3.1 Experimental results
Material removal rates and relative electrode loss rates for the three electrode feed methods for hollowing processing are shown in Fig. 5.
As can be seen in this figure, the highest material removal rate of 29.99 mm3/min is achieved for the lateral reciprocating feed, which is a 135% higher rate than for the S-shaped feed and 255% higher than for the linear feed. Also, its relative electrode loss rate is the lowest among the three feed trajectories: 47% lower than for the S-shaped feed and 61% lower than for the linear feed.
There are obvious differences among the workpieces and electrodes after the three feeding methods are completed, as shown in Fig. 6.
From Fig. 6(a), it can be seen that there are five residual pieces in the groove after the electrode linear feed machining, which is due to the fact that the five spouts reserved on the electrode did not completely cover the machining area on the workpiece. The surface of the workpiece after the electrode transverse reciprocating feed machining did not show obvious bumps and was flatter than after the first two methods. The electrode loss was highest for the linear feed, followed by the S-shaped feed, and smallest for the transverse reciprocating feed, as shown in Fig. 6(b).
3.2 Experimental discussion
3.2.1 Processing waveform analysis
The waveforms of EDM can visually reflect the microscopic discharge state during the machining process, and the stability of the discharge state between poles is directly related to the material removal rate. In order to further investigate the reasons for the 255% and 135% increase in material removal rate for transverse reciprocating feed compared with the linear feed and S-shaped feed, respectively, the discharge waveforms of the three feed methods were collected, as shown in Figs. 7, 8, and 9.
Comparing multiple waveform plots, such as in Figs. 7(a), 8(a), and 9(a), there is almost no idle time in the lateral reciprocating feed waveform. The proportion of normal discharge waveform is also higher, and the discharge is more stable, which is the fundamental reason why the material removal rate of the lateral reciprocating feed is the highest. The chance of arc is greater for linear feed than for S-shaped feed and transverse reciprocating feed, resulting in the highest relative electrode loss rate for linear feed.
Comparing individual waveforms, such as those in Figs. 7(b), 8(b), and 9(b), the breakdown voltages of linear feed, S-shaped feed, and transverse reciprocating feed are, respectively, 10, 140, and 180 V. The different breakdown voltages for the three feeds are caused by the different forms of media between the poles. During the processing of linear feed, a small groove is formed between the two pieces of residue formed on the surface of the workpiece, which plays a role in the working media. aggregation effect, so that the inter-pole filled with deionized water, and lateral reciprocating feed processing process, will not form a bump on the workpiece, deionized water from the injection port shot with air to form a gas-liquid mixed media, deionized water gas-liquid mixed media dielectric constant is greater than the dielectric constant of deionized water, linear feed inter-pole equivalent capacitance is greater than the equivalent capacitance of the lateral reciprocating feed inter-pole.So voltage required for dielectric breakdown is the smallest for linear feed [8], larger for S-feed, and the largest for transverse reciprocating feed.
3.2.2 Surface microcracks
In order to observe the microscopic morphology of the workpiece surface after the three feeding methods, the workpiece surface was microscopically analyzed. Figures 10(a), (b) and (c) show scanning electron microscope (SEM) images of the workpiece surface after linear feed, S-shaped feed, and lateral reciprocating feed machining, respectively.
From Figs. 10(a) and (b), it can be seen that the surface of the workpiece after linear feed is mostly rounded with small craters and rounded molten solids, while the surface of the workpiece after lateral reciprocating feed is mostly flat craters with very few molten solids. As shown in Fig. 10(c), the craters on the workpiece surface after transverse reciprocating feed are larger in area and smaller in depth than the craters on the workpiece surface after the other two feeding mechanisms.
The microscopic structure of the workpiece surface after the lateral reciprocating feed differs greatly from that after the other two feeding methods, a difference that is due to the different chip removal between the poles in the three feeding methods, as shown in Fig. 11 and Fig. 12.
During linear feed machining, because the spray port on the electrode does not completely cover the workpiece machining area, a flaky residue is formed on the workpiece surface, as shown in Fig. 11. A flaky residue is embedded in the electrode spray port, resulting in a smaller electrode outlet, seriously affecting the flow of working fluid. At the same time, after the flaky residue on the workpiece is embedded in the electrode spray port, a closed space is formed between the two flaky residues. When the space is filled with working fluid, the working fluid flow rate is greatly reduced. The molten particles generated after the discharge are solidified on the workpiece surface again because they are not scoured by the high-flow-rate working fluid. As shown in Fig. 10(a), there are a large number of molten solidified particles on the workpiece surface. Although the S-shaped feed also forms a confined space between the residue and the electrode and affects the working fluid flow rate, the electrode feed in the axial direction etches away the residue, so the S-shaped feed will reduce the molten solidified particles on the surface to a level less than that with a linear feed. In the transverse reciprocating feed, since the electrode is set into transverse reciprocating motion during linear feeding, it will not form a residue to block the working fluid. As shown in Fig. 12, the flow rate of working fluid is higher. Meanwhile, the electrode will form a two-phase mixture of air and working fluid between the poles during the motion, and the mixed medium of air and liquid needs a larger electric field strength to be penetrated, so transverse reciprocating feeding has a larger discharge gap. In summary, the transverse reciprocating feed has better inter-pole chip discharge conditions [9, 10], and the machined workpiece surface is almost free of molten solidified particulate matter, as shown in Fig. 10(c).
3.2.3 Electrode surface topography and energy spectrum analysis
An energy spectrum analysis of the elements on the surface of the electrode after machining is shown in Fig. 13, Fig. 14, and Fig. 15. Comparing the energy spectrum analysis graphs of the electrode surface after processing by the three feeding methods, it can be seen that the highest oxygen element content is 12.99% on the surface of the lateral reciprocating feeding electrode, and the oxygen element content on the surface of the remaining two feeding electrodes is lower, by 4.30% and 4.04%, respectively.
This is due to the fact that when the linear feed and S-shaped feed are used, a strip residue is formed on the surface of the workpiece due to the presence of the liquid injection port, and the unetched strip residues form a relatively closed space, together with the electrode surface, which is nearly filled with working fluid. In addition to the working fluid, there is also some air, and the high-speed working fluid ejected from the nozzle will form a gas-liquid mixture with the air. The electrode surface after processing by transverse reciprocating feed therefore has more oxygen content. Such a mixed gas-liquid medium needs higher energy to break down, so the breakdown voltage is higher than 160 V when using transverse reciprocating feed, which, in turn, is higher than the 120 V and 130 V when using linear feed and S-shaped feed, respectively (see Figs. 7, 8, and 9).
In addition, when using transverse reciprocating feed, the Ni and Cr content on the electrode surface is higher than with the other two processing methods. Since Ni and Cr are elements in the workpiece nickel-based high-temperature alloy GH4169, there is more workpiece material melted in the process and part of the backplating on the electrode surface, forming a certain degree of compensation for the electrode loss, which is also one of the reasons for the lower electrode loss rate using transverse reciprocating feed processing. This is one of the reasons why the electrode loss rate is somewhat less with transverse reciprocating feed machining. In addition, the probability of arc discharge in the discharge waveform of transverse reciprocating feed is lower compared to other feeding methods, which also leads to lower electrode loss in transverse reciprocating feed.
3.2.4 Analysis of machining length and time for lateral reciprocating feed
From analysis of the test results, it can be seen that the material removal rate is the highest and the relative electrode loss rate is the lowest for transverse reciprocating feed machining. Next, the relationship between the machining length and time of transverse reciprocating feed machining is further analyzed to provide theoretical guidance for sample preparation. The machining time and the corresponding machining length were recorded during the test and plotted as a machining time-machining length curve, as shown in Fig. 16.
The curve is divided into five stages according to the varying slope of the curve (i.e., the processing rate). The first stage is for processing length less than 3 mm, when the electrode is quite close to the workpiece. Because the electrode outer diameter is 3 mm, the electrode is not completely embedded in the workpiece at this time, and the working fluid from the electrode nozzle is dispersed and sprayed onto the surface of the workpiece. At this stage, the working fluid is dispersed on the outer surface of the workpiece and is not used effectively. Only a part of the working fluid plays the role of scouring and etching products, as shown in Fig. 17(a). The second stage is the part whose processing length is between 3 and 10.5 mm. Here the processing rate is higher than in the first stage, because the electrode has been completely embedded in the workpiece, and the workpiece etches out a U-shaped groove by the electrode. At this time the U-shaped groove plays a gathering role for the working fluid, which increases the working fluid between the poles. Pressure and flow rate of the working fluid between the poles, as well as the chip removal condition, are improved over conditions in the first stage, as shown in Fig. 17(b).
The third stage is the part of the processing length between 10.5 and 17 mm. The processing rate decreases in this stage to less than that of the second stage. At this time, there are more etching products in the working fluid, and the probability of secondary discharge increases. The fourth stage is the part of the processing length between 17 and 25 mm. Here, the processing rate is less than in the third stage. At this time, the front wall of the electrode has been broken, discharging the electrode from the wall surface. After the electrode wall is broken, the pressure and flow rate of the working fluid decrease, and the products of inter-pole etching accumulate, which affects the normal discharge, as shown in Figs. 18(a) and (b). The fifth stage is the part larger than 25 mm. At that time, the electrode processing area has been completely etched, and the middle part has been etched by the workpiece and the workpiece is processed out of triangle, as shown in Fig. 18(c).
The workpiece after using transverse reciprocating machining is shown in Fig. 19. The machining depth is around 20 mm, and the electrode breaks after 20 mm.