Wire-EDM cutting strategies of WC-Co hardmetals: Effect of number of EDM pass on surface integrity

In this study, the effect of the number of wire-EDM passes on the surface integrity of WC-Co hardmetals was investigated. As-received doughnut-shaped WC-25 wt%Co samples were wire EDM’ed with one, two, three and five passes. The resulting surface conditions were investigated in terms of macroscopic and microscopic examinations. Surface roughness and hardness measurements were carried out. Detailed SEM investigations were coupled with EDS analyses. Based on the experimental findings, a detailed cutting strategy was offered for WC-Co hardmetals depending on the expectations of surface conditions, production route, cost and productivity. If there is at least one subsequent process including material removal, wire-EDM with one pass is suggested to decrease cost and increase productivity. However, heat-affected zone (HAZ), recast layer and lower surface hardness region should be removed at subsequent production steps. If there are no production steps to remove HAZ, recast layer and lower surface hardness region, the number of wire-EDM passes should be maximised.


Introduction
WC-Co metal-ceramic composite materials are one of the important engineering materials used in various applications in manufacturing. 1 Tungsten Carbide (WC) particles are sintered with cobalt (Co) binder to have both high wear resistance and high toughness.Owing to studies concentrating on the effects of WC particle size and cobalt content on the final performance of materials, the demand for this material type to be used in different applications has been increasing. 2Knives, mandrels, punches, drawing and forming dies and inserts are some of the tools made of WC-Co hardmetals and are widely used in different metal shaping processes such as forging and machining. 3onsidering various forms, the manufacturing of final products made of WC-Co is carried out in multiple steps to obtain geometrical shapes within tolerances and desired surface conditions.For instance, the production route starts with rough machining or roughening to obtain an approximate geometry from pre-shaped solid or hollow cylindrical raw materials for cold forging dies made of WC-Co.After roughening, further steps including fine machining, grinding and polishing are employed depending on the final shape and expected surface conditions.Since WC-Co metal matrix composite materials are very hard, required cutting forces are very demanding.Due to high hardness, tool wear and tool cost are higher and production time is longer compared to machining of conventional steels.Since the machining of hardmetals differs from other materials in terms of operation parameters, a different term, i.e., hard machining, is often used. 4wing to the high material removal rate, Electro Discharging Machining (EDM) is one of the widely preferred methods for machining of WC-Co hardmetals.However, different artefacts such as surface cracks, heat-affected zone (HAZ), recast layer, additional surface roughness and tensile residual stresses can be introduced during processing.Based on the literature review on the machining of WC-Co hardmetals, it was concluded that the issues related to the EDM of WC-Co could be attributed to differences in physical properties such as melting and evaporation temperatures, thermal expansion and contraction coefficient and electrical conductivity. 5EDM is based on high-voltage electrical current passing through an electrode and workpiece.Considering the principle of EDM, where the dielectric fluid enables current to be passed to the workpiece after ionisation temperature is reached.The resulting spark removes material from the workpiece so that cutting is carried out.Due to high local temperatures originating from high-voltage sparks, microstructural modifications are introduced during EDM.One of the widely-known features is the recast layer which is a µm-sized layer observed at the surface of the materials.Due to EDM, tensile residual stresses are formed with other inherently-introduced features such as binder depletion and thermal grain cracking lowering the performance of EDM'ed WC-Co hardmetals. 6It was underlined that ultrasonic vibration of the tool in die sinking-EDM led to a decrease in the thickness of the HAZ and recast layer. 7onsidering the studies focusing on the EDM of WC-Co hardmetals, most of the attention was directed to the effect of EDM parameters, such as applied intensity, pulse time, wire feed rate etc.For example, the effect of pulse time on various WC-Co grades was investigated. 8It was concluded that an increase in pulse time led to machining instability.The effect of peak current and pulse duration was explored for WC-25wt% Co. 9 At high peak current and pulse duration, EDM-affected layers with cracks were claimed to be observed.The effect of pulse duration, dielectric level, current, flushing pressure and different dielectric fluids on the material removal rate of WC-10wt% Co samples were investigated. 10Increase in pulse duration, current and flushing pressure were found to lead higher material removal rate.The effects of pulse intensity, pulse duration, duty cycle and open-circuit voltage were examined in order to have low surface roughness, low electrode wear and high material removal rate for WC-6wt%Co samples. 11t was suggested that low intensity and low pulse duration should be preferred for low surface roughness.On the other hand, high intensity, duty cycle and open-circuit voltage values with low pulse duration should be chosen for high material removal rate.High intensity as well as low open-circuit voltage and pulse values were offered for low electrode wear.A study focused on determining the most influential factors for surface roughness, electrode wear and material removal rate. 12It was observed that intensity was the most influential factor for all cases within the limits studied in the study.A review study on the EDM of WC-Co hardmetals was carried out to tabulate the EDM process parameters with respect to achieved material removal rate, electrode wear, tool wear rate and surface roughness. 5Another review study was conducted on the EDM of WC-Co hardmetals. 13Process parameters such as spark gap, gap voltage, peak current, pulse duration, pulse interval and pulse intensity were discussed.These literature reviews summarised the studies concentrating on EDM process parameters for WC-Co hardmetals.
Multi-pass EDM cutting is one of the procedures that can be applied to reduce or eliminate the effects of single wire-EDM cutting artefacts such as high surface roughness, recast layer and HAZ as well as tensile residual stresses.Initial rough wire-EDM cutting can be followed by one or more fine trim cuttings to achieve desired surface conditions.The process of the multi-pass wire-EDM procedure was explained in detail. 14xperimental and statistical analyses were carried out for gamma titanium aluminide material.In this study, one and two passes were used and a selection procedure based on productivity and expected surface roughness was offered.Another study focussed on the effects of multi-pass wire-EDM cutting for Ti-6Al-4V alloy. 15ough cutting followed by two additional trim cuttings with different coated EDM wires were investigated.For all wire types, a reduction in surface roughness and recast layer thickness was observed after multi-pass wire-EDM.ANOVA and Taguchi analyses were conducted to determine the influence of process parameters including multi-pass wire-EDM of WC-5.3wt%Co. 16nvestigations on fracture and fatigue behaviour of wire EDM'ed WC-10wt%Co hardmetals were carried out. 17ulti-pass wire-EDM'ed samples were compared with samples processes with mechanical grinding and polishing.It was revealed that fatigue sensitivity of the wire-EDM'ed specimens was obtained as higher compared to reference cutting due to tensile residual stresses after EDM.However, there was no specific information regarding the details of multi-pass wire-EDM.The wear and adhesion performance of samples after dry blasting and TiN coating applied to samples produced with different EDM passes were investigated. 18WC-10wt%Co samples were prepared with one, four, five and seven numbers of wire-EDM passes and it was concluded that applying multi-pass wire-EDM up to 5 passes could create fine TiN crystals.However, it was stated that due to the longer process duration of the multi-pass wire-EDM process, it would not be cost-effective.A review study on the EDM of WC and WC-Co suggested multi-pass EDM process with lower depth of cut. 19owever, there was no further information regarding the multi-pass wire-EDM process.In addition, the effect of number of wire-EDM pass on breaking strength obtained by four-point bending test was investigated. 20t was found that the post-cuts were necessary only to improve surface roughness.It was also stated that decrease in the number of post-cuts would decrease the cost of manufacturing.
Despite some studies mentioning multi-pass wire-EDM of WC-Co hardmetals, a systematic study is needed to explore the resulting changes due to the multi-pass wire-EDM and to suggest cutting strategies that can be implemented on the production floor.In addition, most of the studies in the open literature are concentrating on low Co-content hardmetals, i.e., up to 10wt%.In this study, the effect of wire-EDM passes on surface integrity was investigated for WC-25wt%Co hardmetals.Detailed macro and microstructural examinations were carried out after one, two, three and five passes.After macroscopic examinations, surface roughness and hardness measurements were conducted.Detailed investigations including EDS analysis and microstructural examinations by SEM images were carried out to investigate the surface integrity.Finally, a cutting strategy was offered for wire-EDM cutting of WC-Co hardmetals in terms of surface integrity and production efficiency.Since the final performance of WC-Co hardmetals is directly related to surface integrity, the cutting strategy offered in this study presents the most cost-effective wire-EDM production step, depending on the complete production route without any sacrifice from surface integrity.

Sample preparation
In this study, doughnut-shaped WC-Co samples were employed.As received samples were produced from WC and Co powders by milling, powder pressing, preforming and sintering.The inner and outer diameters of the as-received samples were 13.30 mm and 30.80 mm, respectively.The as-received sample was cut into four pieces so that each sample had a thickness of 5.50 mm.The cobalt content of the samples was 25 wt% and the grain size of WC particles was higher than 6 µm.The mechanical and physical properties of WC-Co samples used in this study are given in Table 1.
After obtaining four samples with the same geometry, the inner hole size was increased to 14.20 mm by using SPM z-Cut wire-EDM.0.25 mm-thick brass wires were employed for cutting.For each sample, desired final hole diameter was obtained by one, two, three and five passes.The wire-EDM process has various parameters to control and wire-EDM machines have software and hardware to optimise these parameters and their complex interactions depending on the samples to be cut.Parameter optimisation requires a substantial amount of work and is well beyond the scope of this work.Therefore, the wire-EDM parameters were automatically defined and controlled by the wire-EDM machine used in this study depending on the number of passes.
After wire-EDM cutting, each sample was further cut into two pieces to obtain semi-doughnut-shaped samples for each configuration.Then, one of the half pieces of each set was polished for further optical investigations.Since, semi-doughnut-shaped samples were obtained from full doughnut-shaped samples by wire-EDM, both one and five passes regions can be seen simultaneously in Figure 1.

Methods
Macroscopic examinations were initially carried out for all samples.Surface roughness measurements were carried out by Mitutoyo Surftest SJ-400.For each configuration, measurements were repeated three times and standard deviations were obtained.Micro hardness measurements were carried out by employing KB 30S and tests were repeated five times.The SEM images were obtained by Carl Zeiss 300VP.Only as wire-EDM'ed samples were used and detailed surface images were recorded from different locations of samples with different wire-EDM passes.EDS analyses were also carried out to investigate the elemental composition of the recast layer formed after wire-EDM.

Macroscopic examinations
Macroscopic examinations were carried out for wire-EDM'ed and polished samples (Figure 2).Optical investigations revealed critical information regarding the effect of pass.First of all, after a rough polishing, i.e., cleaning the samples from residues of the wire-EDM process, a yellowish recast layer was observed very clearly at the processed surface of the samples with one and two passes.For the wire-EDM'ed sample after three passes, a yellowish recast layer was still observed.
Based on macroscopic examinations, the five-passes sample was almost free from the yellowish recast layer.Therefore, as the number of EDM passes was increased, the yellowish layer formed during wire-EDM faded away.

Surface roughness
According to surface roughness measurements, it was revealed that both R a and R z values were significantly affected by the number of passes.Figure 3 shows the average R a and R z values with corresponding standard deviations for the as-received sample as well as one, two, three and five passes samples.First of all, the average surface roughness values of the as-received sample were measured as 2.04 and 12.30 for R a and R z , respectively.The average surface roughness values after one pass, i.e., R a and R z , were increased to 4.07 and 22.56 µm, respectively.The percent increase after one pass was 99 and 83% for R a and R z values compared to the as-received sample.Therefore, it was revealed that  surface roughness significantly increased for the one-pass sample.When the two-passes sample was investigated, R a and R z values were decreased to 3.35 and 18.38 µm compared to the one-pass sample, however, surface roughness values were still higher compared to the as-received sample.For the three-passes sample, surface roughness values were obtained as 0.83 and 5.86 µm for R a and R z respectively.Therefore, after three passes, the surfaces started to be smoother compared to the initial conditions of the as-received sample.The percent decreases in R a and R z values of the three-passes sample were about 59 and 52%, respectively.Considering macroscopic examinations given in Figure 1, the dramatic surface roughness change from two passes to three can also be incorporated with the change in colour of the recast layer.Finally, R a and R z values were obtained as 0.42 and 3.32 µm, after five passes.The percent decrease in R a and R z from three passes to five passes was about 49 and 43%.The total surface roughness improvements from the as-received sample to the five-passes sample were around 79 and 73% for R a and R z , respectively.Based on the surface roughness measurements, it was shown that for the one and two-passes samples, the surface roughness values were increased compared to the as-received sample.However, after three passes, the surface roughness values started to decrease leading to a better surface finish.When the number of passes was five, significant improvements in terms of R a and R z were achieved.Therefore, to improve the surface integrity after wire-EDM, the number of passes should be increased to at least three, based on the set-up used in this study.If the number of passes is lower, surface roughness should be decreased with a subsequent process such as polishing.As known, high surface roughness increases the probability of crack initiation leading to a lower performance for fatigue.Therefore, surface roughness should be decreased for better surface integrity, particularly against fatigue failures.

Hardness
Micro-hardness measurements were obtained from wire-EDM'ed surfaces.The as-received sample had an average hardness of 834 HV as can be seen in Figure 4.
The one-pass sample showed a significant reduction of 44% in hardness.The main reason for the hardness decrease was associated with the recast layer after wire-EDM.Since WC-Co materials are very hard, it was shown that the recast layer has lower hardness compared to the base material.As the number of passes increased, the surface hardness also increased.For the five-passes sample, surface hardness was obtained very close to that of the as-received samples.As shown in this study, processing with a low wire-EDM pass causes a decrease in the hardness of WC-Co materials.Therefore, premature failures can be observed due to hardness decrease at the surface of the material.As a result, performance loss can be expected as a result of lower hardness values for applications such as cold forging dies, where surface hardness is very important to increase wear resistance.

SEM and EDS analysis
Detailed microstructural examinations were carried out by using SEM for the as wire-EDM'ed samples without any polishing.After one pass, the so-called recast layer (the yellowish layer for macro examination) was observed at the processed surface (Figure 5).The average thickness of the layer was about 12 µm.In addition to the recast layer, the heat-affected zone (HAZ), as seen in welding operations, was also observed.The thickness of the HAZ was about 20 µm.Therefore, the average thickness of the wire-EDM-affected zone for the one-pass sample was about 35 µm.Similar to the HAZ seen in this study, a so-called "damaged layer" was observed in the literature. 9When the as wire-EDM'ed sample with two passes was investigated, the average thickness of the recast layer was obtained as 8 µm.No significant HAZ was observed compared to the sample with one pass.For the three-passes sample, the recast layer was still observed with an average thickness of 5 µm.And similar to the two-passes sample, no significant HAZ was observed.As for the five-passes sample, there were still local recast layers observed at particular locations and there were some regions with no recast layer.Therefore, SEM image analysis revealed that after one pass, a significant recast layer and HAZ were obtained.With the increase in the number of passes, the thickness of the recast layer was decreased with no significant heat-affected region.After five passes, the recast layer started to be diminished, however, there were some locations with very thin and local recast layers.The main mechanism for the formation of the recast layer and HAZ can be interrelated with the sudden temperature increase and decrease during wire-EDM cutting.Due to the heating and cooling cycle taking over a very short period, cracking was also observed at HAZ, which is also similar to HAZ cracking observed in welding (Figure 6).This subsurface cracked region is very   detrimental to the service life of the parts that are particularly working under loading.Therefore, the recast layer and HAZ should be removed for better surface integrity.
Along with SEM images, EDS analyses were also conducted to understand the effect of the EDM passes and the mechanism of formation of the recast layer.As shown in Figure 7, four areas and one spot EDS analyses were conducted on the one-pass sample.The initial area scan (named as Selected Area 1) was selected so that elemental analysis was obtained at the recast layer.Based on EDS analysis on Selected Area 1, significant amounts of Cu and Zn were observed at the recast layer.Since EDM cuttings were conducted with 0.25 mm thick brass (an alloy of Cu and Zn) wires, it was revealed that the recast layer was formed due to the deposition of brass alloy on the cutting surface during wire-EDM cutting.Therefore, it can be concluded that the colour of the formed layer after wire-EDM cuttings, known as the recast layer, originated from the brass wire used in the wire-EDM processes (Figure 1).The recast layer consisting of mostly Cu and Zn was also the main reason for hardness decrease as obtained in Figure 4.In addition to Cu and Zn, there were W and C peaks due to WC phase in the base material.Considering initial particle morphology and high temperatures during EDM cutting, it can be inferred that WC particles are melted and solidified in a very short time scale and formed the recast layers with Cu and Zn.Moreover, there was no Co peak recorded from Selected Area 1.The main reason can be attributed to the lower melting and evaporation temperatures of cobalt compared to WC. 8 During wire-EDM, it was stated that cobalt could reach the evaporation temperature before melting of WC particles.Therefore, a cobalt-free region can be formed during wire-EDM.
Another area from the recast layer, named as Selected Area 2, was analysed.Similar elements were observed as compared to Selected Area 1.An additional area was investigated from the HAZ, named as Selected Area 3. The weight percent of Cu and Zn decreased significantly compared to Selected Area 1 and Selected Area 2. On the other hand, the percent of W was increased and significant Co content was obtained.Therefore, it can be inferred that residues of wire in terms of elements were very slight at the HAZ.The final area labelled as Selected Area 4 from the HAZ was analysed and similar elemental composition was obtained compared to Selected Area 3. The point analysis labelled as "EDS Spot 1" was conducted at the cracked region at the HAZ.Elemental analysis showed significant Cu and Zn content in addition to W and C, i.e., very close to the elemental distribution observed at the recast layer with very slight Co content.

Wire-EDM cutting strategies for Wc-Co hardmetals
In order to determine the wire-EDM cutting strategy of WC-Co hardmetals for a particular application, the production route should be taken into account.Depending on the processes after wire-EDM cutting and desired surface conditions, the number of EDM passes should be determined.In addition, due to tight tolerances expected from particularly tools and inserts made of WC-Co hardmetals, the amount of material to be removed at each step should be projected before production.Moreover, considering the running costs and productivity, parameters such as material removal rate and wire consumption should also be taken into account.The flowchart of the cutting strategies of WC-Co hardmetals offered in this study is given in Figure 8.
First of all, cracking, recast layer and heat-affected zone (HAZ) observed after one pass should be removed for better surface integrity.Therefore, if there is no further production step in terms of material removal for the wire-EDM'ed surface such as grinding, the number of passes should be maximised.As shown in this study, there were local recast layer zones and residues after five passes.Therefore, after wire-EDM cutting with a maximum number of passes, polishing is suggested to be applied to ensure that all residues of the recast layer are removed.In addition, surface hardness was affected by the number of EDM passes.Decrease in surface hardness was observed for the samples with lower passes.To recover the surface hardness of WC-Co hardmetals, the number of EDM passes should be maximised.
Increasing the number of passes enhances surface integrity, as proven in this study.However, as higher number of passes is employed in serial production, increase in wire-EDM processing time per part and increase in running costs such as wire consumption are inevitable.Therefore, the cost of the wire-EDM process, i.e., the cost of the production will be higher and the productivity will be lower.Therefore, if the production steps after wire-EDM cutting include grinding, i.e., material removal is possible, wire-EDM cutting can be conducted with one pass.However, in the subsequent processes, material removal should be conducted so that the recast layer and HAZ are completely removed from the wire-EDM'ed surface.The thickness of this area was proven to be about 35 µm for the material-EDM set-up used in this study.In addition, surface hardness decrease should be avoided due to one pass.Therefore, while creating a production plan, material removal at each production step should be considered and after wire-EDM cutting with one pass, enough material should be grinded and polished to obtain the final dimensions of the machined part.Hereby, the final surface can be obtained without recast layer, HAZ and lower surface hardness region.
In terms of surface roughness, an increase in the number of EDM passes leads to a better surface finish.As shown in this study, an average R a of 0.42 µm can be obtained with wire-EDM cutting with five passes.Depending on the application, better surface quality can even be achieved by combining multi-pass wire-EDM with applying subsequent processes such as polishing.For some complex-shaped parts, grinding and polishing could be very difficult to apply.In these cases, the number of passes can be maximised to achieve desired surface conditions.However, depending on the expectations from the surface quality, an increase in the number of passes may not be sufficient.

Conclusions
Based on this study, the following conclusions can be drawn: 1.A yellowish recast layer was found on the surfaces of wire-EDM'ed samples.As the number of passes increased, the thickness of the recast layer decreased, i.e., yellowish recast layer faded away as the number of passes increased.2. Surface roughness values were observed to be decreased with increase in number of passes from one to five.However, after one and two passes, the surface roughness values were higher compared to the as-received condition.3. Surface micro-hardness values after one pass decreased significantly due to the recast layer.
Increase in the number of passes led to increase in micro-hardness reaching the as-received sample hardness value.4. The effect of the number of passes on the surface integrity of processed materials was very significant.The heat-affected zone (HAZ) with micro-cracks observed after one pass can cause premature failures during service conditions.The tendency of observation of the recast layer decreased with increase in the number of passes.Considering the set-up used in this study, there were some recast layer zones even after five passes.5. Depending on the expectations from surface integrity, the production route should be determined including the number of wire-EDM passes.Surface quality, running costs, productivity should be taken into account and depending on each case, production planning should be made.The cutting strategy offered in this study can be implemented for the production of any parts made of WC-Co hardmetals including wire-EDM.

Table 1 .Figure 1 .
Figure 1.Wire-EDM'ed and polished semi-doughnut-shaped sample after five EDM passes.The Doughnut-shaped sample was cut into two halves by wire-EDM.Therefore, one pass (thickness) and five passes (inner-hole) regions can be seen simultaneously.

Figure 2 .
Figure 2. Surface appearance of wire-EDM'ed and polished samples after (a) one, (b) two, (c) three and (d) five passes.

Figure 3 .
Figure 3. (a) R a and (b) R z surface roughness measurements of wire-EDM'ed samples.

Figure 5 .
Figure 5. Cross-sectional SEM images of samples with (a) one, (b) two, (c) three and (d) five wire-EDM passes.

Figure 6 .
Figure 6.Cracking (shown with arrows) observed at heat-affected zone (HAZ) for one-pass sample.

Figure 7 .
Figure 7. (a) EDS locations of one-pass sample.Intensity and percentage of elemental results of locations as depicted in (a): (b) Selected Area 1, (c) Selected Area 2, (d) Selected Area 3, (e) Selected Area 4 and (f) EDS Spot.

Figure 8 .
Figure 8. Flowchart of the wire-EDM cutting strategy of WC-Co hardmetals for maximum surface integrity with lower production cost and higher productivity.