- Email: [email protected]
Derivation of Guidelines for Reliable Finishing of Aluminium Matrix Composites by Jet Electrochemical Machining
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 471 – 476
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Derivation of Guidelines for Reliable Finishing of Aluminium Matrix Composites by Jet Electrochemical Machining Norbert Lehnerta,*, Matthias Hackert-Oschätzchena, André Martina, Andreas Schuberta,b a
Professorship Micromanufacturing Technology, Chemnitz University of Technology, 09107 Chemnitz, Germany b Fraunhofer Institute for Machine Tools and Forming Technology, 09126 Chemnitz, Germany
* Corresponding author. Tel.: +49-371-35132862; fax: +49-371-351832862. E-mail address: [email protected]
Abstract The Collaborative Research Centre SFB 692 at Chemnitz University of Technology is focused on aluminium-based light-weight materials in safety-related parts and components. One field of activity are aluminium matrix composites (AMCs). Thereby, the manufacturing, the mechanical properties, the economic opportunities and challenges as well as the machinability are investigated. One of the research focusses is the finishmachining of AMCs by Jet Electrochemical Machining (Jet-ECM). Therefore, one objective is to achieve guidelines for reliable machining of AMCs by Jet-ECM. In a first step the dissolution characteristic of EN AW 2017 with 10% of SiC particles, which was manufactured within the SFB 692, was analysed by help of a microcapillary cell, applying a sodium nitrate electrolyte. Furthermore, Jet-ECM was used to generate point erosions using different pH-neutral electrolytes, namely NaNO3 and NaCl. AMCs made of the aluminium alloy EN AW 2017 with 0%, 5% and 10% SiC particles were machined with a maximal voltage of 25 V. In the current study the parameter field is extended further. The dissolution characteristic by using the pH-neutral electrolyte sodium-bromide is analysed and the voltage range is set to 60 V. Those experiments show, that each electrolyte causes a specific dissolution characteristic. Based on the experimental data, guidelines for a reliable finishing of AMCs by Jet-ECM are derived. The resulting guidelines can be used to predict the depth of the point erosions as function of particle fraction, the applied voltage and the machining time, with respect to the used electrolyte. The statistical evaluation of the guidelines show, that the applied voltage and the machining time are the dominant factors to control the dimensions of the point erosions. Whereas the particle fraction has a minor effect on depth and width. © 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: Electro chemical machining (ECM); Metal matrix composite; Aluminium, Silicon carbide
1. Introduction At Chemnitz University of Technology in the Collaborative Research Centre SFB 692 aluminium-based light-weight materials were investigated with focus on their use in safetyrelated applications. Among other material, several of the involved academic institutions work on aluminium matrix composites (AMCs) to investigate different ways of production, machining and furthermore the economic potential. Within the Collaborative Research Centre, AMCs were manufactured to adjust the properties by the characteristic and the fraction of the reinforcement. The investigated AMCs in this study consist of the aluminium alloy EN AW 2017
reinforced by SiC particles. The SiC particles in the AMC have an average diameter of 1 µm and a fraction up to 10%. [1–4] Electrochemical machining (ECM) is one of the investigated methods for machining AMCs. Thereby, Electrochemical Precision Machining (PECM) and ECM with continuous electrolytic free jet (Jet-ECM) were used in one sub-project of the Collaborative Research Centre. [5–10] One goal of the research was to determine guidelines for reliable finish machining of AMCs by Jet-ECM. This study presents results of the investigation on the electrochemical dissolution characteristic of AMCs by Jet-ECM. Here, the aluminium alloy EN AW 2017 is reinforced by 0%, 5% and 10%of SiC particles. Three pH-neutral electrolytes, namely
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.2017.12.086
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sodium bromide, sodium chloride and sodium nitrate, each with an average conductivity of 185 mS/cm, were used. Subsequently, the result of multiple linear regression analysis for each of the used electrolytes will be discussed. The results of previous studies [6, 10] were also considered in the regression analysis.
3.1. Point erosions machined by using the NaBr electrolyte First, the results of the experiments with NaBr electrolyte will be discussed. Figure 2 shows cross sections of point erosions, machined by using the NaBr electrolyte, with a voltage of 60 V and a machining time of 2 s, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles.
2. Jet-ECM Principle In this study electrochemical machining with continuous electrolytic free jet is used. Figure 1 shows the principle of JetECM.
Fig. 2. Cross sections of point erosions in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles using NaBr electrolyte (U = 60 V; t = 2 s) Fig. 1. Principle of Jet-ECM [6]
During the Jet-ECM process fresh electrolyte is pumped through a micro nozzle and ejected with a mean velocity of approximately 20 m/s. The electrolyte jet is aligned vertically to the work piece surface. Due to the high velocity, a closed free jet is formed in the surrounding atmospheric air. The current density is locally confined by the electrolyte jet. This leads to a highly localized machining area. In the jet, current densities up to 2000 A/cm2 are realized [11]. As a result, high surface qualities are achievable. [11–15]
All three cross sections have almost similar shapes, which can be described as calottes. Such calotte geometries are common in Jet-ECM [11–13]. While the calottes on the specimens reinforced by 5% and 10 % SiC particles are almost identical, the calotte on the pure EN AW 2017 with 0% SiC particles is wider and deeper. Figure 3 shows the width of the calottes as a function of the machining time. Each point erosion was machined 4 times, so every data point is based on measurements of 4 point erosions.
3. Results and Discussion The Jet-ECM parameters used in this study are shown in table 1. Table 1. Process parameters Parameter Electrolytes
Symbol
Value
NaBr, NaCl, NaNO3
Conductivity of the electrolyte
σ
Temperature of the electrolyte
ϑ
20 °C
Nozzle diameter
dn
100 µm
Working gap
sg
100 µm
Applied voltage
U
60 V
t
0.5 s, 1 s, 1.5 s, 2 s
Machining times
185 mS/cm
A confocal microscope of Walter Uhl GmbH was used to measure the machined point erosions optically.
Fig. 3. Width of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaBr electrolyte (U = 60 V)
It can be seen, that the width increases with increasing time. The increase of the width over time seems to be declining. The width on the pure aluminium alloy is the largest. With increasing machining time the maximal difference between the specimens decreases from 20 µm at 0.5 s to 2 µm at 2 s. It is
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known, that the width of point erosions in Jet-ECM is mainly determined by the nozzle diameter. Previous studies on JetECM of steel showed, that the maximal diameter of point erosions is about 2 times the nozzle diameter, since the current density is almost reduced to zero at this point [12, 13]. Here, the width is in a range of approximately 2.0 to 2.8 times the nozzle diameter, which is larger compared to machining steel. The dissolution characteristic of steel in previous studies could be determined as trans-passive dissolution, where a certain amount of current density is needed to dissolve the anode material. Since the width in this study is larger than 2 times the nozzle diameter, it can be stated that the dissolution characteristic of the aluminium alloy is active, where minimal current densities lead to an electrochemical dissolution. The measurement results of the depth of the point erosions are shown in figure 4.
473
the electrolyte jet, the highest removal rate was expected in the centre, too [13]. But it seems that high current densities lead to passivation effects and to a reduction of the dissolution rate in the centre of the point erosion.
Fig. 5. Cross sections of point erosions in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaCl electrolyte (U = 60 V; t = 2 s)
Fig. 4. Depth of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaBr electrolyte (U = 60 V)
As a result it can be derived, that the presence of particles reduces the passivation effect, since the point erosions in the specimens with 5% and 10% SiC particles do not have a conical pin in the centre and are significantly deeper. A possible reason could be, that the current density increases around the particles [8], which lead to slightly reduce width due to increased passivation. The increased depth is expected to be caused by flushing out the particles, without consuming any charge. Details were not analysed in this study. Therefore, the influence of particles on current density and removal characteristic should be investigated in further experiments and simulation In figure 6, the width of the point erosions as a function of the machining time is shown.
Similar to the width, the depth increases with increasing machining time. The point erosions in the specimens with 0% SiC particles are approximately 10 µm to 20 µm deeper, compared to the specimens with 5% and 10% SiC particles. The achieved depths in the specimens with 5% and 10% SiC particles are nearly similar with a maximal difference below 10%. As a result it can be stated, that the presence of reinforcement particles decreases the depth but the fraction of particles did not lead to a systematic influence.
3.2. Point erosions machined by using the NaCl electrolyte Figure 5 shows cross sections of point erosions, machined by using NaCl as electrolyte at a voltage of 60 V and machining time of 2 s. The cross sections of the reinforced specimens have the typical calotte-shaped geometry, which are nearly similar in width and depth. On the other hand, the cross section of the point erosion in the specimens with 0% SiC particles differs significantly in shape. It is slightly wider and about 40 % less deep. Additionally, in the centre of the point erosion is a conical pin. Since the highest current density appears in the centre of
Fig. 6. Width of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaCl electrolyte (U = 60 V)
Similar to the results with the NaBr electrolyte, the width increases at increasing time, when using the NaCl electrolyte. The specimens without SiC particles have the highest width, whereas the point erosions in the specimens with 10% SiC
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particles mostly have the smallest width. In general, using the NaCl electrolyte results in a width of approximately 2.2 to 3.2 times the nozzle diameter. Figure 7 shows the depth of the point erosions as a function of machining time.
In case of using the NaNO3 electrolyte, the point erosions did not lead to calotte–shaped geometries. Indeed, all the geometries are comparable to point erosion generated previously in EN AW 2017 with the NaCl electrolyte. All point erosions have similar shapes as well as nearly similar depths and widths. So, the presence and the fraction of the SiC particles did not influence the resulting removal geometry. The measurement results of the width as a function of machining time are shown in figure 9.
Fig. 7. Depth of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaCl electrolyte (U = 60 V)
As it can be seen, an increasing time leads to an increasing depth. The specimens with reinforcement show nearly similar results and higher values than in the pure aluminium alloy. The difference between the specimens with and without SiC particles increases from 20 µm at 0.5 s to 50 µm at 2.0 s. Since the results in the specimens with SiC particles show similar depths, it can be stated that the sheer presence of SiC particles leads to an increase of the depth of the point erosions, regardless of their fraction.
3.3. Point erosions machined by using the NaNO3 electrolyte
Fig. 9. Width of the point erosions as a function of the machining time in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles by using NaNO3 electrolyte (U = 60 V)
Here, the width slightly increases with increasing machining time, whereas the specimen with 10% SiC particles has the highest values and the specimens with 0% SiC particles has the smallest values. As a consequence, the width increases with increasing particle fraction. In general, the achieved width using the NaNO3 electrolyte is approximately 2.4 to 3.0 times the nozzle diameter. Figure 10 shows the corresponding depths of the point erosion as a function of the machining time.
The cross sections of point erosions machined with the NaNO3 electrolyte at a voltage of 60 V and a machining time of 2 s are shown in Figure 8.
Fig. 10. Depth of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaNO3 electrolyte (U = 60 V) Fig. 8. Cross sections of point erosions in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles by using NaNO3 electrolyte (U = 60 V; t = 2 s)
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Again, the depth of the point erosions increases with increasing machining time. Considering the different particle fractions, the achieved depths are almost similar in the three materials. Therefore, hardly any effect of the particles on the depth was detected. Summarizing the results of the measurements, it can be said, that using the NaCl electrolyte leads to the widest point erosions, while the NaBr electrolyte results in the narrowest point erosions. Considering the depth, again the NaCl electrolyte leads to the largest values, while the lowest depths were achieved using the NaNO3 electrolyte. The reinforcement particles have their most significant influence when using the NaCl electrolyte and the lowest influence when using the NaNO3 electrolyte.
3.4. Multiple Regression Analysis Using the data of this study and previous studies, a multiple linear regression analysis has been done to generate empirical models[6, 10]. The voltage range goes from 10 V to 60 V, considering all experiments done so far. The used specimens and machining times are similar to the specifications in this study. Hence, the multiple regression analysis is based on 132 data points for each of the three electrolytes. Since the width of the point erosions is affected majorly by the nozzle geometry, values between 2 and 3 times of the nozzle inner diameter are expected, as mentioned earlier. Thus, the analyses of the multiple regression in this study focus on the depth of the resulting point erosions. Equation 1 shows the used linear regression model, which is the adjusted version of the standard model for multiple linear regressions [16]. ݀ ൌ ܣଵ ߱ ڄ ܣଶ ܷ ڄ ܣଷ ݐ ڄ
(1)
In this model, the depth depends linearly on the fraction of particles ߱ in percent, the applied voltage ܷ in V and the machining time ݐin s. A constant coefficient ܣ is not included, since the initial depth before the machining is 0 µm. The coefficient of determination R2 was calculated in order to analyse, how well the developed regression model represents the real data points. It has a range from 0 to 1, while 0 means, the regression model cannot be used to describe the measured data and 1 means, that every data point can be calculated by the regression model [16]. Table 2 presents the regression coefficients for the NaBr electrolyte. Table 2. Regression coefficients NaBr Coefficient
Value
p-Value
ܣଵ ሾɊሿ
-0.70731
0.00842
ܣଶ ሾɊȀሿ
0.82544
0
ܣଷ ሾɊȀሿ
32.51437
0
The multiple regression model of the depth of point erosion using the NaBr electrolyte has an R2 of 0.969. This indicates, that the regression model represents nearly every measured data point.
Since the coefficient A1 is negative, an increasing particle fraction reduces the achievable depth. This is similar to the observations mentioned earlier. The p-Value is a statistical coefficient, which can be used to evaluate the significance of the input value to the regression analysis. By definition, p-Value is always positive and the smaller it is, the more significant is the corresponding input parameter [16]. Here, the voltage and the machining time are equally significant. The particle fraction is the least significant parameter in this case. Table 3 presents the regression coefficients for predicting the depth of point erosions using the NaCl electrolyte. Table 3. Regression coefficients NaCl Coefficient
Value
p-Value
ܣଵ ሾɊ݉ሿ
0.49779
0.12568
ܣଶ ሾɊ݉Ȁܸሿ
0.64956
0
ܣଷ ሾɊ݉Ȁݏሿ
27.81428
0
The R2 is 0.944, which means, that the developed regression model matches with approximately 94.4% of the data points. In contrast to NaBr, A1 is positive, so an increasing particle fraction increases the achievable depth. This is similar to what is shown in figure 7. Considering the p-value, again the machining time and the voltage have the highest significance. The particle fraction is the least significant parameter, with even less significance than in case of the NaBr electrolyte. The regression coefficients of the multiple regression analysis of the experiments using the NaNO3 electrolyte, are shown in table 4. Table 4. Regression coefficients NaNO3 Coefficient
Value
p-Value
ܣଵ ሾɊ݉ሿ
-0.39149
0,07593
ܣଶ ሾɊ݉Ȁܸሿ
0.39497
0
ܣଷ ሾɊ݉Ȁݏሿ
22.08199
0
The developed regression model is characterized by an R2 of 0.941. Here, the coefficient A1 is negative again, so an increasing particle fraction decreases the achievable depth. The parameters machining time and voltage are most significant. The particle fraction is the least significant input parameter, again. The analysis of the experiments presented in section 3.1 to 3.3 indicates that the reinforcement particles influence the shape, width and depth of point erosion, especially in the experiments using the NaCl electrolyte. On the other hand, the measurement results implied that the particle fraction itself is less important than the sheer presence of reinforcement particles. The analysis of the multiple regression of all associated experiments showed, that the particle fraction is the least significant input parameter. This means, that the particle fraction barely influences the result of the point erosions. As a consequence, the electrochemical dissolution characteristic of the investigated AMCs is primarily defined by the dissolution characteristic of the aluminium alloy EN AW 2017. Possible
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causes, could either be attributed to the small size of the particles, with an average diameter of 1 µm, or the small fraction in the AMC, with 5% or 10 %. [4]
4. Conclusion In this study Jet-ECM point erosions were realized in AMCs and guidelines for reliable finishing were derived by multiple linear regression analyses. The AMCs, consisting of the aluminium alloy EN AW 2017 reinforced with 0%, 5% and 10% SiC particles, were machined using three pH-neutral aqueous electrolytes of sodium bromide, sodium chloride and sodium nitrate. Results of point erosion, applying a voltage of 60 V and a machining time of 0.5 s to 2 s, were presented and discussed. Subsequently, multiple linear regression analyses, considering the measurement results of this and previous studies, were carried out. The analysis of the point erosions showed, that the dissolution characteristic differs, depending on the used electrolyte. The deepest point erosions were generated using NaCl as electrolyte, whereas the point erosion generated by using NaNO3 have the lowest depth. The width of the point erosions depends most on the used nozzle diameter. So the achievable width is in a range of 2 times to 3 times the nozzle diameter. The effect of the reinforcement particles was investigated in the three applied electrolytes, too. When using NaNO3, the depth and width were hardly affected by the particle fraction. But in case of NaCl and NaBr, the particles significantly influenced both the width and the depth. In the NaBr electrolyte, solely the presence of reinforcement particles affected the machining result, but the fraction of particles had less influence. But in NaCl a systematic influence was found, since the width of the erosions was decreased and the depth was increased by the increasing fraction of reinforcement particles. The results of the multiple linear regression analyses can be used as guidelines for reliable finishing of AMCs by Jet-ECM. One regression model was developed for each of the three used electrolytes. While the depth was used as the output parameter, the particle fraction, the voltage and the machining time were analysed as the input parameters. The R2 of each model was higher than 0.98, which means that more than 98% of the 132 underlying data points are predictable by the regression model. The analysis of the p-Value indicated the significance of each of the input parameters. For all three electrolytes, the particle fraction was the least significant parameter. Acknowledgements The authors acknowledge the DFG (German Research Foundation) for supporting this work carried out within the framework of project SFB 692 HALS. References [1] [2] [3]
K. K. Chawla, Composite Materials. New York, NY: Springer New York, 2012, ISBN: 978-0-387-74364-6, DOI: 10.1007/978-0-38774365-3 D. D. L. Chung, Composite Materials : Science and Applications. Springer London Dordrecht Heidelberg, 2010, ISBN: 978-1-84882830-8, DOI: 10.1007/978-1-84882-831-5 D. Nestler, S. Siebeck, H. Podlesak et al., "Powder Metallurgy of
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15] [16]
Particle-Reinforced Aluminium Matrix Composites (AMC) by Means of High-Energy Ball Milling", in Integrated Systems, Design and Technology 2010, Berlin, Heidelberg, 2011, pp. 93–107, DOI: 10.1007/978-3-642-17384-4_9 S. Wagner, S. Siebeck, M. Hockauf et al., "Effect of SiCReinforcement and Equal-Channel Angular Pressing on Microstructure and Mechanical Properties of AA2017", Adv. Eng. Mater., vol. 14, no. 6, pp. 388–393, Jun. 2012, ISSN: 14381656, DOI:10.1002/adem.201100253 A. Schubert, U. Götze, M. Hackert-Oschätzchen et al., "Evaluation of the Technical-Economic Potential of Particle- Reinforced Aluminum Matrix Composites and Electrochemical Machining", IOP Conf. Ser. Mater. Sci. Eng., vol. 118, p. 10, 2016, ISSN: 1757-8981, DOI:10.1088/1757-899X/118/1/012035 M. Hackert-Oschätzchen, N. Lehnert, A. Martin et al., "Jet Electrochemical Machining of Particle Reinforced Aluminum Matrix Composites with Different Neutral Electrolytes", IOP Conf. Ser. Mater. Sci. Eng., vol. 118, p. 6, 2016, ISSN: 1757-899X, DOI:10.1088/1757-899X/118/1/012036 M. Hackert-Oschätzchen, N. Lehnert, A. Martin et al., "Surface Characterization of Particle Reinforced Aluminum-matrix Composites Finished by Pulsed Electrochemical Machining", Procedia CIRP, vol. 45, pp. 351–354, 2016, ISSN: 2212-8271, DOI:10.1016/j.procir.2016.02.078 M. Hackert-Oschätzchen, N. Lehnert, M. Kowalick et al., "Analysis of an Electrochemical Machining Process for Particle Reinforced Aluminium Matrix Composites", in Proceeding of the European COMSOL Conference, München, 2016, p. 5 A. Schubert, M. Hackert-Oschätzchen, N. Lehnert et al., "Analysis of the Electrochemical Dissolution Characteristics of High-Strength Particle Reinforced Aluminium Matrix Composites", in 17th International Conference on the Strength of Materials (ICSMA-17), Brno, 2015, vol. 17, DOI: 10.13140/RG.2.1.3929.7764 M. Hackert-Oschätzchen, R. Morgenstern, N. Lehnert et al., "Basic Research on Electrochemical Machining of Particle Reinforced Aluminium Matrix Composites", in International Symposium on Electrochemical Machining Technology (INSECT ), 2015, pp. 35–42 A. Schubert, M. Hackert-Oschätzchen, G. Meichsner et al., "Evaluation of the Influence of the Electric Potential in Jet Electrochemical Machining", in Proceedings of the 7th International Symposium on Electrochemical Machining Technology, 2011, pp. 47– 54, ISBN: 978-3-00-036247-7 M. Hackert-Oschätzchen, G. Meichsner, M. Zinecker et al., "Micro machining with continuous electrolytic free jet", Precis. Eng., vol. 36, no. 4, pp. 612–619, 2012, ISSN: 0141-6359, DOI:10.1016/j.precisioneng.2012.05.003 M. Hackert-Oschätzchen, R. Paul, A. Martin et al., "Study on the dynamic generation of the jet shape in Jet Electrochemical Machining", J. Mater. Process. Technol., vol. 223, pp. 240–251, Sep. 2015, ISSN: 0924-0136, DOI:10.1016/j.jmatprotec.2015.03.049 W. Natsu, T. Ikeda, and M. Kunieda, "Generating complicated surface with electrolyte jet machining", Precis. Eng., vol. 31, no. 1, pp. 33–39, Jan. 2007, ISSN: 01416359, DOI:10.1016/j.precisioneng.2006.02.004 W. Natsu, T. Ikeda, and M. Kunieda, "Machining complicated patterns with electrolyte jet machining", in Proceedings of the 5th Euspen International Conference, 2005, pp. 649–652 N. R. Draper and H. Smith, Applied Regression Analysis. John Wiley & Sons, 2014, ISBN: 978-1-118-62568-2
ScienceDirect Procedia CIRP 68 (2018) 471 – 476
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Derivation of Guidelines for Reliable Finishing of Aluminium Matrix Composites by Jet Electrochemical Machining Norbert Lehnerta,*, Matthias Hackert-Oschätzchena, André Martina, Andreas Schuberta,b a
Professorship Micromanufacturing Technology, Chemnitz University of Technology, 09107 Chemnitz, Germany b Fraunhofer Institute for Machine Tools and Forming Technology, 09126 Chemnitz, Germany
* Corresponding author. Tel.: +49-371-35132862; fax: +49-371-351832862. E-mail address: [email protected]
Abstract The Collaborative Research Centre SFB 692 at Chemnitz University of Technology is focused on aluminium-based light-weight materials in safety-related parts and components. One field of activity are aluminium matrix composites (AMCs). Thereby, the manufacturing, the mechanical properties, the economic opportunities and challenges as well as the machinability are investigated. One of the research focusses is the finishmachining of AMCs by Jet Electrochemical Machining (Jet-ECM). Therefore, one objective is to achieve guidelines for reliable machining of AMCs by Jet-ECM. In a first step the dissolution characteristic of EN AW 2017 with 10% of SiC particles, which was manufactured within the SFB 692, was analysed by help of a microcapillary cell, applying a sodium nitrate electrolyte. Furthermore, Jet-ECM was used to generate point erosions using different pH-neutral electrolytes, namely NaNO3 and NaCl. AMCs made of the aluminium alloy EN AW 2017 with 0%, 5% and 10% SiC particles were machined with a maximal voltage of 25 V. In the current study the parameter field is extended further. The dissolution characteristic by using the pH-neutral electrolyte sodium-bromide is analysed and the voltage range is set to 60 V. Those experiments show, that each electrolyte causes a specific dissolution characteristic. Based on the experimental data, guidelines for a reliable finishing of AMCs by Jet-ECM are derived. The resulting guidelines can be used to predict the depth of the point erosions as function of particle fraction, the applied voltage and the machining time, with respect to the used electrolyte. The statistical evaluation of the guidelines show, that the applied voltage and the machining time are the dominant factors to control the dimensions of the point erosions. Whereas the particle fraction has a minor effect on depth and width. © 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: Electro chemical machining (ECM); Metal matrix composite; Aluminium, Silicon carbide
1. Introduction At Chemnitz University of Technology in the Collaborative Research Centre SFB 692 aluminium-based light-weight materials were investigated with focus on their use in safetyrelated applications. Among other material, several of the involved academic institutions work on aluminium matrix composites (AMCs) to investigate different ways of production, machining and furthermore the economic potential. Within the Collaborative Research Centre, AMCs were manufactured to adjust the properties by the characteristic and the fraction of the reinforcement. The investigated AMCs in this study consist of the aluminium alloy EN AW 2017
reinforced by SiC particles. The SiC particles in the AMC have an average diameter of 1 µm and a fraction up to 10%. [1–4] Electrochemical machining (ECM) is one of the investigated methods for machining AMCs. Thereby, Electrochemical Precision Machining (PECM) and ECM with continuous electrolytic free jet (Jet-ECM) were used in one sub-project of the Collaborative Research Centre. [5–10] One goal of the research was to determine guidelines for reliable finish machining of AMCs by Jet-ECM. This study presents results of the investigation on the electrochemical dissolution characteristic of AMCs by Jet-ECM. Here, the aluminium alloy EN AW 2017 is reinforced by 0%, 5% and 10%of SiC particles. Three pH-neutral electrolytes, namely
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.2017.12.086
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Norbert Lehnert et al. / Procedia CIRP 68 (2018) 471 – 476
sodium bromide, sodium chloride and sodium nitrate, each with an average conductivity of 185 mS/cm, were used. Subsequently, the result of multiple linear regression analysis for each of the used electrolytes will be discussed. The results of previous studies [6, 10] were also considered in the regression analysis.
3.1. Point erosions machined by using the NaBr electrolyte First, the results of the experiments with NaBr electrolyte will be discussed. Figure 2 shows cross sections of point erosions, machined by using the NaBr electrolyte, with a voltage of 60 V and a machining time of 2 s, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles.
2. Jet-ECM Principle In this study electrochemical machining with continuous electrolytic free jet is used. Figure 1 shows the principle of JetECM.
Fig. 2. Cross sections of point erosions in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles using NaBr electrolyte (U = 60 V; t = 2 s) Fig. 1. Principle of Jet-ECM [6]
During the Jet-ECM process fresh electrolyte is pumped through a micro nozzle and ejected with a mean velocity of approximately 20 m/s. The electrolyte jet is aligned vertically to the work piece surface. Due to the high velocity, a closed free jet is formed in the surrounding atmospheric air. The current density is locally confined by the electrolyte jet. This leads to a highly localized machining area. In the jet, current densities up to 2000 A/cm2 are realized [11]. As a result, high surface qualities are achievable. [11–15]
All three cross sections have almost similar shapes, which can be described as calottes. Such calotte geometries are common in Jet-ECM [11–13]. While the calottes on the specimens reinforced by 5% and 10 % SiC particles are almost identical, the calotte on the pure EN AW 2017 with 0% SiC particles is wider and deeper. Figure 3 shows the width of the calottes as a function of the machining time. Each point erosion was machined 4 times, so every data point is based on measurements of 4 point erosions.
3. Results and Discussion The Jet-ECM parameters used in this study are shown in table 1. Table 1. Process parameters Parameter Electrolytes
Symbol
Value
NaBr, NaCl, NaNO3
Conductivity of the electrolyte
σ
Temperature of the electrolyte
ϑ
20 °C
Nozzle diameter
dn
100 µm
Working gap
sg
100 µm
Applied voltage
U
60 V
t
0.5 s, 1 s, 1.5 s, 2 s
Machining times
185 mS/cm
A confocal microscope of Walter Uhl GmbH was used to measure the machined point erosions optically.
Fig. 3. Width of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaBr electrolyte (U = 60 V)
It can be seen, that the width increases with increasing time. The increase of the width over time seems to be declining. The width on the pure aluminium alloy is the largest. With increasing machining time the maximal difference between the specimens decreases from 20 µm at 0.5 s to 2 µm at 2 s. It is
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known, that the width of point erosions in Jet-ECM is mainly determined by the nozzle diameter. Previous studies on JetECM of steel showed, that the maximal diameter of point erosions is about 2 times the nozzle diameter, since the current density is almost reduced to zero at this point [12, 13]. Here, the width is in a range of approximately 2.0 to 2.8 times the nozzle diameter, which is larger compared to machining steel. The dissolution characteristic of steel in previous studies could be determined as trans-passive dissolution, where a certain amount of current density is needed to dissolve the anode material. Since the width in this study is larger than 2 times the nozzle diameter, it can be stated that the dissolution characteristic of the aluminium alloy is active, where minimal current densities lead to an electrochemical dissolution. The measurement results of the depth of the point erosions are shown in figure 4.
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the electrolyte jet, the highest removal rate was expected in the centre, too [13]. But it seems that high current densities lead to passivation effects and to a reduction of the dissolution rate in the centre of the point erosion.
Fig. 5. Cross sections of point erosions in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaCl electrolyte (U = 60 V; t = 2 s)
Fig. 4. Depth of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaBr electrolyte (U = 60 V)
As a result it can be derived, that the presence of particles reduces the passivation effect, since the point erosions in the specimens with 5% and 10% SiC particles do not have a conical pin in the centre and are significantly deeper. A possible reason could be, that the current density increases around the particles [8], which lead to slightly reduce width due to increased passivation. The increased depth is expected to be caused by flushing out the particles, without consuming any charge. Details were not analysed in this study. Therefore, the influence of particles on current density and removal characteristic should be investigated in further experiments and simulation In figure 6, the width of the point erosions as a function of the machining time is shown.
Similar to the width, the depth increases with increasing machining time. The point erosions in the specimens with 0% SiC particles are approximately 10 µm to 20 µm deeper, compared to the specimens with 5% and 10% SiC particles. The achieved depths in the specimens with 5% and 10% SiC particles are nearly similar with a maximal difference below 10%. As a result it can be stated, that the presence of reinforcement particles decreases the depth but the fraction of particles did not lead to a systematic influence.
3.2. Point erosions machined by using the NaCl electrolyte Figure 5 shows cross sections of point erosions, machined by using NaCl as electrolyte at a voltage of 60 V and machining time of 2 s. The cross sections of the reinforced specimens have the typical calotte-shaped geometry, which are nearly similar in width and depth. On the other hand, the cross section of the point erosion in the specimens with 0% SiC particles differs significantly in shape. It is slightly wider and about 40 % less deep. Additionally, in the centre of the point erosion is a conical pin. Since the highest current density appears in the centre of
Fig. 6. Width of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaCl electrolyte (U = 60 V)
Similar to the results with the NaBr electrolyte, the width increases at increasing time, when using the NaCl electrolyte. The specimens without SiC particles have the highest width, whereas the point erosions in the specimens with 10% SiC
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particles mostly have the smallest width. In general, using the NaCl electrolyte results in a width of approximately 2.2 to 3.2 times the nozzle diameter. Figure 7 shows the depth of the point erosions as a function of machining time.
In case of using the NaNO3 electrolyte, the point erosions did not lead to calotte–shaped geometries. Indeed, all the geometries are comparable to point erosion generated previously in EN AW 2017 with the NaCl electrolyte. All point erosions have similar shapes as well as nearly similar depths and widths. So, the presence and the fraction of the SiC particles did not influence the resulting removal geometry. The measurement results of the width as a function of machining time are shown in figure 9.
Fig. 7. Depth of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaCl electrolyte (U = 60 V)
As it can be seen, an increasing time leads to an increasing depth. The specimens with reinforcement show nearly similar results and higher values than in the pure aluminium alloy. The difference between the specimens with and without SiC particles increases from 20 µm at 0.5 s to 50 µm at 2.0 s. Since the results in the specimens with SiC particles show similar depths, it can be stated that the sheer presence of SiC particles leads to an increase of the depth of the point erosions, regardless of their fraction.
3.3. Point erosions machined by using the NaNO3 electrolyte
Fig. 9. Width of the point erosions as a function of the machining time in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles by using NaNO3 electrolyte (U = 60 V)
Here, the width slightly increases with increasing machining time, whereas the specimen with 10% SiC particles has the highest values and the specimens with 0% SiC particles has the smallest values. As a consequence, the width increases with increasing particle fraction. In general, the achieved width using the NaNO3 electrolyte is approximately 2.4 to 3.0 times the nozzle diameter. Figure 10 shows the corresponding depths of the point erosion as a function of the machining time.
The cross sections of point erosions machined with the NaNO3 electrolyte at a voltage of 60 V and a machining time of 2 s are shown in Figure 8.
Fig. 10. Depth of the point erosions as a function of the machining time, in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles, using NaNO3 electrolyte (U = 60 V) Fig. 8. Cross sections of point erosions in EN AW 2017 reinforced by 0%, 5% or 10% SiC particles by using NaNO3 electrolyte (U = 60 V; t = 2 s)
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Again, the depth of the point erosions increases with increasing machining time. Considering the different particle fractions, the achieved depths are almost similar in the three materials. Therefore, hardly any effect of the particles on the depth was detected. Summarizing the results of the measurements, it can be said, that using the NaCl electrolyte leads to the widest point erosions, while the NaBr electrolyte results in the narrowest point erosions. Considering the depth, again the NaCl electrolyte leads to the largest values, while the lowest depths were achieved using the NaNO3 electrolyte. The reinforcement particles have their most significant influence when using the NaCl electrolyte and the lowest influence when using the NaNO3 electrolyte.
3.4. Multiple Regression Analysis Using the data of this study and previous studies, a multiple linear regression analysis has been done to generate empirical models[6, 10]. The voltage range goes from 10 V to 60 V, considering all experiments done so far. The used specimens and machining times are similar to the specifications in this study. Hence, the multiple regression analysis is based on 132 data points for each of the three electrolytes. Since the width of the point erosions is affected majorly by the nozzle geometry, values between 2 and 3 times of the nozzle inner diameter are expected, as mentioned earlier. Thus, the analyses of the multiple regression in this study focus on the depth of the resulting point erosions. Equation 1 shows the used linear regression model, which is the adjusted version of the standard model for multiple linear regressions [16]. ݀ ൌ ܣଵ ߱ ڄ ܣଶ ܷ ڄ ܣଷ ݐ ڄ
(1)
In this model, the depth depends linearly on the fraction of particles ߱ in percent, the applied voltage ܷ in V and the machining time ݐin s. A constant coefficient ܣ is not included, since the initial depth before the machining is 0 µm. The coefficient of determination R2 was calculated in order to analyse, how well the developed regression model represents the real data points. It has a range from 0 to 1, while 0 means, the regression model cannot be used to describe the measured data and 1 means, that every data point can be calculated by the regression model [16]. Table 2 presents the regression coefficients for the NaBr electrolyte. Table 2. Regression coefficients NaBr Coefficient
Value
p-Value
ܣଵ ሾɊሿ
-0.70731
0.00842
ܣଶ ሾɊȀሿ
0.82544
0
ܣଷ ሾɊȀሿ
32.51437
0
The multiple regression model of the depth of point erosion using the NaBr electrolyte has an R2 of 0.969. This indicates, that the regression model represents nearly every measured data point.
Since the coefficient A1 is negative, an increasing particle fraction reduces the achievable depth. This is similar to the observations mentioned earlier. The p-Value is a statistical coefficient, which can be used to evaluate the significance of the input value to the regression analysis. By definition, p-Value is always positive and the smaller it is, the more significant is the corresponding input parameter [16]. Here, the voltage and the machining time are equally significant. The particle fraction is the least significant parameter in this case. Table 3 presents the regression coefficients for predicting the depth of point erosions using the NaCl electrolyte. Table 3. Regression coefficients NaCl Coefficient
Value
p-Value
ܣଵ ሾɊ݉ሿ
0.49779
0.12568
ܣଶ ሾɊ݉Ȁܸሿ
0.64956
0
ܣଷ ሾɊ݉Ȁݏሿ
27.81428
0
The R2 is 0.944, which means, that the developed regression model matches with approximately 94.4% of the data points. In contrast to NaBr, A1 is positive, so an increasing particle fraction increases the achievable depth. This is similar to what is shown in figure 7. Considering the p-value, again the machining time and the voltage have the highest significance. The particle fraction is the least significant parameter, with even less significance than in case of the NaBr electrolyte. The regression coefficients of the multiple regression analysis of the experiments using the NaNO3 electrolyte, are shown in table 4. Table 4. Regression coefficients NaNO3 Coefficient
Value
p-Value
ܣଵ ሾɊ݉ሿ
-0.39149
0,07593
ܣଶ ሾɊ݉Ȁܸሿ
0.39497
0
ܣଷ ሾɊ݉Ȁݏሿ
22.08199
0
The developed regression model is characterized by an R2 of 0.941. Here, the coefficient A1 is negative again, so an increasing particle fraction decreases the achievable depth. The parameters machining time and voltage are most significant. The particle fraction is the least significant input parameter, again. The analysis of the experiments presented in section 3.1 to 3.3 indicates that the reinforcement particles influence the shape, width and depth of point erosion, especially in the experiments using the NaCl electrolyte. On the other hand, the measurement results implied that the particle fraction itself is less important than the sheer presence of reinforcement particles. The analysis of the multiple regression of all associated experiments showed, that the particle fraction is the least significant input parameter. This means, that the particle fraction barely influences the result of the point erosions. As a consequence, the electrochemical dissolution characteristic of the investigated AMCs is primarily defined by the dissolution characteristic of the aluminium alloy EN AW 2017. Possible
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causes, could either be attributed to the small size of the particles, with an average diameter of 1 µm, or the small fraction in the AMC, with 5% or 10 %. [4]
4. Conclusion In this study Jet-ECM point erosions were realized in AMCs and guidelines for reliable finishing were derived by multiple linear regression analyses. The AMCs, consisting of the aluminium alloy EN AW 2017 reinforced with 0%, 5% and 10% SiC particles, were machined using three pH-neutral aqueous electrolytes of sodium bromide, sodium chloride and sodium nitrate. Results of point erosion, applying a voltage of 60 V and a machining time of 0.5 s to 2 s, were presented and discussed. Subsequently, multiple linear regression analyses, considering the measurement results of this and previous studies, were carried out. The analysis of the point erosions showed, that the dissolution characteristic differs, depending on the used electrolyte. The deepest point erosions were generated using NaCl as electrolyte, whereas the point erosion generated by using NaNO3 have the lowest depth. The width of the point erosions depends most on the used nozzle diameter. So the achievable width is in a range of 2 times to 3 times the nozzle diameter. The effect of the reinforcement particles was investigated in the three applied electrolytes, too. When using NaNO3, the depth and width were hardly affected by the particle fraction. But in case of NaCl and NaBr, the particles significantly influenced both the width and the depth. In the NaBr electrolyte, solely the presence of reinforcement particles affected the machining result, but the fraction of particles had less influence. But in NaCl a systematic influence was found, since the width of the erosions was decreased and the depth was increased by the increasing fraction of reinforcement particles. The results of the multiple linear regression analyses can be used as guidelines for reliable finishing of AMCs by Jet-ECM. One regression model was developed for each of the three used electrolytes. While the depth was used as the output parameter, the particle fraction, the voltage and the machining time were analysed as the input parameters. The R2 of each model was higher than 0.98, which means that more than 98% of the 132 underlying data points are predictable by the regression model. The analysis of the p-Value indicated the significance of each of the input parameters. For all three electrolytes, the particle fraction was the least significant parameter. Acknowledgements The authors acknowledge the DFG (German Research Foundation) for supporting this work carried out within the framework of project SFB 692 HALS. References [1] [2] [3]
K. K. Chawla, Composite Materials. New York, NY: Springer New York, 2012, ISBN: 978-0-387-74364-6, DOI: 10.1007/978-0-38774365-3 D. D. L. Chung, Composite Materials : Science and Applications. Springer London Dordrecht Heidelberg, 2010, ISBN: 978-1-84882830-8, DOI: 10.1007/978-1-84882-831-5 D. Nestler, S. Siebeck, H. Podlesak et al., "Powder Metallurgy of
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15] [16]
Particle-Reinforced Aluminium Matrix Composites (AMC) by Means of High-Energy Ball Milling", in Integrated Systems, Design and Technology 2010, Berlin, Heidelberg, 2011, pp. 93–107, DOI: 10.1007/978-3-642-17384-4_9 S. Wagner, S. Siebeck, M. Hockauf et al., "Effect of SiCReinforcement and Equal-Channel Angular Pressing on Microstructure and Mechanical Properties of AA2017", Adv. Eng. Mater., vol. 14, no. 6, pp. 388–393, Jun. 2012, ISSN: 14381656, DOI:10.1002/adem.201100253 A. Schubert, U. Götze, M. Hackert-Oschätzchen et al., "Evaluation of the Technical-Economic Potential of Particle- Reinforced Aluminum Matrix Composites and Electrochemical Machining", IOP Conf. Ser. Mater. Sci. Eng., vol. 118, p. 10, 2016, ISSN: 1757-8981, DOI:10.1088/1757-899X/118/1/012035 M. Hackert-Oschätzchen, N. Lehnert, A. Martin et al., "Jet Electrochemical Machining of Particle Reinforced Aluminum Matrix Composites with Different Neutral Electrolytes", IOP Conf. Ser. Mater. Sci. Eng., vol. 118, p. 6, 2016, ISSN: 1757-899X, DOI:10.1088/1757-899X/118/1/012036 M. Hackert-Oschätzchen, N. Lehnert, A. Martin et al., "Surface Characterization of Particle Reinforced Aluminum-matrix Composites Finished by Pulsed Electrochemical Machining", Procedia CIRP, vol. 45, pp. 351–354, 2016, ISSN: 2212-8271, DOI:10.1016/j.procir.2016.02.078 M. Hackert-Oschätzchen, N. Lehnert, M. Kowalick et al., "Analysis of an Electrochemical Machining Process for Particle Reinforced Aluminium Matrix Composites", in Proceeding of the European COMSOL Conference, München, 2016, p. 5 A. Schubert, M. Hackert-Oschätzchen, N. Lehnert et al., "Analysis of the Electrochemical Dissolution Characteristics of High-Strength Particle Reinforced Aluminium Matrix Composites", in 17th International Conference on the Strength of Materials (ICSMA-17), Brno, 2015, vol. 17, DOI: 10.13140/RG.2.1.3929.7764 M. Hackert-Oschätzchen, R. Morgenstern, N. Lehnert et al., "Basic Research on Electrochemical Machining of Particle Reinforced Aluminium Matrix Composites", in International Symposium on Electrochemical Machining Technology (INSECT ), 2015, pp. 35–42 A. Schubert, M. Hackert-Oschätzchen, G. Meichsner et al., "Evaluation of the Influence of the Electric Potential in Jet Electrochemical Machining", in Proceedings of the 7th International Symposium on Electrochemical Machining Technology, 2011, pp. 47– 54, ISBN: 978-3-00-036247-7 M. Hackert-Oschätzchen, G. Meichsner, M. Zinecker et al., "Micro machining with continuous electrolytic free jet", Precis. Eng., vol. 36, no. 4, pp. 612–619, 2012, ISSN: 0141-6359, DOI:10.1016/j.precisioneng.2012.05.003 M. Hackert-Oschätzchen, R. Paul, A. Martin et al., "Study on the dynamic generation of the jet shape in Jet Electrochemical Machining", J. Mater. Process. Technol., vol. 223, pp. 240–251, Sep. 2015, ISSN: 0924-0136, DOI:10.1016/j.jmatprotec.2015.03.049 W. Natsu, T. Ikeda, and M. Kunieda, "Generating complicated surface with electrolyte jet machining", Precis. Eng., vol. 31, no. 1, pp. 33–39, Jan. 2007, ISSN: 01416359, DOI:10.1016/j.precisioneng.2006.02.004 W. Natsu, T. Ikeda, and M. Kunieda, "Machining complicated patterns with electrolyte jet machining", in Proceedings of the 5th Euspen International Conference, 2005, pp. 649–652 N. R. Draper and H. Smith, Applied Regression Analysis. John Wiley & Sons, 2014, ISBN: 978-1-118-62568-2