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Effect of white layer on the damping capacity of metal matrix composites
Materials Science & Engineering A 591 (2014) 78–81
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Effect of white layer on the damping capacity of metal matrix composites D. Siva Prasad a,n, Ch. Shoba b, B. Srinivasa Prasad a a b
Department of Mechanical Engineering, GITAM University, Visakhapatnam, India Department of Industrial Engineering, GITAM University, Visakhapatnam, India
art ic l e i nf o
a b s t r a c t
Article history: Received 12 August 2013 Received in revised form 19 September 2013 Accepted 22 October 2013 Available online 31 October 2013
Damping is the material's ability to dissipate elastic strain energy during mechanical vibration. Even though, damping measurements can be performed using different methods, dynamic mechanical analyzer is one of the best equipment used to find the damping capacity of metal matrix composites. In general, the samples for the damping measurements will be machined using the wire cut Electrical Discharge Machining (EDM) process to study the damping behavior. In the present work, the specimens for damping measurements were prepared by using the wire cut EDM process and the milling process and the results are compared. Damping measurements of all the specimens were measured by dynamic mechanical analyzer (DMA) at different frequencies in air atmosphere. It was observed that, the specimens machined using the wire cut EDM process exhibit lower damping values compared with milling machined specimens. The lower damping values for the wire cut EDM machined specimens are probably due to the formation of the white layer on the surface of the specimens. & 2013 Elsevier B.V. All rights reserved.
Keywords: White layer Damping Rice husk ash DMA
1. Introduction In response to a dynamic loading, structures can experience severe vibrations which cause human discomfort and high noise levels. An understanding of various aspects of vibration in heavy structures is essential to avoid or minimize these potential problems. Hence materials which possess a high damping capacity with high stiffness and low density are very much required for the present day scenario. Metal matrix composites (MMCs) can be a better choice which simultaneously exhibit high stiffness and high damping values. Rice husk ash (RHA) is one of the inexpensive and low density reinforcement available abundantly throughout the world. RHA contains above 90% of silica, which makes the possible use of it as a reinforcement for widespread applications in automotive and small engine applications. As RHA is an agricultural waste byproduct, the utilization of RHA has an additional benefit for decreasing the pollution. Prasad et al. [1,2] studied the mechanical behavior and tribological characteristics of Al/RHA composites and the results show improved mechanical and tribological properties compared with base alloy. The damping behavior of Al–Li–SiCp composites has been studied by Bauri et al. [3]. The results show that the composites exhibit higher damping capacity than the unreinforced alloy. Damping capacity is found to increase with decreasing frequency and this can be attributed to the higher dislocation density, grain boundary, and interface n
Corresponding author. Tel.: þ 91 98 4848 0122; fax: þ 91 89 1279 0250. E-mail address: [email protected] (D.S. Prasad).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.10.075
damping. Wang et al. [4] showed that the presence of 13 vol% of SiCp clearly increases the damping characteristics compared to the monolithic material condition, especially at higher temperature and the effect of grain boundary sliding is also clearly visible at a temperature of 475 K. Wei et al. [5] pointed out that the damping capacity of the macroscopic graphite particulates reinforced pure aluminum composite is increased with a larger volume fraction of the reinforcements. However, this is accompanied with a decrease in dynamic modulus. Rohatgi et al. [6] investigated the damping capacity of graphite and silicon carbide particulate reinforced Al alloy composites. The damping capacity of graphite/Al alloy composites increased with the volume percentage of graphite within the range studied. However, no obvious improvements in damping capacity were observed by dispersion of silicon carbide in aluminum alloy. In contrast, Srikanth et al. [7] reported that the damping capacity of the pure magnesium matrix was improved in the presence of SiC particulates, and increased with the increase of the proportion of SiC particulates. Ceramic hollow sphere fly ash particulates are another kind of reinforcement, which are inexpensive, low density and available abundantly in large quantities as solid by-products during combustion of coal in thermal power plants. Sudarshan et al. [8] showed that fly ash/A356 MMCs, with a proper volume fraction of fly ash particulate, exhibited improved ambient temperature damping capacity. In the present work, Al/RHA composites were fabricated and the damping measurements were done on the specimens machined by the wire cut EDM process and the milling process and the results were compared and analyzed.
D.S. Prasad et al. / Materials Science & Engineering A 591 (2014) 78–81
Table 1 Chemical composition of A356.2 Al Alloy matrix.
79
0.05
Wire cut EDM
Si
Fe
Cu
Mn
Mg
Zn
Ni
Ti
6.5–7.5
0.15
0.03
0.10
0.4
0.07
0.05
0.1
Damping capacity
0.04
Table 2 Chemical composition of RHA. Constituent Silica Graphite Calcium oxide
Magnesium oxide
Potassium oxide
Ferric oxide
%
0.53
0.39
0.21
90.23 4.77
1.58
0.03
0.02
unreinforced alloy Al/4HA composite Al/6HA composite Al/8HA composite
0.01
0.00 0
5
2. Experimentation
The damping capacity of the base alloy and Al/RHA composites for two kinds of specimens (wire cut EDM machined and milling machined) is shown in Fig. 1a and b respectively. From Fig. 1a and b, it was observed that the damping capacity has been improved with the addition of the reinforcement and no distinct peaks were found in both the cases. The increase in damping capacity for the composites can be attributed to the increase in the dislocation densities due the difference in the coefficient of thermal expansions (CTE) of the reinforcement and the matrix alloy. The CTE values of A356.2 alloy and RHA particulates are 21.4 10 6 1C 1 and 10.1 10 6 1C 1 respectively [9]. Due to this difference in CTE the dislocation density generated can be quite significant at the interface and can be predicted using the following equation: ð1Þ
where B is a geometric constant, ε is the thermal mismatch strain, VRHA is the volume fraction of the RHA particulates, b is the Burgers vector, and d is the grain diameter of reinforcements. The volume fraction (VRHA) is given by the equation: V RHA ¼
ρAl ρmmc ρAl ρRHA
20
25
Milling
0.050 0.045 0.040 0.035 0.030 0.025 0.020 0.015
Unreinforced Al/4HA composite Al/6HA composite Al/8HA composite
0.010 0.005 0
5
10
15
20
25
Frequency in Hertz (Hz) Fig. 1. Tan δ values for unreinforced, 4%, 6% and 8% RHA composites. (a) Wire cut EDM machined specimen and (b) milling machined specimen.
Table 3 Dislocation densities of unreinforced and Al/RHA composites.
3. Results and discussions
BεV RHA bdð1 V RHA Þ
15
0.055
Damping capacity
The matrix material used in this study is an A356.2 alloy whose composition is given in Table 1. The reinforcement is RHA particulates whose chemical composition is given in Table 2. The composites were fabricated by the stir casting technique. Before incorporating the RHA particulates in the alloy the ash particulates are pre-treated in order to free them from inorganic matter and carbonaceous material. The composites with 4, 6 and 8 wt% RHA particulates were fabricated using the stir casting method and the detailed fabrication process of the composites was presented in earlier works [9]. A GABO Eplexor dynamic mechanical analyzer is used to study the damping behavior of the base alloy and the composites. Tests were carried under a static load of 50 N and a dynamic load of 40 N at room temperature for the frequencies ranging from 1 Hz to 25 Hz and a strain amplitude (ε) was 1 10 5 using the three-point bending mode. The samples for damping measurements were machined using an Ultra cut 843/ Ultra cut f2 CNC wire electrical discharge machine and MTAB XL milling machine. The dimensions of the specimen for the damping measurements are 30 12 15 mm3. Microstructural characterization of the samples was done using JSM-6610LV scanning electron microscope (SEM) equipped with energy dispersive X-ray analyzer.
ρ¼
10
Frequency in Hertz (Hz)
ð2Þ
% RHA
Volume fraction
Estimated dislocation density, ρ (m 2)
0.0 4.0 6.0 8.0
– 0.028 0.065 0.086
– 8.81 1010 3.96 1011 7.32 1011
where ρAl is the density of A356.2, ρmmc is the density of composites and ρRHA is the density of rice husk ash. Based on Eqs. (1) and (2) the dislocation density was calculated with an assumption for the Burgers vector of 0.32 nm for Al [9] and are tabulated in Table 3. According to Granato–Lucke mechanism, the dislocation-based damping is mainly due to frequency dependent part and is directly proportional to the dislocation density and is expressed as follows: Q 1 ¼
a 0 B ρL 4 ω2
π 2 Cb2
ð3Þ
where a0 is a numerical factor of order 1, B is the damping constant, ω is the operating frequency, L is the effective dislocation loop length, C is the dislocation line tension (E0.5 Gb2), and G is the shear modulus.
80
D.S. Prasad et al. / Materials Science & Engineering A 591 (2014) 78–81
From Eq. (3) it could be observed that the inverse quality factor Q 1 or damping capacity is directly proportional to the dislocation density. From Table 3 it was observed that the dislocation density increases with the increase in RHA content and hence an increase in damping has been observed in Al/RHA composites. In addition to the dislocation density based damping, generation of plastic zone is another mechanism observed in the present study for the increase of damping capacity of Al/RHA composites. In particle reinforced MMCs, the incorporation of hard particulates in the matrix causes residual stresses in the form of an annular plastic zone due to differences in the CTE of the matrix and the reinforcement, which are responsible for the generation of plastic zone at the Al/RHA interface. The size of the plastic zone Cs can be calculated using C s ¼ rs
ΔαEΔT ð1 νÞsy
The damping can be expressed as a function of plastic zone and is given by tan δ ¼
R C s G sdε π s2o
ð5Þ
where G is the shear modulus of the composite sample, so is the alternating shear stress amplitude and ε is the corresponding strain acting on the specimen. Eq. (5) shows that the damping is a direct measure of the plastic zone radius and from Table 4 it was observed that the plastic zone increases with increase in RHA content and hence the damping capacity of the composites increases with weight percentage of the reinforcement.
ð4Þ
where Δα is the difference in CTEs, ΔT is the temperature difference, E and ν are the matrix elastic modulus and Poisson's ratio, sy is the matrix yield stress and rs is the particulate radius. From Eq. (4) the radius of plastic zone for different weight percentages of RHA was calculated and given in Table 4. Table 4 Radius of plastic zone of unreinforced and Al/RHA composites. % RHA 0.0 4.0 6.0 8.0
CTE (α) (1C 1)
Radius of plastic zone (mm)
6
– 17.64 31.92 42.84
21.4 10 19.3 10 6 17.6 10 6 16.3 10 6
Fig. 3. SEM micrograph showing white layer.
0.026 0.024
Al/6%RHA composite
0.035 for Unreinforced alloy
0.020
Damping capacity
Damping capacity
0.022
0.018 0.016 0.014 0.012 0.010
Wire cut EDM Milling
0.008
0.030
0.025
0.020 Wire cut EDM Milling
0.015
0.006 0.010
0.004 0
5
10
15
20
25
0
Frequency in Hertz (Hz)
0.034
Al/4%RHA composite
15
20
25
Al/8%RHA composite
0.055
0.030
0.050
0.028
0.045
Dampaing capacity
Damping capacity
10
Frequency in Hertz (Hz)
0.032
0.026 0.024 0.022 Wire cut EDM Milling
0.020
5
0.018
0.040 0.035 0.030 Wire cut EDM Milling
0.025
0.016 0.020
0.014
0.015
0.012 0
5
10
15
Frequency in Hertz (Hz)
20
25
0
5
10
15
20
25
Frequency in Hertz (Hz)
Fig. 2. Comparison of damping capacities for the wire cut EDM machined specimen and milling specimen for (a) unreinforced alloy, (b) 4% RHA composite, (c) 6% RHA composite, and (d) 8% RHA composite.
D.S. Prasad et al. / Materials Science & Engineering A 591 (2014) 78–81
Hence dislocation damping and generation of plastic zone at the interface are the operating mechanisms in the RHA reinforced composites although there may be other mechanisms such as grain boundary damping, thermoelastic damping, intrinsic damping, interfacial damping, etc. which are responsible for the increase in the damping capacity; the author is more interested in comparison of the damping capacity of the wire cut EDM machined and milling machined specimens and hence it is not discussed in detail. Fig. 2a–d shows the comparison of the damping values between the wire cut EDM machined and milling machined specimens for the unreinforced and RHA reinforced composites. It was observed that the damping capacity for wire cut EDM machined specimens exhibits low damping values in all the cases when compared with milling machined specimens. This can be attributed to the formation of a white layer when the specimens are machined with the wire cut EDM process. When a specimen is machined using the wire cut EDM process, the current from the process melts the material and a thin layer on the surface of the specimen is formed called the white layer or recast layer. This white layer is because of the molten metal which cannot be expelled and has instead been rapidly quenched by the dielectric oil. From the scanning electron micrograph (Fig. 3), there is clear evidence of white layer formation with an average thickness of 21 mm. Due to the formation of this white layer the energy dissipated to the surroundings will be reduced which is a direct measure of damping capacity and hence a decrease in damping capacity has been observed. 4. Conclusions
considered for the preparation of the damping specimens. The increase in damping capacity of the composite is due the increase in the dislocation density and plastic zone with the percentage increase in the reinforcement. It was concluded that the wire cut EDM machined specimen exhibits low damping values which can be attributed to the formation of white layer which acts like a protective layer to dissipate elastic strain energy which results in reduction of damping values. The formation of white layer contributes 8–15% of the overall damping and this should be considered when the specimen is machined with wire cut EDM.
Novelty statement The enhancement of damping capacity in metal matrix composites can be attributed to the generation of plastic zone at the interface, grain boundary damping, thermoelastic damping, intrinsic damping, interfacial damping, etc. Many researches machined the damping specimen by the wire cut EDM process. When the metal–matrix composites are machined with EDM the formation of the white layer is a common phenomenon and the effect of this white layer on damping is not explained.
References [1] [2] [3] [4] [5] [6]
The Dynamic Mechanical Analyzer is used to predict the damping behavior of the base alloy and the RHA reinforced composites. Wire cut EDM and milling machines have been
81
[7] [8] [9]
D. Siva Prasad, A. Rama Krishna, Int. J. Adv. Sci. Technol. 33 (2011) 51–57. D. Siva Prasad, A.Rama Krishna, J. Mater. Sci. Technol. 28 (2012) 367–372. Ranjit Bauri, M.K. Surappa, Metall. Mater. Trans. A 36A (2005) 667–673. C. Wang, Z. Zhu, Scr. Mater. 38 (1998) 1739–1745. J.N. Wei, H.F. Cheng, Y.F. Zhang, F.S. Han, Z.C Zhou, J.P. Shui, Mater. Sci. Eng. A 325 (2002) 444–453. P.K. Rohatgi, D. Nath, S.S. Singh, B.N. Keshavaram, J. Mater. Sci. 29 (1994) 5975–5984. N. Srikanth, M. Gupta, Mater. Res. Bull. 37 (2002) 1149–1162. K. Sudarshan, M.K. Surappa, Mater. Sci. Eng. A 480 (2008) 117–124. D. Siva Prasad, A. Rama Krishna, Bull. Mater. Sci. 35 (2012) 989–995.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Effect of white layer on the damping capacity of metal matrix composites D. Siva Prasad a,n, Ch. Shoba b, B. Srinivasa Prasad a a b
Department of Mechanical Engineering, GITAM University, Visakhapatnam, India Department of Industrial Engineering, GITAM University, Visakhapatnam, India
art ic l e i nf o
a b s t r a c t
Article history: Received 12 August 2013 Received in revised form 19 September 2013 Accepted 22 October 2013 Available online 31 October 2013
Damping is the material's ability to dissipate elastic strain energy during mechanical vibration. Even though, damping measurements can be performed using different methods, dynamic mechanical analyzer is one of the best equipment used to find the damping capacity of metal matrix composites. In general, the samples for the damping measurements will be machined using the wire cut Electrical Discharge Machining (EDM) process to study the damping behavior. In the present work, the specimens for damping measurements were prepared by using the wire cut EDM process and the milling process and the results are compared. Damping measurements of all the specimens were measured by dynamic mechanical analyzer (DMA) at different frequencies in air atmosphere. It was observed that, the specimens machined using the wire cut EDM process exhibit lower damping values compared with milling machined specimens. The lower damping values for the wire cut EDM machined specimens are probably due to the formation of the white layer on the surface of the specimens. & 2013 Elsevier B.V. All rights reserved.
Keywords: White layer Damping Rice husk ash DMA
1. Introduction In response to a dynamic loading, structures can experience severe vibrations which cause human discomfort and high noise levels. An understanding of various aspects of vibration in heavy structures is essential to avoid or minimize these potential problems. Hence materials which possess a high damping capacity with high stiffness and low density are very much required for the present day scenario. Metal matrix composites (MMCs) can be a better choice which simultaneously exhibit high stiffness and high damping values. Rice husk ash (RHA) is one of the inexpensive and low density reinforcement available abundantly throughout the world. RHA contains above 90% of silica, which makes the possible use of it as a reinforcement for widespread applications in automotive and small engine applications. As RHA is an agricultural waste byproduct, the utilization of RHA has an additional benefit for decreasing the pollution. Prasad et al. [1,2] studied the mechanical behavior and tribological characteristics of Al/RHA composites and the results show improved mechanical and tribological properties compared with base alloy. The damping behavior of Al–Li–SiCp composites has been studied by Bauri et al. [3]. The results show that the composites exhibit higher damping capacity than the unreinforced alloy. Damping capacity is found to increase with decreasing frequency and this can be attributed to the higher dislocation density, grain boundary, and interface n
Corresponding author. Tel.: þ 91 98 4848 0122; fax: þ 91 89 1279 0250. E-mail address: [email protected] (D.S. Prasad).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.10.075
damping. Wang et al. [4] showed that the presence of 13 vol% of SiCp clearly increases the damping characteristics compared to the monolithic material condition, especially at higher temperature and the effect of grain boundary sliding is also clearly visible at a temperature of 475 K. Wei et al. [5] pointed out that the damping capacity of the macroscopic graphite particulates reinforced pure aluminum composite is increased with a larger volume fraction of the reinforcements. However, this is accompanied with a decrease in dynamic modulus. Rohatgi et al. [6] investigated the damping capacity of graphite and silicon carbide particulate reinforced Al alloy composites. The damping capacity of graphite/Al alloy composites increased with the volume percentage of graphite within the range studied. However, no obvious improvements in damping capacity were observed by dispersion of silicon carbide in aluminum alloy. In contrast, Srikanth et al. [7] reported that the damping capacity of the pure magnesium matrix was improved in the presence of SiC particulates, and increased with the increase of the proportion of SiC particulates. Ceramic hollow sphere fly ash particulates are another kind of reinforcement, which are inexpensive, low density and available abundantly in large quantities as solid by-products during combustion of coal in thermal power plants. Sudarshan et al. [8] showed that fly ash/A356 MMCs, with a proper volume fraction of fly ash particulate, exhibited improved ambient temperature damping capacity. In the present work, Al/RHA composites were fabricated and the damping measurements were done on the specimens machined by the wire cut EDM process and the milling process and the results were compared and analyzed.
D.S. Prasad et al. / Materials Science & Engineering A 591 (2014) 78–81
Table 1 Chemical composition of A356.2 Al Alloy matrix.
79
0.05
Wire cut EDM
Si
Fe
Cu
Mn
Mg
Zn
Ni
Ti
6.5–7.5
0.15
0.03
0.10
0.4
0.07
0.05
0.1
Damping capacity
0.04
Table 2 Chemical composition of RHA. Constituent Silica Graphite Calcium oxide
Magnesium oxide
Potassium oxide
Ferric oxide
%
0.53
0.39
0.21
90.23 4.77
1.58
0.03
0.02
unreinforced alloy Al/4HA composite Al/6HA composite Al/8HA composite
0.01
0.00 0
5
2. Experimentation
The damping capacity of the base alloy and Al/RHA composites for two kinds of specimens (wire cut EDM machined and milling machined) is shown in Fig. 1a and b respectively. From Fig. 1a and b, it was observed that the damping capacity has been improved with the addition of the reinforcement and no distinct peaks were found in both the cases. The increase in damping capacity for the composites can be attributed to the increase in the dislocation densities due the difference in the coefficient of thermal expansions (CTE) of the reinforcement and the matrix alloy. The CTE values of A356.2 alloy and RHA particulates are 21.4 10 6 1C 1 and 10.1 10 6 1C 1 respectively [9]. Due to this difference in CTE the dislocation density generated can be quite significant at the interface and can be predicted using the following equation: ð1Þ
where B is a geometric constant, ε is the thermal mismatch strain, VRHA is the volume fraction of the RHA particulates, b is the Burgers vector, and d is the grain diameter of reinforcements. The volume fraction (VRHA) is given by the equation: V RHA ¼
ρAl ρmmc ρAl ρRHA
20
25
Milling
0.050 0.045 0.040 0.035 0.030 0.025 0.020 0.015
Unreinforced Al/4HA composite Al/6HA composite Al/8HA composite
0.010 0.005 0
5
10
15
20
25
Frequency in Hertz (Hz) Fig. 1. Tan δ values for unreinforced, 4%, 6% and 8% RHA composites. (a) Wire cut EDM machined specimen and (b) milling machined specimen.
Table 3 Dislocation densities of unreinforced and Al/RHA composites.
3. Results and discussions
BεV RHA bdð1 V RHA Þ
15
0.055
Damping capacity
The matrix material used in this study is an A356.2 alloy whose composition is given in Table 1. The reinforcement is RHA particulates whose chemical composition is given in Table 2. The composites were fabricated by the stir casting technique. Before incorporating the RHA particulates in the alloy the ash particulates are pre-treated in order to free them from inorganic matter and carbonaceous material. The composites with 4, 6 and 8 wt% RHA particulates were fabricated using the stir casting method and the detailed fabrication process of the composites was presented in earlier works [9]. A GABO Eplexor dynamic mechanical analyzer is used to study the damping behavior of the base alloy and the composites. Tests were carried under a static load of 50 N and a dynamic load of 40 N at room temperature for the frequencies ranging from 1 Hz to 25 Hz and a strain amplitude (ε) was 1 10 5 using the three-point bending mode. The samples for damping measurements were machined using an Ultra cut 843/ Ultra cut f2 CNC wire electrical discharge machine and MTAB XL milling machine. The dimensions of the specimen for the damping measurements are 30 12 15 mm3. Microstructural characterization of the samples was done using JSM-6610LV scanning electron microscope (SEM) equipped with energy dispersive X-ray analyzer.
ρ¼
10
Frequency in Hertz (Hz)
ð2Þ
% RHA
Volume fraction
Estimated dislocation density, ρ (m 2)
0.0 4.0 6.0 8.0
– 0.028 0.065 0.086
– 8.81 1010 3.96 1011 7.32 1011
where ρAl is the density of A356.2, ρmmc is the density of composites and ρRHA is the density of rice husk ash. Based on Eqs. (1) and (2) the dislocation density was calculated with an assumption for the Burgers vector of 0.32 nm for Al [9] and are tabulated in Table 3. According to Granato–Lucke mechanism, the dislocation-based damping is mainly due to frequency dependent part and is directly proportional to the dislocation density and is expressed as follows: Q 1 ¼
a 0 B ρL 4 ω2
π 2 Cb2
ð3Þ
where a0 is a numerical factor of order 1, B is the damping constant, ω is the operating frequency, L is the effective dislocation loop length, C is the dislocation line tension (E0.5 Gb2), and G is the shear modulus.
80
D.S. Prasad et al. / Materials Science & Engineering A 591 (2014) 78–81
From Eq. (3) it could be observed that the inverse quality factor Q 1 or damping capacity is directly proportional to the dislocation density. From Table 3 it was observed that the dislocation density increases with the increase in RHA content and hence an increase in damping has been observed in Al/RHA composites. In addition to the dislocation density based damping, generation of plastic zone is another mechanism observed in the present study for the increase of damping capacity of Al/RHA composites. In particle reinforced MMCs, the incorporation of hard particulates in the matrix causes residual stresses in the form of an annular plastic zone due to differences in the CTE of the matrix and the reinforcement, which are responsible for the generation of plastic zone at the Al/RHA interface. The size of the plastic zone Cs can be calculated using C s ¼ rs
ΔαEΔT ð1 νÞsy
The damping can be expressed as a function of plastic zone and is given by tan δ ¼
R C s G sdε π s2o
ð5Þ
where G is the shear modulus of the composite sample, so is the alternating shear stress amplitude and ε is the corresponding strain acting on the specimen. Eq. (5) shows that the damping is a direct measure of the plastic zone radius and from Table 4 it was observed that the plastic zone increases with increase in RHA content and hence the damping capacity of the composites increases with weight percentage of the reinforcement.
ð4Þ
where Δα is the difference in CTEs, ΔT is the temperature difference, E and ν are the matrix elastic modulus and Poisson's ratio, sy is the matrix yield stress and rs is the particulate radius. From Eq. (4) the radius of plastic zone for different weight percentages of RHA was calculated and given in Table 4. Table 4 Radius of plastic zone of unreinforced and Al/RHA composites. % RHA 0.0 4.0 6.0 8.0
CTE (α) (1C 1)
Radius of plastic zone (mm)
6
– 17.64 31.92 42.84
21.4 10 19.3 10 6 17.6 10 6 16.3 10 6
Fig. 3. SEM micrograph showing white layer.
0.026 0.024
Al/6%RHA composite
0.035 for Unreinforced alloy
0.020
Damping capacity
Damping capacity
0.022
0.018 0.016 0.014 0.012 0.010
Wire cut EDM Milling
0.008
0.030
0.025
0.020 Wire cut EDM Milling
0.015
0.006 0.010
0.004 0
5
10
15
20
25
0
Frequency in Hertz (Hz)
0.034
Al/4%RHA composite
15
20
25
Al/8%RHA composite
0.055
0.030
0.050
0.028
0.045
Dampaing capacity
Damping capacity
10
Frequency in Hertz (Hz)
0.032
0.026 0.024 0.022 Wire cut EDM Milling
0.020
5
0.018
0.040 0.035 0.030 Wire cut EDM Milling
0.025
0.016 0.020
0.014
0.015
0.012 0
5
10
15
Frequency in Hertz (Hz)
20
25
0
5
10
15
20
25
Frequency in Hertz (Hz)
Fig. 2. Comparison of damping capacities for the wire cut EDM machined specimen and milling specimen for (a) unreinforced alloy, (b) 4% RHA composite, (c) 6% RHA composite, and (d) 8% RHA composite.
D.S. Prasad et al. / Materials Science & Engineering A 591 (2014) 78–81
Hence dislocation damping and generation of plastic zone at the interface are the operating mechanisms in the RHA reinforced composites although there may be other mechanisms such as grain boundary damping, thermoelastic damping, intrinsic damping, interfacial damping, etc. which are responsible for the increase in the damping capacity; the author is more interested in comparison of the damping capacity of the wire cut EDM machined and milling machined specimens and hence it is not discussed in detail. Fig. 2a–d shows the comparison of the damping values between the wire cut EDM machined and milling machined specimens for the unreinforced and RHA reinforced composites. It was observed that the damping capacity for wire cut EDM machined specimens exhibits low damping values in all the cases when compared with milling machined specimens. This can be attributed to the formation of a white layer when the specimens are machined with the wire cut EDM process. When a specimen is machined using the wire cut EDM process, the current from the process melts the material and a thin layer on the surface of the specimen is formed called the white layer or recast layer. This white layer is because of the molten metal which cannot be expelled and has instead been rapidly quenched by the dielectric oil. From the scanning electron micrograph (Fig. 3), there is clear evidence of white layer formation with an average thickness of 21 mm. Due to the formation of this white layer the energy dissipated to the surroundings will be reduced which is a direct measure of damping capacity and hence a decrease in damping capacity has been observed. 4. Conclusions
considered for the preparation of the damping specimens. The increase in damping capacity of the composite is due the increase in the dislocation density and plastic zone with the percentage increase in the reinforcement. It was concluded that the wire cut EDM machined specimen exhibits low damping values which can be attributed to the formation of white layer which acts like a protective layer to dissipate elastic strain energy which results in reduction of damping values. The formation of white layer contributes 8–15% of the overall damping and this should be considered when the specimen is machined with wire cut EDM.
Novelty statement The enhancement of damping capacity in metal matrix composites can be attributed to the generation of plastic zone at the interface, grain boundary damping, thermoelastic damping, intrinsic damping, interfacial damping, etc. Many researches machined the damping specimen by the wire cut EDM process. When the metal–matrix composites are machined with EDM the formation of the white layer is a common phenomenon and the effect of this white layer on damping is not explained.
References [1] [2] [3] [4] [5] [6]
The Dynamic Mechanical Analyzer is used to predict the damping behavior of the base alloy and the RHA reinforced composites. Wire cut EDM and milling machines have been
81
[7] [8] [9]
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