Study on Recast Layer Thickness of Microstructures Machined in micro-EDM with Different Electrodes

Functional-surface microstructures are widely used in industrial practice. During the fabrication of microstructures in micro-electrical discharge machining (micro-EDM), the thermal and physical characteristics of both workpieces and electrode materials at room temperature and high temperatures have an important in�uence on surface quality and distribution of recast layer. In order to study the in�uence of different electrode material characteristics on the surface integrity of microstructures machined using micro-EDM, red copper, brass, copper-tungsten and tungsten electrode were used to perform micro-EDM on both Ti-6Al-4V alloy and 304 stainless steel. In the experiment, electrode with groove arrays featuring high copying accuracy and surface quality was designed to carry out powder mixed electrical discharge machining (PMEDM) on Ti-6Al-4V alloy, and the machining results were evaluated based on four indicators: microstructure morphology, tool electrode wear (TEW), material removal rate (MRR), and recast layer thickness (RLT). Simultaneously, the surface morphology and recast layer thickness changes of 304 stainless steel workpieces machined using the above four types of electrodes, using both normal polarity and negative polarity micro-EDM were quantitatively analyzed. The results showed that copper-tungsten electrode is recommended to machine Ti-6Al-4V alloy because it has a smaller TEW (139 µm), the highest MRR (255.39 mm 3 /min), and a thinner recast layer thickness (3.35 µm). This was followed by copper electrode, which featured good machining performance and machinability. When machining 304 stainless steel with negative polarity, the TEW of copper electrode and tungsten electrode was the smallest, and the thickness of recast layer was able to be effectively reduced to about 3 µm.


Introduction
Functional-surface microstructures possess the advantages of improved lubrication conditions, reduced friction, and enhanced heat dissipation performance, and have signi cant application value for practical industrial production [1][2][3].Micro-EDM is one of the most common processes for fabricating functionalsurface microstructures.However, surface machined using micro-EDM inevitably feature recast layer and micro-cracks.These disadvantages affect surface quality (Fig. 1a) and performance of the components, and in severe cases, it will cause the parts to fail [4][5].As shown in the Fig. 1, on the surface of the titanium alloy workpiece after EDM, there are obvious recast layer, surface damage caused by microcracks and microporous (Fig. 1b).The combination of recast layer and base material is not reliable, and it is easy to cause peeling and accelerate wear.Therefore, it is necessary to try to eliminate recast layer.
Different material electrodes exhibit various wear rates in EDM.To make full use of such characteristic, Lei et al. [6] used copper, silicon, and copper-silicon alloy disc foils to construct a laminated disc electrode, and then used these electrodes to produce microgroove structures and micro-columnar structures on the titanium alloy workpieces.To obtain ne surface nish from their own micro-EDM processes, Jahan et al. [7] used tungsten, copper-tungsten, and silver-tungsten electrodes to fabricate WC, and found that silver-tungsten electrodes appeared to be the best choice to perform die-sinking micro-EDM of WC.The surface nish of these pieces was found to be greatly in uenced by the electrical and thermal properties of the electrode material.In order to identify correlations between the material characteristics and the EDM processing results, four different grain sizes and ve different cobalt contents were applied in S-EDM experimental analysis.The experimental studies showed the general suitability of tungsten carbide-cobalt tool electrodes for EDM [8].
There have been a number of studies performed by different researchers on the effect of different electrode materials on machining e cacy. Lee and Li [9] explored the effect during EDM of tungsten carbide using copper-tungsten, graphite, and copper electrodes.The effectiveness of these was evaluated in terms of the MRR, the TWR, and the surface quality.They concluded that copper-tungsten was the most suitable tool electrode for the EDM of tungsten carbide.Hasçalık and Çaydaş [10] used graphite, electrolytic copper and aluminum on Ti-6Al-4V to perform EDM.The experimental results showed that the surface recast layer processed by the copper electrode contained obvious micro-cracks.The graphite electrode featured improved MRR, TEW, and surface crack density, but relatively poorer surface nish.Boujelbene et al. [11] analyzed the effect of machining parameters on the surface characteristics of X200Cr15 and 50CrV4 using copper and graphite electrodes during EDM.The results showed that, with a lower pulse duration and a lower discharge current, a thinner recast layer thickness and heat affected zone (HAZ) can be achieved, and MRR will be reduced.Khan et al. [12] studied the surface characteristics of machined Ti-5Al-2.5Snusing copper-tungsten, copper, and graphite electrode.They reported that at low discharge energy, the copper-tungsten electrode produced the nest surface structure whilst graphite delivered worst surface characteristics.They also used these three types of electrodes to process Ti-5Al-2.5Sntitanium alloy with reverse polarity, and the results showed that among them, the copper electrode produced the lowest Ra whilst graphite electrode produced the highest [13].Choudhary et al. [14] developed a new model for EDM-processed stainless steel 316 machined by copper, brass, and graphite electrodes, they revealed that MRR was higher for copper electrode compared to brass.Brass provided a better surface quality than copper electrode.Büttner et al. [15] explored the key factors of micro-features of electrodes which have to be considered in the micro-milling of pure copper and tungsten-reinforced copper.Bai et al. [16] found that, when using brass and copper electrode to process 45-carbon steel and W18Cr4V, brass electrode were able to achieve higher MRR compared to copper electrode.Munmun and Kalipada [17] used copper, brass, and zinc electrode subjected to electrical discharge machining of Ti-5Al-2.5Sntitanium alloy, comparing the machining results, it was found that thinner and more uniform recast layer and higher surface crack densities were found on surfaces which had undergone EDM surface machining by copper electrode, as compared to brass and zinc electrode.Therefore, they recommend using copper tool electrode in EDM products which require higher precision and superior surface nish.Carlini et al. [18] researched the in uence of two different grades of copper-tungsten (CuW) electrode with 65% and 85% volumes of EDM cemented tungsten carbide.Based on the results, the CuW85 electrode presented lower TEW and higher MRR than that of CuW65 electrode.It was also found that there were obvious micro-cracks stretching from recast layer to HAZ.Selvarajan et al. [19] studied micro-EDM processing of SS316 material using copper and graphite electrode, and the results showed that graphite electrode performed better in this scenario.Raza et al. [20] studied the in uence of three input parameters on the MRR, Ra, and EWR using copper, brass, and stainless steel electrode.Comparative analysis revealed the brass electrode as a superior option for MRR and Ra, while stainless steel was identi ed as a better alternative for EWR.The above study shows that the thermal physical characteristics of workpieces and electrode materials at room temperature and high temperature have an important impact on the machining results.Therefore, it is of great signi cance to study the EDM performance with different electrodes.
Most research in this eld has focused on the in uence of different electrode materials on EDM performance, and they mainly emphasize MRR, EWR, and surface quality as consideration indicators.However, no extant studies have considered the copying accuracy and recast layer integrity in the EDM of speci c microstructures, and none have conducted quantitative evaluations of reduced recast layer thickness.
Xu et al. [21] studied techniques for reducing recast layer and micro-cracks on the surface of 304 stainless steel workpieces machined by reversed polarity micro-EDM.Prior experimental results showed that the machining effect is the best when the pulse width is 0.5 µs, pulse interval is 15 µs, and voltage is 110 V.They also used PMEDM with B 4 C powder to modify the surface of Ti-6Al-4V alloy workpieces (the electrical parameters included a voltage of 130 V, a pulse width of 1 µs, and a pulse interval of 10 µs).With the shaking of the tool electrode, micro-grooves (Ra = 0.205 µm) were machined with high shape precision, and noticeably reduced thickness of recast layer.The electrode used in the above experiments was of red copper.On this basis, in this paper, brass, copper-tungsten, and tungsten electrodes were used to carry out EDM on separate Ti-6Al-4V alloy and 304 stainless steel workpiece to study the in uence of different electrode materials on recast layer integrity of the machined microstructures, and an effective quantitative analysis of the experimental results was conducted.

Experimental Materials And Equipment
V-groove array structures were cut on red copper, brass, copper-tungsten, and tungsten electrode via LSWEDM (AP250LS, Sodick Company, Japan).The physical properties of electrode materials are shown in Table 1 below.The workpiece materials were of 304 stainless steel and Ti-6Al-4V (α + β type) alloy, their main properties are shown in Table 2 The pulse power supply was developed by the research group and the maximum pulse frequency was 5 MHz (Fig. 2a).The insulating medium was a suspension of spark oil and B 4 C with 3000 mesh, and the concentration of B 4 C powder in the working solution was 6 g/L.
After machining, the roughness and microstructure pro le of the workpiece surface were measured via laser confocal microscope (VK-X250K, Keyence Company, Japan).The surface morphology of workpieces and the integrity of recast layer were observed using a SEM (Quanta FEG 450, FEI Company, USA).

Microstructure Copying Accuracy
In EDM, the physical characteristics of the electrode and workpiece material have an important in uence on material removal amount per discharge.Generally speaking, the higher the corrosion resistance of tool electrode, the smaller TEW and the higher copying accuracy.
Microstructures with high aspect ratio have a wide range of application scenarios, and usually they are more di cult to machine.In order to obtain microstructures with high aspect ratio and high copying accuracy, while considering the uncertainty of machining process and TEW, it is necessary to design microstructures with different sizes to verify their feasibility.In this experiment, microstructure maintained a height of 550 µm, design widths of electrodes used to perform EDM were 150 µm, 250 µm, 350 µm, 450 µm, 550 µm, 650 µm, 750 µm, and 850 µm, and then TEW and reproduction rate were calculated.The results are shown in Fig. 3 below.
According to the processing results, with the decrease of aspect ratio, the reproduction rate rst gradually increased and then tended to stabilize (Fig. 3a), while the TEW gradually decreased (Fig. 3b).Because when machining microstructures with a large aspect ratio, the narrow gap uid resistance between electrode and workpiece was large, making it di cult to discharge bubbles and chips from the machining, and anomalous discharge could easily occur, resulting in low microstructure copying accuracy and large TEW.When aspect ratio was 3.7, the reproduction rate was only 43.8%, and the TEW reached 238 µm.
Aspect ratio of 0.85 was the turning point of the polyline, at this time, the reproduction rate was 73.5% and the TEW was 118 µm.If aspect ratio continued to decrease, the reproduction rate and TEW would both undergo little change.
After several experiments were carried out, the V-groove array structure shown in Fig. 4a below was nally machined on electrode.The transverse spacing of microstructures on electrode was 650 µm, the longitudinal height was 550 µm.Using this electrode, PMEDM was conducted on Ti-6Al-4V alloy under voltage of 130 V, pulse width of 1 µs, and pulse interval of 10 µs.The microstructure pro le is shown in Fig. 4b, with transverse spacing of 608 µm, and longitudinal height of 432 µm.The results showed that the reproduction rate of microstructure was about 73.5%, and the length of microstructure was reduced by 42 µm and its height was reduced by 118 µm.The size reduction in height was about 3 times that in length, which was due to the small contact area at the tip of microstructure, resulting in accumulated energy and greater TEW.Therefore, only the vertical direction is considered for the TEW referred to in the following section.

Results And Discussion
4.1 Microstructure morphology on Ti-6Al-4V alloy machined via PMEDM Keeping other parameters unchanged, PMEDM was carried out on Ti-6Al-4V alloy using copper, brass, copper-tungsten, and tungsten electrode, and the obtained microstructure morphologies are shown in Fig. 5.The machining conditions are shown in Table 3.The reproduction rates of microstructures machined with four electrodes were 73.5%, 66.9%, 75.2% and 62.3% respectively (Fig. 5d).Among them, the surface roughness of workpiece machined with tungsten electrode was the largest (Ra = 0.401 µm), followed by brass electrode (Ra = 0. 383 µm), and coppertungsten electrode (Ra = 0. 203 µm), which was similar to the surface quality of workpiece machined with copper electrode (Ra = 0. 205 µm).Compared with brass and tungsten electrode, the microstructures processed with copper-tungsten electrode has higher shape precision and higher surface quality (Fig. 5b).

Table 3 Experimental conditions for processing
That is because copper-tungsten electrode combines the good properties of tungsten and copper, featuring high temperature resistance, high strength, arc erosion resistance, good electric conductivity, heat conductivity and fast heat dissipation, which ensures the stability of machining process.4.2 Machining performance of Ti-6Al-4V alloy via PMEDM Figure 6 shows the contour of the electrode after processing.It can be seen that the TEW of brass electrode (Fig. 6a) is larger than that of copper-tungsten and tungsten electrode.The tungsten electrode (Fig. 6c) has high surface nish and experienced less thermal damage after EDM.That is because tungsten has good arcing performance and high arc column stability, so the tungsten electrode is more resistant to arc erosion.There was obvious electrode erosion present on the micro-grooves of brass and copper-tungsten electrode, which may have been caused by the concentration of discharge energy in this area.It was also observed that electrode wear was inversely proportional to the melting point of electrode material, and materials with good tool wear characteristics generally have high melting points and high wear resistance.
Melting and vaporization lead to the removal of material molecules from the workpiece in EDM, and machining performance is greatly affected by the physical properties of electrode materials.Figure 7 above shows the machining performance of Ti-6Al-4V alloy machined with different electrodes in terms of TEW and MRR.As can be seen from the gure, the TEW of copper electrode was the smallest (129 µm), the TEW of brass electrode was the largest (181 µm), while copper-tungsten and tungsten electrode had similar TEW.In terms of MRR, copper electrode had the smallest MRR, which was 218.72 mm 3 /min, and copper-tungsten electrode had the highest MRR, reaching 255.39 mm 3 /min, which was 16.8% higher than that of copper electrode.
The analysis shows that copper electrode has a high thermal conductivity (401 W/m•K) and temperature transfer coe cient.This high thermal conductivity makes it di cult to accumulate heat in machining process and produce thermal damage, reducing the TEW.At the same time, the rapid loss of energy shortens the duration of spark discharge, and reduces the energy transferred to workpiece.Coppertungsten and tungsten electrodes have high melting points, high erosion resistance, so it has low TEW, and high machining speed.Generally speaking, copper, copper-tungsten and tungsten electrodes have smaller TEW, while brass electrodes have higher TEW.Compared with copper and brass electrodes, copper-tungsten and tungsten electrodes can achieve higher MRR.

Recast layer integrity of Ti-6Al-4V alloy machined via PMEDM
The process of discharge erosion is very complex, and is not only manifested in the in uence of thermal physical constants of material on its corrosion resistance, but also in the interaction between electrode and workpiece on the energy distribution and transfer, the dispersal and re-solidi cation of electrical erosion products, etc.Therefore, the use of different materials not only affects TEW and MRR, but also affects the distribution of recast layer and micro-cracks.From Fig. 7 above, the recast layer on microstructure surface machined with brass electrode was 7.21 µm at its thickest, and the recast layer contains thin micro-cracks.The maximum thickness of recast layer on microstructure surface machined using copper-tungsten electrode was 3.35 µm, and recast layer contained microporous.The maximum thickness of recast layer on microstructure surface machined using tungsten electrode was 10.78 µm, and its recast layer also contained obvious micro-cracks.The recast layer was the main host of the microcracks, and its thickness affects the intensity, density, and distribution of micro-cracks.Therefore, the effective thinning of the thickness of recast layer has a positive effect on the suppression of the microcrack phenomenon, and can also greatly improve the surface quality. .
To sum up, in order to obtain microstructures with a high copying accuracy and thin recast layer when machining Ti-6Al-4V alloy via PMEDM, it is recommended to use copper-tungsten electrode, which have a smaller TEW (139 µm) and the greatest MRR (255.39 mm 3 /min).This is followed by copper electrode, which feature good machining performance and machinability.

Recast layer integrity of machined 304 stainless steel
The machining conditions for 304 stainless steel under normal polarity are shown in Table 4, and the machined surface morphologies are shown in Fig. 9 below.Figure 9 below shows the surface morphology of 304 stainless steel workpieces machined at normal polarity using different electrodes.It can be seen that the surface quality of the workpiece machined by this method is not ideal, and the surface contains many discharge pits, accompanied by surface erosion.
Due to residual stress during the discharge process, cracks were produced on the surface.Measurement showed that their Ra values were above 0.8 µm.
In order to improve the surface quality, continue to nish machining the workpiece through the negative polarity by micro-EDM.The electrical parameters used in this process included a pulse width of 0.5 µs, a pulse interval of 15 µs, a voltage of 110 V, and a machining depth of 50 µm.The obtained results are shown in Fig. 10 below.The processing environment is as shown in Table 5.As can be seen from Fig. 10 above, the machined surface of brass electrode (Fig. 10a) contains slender cracks, a surface roughness (Ra) of 0. 58 µm, and a recast layer of 11.16 µm at its thickest point (Fig. 10b).That is because brass has low thermal conductivity, and drastic temperature changes deform the material and generate thermal stress, thus producing micro-cracks.The diameter and density of discharge pits on the machined surface of copper-tungsten electrode (Fig. 10c) are highly noticeable; the surface roughness (Ra) is 0. 62 µm, the recast layer is 6.97 µm at its thickest point, and it contains obvious micro-cracks and micro-pores.Compared with brass and copper-tungsten electrodes, the machined surface of tungsten electrode (Fig. 10e) is smooth, with almost no erosion visible, which indicates that there was little destructive arc discharge during its machining performance.Measurement shows that its surface roughness (Ra) was 0. 35 µm, and its recast layer was 3.65 µm at its thickest point.
To sum up the results, when machining 304 stainless steel with positive polarity micro-EDM, the surface quality of the workpiece is poor and it is di cult to meet the use requirements.So it is recommended to use reversed polarity micro-EDM for machining, which can effectively improve surface quality, and the use of copper electrode and tungsten electrode can reduce surface micro-cracks and the thickness of recast layer.

Conclusions
During the fabrication of microstructures produced through the utilization of micro-EDM, the thermal and physical characteristics of both workpieces and electrode materials at room and high temperatures have considerable in uence on the material removal per discharge, the quality surface, and the distribution of recast layer.Materials with good tool wear characteristics generally have high melting points and wear resistance, so improved MRR, reduced TEW, thinner recast layers, and ner surface quality can be

Figure 5
Figure5above shows the microstructure morphology of Ti-6Al-4V alloy machined using different electrodes.It can be seen that the workpieces all possessed microstructures high good copying accuracy.

Figures
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Figure 10 Surface
Figure 10

Table 1
The main physical properties of electrode materials