Micro arc oxidation and electrophoretic deposition effect on damping ...

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J. Cent. South Univ. (2014) 21: 3419−3425 DOI: 10.1007/s11771-014-2317-5

Micro arc oxidation and electrophoretic deposition effect on damping and sound transmission characteristics of AZ31B magnesium Alloy LUO Zhi(罗智)1, HAO Zhi-yong(郝志勇)1, JIANG Bai-ling(蒋百灵)2, GE Yan-feng(葛延峰)2, ZHENG Xu(郑旭)1 1. Department of Energy Engineering, Zhejiang University, Hangzhou 310027, China; 2. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2014 Abstract: Micro arc oxidation (MAO) and electrophoretic deposition (EPD) process are employed to fabricate a dense coating on magnesium alloy to protect it from corrosion in engineering application. The EPD film changes the damping characteristic of magnesium alloy, and both the MAO and EPD process change the bending stiffness of samples being treated. Damping loss factor (DLF) test and sound transmission experiments were carried out for AZ31B magnesium alloy with coating fabricated by MAO and EPD processes. The results indicate that DLF is improved in frequency range from 0−850 Hz. Bending stiffness of the samples is improved with MAO and EPD treatment. As a result, the sound transmission loss (LST) is improved in the stiffness control stage of the sound transmission verse frequency curve. To the samples by electrophoresis process, the LST is improved in frequency range from 2500−3200 Hz, because the damping loss factor is improved with EPD process. The results are useful for the surface treatment to enhance the damping loss factor, LST and widespread application of magnesium alloy while improving the corrosion resistance. Key words: magnesium alloy; micro arc oxidation; electrophoretic deposition; damping loss factor; sound transmission loss; bending stiffness

1 Introduction Magnesium alloys have excellent physical and mechanical properties, such as low density, high stiffness, good electromagnetic shielding, high strength to weight ratio, and good damping characteristics, which suggest that widespread application of magnesium alloys is expected in the fields of communication, automobile and aerospace industry [1−3]. Besides, various studies [4−6] have shown that magnesium alloys may have a promising future as potential degradable implant materials, due to their low toxicity, their unique biodegradation characteristics, their elastic modulus (about 45 GPa) being closer to that of bone than the currently used medical metals, such as titanium alloy and stainless steel, and their better comprehensive mechanical properties than those of bioceramics and biodegradable polymers [7]. However, the high chemical reactivity of magnesium [8] hindered their widespread use in many applications, especially in acidic environment and in salt-water conditions [9]. Micro arc oxidation (MAO) or both MAO and electrophoretic

deposition (EPD) treatment are always adopted to fabricate a ceramic coating or composite coating. The ceramic coating formed on magnesium alloy with MAO treatment are composed of two layers, an outer loose layer and an inner dense layer which is firmly bonded to the magnesium alloy substrate. Because of the outer loose layer of the MAO coating, the EPD film can be embedded in it and a strong bonding between the ceramic coating and the EPD film can be formed [10−13]. Both the ceramic coating and the EPD film change the bending stiffness of magnesium alloy samples that being treated. In addition, the EDP film also modifies the damping character of magnesium alloy. As a result, the sound transmission character will be different with those without the treatments. To the author’s best knowledge, there is no literature published on damping and sound transmission study of magnesium alloy with MAO and EPD treatment. However, this is important for the application of magnesium alloy, especially in vibration attenuation and sound insulation aspect. In the present work, AZ31B magnesium alloy samples with ceramic coating and composite coating for

Foundation item: Project(2011BAE22B05) supported by National Technology R&D Program in the 12th Five year Plan of China; Project(2011DFA50900) supported by the Canada-China-USA Collaborative Research & Development Project; Project(51071121) supported by the National Natural Science Foundation of China Received date: 2013−05−29; Accepted date: 2013−09−02 Corresponding author: LUO Zhi, PhD Candidate; Tel: +86−571−87953286; E-mail: [email protected]

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different thicknesses with MAO treatment and the combination of MAO and EPD process were prepared. The damping loss factor (DLF) test of the samples with and without composite coating was carried out with the impulse energy method. And the sound transmission characteristics for different coatings with various thicknesses were obtained with four-microphone standing wave tube method.

2 Samples preparation Chemical composition of AZ31B magnesium alloy is listed in Table 1.

surface of AZ31B samples under peak current of 50 A/dm2, frequency of 500 Hz, pulse width of 30 μm and processing time of 10 min. After MAO process, the samples were cleaned to ensure that there were no remnant solutions on the coating, before drying. The E-coat process applies a cathodic 185 V voltage for 2 min to MAO coated AZ31B magnesium alloy panels immersed in a bath of positively charged paint particles to deposit polymeric paint onto the part surface. The weak bonding of the polymers both to the substrate and surrounding polymer particles is strengthened with a post-deposition cure at 170 °C for 30 min.

Table 1 Chemical composition of AZ31B magnesium alloy (mass fraction, %) Al

Zn

Mn

Si

Cu

Fe

3.01

0.98

0.029

0.0019

0.0012

0.0022

The specimens were manually polished with sand papers from 600 to 1500 grade. The MAO coating facility was manufactured by the research team at Xi’an University of Technology (XAUT). Pulse current anodizing, the peak current, frequency and pulse width can be adjusted independently. The solutions of MAO were prepared using Na2SiO3, KOH and KF which can be dissolved in deionized water, and electrolyte temperature was maintained at approximately 25 °C to 40 °C during processing. The electrophoresis paints were cathodic-type with bi-component epoxy resin. The ceramic coating with MAO process was formed on the

Fig. 1 Cross-section of samples (a) and coated samples (b)

Samples with ceramic coating and composite coating are labeled and listed in Table 2. Three other

Table 2 Labels and thickness of coatings Label

Size/mm

Process and coating (film thickness)

BARE 2

None

MAO 21

Ceramic coating(5 μm)

MAO 22 MAO 23

Ceramic coating(10 μm) Thickness: 2 mm; diameter: 150 mm

Ceramic coating(14 μm)

MAOE 21

Ceramic coating(10 μm)+resin epoxy film(20 μm)

MAOE 22

Ceramic coating(10 μm)+ resin epoxy film(50 μm)

MAOE 23

Ceramic coating(10 μm)+ resin epoxy film(70 μm)

BARE 20 MAOE 20

500 mm×370 mm×2 mm

None Ceramic coating(5 μm)+ resin epoxy film(70 μm)

Bare 4

None

MAO 41

Ceramic coating(5 μm)

MAO 42 MAO 43

Testing content

Sound transmission loss

Damping loss factor

Ceramic coating(10 μm) Thickness: 4 mm; diameter: 150 mm

Ceramic coating(14 μm)

MAOE 41

Ceramic coating(10 μm)+ resin epoxy film(20 μm)

MAOE 42

Ceramic coating(10μm)+ resin epoxy film(50 μm)

MAOE 43

Ceramic coating(10 μm)+ resin epoxy film(70 μm)

Sound transmission loss

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samples, Bare 2, Bare 20 and Bare 4, without any treatment are given in Table 2 as well.

3 Damping loss factor test As described in Section 2, EPD film consists of epoxy resin, which has good damping character for their long-chain molecules that exhibit viscoelastic behavior [14−15]. Therefore, the damping performance of the samples with EDP film changes. Impulse response decay method was employed to test the DLF of the sample MAOE 20, the thickness and other parameters of which are given in Table 2. For comparison, the test also carried out on BARE 20, having the same size and thickness of MAOE 20. DLF of sample with ceramic coating only is not studied here, for the thickness of the ceramic coating is smaller than that of the epoxy resin film. On the other hand, the damping loss factor of ceramic coating is much lower than that of epoxy resin [16]. The schematic diagram is shown in Fig. 2. In order to avoid the energy exchange between samples and the roof, a soft elastic rope was used to hang up the samples, which also ensures the free-free vibration of those samples.

Fig. 2 Schematic diagram of damping loss factor test

Shock from the hammer induced the vibration of the samples, and the acceleration of the samples was collected with an accelerometer, which attached to the samples being tested. The accelerometer is only 3 g in mass, while the masses of the samples are greater than 647 g. Therefore, the influence of the accelerometer of the results can be neglected. The DLF of the samples can

Fig. 4 Set-up of sound transmission loss experiment

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be obtained from the decay character of the vibration acceleration. More details on the theory of the damping loss factor test were presented in Refs. [17−18]. Ten different test points were selected to test the DLF so to eliminate the error induced in the test, while the load position was fixed. The test points of the two samples were the same for comparison. The DLF curves of MAOE 20 and BARE 20 are plotted in Fig. 3, in which the result is the average of the 10 test points. The solid line represents the DLF of MAOE 20, and the red dash line represents the DLF of BARE 20. It is clearly seen that the damping loss factor of MAOE 20 is much greater than that of BARE 20 at frequencies below 850 Hz. The DLF of BARE 20 is 0.836×10−3, and 1.807×10−3 for MAOE 20 at 850 Hz. The latter is 1.16 times that of BARE 20. This is principally induced by the epoxy resin film.

Fig. 3 Damping loss factor of MAOE 20 and bare sample without treatment

4 Sound transmission 4.1 Basic theory Experiments on sound transmission loss (LST) of the samples were carried out in standing wave tube with four-microphone. Figure 4 shows the schematic graph of the experiment.

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The white noise signal generated by generator is amplified by the power amplifier and emitted with the speaker to generate a plane wave field in a standing wave tube. The plane sound wave propagates toward the sample and reflected. While the sound transmitted propagates toward the terminal, approximating anechoic termination that was created by loosely packed soundabsorbing material. The re-reflected wave can be neglected for it is very weak [19]. As shown in Fig. 4, there are two microphones, respectively, in the upstream and downstream of the standing wave tube. The sound pressure in the four positions are P1, P2, P3 and P4 as follows: P1  ( A jkx1  B jkx1 )e jt

(1a)

P2  ( A jkx2  B jkx2 )e jt

(1b)

P3  (C  jkx3  D jkx3 )e jt

(1c)

P4  (C  jkx4  D jkx4 )e jt

(1d)

where k is the wave number in the ambient fluid, ω is the circular frequency and j  1. x1, x2, x3 and x4 are the coordinates of microphones 1 to 4, respectively, as shown in Fig. 4. A to D are the complex amplitude of the four waves, and they can be derived by the combination of equations (1a)−(1d), i.e.: jkx2

jkx1

j( P1e  P2 e ) 2sin[k ( x1  x2 )]

(2a)

j( P e jkx4  P4 e jkx3 ) C 3 2sin k[( x3  x4 )]

(2b)

A

Then, the LST (sound transmission loss, LST) can be given by LST  20 log

C A

(3)

4.2 Results and discussion 4.2.1 MAO treatment effect The LST of sample MAO 21, MAO 22 and MAO 23, which have different coating thicknesses of 2 mm thick AZ31B magnesium alloy with MAO process, as listed in Table 2, are shown in Fig. 5. Furthermore, the LST of BARE2 is also shown in Fig. 5 for comparative studies. It is obviously that the LST increases as the thickness of the coating increases. The LST of the BARE 2 is lower than that of the other three samples with ceramic coating in the frequency range from 47 to 1352 Hz. The LST of the BARE 2 is about 5 dB lower than that of MAO 21, while about 6−7 dB lower than that of MAO 23. It is even 20.8 dB lower than that of MAO 23 at the anti-resonant frequency of 1333 Hz.

Fig. 5 LST of 2 mm thick samples with ceramic coating

Samples with MAO process can be considered as multilayer structure for the coating has a strong adhesive with substrate [20]. The bending stiffness D of the structure is D   Dk

(4)

k

while Dk is the bending stiffness of each layer and can be expressed by Dk 

hk3  hk31 3(1  k2 )

Ek

(5)

where hk, Ek and υk are thickness, elastic modulus and Poisson ratio of the k-th layer, respectively, hk−1 is the thickness of the (k−1)-th layer. The elastic modulus and Poisson ratio of AZ31B magnesium alloy are 45 GPa and 0.35, respectively. The elastic modulus and Poisson ratio of magnesium oxide, which is more than 95% in mass of the ceramic coating, are 250 GPa and 0.178, respectively [21]. The values of thicknesses of the coatings are listed in Table 2. Then, the bending stiffness of the bare sample, MAO 21, MAO 22 and MAO 23 can be obtained by combining Eq. (4) and Eq. (5), and they are 34.188, 35.485, 36.256 and 37.853 N/m, respectively. Namely, the stiffnesses of the samples with MAO are enhanced by 3.8%, 6.1% and 10.7%, respectively. While in Fig. 5, the frequency range of 400 Hz up to the anti-resonant frequency of BARE 2 is the stiffness control region of the LST verse frequency curve [22]. And this is the reason for enhancement of the LST in the frequency range from 400 Hz to 1352 Hz. Figure 6 shows the L ST of sample MAO 41, MAO 42 and MAO 43, which are the 4 mm thick AZ31B magnesium alloy samples with same process as that of MAO 21, MAO 22 and MAO 23, as listed in Table 2. In the frequency range from 859 to 933 Hz and 1840 to 2000 Hz, as box 1 and box 2 shown in Fig. 6, LST of the

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DLFs of those samples are enhanced. On the other hand, it is easy to find that the bending stiffness of the samples with EPD process must be higher than those samples without the process from Eq. (4) and Eq. (5), although the elastic modulus of the deposition film is lower than the substrate or the ceramic coating. The bending stiffness of the samples increases as the thickness of the film increasing. From Fig. 7, we can see that the LST differences in 2400−3200 Hz frequency range are very noticeable. Maximum differences occur at 2868 Hz, where the LST of MAOE 23 is 12.07 dB higher than that of MAOE 22 and even 18.61 dB higher than that of MAOE 21. Fig. 6 LST of 4 mm thick samples with ceramic coating

samples increase as the thickness of the coating increases. While in the rest frequencies, the values of LST are almost the same. The bending stiffnesses of sample MAO 41, MAO 42 and MAO 43 are enhanced by 1.9%, 3.8% and 5.3%, respectively. Therefore, MAO process has little influence on the bending stiffness of 4 mm thick samples, so the LST of this group samples are almost the same. 4.2.2 EPD process effect Three 2 mm thick samples were processed with same MAO process, and the thicknesses of the coatings are 10 μm. Then, EPS was used to fabricate an epoxy resin film on them with different thicknesses. The samples are identified with MAOE 21, MAOE 22 and MAOE 23, and the thicknesses of those samples are listed in Table 2. The values of LST of those samples are shown in Fig. 7. It is obvious that sample with thicker coating has higher LST in frequency range from 400 to 2000 Hz, which can also be found in 2400−3200 Hz frequency range.

Fig. 7 LST of 2 mm thick samples with composite coating

The values of LST of MAOE 23 are 5.06 dB, 10.34 dB higher than those of MAOE 22 and MAOE 21 at 1300 Hz, respectively. As revealed in Section 3, the

Fig. 8 LST of 4 mm thick samples with composite coating

To investigate the sound transmission characteristic of thicker sample, three 4 mm thick samples were processed with same MAO treatment, and the coating is 10 μm thick. Then, epoxy resin film with different thickness was fabricated on them by EPD process. The samples are labeled with MAOE 41, MAOE 42 and MAOE 43, and the thickness of those samples are listed in Table 2. The values of LST of those samples are almost the same in frequency range of 400−1131 Hz in Fig. 8. The composite coatings have little effect on the bending stiffness of those samples. Therefore, the values of LST of those samples are almost the same. However, sample with thicker epoxy resin film has higher LST in the frequency range of 1131−2589 Hz. 4.2.3 Comparative study of MAO and EPD treatment Same MAO process was performed on two 2 mm thick samples. In addition, one of them was selected to do EPD process. The samples are identified as MAO 22 and MAOE 22, as listed in Table 2. Figure 9 gives the LST of samples MAO 22 and MAOE 22. MAOE 22 has higher bending stiffness for its EPS process. As a result, LST of MAOE 22 is about 2 dB higher than that of MAO 22 at the frequency range from 400 to 1423 Hz, while the LST of MAO 22 is a little higher than that of MAOE 22 above the anti-resonant

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Fig. 9 LST of 2 mm thick samples with ceramic coating and composite coating

frequency and below 2000 Hz. The LST of MAOE 22 is higher than that of MAO 22 in the frequency range from 2528 Hz to 2902 Hz for the damping effect of its epoxy resin film. Similar to MAO 22 and MAOE 22, same MAO process was conducted on two 4 mm thick samples, and EPD film was fabricated on one of them with EPD process. Their LST curves are plotted in Fig. 10. It is clear that the values of LST of MAO 42 and MAOE 42 are almost the same in frequency range of 0.4 to 2.264 kHz. This mainly because the thicknesses of the ceramic coatings are the same, the samples are too thick before processing, so epoxy resin film almost has little influence on the bending stiffness of them. However, at the frequency range from 2579 to 3200 Hz, MAOE 42 has a higher LST than that of MAO 42. As mentioned above, this is induced by damping effect of epoxy resin film.

treatment at 850 Hz below. This is mainly because the EPD film consists of epoxy resin, which has good damping character. And the ceramic coating may also contribute to that. 2) Bending stiffness of samples can be enhanced with MAO process and EPD treatment, which induced the improvement of LST of AZ31B magnesium alloy with the process. 3) MAO process has little effect on bending stiffness of 4 mm thick samples. As a result, the LST values of those samples are almost the same even the thickness of their coatings are different. However, EPD process still has positive effect on sound insulation of magnesium alloy in mid-high frequency range.

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Fig. 10 LST of 4 mm thick samples with ceramic coating and composite coating

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(Edited by HE Yun-bin)