1. Introduction
Wire-electric discharge machining (W-EDM) is an electro-thermal non-conventional machining process used to machine hard-to-cut materials. An electrical circuit is created between two electrodes, the workpiece and the wire tool, separated by a dielectric fluid, typically deionised water. The process consists of recurring sparks when an electrical voltage is applied between the workpiece and the wire [
1]. Where the discharge takes place, the material is removed by evaporation or ejection in the liquid phase. The removed material cools down rapidly because of the presence of deionised water and solidifies into small spherical particles, called debris, which are flushed away by the dielectric fluid [
2]. On the surface of the workpiece, small craters can be observed where the material has been removed. The presence of such craters defines the technological signature left by W-EDM machining. Because of the absence of mechanical contact between the two electrodes, no cutting tool forces are created while machining [
2]. Therefore, only the workpiece electrical and thermal properties are relevant for the process [
3]. This aspect allows for its use with difficult-to-cut materials [
4] and represents one of the most significant advantages of W-EDM over other cutting machining processes.
Commonly, the absence of both wire breakage and spark indicates stable process conditions [
5], and quantitative indicators such as the material removal rate (MRR), the cutting speed (V
c), and the surface roughness of the machined workpiece (SR) are used to measure the process characteristics under specific processing parameters. As an example, combination of high pulse on-time (t
on) and discharge current (I) make the process faster, but is detrimental to the surface roughness, the kerf width (KW), and the wire wear ratio (WWR) [
6,
7,
8,
9,
10,
11,
12,
13,
14]. Higher pulse off-time (t
off) values lower the MRR and the cutting speed, but are beneficial for the surface roughness [
15,
16,
17,
18]. However, in some cases, the surface roughness worsens over a certain t
off value [
19,
20]. Higher servo voltage (SV) values increase KW and enhance the surface finishing, but reduce the MRR, the cutting speed, and the WWR [
6,
13,
18,
21,
22,
23,
24,
25,
26,
27,
28]. Higher open-circuit voltage (V) causes higher MRR, cutting speed, KW, and surface roughness [
29]. Other factors, such as wire feed rate (WF), wire tension (WT), and dielectric pressure (DP), or dielectric flow rate, less significantly affect the W-EDM performance [
27,
28,
29,
30,
31,
32]. The workpiece thickness has been scarcely investigated, although it has been demonstrated to affect the process results [
31,
33]. A cross-comparison among the literature studies revealed some contrasting findings. For example, some work [
10,
17,
21] demonstrated that higher t
off values could reduce the surface roughness, but the opposite result has been shown in refs. [
19,
20]. In addition, material processing technology has never been considered as a determining factor.
However, using different processing technologies may affect the W-EDM performance, especially in the case of additive manufacturing (AM), which can process conventional metallic materials using a completely new approach. On the other hand, the advent of AM for metallic components led to significant W-EDM development and market growth. In fact, W-EDM is the preferable option for removing additive manufactured parts from the build platform onto which they were produced. However, few studies have investigated the working conditions during the W-EDM of AM parts [
34,
35], and these have focused only on specimens of AlSi10Mg and produced by laser powder bed fusion (PBF-LB) based AM systems [
34,
35].
This work investigates the performance of W-EDM when cutting the same nominal material, Ti6Al4V, produced with different processing technologies: casting, PBF-LB, and electron beam powder bed fusion (PBF-EB). The quality of the cut surface is evaluated using the surface roughness index, while the process efficiency is evaluated using the cutting speed. The effects of the process parameters, including the workpiece thickness, on these indexes are analysed using descriptive and inferential statistical tools, and regression models are inferred from the collected data.
3. Results and Discussion
Figure 3 compares the surface texture obtained under the same processing condition for each production technology. The surface texture is clearly different among the manufacturing technologies. The surface belonging to the casted sample (
Figure 3a) is characterised by small and almost uniform craters. Both the surfaces belonging to the pieces produced by PBF-EB (
Figure 3b) and L-PBF (
Figure 3c) are characterised by a wave pattern in the wire advancement direction, which could be mainly caused by the periodic inversion of the wire rotation. For the casted sample (
Figure 3a,d), the dimensions of the craters are smaller compared to the ones on PBF-EB (
Figure 3b,e) and PBF-LB (
Figure 3c,f).
In addition, the surfaces show a certain quantity of spherical debris (examples are indicated with white arrows), which vary in size and concentration according to the material processing technology. They appear to be smaller in dimensions and number on casted sample surfaces than on PBF-LB or PBF-EB surfaces, where a large amount of the particles is attached to the surface on which the wire inversions took place. This effect is more evident for the PBF-LB sample, in which the craters on the surface are more prominent than in other processes. In fact, the average size of these debris is 14.75 ± 4.02 µm for PBF-LB, 17.73 ± 4.87 µm for PBF-EB, and 12.83 ± 2.13 µm for casting.
The white spherical particles attached to the sample surface (examples are indicated with red arrows) are residual from the electrode, in this case, Molybdenum. The average size of these residual is 3.69 ± 0.69 µm for PBF-LB, 3.30 ± 0.55 µm for PBF-EB, and 3.32 ± 0.72 µm for casting. No significant difference has been detected among the processes.
From the SEM images of the workpiece surface (
Figure 3d–f), it is also possible to obtain an indication regarding the craters typically produced by the W-EDM process (examples are indicated by yellow arrows).
Some superficial cracks are also visible, which are caused by high temperature and subsequent rapid cooling.
As an example, analysing the surfaces in
Figure 3, the roughness of the surfaces is described by the root mean square value within the cut area, Sq. In this case, the measured value of the entire surface was 8.17 µm for PBF-LB, 4.41 µm for PBF-EB, and 4.60 µm for casting. A strong difference between PBF-LB and the other processes is evident. However, the surfaces show grossly different spatial features, which are revealed by measuring the Ra
x and Ra
y (
Table 4). Moreover, the more the pattern is evident, as in the case of PBF-LB (
Figure 3c), the higher the measured surface roughness is, especially in the wire advancement direction (Ra
x), as can be observed in
Table 4. The pattern can be also noted in the topographies reported in
Figure 4.
This point was confirmed by an ANOVA analysis (
Table 5), which highlighted, with a risk level of 0.05, a significant difference between the surface roughness measured in the two directions for PBF-EB and PBF-LB. In addition to the above conclusion, the more the pattern is evident, the more significant the difference between the two values. Since the surface roughness is always higher in the wire advancement direction (
Table 4), Ra
x represents the most critical index for describing the surface finish of the machined surface. For this reason, only Ra
x is discussed. For completeness, the results concerning Ra
y are presented in
Appendix A.
As can be observed in
Table 4, the cut surface of samples manufactured by the PBF-LB process are more dispersed, and the surfaces are rougher than the corresponding PBF-EB and casted counterparts. In particular, the average value of Ra
x for the casted workpiece (6.88 µm) was significantly lower than those registered for their PBF-LB and PBF-EB counterparts (9.99 µm and 8.63 µm, respectively). This result indicates the presence of a systematic effect of the material manufacturing technology on the surface roughness. This result is also confirmed by the analysis of variance (ANOVA) in
Table 6, which also showed no significant differences for the V
c values using various sample production methods.
Figure 5 shows the main effects of process parameters on V
c and Ra
x for each technology. In agreement with the previous findings, no significant differences among the technologies can be noticed in the effect of the investigated factors on V
c. An increasing discharge current value causes an increase in the discharge energy and thus, the cutting speed or MRR, in agreement with refs. [
7,
10,
12,
29]. A thicker cut determines a lower value of cutting speed. This could be explained by the more considerable amount of material to be removed with the same discharge energy when increasing the thickness, thus causing a cutting speed reduction. Compared to DC, even if t
on determines the discharge duration and the productive phase, it does not significantly affect the cutting speed. This could be explained by the multiplicative relationship between the t
off and t
on, valid for high-speed W-EDM, presented in Equation (2). This relationship implies the presence of high unproductive times (t
off) that are always larger than the productive examples. Consequently, DC is low and limits the machine productivity. For this reason, if an increasing erosion rate does not compensate for an increased t
off during the productive phase, the unproductive phase becomes the main factor affecting the cutting speed and thus, the process productivity. The effect of t
off agrees with that noted in refs. [
15,
18].
Regarding surface roughness, contrasting findings can be detected and are peculiar to the material manufacturing process. For the PBF-LB, the increasing discharge current seems to have a quadratic effect on the surface roughness, with a maximum value at 3 A. For PBF-EB instead, only slight Ra
x variations were observed for currents between 3 A and 4 A. According to Equation (3), an increase in DC and a corresponding reduction of t
off produces smoother surfaces in the case of parts produced by PBF-LB. Instead, for parts manufactured by PBF-EB, an increase in DC and thus, a reduction of t
off, increases the surface roughness. As noted the literature review, the effect of t
off on the final surface roughness is contrasting, and the reasons are unclear. The behaviour of surface roughness in the case of PBF-LB agreed with the results reported by refs. [
19,
20], in which conventional technologies were used to manufacture the W-EDM machined workpieces. However, the results when machining PBF-EB specimens are corroborated in several works, such as refs. [
10,
18,
21]. For casted samples, similarly to the PBF-LB, the effect of I is almost parabolic, with a maximum at 3A, and an increase in t
off* was found detrimental for Ra
x. Therefore, Despite the same nominal composition, the material behaves as if it were processing different materials [
25]. This result may be also explained by the fact that each production technology generates a different thermal history on the material that modifies the material microstructure and the corresponding thermal and electrical conductivities. This effect has been recently demonstrated by Altug et al. [
41] when analysing the modification of material properties subjected to different heat treatments. This finding can also be supported by combining the results of Eshkabilov et al. [
42] and Strumza et al. [
43], which found that the thermal properties of the components are also affected by the material manufacturing technology, and the findings of Han et al. [
44] and Choudhuri et al. [
45], which demonstrated that the surface morphology strongly depends on the thermal properties of the material. The results presented in these works may also explain the findings of Refs. [
46,
47], which analysed the surface morphology and topography obtained after W-EDM of a material subjected to different heat treatments. In these works, the authors found that Ra depended on the heat treatment performed on the workpiece.
The cut Thickness and t
on variations generate almost the same effect on Ra
x for both PBF-LB and PBF-EB. The increasing t
on has a detrimental impact on Ra in both samples, according to refs. [
7,
10,
12,
29]. This could be explained by the fact that an increase in the discharge energy leads to larger and irregularly distributed craters that affect the surface smoothness. On the contrary, since the increased cut thickness causes a reduction in the discharge energy dispersion, Ra
x decreases, in agreement with the results of Ikram et al. [
33].
To determine whether the process parameter variation significantly affects the variation of V
c and Ra
x in the case of AM parts, an ANOVA was performed with a fixed confidence level equal to 95%. Selecting a
p-value threshold equal to 0.05,
Table 7 and
Table 8 highlight the process parameters and their interaction, which significantly affect the V
c and Ra, respectively.
For the cutting speed (
Table 7), the most significant process parameters are the same for all processes, which confirms the results of
Figure 5. The most influential parameter is DC, while all the other process parameters were non-significant. The interactions between I and DC, and I and thickness, even with a total sequential sum of squares below 1%, and DC and thickness were found to be statistically significant. Further statistical tests were performed, removing the non-significant interactions. In this case, thickness and discharge current (I) also significantly affected the cutting speed, while t
on remained the only process parameter not to significantly affect the cutting speed. The only significant interaction was found between DC and thickness. Considering these findings, the regression model was determined considering only the DC, I, thickness, and the interaction between DC and thickness.
Regarding Ra
x, in the case of PBF-LB manufactured parts (
Table 8), the discharge current, particularly its quadratic relationship with Ra
x, is the parameter that affects the surface roughness the most. Thickness and duty factor are non-significant parameters, but their interaction has a significant effect (
p-value lower than 0.05). The quadratic impact of thickness had a p-value below 0.05, but its contribution to the overall sum of squares was below 1%; therefore, it was excluded from the significant parameters.
For the sample manufactured by the PBF-EB process (
Table 8), the discharge current, DC, and their quadratic relationship with Ra
x were found to be significant. Moreover, the interaction between DC and discharge current was significant for the roughness variation.
Because of the high contribution to experimental variability of some process parameters (e.g., t
on for PBF-EB,
Table 8), additional statistical tests were performed for both PBF-LB and PBF-EB, removing the non-significant interactions. These tests showed thickness as a significant factor for both PBF-LB and PBF-EB. Additionally, t
on also became significant for PBF-EB. Those parameters were used in the next section to infer the regression models for Ra
x. The fact that models on surface roughness must be inferred using different inputs demonstrates the remarkable effect of the material manufacturing technologies on the final surface quality after W-EDM cuts.
Regression Models
The regression model that better fits the V
c data for the PBF-LB and PBF-EB process is presented in Equations (5) and (6), respectively.
From the remarkably high R
2adj and low S values, it is possible to affirm that the adopted models well explain the experimental data. This observation is confirmed by the residual values and their distribution (
Figure 6). The positive or negative sign of residuals can be explained by the experimental cutting order, which was always from the smallest to the largest thickness, and could have biased the results. Additionally, certain interactions between process parameters were observed to lead to unstable machining conditions that may have affected the V
c measurements. These instabilities were causal and mainly caused by short circuits, electrode retraction, and poor gap conditions.
Comparing the regression models of Vc determined for parts produced by different AM technologies, it is possible to notice that the difference between the two experimental models is almost negligible because it only consists of a slight variation in the regression coefficients. This fact again underlines that different production technologies do not significantly affect the W-EDM cutting speed index.
The models inferred for Ra
x reflect the results of ANOVAs. The two equations are characterised by different terms and coefficients (Equations (7) and (8), for PBF-LB and PBF-EB parts, respectively.
The high R
2 and low S values for both models affirm that the models represent the experimental data well. This observation is confirmed by the distribution of the residuals in
Figure 7, where the errors are normally distributed, and no data clustering or sign tendencies have been detected.