Enhancing the machining productivity in PMEDM for titanium alloy with low-frequency vibrations associated with the workpiece

1 Introduction

Electric discharge machining (EDM) is still the needed technique when it comes to the area of machining complicated three-dimensional surfaces, in comparison to other non-traditional ways of machining. On the other hand, the necessity that leads to the application of EDM in practice is the restriction that this technique has in terms of productivity and the quality of the machined surface. At this time, powder-mixed EDM (PMEDM) has been utilized with dielectric fluid solution mingled with powder particles that have the potential to overcome these restrictions. The efficacy of this solution in boosting both productivity and the quality of the machined surface is rather substantial [1]. PMEDM is a solution that has a great deal of promise for the future. PMEDM does, however, have a few drawbacks, including the fact that powder particles in the dielectric fluid are not uniform, the presence of powder particles in the discharge gap, and the fact that the process of attracting and repelling the dielectric fluid using powder is quite difficult. Experts in this industry are paying attention to this matter since it is a problem that is catching their notice. Multiple investigations have shown that the combination of conductive powder and dielectric fluid is a successful method for achieving the desired results. As the concentration of chromium powder in PMEDM increases from 2 to 6 g/l, the quality metrics are significantly impacted. Specifically, the metal removal rate (MRR) increases while the TWR decreases as the powder concentration increases [1]. There is a significant difference in the efficacy of Gr powder in PMEDM and that of H₃BO₃ powder. Additionally, the surface roughness (SR), MRR, and tool wear rate (TWR) indicators are improved when graphite (Gr) powder is used [2]. There is a direct correlation between the rise in powder concentration and the decrease in SR, as well as the increase in MRR, TWR, and the micro-hardness of the surface area. Aluminum (Al) powder that was combined with dielectric fluid resulted in a rise in MRR, a reduction in TWR, and a reduced SR [3,4,5]. There is a substantial relationship between MRR and the technical parameters of silicon powder concentration and current intensity in PMEDM [6]. Using Gr powder, it was shown that the TWR of heat-treated copper was lower than that of copper in the PMEDM investigation [7]. Taking into account the contributions of Gr powder, SiC powder, and Al₂O₃ powder in PMEDM, it was discovered that Gr powder would result in the highest MRR, while SiC powder will result in the lowest TWR [8]. Increasing the amount of SiC powder that was present in the dielectric fluid led to a 90% increase in the MRR [9]. The PMEDM approach has been shown to have a great deal of potential in terms of enhancing both production and quality, according to literatures. The efficacy of low-frequency vibration used in PMEDM has been investigated in a number of research works, and the study of integrated vibration in EDM and PMEDM has been presented in a number of studies as well. The use of PMEDM in conjunction with vibration helped to further enhance the equal distribution of powder particles inside the discharge gap, which ultimately resulted in improved spark generation. A further point to consider is that the constant movement of the workpiece or electrode in an upward and downward direction could result in an increase in pressure, which will force the chip particles out of the discharge gap [10]. As a result, the discharge process becomes more steady, which in turn leads to an increase in the effectiveness of EDM machining. The integrated vibration in PMEDM that makes use of Gr powder has led to a reduction in the amount of time required for machining that is three times shorter [11]. Low-frequency vibration applied to the workpiece in PMEDM utilizing titanium powder results in increased efficiency [12]. This is in contrast to EDM, which uses high-frequency vibration. When compared to EDM with vibration, the MRR associated with PMEDM with vibration is much greater (≈95.89%), but the SR is greatly lower (63.2%). The quality indicators MRR, TWR, and topography of the machined surface in PMEDM with low-frequency vibration have been greatly improved [13]. There is a 30% improvement in MRR in PMEDM when integrated duplex ultrasonography was used in the process [14]. When compared to PMEDM, which uses powders of TiO₂ and Al₂O₃ in conjunction with ultrasonic waves, the machining efficiency of ZnO powder is much greater [15].

On the other hand, the number of studies that were published in reputable publications is very low, and the findings of the study are only applicable to issues with a single parameter. As a result, the process of machining in this sector requires more clarification to be understood. An investigation into the single-parameter optimization of the MRR of PMEDM with low-frequency vibration connected to the workpiece for the purpose of completing SKD61 mold steels was carried out in this research. Several studies have been conducted to investigate the significant technical factors and the relationships between them that have a significant impact on the excessive productivity of PMEDM. An analysis was conducted to determine the impact that several factors have on the MRR in PMEDM. These characteristics include the current intensity, pulse generation duration, titanium powder concentration, nozzle pressure, vibration frequency, and vibration amplitude.

2 Experimental setup

A CNC-CM323C die-Sinking machining machine (CHMER, Taiwan) has been used for the purpose of conducting experiments. For the purpose of this investigation, the following parameters were chosen: the current (I), the pulse on-time (T _on), the powder concentration (C), the flushing pressure (P), the amplitude (A), and the frequency (F). For the workpiece, which measures 25 mm × 25 mm × 30 mm, SKD61 steel was used, and copper (Cu) electrode was utilized. Oil labeled D323 was used as the dielectric fluid. Titanium powder was used with a particle size of 45 μm. For the purpose of assigning workpieces in EDM, the vibration unit, which was manufactured by Brüel & Kjaer in Denmark (Exciter 4824), was used. Table 1 presents the outcomes of the experiments as well as the characteristics of the manufacturing process. The PMEDM method is shown in a schematic design that may be seen in Figure 1. It was manufactured of steel CT3, and the dimensions of the container were 330 mm in length, 180 mm in width, and 320 mm in height. To ensure an even distribution of titanium powder throughout the dielectric fluid, two stirring vanes were moved in opposing directions at a speed of 200 revolutions per minute. A solvent pump was used to provide a consistent flow into the processing area at a rate of 24 l/min. To prevent the agglomeration from combining with the titanium powder and entering the machining zone, which would have a detrimental impact on the process of electrical discharges. The magnets were used to attract the swarf that was produced during the machining process. The material removal rate (MRR) was used as the performance measure and it has been denoted by g/min. MRR was computed by finding weight difference. Prior to and during the processing of the workpiece, the mass of the workpiece was measured using an electronic scale of the AJ 203 model manufactured by Shinko Denshi Co., Ltd, in Japan. This scale has the capability of weighing up to a maximum of 200 g with an accuracy of 0.001 g. It was necessary to conduct each experiment three times to ascertain the outcome of the data that was used in the computation.

Figure 1

Experimental diagram.

3 Results and discussion

3.1 Influence of parameters on MRR

The effect of technological parameters on MRR in PMEDM with vibration assigned to the workpiece was evaluated by analysis of variance (ANOVA). The reliability of the analysis is 95%. The results for MRR were analyzed using ANOVA to determine the influence of technological parameters on MRR. ANOVA for mean MRR at 95% confidence intervals is given in Table 2. Current intensity (I) had the strongest influence and pulse duration (T _on) had a low influence most to MRR. The variation in MRR in PMEDM with vibration integrated with the workpiece according to the parameters is shown in Figure 2; the results show current intensity (I): MRR increases very strongly when increasing current intensity I from 2 to 6 A, Figure 2a. The reason is that the energy of the spark will increase as I increases, and this leads to the amount of workpiece material being melted and evaporated also increasing. Compared with I = 2 A, the MRR in PMEDM with vibration integrated with the workpiece at I = 6 A increases the largest by 229.6%.

Figure 2

Minitab view of the influence of parameters on MRR.

Table 2

The influence of technological parameters on MRR

Levels	Technology parameters
	I	T on	C	P	F	A
1	5.319	10.367	5.347	13.565	10.122	11.348
2	8.219	10.399	11.038	14.281	11.238	12.137
3	10.922	12.775	11.573	10.262	16.153	12.510
4	13.430	11.314	12.347	8.552	11.345	9.632
5	17.531	10.566	15.116	8.761	10.563	9.794
Influence level	1	6	2	4	3	5

Pulse generation time (T _on): T _on = 18–37 µs produced a rise in spark energy, which in turn led to an increase in MRR, as seen in Figure 2b. On the other hand, at T _on = 37–75 µs, it resulted in a drop in MRR. This is due to the fact that the pulse duration is too long, which also results in a reduction in the amount of time required for the formation of pulses. The amount of time required to expel the chip from the cutting area and for the dielectric fluid to recover will be decreased as a result of this. As a result, the machining process will be unstable, and a large number of short-circuit pulses will arise, which will result in a reduction in the efficiency of the machining process. The change in MRR in PMEDM with vibration applied to the workpiece is not substantial when compared to the value of T _on = 18 µs. The MRR rises the most by 23.2% when the T _on is equal to 37 µs.

Titanium powder concentration (C): This is the parameter with the second strongest influence on MRR, Table 2. Powder mixed into the dielectric fluid led to improved MRR, Figure 2c. The reason may be that the titanium powder entering the discharge gap has led to the formation of many discharge bridges, which leads to a significant increase in the number of sparks. On the other hand, powder mixed into the dielectric fluid will reduce the dielectric strength of the solvent, so the pulse energy loss for the breakdown of the dielectric fluid will also decrease. This results in increased useful energy for the melting and evaporation of the workpiece material. On the other hand, the discharge gap size also increases when the powder is mixed into the dielectric fluid. This will facilitate the process of pushing chips out of the discharge gap and lead to a reduction in the number of short circuit pulses and arc generation in the machining gap, so the machining process is more stable. Compared to no powder, powder mixed in a dielectric solution resulted in the largest increase in MRR of 182.7% at C = 4 g/l. The increase in powder concentration from 1 to 4 g/l led to an increase in MRR.

Dielectric fluid flow pressure (P): Dielectric fluid flow pressure has a strong influence on MRR in PMEDM with vibration associated with the workpiece (fourth strongest) (Table 2). Figure 2d shows that the fluid flow pressure dielectric fluid can lead to different MRR changes. The reason is that the pressure of the dielectric fluid flow affects the ability to push the chip out of the discharge gap, the stability of the dielectric fluid in the machining area, and the appearance of powder particles in the discharge gap. This has a very strong influence on the machining ability of PMEDM. When the dielectric fluid flow pressure is small, it will lead to the ejection of chip particles, powder, and solvent being affected by a slow electric spark, so the machining process will be less stable. On the other hand, when P is too large, it will lead to the existence of powder particles in a gap that is too short, and the formation of electric sparks with unstable high pressure. These things have an adverse effect on the machining process. P = 15 kPa will result in the largest MRR.

Vibration frequency (F): Is the parameter that has the third strongest influence on MRR (Table 2). Different vibration frequencies have led to MRR in PMEDM with vibration assigned to the workpiece changing differently (Figure 2e). The influence of vibration frequency will affect the formation and quantity of electric sparks and the stability of the machining process. F = 200–400 Hz leads to an increase in MRR and F = 500–600 Hz leads to a decrease in MRR. The reason for the influence of F on MRR has been clarified, MRR is largest at F = 400 Hz and smallest at F = 200 Hz.

Vibration amplitude (A): A parameter that does not greatly affect MRR in PMEDM (fifth level of influence) (Table 2). A = 0.5–1.5 µm led to increased MRR (Figure 2f). The reason may be that the amplitude affects the dielectric fluid pressure created between the electrode and the workpiece. When the amplitude increases in this range, the pressure increases to push the dielectric fluid out of the discharge gap, which will contribute to increasing the efficiency of pushing chips, powder, and dielectric fluid out of the machining area. Hence the machining process is more stable. However, A = 2.0–2.5 µm led to a sharp decrease in MRR, due to the large change in A leading to instability of the spark formation process at the discharge gap. MRR in PMEDM is largest at A = 1.5 µm.

3.2 Determine a set of technological parameters for machining productivity

The S/N ratio of MRR is analyzed in terms of larger is better. S/N analysis of MRR is shown in Figure 3. MRR in PMEDM with the largest vibration assigned to the workpiece with a reasonable set of parameters as follows: I = 6 A, T _on = 37 µs, C = 4 g/l, P = 15 kPa, F = 400 Hz, A = 1.5 µm. The optimal result of MRR is determined according to formula (1) and MRR_opt = 28.95 mm³/min. Experimental verification has obtained MRR_opt = 31.29 mm³/min with an error of 8.08%.

(1) MRR opt = I 5 + T on 3 + C 5 + P 2 + F 3 + A 3 – 5 ⋅ T ¯ .

Figure 3

Minitab view of the influence of technology parameters on S/N of MRR.

4 Conclusion

This study aims to determine the effect of adjusted technological parameters in PMEDM with low-frequency vibrations attached to the workpiece on MRR. The research results have drawn some conclusions as follows:

• Since the current intensity significantly affects MRR, it is found as the most influential parameter.

• The increase in I led to an increase in MRR, and the largest increase in MRR was 229.6% at I = 6 A.

• T _on has the weakest influence on MRR, and MRR is largest at T _on = 37 µs.

• Increasing C leads to increased MRR, and it is a parameter that significantly affects MRR.

• MRR can be increased the largest by 182.7% at C = 4 g/l.

• The MRR_opt was 31.29 mm³/min in PMEDM using Cu electrode, SKD61 workpiece with I = 6 A, T _on = 37 µs, C = 4 g/l, P = 15 kPa, F = 400 Hz, A = 1.5 µm.