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Study of the Recast Layer of Particulate Reinforced Metal Matrix Composites machined by EDM
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 3243–3251
www.materialstoday.com/proceedings
5th International Conference of Materials Processing and Characterization (ICMPC 2016)
Study of the Recast Layer of Particulate Reinforced Metal Matrix Composites machined by EDM Sarabjeet Singh Sidhu*, Preetkanwal Singh Bains Department of Mechanical Engineering, Beant college of Engineering and Technology, Gurdaspur-143521, India
Abstract This study explores the relationship between the recast layer zone and the electrical discharge machining (EDM) process parameters behaviors on metal matrix composites (MMCs). The characteristics of recast layer are investigated by using a L27 Taguchi’s design of experiments. The formation of recast layer in three types of aluminum based particulate reinforced MMCs have been examined in combination with process parameters in the variant dielectric medium. The influence of each selected parameter is obtained by ANOVA analysis. It is found that the formation of recast layer is significantly affected by the reinforcement architectures in matrix phase and the spark energy of EDM. The cross-sectional micrograph of recast layer is assessed for the metal removal mechanism and EDM-induced surface alterations on the machined zone. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Keywords: Electrical discharge machining; Metal Matrix Composites; Recast layer; XRD; SEM.
1. Introduction Researchers have continuously strived for the development of advanced materials for industrial applications that have better mechanical and physical properties. The ability of high strength to weight ratio, wear resistant, and stability at high operating temperature are attractive properties of composite materials. These composites result in attaining synergetic improved properties and thus they have the ability to replace the expensive alloy materials that are being currently used in manufacturing industries. The current research topic is mainly focused on the machining of particulate reinforced MMCs. The versatile properties of composite materials have made them desirable for the ______________ * Corresponding author. Tel.:+91-98558-88828; E-mail address: [email protected]
2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
3244
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
engineering applications [1]. The commonly used matrix materials such as copper, magnesium, Aluminum and titanium may be reinforced with ceramics or whiskers depending upon their compatibility with the matrix material and bonding characteristics [2]. However, these hard reinforcement particles in metal matrix make it difficult to be machined by conventional machining processes. Inability to produce complex shapes with high accuracy in these materials limits its application in different sectors. Towards this end, electric discharge machining (EDM) is emerging as a viable machining process that has the potential to produce intricate shapes with high accuracy and precision in these materials [3]. In this process, electrical energy in the form of pulses produces a series of sparks between two electrodes at the smallest interelectrode gap. The electrodes, as well as the workpiece, remain immersed in a dielectric. During machining, the eroded material is flushed away by the flowing dielectric but some amount of this material may be deposited back on the machined surface during pulse off-time that is known as the recast layer or re-solidified layer. Since the cycle time is only a few micro-seconds, this fast process of heating and cooling damages the surface and produces defects like micro-cracks, voids, pores or undesirable phase transformations [4]. The MMCs are the combination of two or more materials hence; it may alter the properties of the workpiece in the machine area due to different erosion mechanism. Recently, Kandpal and his teams reviewed the effect of process parameters of EDM on aluminum matrix composite. They also addressed the advantages of hybrid EDM for improving the machining capabilities related to MMCs [5]. Rajendran et al. presented machining performance during EDM of T90Mn2W50Cr45 tool steel. It was found that re-solidified layer thickness is directly proportional to the current, which results in evaporation of dielectric fluid [6]. Ramulu et al. examined the recast layers formed during EDM of 15vol% SiC/ A356 composite. They observed the damage in recast layer due to SiC particulate pullout for fine and coarse EDM [7]. Senthilkumar et al. conducted the study on recast layer evolution during EDM of Al/(2.5% & 5%) TiC composites. They found that white/ recast layer appears on the machined surface with crack and micro-voids, but TiC particles pull-out of the matrix had not recast on this zone [8]. The presence of cracks, voids, residual stress, phase transformation, in recast layer results in a drastic change in surface properties. For the components used in high-stress concentration environment, researcher advocates for modified the spark energy to reduce the recast layer formation [9] or in many cases secondary operation was suggested for its removal from the surface for uniform surface integrity [10]. A few class researchers centered their efforts to study the advantageous the surface modification due to metal deposition /recast layer formation during EDM process. Parkash and his team presented the application of EDM process for the surface modifications of biomaterials in orthopedics implants [11]. Hung et al. also suggested the application of EDM for surface modification of shape memory alloy for biomedical application [12]. Patowari et al. enhanced the surface hardness of C-40 grade plain carbon steel by using W-Cu electrode prepared by powder metallurgy route [13]. The authors explored the effect of EDM process parameters on the surface properties of machined MMCs in their previous published work [14]. However, in the present work the detailed analysis of recast layer formation during EDM process was capitalized by using Taguchi’s experimental design. In the final step, SEM was also conducted to underline the metal removal mechanism and its effect on recast layer thickness in the selected range of machining. The selected MMC (Material I) is the 65% SiC/A356.2 composite reinforced with three different SiC-particulates sizes (Approx. 50, 25, 5 µm). The specimen was supplied by Ceramic process system (CPS), USA, prepared by preform infiltration process (Commercial Application: Thermal management of electronic components). Materials II is the aluminum matrix bimodal (10% SiC and 5% quartz) reinforced composite having SiC (~50 µm) and quartz (~14 µm) particulates. The specimen was prepared by conventional stir casting method [15]. Material III is the 30% SiC/ A359, (SiC size: ~17 µm) Aluminum composite procured from Metallic composite (MC-21) USA and prepared by stir casting followed by rolling process (Commercial Application: Automotive applications such as disc brakes, engine components). 2.
Materials and Methods
The experimentation was carried out on the MMCs that are fast gaining acceptance as industrial materials. Material I and Material III are readily available in the market in various standard shapes while Material II was prepared in-house by using stir casting method. The properties of these three MMCs are presented in Table 1.
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251 Table 1: Properties of MMCs Properties
Material I
Material II
Material III
Density (g/cm3)
3.01
2.48
2.8
Thermal Conductivity (W/mk)
200
215
170
Coefficient of Thermal expansion (ppm/oC)
8.00
25.6
14.5
Modulus of Elasticity (GPa)
179
78
110
Poisson Ratio
0.25
0.33
0.29
Flexural Strength (MPa)
488
240
318
Thermal shock resistance (0C)
255.5
80.5
141.5
Table 2. L27 experimental design layout and results Trial No.
Control parameters Workpiece
Electrode
Current
Response
Pulse-on
(A)
Pulse-off (μs)
(μs)
Dielectric
Recast layer thickness (µm)
medium
1
1
1
1
1
1
1
33.72
2
1
1
2
2
2
2
28.79
3
1
1
3
3
3
3
89.09
4
1
2
3
3
1
2
61.9
5
1
2
1
1
2
3
37.0
6
1
2
2
2
3
1
125.0
7
1
3
2
2
1
3
100.36
8
1
3
3
3
2
1
31.91
9
1
3
1
1
3
2
28.19
10
2
1
2
3
1
3
71.7
11
2
1
3
1
2
1
36.0
12
2
1
1
2
3
2
10.4
13
2
2
1
2
1
1
6.1
14
2
2
2
3
2
2
35.8
15
2
2
3
1
3
3
23.0
16
2
3
3
1
1
2
17.94
17
2
3
1
2
2
3
8.3
18
2
3
2
3
3
1
39.8
19
3
1
3
2
1
2
33.16
20
3
1
1
3
2
3
19.17
21
3
1
2
1
3
1
6.0
22
3
2
2
1
1
3
12.40
23
3
2
3
2
2
1
31.26
24
3
2
1
3
3
2
26.48
25
3
3
1
3
1
1
7.67
26
3
3
2
1
2
2
4.7
27
3
3
3
2
3
3
6.3
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Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
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The parametric settings of machining used in this work were based on pilot experimentation and detailed literature survey. The effect of these variables on re-solidified layer was studied during machining as per L27 Taguchi’s design of experiment technique [16]. The allocation of six factors in the array and their levels were; MMCs type (Material I: Level 1, Material II: Level 2, Material III: Level 3); Electrode material (Cu: Level 1, Gr: Level 2, Cu-Gr: Level 3); Current (4 A: Level 1, 8 A: Level 2, 12 A: Level 3); Pulse-on time (10 μs: Level 1, 30 μs: Level 2, 45 μs: Level 3); Pulse-off time (15 μs: Level 1, 30 μs: Level 2, 45 μs: Level 3); and Dielectric medium EDM oil ( Level 1), copper powder mixed in EDM oil ( Level 2) and graphite powder mixed in EDM oil (Level 3). The values of remaining machining parameters were kept fixed, for example, sparking voltage was 135 V and flushing pressure was 0.6 kg/cm2 throughout the experimentation. The experiments were conducted an OSCARMAX (SD550 ZNC, Taiwan) die sinking EDM machine using a conventional polarity. The selected levels in each experimental setup and response parameters are summarized in Table 2. 3. Results and Discussion The specimen obtained after machining as per the 27 treatment conditions laid down by Taguchi’s L27 matrix are given in the last column of Table 2. To study the re-solidified layer, the workpieces were cut through the crosssection with the help of water cooled precision cutting machine. Before taking micrographs, the cross-section was polished as per standard polishing procedure using different grit sizes of emery papers. The recast layer was identified by the curved topography and homogenous structure. Out of selected variables, the one that is affecting the recast layer thickness were identified by using the Analysis of Variance (ANOVA) statistical technique. The results of this analysis are presented in Table 3, and the main effect plot is shown in Fig. 1. Table 3: Analysis of means for recast layer thickness. Factors
Degree of freedom
Sum of Squares
Work piece
2
9032.1
Variance
F-value
p-value
4516
9
0.003**
Electrode
2
768.4
384.2
0.77
0.483
Current
2
3473.6
1736.8
3.46
0.060*
Pulse-on
2
2148.1
1074
2.14
0.154*
Pulse-off
2
1011.0
505.5
1.01
0.390
0.80
0.467
Dielectric
2
806.7
403.3
Error
14
7023.1
501.6
Total
26
24162.9
** Most significant *Significant
Comparing the p-value from ANOVA table, the MMC reinforced with dense and bigger size particles plays a significant role (95%) in developing the recast layer thickness. These particulates exhibit barrier to the heat transportation in the metallic phase which subsequently contributes to thick recast layer. The peak current setting was also found to be a significant factor (90%) affecting recast layer thickness that is in-line with the findings of Hourmand et al. [17]. The variation of recast layer thickness on each MMC with respect to current and pulse-on time levels is represented in Fig. 2 (a, b) respectively. It was found that Material I formed thicker recast layer at an intermediate level of current (8 A) and pulse-on time (30 µs). Material III exhibits minimum the recast layer thickness at current at 8 A and pulse-on time at 10 µs. However, for Material II, the thickness reduced drastically at current at 4 A and pulse-on time at 30 µs. The contribution of current and pulse-on time settings in the formation of recast layer can be identified from Fig. 3.
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
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Main effects plot for Means Data Means
W ork piece
60
Electrode
47.2
50
39.9
36.4
40
Mean of Means
C urrent (A mp)
59.5
30
36.7
27.7
20
27.2
16.3 Sample I
Sample II
Sample III
Cu
Pulse-on (µs)
60
Gr
4
Pulse-off (µs) 42.6
50
C u-Gr
19.6
38.87
38.3
8
12
Dielectric + Powder
39.3
40
40.8 35.2
30 22.12
20 10
27.5
25.9 30
45
15
30
45
D
D+C u
D+Gr
Fig. 1. Main effects plots for recast layer thickness.
Fig. 2: Variation of Recast layer thickness (a) Effect of Current (b) Effect Pulse-on time
The material removal mechanism in EDM process may be due thermal spalling (i.e. breaking away the material part due to the temperature gradient); thermal shocks/cracks (i.e. sudden change in transient temperature); melting and evaporation or oxidation. The typical fracture resistant strength R (resistance to thermal shocks) against the thermal gradient is represented as [18]. 𝑅=
𝜎𝑓 (1 − 𝜐) 𝛼𝐸
Where σf = Fracture strength, υ = Poisson’s ratio, α = coefficient of thermal expansion and E= Young’s modulus. A lower thermal resistance (R) value indicates that the thermal shock due to heating and cooling dominate the material removal. The MMCs with lower thermal conductivity have localized heat generation at the surface; resulting in the removal of material due to melting and evaporation. The density of cracks generated within the MMCs is related to the property of coefficient of thermal expansion of the constituents and its elastic properties. The properties of different constituents results in the development of micro-stresses or tessellated stresses [8].
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Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
85
Recat layer Thickness
68 51 34 17
45 30
4 Curr 8 ent
Puls e-on
0
10
12
Fig. 3: Variation of Current and Pulse-on time on Recast layer thickness.
The microstructure analysis for each MMC is explained corresponding to its trial. Material I: The SEM cross-sectional micrographs were analyzed for the trials to examine the possible metal removal mechanism. According to Trial 1, it is observed that there is a formation of deep irregular crater (Fig. 4a). This may be due to lowest EDM parameters settings results in lower spark energy plasma between the electrodes (work piece and tool electrode). On increasing the spark energy the formation of craters increases, i.e. roughness enhances [19], and also shown the sign of thermal shocks with deep pits and larger size grains on the surface (Fig. 4b). The increase in spark energy results in the occurrence of cracks in the recast layer and traces of melting, thermal shocks appears (Fig. 4c), hence increase in metal removal rate. b
a
c
Cracks
Spalling
d
Spalling
Fig. 4: SEM micrograph of 65%SiC/ A356.2 (a) Trial 1(b) Trial 2 (c) Trial 3 (d) Trial 8.
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
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It was established by the researcher, that the spark energy is directly proportional to the material removal rate due to cracks formation in dense ceramic reinforced composites [20]. The formation of recast layer was maximum at the intermediate level of current and pulse-on time setting (i.e. Trial 6). SEM corresponding to trial 8 reveals that there is a formation of minimum re-solidified layer deposition on the surface (Fig. 4d). This was attributed to non-uniform plasma formation in the EDM oil (without additive) that results in intense thermal spalling. In the case of trial 9 although the spark energy was set at its lowest level, but cracks were observed around the circumference grain deposited due to lower surface energy as shown in Fig. 5a. These cracks were formed due to improper wettability of disintegrated molten material on the machined surface. In this type of MMC, no cracks were noted in the intermediate or the parent matrix due to the presence of densely reinforced particle that blocks the propagation of the cracks as shown in Fig. 5b. The cross-sectional micrograph (Fig. 5b) featured the absence of reinforced particles in recast layer, hence results in non- uniform properties of machined surface and base material. a
b
Fig. 5. (a) Topography of machined surface (b) Cross-sectional view of recast layer (Material I)
Material II: The material removal mechanism in this type of MMCs was mainly due to melting and brittle fracture that is evident from major cracks/texture of the machined surface as shown in Fig. 6. This is due to least value of thermal fracture resistance (R). It is observed that, when spark energy increased to its highest level, it leads to severe flow lines and grain growth on the topography of the machined surface.
Fig. 6: Micrograph shows the formation of wide cracks (Trial 12)
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Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
Material III: The mechanism of material removal in this class of MMC was due to melting and spalling process. Thermal shock found in this material was minimum. Although the fracture resistance (R= 141.50C) of the Material III is less as compared to Material I, the size of reinforced particle (17 µm) play a major role to enhanced the resistance against the thermal shocks. These fine reinforced particulates reduce the porosity thus enhances the density of the composites. The recast layer formed is uniform for all the trial (trial 19-27) due to small pullout pits formation. The dense voids and cracks with oxidation signs were observed in trial 23 when the specimen was machined in EDM oil (absence of suspended additive). The enlarged plasma in powder mixed EDM process results in the even distribution of spark energy, hence comparatively uniform machined surface as shown in Fig. 7. The formations of pits on the surface were broader and shallower, hence good surface finish. The high pulse-off time results in enlarged re-solidified spherical grains formation; hence, it was also evident that at high spark energy melting dominated the thermal spalling mechanism in the material removal process. In this class of material [ref. Table 2] at maximum spark energy and high pulse-off time, the recast layer thickness is minimum due to effective flushing of molten metal. At lower values of peak current and higher values of pulse-on time, these grains are very small. The size of grain depends upon the current rate that is also reported ref. therein [6].
Fig. 7: Topography of specimen obtained in Cu- mixed dielectric medium (Trial 26).
4. Conclusions In this experimental work, three types of aluminum matrix based MMCs were machined by the EDM process under different machining conditions to assess their impact on the recast layer thickness. A selection of percentile reinforcements and peak current were found to be the two most significant factors affecting the quality and thickness of recast layer. The MMC whose coefficient of thermal resistance was lowest, experienced the high magnitude of fracture during the process and hence, higher MRR. The recast layer thickness formation was prominent in the densely reinforced matrix (Material I) due to the high rate of thermal spalling in metal removal mechanism. The recast layer increases with the increase in current from 4 A to 8 A, but after that it further drops, due to effective flushing of a molten metallic portion at high spark energy. The addition of powder in the dielectric lowers crater depth, resulting in smooth recast layer. The formation of recast layer and its properties depend mainly on the architecture of reinforced particles and the fabrication route of composites. Due to fine reinforced particles (~17 µm), the material removal mechanism in Material III is mainly due to melting as compared to thermal spalling, hence resulting in smoother recast layer. The machined surface cracks in Materials I and III are fewer, due to effective diffusion of constituents during the initial fabrication process, which reduces porosity and enhanced its
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
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elastic properties. It was also concluded that the reinforcement architecture of Material I is tailored for heat sinking application of miniature high-energy electronic parts. Thus, the formation of recast layer on the surface of this class of MMC with the conductive material was advantageous. However, Material II, III was mostly utilized in automobile industries. Thus, removal of recast layer with the secondary operation is required for superior surface integrity. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Ahamed, A. Riaz, P. Asokan, S. E. D. M. Aravindan, Int. J. Adv. Manuf. Tech. 44 (2009) 520-528. D. D. Chung, Composite Materials: Science and Applications, Springer-Verlag London, 2010, pp. 1-34. P. Narender Singh, K. Raghukandan, M. Rathinasabapathi, B. C. Pai, J. Mater. Process. Technol. 155 (2004) 1653-1657. J. W. Lee, J. Mater. Sci. 38 (2003) 1679-1687. B.C. Kandpal, J. Kumar, H. Singh, Mater. Today : Proc. 2 (2015) 1665-1671. S. Rajendran, K. Marimuthu, and M. Sakthivel, Mater. Manuf. Process. 28 (2013) 664-669. M. Ramulu, , G. Paul, and J. Patel, Compos. Struct. 54 (2001) 79-86. V. Senthilkumar, B. U. Omprakash, J. Manuf. Process. 13 (2011) 60-66. S. Vignesh, , B. Mohan, T. Muthuramalingam, S. Karthikeyan, Appl. Mech. Mater. 766 (2015) 518-522. Sasan Khalaj Amineh, Alireza Fadaei Tehrani, Aminollah Mohammadi, Int. J. Adv. Manuf. Tech. 66 (2013) 1793-1803. Chander Prakash, Harmesh K. Kansal, B. S. Pabla, Sanjeev Puri, Aditya Aggarwal, P. I. Mech. Eng. B-J. Eng. (2015) doi: 10.1177/0954405415579113. Tyau-Song Huang, Shy-Feng Hsieh, Sung-Long Chen, Ming-Hong Lin, Shih-Fu Ou, Wei-Tse Chang, J. Mater. Process. Technol. 221 (2015) 279-284. Promod K. Patowari, , Partha Saha, Prasanta K. Mishra, Int. J. Adv. Manuf. Tech. 54 (2011) 593-604. Sarabjeet Singh Sidhu, Ajay Batish, Sanjeev Kumar, Mater. Sci. Technol. (2015) doi: 1743284715Y-0000000033. Preetkanwal Singh, Sarabjeet Singh Sidhu, H.S. Payal, Mater. Manuf. Process. (2015) doi: 10.1080/10426914.2015.1025976. G. Taguchi, Introduction to Quality Engineering, Asian Productivity Organization, Tokyo, 1990. M. Hourmand, S. Farahany, A. A. Sarhan, M. Y. Noordin, Int. J. Adv. Manuf. Tech. 77 (2015) 831-841. W. D Kingery, J. Am. Ceram. Soc. 38 (1955) 3-15. S. Assarzaden, M. Ghoreishi, Adv. Mater. Manuf. & Charac. 3 (2013) 487-498. Sarabjeet Singh Sidhu, Ajay Batish, Sanjeev Kumar, J. Reinf. Plast. Comp. 32 (2013) 1310-1320.
ScienceDirect Materials Today: Proceedings 4 (2017) 3243–3251
www.materialstoday.com/proceedings
5th International Conference of Materials Processing and Characterization (ICMPC 2016)
Study of the Recast Layer of Particulate Reinforced Metal Matrix Composites machined by EDM Sarabjeet Singh Sidhu*, Preetkanwal Singh Bains Department of Mechanical Engineering, Beant college of Engineering and Technology, Gurdaspur-143521, India
Abstract This study explores the relationship between the recast layer zone and the electrical discharge machining (EDM) process parameters behaviors on metal matrix composites (MMCs). The characteristics of recast layer are investigated by using a L27 Taguchi’s design of experiments. The formation of recast layer in three types of aluminum based particulate reinforced MMCs have been examined in combination with process parameters in the variant dielectric medium. The influence of each selected parameter is obtained by ANOVA analysis. It is found that the formation of recast layer is significantly affected by the reinforcement architectures in matrix phase and the spark energy of EDM. The cross-sectional micrograph of recast layer is assessed for the metal removal mechanism and EDM-induced surface alterations on the machined zone. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Keywords: Electrical discharge machining; Metal Matrix Composites; Recast layer; XRD; SEM.
1. Introduction Researchers have continuously strived for the development of advanced materials for industrial applications that have better mechanical and physical properties. The ability of high strength to weight ratio, wear resistant, and stability at high operating temperature are attractive properties of composite materials. These composites result in attaining synergetic improved properties and thus they have the ability to replace the expensive alloy materials that are being currently used in manufacturing industries. The current research topic is mainly focused on the machining of particulate reinforced MMCs. The versatile properties of composite materials have made them desirable for the ______________ * Corresponding author. Tel.:+91-98558-88828; E-mail address: [email protected]
2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
3244
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
engineering applications [1]. The commonly used matrix materials such as copper, magnesium, Aluminum and titanium may be reinforced with ceramics or whiskers depending upon their compatibility with the matrix material and bonding characteristics [2]. However, these hard reinforcement particles in metal matrix make it difficult to be machined by conventional machining processes. Inability to produce complex shapes with high accuracy in these materials limits its application in different sectors. Towards this end, electric discharge machining (EDM) is emerging as a viable machining process that has the potential to produce intricate shapes with high accuracy and precision in these materials [3]. In this process, electrical energy in the form of pulses produces a series of sparks between two electrodes at the smallest interelectrode gap. The electrodes, as well as the workpiece, remain immersed in a dielectric. During machining, the eroded material is flushed away by the flowing dielectric but some amount of this material may be deposited back on the machined surface during pulse off-time that is known as the recast layer or re-solidified layer. Since the cycle time is only a few micro-seconds, this fast process of heating and cooling damages the surface and produces defects like micro-cracks, voids, pores or undesirable phase transformations [4]. The MMCs are the combination of two or more materials hence; it may alter the properties of the workpiece in the machine area due to different erosion mechanism. Recently, Kandpal and his teams reviewed the effect of process parameters of EDM on aluminum matrix composite. They also addressed the advantages of hybrid EDM for improving the machining capabilities related to MMCs [5]. Rajendran et al. presented machining performance during EDM of T90Mn2W50Cr45 tool steel. It was found that re-solidified layer thickness is directly proportional to the current, which results in evaporation of dielectric fluid [6]. Ramulu et al. examined the recast layers formed during EDM of 15vol% SiC/ A356 composite. They observed the damage in recast layer due to SiC particulate pullout for fine and coarse EDM [7]. Senthilkumar et al. conducted the study on recast layer evolution during EDM of Al/(2.5% & 5%) TiC composites. They found that white/ recast layer appears on the machined surface with crack and micro-voids, but TiC particles pull-out of the matrix had not recast on this zone [8]. The presence of cracks, voids, residual stress, phase transformation, in recast layer results in a drastic change in surface properties. For the components used in high-stress concentration environment, researcher advocates for modified the spark energy to reduce the recast layer formation [9] or in many cases secondary operation was suggested for its removal from the surface for uniform surface integrity [10]. A few class researchers centered their efforts to study the advantageous the surface modification due to metal deposition /recast layer formation during EDM process. Parkash and his team presented the application of EDM process for the surface modifications of biomaterials in orthopedics implants [11]. Hung et al. also suggested the application of EDM for surface modification of shape memory alloy for biomedical application [12]. Patowari et al. enhanced the surface hardness of C-40 grade plain carbon steel by using W-Cu electrode prepared by powder metallurgy route [13]. The authors explored the effect of EDM process parameters on the surface properties of machined MMCs in their previous published work [14]. However, in the present work the detailed analysis of recast layer formation during EDM process was capitalized by using Taguchi’s experimental design. In the final step, SEM was also conducted to underline the metal removal mechanism and its effect on recast layer thickness in the selected range of machining. The selected MMC (Material I) is the 65% SiC/A356.2 composite reinforced with three different SiC-particulates sizes (Approx. 50, 25, 5 µm). The specimen was supplied by Ceramic process system (CPS), USA, prepared by preform infiltration process (Commercial Application: Thermal management of electronic components). Materials II is the aluminum matrix bimodal (10% SiC and 5% quartz) reinforced composite having SiC (~50 µm) and quartz (~14 µm) particulates. The specimen was prepared by conventional stir casting method [15]. Material III is the 30% SiC/ A359, (SiC size: ~17 µm) Aluminum composite procured from Metallic composite (MC-21) USA and prepared by stir casting followed by rolling process (Commercial Application: Automotive applications such as disc brakes, engine components). 2.
Materials and Methods
The experimentation was carried out on the MMCs that are fast gaining acceptance as industrial materials. Material I and Material III are readily available in the market in various standard shapes while Material II was prepared in-house by using stir casting method. The properties of these three MMCs are presented in Table 1.
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251 Table 1: Properties of MMCs Properties
Material I
Material II
Material III
Density (g/cm3)
3.01
2.48
2.8
Thermal Conductivity (W/mk)
200
215
170
Coefficient of Thermal expansion (ppm/oC)
8.00
25.6
14.5
Modulus of Elasticity (GPa)
179
78
110
Poisson Ratio
0.25
0.33
0.29
Flexural Strength (MPa)
488
240
318
Thermal shock resistance (0C)
255.5
80.5
141.5
Table 2. L27 experimental design layout and results Trial No.
Control parameters Workpiece
Electrode
Current
Response
Pulse-on
(A)
Pulse-off (μs)
(μs)
Dielectric
Recast layer thickness (µm)
medium
1
1
1
1
1
1
1
33.72
2
1
1
2
2
2
2
28.79
3
1
1
3
3
3
3
89.09
4
1
2
3
3
1
2
61.9
5
1
2
1
1
2
3
37.0
6
1
2
2
2
3
1
125.0
7
1
3
2
2
1
3
100.36
8
1
3
3
3
2
1
31.91
9
1
3
1
1
3
2
28.19
10
2
1
2
3
1
3
71.7
11
2
1
3
1
2
1
36.0
12
2
1
1
2
3
2
10.4
13
2
2
1
2
1
1
6.1
14
2
2
2
3
2
2
35.8
15
2
2
3
1
3
3
23.0
16
2
3
3
1
1
2
17.94
17
2
3
1
2
2
3
8.3
18
2
3
2
3
3
1
39.8
19
3
1
3
2
1
2
33.16
20
3
1
1
3
2
3
19.17
21
3
1
2
1
3
1
6.0
22
3
2
2
1
1
3
12.40
23
3
2
3
2
2
1
31.26
24
3
2
1
3
3
2
26.48
25
3
3
1
3
1
1
7.67
26
3
3
2
1
2
2
4.7
27
3
3
3
2
3
3
6.3
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The parametric settings of machining used in this work were based on pilot experimentation and detailed literature survey. The effect of these variables on re-solidified layer was studied during machining as per L27 Taguchi’s design of experiment technique [16]. The allocation of six factors in the array and their levels were; MMCs type (Material I: Level 1, Material II: Level 2, Material III: Level 3); Electrode material (Cu: Level 1, Gr: Level 2, Cu-Gr: Level 3); Current (4 A: Level 1, 8 A: Level 2, 12 A: Level 3); Pulse-on time (10 μs: Level 1, 30 μs: Level 2, 45 μs: Level 3); Pulse-off time (15 μs: Level 1, 30 μs: Level 2, 45 μs: Level 3); and Dielectric medium EDM oil ( Level 1), copper powder mixed in EDM oil ( Level 2) and graphite powder mixed in EDM oil (Level 3). The values of remaining machining parameters were kept fixed, for example, sparking voltage was 135 V and flushing pressure was 0.6 kg/cm2 throughout the experimentation. The experiments were conducted an OSCARMAX (SD550 ZNC, Taiwan) die sinking EDM machine using a conventional polarity. The selected levels in each experimental setup and response parameters are summarized in Table 2. 3. Results and Discussion The specimen obtained after machining as per the 27 treatment conditions laid down by Taguchi’s L27 matrix are given in the last column of Table 2. To study the re-solidified layer, the workpieces were cut through the crosssection with the help of water cooled precision cutting machine. Before taking micrographs, the cross-section was polished as per standard polishing procedure using different grit sizes of emery papers. The recast layer was identified by the curved topography and homogenous structure. Out of selected variables, the one that is affecting the recast layer thickness were identified by using the Analysis of Variance (ANOVA) statistical technique. The results of this analysis are presented in Table 3, and the main effect plot is shown in Fig. 1. Table 3: Analysis of means for recast layer thickness. Factors
Degree of freedom
Sum of Squares
Work piece
2
9032.1
Variance
F-value
p-value
4516
9
0.003**
Electrode
2
768.4
384.2
0.77
0.483
Current
2
3473.6
1736.8
3.46
0.060*
Pulse-on
2
2148.1
1074
2.14
0.154*
Pulse-off
2
1011.0
505.5
1.01
0.390
0.80
0.467
Dielectric
2
806.7
403.3
Error
14
7023.1
501.6
Total
26
24162.9
** Most significant *Significant
Comparing the p-value from ANOVA table, the MMC reinforced with dense and bigger size particles plays a significant role (95%) in developing the recast layer thickness. These particulates exhibit barrier to the heat transportation in the metallic phase which subsequently contributes to thick recast layer. The peak current setting was also found to be a significant factor (90%) affecting recast layer thickness that is in-line with the findings of Hourmand et al. [17]. The variation of recast layer thickness on each MMC with respect to current and pulse-on time levels is represented in Fig. 2 (a, b) respectively. It was found that Material I formed thicker recast layer at an intermediate level of current (8 A) and pulse-on time (30 µs). Material III exhibits minimum the recast layer thickness at current at 8 A and pulse-on time at 10 µs. However, for Material II, the thickness reduced drastically at current at 4 A and pulse-on time at 30 µs. The contribution of current and pulse-on time settings in the formation of recast layer can be identified from Fig. 3.
Sarabjeet Singh Sidhu and Preetkanwal Singh Bains / Materials Today: Proceedings 4 (2017) 3243–3251
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Main effects plot for Means Data Means
W ork piece
60
Electrode
47.2
50
39.9
36.4
40
Mean of Means
C urrent (A mp)
59.5
30
36.7
27.7
20
27.2
16.3 Sample I
Sample II
Sample III
Cu
Pulse-on (µs)
60
Gr
4
Pulse-off (µs) 42.6
50
C u-Gr
19.6
38.87
38.3
8
12
Dielectric + Powder
39.3
40
40.8 35.2
30 22.12
20 10
27.5
25.9 30
45
15
30
45
D
D+C u
D+Gr
Fig. 1. Main effects plots for recast layer thickness.
Fig. 2: Variation of Recast layer thickness (a) Effect of Current (b) Effect Pulse-on time
The material removal mechanism in EDM process may be due thermal spalling (i.e. breaking away the material part due to the temperature gradient); thermal shocks/cracks (i.e. sudden change in transient temperature); melting and evaporation or oxidation. The typical fracture resistant strength R (resistance to thermal shocks) against the thermal gradient is represented as [18]. 𝑅=
𝜎𝑓 (1 − 𝜐) 𝛼𝐸
Where σf = Fracture strength, υ = Poisson’s ratio, α = coefficient of thermal expansion and E= Young’s modulus. A lower thermal resistance (R) value indicates that the thermal shock due to heating and cooling dominate the material removal. The MMCs with lower thermal conductivity have localized heat generation at the surface; resulting in the removal of material due to melting and evaporation. The density of cracks generated within the MMCs is related to the property of coefficient of thermal expansion of the constituents and its elastic properties. The properties of different constituents results in the development of micro-stresses or tessellated stresses [8].
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Recat layer Thickness
68 51 34 17
45 30
4 Curr 8 ent
Puls e-on
0
10
12
Fig. 3: Variation of Current and Pulse-on time on Recast layer thickness.
The microstructure analysis for each MMC is explained corresponding to its trial. Material I: The SEM cross-sectional micrographs were analyzed for the trials to examine the possible metal removal mechanism. According to Trial 1, it is observed that there is a formation of deep irregular crater (Fig. 4a). This may be due to lowest EDM parameters settings results in lower spark energy plasma between the electrodes (work piece and tool electrode). On increasing the spark energy the formation of craters increases, i.e. roughness enhances [19], and also shown the sign of thermal shocks with deep pits and larger size grains on the surface (Fig. 4b). The increase in spark energy results in the occurrence of cracks in the recast layer and traces of melting, thermal shocks appears (Fig. 4c), hence increase in metal removal rate. b
a
c
Cracks
Spalling
d
Spalling
Fig. 4: SEM micrograph of 65%SiC/ A356.2 (a) Trial 1(b) Trial 2 (c) Trial 3 (d) Trial 8.
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It was established by the researcher, that the spark energy is directly proportional to the material removal rate due to cracks formation in dense ceramic reinforced composites [20]. The formation of recast layer was maximum at the intermediate level of current and pulse-on time setting (i.e. Trial 6). SEM corresponding to trial 8 reveals that there is a formation of minimum re-solidified layer deposition on the surface (Fig. 4d). This was attributed to non-uniform plasma formation in the EDM oil (without additive) that results in intense thermal spalling. In the case of trial 9 although the spark energy was set at its lowest level, but cracks were observed around the circumference grain deposited due to lower surface energy as shown in Fig. 5a. These cracks were formed due to improper wettability of disintegrated molten material on the machined surface. In this type of MMC, no cracks were noted in the intermediate or the parent matrix due to the presence of densely reinforced particle that blocks the propagation of the cracks as shown in Fig. 5b. The cross-sectional micrograph (Fig. 5b) featured the absence of reinforced particles in recast layer, hence results in non- uniform properties of machined surface and base material. a
b
Fig. 5. (a) Topography of machined surface (b) Cross-sectional view of recast layer (Material I)
Material II: The material removal mechanism in this type of MMCs was mainly due to melting and brittle fracture that is evident from major cracks/texture of the machined surface as shown in Fig. 6. This is due to least value of thermal fracture resistance (R). It is observed that, when spark energy increased to its highest level, it leads to severe flow lines and grain growth on the topography of the machined surface.
Fig. 6: Micrograph shows the formation of wide cracks (Trial 12)
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Material III: The mechanism of material removal in this class of MMC was due to melting and spalling process. Thermal shock found in this material was minimum. Although the fracture resistance (R= 141.50C) of the Material III is less as compared to Material I, the size of reinforced particle (17 µm) play a major role to enhanced the resistance against the thermal shocks. These fine reinforced particulates reduce the porosity thus enhances the density of the composites. The recast layer formed is uniform for all the trial (trial 19-27) due to small pullout pits formation. The dense voids and cracks with oxidation signs were observed in trial 23 when the specimen was machined in EDM oil (absence of suspended additive). The enlarged plasma in powder mixed EDM process results in the even distribution of spark energy, hence comparatively uniform machined surface as shown in Fig. 7. The formations of pits on the surface were broader and shallower, hence good surface finish. The high pulse-off time results in enlarged re-solidified spherical grains formation; hence, it was also evident that at high spark energy melting dominated the thermal spalling mechanism in the material removal process. In this class of material [ref. Table 2] at maximum spark energy and high pulse-off time, the recast layer thickness is minimum due to effective flushing of molten metal. At lower values of peak current and higher values of pulse-on time, these grains are very small. The size of grain depends upon the current rate that is also reported ref. therein [6].
Fig. 7: Topography of specimen obtained in Cu- mixed dielectric medium (Trial 26).
4. Conclusions In this experimental work, three types of aluminum matrix based MMCs were machined by the EDM process under different machining conditions to assess their impact on the recast layer thickness. A selection of percentile reinforcements and peak current were found to be the two most significant factors affecting the quality and thickness of recast layer. The MMC whose coefficient of thermal resistance was lowest, experienced the high magnitude of fracture during the process and hence, higher MRR. The recast layer thickness formation was prominent in the densely reinforced matrix (Material I) due to the high rate of thermal spalling in metal removal mechanism. The recast layer increases with the increase in current from 4 A to 8 A, but after that it further drops, due to effective flushing of a molten metallic portion at high spark energy. The addition of powder in the dielectric lowers crater depth, resulting in smooth recast layer. The formation of recast layer and its properties depend mainly on the architecture of reinforced particles and the fabrication route of composites. Due to fine reinforced particles (~17 µm), the material removal mechanism in Material III is mainly due to melting as compared to thermal spalling, hence resulting in smoother recast layer. The machined surface cracks in Materials I and III are fewer, due to effective diffusion of constituents during the initial fabrication process, which reduces porosity and enhanced its
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elastic properties. It was also concluded that the reinforcement architecture of Material I is tailored for heat sinking application of miniature high-energy electronic parts. Thus, the formation of recast layer on the surface of this class of MMC with the conductive material was advantageous. However, Material II, III was mostly utilized in automobile industries. Thus, removal of recast layer with the secondary operation is required for superior surface integrity. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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