Measurement 46 (2013) 2705–2715
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Application of response surface method on machining of Al–SiC nano-composites S. Gopalakannan a,⇑, T. Senthilvelan b a b
Department of Mechanical Engineering, Adhiparasakthi Engineering College, Melmaruvathur, Tamil Nadu 603319, India Department of Mechanical Engineering, Pondicherry Engineering College, Puducherry 605014, India
a r t i c l e
i n f o
Article history: Received 16 August 2012 Received in revised form 20 January 2013 Accepted 25 April 2013 Available online 14 May 2013 Keywords: Metal matrix nano-composite Ultrasonic cavitation method EDM RSM Desirability
a b s t r a c t The newly fabricated metal matrix nano-composite (MMNC) of Al 7075 reinforced with 1.5 wt% SiC nano-particles was prepared by a novel ultrasonic cavitation method. The high resolution scanning electron micrograph (SEM) and field emission scanning electron micrograph (FESEM) shows uniform distribution and good dispersion of the SiC nanoparticles within the aluminum metal matrix. Electrical discharge machining (EDM) was employed to machine MMNC with copper electrode by adopting face centered central composite design of response surface methodology. Analysis of variance was applied to investigate the influence of process parameters and their interactions. Further a mathematical model has been formulated in order to estimate the machining characteristics. It has been observed that pulse current was found to be the most important factor affecting all the three output parameters such as material removal rate (MRR), electrode wear rate (EWR) and surface roughness (SR). The optimum parameter of combination setting has been identified for the MMNC are voltage 50.00 V, pulse current 8.00 A, Pulse on time 8.00 ls and pulse off time 9.00 ls. Finally the parameters were optimized for maximizing MRR, minimizing EWR and SR using desirability function approach. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The growing need for light weight high strength materials in technologically advanced industries supported by the advances in the field of material science. Aluminum based metal matrix composites (MMCs) have been extensively studied as an attractive choice for automotive, aerospace and military applications due to their light-weight, high strength, stiffness and resistance to high temperature [1]. MMCs with silicon carbide (SiC), boron carbide (B4C) and aluminum oxide (Al2O3) as reinforcement have significant advantage over conventional materials. Generally, the micro-ceramic particles are used to improve the yield and ultimate strength of the metal. However the ductility of the MMCs deteriorates with high ceramic particle con⇑ Corresponding author. E-mail addresses:
[email protected] (S. Gopalakannan),
[email protected] (T. Senthilvelan). 0263-2241/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.measurement.2013.04.036
centration. Nanoparticle reinforcements can significantly improve the mechanical properties of the matrix by more effectively promoting the particle hardening mechanisms than micron size particles [2]. It is expected that MMCs reinforced with ceramic nanoparticles (less than 100 nm), termed as metal matrix nanocomposites (MMNCs), can overcome those disadvantages associated with the conventional MMCs. The properties of MMNCs would be improved considerably even with lower volume fraction of nanoparticles. MMNCs could especially provide a significantly improved performance at elevated temperatures [3]. However to produce MMNCs, it is extremely challenging for the conventional mechanical stirring method to distribute and disperse nanoparticles uniformly in metals because of higher specific surface areas in nanoparticles. In order to achieve a uniform dispersion and distribution of nanoparticles in aluminum metal matrix composites, Lan et al. and Yang et al. developed a new technique that
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combined solidification processes with ultrasonic cavitation based dispersion of nanoparticles in metal melts [4,5]. Traditional nanomanufacturing methods for nanocomposites, such as high energy ball milling, rapid solidification, electroplating, sputtering, etc., cannot be used for mass production and net shape fabrication of complex structural components. These composite materials are extensively used in structural, aerospace and automotive industries. Whereas the machinability is concerned, the applications of existing MMCs are limited because of their poor machinability which results in poor surface finish and excessive tool wear. Due to possession of higher hardness and reinforcement strength, MMCs are difficult to be machined by traditional techniques. Hence electrical discharge machining (EDM) process becomes viable method to these kinds of MMCs. EDM process does not involve mechanical energy, the material removal rate is not influenced by the material properties like hardness, strength, toughness, etc. Materials with poor machinability such as cemented tungsten carbide and composites can also be processed without much difficulty by the EDM process [6,7]. The investigations on the machining aspects of MMCs with particulate reinforcement have been carried out and reported. George et al. investigated the carbon–carbon composites considering three parameters at two levels and reported that pulse current and pulse on time are significant for EWR and MRR [8]. The effect of percentage volume of SiC and other machining characteristics were studied while machining Al–SiC, and concluded that increase in SiC decreases the MRR, where as increases EWR and SR [9,10]. The effect of rotation of electrode on EDM of Al–SiC and Al–Al2O3 composites yielded positive effect on MRR, EWR and SR [11,12]. Gopalakannan et al. experimented EDM characteristics of Al 7075/10wt% SiC composites in order to assess the machinability and workpiece quality and the results reveal that, the MRR showed an increasing trend with increase in product of pulse current and pulse on-time up to the optimum value and then decreases [13]. Kumar and Paulo Davim have carried out an experimental study on the machining parameters in powder mixed electric discharge machining of Al–10%SiC MMC. They mixed silicon powder into the dielectric fluid and reported that the addition of silicon powder into the dielectric fluid of EDM increases MRR and decreases SR [14]. However EDM of nanocomposites is concerned the magnesium nano-alumina composites made by powder metallurgy route were investigated in the machining characteristics of EDM process and reported that the pulse on time showed the significant effect [15]. Literatures on the machining of MMNCs are scarce in general and investigations on the characteristics of EDM in particular. In recent years, the application of both Taguchi method and response surface method have adopted for evaluation of machining performance [16]. Agarwal et al. carried out a comparative analysis of CNC turned parts using both Taguchi method and response surface method. They used four parameters at three levels and conducted 27 experiments for Taguchi method and 30 experiments for face centered central composite design of response surface method.
The results revealed that (i) significance of interactions and square terms of parameters is more clearly predicted in RSM than Taguchi method. (ii) RSM technique can model the response in terms of significant parameters, their interactions and square terms, which is not provided by Taguchi method. (iii) 3D surfaces generated by RSM can help in visualizing the effect of parameters on response in the entire range specified whereas Taguchi method gives the average values of response at given level of parameters. Thus RSM can better predict the effect of parameters on response and is a better tool for optimization [17]. Hence in this research work the face centered central composite design of RSM is used for the plan of experiments. The present work is envisaged to study the mechanical properties of MMNC and EDM studies by developing mathematical model and analyzing the effects of parameters on the performance characteristics of MMNC using response surface methodology (RSM). Accordingly, the quantitative mathematical models have been carried out to study influence of pulse current (Ip), voltage (V), pulse on time (Ton) and pulse off time (Toff) on the material removal rate (MRR), electrode wear rate (EWR) and surface roughness (SR) by using RSM [18]. 2. Preparation of nano-composites The SiC nanoparticles reinforced with aluminum was fabricated by ultrasonic cavitation based solidification processing is shown in Fig. 1. The microstructures and mechanical properties of nanocomposites were studied to understand the effect of nanoparticles in as cast Al 7075 alloys. Aluminum (Al 7075) is used as the base matrix alloy. Its chemical composition (%) is Si = 0.2, Fe = 0.22, Cu = 2.0
Argon gas
Furnace
Ultrasonic probe
Fig. 1. Ultrasonic cavitation setup.
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Nano SiC particles
Fig. 2. FESEM of Al7075 reinforced with 1.5 wt% of SiC.
Table 1 Machining parameters with their levels. Parameters
Voltage, V (V) Pulse current, Ip (A) Pulse on time, Ton (ls) Pulse off time, Toff (ls)
Labels
A B C D
Levels 1
0
+1
40 6 4 5
50 10 6 7
60 14 8 9
max, Mn = 0.1, Mg = 2.1–2.9, Zn = 5.1–6.1, Ti = 0.1 max, Cr = 0.2, and balance as Al. The aluminum matrix was reinforced with 0.5 wt% of SiC nano-particles with an average particle size of 50 nm. The ultrasonic cavitation setup mainly consists of, a resistance heating furnace for melting aluminum, protection gas system and ultrasonic processing system. A stainless steel crucible of size 110 mm inside diameter and 150 mm height was used for melting. The ultrasonic probe made of niobium alloy was used to generate an 18 kHz and a maximum of 4 kW power output for melt processing. The melt temperature for ultrasonic processing was controlled at about 150 °C above the alloy melting temperature (610 °C). Niobium is a high temperature element and does not react with aluminum at the melt temperature. Nano-sized SiC particles were fed into the aluminum melt through a steel tube. For each casting, about 1 kg of Al 7075 was first melted in the crucible to a temperature of 750 °C. The aluminum alloy melt pool was protected by argon gas. SiC nanoparticles of 1.5 wt% (15 g) preheated to 800 °C for 1 h in a muffle furnace to improve the wettability [19]. When nanoparticles were added in the Al alloy melts, the viscosity of the Al alloy significantly increased. Thus, after efficient ultrasonic processing, a higher casting temperature of 750 °C was used to ensure the flowability inside the mold. The aluminum melt was cast into a steel permanent mold that was preheated to 500 °C. Al MMNCs with 0.5 wt% of SiC was fabricated.
Table 2 Design layout and experimental results. Exp. no.
Factor 1 A:voltage
Factor 2 B:current
Factor 3 C:Ton
Factor 4 D:Toff
Response 1 MRR (g/ min)
Response 2 EWR (g/ min)
Response 3 SR (lm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
40 60 50 50 50 60 40 40 50 40 60 60 50 50 40 50 50 60 60 60 40 60 50 50 60 50 40 40 50 40
6 6 10 10 6 14 14 6 10 6 10 6 10 10 14 10 10 6 14 6 14 14 14 10 6 10 14 6 10 10
4 8 4 6 6 4 8 8 6 4 6 8 6 8 4 6 6 4 8 4 8 4 6 6 8 6 4 8 6 6
9 9 7 7 7 5 9 9 9 5 7 9 7 7 9 7 7 9 5 5 5 9 7 7 5 7 5 5 5 7
0.018 0.103 0.122 0.321 0.163 0.277 0.689 0.106 0.325 0.108 0.512 0.642 0.416 0.485 0.122 0.321 0.329 0.098 0.721 0.105 0.752 0.109 0.586 0.321 0.152 0.318 0.341 0.179 0.564 0.427
0.002 0.004 0.07 0.010 0.006 0.008 0.017 0.003 0.009 0.004 0.015 0.009 0.008 0.012 0.004 0.006 0.007 0.002 0.015 0.004 0.017 0.002 0.011 0.006 0.004 0.008 0.007 0.005 0.012 0.011
4.024 5.826 6.432 9.726 4.452 7.824 12.538 4.234 7.452 4.252 17.024 20.282 6.456 3.428 4.248 6.846 10.22 3.428 19.524 4.026 20.824 5.246 15.263 8.868 6.728 11.215 9.725 7.013 18.217 10.521
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Table 3 The ANOVA table for MRR. Source
Sum of squares
df
Mean square
F Value
p-Value Prob. > F
Model A-Voltage B-Current C-Ton D-Toff AB AC AD BC BD CD A2 B2 C2 D2 Residual Lack of fit Pure Error Cor total
1.27 2.939E005 0.57 0.36 0.054 2.550E003 7.290E004 1.260E003 0.19 6.006E003 3.025E003 6.130E003 5.568E003 0.036 1.448E003 0.059 0.051 7.431E003 1.32
14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 15 10 5 29
0.090 2.939E005 0.57 0.36 0.054 2.550E003 7.290E004 1.260E003 0.19 6.006E003 3.025E003 6.130E003 5.568E003 0.036 1.448E003 3.911E003 5.124E003 1.486E003
23.11 7.514E003 146.08 90.85 13.84 0.65 0.19 0.32 48.60 1.54 0.77 1.57 1.42 9.12 0.37
