- Email: [email protected]
Surface Integrity Evaluation of Modified EDM Surface Structure
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 308 – 312
2018, Bilbao, Spain 19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2017,
Surface Integrity Evaluation of Modified EDM Surface Structure Mohammad Antara*, Phillip Haywarda, Justin Dunleaveya, Paul Butler-Smithb a
The Manufacturing Technology Centre Ltd, Ansty Business Park, Coventry CV7 9JU, United Kingdom b University of Nottingham, University Park, Nottingham, NG9 2SH, United Kingdom
* Corresponding author. Tel.: +442476701716; E-mail address: [email protected]
Abstract
Surface engineering, through either texturing or surface modification, for enhanced functionality is currently one of the most emerging research topic/technologies. This is mainly related to its huge commercial potential in terms of added value or product customisation, with applications extending from tooling, to mould & die, medical, food & drink, aerospace and others. With the recent advances in EDM generator design and process control, a number of commercial solutions were released by leading technology providers where it is deemed possible to apply a certain surface finish/structure, resulting in enhanced functionality (e.g. reduced friction or reduced/controlled adhesion) via the EDM process itself and without the need for a secondary operation. This paper investigates the surface integrity effects for this modified surface structuring concept (3DS). Experimental results showed that it was possible to reduce the friction coefficient (consequently ejection forces in the injection moulding process, for example) by as much as 60%, compared to samples machined using standard EDM settings while both having similar Ra values. 3DS samples however showed slightly lower fatigue performance which is likely to be the result of surface micro-cracking which appeared to be more prominent on these, compared to the standard EDM samples. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2018 2018The 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; Surface Engineering; Friciton; Fatigue
1. Introduction Surface texture has become an important aspect of product design in a number of industries. Not only does it provide the means to refine the physical appearance of an item, but it also allows a company to influence users’ tactile impressions of their products. Therefore, it is of no surprise that mould makers are increasingly seeking ways to improve the level of control exerted in surface texturing. Since its development in the 1940s, EDM has become an established method for applications like mould and die which require a high level of accuracy and surface quality [1]. However, in some cases post-EDM treatments are employed which usually involve smoothing or polishing of the mould surface for lower adhesion of the injected media, aesthetic
effects or some combination of both. A specific need for control over surface finish is to form surfaces that will allow suitable ease in releasing the formed component from the mould. In an automated or partly-automated injection moulding production setting ejector pins are used to remove the cooled production from the mould encasing – a process known as demoulding. Recent developments in EDM generator technology have enabled full manipulation and control of pulse profile and spatial discharge phenomena with concomitant benefits to productivity, workpiece quality and integrity [2]. These developments have allowed manipulation of the “standard” EDM crater topology and the formation of laterally enlarged, vertically flattened craters while maintaining the same average roughness (Ra) value. This can potentially lead to introducing surface functionalities such as reduced friction, improved wear resistance etc. without the need for a secondary operation, with applications extending beyond mould making
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.2017.12.069
309
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
into a wide range of industrial sectors. For instance, orthopedic implants are subject to numerous considerations when selecting and optimising a manufacturing technique/method. In addition to the standard mechanical performance requirements such as tensile strength, formability and endurance/fatigue, there is a major consideration for biocompatibility with aspects including corrosion resistance, cell adhesion and osseointegration. In this vein, a number of studies [3-6] examined the suitability of EDM machined metallic implants as an alternative to polymers. Here, EDM offers the possibility of machining intricate, repeated features for bone matter adhesion with the advantage of limited fixturing/cutting forces and high precision. For these components, there is also a requirement for generating an oxide which would act as a ceramic-like/chemically inert layer to reduce the possibility of undesired further chemical reactions on the surface. Due to the nature of the process, EDM would complement existing processes currently used to achieve this such as plasma electrolytic conversion (PEC). A study by Strasky et al [5] suggested that EDM surfaces provide an even better substrate for bone-derived cell adhesion compared to those plasma sprayed (TiO2) with ~50% more cells/cm2 formed on the former. Therefore due to a lack of a complete metrological definition of these engineered surfaces in relation to functional performance and with the increasing of various industrial requirements for functional surfaces and the development of new machining concepts to achieve this, it has become apparent that more comprehensive surface characterisation tools and standards, (i.e. rather than the simplistic, traditional Ra) are necessary. A clear example is shown in Figure 1 where both surfaces have the same Ra (1.6µm) value even though the surface structures vary significantly.
topography to gain a better understanding of the effect of these technology trends (i.e. modified crater morphology) surface quality, friction behavior and fatigue performance. 2. Experimental Work 2.1. Material and equipment Workpiece material was annealed tool steal (AISI Type 01 Gauge Plate) with bulk hardness of 218HB. Chemical composition is provided in Table 1. Table 1. Workpiece material composition by mass percentage.
Element %
C 0.851.05
Mn 1.001.40
W 0.400.60
Cr 0.400.6
Va 0.150.30
Fe Bal.
All experimental trials were carried out at the Manufacturing Technology Centre using a GF AgieCharmilles Form 3000 VHP machine. An isotropic superfine particle graphite POCO EDM 3 Electrodes (50X10mm) were utilised. Dielectric was IonoPlus IME-MH supplied by Oelheld GmbH. Surface roughness measurements (both 2D and 3D) were taken using an Alicona InfiniteFocus optical roughness measurement system. EDM treated surfaces were scanned and the output for a range of roughness characteristics was gathered. Friction trials were undertaken on a Bruker Tribometer based at the University of Nottingham. A 6mm diameter hardened chromium steel ball to AISA 490C was used. Tests were run for 10 minutes in dry contact conditions using a 10N load, 5mm stroke and 1Hz frequency. Fatigue cycling was carried out using a RUMUL Testronic Fatigue resonance testing machine. 2.2. Test procedure and operating parameters
Figure 1: EDM surfaces with similar Ra values and different topographies
To quantify this, other surface parameters including Rsm and Rdq can be used. Rsm is the mean width of the profile double curve element along a specific sampling length, while Rdq describes the root-mean-square value of the ‘tile’ of surface features. In EDM surface terms, Rsm is used to describe the crater’s width while and Rdq is for the crater’s effective flatness. In a recent paper by Klink et al [7] the authors have thoroughly analysed modern EDM surface structures established by various technology providers using current and alternative surface measurement standards and highlighted the needs for adopting these at a wider industrial scale. This paper complements the published research in this area by covering other surface integrity aspects beyond surface
Two sets of test coupons were manufactured using a) standard EDM die-sinking strategy (STD) and b) modified strategy (3DS). Both settings were pre-defined by the technology provider for tool steel (the specified test material). The operating parameters were selected to achieve a target Ra value of 1.4µm for both settings. Machining depth was set to 0.5mm for all samples. This was achieved through a series of iterations (passes) where the machining parameters vary for each iteration (typically starting with high level at the first pass and ending with low level in the final one). The range for each of the key parameter is given in the Table 2. The total machining times were ~18 minutes and ~12 minutes for the STD and 3DS samples respectively. Selected workpiece samples were sectioned and subsequently set in bakelite using a Buehler Ltd. mounting press and then ground and polished. Samples preparation was done according to ASTM E3-11 Standard and Kalling’s reagent (33ml HCL, 33ml EOH, 33ml H2O and 1.5g CuCl2) was used to reveal the recast layer. A Zeiss Imager M2m digital microscope was used to examine the recast layer and subsurface microstructure.
310
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
Table 2. EDM operating parameters
Parameter
Unit
Current Pulse Duration Off-time Voltage Approach speed Withdrawal speed
A µs µs V mm /min mm /min
STD Min 26.0 7.5 70.2
3DS Max 2.5 56.0
Min 19.0 10.6
Max 5.6 132.8
11.2
24.0 140
10.0 100
100 6000
1600
5900
1600
1000 1000
(1.4 µm) with a slightly smoother surfaces for the 3DS samples. On the other hand, higher Rsm values associated with the 3DS samples reflect the increased crater width whereas the lower Rdq values represents the increased flatness. This can also be inferred from the lower Rz and Rt values for 3DS compared to STD samples. Figure 3 shows the 3D scans of both samples which reveal the significant variation in surface morphology despite the similar Ra values where larger, shallower craters can be observed on the 3DS sample. Table 3. Surface roughness measurements
600
For fatigue performance evaluation, sample geometry was set to 10 mm x 10 mm x 50 mm. The test face (face in maximum tensile stress during cyclic loading) was produced by die-sink EDM using both STD and 3DS settings while the three remaining faces were produced using conventional machining methods. Specimens were tested at ambient temperature using a three point bending arrangement, see Figure 2. Frequencies of oscillation ranged from 78-88 Hz. Stresses induced were calculated using the bending stress equation below:
Parameter
STD
3DS
Ra (µm)
1.37
1.31
Rt (µm)
13.83
11.34
Rz (µm)
10.17
8.74
Rsm (µm)
151.6
201.6
Rdq
0.19
0.13
(1) Where ߪ݉ܽ ݔis the maximum stress level, Pmax is the maximum applied load, L is the distance between supports, b is specimen width and t is specimen height.
Figure 2. (a) Fatigue testing setup (b) fatigue specimens
Fatigue run-out was set to 1,000,000 cycles and stress ratio (R) was set to 0.1. Stress ratio describes the relationship between the maximum and minimum applied loads during repeated cycling loadings. 3. Results Table 3 shows the average surface roughness parameters values (8 different measurements) for STD and 3DS samples which includes average roughness (Ra), Maximum peak to valley height of roughness (Rt), Mean peak to valley height of roughness (Rz), Mean spacing of profile irregularities (Rsm) and Root-mean-square slope of roughness (Rdq). The Ra values were very close to the target as set by the machine-tool
Figure 3. Surface morphology for samples machined using STD and 3DS settings
Attempts to mathematically characterise the ejection force for injection moulded parts have been made, mostly based around the following equation [8]: ܨோ ൌ ݂ ൈ ൈ ܣ
(2)
Where FR is the ejection force, f is the coefficient of friction between the mould and the part, PA is the contact pressure between the mould and part and A is the area of contact. Various other forms and equations also exist to reflect different components’ shapes and geometries, yet for all of these the
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
friction coefficient is perhaps the most significant factor [9-10]. Any reduction or increase in the coefficient of friction has a direct and proportional effect on the required demoulding force. In other words, a 20% reduction in this factor would theoretically lead to a 20% reduction in the overall output variable (i.e. ejection force) and vice-versa. Figure 4 shows friction coefficient measurements for both test samples which were undertaken for 10 minutes on a Bruker Tribometer using a 6mm diameter hardened chromium steel ball. The results reveal a significant (~60%) reduction in friction coefficient for the 3DS samples compared to the standard one despite the similar Ra values. This was in line with a study by Ivkovic et al [11] who reported a large variance between friction coefficients for processed surfaces having similar Ra values using different operations (milling, lapping and polishing). This suggests that the surface microgeometry has a considerable influence in addition to the overall roughness, yet does not necessarily affect the working assumption that higher surface roughness will lead to increased friction coefficients between surfaces in contact.
311
methodically analyse fatigue data. When cycles to failure and applied stresses are both plotted on logarithmic X and Y axes, a linear relation between the values can be described using simple linear regression. (BS 2864, 1976.) (ASTM E-739-91, 2004), see Figure 6. The graph also shows the equation of the linear regression and the coefficient of determination (R2, value range of 0 to 1), which indicate the relative strength of the linear relationship between the variables. For all groups tested, there was a negative correlation between applied stress and cycles to failure, which would indicate a negative R2 value. As negative correlation is the norm for this relation, absolute values are displayed on the graphs. Correlations greater than 0.8 in value are generally considered strong, whereas values less than 0.5 are considered weak. The regression line can be used to predict performance at other stress levels within the linear region, assuming that a sufficient number of tests have been carried out prior to applying regression.
Figure 6. Log10 fatigue curves for STD and 3DS samples
Figure 4. Friction test (coefficient) measurements for STD and 3DS samples
Figure 5 shows the S-N curves for the STD and 3DS specimens. The results show similar plot points for both groups at the highest applied stress level, however for lower stress values the variation is more noticeable. At intermediate stress levels, a shift is evident in the response for STD suggesting a better fatigue life. The fatigue run out of 1x106 cycles was recorded at ~0.68 of the ultimate strength for STD specimens, while the ratio was ~0.6 for 3DS samples. The S-N curves of both also suggest low variation in the number of cycles to failure at the same stress levels, indicating consistency of the machining operations. Regression analysis can be used to more
Figure 5. S-N fatigue curves for STD and 3DS samples
Figure 7 shows SEM micrographs for top (machined) surface of STD and 3DS fatigue specimens (prior to testing). Surface micro-cracking can clearly be seen on the 3DS samples which may have contributed to the lower fatigue performance compared to the STD samples where very few cracks were observed. This may be due to the variation in operating parameters/conditions between the 2 machining strategies, where in 3DS pulse duration is a lot longer than that of STD even though peak current gradually drops down to low levels shortly after spark initiation in the former. Further EDX analyses were undertaken to assess the dark spots on the 3DS sample. The results revealed the presence of high level of carbon in those regions which is very likely to be depleted from the graphite electrode as the polarity is switched towards the end of the machining operation (which only occurs in the 3DS settings).
312
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
x
x
x
x
Figure 7. SEM surface micrographs for machined samples
Figure 8 shows cross-sectional SEM micrographs for STD and 3DS samples. A thin, mostly continuous recast layer (less than 10µm) can be observed in both cross-sections with relatively small globules also present. Micro-cracks are evident in the 3DS sample even though they were confined within the recast layer and seemed not to extend into the parent material (in all assessed samples). No signs of heat affected zones or microstructural changes were noticed underneath the recast layer.
Figure 8. SEM cross-sectional micrographs for machined samples
4. Conclusions Recent advances in EDM generator technology have enabled full control over the discharge phenomena which, therefore, enabled much better control over the process itself and the structure of the treated surfaces. This primarily involves manipulating the pulse profile, along with other key process parameters, which results in a modified crater morphology. From the experimental work undertaken in this research the following conclusions could be drawn: x Two different surface structures, yet with the same average roughness (Ra) values were achieved mainly by altering current slope and pulse time. This highlights the need for more comprehensive surface characterization methodologies and standards. In depth analysis and suggestions on this specific point were covered in a recent research by Klink et al [7].
x
Tribology tests showed that the modified surface structure (enlarged and flatter craters – denoted by 3DS) resulted in significantly lower friction coefficient compared to the standard EDM surface. This may be attributed to the “smoother” surface, featuring shallower peaks/craters in the 3DS samples. SEM analysis revealed the presence of significantly more surface micro-cracks on the 3DS samples compared to the STD ones. One likely reason could be the longer pulse on-time and shorter off-time (i.e. thermal cycle. The presence of surface micro-cracking seemed to have a direct effect on the fatigue performance of the 3DS samples as the endurance limit was ~10% lower than that of STD samples where much fewer cracks were observed. Cross-sectional analysis of both samples (STD and 3DS) revealed the presence of thin (5-10µm) layer of recast. Surface micro-cracking were mainly confined within the recast layer and no heat affected zones were evident underneath that. Further work is planned using copper electrode material to assess the impact on the surface integrity (particularly fatigue performance and surface microcracking) on the machined samples
References [1] Simao J, Lee H G, Aspinwall D K, Dewes R C, Aspinwall E M, Workpiece surface modification using electrical discharge machining, International Journal of Machine Tools and Manufacture, Volume 43, Issue 2, 2003, Pages 121-128. [2] Antar M T, Soo S L, Aspinwall D K, Cuttell M, Perez R., and Winn, A.J. WEDM of Aerospace Alloys Using ‘Clean Cut’ Generator Technology, Proceedings of the 16th International Symposium on Electromachining (ISEM XVI), Shanghai, 2010, Pages 285-290 [3] Klocke, F., Scwade, M., Welling, D., & Kopp, A., Multi-scale directed surface topography machined by electro discharge machining in combination with plasma electrolytic conversion for improved osseointegration. International Journal of Mechatronics and Manufacturing Systems 6(3), 2013, Pages 254-269 [4] Nayab, S., Shinawi, L., Hobkirk, J., Tate, T. J., Olsen , I., & Jones , F. H., Adhesion of bone cells to ion-implanted titanium. Journal of Materials Cience: Materials in Medicine, 2003, Pages 991-997. [5] Strasky, J., Janecek, M., & Harcuba , P.,Electric Discharge Machining of Ti-6Al-4V Alloy for Biomedical Use. WDS'11 Proceedings of Contributed Papers, 2011, Pages127-131 [6] Novaes Jr, A. B., de Souza, S. L., de Barros, R. M., Pereira, K. Y., Iezzi, G., & Piatelli, A., Influence of Implant Surfaces on Osseointegration. Braz Dent J 21(6), 2010, Pages 471-481 [7] Klink A, Holsten M, Hensgen L, Crater morphology evaluation of contemporary advanced EDM generator technology, CIRP Annals Manufacturing Technology, Volume 66, Issue 1, 2017, Pages 197-200 [8] Kinsella, M. E.,Ejection forces and static friction coefficients for rapid tooled injection mold inserts, Dissertaion submitted for Doctoral Thesis. 2004,Ohio State University [9] Menges, G., & Bangert, H. Measurement of Coefficients of Static Friction as a Means of Determining Opening and Demoulding Forces in Injection Moulds. Kunststoffe, vol. 71,1981. Pages 552-557 [10] Burke , C., & Malloy, R. An Experimental Study of the Ejection Forces Encountered During Injection Molding. ANTEC, 1991, Pages 1781-1787. [11] Ivkovic, B., Djukdjanvic, M., & Stamenkovic, D. The Influence of the Contact Surface Roughness on the Static Friction Coefficient. Tribology in industry, Volume 22, 2000
ScienceDirect Procedia CIRP 68 (2018) 308 – 312
2018, Bilbao, Spain 19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2017,
Surface Integrity Evaluation of Modified EDM Surface Structure Mohammad Antara*, Phillip Haywarda, Justin Dunleaveya, Paul Butler-Smithb a
The Manufacturing Technology Centre Ltd, Ansty Business Park, Coventry CV7 9JU, United Kingdom b University of Nottingham, University Park, Nottingham, NG9 2SH, United Kingdom
* Corresponding author. Tel.: +442476701716; E-mail address: [email protected]
Abstract
Surface engineering, through either texturing or surface modification, for enhanced functionality is currently one of the most emerging research topic/technologies. This is mainly related to its huge commercial potential in terms of added value or product customisation, with applications extending from tooling, to mould & die, medical, food & drink, aerospace and others. With the recent advances in EDM generator design and process control, a number of commercial solutions were released by leading technology providers where it is deemed possible to apply a certain surface finish/structure, resulting in enhanced functionality (e.g. reduced friction or reduced/controlled adhesion) via the EDM process itself and without the need for a secondary operation. This paper investigates the surface integrity effects for this modified surface structuring concept (3DS). Experimental results showed that it was possible to reduce the friction coefficient (consequently ejection forces in the injection moulding process, for example) by as much as 60%, compared to samples machined using standard EDM settings while both having similar Ra values. 3DS samples however showed slightly lower fatigue performance which is likely to be the result of surface micro-cracking which appeared to be more prominent on these, compared to the standard EDM samples. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2018 2018The 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; Surface Engineering; Friciton; Fatigue
1. Introduction Surface texture has become an important aspect of product design in a number of industries. Not only does it provide the means to refine the physical appearance of an item, but it also allows a company to influence users’ tactile impressions of their products. Therefore, it is of no surprise that mould makers are increasingly seeking ways to improve the level of control exerted in surface texturing. Since its development in the 1940s, EDM has become an established method for applications like mould and die which require a high level of accuracy and surface quality [1]. However, in some cases post-EDM treatments are employed which usually involve smoothing or polishing of the mould surface for lower adhesion of the injected media, aesthetic
effects or some combination of both. A specific need for control over surface finish is to form surfaces that will allow suitable ease in releasing the formed component from the mould. In an automated or partly-automated injection moulding production setting ejector pins are used to remove the cooled production from the mould encasing – a process known as demoulding. Recent developments in EDM generator technology have enabled full manipulation and control of pulse profile and spatial discharge phenomena with concomitant benefits to productivity, workpiece quality and integrity [2]. These developments have allowed manipulation of the “standard” EDM crater topology and the formation of laterally enlarged, vertically flattened craters while maintaining the same average roughness (Ra) value. This can potentially lead to introducing surface functionalities such as reduced friction, improved wear resistance etc. without the need for a secondary operation, with applications extending beyond mould making
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.2017.12.069
309
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
into a wide range of industrial sectors. For instance, orthopedic implants are subject to numerous considerations when selecting and optimising a manufacturing technique/method. In addition to the standard mechanical performance requirements such as tensile strength, formability and endurance/fatigue, there is a major consideration for biocompatibility with aspects including corrosion resistance, cell adhesion and osseointegration. In this vein, a number of studies [3-6] examined the suitability of EDM machined metallic implants as an alternative to polymers. Here, EDM offers the possibility of machining intricate, repeated features for bone matter adhesion with the advantage of limited fixturing/cutting forces and high precision. For these components, there is also a requirement for generating an oxide which would act as a ceramic-like/chemically inert layer to reduce the possibility of undesired further chemical reactions on the surface. Due to the nature of the process, EDM would complement existing processes currently used to achieve this such as plasma electrolytic conversion (PEC). A study by Strasky et al [5] suggested that EDM surfaces provide an even better substrate for bone-derived cell adhesion compared to those plasma sprayed (TiO2) with ~50% more cells/cm2 formed on the former. Therefore due to a lack of a complete metrological definition of these engineered surfaces in relation to functional performance and with the increasing of various industrial requirements for functional surfaces and the development of new machining concepts to achieve this, it has become apparent that more comprehensive surface characterisation tools and standards, (i.e. rather than the simplistic, traditional Ra) are necessary. A clear example is shown in Figure 1 where both surfaces have the same Ra (1.6µm) value even though the surface structures vary significantly.
topography to gain a better understanding of the effect of these technology trends (i.e. modified crater morphology) surface quality, friction behavior and fatigue performance. 2. Experimental Work 2.1. Material and equipment Workpiece material was annealed tool steal (AISI Type 01 Gauge Plate) with bulk hardness of 218HB. Chemical composition is provided in Table 1. Table 1. Workpiece material composition by mass percentage.
Element %
C 0.851.05
Mn 1.001.40
W 0.400.60
Cr 0.400.6
Va 0.150.30
Fe Bal.
All experimental trials were carried out at the Manufacturing Technology Centre using a GF AgieCharmilles Form 3000 VHP machine. An isotropic superfine particle graphite POCO EDM 3 Electrodes (50X10mm) were utilised. Dielectric was IonoPlus IME-MH supplied by Oelheld GmbH. Surface roughness measurements (both 2D and 3D) were taken using an Alicona InfiniteFocus optical roughness measurement system. EDM treated surfaces were scanned and the output for a range of roughness characteristics was gathered. Friction trials were undertaken on a Bruker Tribometer based at the University of Nottingham. A 6mm diameter hardened chromium steel ball to AISA 490C was used. Tests were run for 10 minutes in dry contact conditions using a 10N load, 5mm stroke and 1Hz frequency. Fatigue cycling was carried out using a RUMUL Testronic Fatigue resonance testing machine. 2.2. Test procedure and operating parameters
Figure 1: EDM surfaces with similar Ra values and different topographies
To quantify this, other surface parameters including Rsm and Rdq can be used. Rsm is the mean width of the profile double curve element along a specific sampling length, while Rdq describes the root-mean-square value of the ‘tile’ of surface features. In EDM surface terms, Rsm is used to describe the crater’s width while and Rdq is for the crater’s effective flatness. In a recent paper by Klink et al [7] the authors have thoroughly analysed modern EDM surface structures established by various technology providers using current and alternative surface measurement standards and highlighted the needs for adopting these at a wider industrial scale. This paper complements the published research in this area by covering other surface integrity aspects beyond surface
Two sets of test coupons were manufactured using a) standard EDM die-sinking strategy (STD) and b) modified strategy (3DS). Both settings were pre-defined by the technology provider for tool steel (the specified test material). The operating parameters were selected to achieve a target Ra value of 1.4µm for both settings. Machining depth was set to 0.5mm for all samples. This was achieved through a series of iterations (passes) where the machining parameters vary for each iteration (typically starting with high level at the first pass and ending with low level in the final one). The range for each of the key parameter is given in the Table 2. The total machining times were ~18 minutes and ~12 minutes for the STD and 3DS samples respectively. Selected workpiece samples were sectioned and subsequently set in bakelite using a Buehler Ltd. mounting press and then ground and polished. Samples preparation was done according to ASTM E3-11 Standard and Kalling’s reagent (33ml HCL, 33ml EOH, 33ml H2O and 1.5g CuCl2) was used to reveal the recast layer. A Zeiss Imager M2m digital microscope was used to examine the recast layer and subsurface microstructure.
310
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
Table 2. EDM operating parameters
Parameter
Unit
Current Pulse Duration Off-time Voltage Approach speed Withdrawal speed
A µs µs V mm /min mm /min
STD Min 26.0 7.5 70.2
3DS Max 2.5 56.0
Min 19.0 10.6
Max 5.6 132.8
11.2
24.0 140
10.0 100
100 6000
1600
5900
1600
1000 1000
(1.4 µm) with a slightly smoother surfaces for the 3DS samples. On the other hand, higher Rsm values associated with the 3DS samples reflect the increased crater width whereas the lower Rdq values represents the increased flatness. This can also be inferred from the lower Rz and Rt values for 3DS compared to STD samples. Figure 3 shows the 3D scans of both samples which reveal the significant variation in surface morphology despite the similar Ra values where larger, shallower craters can be observed on the 3DS sample. Table 3. Surface roughness measurements
600
For fatigue performance evaluation, sample geometry was set to 10 mm x 10 mm x 50 mm. The test face (face in maximum tensile stress during cyclic loading) was produced by die-sink EDM using both STD and 3DS settings while the three remaining faces were produced using conventional machining methods. Specimens were tested at ambient temperature using a three point bending arrangement, see Figure 2. Frequencies of oscillation ranged from 78-88 Hz. Stresses induced were calculated using the bending stress equation below:
Parameter
STD
3DS
Ra (µm)
1.37
1.31
Rt (µm)
13.83
11.34
Rz (µm)
10.17
8.74
Rsm (µm)
151.6
201.6
Rdq
0.19
0.13
(1) Where ߪ݉ܽ ݔis the maximum stress level, Pmax is the maximum applied load, L is the distance between supports, b is specimen width and t is specimen height.
Figure 2. (a) Fatigue testing setup (b) fatigue specimens
Fatigue run-out was set to 1,000,000 cycles and stress ratio (R) was set to 0.1. Stress ratio describes the relationship between the maximum and minimum applied loads during repeated cycling loadings. 3. Results Table 3 shows the average surface roughness parameters values (8 different measurements) for STD and 3DS samples which includes average roughness (Ra), Maximum peak to valley height of roughness (Rt), Mean peak to valley height of roughness (Rz), Mean spacing of profile irregularities (Rsm) and Root-mean-square slope of roughness (Rdq). The Ra values were very close to the target as set by the machine-tool
Figure 3. Surface morphology for samples machined using STD and 3DS settings
Attempts to mathematically characterise the ejection force for injection moulded parts have been made, mostly based around the following equation [8]: ܨோ ൌ ݂ ൈ ൈ ܣ
(2)
Where FR is the ejection force, f is the coefficient of friction between the mould and the part, PA is the contact pressure between the mould and part and A is the area of contact. Various other forms and equations also exist to reflect different components’ shapes and geometries, yet for all of these the
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
friction coefficient is perhaps the most significant factor [9-10]. Any reduction or increase in the coefficient of friction has a direct and proportional effect on the required demoulding force. In other words, a 20% reduction in this factor would theoretically lead to a 20% reduction in the overall output variable (i.e. ejection force) and vice-versa. Figure 4 shows friction coefficient measurements for both test samples which were undertaken for 10 minutes on a Bruker Tribometer using a 6mm diameter hardened chromium steel ball. The results reveal a significant (~60%) reduction in friction coefficient for the 3DS samples compared to the standard one despite the similar Ra values. This was in line with a study by Ivkovic et al [11] who reported a large variance between friction coefficients for processed surfaces having similar Ra values using different operations (milling, lapping and polishing). This suggests that the surface microgeometry has a considerable influence in addition to the overall roughness, yet does not necessarily affect the working assumption that higher surface roughness will lead to increased friction coefficients between surfaces in contact.
311
methodically analyse fatigue data. When cycles to failure and applied stresses are both plotted on logarithmic X and Y axes, a linear relation between the values can be described using simple linear regression. (BS 2864, 1976.) (ASTM E-739-91, 2004), see Figure 6. The graph also shows the equation of the linear regression and the coefficient of determination (R2, value range of 0 to 1), which indicate the relative strength of the linear relationship between the variables. For all groups tested, there was a negative correlation between applied stress and cycles to failure, which would indicate a negative R2 value. As negative correlation is the norm for this relation, absolute values are displayed on the graphs. Correlations greater than 0.8 in value are generally considered strong, whereas values less than 0.5 are considered weak. The regression line can be used to predict performance at other stress levels within the linear region, assuming that a sufficient number of tests have been carried out prior to applying regression.
Figure 6. Log10 fatigue curves for STD and 3DS samples
Figure 4. Friction test (coefficient) measurements for STD and 3DS samples
Figure 5 shows the S-N curves for the STD and 3DS specimens. The results show similar plot points for both groups at the highest applied stress level, however for lower stress values the variation is more noticeable. At intermediate stress levels, a shift is evident in the response for STD suggesting a better fatigue life. The fatigue run out of 1x106 cycles was recorded at ~0.68 of the ultimate strength for STD specimens, while the ratio was ~0.6 for 3DS samples. The S-N curves of both also suggest low variation in the number of cycles to failure at the same stress levels, indicating consistency of the machining operations. Regression analysis can be used to more
Figure 5. S-N fatigue curves for STD and 3DS samples
Figure 7 shows SEM micrographs for top (machined) surface of STD and 3DS fatigue specimens (prior to testing). Surface micro-cracking can clearly be seen on the 3DS samples which may have contributed to the lower fatigue performance compared to the STD samples where very few cracks were observed. This may be due to the variation in operating parameters/conditions between the 2 machining strategies, where in 3DS pulse duration is a lot longer than that of STD even though peak current gradually drops down to low levels shortly after spark initiation in the former. Further EDX analyses were undertaken to assess the dark spots on the 3DS sample. The results revealed the presence of high level of carbon in those regions which is very likely to be depleted from the graphite electrode as the polarity is switched towards the end of the machining operation (which only occurs in the 3DS settings).
312
Mohammad Antar et al. / Procedia CIRP 68 (2018) 308 – 312
x
x
x
x
Figure 7. SEM surface micrographs for machined samples
Figure 8 shows cross-sectional SEM micrographs for STD and 3DS samples. A thin, mostly continuous recast layer (less than 10µm) can be observed in both cross-sections with relatively small globules also present. Micro-cracks are evident in the 3DS sample even though they were confined within the recast layer and seemed not to extend into the parent material (in all assessed samples). No signs of heat affected zones or microstructural changes were noticed underneath the recast layer.
Figure 8. SEM cross-sectional micrographs for machined samples
4. Conclusions Recent advances in EDM generator technology have enabled full control over the discharge phenomena which, therefore, enabled much better control over the process itself and the structure of the treated surfaces. This primarily involves manipulating the pulse profile, along with other key process parameters, which results in a modified crater morphology. From the experimental work undertaken in this research the following conclusions could be drawn: x Two different surface structures, yet with the same average roughness (Ra) values were achieved mainly by altering current slope and pulse time. This highlights the need for more comprehensive surface characterization methodologies and standards. In depth analysis and suggestions on this specific point were covered in a recent research by Klink et al [7].
x
Tribology tests showed that the modified surface structure (enlarged and flatter craters – denoted by 3DS) resulted in significantly lower friction coefficient compared to the standard EDM surface. This may be attributed to the “smoother” surface, featuring shallower peaks/craters in the 3DS samples. SEM analysis revealed the presence of significantly more surface micro-cracks on the 3DS samples compared to the STD ones. One likely reason could be the longer pulse on-time and shorter off-time (i.e. thermal cycle. The presence of surface micro-cracking seemed to have a direct effect on the fatigue performance of the 3DS samples as the endurance limit was ~10% lower than that of STD samples where much fewer cracks were observed. Cross-sectional analysis of both samples (STD and 3DS) revealed the presence of thin (5-10µm) layer of recast. Surface micro-cracking were mainly confined within the recast layer and no heat affected zones were evident underneath that. Further work is planned using copper electrode material to assess the impact on the surface integrity (particularly fatigue performance and surface microcracking) on the machined samples
References [1] Simao J, Lee H G, Aspinwall D K, Dewes R C, Aspinwall E M, Workpiece surface modification using electrical discharge machining, International Journal of Machine Tools and Manufacture, Volume 43, Issue 2, 2003, Pages 121-128. [2] Antar M T, Soo S L, Aspinwall D K, Cuttell M, Perez R., and Winn, A.J. WEDM of Aerospace Alloys Using ‘Clean Cut’ Generator Technology, Proceedings of the 16th International Symposium on Electromachining (ISEM XVI), Shanghai, 2010, Pages 285-290 [3] Klocke, F., Scwade, M., Welling, D., & Kopp, A., Multi-scale directed surface topography machined by electro discharge machining in combination with plasma electrolytic conversion for improved osseointegration. International Journal of Mechatronics and Manufacturing Systems 6(3), 2013, Pages 254-269 [4] Nayab, S., Shinawi, L., Hobkirk, J., Tate, T. J., Olsen , I., & Jones , F. H., Adhesion of bone cells to ion-implanted titanium. Journal of Materials Cience: Materials in Medicine, 2003, Pages 991-997. [5] Strasky, J., Janecek, M., & Harcuba , P.,Electric Discharge Machining of Ti-6Al-4V Alloy for Biomedical Use. WDS'11 Proceedings of Contributed Papers, 2011, Pages127-131 [6] Novaes Jr, A. B., de Souza, S. L., de Barros, R. M., Pereira, K. Y., Iezzi, G., & Piatelli, A., Influence of Implant Surfaces on Osseointegration. Braz Dent J 21(6), 2010, Pages 471-481 [7] Klink A, Holsten M, Hensgen L, Crater morphology evaluation of contemporary advanced EDM generator technology, CIRP Annals Manufacturing Technology, Volume 66, Issue 1, 2017, Pages 197-200 [8] Kinsella, M. E.,Ejection forces and static friction coefficients for rapid tooled injection mold inserts, Dissertaion submitted for Doctoral Thesis. 2004,Ohio State University [9] Menges, G., & Bangert, H. Measurement of Coefficients of Static Friction as a Means of Determining Opening and Demoulding Forces in Injection Moulds. Kunststoffe, vol. 71,1981. Pages 552-557 [10] Burke , C., & Malloy, R. An Experimental Study of the Ejection Forces Encountered During Injection Molding. ANTEC, 1991, Pages 1781-1787. [11] Ivkovic, B., Djukdjanvic, M., & Stamenkovic, D. The Influence of the Contact Surface Roughness on the Static Friction Coefficient. Tribology in industry, Volume 22, 2000