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Optimizations of electric current assisted chemical milling condition of 2219 aluminum alloy
Journal of Materials Processing Tech. 249 (2017) 379–385
Contents lists available at ScienceDirect
Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Research Paper
Optimizations of electric current assisted chemical milling condition of 2219 aluminum alloy
MARK
⁎
Qiushi Li, Jihui Wang , Wenbin Hu Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical milling Aluminum alloy Electric current Milling rate Surface roughness
The chemical milling condition of 2219 aluminum alloy is investigated in the NaOH + Na2S + triethanolamine + Al3+ alkaline system by the assistance of electric currents. The purpose is to study the influences of electric current densities on both the milling rate and the surface roughness of 2219 alloy under different temperatures, NaOH concentrations and additions of inhibitors. The experiments are carried out under the given requirement that the milling rate is from 0.08 mm/min to 0.14 mm/min and the surface roughness is less than 0.65 μm. The reaction temperature of the chemical milling process can be reduced by 10 °C with the application of current densities in the range of 0–20 mA/cm2. The concentration of NaOH in the alkaline system can be reduced from 180 g/L to 120 g/L with the application of current densities in the range of 0–60 mA/cm2. With the addition of Na2SiO3, Na2CO3 and Na2SnO3 inhibitors in the alkaline system, both the milling rate and the surface roughness of 2219 alloy decrease. The region enclosed with the suitable NaOH concentration and current density expands and moves towards the high current density.
1. Introduction Chemical milling is a non-traditional machining process, by which materials are removed in strong corrosive solutions and components with complex geometries and accurate dimensions can be machined (Çakir, 2008). In addition, using chemical milling to realize a highly efficient and environmentally-benign manufacturing is presented and evaluated by McCallion (1987). As a result, chemical milling is widely applied in the manufacturing of steel or aluminum alloy products. Sanz (1956) has firstly introduced chemical milling for industry, classifying the chemical milling solutions used for aluminum alloys into two groups. One group is the acidic system, which is mainly composed by ferric nitrate or ferric chloride. Chambers (2000) has invented a viable etchant composed of ferric nitrate and disclosed the relationship between the etch rate and the sufficient concentration of ferric compounds. However, researches on the surface quality affected by operating conditions are lacking. Çakir (2008) has described the chemical milling of aluminum alloys in ferric chloride at different operating temperatures. In the acidic system, the chemical milling of aluminum alloys can be operated at a relatively low temperature (20–50 °C), while the processed aluminum alloy has a high surface roughness (about 7–10 μm). The other group is the alkaline system, which consists of sodium hydroxide and dissolved aluminum with the addition of Na2S, triethanolamine (TEA), NaNO3, Na2CO3, etc. Gross (1986) has reported
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a chemical milling solution which contains sodium hydroxide, nitrate and ethylene glycol. This solution has an advantage in machining aluminum alloys with high compositions of copper or zinc, e.g., 2219 aluminum alloy. However, the operating temperature needs to be maintained at 90 °C in this chemical milling solution. Matsumoto et al. (2007) have demonstrated a chemical milling method to produce density-graded aluminum foams by using a pH-temperature controlled NaOH bath. This work focuses on researching the influences of processing parameters on the microstructure. The result shows that a delicate morphology can be obtained by elevating the pH. A comparison made by Chandler (2008) has also revealed the same trend. Compared with the chemical milling process in the acidic system, the process in the alkaline system results a better surface roughness with a higher temperature (55–65 °C). To further use the advantage of the alkaline system in the surface quality, efforts have been made on promoting the surface quality of samples processed in the alkaline system by altering solution compositions and operating conditions. The works in the literature have contributed to the knowledge of the influences of processing parameters on the machining quality of chemical milling. Smooth surfaces and applicable milling rates can be obtained by adjusting parameters. Li et al. (2015) have recently presented the optimization of a chemical milling solution for 2219 aluminum alloy. This solution is composed of NaOH, Na2S, TEA, and Al3+. In this solution, a low surface roughness (0.5–0.8 μm) of 2219 alloy can be obtained.
Corresponding author. E-mail addresses: [email protected] (Q. Li), [email protected] (J. Wang), [email protected] (W. Hu).
http://dx.doi.org/10.1016/j.jmatprotec.2017.06.028 Received 16 December 2016; Received in revised form 14 June 2017; Accepted 19 June 2017 Available online 20 June 2017 0924-0136/ © 2017 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 249 (2017) 379–385
Q. Li et al.
as illustrated by Rosilda et al. (1994). Three inhibitors with different compositions of 0.02% Na2SiO3, 0.02% Na2SiO3 + 10% Na2CO3 and 0.03% Na2SnO3 were selected as the additives in the proposed experiment. The results from the immersion tests indicated that the inhibition efficiencies of 0.02% Na2SiO3, 0.02% Na2SiO3 + 10% Na2CO3 and 0.03% Na2SnO3 for 2219 alloy in the above alkaline solution were 15%, 20% and 32%, respectively. Electric currents with the density of 20, 40, 60, 80, 100 mA/cm2 were imposed on the 2219 aluminum alloy. Pulse currents were used in the proposed experiment, following a cycle of 15 s forward current and 5 s backward current. The sample of 2219 aluminum alloy with a working area of 12.8 cm2 was used as the working electrode, and a graphite rod was applied as the counter electrode. The reaction temperatures of the milling process were 20, 40, 60 and 80 °C controlled by a water bath. After processed for 60 min, the samples were taken out from the solution, and then treated in 30% (wt. %) HNO3 solution for glaring until the black precipitations absorbed on the surface were dissolved completely. All the chemical reagents used above were in analytical grade, and the chemical milling solution was prepared with distilled water. After cleaned with water and dried, the samples were well prepared for surface characterizations.
However, a high reaction temperature (80 °C) and a high NaOH concentration (160–200 g L−1) are still required for the chemical milling process. In order to improve the operating condition of chemical milling processes, the reaction temperature and the NaOH concentration for the alkaline system need to be reduced. Electrochemical machining (ECM) is a well-established technology for shaping metals to produce complex parts in aerospace, defense and medical industries (Pajak et al., 2006). Lohrengel et al. (2016) have recently reported the mechanisms of the anodic dissolution during electrochemical machining. The mechanisms can be classified by the changing of surface conditions during ECM. The removal rate of the ECM process is controlled by the anodic dissolution of materials at high current densities of up to 100 A/cm2. In the study reported by Meichsner et al. (2016), the pulse current is introduced into ECM to achieve fast removal rate and precise control. The machining precision of products can be improved by the application of pulsed currents, laser beams, charge modulations, etc. However, the research on the combination of chemical milling and electrochemical machining for shaping materials remains an open direction. The objective of this work is to explore the chemical milling conditions of 2219 aluminum alloy in the alkaline system under different electric current densities. The milling rate and the surface roughness of 2219 alloy are investigated under different milling conditions. With the combination of chemical milling and electrochemical machining, not only the operating conditions such as the reaction temperature and the NaOH concentration but also the composition of alkaline system can be optimized to improve the safety and the operability of the chemical milling process.
2.3. Characterizations The surface roughness of 2219 aluminum alloy after chemical milling was measured by the Talysurf tester (Taylor-Hobson Form Talysurf i120). The chemical milling rate v was calculated by the equation formulated as follows,
v = (m1 − m2)/(ρ ·a·b·t )
2. Experimental
(1)
The chemical compositions of 2219 aluminum alloy used in this paper were shown in Table 1. A plate of 2219 aluminum alloy with the thickness of 6 mm was firstly machined to samples with the dimensions of 20 mm × 20mm × 6 mm, and then abraded by water emery papers from 200 to 1500 SiC grit. After cleaned with ethanol and acetone, the samples were dried for chemical milling.
where m1 (g) and m2 (g) represented the weights of the samples before and after milling, respectively; ρ (g cm−3) represented the density of 2219 aluminum alloy; a (cm) and b (cm) represent the length and width of the samples after milling, respectively; t (min) represented the processing time of chemical milling. The unit of the chemical milling rate obtained from the proposed equation was cm/min. For the simplicity of the following discussion, the unit was changed from cm/min to mm/ min. After the characterizations, the experimental results and further discussions were presented in the following section.
2.2. Electric current assisted chemical milling
3. Results and discussion
The reaction temperature and the NaOH concentration needed to be reduced for the safety and the operability of the chemical milling process. The purpose of this design was to explore the methods to reduce the temperature and the NaOH concentration. Thus, the ECM method of imposing electric currents was worth a try. In the experiment, the electrolyte solution for chemical milling was mainly composed by 120–180 g L−1 NaOH, 5 g L−1 Na2S, 60 g L−1 TEA and 25 g L−1 Al3+. This solution was the optimal milling solution with the lowest surface roughness of 2219 aluminum alloy as demonstrated by Li et al. (2015). In the proposed experiment, inhibitors were added into the above solution to adjust the milling rate of the alloy. As reviewed in the literature, both Triki et al. (1979) and Lopez-Garrity and Frankel (2014) investigated that sodium silicate could be an effective inhibitor for aluminum in the alkaline system due to the formation of films on metal surfaces. Sodium carbonate was reported as a surface-finishing agent in the etching solution for aluminum alloys by An et al. (2002). Sodium stannate was also an inhibitor for aluminum in alkaline media
3.1. Effects of the temperature under electric currents
2.1. Materials
The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and temperatures are illustrated in Fig. 1. As shown in Fig. 1a, the milling rate of 2219 alloy varies from 0.03 mm/min to 0.27 mm/min. The milling rate increases obviously as the current density and the temperature increase. As shown in Fig. 1b, the surface roughness of 2219 alloy varies from 0.55 μm to 1.39 μm. The surface roughness increases as the current density increases under the same temperature. While under the same current density, the surface roughness fluctuates as the temperature increases. The surface roughness at 20 °C and 60 °C is larger than that at 40 °C and 80 °C. Based on the data in Fig. 1, three-dimensional graphs for the behaviors of the milling rate and the surface roughness of the 2219 alloy with respect to the current density and the temperature are illustrated in Fig. 2. By considering the practical applications in the industry, DeGarmo et al. (2003) have described in the publication that the
Table 1 Chemical compositions of 2219 aluminum alloy (wt.%). Cu
Si
Fe
Mn
Mg
V
Zr
Zn
Ti
Others
Al
5.8–6.8
≤0.20
≤0.30
0.20–0.40
≤0.02
0.05–0.15
0.10–0.25
≤0.10
0.02–0.10
≤0.15
Bal.
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Fig. 1. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and temperatures.
material removal rate is among the range of 0.013–0.076 mm/min in general. The surface roughness varying from 1.6 μm to 6.3 μm is normally required for chemical milling as mentioned by Field et al. (1989). Based on the chemical milling results of 2219 aluminum alloy in the alkaline system (Li et al., 2015), the milling rate in the range of 0.08–0.14 mm/min and the surface roughness less than 0.65 μm are appropriate. The milling rate and the surface roughness in these ranges are chosen as the required milling rate and surface roughness in the present paper for electric current assisted chemical milling of 2219 aluminum alloy. The regions of the bottom projections under the required milling rate and surface roughness represent the adequate ranges of current densities and temperatures, shown in Fig. 2. Fig. 3 shows two adequate ranges of the current density and the temperature. One is obtained under the required milling rate, and the other is obtained under the required surface roughness. The overlapping region satisfies both of the required milling rate and surface roughness. The suitable operating conditions are located in this overlapping region with the temperature varying from 70 °C to 80 °C and the current density varying from 0 mA/cm2 to 20 mA/cm2. As shown in Fig. 3, with the application of an appropriate current density in the range of 0–20 mA/cm2, the reaction temperature can be reduced by approximately 10 °C. Thus, the safety of the chemical milling process is improved.
Fig. 3. Ranges of the current density and the temperature of 2219 aluminum alloy under the milling rate of 0.08–0.14 mm/min and surface roughness less than 0.65 μm.
Fig. 5 shows two adequate ranges of the current density and the NaOH concentration. One is obtained under the required milling rate, and the other is obtained under the required surface roughness. The overlapping region satisfies both of the required milling rate and surface roughness. From the aspect of the milling rate, the concentration of NaOH can be reduced from 180 g/L to 120 g/L with the increasing current density from 0 to 100 mA/cm2, shadowed with the black vertical lines in Fig. 5. From the aspect of surface roughness, the concentration of NaOH can be reduced from 180 g/L to 120 g/L with the increasing current density from 0 to 57 mA/cm2, shadowed with the red slashes in Fig. 5. Under the requirement of the milling rate and the surface roughness, the 2219 alloy can be processed by electric current assisted chemical milling with a reduced NaOH concentration range of 180 g/L to 120 g/L. Thus, the safety of the chemical milling process is also improved.
3.2. Effects of the NaOH concentration under electric currents Fig. 4 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations. As shown in Fig. 4a, the milling rate varies from 0.05 mm/ min to 0.27 mm/min. The milling rate increases obviously as the current density and the NaOH concentration increase. As shown in Fig. 4b, the surface roughness of 2219 alloy varies from 0.50 μm to 1.02 μm. Under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH.
Fig. 2. Three-dimensional graphs of the milling rate and the surface roughness of 2219 aluminum alloy with respect to the current density and the temperature.
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Fig. 4. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations.
roughness (Fig. 6b) is slightly lower than that in the solution without 0.02% Na2SiO3 (Fig. 4b). Fig. 7 shows the regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 under the required milling rate and surface roughness. Compared with the milling rate boundaries without inhibitors, both of the milling rate boundaries at 0.14 mm/min and 0.08 mm/min with 0.02% Na2SiO3 move towards the high current density and the high NaOH concentration. Meanwhile, the surface roughness boundary at 0.65 μm with 0.02% Na2SiO3 also moves to the high current density, compared with the boundary without inhibitors. Thus, with the addition of 0.02% Na2SiO3, the region of the suitable operating conditions moves to the high current density and the high NaOH concentration, i.e., from the gray shaded area to the green shaded area shown in Fig. 7. Compared with the region of the suitable operating conditions without the inhibitor (grey shaded area), the region of the suitable operating conditions with the inhibitor (green shaded area) for the current density and the NaOH concentration expands. With the application of the current density of 45–60 mA/cm2, the chemical milling of 2219 alloy can be processed in the solution with the addition of 0.02% Na2SiO3 and a low NaOH concentration of 120 g/L. The addition of the inhibitor slows down the changes of the milling rate and the surface roughness. With the addition of 0.02% Na2SiO3, the selection of the operating conditions for the current density and the NaOH concentration becomes more flexible.
Fig. 5. Ranges of the current density and the NaOH concentration of 2219 aluminum alloy under the milling rate of 0.08–0.14 mm/min and surface roughness less than 0.65 μm. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
3.3. Effects of the inhibitors under electric currents 3.3.1. Na2SiO3 Fig. 6 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations in the solution with 0.02% Na2SiO3. As shown in Fig. 6a, the milling rate increases obviously as the current density and the NaOH concentration increase. Under the same current density and NaOH concentration, the milling rate in the solution with 0.02% Na2SiO3 (Fig. 6a) is lower than the milling rate in the solution without inhibitors (Fig. 4a). As shown in Fig. 6b, under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH. Under the same current density and NaOH concentration, the surface
3.3.2. Na2SiO3 + Na2CO3 Though the chemical milling of 2219 alloy can be processed in the solution with the addition of 0.02% Na2SiO3 and a low NaOH concentration of 120 g/L, the alternative range of the current density is narrow. Sodium carbonate can be used as the surface-finishing agent to adjust the milling rate and the surface roughness for the chemical milling process. In the proposed experiment, 10% Na2CO3 is added into the solution with 0.02% Na2SiO3 for further adjustment. Fig. 8 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations in the Fig. 6. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations with 0.02% Na2SiO3.
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Fig. 7. Regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 under the required milling rate and surface roughness.
Fig. 9. Regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 + 10% Na2CO3 under the required milling rate and surface roughness.
solution with 0.02% Na2SiO3 + 10% Na2CO3. As shown in Fig. 8a, the milling rate increases obviously as the current density and the NaOH concentration increase. Under the same current density and NaOH concentration, the milling rate in the solution with 0.02% Na2SiO3 + 10% Na2CO3 (Fig. 8a) is lower than the milling rates in the solution with 0.02% Na2SiO3 (Fig. 6a) and the solution with no inhibitors (Fig. 4a). As shown in Fig. 8b, under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH. Under the same current density and NaOH concentration, the surface roughness (Fig. 8b) is slightly lower than that in the condition with 0.02% Na2SiO3 (Fig. 6b) as well as the surface roughness in the condition without inhibitors (Fig. 4b). Fig. 9 shows the regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 + 10% Na2CO3 under the required milling rate and surface roughness. Compared with the milling rate boundaries without inhibitors, both of the milling rate boundaries at 0.14 mm/min and 0.08 mm/min with 0.02% Na2SiO3 + 10% Na2CO3 move towards the high current density and the high NaOH concentration. Meanwhile, the surface roughness boundary at 0.65 μm with 0.02% Na2SiO3 + 10% Na2CO3 also moves to the high current density, compared with the boundary without inhibitors. Thus, with the addition of 0.02% Na2SiO3 + 10% Na2CO3, the region of the suitable operating conditions moves to the high current density and the high NaOH concentration. Compared with the region of the suitable operating conditions without the inhibitor (grey shaded area), the region of the suitable operating conditions with the inhibitor (green shaded area) for the current density and the NaOH concentration expands. With the application of the
current density of 40–70 mA/cm2, the chemical milling of 2219 alloy can be also processed in the solution with the addition of 0.02% Na2SiO3 + 10% Na2CO3 and a low NaOH concentration of 120 g/L. The addition of the inhibitor slows down the changes of the milling rate and the surface roughness. With the addition of 0.02% Na2SiO3 + 10% Na2CO3, the selection of the operating conditions for the current density and the NaOH concentration becomes more flexible. 3.3.3. Na2SnO3 Fig. 10 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations in the solution with 0.03% Na2SnO3. As shown in Fig. 10a, the milling rate increases obviously as the current density and the NaOH concentration increase. Under the same current density and NaOH concentration, the milling rate in the solution with 0.03% Na2SnO3 (Fig. 10a) is lower than the milling rate in the solution without inhibitors (Fig. 4a). As shown in Fig. 10b, under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH. Under the same current density and NaOH concentration, the surface roughness (Fig. 10b) is slightly lower than that in the solution without inhibitors (Fig. 4b). Fig. 11 shows the regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.03% Na2SnO3 under the required milling rate and surface roughness. Compared with the milling rate boundaries without the inhibitor, both of the milling rate boundaries at 0.14 mm/min and 0.08 mm/min with 0.03% Na2SnO3 move towards the high current density and the high NaOH concentration. Meanwhile, the surface roughness boundary at 0.65 μm with 0.03% Na2SnO3 also moves to the high current density, Fig. 8. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations with 0.02% Na2SiO3 + 10% Na2CO3.
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Fig. 10. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations with 0.03% Na2SnO3.
gradually increase in the sequence of Na2SiO3, Na2SiO3 + Na2CO3 and Na2SnO3, because of the increasing inhibition efficiency. By adding inhibitors into the chemical milling solution, the selection of the suitable operating conditions becomes more flexible. Thus, the operability of the electric current assisted chemical milling process is further improved. 4. Conclusions (1) For the chemical milling process of 2219 aluminum alloy in the alkaline system, the application of the current density in a range of 0–20 mA/cm2 has reduced the reaction temperature by 10 °C. This reduction in temperatures improves the safety and the operability of the chemical milling process. (2) Under the given requirement of the milling rate and the surface roughness, 2219 aluminum alloy has been successfully processed under a reduced concentration of NaOH with a current density of 0–60 mA/cm2. The concentration of NaOH has been reduced down to 120 g/L with the current density of 28–57 mA/cm2. This reduction in NaOH concentrations also improves the safety and the operability of the chemical milling process. (3) With the addition of Na2SiO3, Na2CO3 and Na2SnO3 inhibitors in the alkaline system, the milling rate and the surface roughness of 2219 aluminum alloy decrease. The adequate region of the NaOH concentration and the current density expands and moves towards the direction of the high current density. By adding inhibitors into the chemical milling solution, the selection of the suitable operating conditions becomes more flexible, which further improves the operability of the electric current assisted chemical milling process.
Fig. 11. Regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.03% Na2SnO3 under the required milling rate and surface roughness.
compared with the boundary without inhibitors. Thus, with the addition of 0.03% Na2SnO3, the region of the suitable operating conditions moves to the high current density and the high NaOH concentration. Compared with the region of the suitable operating conditions without the inhibitor (grey shaded area), the region of the suitable operating conditions with the inhibitor (green shaded area) for the current density and the NaOH concentration expands. With the application of the current density of 74 mA/cm2, the chemical milling of 2219 alloy can be also processed in the solution with the addition of 0.03% Na2SnO3 and a low NaOH concentration of 121 g/L. The addition of the inhibitor slows down the changes of the milling rate and the surface roughness. With the addition of 0.03% Na2SnO3, the selection of the operating conditions for the current density and the NaOH concentration becomes more flexible. From the above experimental results, it can be concluded that with the assistance of electric currents, the reaction temperature for chemical milling of 2219 aluminum alloy in the alkaline system can be reduced by 10 °C. With the application of the current density in a range of 28–57 mA/cm2, the concentration of NaOH can be significantly reduced from 180 g/L to 120 g/L. This reduction in temperatures and NaOH concentrations improve the safety and the operability of the chemical milling process of 2219 aluminum alloy in the alkaline system. With the addition of inhibitors in the alkaline system, both of the milling rate and surface roughness decrease due to the inhibition effects of Na2SiO3, Na2SiO3 + Na2CO3 and Na2SnO3 on aluminum alloys. Under the required milling rate and surface roughness, the adequate regions of the NaOH concentration and the current density for adding all the three kinds of inhibitors expand and move towards the direction of the high current density, shown in Figs. 7, 9 and 11. The moving distances of the adequate regions for adding different inhibitors
Acknowledgements This work was supported by National Basic Research Program of China (2014CB046801); National Natural Science Foundation of China (51471117); and Key Project of Tianjin Natural Science Foundation (13JCZDJC29500). References Çakir, O., 2008. Chemical etching of aluminum. J. Mater. Process. Technol. 199, 337–340. An, M.Z., Zhao, L.C., Tu, Z.M., 2002. Graining technology by electrolytic etching on the surface of aluminum alloys. Mater. Chem. Phys. 77, 170–178. Chambers, B., 2000. Etching of aluminum alloys by ferric ion. Metal Finish. 98, 26–29. Chandler, W., 2008. Pros and cons of alkaline vs. acid etching of aluminum: method chosen boils down to desired appearance and physical attributes of finished product. Metal Finish. 106 (15), 18. DeGarmo, E.P., Black, J.T., Kohser, R.A., 2003. Materials and Process in Manufacturing, ninth ed. John Wiley & Sonspp. 242–246. Field, M., Kahles, J.F., Koster, W.P., 1989. Surface finish and surface integrity. In: Davis, J.R., SocietyAmerican Society for Metals (Eds.), ASM handbook vol. 16: Machining. ASM International, pp. 43–84. Gross, D.W., 1986. Chemical milling processes and etchants therefor, US Patent no.: 4, 588, 474 pp.
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electrochemical machining. Procedia CIRP 46, 123–126. Pajak, P.T., Harrison, D.K., McGeough, J.A., Desilva, A.K.M., 2006. Precision and efficiency of laser assisted jet electrochemical machining. Prec. Eng. 20, 288–298. Rosilda, L.G.S., Ganesan, M., Kulandainathan, M.A., Kapali, V., 1994. Influence of inhibitors on corrosion and anodic behaviour of different grades of aluminium in alkaline media. J. Power Sour. 50, 321–329. Sanz, M.C., 1956. Process of chemically milling structural shapes and resultant article, US Patent no.: 2, 739, 047 pp. Triki, E., Daufin, G., Labbé, J.P., Pagetti, J., 1979. Mechanisms of corrosion inhibition for an aluminum-silicon-magnesium alloy in 0.1 N NaOH solutions at 60°C. Mater. Corros. 30, 259–265.
Li, Q., Wang, J., Hu, W., 2015. Optimization of chemical milling solution for 2219 aluminum alloy. Trans. Mater. Heat. Treat. 36, 243–249. Lohrengel, M.M., Rataj, K.P., Munninehogg, T., 2016. Electrochemical machining-mechanisms of anodic dissolution. Electrochim. Acta 201, 348–353. Lopez-Garrity, O., Frankel, G.S., 2014. Corrosion inhibition of AA2024-T3 by sodium silicate. Electrochim. Acta 130, 9–21. Matsumoto, Y., Brothers, A.H., Stock, S.R., Dunand, D.C., 2007. Uniform and graded chemical milling of aluminum foams. Mater. Sci. Eng. A 447, 150–157. McCallion, H., 1987. High production chemical milling. Prod. Eng. 66, 14–16. Meichsner, G., Hackert-Oschatzchen, M., Kronert, M., Edelmann, J., Schubert, A., Putz, M., 2016. Fast determination of the material removal characteristics in pulsed
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Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Research Paper
Optimizations of electric current assisted chemical milling condition of 2219 aluminum alloy
MARK
⁎
Qiushi Li, Jihui Wang , Wenbin Hu Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical milling Aluminum alloy Electric current Milling rate Surface roughness
The chemical milling condition of 2219 aluminum alloy is investigated in the NaOH + Na2S + triethanolamine + Al3+ alkaline system by the assistance of electric currents. The purpose is to study the influences of electric current densities on both the milling rate and the surface roughness of 2219 alloy under different temperatures, NaOH concentrations and additions of inhibitors. The experiments are carried out under the given requirement that the milling rate is from 0.08 mm/min to 0.14 mm/min and the surface roughness is less than 0.65 μm. The reaction temperature of the chemical milling process can be reduced by 10 °C with the application of current densities in the range of 0–20 mA/cm2. The concentration of NaOH in the alkaline system can be reduced from 180 g/L to 120 g/L with the application of current densities in the range of 0–60 mA/cm2. With the addition of Na2SiO3, Na2CO3 and Na2SnO3 inhibitors in the alkaline system, both the milling rate and the surface roughness of 2219 alloy decrease. The region enclosed with the suitable NaOH concentration and current density expands and moves towards the high current density.
1. Introduction Chemical milling is a non-traditional machining process, by which materials are removed in strong corrosive solutions and components with complex geometries and accurate dimensions can be machined (Çakir, 2008). In addition, using chemical milling to realize a highly efficient and environmentally-benign manufacturing is presented and evaluated by McCallion (1987). As a result, chemical milling is widely applied in the manufacturing of steel or aluminum alloy products. Sanz (1956) has firstly introduced chemical milling for industry, classifying the chemical milling solutions used for aluminum alloys into two groups. One group is the acidic system, which is mainly composed by ferric nitrate or ferric chloride. Chambers (2000) has invented a viable etchant composed of ferric nitrate and disclosed the relationship between the etch rate and the sufficient concentration of ferric compounds. However, researches on the surface quality affected by operating conditions are lacking. Çakir (2008) has described the chemical milling of aluminum alloys in ferric chloride at different operating temperatures. In the acidic system, the chemical milling of aluminum alloys can be operated at a relatively low temperature (20–50 °C), while the processed aluminum alloy has a high surface roughness (about 7–10 μm). The other group is the alkaline system, which consists of sodium hydroxide and dissolved aluminum with the addition of Na2S, triethanolamine (TEA), NaNO3, Na2CO3, etc. Gross (1986) has reported
⁎
a chemical milling solution which contains sodium hydroxide, nitrate and ethylene glycol. This solution has an advantage in machining aluminum alloys with high compositions of copper or zinc, e.g., 2219 aluminum alloy. However, the operating temperature needs to be maintained at 90 °C in this chemical milling solution. Matsumoto et al. (2007) have demonstrated a chemical milling method to produce density-graded aluminum foams by using a pH-temperature controlled NaOH bath. This work focuses on researching the influences of processing parameters on the microstructure. The result shows that a delicate morphology can be obtained by elevating the pH. A comparison made by Chandler (2008) has also revealed the same trend. Compared with the chemical milling process in the acidic system, the process in the alkaline system results a better surface roughness with a higher temperature (55–65 °C). To further use the advantage of the alkaline system in the surface quality, efforts have been made on promoting the surface quality of samples processed in the alkaline system by altering solution compositions and operating conditions. The works in the literature have contributed to the knowledge of the influences of processing parameters on the machining quality of chemical milling. Smooth surfaces and applicable milling rates can be obtained by adjusting parameters. Li et al. (2015) have recently presented the optimization of a chemical milling solution for 2219 aluminum alloy. This solution is composed of NaOH, Na2S, TEA, and Al3+. In this solution, a low surface roughness (0.5–0.8 μm) of 2219 alloy can be obtained.
Corresponding author. E-mail addresses: [email protected] (Q. Li), [email protected] (J. Wang), [email protected] (W. Hu).
http://dx.doi.org/10.1016/j.jmatprotec.2017.06.028 Received 16 December 2016; Received in revised form 14 June 2017; Accepted 19 June 2017 Available online 20 June 2017 0924-0136/ © 2017 Elsevier B.V. All rights reserved.
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as illustrated by Rosilda et al. (1994). Three inhibitors with different compositions of 0.02% Na2SiO3, 0.02% Na2SiO3 + 10% Na2CO3 and 0.03% Na2SnO3 were selected as the additives in the proposed experiment. The results from the immersion tests indicated that the inhibition efficiencies of 0.02% Na2SiO3, 0.02% Na2SiO3 + 10% Na2CO3 and 0.03% Na2SnO3 for 2219 alloy in the above alkaline solution were 15%, 20% and 32%, respectively. Electric currents with the density of 20, 40, 60, 80, 100 mA/cm2 were imposed on the 2219 aluminum alloy. Pulse currents were used in the proposed experiment, following a cycle of 15 s forward current and 5 s backward current. The sample of 2219 aluminum alloy with a working area of 12.8 cm2 was used as the working electrode, and a graphite rod was applied as the counter electrode. The reaction temperatures of the milling process were 20, 40, 60 and 80 °C controlled by a water bath. After processed for 60 min, the samples were taken out from the solution, and then treated in 30% (wt. %) HNO3 solution for glaring until the black precipitations absorbed on the surface were dissolved completely. All the chemical reagents used above were in analytical grade, and the chemical milling solution was prepared with distilled water. After cleaned with water and dried, the samples were well prepared for surface characterizations.
However, a high reaction temperature (80 °C) and a high NaOH concentration (160–200 g L−1) are still required for the chemical milling process. In order to improve the operating condition of chemical milling processes, the reaction temperature and the NaOH concentration for the alkaline system need to be reduced. Electrochemical machining (ECM) is a well-established technology for shaping metals to produce complex parts in aerospace, defense and medical industries (Pajak et al., 2006). Lohrengel et al. (2016) have recently reported the mechanisms of the anodic dissolution during electrochemical machining. The mechanisms can be classified by the changing of surface conditions during ECM. The removal rate of the ECM process is controlled by the anodic dissolution of materials at high current densities of up to 100 A/cm2. In the study reported by Meichsner et al. (2016), the pulse current is introduced into ECM to achieve fast removal rate and precise control. The machining precision of products can be improved by the application of pulsed currents, laser beams, charge modulations, etc. However, the research on the combination of chemical milling and electrochemical machining for shaping materials remains an open direction. The objective of this work is to explore the chemical milling conditions of 2219 aluminum alloy in the alkaline system under different electric current densities. The milling rate and the surface roughness of 2219 alloy are investigated under different milling conditions. With the combination of chemical milling and electrochemical machining, not only the operating conditions such as the reaction temperature and the NaOH concentration but also the composition of alkaline system can be optimized to improve the safety and the operability of the chemical milling process.
2.3. Characterizations The surface roughness of 2219 aluminum alloy after chemical milling was measured by the Talysurf tester (Taylor-Hobson Form Talysurf i120). The chemical milling rate v was calculated by the equation formulated as follows,
v = (m1 − m2)/(ρ ·a·b·t )
2. Experimental
(1)
The chemical compositions of 2219 aluminum alloy used in this paper were shown in Table 1. A plate of 2219 aluminum alloy with the thickness of 6 mm was firstly machined to samples with the dimensions of 20 mm × 20mm × 6 mm, and then abraded by water emery papers from 200 to 1500 SiC grit. After cleaned with ethanol and acetone, the samples were dried for chemical milling.
where m1 (g) and m2 (g) represented the weights of the samples before and after milling, respectively; ρ (g cm−3) represented the density of 2219 aluminum alloy; a (cm) and b (cm) represent the length and width of the samples after milling, respectively; t (min) represented the processing time of chemical milling. The unit of the chemical milling rate obtained from the proposed equation was cm/min. For the simplicity of the following discussion, the unit was changed from cm/min to mm/ min. After the characterizations, the experimental results and further discussions were presented in the following section.
2.2. Electric current assisted chemical milling
3. Results and discussion
The reaction temperature and the NaOH concentration needed to be reduced for the safety and the operability of the chemical milling process. The purpose of this design was to explore the methods to reduce the temperature and the NaOH concentration. Thus, the ECM method of imposing electric currents was worth a try. In the experiment, the electrolyte solution for chemical milling was mainly composed by 120–180 g L−1 NaOH, 5 g L−1 Na2S, 60 g L−1 TEA and 25 g L−1 Al3+. This solution was the optimal milling solution with the lowest surface roughness of 2219 aluminum alloy as demonstrated by Li et al. (2015). In the proposed experiment, inhibitors were added into the above solution to adjust the milling rate of the alloy. As reviewed in the literature, both Triki et al. (1979) and Lopez-Garrity and Frankel (2014) investigated that sodium silicate could be an effective inhibitor for aluminum in the alkaline system due to the formation of films on metal surfaces. Sodium carbonate was reported as a surface-finishing agent in the etching solution for aluminum alloys by An et al. (2002). Sodium stannate was also an inhibitor for aluminum in alkaline media
3.1. Effects of the temperature under electric currents
2.1. Materials
The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and temperatures are illustrated in Fig. 1. As shown in Fig. 1a, the milling rate of 2219 alloy varies from 0.03 mm/min to 0.27 mm/min. The milling rate increases obviously as the current density and the temperature increase. As shown in Fig. 1b, the surface roughness of 2219 alloy varies from 0.55 μm to 1.39 μm. The surface roughness increases as the current density increases under the same temperature. While under the same current density, the surface roughness fluctuates as the temperature increases. The surface roughness at 20 °C and 60 °C is larger than that at 40 °C and 80 °C. Based on the data in Fig. 1, three-dimensional graphs for the behaviors of the milling rate and the surface roughness of the 2219 alloy with respect to the current density and the temperature are illustrated in Fig. 2. By considering the practical applications in the industry, DeGarmo et al. (2003) have described in the publication that the
Table 1 Chemical compositions of 2219 aluminum alloy (wt.%). Cu
Si
Fe
Mn
Mg
V
Zr
Zn
Ti
Others
Al
5.8–6.8
≤0.20
≤0.30
0.20–0.40
≤0.02
0.05–0.15
0.10–0.25
≤0.10
0.02–0.10
≤0.15
Bal.
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Fig. 1. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and temperatures.
material removal rate is among the range of 0.013–0.076 mm/min in general. The surface roughness varying from 1.6 μm to 6.3 μm is normally required for chemical milling as mentioned by Field et al. (1989). Based on the chemical milling results of 2219 aluminum alloy in the alkaline system (Li et al., 2015), the milling rate in the range of 0.08–0.14 mm/min and the surface roughness less than 0.65 μm are appropriate. The milling rate and the surface roughness in these ranges are chosen as the required milling rate and surface roughness in the present paper for electric current assisted chemical milling of 2219 aluminum alloy. The regions of the bottom projections under the required milling rate and surface roughness represent the adequate ranges of current densities and temperatures, shown in Fig. 2. Fig. 3 shows two adequate ranges of the current density and the temperature. One is obtained under the required milling rate, and the other is obtained under the required surface roughness. The overlapping region satisfies both of the required milling rate and surface roughness. The suitable operating conditions are located in this overlapping region with the temperature varying from 70 °C to 80 °C and the current density varying from 0 mA/cm2 to 20 mA/cm2. As shown in Fig. 3, with the application of an appropriate current density in the range of 0–20 mA/cm2, the reaction temperature can be reduced by approximately 10 °C. Thus, the safety of the chemical milling process is improved.
Fig. 3. Ranges of the current density and the temperature of 2219 aluminum alloy under the milling rate of 0.08–0.14 mm/min and surface roughness less than 0.65 μm.
Fig. 5 shows two adequate ranges of the current density and the NaOH concentration. One is obtained under the required milling rate, and the other is obtained under the required surface roughness. The overlapping region satisfies both of the required milling rate and surface roughness. From the aspect of the milling rate, the concentration of NaOH can be reduced from 180 g/L to 120 g/L with the increasing current density from 0 to 100 mA/cm2, shadowed with the black vertical lines in Fig. 5. From the aspect of surface roughness, the concentration of NaOH can be reduced from 180 g/L to 120 g/L with the increasing current density from 0 to 57 mA/cm2, shadowed with the red slashes in Fig. 5. Under the requirement of the milling rate and the surface roughness, the 2219 alloy can be processed by electric current assisted chemical milling with a reduced NaOH concentration range of 180 g/L to 120 g/L. Thus, the safety of the chemical milling process is also improved.
3.2. Effects of the NaOH concentration under electric currents Fig. 4 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations. As shown in Fig. 4a, the milling rate varies from 0.05 mm/ min to 0.27 mm/min. The milling rate increases obviously as the current density and the NaOH concentration increase. As shown in Fig. 4b, the surface roughness of 2219 alloy varies from 0.50 μm to 1.02 μm. Under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH.
Fig. 2. Three-dimensional graphs of the milling rate and the surface roughness of 2219 aluminum alloy with respect to the current density and the temperature.
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Fig. 4. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations.
roughness (Fig. 6b) is slightly lower than that in the solution without 0.02% Na2SiO3 (Fig. 4b). Fig. 7 shows the regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 under the required milling rate and surface roughness. Compared with the milling rate boundaries without inhibitors, both of the milling rate boundaries at 0.14 mm/min and 0.08 mm/min with 0.02% Na2SiO3 move towards the high current density and the high NaOH concentration. Meanwhile, the surface roughness boundary at 0.65 μm with 0.02% Na2SiO3 also moves to the high current density, compared with the boundary without inhibitors. Thus, with the addition of 0.02% Na2SiO3, the region of the suitable operating conditions moves to the high current density and the high NaOH concentration, i.e., from the gray shaded area to the green shaded area shown in Fig. 7. Compared with the region of the suitable operating conditions without the inhibitor (grey shaded area), the region of the suitable operating conditions with the inhibitor (green shaded area) for the current density and the NaOH concentration expands. With the application of the current density of 45–60 mA/cm2, the chemical milling of 2219 alloy can be processed in the solution with the addition of 0.02% Na2SiO3 and a low NaOH concentration of 120 g/L. The addition of the inhibitor slows down the changes of the milling rate and the surface roughness. With the addition of 0.02% Na2SiO3, the selection of the operating conditions for the current density and the NaOH concentration becomes more flexible.
Fig. 5. Ranges of the current density and the NaOH concentration of 2219 aluminum alloy under the milling rate of 0.08–0.14 mm/min and surface roughness less than 0.65 μm. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
3.3. Effects of the inhibitors under electric currents 3.3.1. Na2SiO3 Fig. 6 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations in the solution with 0.02% Na2SiO3. As shown in Fig. 6a, the milling rate increases obviously as the current density and the NaOH concentration increase. Under the same current density and NaOH concentration, the milling rate in the solution with 0.02% Na2SiO3 (Fig. 6a) is lower than the milling rate in the solution without inhibitors (Fig. 4a). As shown in Fig. 6b, under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH. Under the same current density and NaOH concentration, the surface
3.3.2. Na2SiO3 + Na2CO3 Though the chemical milling of 2219 alloy can be processed in the solution with the addition of 0.02% Na2SiO3 and a low NaOH concentration of 120 g/L, the alternative range of the current density is narrow. Sodium carbonate can be used as the surface-finishing agent to adjust the milling rate and the surface roughness for the chemical milling process. In the proposed experiment, 10% Na2CO3 is added into the solution with 0.02% Na2SiO3 for further adjustment. Fig. 8 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations in the Fig. 6. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations with 0.02% Na2SiO3.
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Fig. 7. Regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 under the required milling rate and surface roughness.
Fig. 9. Regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 + 10% Na2CO3 under the required milling rate and surface roughness.
solution with 0.02% Na2SiO3 + 10% Na2CO3. As shown in Fig. 8a, the milling rate increases obviously as the current density and the NaOH concentration increase. Under the same current density and NaOH concentration, the milling rate in the solution with 0.02% Na2SiO3 + 10% Na2CO3 (Fig. 8a) is lower than the milling rates in the solution with 0.02% Na2SiO3 (Fig. 6a) and the solution with no inhibitors (Fig. 4a). As shown in Fig. 8b, under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH. Under the same current density and NaOH concentration, the surface roughness (Fig. 8b) is slightly lower than that in the condition with 0.02% Na2SiO3 (Fig. 6b) as well as the surface roughness in the condition without inhibitors (Fig. 4b). Fig. 9 shows the regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.02% Na2SiO3 + 10% Na2CO3 under the required milling rate and surface roughness. Compared with the milling rate boundaries without inhibitors, both of the milling rate boundaries at 0.14 mm/min and 0.08 mm/min with 0.02% Na2SiO3 + 10% Na2CO3 move towards the high current density and the high NaOH concentration. Meanwhile, the surface roughness boundary at 0.65 μm with 0.02% Na2SiO3 + 10% Na2CO3 also moves to the high current density, compared with the boundary without inhibitors. Thus, with the addition of 0.02% Na2SiO3 + 10% Na2CO3, the region of the suitable operating conditions moves to the high current density and the high NaOH concentration. Compared with the region of the suitable operating conditions without the inhibitor (grey shaded area), the region of the suitable operating conditions with the inhibitor (green shaded area) for the current density and the NaOH concentration expands. With the application of the
current density of 40–70 mA/cm2, the chemical milling of 2219 alloy can be also processed in the solution with the addition of 0.02% Na2SiO3 + 10% Na2CO3 and a low NaOH concentration of 120 g/L. The addition of the inhibitor slows down the changes of the milling rate and the surface roughness. With the addition of 0.02% Na2SiO3 + 10% Na2CO3, the selection of the operating conditions for the current density and the NaOH concentration becomes more flexible. 3.3.3. Na2SnO3 Fig. 10 illustrates the milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations in the solution with 0.03% Na2SnO3. As shown in Fig. 10a, the milling rate increases obviously as the current density and the NaOH concentration increase. Under the same current density and NaOH concentration, the milling rate in the solution with 0.03% Na2SnO3 (Fig. 10a) is lower than the milling rate in the solution without inhibitors (Fig. 4a). As shown in Fig. 10b, under the same concentration of NaOH, the surface roughness increases obviously as the current density increases. While under the same current density, the surface roughness is influenced slightly by the concentration of NaOH. Under the same current density and NaOH concentration, the surface roughness (Fig. 10b) is slightly lower than that in the solution without inhibitors (Fig. 4b). Fig. 11 shows the regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.03% Na2SnO3 under the required milling rate and surface roughness. Compared with the milling rate boundaries without the inhibitor, both of the milling rate boundaries at 0.14 mm/min and 0.08 mm/min with 0.03% Na2SnO3 move towards the high current density and the high NaOH concentration. Meanwhile, the surface roughness boundary at 0.65 μm with 0.03% Na2SnO3 also moves to the high current density, Fig. 8. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations with 0.02% Na2SiO3 + 10% Na2CO3.
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Fig. 10. The milling rate and the surface roughness of 2219 aluminum alloy under different current densities and NaOH concentrations with 0.03% Na2SnO3.
gradually increase in the sequence of Na2SiO3, Na2SiO3 + Na2CO3 and Na2SnO3, because of the increasing inhibition efficiency. By adding inhibitors into the chemical milling solution, the selection of the suitable operating conditions becomes more flexible. Thus, the operability of the electric current assisted chemical milling process is further improved. 4. Conclusions (1) For the chemical milling process of 2219 aluminum alloy in the alkaline system, the application of the current density in a range of 0–20 mA/cm2 has reduced the reaction temperature by 10 °C. This reduction in temperatures improves the safety and the operability of the chemical milling process. (2) Under the given requirement of the milling rate and the surface roughness, 2219 aluminum alloy has been successfully processed under a reduced concentration of NaOH with a current density of 0–60 mA/cm2. The concentration of NaOH has been reduced down to 120 g/L with the current density of 28–57 mA/cm2. This reduction in NaOH concentrations also improves the safety and the operability of the chemical milling process. (3) With the addition of Na2SiO3, Na2CO3 and Na2SnO3 inhibitors in the alkaline system, the milling rate and the surface roughness of 2219 aluminum alloy decrease. The adequate region of the NaOH concentration and the current density expands and moves towards the direction of the high current density. By adding inhibitors into the chemical milling solution, the selection of the suitable operating conditions becomes more flexible, which further improves the operability of the electric current assisted chemical milling process.
Fig. 11. Regions of the current density and the NaOH concentration for 2219 aluminum alloy with and without 0.03% Na2SnO3 under the required milling rate and surface roughness.
compared with the boundary without inhibitors. Thus, with the addition of 0.03% Na2SnO3, the region of the suitable operating conditions moves to the high current density and the high NaOH concentration. Compared with the region of the suitable operating conditions without the inhibitor (grey shaded area), the region of the suitable operating conditions with the inhibitor (green shaded area) for the current density and the NaOH concentration expands. With the application of the current density of 74 mA/cm2, the chemical milling of 2219 alloy can be also processed in the solution with the addition of 0.03% Na2SnO3 and a low NaOH concentration of 121 g/L. The addition of the inhibitor slows down the changes of the milling rate and the surface roughness. With the addition of 0.03% Na2SnO3, the selection of the operating conditions for the current density and the NaOH concentration becomes more flexible. From the above experimental results, it can be concluded that with the assistance of electric currents, the reaction temperature for chemical milling of 2219 aluminum alloy in the alkaline system can be reduced by 10 °C. With the application of the current density in a range of 28–57 mA/cm2, the concentration of NaOH can be significantly reduced from 180 g/L to 120 g/L. This reduction in temperatures and NaOH concentrations improve the safety and the operability of the chemical milling process of 2219 aluminum alloy in the alkaline system. With the addition of inhibitors in the alkaline system, both of the milling rate and surface roughness decrease due to the inhibition effects of Na2SiO3, Na2SiO3 + Na2CO3 and Na2SnO3 on aluminum alloys. Under the required milling rate and surface roughness, the adequate regions of the NaOH concentration and the current density for adding all the three kinds of inhibitors expand and move towards the direction of the high current density, shown in Figs. 7, 9 and 11. The moving distances of the adequate regions for adding different inhibitors
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