 60 g/l, and 2.765%, 2.972%, 3.450% corresponding to Ip = 3A; Ton
= 200 µs; Cp = 20, 40, 60 g/l. Thus, the PMEDM method has significantly improved micro-crack on the surface of the
Figure 13. The cross-section of samples, 500 times magnification: a) Ip = 2A, Ton = 50 µs, Cp = 0 g/l; b) Ip = 2A, Ton = 50 µs, Cp = 20 g/l.
samples after machining. The best improvement result of PSCDA is the reduction of 52.558% as compared with the EDM method at Ip = 2A; Ton = 200 µs; Cp = 40 g/l. Based on the values on the charts of Figs. 14 and 15. The PSCDA of the PMEDM method is better than that of the EDM method. The reason is the participation of conductive pow- der particles in the spark discharge process. This can be shown through the following aspects: Due to the participa- tion of conductive powder particles changes in the spark discharge process. It is a more uniform spark discharge.
This uniform spark discharge causes the residual stresses generated from the melting of the material to the cooling of the material and deposition of them on the surface of the specimen to be small and uniform.[62] The presence of suspended powder particles in the dielectric oil fluid has been reduced the viscosity of the dielectric oil fluid,[18]
which makes the process of transporting the debris to go out better, they are deposited less on the surface. In addi- tion, the ability of transfer heat to the environment is better. These reduce the residual stress on the surface resulting from after the spark discharge process. Hence, the micro-crack is also significantly reduced.
However, at concentration Cp = 60 g/l with Ip = 2A; 3A and all the pulse on time of experiments, it was found that the PSCDA has been increased as compared to the PSCDA at concentrations Cp = 40 g/l. This phenomenon can be due to the high powder concentration in the dielectric oil fluid, which in turn leads to the sparks have been discharged at multiple points on the surface of the specimen. Resulting in a large amount of molten material in the discharge is produced.
Meanwhile, the molten material is not ejected away from the surface much. A large amount of molten material is deposited on the surface. Therefore, this has created a high residual stress on the surface, which is the cause of more micro-crack.
In Figs. 14 and 15, the PSCDA of Ip = 2A is lower than the PSCDA of Ip = 3A respectively with Ton = 50 µs; 200 µs and Cp
= 20, 40, 60 g/l. Since Ip = 3A, the thermal energy in the discharge channel is larger than that of Ip = 2A, which results in a large amount of material has been deposited on the sur- face. The molten material is not transported away from the surface of the specimen well.[63] Hence, this has created a high residual stress on the surface to facilitate the development of more micro-crack. In addition, the large energy in the dis- charge channel, which results in the spark discharge energy is
Figure 14. The percentage of surface crack density acreage at Ip = 2A.
Figure 15. The percentage of surface crack density acreage at Ip = 3A.
greater, which results in the change in the local residual stress in the discharge and un-discharged region.[64] Therefore, it has formed the residual stress on the surface, and the micro-crack has been developed more.
The EDM process with the participation of different pow- ders including SiC, WS2, Cu, Si, Carbon nanotube, Ti, Cr, and CeO2 reduced the micro-crack of the surface.[14–18,25,26,30]
However, these studies just looked at micro-crack of the sur- face in length and by SEM images. While the width of micro- cracks has not been evaluated or has been evaluated in independent form, it has not been combined with the length of micro-crack. These studies have also shown the effect of powders on the micro-crack of surfaces. The effect is more or less, this depends on the combination of the process para- meters and the thermal conductivity of the powders. In this study, the addition of tungsten carbide powder improved the PSCDA of surfaces. The rules of the influence of the process parameters and powder concentration on the PSCDA of the surface are completely consistent with previous researches in different powders.
Conclusions
In this research, the domination of the addition of the powder into the insulating oil solvent to the machinability and the integrity of the recast layer of SKD61 steel were investigated through experiments. The main results can be summarized as follows:
● The MRR of the PMEDM method has been improved as compared to the MRR of the EDM method. The MRR has changed in the uptrend as the technological parameters of change increase. However, at Ip = 3A; Ton = 50 µs when the powder concentration was increased from Cp = 40 g/l to Cp = 60 g/l, the MRR was decreased. At Ip = 3A; Tom
= 200 µs; Cp = 60 g/l, the largest change of the MRR by the PMEDM method is 165.714% as compared with the EDM method.
● The TWR has changed in the uptrend as the technological parameters of change increase. However, at Ip = 3A; Ton
= 50 µs when the powder concentration was increased from Cp = 40 g/l to Cp = 60 g/l, then the TWR was decreased. At Ip = 3A; Ton = 200 µs; Cp = 60 g/l, the largest change of the TWR by the PMEDM method is 163.830% as compared with the EDM method.
● The chemical composition and the percentage of them on the top surface of the recast layer were transformed as compared to that in the substrate of SKD61 steel, and that on the recast layer of the EDM method as follows: The Mn element was completely absent on the top of the surface. The percentage of the Fe element has been reduced as compared to the substrate of SKD61 steel, but it increases as compared to the surface layer of sam- ples by the EDM method. The percentage of the C element increases as compared to that of the substrate layer, but still lower than the percentage of the C element on the recast layer of the EDM method. The Co element has appeared on the surface at Ip = 2A; Ton = 200 µs; Cp
= 20 g/l, and Cp = 40 g/l; and at Ip = 3A; Ton = 200 µs;Cp
= 20 g/l. The Cu element has appeared on the surface in a number of modes such as Ip = 3A; Ton = 50 µs;Cp = 20 and 40 g/l, Ip = 3A; Ton = 200 µs;Cp = 40 and 60 g/l. The W element has appeared on the surface of the recast layer, the exception in mode Ip = 3A; Ton = 200 µs; Cp = 40 g/l.
● The percentage of the C element tends to increase when the powder concentration increases from Cp = 20 g/l to Cp = 60 g/l at all the pulse on times and the peak currents, the exception in mode Ip = 3A; Ton = 50 µs. The percen- tage of Si and W elements tend to increase when the concentration increases from Cp = 20 g/l to Cp = 40 g/l in all electrical parameters, but at Cp = 60 g/l the percen- tage of them tends to reduce as compared to that of Cp
= 40 g/l. The percentage of V, Cr, Fe, and Mo elements tend to increase or reduce differently when the concen- tration increases from Cp = 20 g/l to Cp = 60 g/l, they depend on a combination of the pulse on time, the pulse off time, the peak current, and the powder concentration.
● The value of the RLT result by the PMEDM method has been significantly improved as compared to the RLT by the EDM method, at Ip = 3A; Cp = 20 g/l and Ton = 200 µs the largest improvement of the RLT by the PMEDM method is 63.366% as compared with the EDM method. The RLT by the PMEDM method at Ip = 3A is improved better than the RLT by the PMEDM method at Ip = 2A with all the pulse on time and the powder concentration, respectively.
● The PMEDM method has significantly improved micro- crack on the surface of the samples after machining. The best improvement result of PSCDA is the reduction of 52.558% as compared with the EDM method at Ip = 2A;
Ton = 200 µs; Cp = 40 g/l. The PSCDA of Ip = 2A is lower than the PSCDA of Ip = 3A with all the pulse on time and the powder concentration, respectively.
References
[1] Ho, K.; Newman, S. State of the Art Electrical Discharge Machining (EDM). Int. J. Mach. Tools Manuf. 2003, 43(13), 1287–1300. DOI:
10.1016/S0890-6955(03)00162-7.
[2] Davim Paulo, J. Nontraditional Machining Processes; Springer:
London, 2013. DOI: 10.1007/978-1-4471-5179-1.
[3] Marashi, H.; Jafarlou, D. M.; Sarhan, A. A. D.; Hamdi, M. State of the Art in Powder Mixed Dielectric for EDM Applications. Precis.
Eng. 2016, 46, 11–33. DOI: 10.1016/j.precisioneng.2016.05.010.
[4] Furutani, K.; Saneto, A.; Takezawa, H.; Mohri, N.; Miyake, H.
Accretion of Titanium Carbide by Electrical Discharge Machining with Powder Suspended in Working Fluid. Precis.
Eng. 2001, 25, 138–144. DOI: 10.1016/S0141-6359(00)00068-4.
[5] Schumacher, B. M. About the Role of Debris in the Gap during Electrical Discharge Machining. CIRP Ann. 1990, 39(1), 197–199.
DOI: 10.1016/S0007-8506(07)61034-8.
[6] Tzeng, Y. F.; Lee, C. Y. Effects of Powder Characteristics on Electro Discharge Machining Efficiency. Int. J. Adv. Manuf. Technol. 2001, 17, 586–592. DOI: 10.1007/s001700170142.
[7] Zhao, W. S.; Meng, Q. G.; Wang, Z. L. The Application of Research on Powder Mixed EDM in Rough Machining. J. Mater. Process.
Technol. 2002, 129(1–3), 30–33. DOI: 10.1016/S0924-0136(02) 00570-8.
[8] Kumar, A.; Mandal, A.; Dixit, A. R.; Mandal, D. K. Quantitative Analysis of Bubble Size and Electrodes Gap at Different Dielectric Conditions in Powder Mixed EDM Process. Int. J. Adv. Manuf.
Technol. 2020, 107, 3065–3075. DOI: 10.1007/s00170-020-05189-x.
