Analysis of Fundamental Process Characteristics for Sinking-EDM of Cemented Carbides as a Function of Polarity

Analysis of Fundamental Process Characteristics for Sinking-EDM of Cemented Carbides as a Function of Polarity

Available online at www.sciencedirect.com ScienceDirect Procedia CIRP 68 (2018) 313 – 318 19th CIRP Conference on Electro Physical and Chemical Mach...

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Available online at www.sciencedirect.com

ScienceDirect Procedia CIRP 68 (2018) 313 – 318

19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, Bilbao, Spain

Analysis of Fundamental Process Characteristics for Sinking-EDM of Cemented Carbides as a Function of Polarity Fritz Klockea, Lothar Chrubasika*, Andreas Klinka, Lars Hensgena a

Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Steinbachstraße 19, Aachen 52074, Germany

* Corresponding author. Tel.: +49-241-80-28002; fax: +49-241-80-22293. E-mail address: [email protected]

Abstract

Cemented carbides have a growing potential as a tool material in metal forming due to their great resistance to wear and increased compressive strength. The further spread for active elements in metal forming is conflicted by high costs and a lack of knowledge about the specific performance and interpretation of the applicable regulations. A key technology for the machining of cemented carbide forming dies is Electrical Discharge Machining (EDM) because of a difficult conventional machinability and specific material properties. However, machined carbide tools often show a toughness remaining below expectations. A possible explanation could be the extent of the manufacturing process related thermally affected rim zone. In addition, the polarity needs an evaluation for a basic understanding in regard to MRR. Therefore, a cemented carbide, a version with normal binder phase content, and a common tool steel as standard material are compared focusing on the material removal rate and formation of cracks in the rim zone for different polarizations. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2018 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining

Keywords: EDM; Cemented Carbides; Material Removal Rate; Wear; Forming Dies

1. Introduction Since the introduction of hard metals in the beginning of the 20th century, there has been an ever-growing market for new applications and enhanced material developments. In the past decades increased tool wear, loadability and reliability have been the driving force for the development of tool material in metal forming. Today cemented carbides with a higher tungsten binder phase are already in use in the metal forming industry [1]. They show a higher hardness compared to tool steels and a greater bending strength compared to ceramics. The compressive strength in forming tools has to be above the yield strength of the shaped material. For the steel forming they can reach up to 3000 N/mm² [2]. Due to their great resistance to wear and increased compressive strength, they still have a growing potential, which is afflicted up to now by high costs. Among other reasons, this is because of a lack of knowledge in

machining. Main issues in dies and molds come from poor surface integrity [3]. Machining cemented carbides using conventional technologies like milling is generally not economical because of the workpiece hardness and the resulting tool wear. Though, there are approaches undertaken for cutting of cemented carbides, whereby there is no economic comparison, yet [4,5]. The most established technology for machining of cemented carbide forming dies is Electrical Discharge Machining (EDM) [6]. For wire EDM several experiments have already been conducted to analyze the process force [7] and to improve the surface integrity [8–10]. For sinking-EDM of hard metals fundamental investigation already exist [11–13] as well as an economic investigation of cemented carbide machining [14]. Besides, fundamental technological results investigating polarity for sinking-EDM of cemented carbides using contemporary generator technology are missing. Missing knowledge leads to machined carbide tools still not fulfilling

2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.070

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tool life requirements and remaining below expectations due to thermally damaged rim zones. In EDM, it is possible to decide whether more thermal energy is set free in the workpiece or in the tool by choosing the polarity. A short pulse duration is usually set for machining cemented carbides. In that case, the tool electrode has to be negative as a cathode and the workpiece positive as an anode because more energy flows into the anode. Utilizing prolonged pulse durations, i.e. for steel machining, the polarization is switched because the energy levels of the larger and positive charged particles grow [15]. For long discharge durations, a carbon layer forms on the anode of cemented carbide but not on the surface of steel. This layer is formed by disintegration of long carbon molecules that protect the anode from wear. For cemented carbides though long discharge durations would therefore lead to diminished material removal rates (MRR) [16]. In order to get a direct comparison of the anode and cathode material removal rates as well as the surface qualities for tool materials, this paper examines a cemented carbide in comparison to a common tool steel. This is done in order to get a better understanding of machining processes and their differences due to material properties. The materials are used as anode and cathode at the same time machining themselves. The main reason to use this experimental build is the direct comparability of the machined surfaces and the process signals. Furthermore, it is highly efficient on behalf of time and material use. The following experiments have been conducted with the intention to get a profound understanding of the processes using the latest generator technologies since EDM is in direct competition with other technologies. It is therefore important to get a better understanding of the fundamental processes at hand in order to remain competitive.

further on. The following experiments used square shaped discharges with the process parameters from Table 1. Table 1. Description of the process parameters used. Property Unit

Value

Variable parameters Discharge duration (te)

µs

7, 10, 100, 200

Electrode material

-

G40, HS6-5-3

Electrode geometry

mm2

square 0.16

Discharge current (ie)

A

20

Open circuit voltage (ûi)

V

100

Duty factor (τ)

-

0.08

Dielectric fluid

-

oelheld IME-MH

Constant parameters

The process factors current and voltage have been recorded with an oscilloscope in order to calculate the mean discharge energy per discharge and to analyze the process itself. Erosion time was set to 10 min for every experiment. Afterwards, cathode and anode materials have been weighed for material removal rate calculations. The sample surfaces then were examined using confocal laser scanning microscopy looking for cracks and other distinctive features. ie = 20 A |ûi | = 100 V τ = 0.08 Steel negative electrode

Steel positive electrode

Nomenclature ie τ te ue ûi VW V

Discharge current / A Duty cycle / Discharge duration / μs Discharge voltage / V Open circuit voltage / V Material removal rate / (mm3 / min) Volume / mm3

2. Experimental description In the series of tests for each experiment only one kind of material is used for the negative electrode and for the positive electrode at the same time, see Fig. 1. Past research has shown that a greater part of the energy applied flows in the anode using a copper workpiece and a graphite tool electrode. Furthermore, if the two electrodes are made from copper a pyrolytic carbon layer protects the anode from wear [16]. In a first attempt, very short capacitor discharges were used since it is the common way to machine hard metals. However, the results have not been comparable because of process energy levels fluctuating too much. This attempt was not pursued

Fig. 1. Experimental setup with steel HS6-5-3 as anode and cathode. Table 2. Description of the material characteristics [17–20]. Characteristic Tool Steel HS6-5-3 Cemented Carbide G40 Content wt / %

C 1.28; Cr 4.2; Mo 5.0; W6.4; V3.1

WC 80; Co 20

Thermal conductivity λ / (W / (m·K))

19-24

90

The utilized cemented carbide is a medium grain sized G40 and the tool steel is HS6-5-3 see Table 2. Both are applicable in metal forming processes. All experiments were conducted on a GF Machining Solutions Form 2000 machine tool using

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3. Results In the following, this paper discusses the results of the machining process. First MRR are analyzed and afterwards the condition of the machined surfaces. 3.1. Material removal rates

Mate rial removal rate VW / (mm3 /min)

Henceforth, the term MRR will be used for both negative and positive electrodes. Fig. 2 shows the results for the material removal rates of the anodic- and cathodic-poled materials on a double logarithmic scale. 1

experiment and combination. Observing the machining data of steel there are many short circuit and abnormal discharge impulses. In total short circuit discharges reach 61 % and abnormal discharges 22 % for te = 7 µs as well as even 75 % and 13 % for te = 10 µs discharge duration. The corresponding figures for cemented carbide are 2 % and 29 % as well as 1 % and 28 % respectively. Short circuit and abnormal impulses do not generate any or very few material removal, which in addition to the overall less discharges means a further diminished MRR. 100 %

Ave rage distribution of discharge types in / %

up-to-date generator technology. Short circuit regulations have been turned off. The dielectric utilized was oelheld IME-MH, a standard oil based dielectric, with no additional flushing system, the laser microscope used was a Keyence VK-X 100/150 series and the oscilloscope in use was a Tektronix DPO7104c.

80 %

60 % 40 % 20 % 0%

ie = 20 A |ûi | = 100 V τ = 0.08

ie = 20 A |ûi | = 100 V τ = 0.08

HS G 7

HS G

HS G

HS G

10

100

200

Discharge duration for HS6-5-3 (HS) and G40 (G) t e / µs

effective discharge pulse

abnormal discharge pulse

short circuit pulse

0.1 Fig. 3. Average distribution of discharge types for cemented carbide G40 and steel HS6-5-3 for different discharge durations - similar materials for both electrodes at the same time.

HS6-5-3 (+) HS6-5-3 (-) G40 (+) G40 (-) 0.01 5

50

250

Discharge duration t e / µs Fig. 2. MRR of anodic and cathodic cemented carbide G40 and steel HS6-5-3 as a function of the discharge duration - similar materials for both electrodes at the same time.

Looking at the cemented carbide it has a much higher MRR compared to the steel for the lower discharge duration of te = 7 µs and te = 10 µs for both polarizations. This may be due to lower average discharge energy for steel. More likely though, is an unstable process, which has been observed for the steel specimens. The oscilloscope data shows many short circuit pulses mainly for the steel whereas for the cemented carbide it shows notably fewer. The number of total discharges for steel specimens for all discharge durations was roughly 30 % lower than for the cemented carbide specimens based on the used selfdeveloped algorithm. This is one explanation for the overall low MRR of steel. Another crucial factor for the MRR are the different discharge types displayed in Fig. 3. There it is possible to see that there are huge discrepancies comparing the different discharge types for the two materials and the different discharge durations. For each discharge duration and material combination three oscilloscope data sets of 2 s each were recorded. The data sets were then averaged for each machining

Abnormal impulses for this process show a lower discharge current of about 5 A and shorter discharge duration leading to very low MRR. One explanation for the short circuit pulses could be the debris of the erosion process [21]. Looking at Table 2 steel has a lower thermal conductivity compared to cemented carbides, which means that the material is removed better by the thermal process at hand. Further, bigger particles might be found in the gap and thus conductive bridges are easier created leading to short circuits. Apparently the short pulse interval time is insufficient for the debris to spread in the gap and therefore enables the discharge bridges to form [22]. Moreover, for the short discharge durations the cemented carbide shows no such signs even though the MRR is significantly higher. Following the theory of larger debris for the steel specimens, the debris of the cemented carbide would have to be smaller in order to spread faster. This phenomenon is reduced for the long discharge durations, looking at the extended pulse interval times. Resulting process improvements for steel due to the duty factor have a huge impact. Only 55 % and 44 % for te = 100 µs and te = 200 µs of the discharges are short circuit pulses. Fig. 4 shows the mean removal volume for single discharges in cemented carbide and steel specimens for different discharge durations. For the single discharge removal volume, again the recorded sets of oscilloscope data were used for each discharge duration and material combination. For each combination, the averaged discharge amount then was adapted to 10 min in order to split the overall MRR by the total amount of discharges. The first things standing out are the different graph intersection

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points comparing Fig. 2 and Fig. 4. In Fig. 2 the intersection points for the different materials are found between discharge durations of te = 50 µs and te = 100 µs, whereas in Fig. 4 they are already intersecting at te = 10 µs. This means even though the single discharges above te = 10 µs discharge duration show a better removal volume they still have a reduced MRR until te = 50 µs. The earlier mentioned process conditions might lead to such low MRR in Fig. 2 for steel. For the cathodic steel specimens Fig. 4 shows removal rates of about 2,800 µm³ to 39,100 µm³ for cathodic and for the anodic 1,200 µm³ to 69,900 µm³ as well as 3,600 µm³ to 12,000 µm³ and 2,200 µm³ to 10,100 µm³ for the cemented carbide specimens, respectively.

The graphs though do not intersect. Thus, this means an overall lower energy level for the discharges happening in steel and therefore even lower removal rates. These lower energy levels can be observed in Fig. 6 for the different discharge durations and materials. Looking at the trend lines of steel it shows again reduced energy levels approaching the cemented carbide trend line with advancing discharge duration. Nevertheless, the MRR of the cemented carbides is surpassed by the steel specimens as already discussed. Ave rage discharge energy W e / mJ

316

Removal per discharge V / µm³

100,000 ie = 20 A |ûi | = 100 V τ = 0.08 10,000

0.06

0.04 0.02 0 0

50

100

150

200

250

Discharge duration t e / µs

ie = 20 A |ûi | = 100 V τ = 0.08

HS6-5-3 (+)

1,000

HS6-5-3 (-)

HS6-5-3 Linear (HS6-5-3)

G40 Linear (G40)

G40 (+) G40 (-) 100 5

250

50 Discharge duration t e / µs

Fig. 4. Average removal per discharge for cemented carbide G40 and steel HS6-5-3 comparing different discharge durations from te = 7 µs to te = 200 µs - similar materials for both electrodes at the same time.

Fig. 5 introduces the average discharge voltages of the specimens, which have been measured by the machine. Afterwards, they have been verified by the oscilloscope data. The voltages for the cemented carbide decrease starting from 22 V and for steel increase from 5 V for the shorter discharge duration. A discharge with 5 V roughly represents the conditions of a short circuit, which explains the low MRR in Fig. 2.

Fig. 6. Average discharge energy cemented carbide G40 and steel HS6-5-3 looking at different discharge durations - similar materials for both electrodes at the same time.

The MRR for longer discharge durations of te = 100 µs and te = 200 µs is lower for the cemented carbide. At te = 200 µs, the anodic cemented carbide is machined at a rate of only one eighth of the MRR of te = 7 µs. This is most likely due to the pyrolytic graphite forming on the anodic surface of the cemented carbide see Fig. 7. The figure shows the counterpart surfaces of the steel cathode (a) and anode (b) as well as the cemented carbide surfaces (c) and (d), respectively. There it is possible to see black layers around the discharge crates in picture (d) and partly in (c). Cathode

(a)

Anode t e = 200 µs

(b)

t e = 200 µs

HS6-5-3

25 20

15 100 µm

100 µm (c)

10

5 HS6-5-3

G40

ie = 20 A |ûi | = 100 V τ = 0.08

0 5

50 Discharge duration te / µs

t e = 200 µs

(d)

t e = 200 µs

G40

Ave rage discharge voltage ue / V

0.08

250

Fig. 5. Average discharge voltage for cemented carbide G40 and steel HS6-5-3 as well as different discharge durations - similar materials for both electrodes at the same time.

100 µm

100 µm

Fig. 7. Surface laser images of steel HS6-5-3 cathode (a) and anode (b) as well as cemented carbide G40 cathode (c) and anode (d) machined with discharge duration of te = 200 µs - similar materials for both electrodes at the same time.

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3.2. Surface cracks Growing discharge durations mean an increased energy input into the specimens surface. EDM machined surfaces usually show tensile stress. Especially for higher energy levels, those tensions lead to cracks on the surfaces. The steel and cemented carbide specimens show according results. HS6-5-3 Cathode

HS6-5-3 Anode

(b)

t e = 100 µs

(a)

20 µm

20 µm (d)

t e = 200 µs

(c)

20 µm

20 µm

Fig. 8. Surface laser images of steel HS6-5-3 cathodes (a) and (c) as well as anodes (b) and (d) machined with discharge duration of te = 100 µs and te = 200 µs - similar materials for both electrodes at the same time.

Fig. 8 shows the surfaces of steel for discharge durations of te = 100 µs and te = 200 µs. The cathode surface (a) and anode surface (b) machined at te = 100 µs show fewer and more narrow cracks compared to (c) and (d) machined at te = 200 µs. As mentioned above this is due to the higher energy and the resulting tensile stress. These cracks would grow and finally

result in tool failure. For lower discharge durations of te = 7 µs and te = 10 µs (not shown in figures), the surfaces look spattered especially on the anodes, although no cracks are visible through the laser microscope. The cemented carbide surfaces differ strongly from the steel surfaces. Comparing the surfaces from Fig. 8 and Fig. 9, it is possible to see that the surfaces of the cemented carbide show less and thinner cracks which would reduce tool life all the same, if not removed properly. Looking at the cracks, the cemented carbide surfaces do not show a big difference between the different discharge durations. G40 Cathode

G40 Anode

(a)

t e = 100 µs

(b)

20 µm (c)

20 µm (d)

t e = 200 µs

Most likely the graphite protects the cemented carbide due to its high sublimating temperature and thermal resistance as stated by Kunieda et al. explaining the low MRR [16]. In contrast, the MRR of steel is growing to more than four times in comparison to te = 7 µs discharge duration for the anodic electrode. It might be possible that the graphite is rather forming a layer on the cemented carbide because of the higher thermal conductivity. The dielectric around the discharge might cool down faster. Thus, the sublimated graphite would adhere on the cooler surface forming a layer. Since steel has a lower thermal conductivity, this effect might be limited, explaining the missing layer. Comparing the cathodic-poled electrodes, a peak for cemented carbide at te = 10 µs and a lower MRR for shorter and longer discharge times is visible. For te = 100 µs and te = 200 µs, the MRR is less than one sixth and less than one eighth respectively. For the steel, the MRR is between 0.089 mm³/min and 0.098 mm³/min, which is roughly one eighth of the cemented carbide peak. By only looking at the surfaces, it is impossible to make a well-founded assertion of the erosion process but it provides indications in some cases. As already stated, deposits can have an impact on MRR and further cracks can have an influence on later wear or premature failure.

20 µm

20 µm

Fig. 9. Surface laser images of cemented carbide G40 cathodes (a) and (c) as well as anodes b) and (d) machined with discharge duration of te = 100 µs and te = 200 µs - similar materials for both electrodes at the same time.

Notably different are the cemented carbide surfaces of the cathodes for te = 100 µs and te = 200 µs of the cemented carbides in Fig. 9. They feature porous sponge-like surface with many little holes. These small holes can be found as well on the surface of the cemented carbide specimen machined with a discharge duration of te = 10 µs but cannot be found for te = 7 µs. The small holes could be a tungsten carbide structure where the binder phase has been removed. Considering a grain size of 1.6 µm to 2.5 µm the size of the holes and the structure would support this assumption. A next step would include an evaluation of the surface components in order to investigate the influence of this surface on the EDM process. It might be possible to identify the amount of cobalt left on the surface with an energy dispersive X-ray spectroscopy. Furthermore, (c) shows a big hole looking like a bigger piece broke out during the machining process. Such holes appear more often on the surfaces of these specimens machined at a discharge time of te = 200 µs. Such particles should be bigger and therefore it should be possible to verify their existence with a particle size analysis. 4. Summary and Outlook In this paper results for sinking-EDM of a cemented carbide and a tool steel are compared. In every experiment conducted, only one of the two materials was used as anodic and cathodic

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electrode at the same time. This approach enabled a direct comparison of the MRR and the resulting surface characteristics. It was found that the process was far more stable for cemented carbide specimens than for the steel specimens resulting in a very low MRR for steel. Nevertheless, for longer discharge durations steel shows higher MRR compared to the cemented carbide because of significantly declining MRR. The low performance of steel machining is most likely due to the debris and the pulse interval time leading to many short circuit pulses as well as abnormal discharge pulses. For longer discharge durations, the amount of effective discharge pulses rises for steel and so does the MRR. Cemented carbides though show a reduced MRR most likely because of a protecting pyrolytic graphite layer. One theory for the formation of these layers can be traced back to the thermal conductivity. Furthermore, cracks on the surfaces of the specimens have been spotted for longer discharge durations. The steel specimens show more and thicker cracks on both anodic and cathodic surfaces. Particularly on the anodic and partly on the cathodic electrodes of the cemented carbides black layers and far less cracks are visible. The cathodic cemented carbide electrodes though show a very porous surface for almost all specimens except the one with the lowest discharge duration. A first assumption is that the structure could be made of tungsten carbide with the cobalt being removed. Future experiments need to study the surface integrity more detailed concentrating on the formation of the porous structure on the cemented carbide cathodes as well as their impact on tool life. An energy dispersive X-ray spectroscopy could provide a better understanding of the surface composition. The amount of cobalt left in the porous structure would give a first hint whether the provided theory of removed cobalt is right. Furthermore, a particle size analysis might give insight on the big holes created during the 200 µs discharge duration. Looking at the porous structure as well as the formation of cracks other discharge pulse forms have to be investigated. In addition, other factors like binder phase, grain size and other tool electrodes will have to be examined. 5. Acknowledgments The authors thank the German Research Foundation (DFG) for funding in the DFG project KL 500/153-1 “Analysis of discharge-dependent surface integrity of cemented carbide forming dies and its influence on the tribological characteristics and the resulting fatigue behavior“. Furthermore, they thank CERATIZIT for providing the cemented carbide.

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