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Machining of Metal Foams with Varying Mesostructure Using Wire EDM
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 42 (2016) 263 – 267
18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)
Machining of Metal Foams with varying mesostructure using Wire EDM Alexander Martin Matza, Dennis Kammerera, Norbert Josta and Kai Oßwalda * a
Institute for Materials and Material Technologies, Pforzheim University of Applied Sciences, Tiefenbronner Straße 65, 75175 Pforzheim, Germany
* Corresponding author. Tel.: +49-177-7871999; E-mail address: [email protected]
Abstract Metal foams offer a wide range of applications including heat exchanger and light weight construction applications. The structure of these foams can be designed on different length scales using innovative technologies. Especially in the case of the mesostructure the relative density as well as the pore density can be varied in a relatively large scale [1]. Conventional machining of these materials is difficult due to their sensitivity to cutting forces. However, wire EDM has been proven to be an appropriate process for precise machining in this field [2]. In this study, the influence of the mesostructured parameters of aluminum metal foams on the EDM process is investigated. Herein the cutting rate is of particular interest. Furthermore the specific structure of metallic foams provides nearly ideal flushing conditions to the EDM process. Therefore, the influence of flushing on the EDM process is examined in this research. The results show that very fast feed speeds can be achieved due to the good flushing conditions and the low relative density of the material. Also a strong correlation of the relative density and the material removal rate was found.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: Metal Foams; Wire EDM; EDM; Flushing
1. Introduction 1.1. Metal Foams Metal foams are cellular solids which consist as a multiphase material of a solid and a fluid portion. They can be classified in a close-pore and an open-pore structure. Closepore metal foams consist of single fluid-filled cells which are separated by each other by the metallic portion of the foam. Open-pore metal foams consist of a metallic network of single struts which define the stochastically distributed cells at which these cells are interconnected and gaseous or liquid substances can pass the material. Open-pore metal foams which are considered in this study are highly porous and are hence light weight structural materials with a very large specific surface area [3]. They show outstanding properties that are especially of thermal [4], electrical [5], acoustical [6] and mechanical [7] character. Hence, they are of great interest for a wide variety of applications that are amongst others in the sectors of heat
engineering [8], electrical energy storage [9], biomedical engineering [10] and lightweight design [11]. The processing of open-pore metal foams is mainly based on sintering or casting techniques [12]. The method used in this study is a modified investment casting process which offers great flexibility in selecting the alloy composition and in designing the metal foam’s mesostructure, that is, the structure/geometry of the cells and struts as well as the relative density ρrel which is a measure for the solid portion. For the investment casting process, reticulated polymer foams are used as a precursor (cf. [13]). These foams show a relative density of ρrel = 2 ± 0.2% which results in a too low metallic portion for some applications. For this reason, Matz et al. [1] developed a method by which a homogenous thickening of the foam’s struts and hence an enhancement in ρrel can be achieved. The thickening is a thermal-additive process where the struts of the samples are firstly coated with an adhesive layer, followed secondly by powdering them with polymer granules and thirdly treating the samples above the melting temperature of the polymer granules. By this method, a
2212-8271 © 2016 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 organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.283
264
Alexander Martin Matz et al. / Procedia CIRP 42 (2016) 263 – 267
pronounced flexibility in designing the mesostructure is given which makes open-pore metal foams attractive for a wide range of applications. The usage of metal foams as components in an application requires, however, appropriate methods of machining them. Common procedures that are reported in several investigations [14],[15],[16],[17] are based on sawing techniques. De Giorgi et al. [14] prepared samples for shear deformation tests by circular sawing. Cluff and Esmaeili [15] studied the compressive properties of Al foams and used a band saw to cut their samples. Clark et al. [16] used an abrasive diamond wire to section brittle foams. To study the microstructure of Al alloy foams, [17] partitioned the samples by wet disc grinding. However, due to cutting forces acting on the foam samples, local plastic deformations may occur [7] and to some extend near-surface damage can result. For this reason, wire EDM seems to be an appropriate alternative method.
Miller et al. [2] investigated the material removal rate as a function of the spark cycle for different “advanced materials” amongst those metal foams of stainless steel (316) and FeCr alloy (Fe-Cr-Al). By varying the spark-on time Ton and the Spark cycle T, a process envelop was identified for each material as shown in Fig. 1. Ghose et al. [22] have examined EDM of closed cell aluminum foams using both Taguchi and Fuzzy methodology. The authors conclude that “to achieve higher productivity while machining aluminium foam using EDM process, two parameters pulse current and pulse on time should be set at high along with low setting of duty cycle” [22]. The influence of different mesostructures of metal foams on the EDM process has apparently not been investigated before. 2. Experimental Details 2.1. Materials and Manufacturing
1.2. Wire EDM of Metal Foams Wire Electrical Discharge Machining (WEDM) is a machining process with very low forces [18] compared to chipping processes like grinding or sawing. Accordingly, deformation of the foam structure due to WEDM is little or nonexistent.Moreover, WEDM is a very precise process compared to standard cut-off machining methods. A number of researchers have therefore used EDM to cut their specimens of metal foam raw material [19], [20], [21]. However, relatively little research has been published that investigates the EDM processing of metal foams itself. This might be caused by the process that is very easy to apply due to the excellent flushing conditions provided by the porous structure of the material, at least when dealing with open-pore metal foams.
The staring material used in this investigation is the binary aluminum alloy Al-11Zn which was manufactured by HMW Hauner Metallische Werkstoffe in Röttenbach, Germany by using Al and Zn in a purity of ≥ 99.99%, respectively. The open-pore metal foam samples are in-house manufactured via investment casting using thermal-additively treated precursors [1], as briefly mentioned in Sec. 1.1. The preform is infiltrated by a calcium sulfate-bonded plaster which is in a next step heated in an incineration furnace to fabricate the mold. The material, Al-11Zn, is melted and casted into the mold by a centrifugal casting machine (Vacutherm-3,3-Titan, Linn High Therm). In a final step the samples are cleaned by water jet to remove the residual investment. Thereafter, the as-cast sample is obtained as shown in Fig. 2.
Fig. 2. As-cast open-pore Al-11Zn foam sample with a pore density of ρP = 10 ppi and a relative density of ρrel = 0.125
Fig. 1. EDM process envelops for different metal foams, identified by Miller et al. [2]
Alexander Martin Matz et al. / Procedia CIRP 42 (2016) 263 – 267
The variety in mesostructural specification in this work is given by different pore densities ρP and thermal treatment runs nth (resulting in differnent relative densities ρrel). The pore density that is defined as ߩ ൌ ౌ, (1) where nP is the number of single pores on a certain length l (typically 25.4 mm), is chosen to be 7 ppi (pores per inch), 10 ppi and 13 ppi. The number of thermal treatments on the single samples are varied from nth = 1 to nth = 3. Thus, different relative densities, that is defined as ఘ ߩ୰ୣ୪ ൌ , (2) ఘ౩
wherein ρf is the density of the foam and ρs is the density of the solid material, could be achieved as shown in Fig. 3.
3. Results and Discussion The relative density ρrel that is shown in Fig. 3 is slightly increasing by trend with increasing pore densities ρP. However, the increasing effect of the thermal treatments nth on ρrel is much more significant in all cases. This can be stated as a basis for the results shown in the following. Fig. 4 shows the variation of the speed of the wire electrode vw for two different maximum discharge currents I. The graph shows the effect of these parameters on the cutting rate vc. In WEDM of conventional materials, an improving effect of higher wire speeds would be expected due to better flushing of the gap, considering the high importance of good exchange of the dielectric liquid [18]. However, as the results show, basically no effect of vw can be found for both levels of I. Despite the fact that no flushing pressure was applied, the foam structure seems to allow optimal flushing, already at very low wire speeds.
Fig. 3. Relative density ρrel vs. pore density ρP for the metal foam materials used in this study. Different symbols represent different numbers of thermal treatment runs nth
2.2. Machining The experiments for this study were carried out on a stateof-the-art universal wire EDM machine (CUT20, AgieCharmilles) with no major modifications. Additionally, an oscilloscope and a special EDM process duration measurement device [23] were used. Clamping of metal foams can be challenging due to the low resilience properties of the material. For this study a universal EDM clamping system (Ergospanner) was used. The parameters used for all of the experiments are shown in Table 1. The maximum discharge current I is a machine parameter and is not identical with the discharge current. For experiments with a fixed wire speed a value of 165 m/min was chosen. Table 1. Parameter list Parameter
Value
Maximum discharge current I (A)
65; 120; 380
Spark-on time ton,ref (µs)
9.3
Off time toff (µs)
718
Wire speed vw [m/min]
30; 60; 165; 330
Water pressure p [bar]
0
Fig. 4. Cutting rate vc vs. wire speed vw for two levels of maximum discharge current (I = 120 A and I = 380 A)
To investigate the correlation between the relative density ρrel and the cutting time tc, foams with different mesostructures (one, two and three thermal treatments) have been cut with different values of the maximum discharge current I (65 A, 120 A, 380 A). This parameter was chosen in order to investigate the influence of the puls energy on the cutting time for this particular material. For the experiment, specimens with dimensions of 50 ڄ20 ڄ50 mm3 were cut in a vertical orientation (cutting hight: 50 mm, cutting length: 20 mm). The results are shown in Fig. 5.
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foam. The comparison shows that for I = 65 A and I = 120 A cutting rates are very similar for vertical and horizontal orientation. For I = 380 A the cutting rate is much higher at ρrel = 0.05 for vertical orientation than for horizontal orientation. This effect minimizes for higher relative densities. In the absence of physical reasons for this effect its cause is probably the parameter setup of the feed control.
Fig. 5. Cutting time vc vs. relative density ρrel for specimens in vertical orientation
Larger values of I lead to significant drops in tc. Higher values of I increase the puls energy and therefore accelerate the process. However, higher relative densities of the foam material have a decelerating effect. This seems plausible because more material has to be machined per distance. The same experiment was conducted for specimens of 20 ڄ30 ڄ30 mm3 in horizontal orientation (cutting hight: 20 mm, cutting length: 30 mm) as shown in Fig. 6. The influence of I is similar to the previous experiment. The cutting times are significantly shorter due to the 20% lesser volume that was to be machined. In the horizontal orientation the decelerating effect of higher relative densities is stronger the lower the maximum discharge current is set. The reason for this might be derived from the feed control of the machine. In fact, for the shortest cutting times the machine was temporarily working close to its maximum feed speed of 40 mm/min.
Fig. 7. Cutting rate vc vs. relative density ρrel for specimens in horizontal and vertical orientation
4. Conclusions In this study the influence of varying mesostructures of open-pore metal foams on the WEDM process was investigated. Different parameters, namely wire speed vw and maximum discharge current I were varied and their influence on the cutting rate vc was observed. No influence of the wire speed was found, however the maximum discharge current has a strong influence on the cutting rate. Also the relative density of the metal foams was found to affect the cutting rate. Results show very high cutting rates if compared to massive material. Due to the excellent flushing conditions, the EDM process is easy to set up for this kind of material. Acknowledgements The authors gratefully acknowledge the federal state Baden-Württemberg for financial support. Moreover, they express their gratitude to B.S. Mocker for scientific support und fruitful discussions. References
Fig. 6. Cutting time vc vs. relative density ρrel for specimens in horizontal orientation
In order to compare the cutting in both orientations the cutting rate vc was calculated for both experiments as shown in Fig. 7. The cutting rates all range at high or very high values – compared to values generated by WEDM of massive materials [24]. Corresponding to Fig. 5 and Fig. 6 the cutting rate is the higher the larger values for I are chosen and the lower the higher the relative density of the machined metal
[1] Matz, A. M., Mocker, B. S., Müller, D. W., Jost, N., & Eggeler, G. (2014). Mesostructural design and manufacturing of open-pore metal foams by investment casting. Advances in Materials Science and Engineering, 2014, Art. 421729. [2] Miller, S. F., Shih, A. J., & Qu, J. (2004). Investigation of the spark cycle on material removal rate in wire electrical discharge machining of advanced materials. International Journal of Machine Tools and Manufacture, 44(4), 391-400. [3] Hong, S.-T., & Herling, D. R. (2006). Open-cell aluminum foams filled with phase change materials as compact heat sinks. Scripta Materialia, 55 (10), 887-890.
Alexander Martin Matz et al. / Procedia CIRP 42 (2016) 263 – 267
[4] Matz, A. M., Mocker, B. S., Jost, N., & Krug, P. (2015). Effective thermal conductivity of open-pore metal foams as a function of the base material. Materials Testing, 57(10), 825-836. [5] Dharmasena, K. P., & Wadley, H. N. G. (2002). Electrical conductivity of open-cell metal foams. Journal of Materials Research, 17(3), 625-631. [6] Hinze, B., & Rösler, J. (2014). Measuring and Simulating Acoustic Absorption of Open-Celled Metals. Advanced Engineering Materials, 16(3), 284-288. [7] Zhou, J., Shrotriya, P., & Soboyejo, W. O. (2004). Mechanisms and mechanics of compressive deformation in open-cell Al foams. Mechanics of Materials, 36(8), 781-797. [8] Han, X. H., Wang, Q., Park, Y. G., T’Joen, C., Sommers, A., & Jacobi, A. (2012). A review of metal foam and metal matrix composites for heat exchangers and heat sinks. Heat Transfer Engineering, 33(12), 991-1009. [9] Choi, Y. S., Yu, H. R., & Cheong, H. W. (2015). Electrochemical properties of a lithium-impregnated metal foam anode for thermal batteries. Journal of Power Sources, 276, 102-104. [10] Müller, D. W., Matz, A. M., & Jost, N. (2012). Casting open porous Ti foam suitable for medical applications. Bioinspired, Biomimetic and Nanobiomaterials, 2(2), 76-83. [11] Betts, C. (2012). Benefits of metal foams and developments in modelling techniques to assess their materials behaviour: a review. Materials Science and Technology, 28(2), 129-143 [12] Wadley, H. N. (2002). Cellular metals manufacturing. Advanced Engineering Materials, 4(10), 726-733. [13] Wang, L. C., & Wang, F. (2009). Preparation of the open pore aluminum foams usinginvestment casting process. Acta Metallurgica Sinica (English Letters), 14(1), 27-32. [14] De Giorgi, M., Giancane, S., Nobile, R., & Palano, F. (2010). Digital Image Correlation technique applied to mechanical characterisation of aluminium foam. EPJ Web of Conferences (Vol. 6, Art. 31004).
[15] Cluff, D. R., & Esmaeili, S. (2008). Prediction of the effect of artificial aging heat treatment on the yield strength of an open-cell aluminum foam. Journal of Materials Science, 43(3), 1121-1127. [16] Clark, W. I., Shih, A. J., Lemaster, R. L., & McSpadden, S. B. (2003). Fixed abrasive diamond wire machining—part II: experiment design and results. International Journal of Machine Tools and Manufacture, 43(5), 533-542. [17] Matz, A. M., Mocker, B. S., Christian, U., & Jost, N. (2014). Microstructural evolution in investment casted open-pore aluminumbased alloy foams. Procedia Materials Science, 4, 139-144. [18] Kunieda, M., Lauwers, B., Rajurkar, K. P., & Schumacher, B. M. (2005). Advancing EDM through fundamental insight into the process. CIRP Annals-Manufacturing Technology, 54(2), 64-87. [19] Esmaeelzadeh, S., Simchi, A., & Lehmhus, D. (2006). Effect of ceramic particle addition on the foaming behavior, cell structure and mechanical properties of P/M AlSi7 foam. Materials Science and Engineering: A, 424(1), 290-299. [20] Banhart, J., Bellmann, D., & Clemens, H. (2001). Investigation of metal foam formation by microscopy and ultra small-angle neutron scattering. Acta Materialia, 49(17), 3409-3420. [21] Ramamurty, U., & Paul, A. (2004). Variability in mechanical properties of a metal foam. Acta Materialia, 52(4), 869-876. [22] Ghose, J., Sharma, V., Kumar, N., Krishnamurthy, A., Kumar, S., & Botak, Z. (2011). Taguchi-fuzzy based mapping of EDM-machinability of aluminium foam. Tehnički vjesnik, 18(4), 595-600. [23] Oßwald, K. (2015). Simple EDM Process Duration Measurement. Technical report. doi: 10.13140/RG.2.1.1252.2401 [24] Ho, K. H., Newman, S. T., Rahimifard, S., & Allen, R. D. (2004). State of the art in wire electrical discharge machining (WEDM). International Journal of Machine Tools and Manufacture, 44(12), 1247-1259.
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18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)
Machining of Metal Foams with varying mesostructure using Wire EDM Alexander Martin Matza, Dennis Kammerera, Norbert Josta and Kai Oßwalda * a
Institute for Materials and Material Technologies, Pforzheim University of Applied Sciences, Tiefenbronner Straße 65, 75175 Pforzheim, Germany
* Corresponding author. Tel.: +49-177-7871999; E-mail address: [email protected]
Abstract Metal foams offer a wide range of applications including heat exchanger and light weight construction applications. The structure of these foams can be designed on different length scales using innovative technologies. Especially in the case of the mesostructure the relative density as well as the pore density can be varied in a relatively large scale [1]. Conventional machining of these materials is difficult due to their sensitivity to cutting forces. However, wire EDM has been proven to be an appropriate process for precise machining in this field [2]. In this study, the influence of the mesostructured parameters of aluminum metal foams on the EDM process is investigated. Herein the cutting rate is of particular interest. Furthermore the specific structure of metallic foams provides nearly ideal flushing conditions to the EDM process. Therefore, the influence of flushing on the EDM process is examined in this research. The results show that very fast feed speeds can be achieved due to the good flushing conditions and the low relative density of the material. Also a strong correlation of the relative density and the material removal rate was found.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: Metal Foams; Wire EDM; EDM; Flushing
1. Introduction 1.1. Metal Foams Metal foams are cellular solids which consist as a multiphase material of a solid and a fluid portion. They can be classified in a close-pore and an open-pore structure. Closepore metal foams consist of single fluid-filled cells which are separated by each other by the metallic portion of the foam. Open-pore metal foams consist of a metallic network of single struts which define the stochastically distributed cells at which these cells are interconnected and gaseous or liquid substances can pass the material. Open-pore metal foams which are considered in this study are highly porous and are hence light weight structural materials with a very large specific surface area [3]. They show outstanding properties that are especially of thermal [4], electrical [5], acoustical [6] and mechanical [7] character. Hence, they are of great interest for a wide variety of applications that are amongst others in the sectors of heat
engineering [8], electrical energy storage [9], biomedical engineering [10] and lightweight design [11]. The processing of open-pore metal foams is mainly based on sintering or casting techniques [12]. The method used in this study is a modified investment casting process which offers great flexibility in selecting the alloy composition and in designing the metal foam’s mesostructure, that is, the structure/geometry of the cells and struts as well as the relative density ρrel which is a measure for the solid portion. For the investment casting process, reticulated polymer foams are used as a precursor (cf. [13]). These foams show a relative density of ρrel = 2 ± 0.2% which results in a too low metallic portion for some applications. For this reason, Matz et al. [1] developed a method by which a homogenous thickening of the foam’s struts and hence an enhancement in ρrel can be achieved. The thickening is a thermal-additive process where the struts of the samples are firstly coated with an adhesive layer, followed secondly by powdering them with polymer granules and thirdly treating the samples above the melting temperature of the polymer granules. By this method, a
2212-8271 © 2016 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 organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.283
264
Alexander Martin Matz et al. / Procedia CIRP 42 (2016) 263 – 267
pronounced flexibility in designing the mesostructure is given which makes open-pore metal foams attractive for a wide range of applications. The usage of metal foams as components in an application requires, however, appropriate methods of machining them. Common procedures that are reported in several investigations [14],[15],[16],[17] are based on sawing techniques. De Giorgi et al. [14] prepared samples for shear deformation tests by circular sawing. Cluff and Esmaeili [15] studied the compressive properties of Al foams and used a band saw to cut their samples. Clark et al. [16] used an abrasive diamond wire to section brittle foams. To study the microstructure of Al alloy foams, [17] partitioned the samples by wet disc grinding. However, due to cutting forces acting on the foam samples, local plastic deformations may occur [7] and to some extend near-surface damage can result. For this reason, wire EDM seems to be an appropriate alternative method.
Miller et al. [2] investigated the material removal rate as a function of the spark cycle for different “advanced materials” amongst those metal foams of stainless steel (316) and FeCr alloy (Fe-Cr-Al). By varying the spark-on time Ton and the Spark cycle T, a process envelop was identified for each material as shown in Fig. 1. Ghose et al. [22] have examined EDM of closed cell aluminum foams using both Taguchi and Fuzzy methodology. The authors conclude that “to achieve higher productivity while machining aluminium foam using EDM process, two parameters pulse current and pulse on time should be set at high along with low setting of duty cycle” [22]. The influence of different mesostructures of metal foams on the EDM process has apparently not been investigated before. 2. Experimental Details 2.1. Materials and Manufacturing
1.2. Wire EDM of Metal Foams Wire Electrical Discharge Machining (WEDM) is a machining process with very low forces [18] compared to chipping processes like grinding or sawing. Accordingly, deformation of the foam structure due to WEDM is little or nonexistent.Moreover, WEDM is a very precise process compared to standard cut-off machining methods. A number of researchers have therefore used EDM to cut their specimens of metal foam raw material [19], [20], [21]. However, relatively little research has been published that investigates the EDM processing of metal foams itself. This might be caused by the process that is very easy to apply due to the excellent flushing conditions provided by the porous structure of the material, at least when dealing with open-pore metal foams.
The staring material used in this investigation is the binary aluminum alloy Al-11Zn which was manufactured by HMW Hauner Metallische Werkstoffe in Röttenbach, Germany by using Al and Zn in a purity of ≥ 99.99%, respectively. The open-pore metal foam samples are in-house manufactured via investment casting using thermal-additively treated precursors [1], as briefly mentioned in Sec. 1.1. The preform is infiltrated by a calcium sulfate-bonded plaster which is in a next step heated in an incineration furnace to fabricate the mold. The material, Al-11Zn, is melted and casted into the mold by a centrifugal casting machine (Vacutherm-3,3-Titan, Linn High Therm). In a final step the samples are cleaned by water jet to remove the residual investment. Thereafter, the as-cast sample is obtained as shown in Fig. 2.
Fig. 2. As-cast open-pore Al-11Zn foam sample with a pore density of ρP = 10 ppi and a relative density of ρrel = 0.125
Fig. 1. EDM process envelops for different metal foams, identified by Miller et al. [2]
Alexander Martin Matz et al. / Procedia CIRP 42 (2016) 263 – 267
The variety in mesostructural specification in this work is given by different pore densities ρP and thermal treatment runs nth (resulting in differnent relative densities ρrel). The pore density that is defined as ߩ ൌ ౌ, (1) where nP is the number of single pores on a certain length l (typically 25.4 mm), is chosen to be 7 ppi (pores per inch), 10 ppi and 13 ppi. The number of thermal treatments on the single samples are varied from nth = 1 to nth = 3. Thus, different relative densities, that is defined as ఘ ߩ୰ୣ୪ ൌ , (2) ఘ౩
wherein ρf is the density of the foam and ρs is the density of the solid material, could be achieved as shown in Fig. 3.
3. Results and Discussion The relative density ρrel that is shown in Fig. 3 is slightly increasing by trend with increasing pore densities ρP. However, the increasing effect of the thermal treatments nth on ρrel is much more significant in all cases. This can be stated as a basis for the results shown in the following. Fig. 4 shows the variation of the speed of the wire electrode vw for two different maximum discharge currents I. The graph shows the effect of these parameters on the cutting rate vc. In WEDM of conventional materials, an improving effect of higher wire speeds would be expected due to better flushing of the gap, considering the high importance of good exchange of the dielectric liquid [18]. However, as the results show, basically no effect of vw can be found for both levels of I. Despite the fact that no flushing pressure was applied, the foam structure seems to allow optimal flushing, already at very low wire speeds.
Fig. 3. Relative density ρrel vs. pore density ρP for the metal foam materials used in this study. Different symbols represent different numbers of thermal treatment runs nth
2.2. Machining The experiments for this study were carried out on a stateof-the-art universal wire EDM machine (CUT20, AgieCharmilles) with no major modifications. Additionally, an oscilloscope and a special EDM process duration measurement device [23] were used. Clamping of metal foams can be challenging due to the low resilience properties of the material. For this study a universal EDM clamping system (Ergospanner) was used. The parameters used for all of the experiments are shown in Table 1. The maximum discharge current I is a machine parameter and is not identical with the discharge current. For experiments with a fixed wire speed a value of 165 m/min was chosen. Table 1. Parameter list Parameter
Value
Maximum discharge current I (A)
65; 120; 380
Spark-on time ton,ref (µs)
9.3
Off time toff (µs)
718
Wire speed vw [m/min]
30; 60; 165; 330
Water pressure p [bar]
0
Fig. 4. Cutting rate vc vs. wire speed vw for two levels of maximum discharge current (I = 120 A and I = 380 A)
To investigate the correlation between the relative density ρrel and the cutting time tc, foams with different mesostructures (one, two and three thermal treatments) have been cut with different values of the maximum discharge current I (65 A, 120 A, 380 A). This parameter was chosen in order to investigate the influence of the puls energy on the cutting time for this particular material. For the experiment, specimens with dimensions of 50 ڄ20 ڄ50 mm3 were cut in a vertical orientation (cutting hight: 50 mm, cutting length: 20 mm). The results are shown in Fig. 5.
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foam. The comparison shows that for I = 65 A and I = 120 A cutting rates are very similar for vertical and horizontal orientation. For I = 380 A the cutting rate is much higher at ρrel = 0.05 for vertical orientation than for horizontal orientation. This effect minimizes for higher relative densities. In the absence of physical reasons for this effect its cause is probably the parameter setup of the feed control.
Fig. 5. Cutting time vc vs. relative density ρrel for specimens in vertical orientation
Larger values of I lead to significant drops in tc. Higher values of I increase the puls energy and therefore accelerate the process. However, higher relative densities of the foam material have a decelerating effect. This seems plausible because more material has to be machined per distance. The same experiment was conducted for specimens of 20 ڄ30 ڄ30 mm3 in horizontal orientation (cutting hight: 20 mm, cutting length: 30 mm) as shown in Fig. 6. The influence of I is similar to the previous experiment. The cutting times are significantly shorter due to the 20% lesser volume that was to be machined. In the horizontal orientation the decelerating effect of higher relative densities is stronger the lower the maximum discharge current is set. The reason for this might be derived from the feed control of the machine. In fact, for the shortest cutting times the machine was temporarily working close to its maximum feed speed of 40 mm/min.
Fig. 7. Cutting rate vc vs. relative density ρrel for specimens in horizontal and vertical orientation
4. Conclusions In this study the influence of varying mesostructures of open-pore metal foams on the WEDM process was investigated. Different parameters, namely wire speed vw and maximum discharge current I were varied and their influence on the cutting rate vc was observed. No influence of the wire speed was found, however the maximum discharge current has a strong influence on the cutting rate. Also the relative density of the metal foams was found to affect the cutting rate. Results show very high cutting rates if compared to massive material. Due to the excellent flushing conditions, the EDM process is easy to set up for this kind of material. Acknowledgements The authors gratefully acknowledge the federal state Baden-Württemberg for financial support. Moreover, they express their gratitude to B.S. Mocker for scientific support und fruitful discussions. References
Fig. 6. Cutting time vc vs. relative density ρrel for specimens in horizontal orientation
In order to compare the cutting in both orientations the cutting rate vc was calculated for both experiments as shown in Fig. 7. The cutting rates all range at high or very high values – compared to values generated by WEDM of massive materials [24]. Corresponding to Fig. 5 and Fig. 6 the cutting rate is the higher the larger values for I are chosen and the lower the higher the relative density of the machined metal
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