Procedia CIRP 24 ( 2014 ) 97 – 102

Available online at www.sciencedirect.com

2212-8271 © 2014 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/3.0/).Selection and peer-review under responsibility of the International Scientific Committee of the “New Production Technologies in Aerospace Industry” conference in the person of the Conference Chairs: Prof. Berend Denkena, Prof. Yusuf Altintas, Prof. Pedro J. Arrazola, Prof. Tojiro Aoyama and Prof. Dragos Axintedoi: 10.1016/j.procir.2014.07.136

ScienceDirect

New Production Technologies in Aerospace Industry - 5th Machining Innovations Conference (MIC 2014)

Quality assessment through in-process monitoring of wire-EDM for fir tree slot production

F. Klockea, D. Wellinga*, A. Klinka, R. Perezb aLaboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University, 52074 Aachen, Germany

bAgie Charmilles SA, 1217 Meyrin, Switzerland

* Corresponding author: Tel.: +49 (0) 241 80 28039; fax: +49 (0) 241 80 22293; E-mail address: D.Welling@wzl.rwth-aachen.de

Abstract

Against the background of an increased importance of in-process quality measurement – especially for manufacturing of safety critical jet engine componets – this paper deals with the development of a process monitoring tool for process quality assessment and correlating surface integrity evaluation of Wire-EDM for fir tree slot production. For the last trim cut, which is the relevant cut for the final surface integrity, a correlation between process data and surface integrity aspects is set up. The process data are read out from the machine tool’s controlling device to obtain a position depending signal of mean gap voltage for the quality assessment. Two demonstration runs showed the work capability of the monitoring tool for cutting the often applied nickel-based turbine alloy Inconel 718. On the one hand a failure free slot machining process and on the other hand a manipulated machining process with enforced short circuits were conducted. For the manipulated process these instabilities were successfully detected. A cross section view analysis of both fir tree slots results in best surface integrity qualities for the first run, which shows the potential to machine fir tree slots by Wire-EDM, and failures in the surface of the manipulated run due to short cuts. These failures were additionally correlated with the position depending signal. With the developed monitoring tool a first approach was designed to build up an in-process and position oriented quality assessment for the production of fir tree slots by Wire-EDM. © 2014 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of the International Scientific Committee of the “New Production Technologies in Aerospace Industry” conference in the person of the Conference Chairs: Prof. Berend Denkena, Prof. Yusuf Altintas, Prof. Pedro J. Arrazola, Prof. Tojiro Aoyama and Prof. Dragos Axinte.

Keywords:Wire-EDM; Surface Integrity; Process Monitoring; Nickel Alloy

1. Introduction

Wire-EDM becomes more and more a serious alternative to the established and as critical defined broaching process for fir tree slot production [1]. Klocke et al. [2] summarize in their publication aspects why the broaching process is seen as a critical manufacturing process in the process chain of jet engine disc production. The main aspects are: High machine tool investment costs, large floor space requirements to place broaching machine tools, high energy consumption during machining, long lead in times and high costs of cutting tools.

Within the last decade several publications which deal with alternative manufacturing processes for the fir tree slot production have been released. A summary is given by Curtis

et al. [3]. Especially to the research of the Wire-EDM process an increased significance was given. By passing multiple development steps, high productive Wire-EDM processes were achieved to meet the demanded requirements of the fir tree slots in terms of accuracy and surface integrity aspects. An overview of achievable surface integrities was given by Aspinwall et al. [4] and Li et al. [5]. Klocke et al. [6] presented slot qualities that exhibit geometry tolerances within t ± 5 μm, surface roughness values Ra < 0.8 μm and recast layer thicknesses which tend to zero in thickness. Even the resulting dynamic component strength of produced parts by Wire-EDM was shown to be in the same magnitude as broaching by Welling [7]. To decrease the machining time of the fir tree slot production by Wire-EDM Klocke et al. [2] set

© 2014 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/3.0/).Selection and peer-review under responsibility of the International Scientifi c Committee of the “New Production Technologies in Aerospace Industry” conference in the person of the Conference Chairs: Prof. Berend Denkena, Prof. Yusuf Altintas, Prof. Pedro J. Arrazola, Prof. Tojiro Aoyama and Prof. Dragos Axinte

98 F. Klocke et al. / Procedia CIRP 24 ( 2014 ) 97 – 102

up a comparison of different wire electrodes and technology parameter tables.

Nomenclature

HRC Hardness - Rockwell scale F1 Force on wire towards surface / N F2 Force on wire away from surface / N FD Wire pre tensioning force / N fe Process measurement sampling frequency / Hz ff Filter sampling frequency / Hz fs Sampling frequency / Hz h Height / mm ID Discharge current / A LPF Low pass filter MS Machined surface pfir Fir tree slot cutting path / mm R Rough cut Ra Arithmetic average surface roughness / μm rmin,inner Minimal inner radius / mm rmin,outer Minimal outer radius / mm s Cutting length / nm s Working gap width / μm TC Trim cut t Tolerance / μm t Time / μs tRL Thickness of recast layer / μm TF Measurement interval / s UD Discharge voltage / V ûi Open circuit voltage / V Um Mean gap voltage / V Um,lim+ Upper limit of mean gap voltage / V Um,lim- Lower limit of mean gap voltage / V z Infeed / μm

For the production of safety critical features like fir tree

slots in jet engine discs a stable manufacturing process is essential to ensure a high part quality. Beside the geometrical accuracy, the surface integrity of the features is the significant quality aspect. This is due to the fact that the product life time can be directly related to the surface integrity which is introduced by the manufacturing process [8]. One approach to guarantee a good surface integrity is the monitoring of process data during the manufacturing and the correlation of these data with the resulting surface integrity. In the past a lot of research was conducted in this field for conventional processes like broaching, drilling, turning and milling [9]. In the field of Wire-EDM a lot of publications deal with the monitoring of gap phenomena to prevent wire breakages [10-13] or to analyze the process stability to improve the technology parameters [14, 15]. A fully developed tool which correlates process signals with surface integrity aspects has not been published in the scientific community.

With the approach of process monitoring tools for conventional processes [16], this paper deals with the development of a process monitoring tool for process quality

assessment. Especially the Wire-EDM process to manufacture fir tree slots is addressed.

2. Approach

In Figure 1 the general approach for the process monitoring method of fir tree slot production by Wire-EDM is mapped. This method contains an in-process monitoring of process parameters. With the monitored data an in-line evaluation is conducted by the system to achieve two results. These are on the one hand an online analysis of the process quality with the benefit to stop the process if any significant complications occur and on the other hand a documentary which contains the process quality and the related part quality.

To develop the model which describes the correlation of process signals and the resulting surface integrity aspects two central challenges have to be addressed. The first challenge is to locate process parameters which describe the process in a definite manner and the second is to correlate the monitored signals with the surface integrity. This implies the impact factor of signal variations on the surface integrity. Following, a first approach of a system build up to monitor the final/ second trim cut will be presented.

Figure 1: Approach for process monitoring of fir tree slot production by Wire-EDM.

3. Experimental Work

3.1. Workpiece Material, Equipment and Test Piece Geometry

For the tests the aerospace alloy Inconel 718 was selected. Several publications show the difficult-to-cut material’s properties and their effects on the machinability via cutting [17-19]. The elected solution treated and aged Inconel 718 is characterized by a coarse grain size which varies from 10 μm - 80 μm and possesses a hardness of 45 HRC. The composition of the material was verified with a wet chemical analysis. In Table 1 the result of the analysis is listed.

Table 1: Inconel 718 composition

Element Ni Cr Fe Nb Mo Ti Co oth. Weight % 53.4 18.0 17.9 5.16 3.00 1.00 0.66 0.88

As machine tool a GF AgieCharmilles 440 ccS was used. It

is equipped with an anti-electrolysis ‘clean cut’ generator, which achieves best surfaces due to high-frequency energy-rich pulses. An open interface at the machine tool’s controlling system enables the reading-out of certain process parameters during machining. In Figure 2 the test set up is

Process

Process

In-process monitoring of process parameter

DocumentaryAcquisition

Online evaluating of monitored parameter

Approval

99 F. Klocke et al. / Procedia CIRP 24 ( 2014 ) 97 – 102

shown. By using a patch cable a PC is communicating with the machine tool’s controlling system. For the data acquisition a LabVIEW (National Instruments) based macro is used on the PC.

Surface roughness measurements of the test cuts were conducted with a Mahr MFW-250 measuring head. The measuring procedure was done in accordance to the specifications of the DIN EN ISO 4288. Further analysis of the surface integrities were done using cross-section polishings which are inspected by optical microscopy.

Figure 2: Test set up for monitoring of process parameters.

For the tests a generic fir tree slot geometry was taken from former researches (cp. Figure 3a)). In the publication of Klocke et al. [2] the details about fir tree slot and the technology parameters (cp. Figure 3b) [2]) as well as the boundary conditions of the cutting tests are described.

Figure 3: Generic fir tree slot and technology parameter [2].

3.2. Acquisition of Process Signals of Second Trim Cut

For the present study, in which the second trim cut is analyzed, two process parameters were read out of the machine tool’s controlling board by the PC with a sampling frequency fs = 50 Hz. These are the cutting length s with a resolution of 100 nm and the mean gap voltage Um. In Figure 4 the overall method of the controlling board to determine Um is shown. A voltmeter is measuring the voltage UD of the process with a frequency fe = 20 MHz. The measured voltage U is then down filtered with a frequency fF = 2000 Hz. The result is the mean gap voltage Um. This voltage is collected by

the controlling board of the machine tool (not adaptable by the operator) and scanned with the PC.

For the chosen technology table the machine tool possesses an isofrequent pulse control for the second trim cut. Within this control the generator keeps the discharge duration and the pulse interval time constant. The feed rate is set as constant by the operator and is not controlled by the machine tool as during e.g. the rough cut. So, the controlling system is not able to adapt the working gap width while machining. Due to these facts it is important to monitor the gap width to achieve information about the process and the final part quality.

Considering the issue, that the working gap cannot be measured directly during machining, a process parameter which correlates with the gap width has to be found. A first approach is to use the mean gap voltage Um.

This assumption is traced back on three different situations which can be evaluated by using Um:

Sit. 1: Working gap s too large – Um too high; Sit. 2: Working gap s ideal – Um ideal; Sit. 3: Working gap s too small - Um too small.

If the working gap s between the wire and the workpiece (see Figure 4) is too large, the ignition delay time td increases. It takes longer to move the wire onto the workpiece surface to reach the break down voltage. Due to this fact the mean gap voltage increases as well, because the open circuit voltage ûi is applied a longer time. Vice versa, if the working gap s is too small the mean voltage decreases. This situation is the most critical because the potential of discharges which are characterized as abnormal increases. Due to the fact that these abnormal discharges like short circuit pulses and arc discharge pulses affect the surface integrity negatively, this situation has to be prevented [20].

Figure 4: Determining method of mean gap voltage for second trim cut.

PC for signal acquisitionControlling board of machine tool

Patch cableTest bed machine tool

20

7.955

20

Geometrical dataMin. inner radius rmin,inner 0.3 mmMin. outer radius rmin,outer 0.3 mmCutting length l 76 mm

Pressure flanks

Root section

Inner radius

Outer radius

a) Generic fir tree slot geometry

b) Technology parameter

Tech./ Series

Mode Shape Offs./ mm

ûi/ V

t0/ μs

tr/ μs

Aj/ V

2.BS RC Trapeze 0.221 80 6.0 0.65 50.0adapted 1.TC Triangle 0.143 200 5.4 0.05 70.0

2.TC HF-Mode 0.134 200 0.6 0.40 0.0

Vol

tage

U

D/ V

Cur

rent

I D

/ A

Time t / μs

Discharge voltage UD

LPFfF = 2000 HzV

fe = 20 MHz

Controlling board

LPF

PCM

ean

gap

volta

ge U

m/ V

Time t / ms

TF = 1/fF

Mean gap voltage Um

Process Measurement chain Acquisition

fs = 50 HzWire

Workpiece

UD Um

td

s

100 F. Klocke et al. / Procedia CIRP 24 ( 2014 ) 97 – 102

3.3. Determination of Adequate Tolerance Bands and Correlation to Surface Integrity for Second Trim Cut

The next step is to correlate the working gap distance s of the second trim cut with the mean gap voltage Um and the final surface integrity in terms of surface roughness Ra. A threshold for Um has to be found which ensures a surface integrity quality to meet the jet engine requirements for safety critical parts (Ra < 0.8 μm [2]).

To define a threshold for Um, the influence of varying gap width s of the second trim cut on the surface integrity was analyzed. In Figure 5a) the wire position for the tests is mapped. A workpiece (h = 40 mm) was prepared with a rough and a first trim cut operation. The surface was then cut with varying infeeds. In Figure 5c) the resulting surface which was cut by a rough, a first trim cut and a second trim cut with varying infeeds is pictured. No process was identified at the infeed z = 16 μm. The gap width s was too small so that the wire was in short circuit with the workpiece. For the infeeds z < 6 μm rough areas were detected at the top and bottom of the surface. These areas increase with greater gap widths. In Figure 5b) an explanation for this effect is sketched. As described in section 3.2 the ignition delay time td increases with larger gap widths. During that time an electro static force (F2) acts on the wire, which moves the wire towards the surface [21]. This leads to a bulgy formed wire (string between two wire guides) in the working area so that the discharges operate in the middle of the workpiece (cp. MS within the sketch). At the top and the bottom of the surface no discharges appear. The surface remains as it was before the cut. For ideal gap widths the force F1 (detracting force) and F2 are equal, so the wire is straight and the discharges appear across the whole workpiece height.

Figure 5: Wire position a), wire forces b) and resulting surface c).

After the optical inspection the surface roughness was measured at the top, the middle and the bottom in accordance to the infeed value. In Figure 6 the surface roughness measurement result is shown as a bar chart. For the infeeds 14 μm > z > 6 μm the mean surface roughness meets the required value of Ra < 0.8 μm. For infeeds z < 6 μm the measurements at the boundary areas miss the requirement, due to the bulgy formed wire. In addition, the monitored mean gap voltage Um is plotted in the diagram. The reported magnitude is the average value along the 15 mm test cut length for each infeed. Over the variation of the infeeds a continuous increase can be detected. The value increases from Um = 31.3 V at z = 16 μm to Um = 38.4 V at z = -4 μm. With the information from the surface roughness measurement and the monitoring of the mean gap voltage a threshold for a process monitoring system to assess the resulting surface quality was identified. This threshold implies the requirement for the mean gap voltage of a production process to result in acceptable surface qualities over the complete workpiece height (Ra < 0.8 μm). Due to the fact that the surface roughness of the infeed values 14 μm > z > 6 μm meet the requirements, thresholds were determined for a fir tree slot production with the constraints Um,lim- = 32 V and Um,lim+ = 35 V (compare red lines in chart).

Figure 6: Surface roughness and mean gap voltage according to infeed.

3.4. Approval of Developed Tool, Case Study: Fir Tree Slot Production

By using the monitoring tool a fir tree slot was manufactured with the published brass wire technology (cp. Figure 3b)) by Klocke et al. [2]. In Figure 7 the measured mean gap voltage Um is plotted in accordance to the fir tree slot cutting path. The red lines show the defined threshold. For the whole cut the measured voltage was within the threshold. The process quality has been in an acceptable condition. Solely at the bottom of the slot a small section runs out of the boundaries.

Figure 7: Measured mean gap voltage of fir tree slot production.

FD FD

FDFD

s2

F2 F1 F2

F1 = F2 F1 < F2

s1

WorkpieceInfeed: z

RC1st TC2nd TC

a) Wire position b) Wire forces

Infeed z / μm

40 m

m

c) Resulting surface

Rough area

16 14 12 10 8 6 4 2 0 -2 -4

15 mm Top

Middle

Bottom

Wire

s1 < s2

s

Wire guides

F1

MS

Infeed z / μm

Mea

n ga

p vo

ltage

U

m/ V

Surfa

ce ro

ughn

ess

Ra

/ μm

0

15

30

45

0.00.5

1.52.02.53.0

16 14 12 10 8 6 4 2 0 -2 -4

1.0

Ra top Ra middle Ra bottom Mean voltage

Ra = 0.8 μm

29

33

37

41

45

Fir tree slot cutting path pfir / mm

Threshold32 V – 35 V

0 10 20 30 40 60 7050

Mea

n ga

p vo

ltage

U

m/ V

Start of cut End of cut

Star

t of c

ut

End

of c

ut

Fl. 1Fl. 2

Flank 2 Flank 1

pfir

101 F. Klocke et al. / Procedia CIRP 24 ( 2014 ) 97 – 102

The surface integrity of the produced fir tree slot has been analyzed via rim zone inspection. For this inspection three critical areas were defined. These are on the one hand two process critical areas and on the other hand one part critical area. In Figure 8a) these areas are defined. Area 1 is defined as part critical due to high stress levels during service. The two process critical areas are defined according to the geometry strategy during rough cut machining. At the small radius (area 1) the machining speed and the flushing pressure is decreased by the machine (100% geometry strategy) to avoid inaccuracies at filigree contours due to wire lag. For straight lines like in the root of the fir tree slots (area 2) the geometry strategy is inactive. The flushing pressure is at its maximum and the cutting speed reaches the highest value during cutting of the fir tree slot.

In compliance to these definitions rim zone inspections of the machined slot were established. Figure 8b) shows the microscopies of the two areas. Both recorded pictures show a very small white layer with a maximum thickness of tRL < 3 μm on the top of the surface. At the part critical area as well as at the process critical areas the machining process results in best surfaces without cracks or any other failures on these parts. This achievement confirms the results which were found in the literature [1, 2, 4, 6] and underlines the possibility to substitute the critical broaching process to produce fir tree slots in jet engine discs by Wire-EDM.

Figure 8: Critical areas on fir tree slots a) and rim zone inspections b).

To test the developed monitoring tool in terms of detecting process instabilities a fir tree slot was machined with a

manipulated technology table. The rough cut and the first trim cut were taken from the used table for the first fir tree slot. Solely the offset of the second trim cut was manipulated. It was increased from zstandard = 9 μm (Offset1TC – Offset2TC = zstandard cp. Figure 3) to zmanipulated = 12 μm. Thus, the gap width s was decreased and the potential of instabilities was increased. With the manipulated technology table a fir tree slot has been cut. During cutting the mean gap voltage Um was monitored as within the first test cut. In Figure 9a) the monitored mean gap voltage is plotted in accordance to the cutting path. Additionally, the defined threshold of the pre-test is marked as red lines in the plot. Four distinctive peaks where the mean gap voltage decreased down to Um = 8 V can be detected in the plot. These peaks are deduced to short circuits during machining and indicate bad process qualities, which result in bad surface integrities. Using the information of the cutting path pfir, a correlation to the locations of the fir tree slot was set up. In Figure 9b) the locations are marked. The process instabilities 1 - 3 appeared at the minimal inner radius and the instability 4 was traced back to a location at an outer radius. In addition to the instabilities a reference area where the mean gap voltage is within the threshold is distinguished. In the root of the fir tree slot (area 5) the process does not run out of the threshold.

Figure 9: Measured mean gap voltage of manipulated fir tree slot production a) and location of instabilities b).

In accordance to the instabilities rim zone inspections were prepared at the detected areas two, four and five (cp. Figure 10). In the area two (minimal inner radius) surface defects can be seen at the exposed location. The same anomalies can be discovered for area four (minimal outer radius). In both areas recast material is accumulated on the surface. In the reference area five the surface is devoid of defects. This is in accordance to the pre-tests in which the infeeds were changed. It can be concluded that for straight cuts, gap width variations are not as critical for the machining process as at filigree radii.

1

2

10 mm

Embedded fir tree slot half

Process critical area –100% geometry

strategy

Part critical area

Process critical area –0% geometry strategy

50 μm 20 μm

Area 1

50 μm 20 μm

Area 2

b) Rim zone inspections according to critical areas

a) Definition of critical areas on fir tree slot after wire EDM1234

5515253545

Procesinstabilities1 2 3 4

Mea

n ga

p vo

ltage

U

m/ V

Threshold32 V – 35 V

Fir tree slot cutting path pfir / mm 0 10 20 30 40 60 7050

5

Flank 2 Flank 1

Star

t of c

ut

End

of c

ut

Fl. 1Fl. 2

Process instabilities

a) b)

102 F. Klocke et al. / Procedia CIRP 24 ( 2014 ) 97 – 102

Figure 10: Rim zone inspections of position two a) and four b) (process instabilities) as well as five c) (fault-less manufacture).

4. Conclusions

The conclusions of the quality assessment through in-process monitoring of wire-EDM are: The general approach to detect surface integrity defects

by reading out process signals with a process monitoring tool was successfully demonstrated for the Wire-EDM process.

Machining fir tree slots with a stable Wire-EDM process results in near zero recast layer thickness values. Even at part critical and process critical areas best surface integrities are achievable.

Gap width variations are not as critical for the machining process for straight cuts as at filigree radii.

Acknowledgements

The authors acknowledge the support of the European Commission in the 7th Framework Programme Transport including Aeronautics call, project number 234325 - ADMAP-GAS.

References

[1] Klocke F, Klink A, Veselovac D, Aspinwall DK, Soo SL, Schmidt M, Schilp J, Levy G, Kruth JP. Turbomachinery component manufacture by application of electrochemical, electro-physical and photonic processes. In CIRP Annals - Manufacturing Technology, in press, 2014

[2] Klocke F, Welling D, Klink A, Veselovac D, Nöthe T, Perez R. Evaluation of Advanced Wire-EDM Capabilities for the Manufacture of

Fir Tree Slots in Inconel 718. In: Procedia CIRP, Volume 14, p. 430–435, 2014

[3] Curtis DT, Soo SL, Aspinwall DK, Huber C, Fuhlendorf J, Grimm A. Production of complex blade mounting slots in turbine disks using novel machining techniques. In: Proceedings of the 3rd International CIRP HPC Conference Dublin, Ireland, vol. 1 pp. 219–228. 2008

[4] Aspinwall DK, Soo SL, Berrisford AE, Walder G. Workpiece surface roughness and integrity after WEDM of Ti–6Al–4V and Inconel 718 using minimum damage generator technology. In: CIRP Annals - Manufacturing Technology 57 (1), S. 187–190. 2008

[5] Li L, Guo YB, Wei XT, Li W. Surface Integrity Characteristics in Wire-EDM of Inconel 718 at Different Discharge Energy. In: Procedia CIRP, vol. 6, p. 220–225, 2013

[6] Klocke F, Welling D, Dieckmann J, Veselovac D, Perez R. Developments in Wire-EDM for the manufacturing of fir tree slots in turbine discs made of Inconel 718. In: Key Engineering Materials (1665), p. 1177–1182. 2012

[7] Welling D. Results of Surface Integrity and Fatigue Study of Wire-EDM Compared to Broaching and Grinding for Demanding Jet Engine Components Made of Inconel 718. In: Procedia CIRP, Volume 13, 2014, p. 339-344

[8] Field, M. Surface Integrity – A New Requirement for Improving Reliability of Aerospace Hardware; 18th Annual National SAMPE Symposium, 1973

[9] Teti R, Jemielniak K, O’Donnell G, Dornfeld D. Advanced monitoring of machining operations. In: CIRP Annals - Manufacturing Technology 59, 2010, p. 717–739

[10] Yan MT, Liao YS. Adaptive control of the WEDM process using the fuzzy control strategy. In: Journal of Manufacturing Systems, Vol. 17, Issue 4, p. 263–274, 1998

[11] Lauwers B, Kruth JP, Bleys P, Van Coppenolle B, Stenvens L, Derighetti R. Wire rupture prevention using on-line pulse localisation in WEDM. In: Proc. of ISEM X, p. 410-416, 1992

[12] Yan MT, Chien HAT. Monitoring and control of the micro wire-EDM process. In: International Journal of Machine Tools and Manufacture, Vol. 47, Issue 1, p. 148-157, 2007

[13] Cabanes I, Portillo E, Marcos M, Sánchez JA. An industrial application for on-line detection of instability and wire breakage in wire EDM. In: Journal of Materials Processing Technology, vol. 195, Issues 1–3, p. 101–109, 2008

[14] Watanabe H, Sato T, SuzukiI, Kinoshita N. WEDM Monitoring with a Statistical Pulse-Classification Method. In: CIRP Annals – Manufacturing Technology, Volume 39, Issue 1, p. 175–178, 1990

[15] Kunieda M, Lauwers B, Rajurkar KP, Schumacher BM. Advancing EDM through Fundamental Insight into the Process. In: CIRP Annals - Manufacturing Technology, Vol. 54, Issue 2, p. 64–87, 2005

[16] Klocke F, Veselovac D, Gierlings S, Auerbach T. Prozessüberwachung für zertifizierte Fertigungsprozesses in der Luftfahrtindustrie. In: ZWF Jahrg. 107 (2012), Carl Hanserverlag, München, p. 7-8

[17] Pejryd L, Beno T, Isaksson M. Machinig aerospace materials with room-temperature and cooled minimal- quantity cutting fluids. Proc. ImechE Vol. 225, Part B: J. Engineering Man. 2011

[18] Rahman M, Seah WKH, Teo TT. The machinability of Inconel 718. Journal of Material Processing Technology 63, pp. 199 - 204. 1997

[19] Ezugwu EO, Wang ZM, Machado AR. The machinability of nickel-based alloys: a review. In: Journal of Materials Processing Technology 86, pp. 1 – 16. 1999

[20] Klocke F, König W. Fertigungsverfahren 3 – Abtragen, Generieren und Lasermaterialbearbeitung. Springer 2007, 4. Auflage

[21] Panschow R. Über die Kräfte und ihre Wirkungen beim elektroerosiven Schneiden mit Drahtelektrode, Dissertation, Technischen Universität Hannover, 1974

200 μm 50 μm

Surface defectsArea 2 a)

200 μm 50 μm

Surface defectsArea 4 b)

200 μm 50 μm

Area 5 c)