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Effect of Titanium Carbide particle addition in the aluminium composite on EDM process parameters
Journal of Manufacturing Processes 13 (2011) 60–66
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical paper
Effect of Titanium Carbide particle addition in the aluminium composite on EDM process parameters Velusamy Senthilkumar ∗ , Bidwai Uday Omprakash Department of Production Engineering, National Institute of Technology, Thiruchirappalli-620015, India
article
info
Article history: Received 9 November 2009 Received in revised form 11 June 2010 Accepted 28 October 2010 Available online 27 November 2010
abstract Machining of hard materials such as metal matrix composites (Al/TiC) to a high degree of accuracy and surface finish is difficult. Electrical discharge machining (EDM) is an important process for machining difficult-to-machine materials like metal matrix composites. EDM is an effective tool in shaping such difficult-to-machine materials. The objective of this work is to investigate the effect of current (C ), Pulse On-Time (POT) and flushing pressure (P ) on Metal Removal Rate (MRR), Tool Wear Rate (TWR) during electrical discharge machining of as-sintered Al-MMC with 5% and 2.5% TiC reinforcement. The use of kerosene as a dielectric fluid was employed in the present investigation. A copper tool of diameter 7 mm was used to drill the specimens. An L18 orthogonal array (OA), for the three machining parameters at three levels each, was opted to conduct the experiments. Analysis of variance (ANOVA) was performed to find the validity of the experimental plan followed in the present work. An attempt was also made in the present work to study the effect of TiC particle addition on the Electrode Wear Ratio (EWR), a new parameter taking into consideration both MRR and TWR. Scanning electron microscope (SEM) analysis was conducted to study the recast layer evolved during the electrical discharge machining process. © 2010 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Aluminium alloys reinforced with SiC and TiC particles are often used for various automotive and aerospace applications due to their extreme hardness and temperature resistant properties. However, the full potential of these metal matrix composites was hindered by the high manufacturing cost mainly because of the difficulties in machining such as turning, drilling, sawing, etc. generally results in excessive tool wear due to the very abrasive nature of the material [1,2]. Metal matrix composites reinforced with hard ceramic fibers, particles can be machined with either an electroplated diamond grinding wheel or carbide with Poly Crystalline Diamond (PCD) cutting tools [3,4]. As a consequence, non-conventional machining process like electro discharge machining (EDM) [5–7], laser [8] and other techniques were increasingly being applied for the machining of particle reinforced metal matrix composites. A great deal of investigation is needed to optimize the process parameters in the electro discharge machining process. Experimental investigation was carried out in the past to evaluate the effect of current, pulse on time and gap voltage on metal removal rate (MRR), tool wear rate (TWR) and radial over cut on electrical discharge machining of an Al–4Cu–6Si
∗
Corresponding author. Tel.: +91 431 250 3519; fax: +91 431 250 0133. E-mail address: [email protected] (V. Senthilkumar).
alloy reinforced with silicon carbide particles [9]. Experimental investigations were carried out to study the effect of current, pulse on time and flushing pressure on metal removal rate, tool wear rate and surface roughness in the as cast Aluminium reinforced with 10% SiC particulates composites [10]. Ramulu et al. [11] carried out experimental investigations on the effect of surface roughness generated by the machining process on mechanical properties of a 15 vol.% SiCp/A336 aluminum metal matrix composite. The main objective of the above work was to study the fatigue behavior of the machined surface. Akshay Dvivedi et al. [12] investigated the machinability of an Al6063 SiCp metal matrix composite and obtained an optimal setting of process parameters. The material was developed using the melt stir–squeeze–quench casting route and was characterized for density, porosity and electrical conductivity. Yan and Wang [13] investigated the machining characteristics of the Al2 O3 /6061Al composite using rotary electro-discharge machining with a tube electrode. They concluded that the machining process of the Al2 O3 /6061Al composite by EDM-drilling is feasible in comparison to other machining processes. In the present work, aluminum metal matrix composites reinforced with two different percentages of Titanium carbide particles were prepared using the powder metallurgy route. The electrical discharge machining of Al–TiCp was done using 7 mm electrolytic copper and the effects of various parameters, namely, current, pulse on time and flushing pressure on Tool Wear Rate, Metal Removal Rate and Electrode Wear Ratio (EWR). An attempt
1526-6125/$ – see front matter © 2010 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jmapro.2010.10.005
V. Senthilkumar, B.U. Omprakash / Journal of Manufacturing Processes 13 (2011) 60–66
61
Fig. 1. Microstructure of Al/2.5% TiC composite.
was made in the present investigation to evaluate the effect of TiC reinforcement in the composite on the TWR, MRR and EWR. The effect of addition of titanium carbide particles in the composite on the recast layer thickness has also been studied.
Fig. 2. Microstructure of Al/5% TiC composite. Table 1 Micro hardness values of as sintered aluminium and composite samples. Hardness values (HRc )
2. Experimental and analytical procedure The metal matrix composites used for the tests were Aluminum composites reinforced with varying percentages of titanium carbide particles (TiC) produced through the powder metallurgy route. The composites consisted of pure aluminium (45 µm particle size) reinforced with 2.5% and 5% Titanium Carbide particles in as received condition from M/s Sigma Aldrich (fine size of 45 µm), compacted to a pressure of 300 MPa and sintered at a temperature of 500 °C for two hours in a tube furnace under argon atmosphere. Basic characterization studies such as microstructure, XRD pattern and hardness of sintered composites were conducted on the sintered composites to evaluate the material characteristics. Figs. 1 and 2 show the microstructure of as sintered composite samples of Aluminium composites with 2.5% and 5% TiC particles respectively. A close observation of the above micrographs indicate the uniform distribution of titanium carbide particles in the aluminium matrix in the case of Al-2.5% TiC composites compared to Al-5% TiC which shows slight agglomeration of TiC particles in the matrix. The XRD pattern of the sintered composites in Fig. 3(a) and (b) show the Aluminium and TiC peaks indexed using JCPDS file numbers 04-0787 and 65-7994 respectively. The analysis of the above XRD patterns show the absence of any intermetallic phases in both Al-2.5% TiC and Al-5% TiC composites. Micro hardness tests were carried out on the composite samples provided in Table 1 show increased hardness values for the composite samples as the percentage of titanium carbide increases. The work piece used in this experiment was Al–TiC composites of two different titanium carbide particle reinforcements (2.5% and 5%). A die sinking EDM machine was used for the present experimental works. The specimens were machined by fine grinding to make both ends parallel. The electrode materials were electrolyte copper. During machining, commercial kerosene was circulated as the dielectric fluid in the tank. For the present experimental investigation, three different machining parameters, namely, pulse current (2, 4 and 6 A), pulse on time (300, 400 and 500 µs) and flushing pressure (0.25, 0.5 and 0.75 kgf/cm2 ) were identified and their levels were fixed. For the above experiments, the gap voltage was maintained at 45 V and the pulse off time was kept as constant at 150 µs. The various levels for the individual parameters such as pulse current, pulse on time and flushing pressure were selected based on previous
Al-5TiC%
Al-2.5%
45.8 44.8 41.4
37.1 37.1 39.4
Table 2 L18 experimental plan. Exp no.
Current (A)
Pulse on time (µs)
Pressure (kgf/cm2 )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
2 2 2 4 4 4 6 6 6 2 2 2 4 4 4 6 6 6
300 400 500 300 400 500 300 400 500 300 400 500 300 400 500 300 400 500
0.25 0.5 0.75 0.25 0.5 0.75 0.5 0.75 0.25 0.75 0.25 0.5 0.5 0.75 0.25 0.75 0.25 0.5
literature [9,10] and the size of the hole investigated in the present work which is 7 mm in diameter. A total of 18 experiments were planned based on the Taguchi model as provided in Table 2. Three different responses studied were MRR (Metal Removal Rate), TWR (Tool Wear Rate) and EWR (Electrode Wear Ratio). Material removal rate is expressed as the ratio of the difference of weight of the workpiece before and after the machining to the machining time. MRR = (wjb − wja )/t
(1)
where, wjb and wja are weights of workpiece before and after the machining and t is the machining time. Tool Wear Rate is expressed as the ratio of the difference of weight of the tool before and after the machining to the machining time: TWR = (wtb − wta )/t
(2)
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(a) XRD pattern of Al-2.5% TiC composite.
(b) XRD pattern of Al-5% TiC composite. Fig. 3.
Table 3a ANOVA summary table for 5% composition for MRR.
Table 3b ANOVA summary table for 5% compositions for TWR.
Source
SS
DF
MSD
F
Source
SS
DF
MSD
F
Current POT FP error Total
8 003.862 8 022.243 128.9214 18 642.56 34 797.59
2 2 2 11 17
4001.931 4011.121 64.4607 1694.778
2.36133 2.366753 0.038035
Current POT FP error Total
1.367021 1.362866 2.059122 2.223115 7.012125
2 2 2 11 17
0.683511 0.681433 1.029561 0.202101
3.382019 3.37174 5.094281
where, wtb and wta are weights of tool before and after the machining. Electrode wear ratio (EWR) is expressed as the ratio of MRR and TWR: EWR = MRR/TWR.
(3)
For each composition of the aluminium composites, 18 experimental runs were carried out. For each trial, to calculate MRR and TWR, the weight of the tool and workpiece before and after machining were measured and for every trial, the machining time was also noted down and readings were tabulated in the table. Al-MMC with 2.5% and 5% TiC was drilled using a Copper tool of φ 7 mm. Positive polarity was maintained for the workpiece and negative polarity for the tool. Commercial grade kerosene was used as the dielectric fluid. It was decided to select the trials at random and complete all the three successive repetitions in that trial. Following machining of the composite workpieces, the work pieces were sectioned and microstructure of the material was investigated using a Scanning Electron Microscope (SEM) and a recast layer thickness of the machined composites was evaluated. 3. Results and discussion From the data obtained from the experiments, validity of the results was analysed using the Analysis of Variance (ANOVA) method [14]. The ANOVA results for various response variables such as MRR and TWR are shown in Tables 3a–4b. The analysis of the above ANOVA results show that the calculated F values obtained are significant thereby the experimental plan is valid. 3.1. Effect of TiC addition on MRR Results show that the aluminium composites reinforced with Titanium Carbide particles can be machined using EDM in spite of poor electrical conductivity and high thermal resistance of titanium carbide particles. However, the material removal rate
Table 4a ANOVA summary table for 2.5% composition for MRR. Source
SS
DF
MSD
F
Current POT FP error Total
35 770.2 4 999.91 1 730.8 2 527.58 45 028.5
2 2 2 11 17
17 885.1 2 499.96 865.399 229.78
77.8358 10.8798 3.7662
Table 4b ANOVA summary table for 2.5% compositions for TWR. Source
SS
DF
MSD
F
Current POT FP error Total
7.81054 6.955887 5.02142 5.619218 25.40707
2 2 2 11 17
3.90527 3.477943 2.51071 0.510838
7.64483 6.80831 4.914885
is low when compared to machining other electrical conducting materials [2]. The effect of titanium particle content in the composite and various input parameters such as machining current, pulse on time and flushing pressure on the material removal rate and tool wear rate are shown in Fig. 4(a)–(c). Metal removal rate is found to increase with increasing values of discharge current. Increased rate of material removal from the work piece and the tool electrode is attributed to the higher thermal loading as a result of higher discharge current value. Material removal rate decreases as the percent of titanium carbide particle in the composite increases due to the shielding effect of titanium carbide particle in the composites. As reported [2], the ceramic particles did not melt during the machining process and removal of material in the composite occurs as a result of matrix melting and vaporizing around the ceramic particles. The effect of flushing pressure on the material removal has been evaluated in the present investigation and is shown in Fig. 4(b). Increase in flushing pressure improves the machining process in the form of clearing the ceramic particles removed from the material thereby
V. Senthilkumar, B.U. Omprakash / Journal of Manufacturing Processes 13 (2011) 60–66
(a) Effect of current on MRR.
63
(b) Effect of flushing pressure on MRR.
(c) Effect of pulse on time on MRR. Fig. 4.
(a) Effect of current on TWR.
(b) Effect of flushing pressure on TWR.
(c) Effect of pulse on time on TWR. Fig. 5.
setting the conductive path to continue the formation of ionized bridges. However, as the flushing pressure reaches very high values (0.75 kgf/cm2 ) the flow of dielectric blocks the formation of ionized bridges and reduces the material removal rate. The effect
of pulse on time on MRR is shown in Fig. 4(c). Metal removal rate increases with increasing pulse on time initially and after an optimum value is reached no visible improvement in the material removal rate has been noticed. Longer pulse duration creates
64
V. Senthilkumar, B.U. Omprakash / Journal of Manufacturing Processes 13 (2011) 60–66
(a) Effect of current on Electrode Wear Ratio.
(b) Effect of flushing pressure on Electrode Wear Ratio.
(c) Effect of pulse on time on Electrode Wear Ratio. Fig. 6.
larger removal of material in the form of craters which hinders the mechanism of creation of a conductive path between the electrodes and results in a drop in the material removal rate when pulse on time increases to higher value (500 µs). The amount of ceramic particles present in the crater produced by the longer duration of pulse on time is higher in the case of Al-5% TiC composites which exhibit lower MRR for any pulse on time values when compared to Al-2.5% TiC composites. 3.2. Effect of TiC addition on TWR The effect of discharge current on the tool wear rate is evaluated and a plot showing the trend is as shown in Fig. 5(a). Increase in discharge current has a moderate effect on TWR up to an optimum value and thereafter a significant rise in the wear rate is associated with higher thermal loading. The negligible tool wear in the initial period is due to the deposition of workpiece material on the tool electrode which inhibits the wear of electrode. However, as the current is further increased the tool electrode exhibits substantial wear. When compared to composites with low titanium carbide content (Al-2.5% TiC), composites with higher reinforcement content (Al-5% TiC) exhibit lower tool wear rate as the ceramic particles trapped in the conductive path require more flushing pressure for the machining process to continue. The effect of the flushing pressure on tool wear is shown in Fig. 5(b). The increase in flushing pressure clears the material particles trapped in the conductive path and facilitates the machining as evidenced in the present investigation has been in conformity with the works of earlier researchers [10]. Increase in flushing pressure also reduced tool wear rate substantially in the case of Al2.5% TiC when compared to Al-5% TiC suggests a careful selection of flushing pressure level for the effective machining of composites. Effect of pulse on time on the tool wear rate is shown in Fig. 5(c). Increasing the duration of pulse increases the tool wear during the
Fig. 7. SEM micrograph of machined surface in Al-5% TiC showing formation of different layers during EDM.
initial period. However, as the pulse on time reaches an optimum value a gradual reduction in the tool wear is noticed which is the combined effect of an increase in the flushing pressure particle crater formation due to longer pulse duration. A sudden drop in the tool wear rate has been noticed when machining Al-2.5% TiC composites at very high pulse on time value. 3.3. Effect of TiC addition on Electrode Wear Ratio (EWR) Electrode wear ratio of the composite specimens was evaluated in the present investigation. EDM input parameters such as current, flushing pressure and pulse on time were plotted against electrode wear ratio (MRR/TWR) as shown in Fig. 6(a)–(c). The EWR increases with increasing values of discharge current due to higher thermal loading. The discharge current requirement increasing with increasing levels of reinforcement in the composite materials is in good agreement with earlier reports [10]. A longer duration of pulse increases EWR during the initial period and
V. Senthilkumar, B.U. Omprakash / Journal of Manufacturing Processes 13 (2011) 60–66
65
Fig. 8. (a) Microvoids and cracks on the EDM surface of 5% TiC composites (b) Recast layer white layer on EDM machine surface 5% TiC composites.
Fig. 9. (a) Crater formation on the EDM surface on 2.5% TiC composites. (b) White recast layer in EDM for 2.5% Al/TiC.
thereafter it decreases due to formation of craters and resistance in the conductive path due to removed ceramic particles. A higher percentage of TiC content in the composite decreases EWR due to high electrical resistance. The effect of flushing pressure on EWR was evaluated as shown in Fig. 6(c). Increase in flushing pressure increases EWR continuously in the case of Al-2.5% TiC. However, the trend in Al-5% TiC is different in which during the initial period due to the formation of bigger size craters EWR is slightly reduced and when the flushing pressure is further increased EWR is increased drastically. 3.4. Effect of TiC addition on recast layer thickness Fig. 7 shows various surface layers produced during the electro discharge machining of composite samples. Three different distinct layers formed during electro discharge machining are a white or recast layer at the top of the surface, a heat affected zone in which the structure of the material is slightly altered and an unaffected zone with identical microstructure of the base material. A large number of micro cracks have been observed on the recast layer. Cracks appearing on the recast layer are due to high tensile stress developed due to imperfect joining of molten droplets inducing a lot of stress gradient exceeding the ultimate tensile strength of the composite material. The matrix material melts and recasts on the machining surface with lots of micro-cracks and voids formed, as shown in Figs. 8(a)–9(b), due to the formation of gas bubbles.
However, ceramic particles pulled out of the matrix during the process have not recast on the surface as evidenced from SEM pictures of 2.5% and 5% TiC composites thereby making the recast layer distinct with different surface integrity. The above theory has been further strengthened by the XRD studies carried out, the results of which have been provided in Fig. 10(a)–(b), on the machined surface of 2.5% and 5% TiC reinforced aluminium composites indicating only Aluminium peaks and no visible TiC peaks. It has also been found that the thickness of the recast layer increases with increasing percentage of TiC particle in the composite which is in good conformity with earlier research [10]. 4. Conclusions Al–TiC composites can be machined using electrodischarge machining and by selecting optimum levels for the EDM parameters, namely, discharge current, pulse on time and flushing pressure the effectiveness of process like metal removal rate and electrode wear rate can be improved. A careful investigation into the structure of the material after the machining suggests that ceramic particles (TiC) were not melted during the process and removal of material occurs as a result of matrix melting and ceramic particle pull out thereafter. The above phenomenon results in the reduced metal removal rate with the increased titanium carbide content in the composite material. Material removal rate and tool wear rates are
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(a) XRD pattern of the hole surface of Al-2.5% TiC composite.
(b) XRD pattern of the hole surface of Al-5% TiC composite. Fig. 10.
influenced by discharge current. Flushing pressure plays an important role in continuing the process and improving the material removal rate at higher discharge current and pulse duration levels. Acknowledgement This work has benefited from the use of the facilities from the Project funded by the Department of Science and Technology, Government of India, under Grant No. SR/FTP/ETA-69/07. References [1] Brazil D, Monaghan J, Aspinwall DK, Ng EG. Wear characterization of various diamond tooling when single point turning a particle reinforced metal matrix composite. In: Proceeding of the IMC-14 conference. 1997. p. 143–52. [2] Muller F, Monaghan J. Non conventional machining of particle reinforced metal matrix composites. J Mater Process Technol 2001;118:278–85. [3] Hung NP, Yang LJ, Leong KW. Electro discharge machining of cast metal matrix composites. J Mater Process Technol 1994;44:229–36. [4] Roux TLe, Wise MLH, Aspinwall DK. The effect of electro discharge machining on the surface integrity of an aluminium–silicon carbide metal matrix composite. J Process Adv Mater 1993;3:233–41.
[5] Muller F, Monaghan J. Electro discharge machining of a particle reinforced metal matrix composite. In: Proceedings of the 12th international symposium for electro machining. ISEM XII. VDI-Berichte 1405. 1998. p. 513–22. [6] Roux TLe, Wise MLH, Aspinwall DK. The effect of electro discharge machining on the surface integrity of aluminium–silicon carbide metal matrix composite. J Process Adv Mater 1993;3:233–41. [7] Lau WS, Lee WB, Pang SQ. Pulsed ND:YAG laser cutting of carbon fibre composite materials. Ann CIRP 1990;39(1):179–82. [8] Hung NP, Yang LJ, Leong KW. Laser drilling cast metal matrix composites. AMD vol. 208. MD vol. 59. ASME. Los Angeles (CA); 1995. p. 87. [9] Dhar Sushant, Purohit Rajesh, Saini Nishant, Sharma Akhil, Hemath Kumar G. Mathematical modeling of electric discharge machining of cast Al–4Cu–6Si alloy-10 wt% SiCP composites. J Mater Process Technol 2007;194:24–9. [10] Narender Singh P, Raghukandan K, Rathinasabapathi M, Pai BC. Electric discharge machining of Al–10% SiCP as-cast metal matrix composite. J Mater Process Technol 2004;1653–7. [11] Ramulu M, Paul G, Patel J. EDM surface effects on fatigue strength of 15 vol.% SiCp/Al metal matrix composite material. Compos Struct 2001;54:79–86. [12] Akshay D, Pradeep K, Inderdeep S. Experimental investigation and optimization in EDM of Al 6063 SiCp metal matrix composite. Int J Mach Mach Mater 2008;5(3–4):293–308. [13] Yan BH, Wang CC. The machining characteristics of Al2 O3 /6061Al composite using rotary electro-discharge machining with a tube electrode. J Mater Process Technol 1999;95:222–31. [14] Montgomery DC. Design and analysis of experiments. 5th ed. New York: Wiley; 2001.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical paper
Effect of Titanium Carbide particle addition in the aluminium composite on EDM process parameters Velusamy Senthilkumar ∗ , Bidwai Uday Omprakash Department of Production Engineering, National Institute of Technology, Thiruchirappalli-620015, India
article
info
Article history: Received 9 November 2009 Received in revised form 11 June 2010 Accepted 28 October 2010 Available online 27 November 2010
abstract Machining of hard materials such as metal matrix composites (Al/TiC) to a high degree of accuracy and surface finish is difficult. Electrical discharge machining (EDM) is an important process for machining difficult-to-machine materials like metal matrix composites. EDM is an effective tool in shaping such difficult-to-machine materials. The objective of this work is to investigate the effect of current (C ), Pulse On-Time (POT) and flushing pressure (P ) on Metal Removal Rate (MRR), Tool Wear Rate (TWR) during electrical discharge machining of as-sintered Al-MMC with 5% and 2.5% TiC reinforcement. The use of kerosene as a dielectric fluid was employed in the present investigation. A copper tool of diameter 7 mm was used to drill the specimens. An L18 orthogonal array (OA), for the three machining parameters at three levels each, was opted to conduct the experiments. Analysis of variance (ANOVA) was performed to find the validity of the experimental plan followed in the present work. An attempt was also made in the present work to study the effect of TiC particle addition on the Electrode Wear Ratio (EWR), a new parameter taking into consideration both MRR and TWR. Scanning electron microscope (SEM) analysis was conducted to study the recast layer evolved during the electrical discharge machining process. © 2010 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Aluminium alloys reinforced with SiC and TiC particles are often used for various automotive and aerospace applications due to their extreme hardness and temperature resistant properties. However, the full potential of these metal matrix composites was hindered by the high manufacturing cost mainly because of the difficulties in machining such as turning, drilling, sawing, etc. generally results in excessive tool wear due to the very abrasive nature of the material [1,2]. Metal matrix composites reinforced with hard ceramic fibers, particles can be machined with either an electroplated diamond grinding wheel or carbide with Poly Crystalline Diamond (PCD) cutting tools [3,4]. As a consequence, non-conventional machining process like electro discharge machining (EDM) [5–7], laser [8] and other techniques were increasingly being applied for the machining of particle reinforced metal matrix composites. A great deal of investigation is needed to optimize the process parameters in the electro discharge machining process. Experimental investigation was carried out in the past to evaluate the effect of current, pulse on time and gap voltage on metal removal rate (MRR), tool wear rate (TWR) and radial over cut on electrical discharge machining of an Al–4Cu–6Si
∗
Corresponding author. Tel.: +91 431 250 3519; fax: +91 431 250 0133. E-mail address: [email protected] (V. Senthilkumar).
alloy reinforced with silicon carbide particles [9]. Experimental investigations were carried out to study the effect of current, pulse on time and flushing pressure on metal removal rate, tool wear rate and surface roughness in the as cast Aluminium reinforced with 10% SiC particulates composites [10]. Ramulu et al. [11] carried out experimental investigations on the effect of surface roughness generated by the machining process on mechanical properties of a 15 vol.% SiCp/A336 aluminum metal matrix composite. The main objective of the above work was to study the fatigue behavior of the machined surface. Akshay Dvivedi et al. [12] investigated the machinability of an Al6063 SiCp metal matrix composite and obtained an optimal setting of process parameters. The material was developed using the melt stir–squeeze–quench casting route and was characterized for density, porosity and electrical conductivity. Yan and Wang [13] investigated the machining characteristics of the Al2 O3 /6061Al composite using rotary electro-discharge machining with a tube electrode. They concluded that the machining process of the Al2 O3 /6061Al composite by EDM-drilling is feasible in comparison to other machining processes. In the present work, aluminum metal matrix composites reinforced with two different percentages of Titanium carbide particles were prepared using the powder metallurgy route. The electrical discharge machining of Al–TiCp was done using 7 mm electrolytic copper and the effects of various parameters, namely, current, pulse on time and flushing pressure on Tool Wear Rate, Metal Removal Rate and Electrode Wear Ratio (EWR). An attempt
1526-6125/$ – see front matter © 2010 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jmapro.2010.10.005
V. Senthilkumar, B.U. Omprakash / Journal of Manufacturing Processes 13 (2011) 60–66
61
Fig. 1. Microstructure of Al/2.5% TiC composite.
was made in the present investigation to evaluate the effect of TiC reinforcement in the composite on the TWR, MRR and EWR. The effect of addition of titanium carbide particles in the composite on the recast layer thickness has also been studied.
Fig. 2. Microstructure of Al/5% TiC composite. Table 1 Micro hardness values of as sintered aluminium and composite samples. Hardness values (HRc )
2. Experimental and analytical procedure The metal matrix composites used for the tests were Aluminum composites reinforced with varying percentages of titanium carbide particles (TiC) produced through the powder metallurgy route. The composites consisted of pure aluminium (45 µm particle size) reinforced with 2.5% and 5% Titanium Carbide particles in as received condition from M/s Sigma Aldrich (fine size of 45 µm), compacted to a pressure of 300 MPa and sintered at a temperature of 500 °C for two hours in a tube furnace under argon atmosphere. Basic characterization studies such as microstructure, XRD pattern and hardness of sintered composites were conducted on the sintered composites to evaluate the material characteristics. Figs. 1 and 2 show the microstructure of as sintered composite samples of Aluminium composites with 2.5% and 5% TiC particles respectively. A close observation of the above micrographs indicate the uniform distribution of titanium carbide particles in the aluminium matrix in the case of Al-2.5% TiC composites compared to Al-5% TiC which shows slight agglomeration of TiC particles in the matrix. The XRD pattern of the sintered composites in Fig. 3(a) and (b) show the Aluminium and TiC peaks indexed using JCPDS file numbers 04-0787 and 65-7994 respectively. The analysis of the above XRD patterns show the absence of any intermetallic phases in both Al-2.5% TiC and Al-5% TiC composites. Micro hardness tests were carried out on the composite samples provided in Table 1 show increased hardness values for the composite samples as the percentage of titanium carbide increases. The work piece used in this experiment was Al–TiC composites of two different titanium carbide particle reinforcements (2.5% and 5%). A die sinking EDM machine was used for the present experimental works. The specimens were machined by fine grinding to make both ends parallel. The electrode materials were electrolyte copper. During machining, commercial kerosene was circulated as the dielectric fluid in the tank. For the present experimental investigation, three different machining parameters, namely, pulse current (2, 4 and 6 A), pulse on time (300, 400 and 500 µs) and flushing pressure (0.25, 0.5 and 0.75 kgf/cm2 ) were identified and their levels were fixed. For the above experiments, the gap voltage was maintained at 45 V and the pulse off time was kept as constant at 150 µs. The various levels for the individual parameters such as pulse current, pulse on time and flushing pressure were selected based on previous
Al-5TiC%
Al-2.5%
45.8 44.8 41.4
37.1 37.1 39.4
Table 2 L18 experimental plan. Exp no.
Current (A)
Pulse on time (µs)
Pressure (kgf/cm2 )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
2 2 2 4 4 4 6 6 6 2 2 2 4 4 4 6 6 6
300 400 500 300 400 500 300 400 500 300 400 500 300 400 500 300 400 500
0.25 0.5 0.75 0.25 0.5 0.75 0.5 0.75 0.25 0.75 0.25 0.5 0.5 0.75 0.25 0.75 0.25 0.5
literature [9,10] and the size of the hole investigated in the present work which is 7 mm in diameter. A total of 18 experiments were planned based on the Taguchi model as provided in Table 2. Three different responses studied were MRR (Metal Removal Rate), TWR (Tool Wear Rate) and EWR (Electrode Wear Ratio). Material removal rate is expressed as the ratio of the difference of weight of the workpiece before and after the machining to the machining time. MRR = (wjb − wja )/t
(1)
where, wjb and wja are weights of workpiece before and after the machining and t is the machining time. Tool Wear Rate is expressed as the ratio of the difference of weight of the tool before and after the machining to the machining time: TWR = (wtb − wta )/t
(2)
62
V. Senthilkumar, B.U. Omprakash / Journal of Manufacturing Processes 13 (2011) 60–66
(a) XRD pattern of Al-2.5% TiC composite.
(b) XRD pattern of Al-5% TiC composite. Fig. 3.
Table 3a ANOVA summary table for 5% composition for MRR.
Table 3b ANOVA summary table for 5% compositions for TWR.
Source
SS
DF
MSD
F
Source
SS
DF
MSD
F
Current POT FP error Total
8 003.862 8 022.243 128.9214 18 642.56 34 797.59
2 2 2 11 17
4001.931 4011.121 64.4607 1694.778
2.36133 2.366753 0.038035
Current POT FP error Total
1.367021 1.362866 2.059122 2.223115 7.012125
2 2 2 11 17
0.683511 0.681433 1.029561 0.202101
3.382019 3.37174 5.094281
where, wtb and wta are weights of tool before and after the machining. Electrode wear ratio (EWR) is expressed as the ratio of MRR and TWR: EWR = MRR/TWR.
(3)
For each composition of the aluminium composites, 18 experimental runs were carried out. For each trial, to calculate MRR and TWR, the weight of the tool and workpiece before and after machining were measured and for every trial, the machining time was also noted down and readings were tabulated in the table. Al-MMC with 2.5% and 5% TiC was drilled using a Copper tool of φ 7 mm. Positive polarity was maintained for the workpiece and negative polarity for the tool. Commercial grade kerosene was used as the dielectric fluid. It was decided to select the trials at random and complete all the three successive repetitions in that trial. Following machining of the composite workpieces, the work pieces were sectioned and microstructure of the material was investigated using a Scanning Electron Microscope (SEM) and a recast layer thickness of the machined composites was evaluated. 3. Results and discussion From the data obtained from the experiments, validity of the results was analysed using the Analysis of Variance (ANOVA) method [14]. The ANOVA results for various response variables such as MRR and TWR are shown in Tables 3a–4b. The analysis of the above ANOVA results show that the calculated F values obtained are significant thereby the experimental plan is valid. 3.1. Effect of TiC addition on MRR Results show that the aluminium composites reinforced with Titanium Carbide particles can be machined using EDM in spite of poor electrical conductivity and high thermal resistance of titanium carbide particles. However, the material removal rate
Table 4a ANOVA summary table for 2.5% composition for MRR. Source
SS
DF
MSD
F
Current POT FP error Total
35 770.2 4 999.91 1 730.8 2 527.58 45 028.5
2 2 2 11 17
17 885.1 2 499.96 865.399 229.78
77.8358 10.8798 3.7662
Table 4b ANOVA summary table for 2.5% compositions for TWR. Source
SS
DF
MSD
F
Current POT FP error Total
7.81054 6.955887 5.02142 5.619218 25.40707
2 2 2 11 17
3.90527 3.477943 2.51071 0.510838
7.64483 6.80831 4.914885
is low when compared to machining other electrical conducting materials [2]. The effect of titanium particle content in the composite and various input parameters such as machining current, pulse on time and flushing pressure on the material removal rate and tool wear rate are shown in Fig. 4(a)–(c). Metal removal rate is found to increase with increasing values of discharge current. Increased rate of material removal from the work piece and the tool electrode is attributed to the higher thermal loading as a result of higher discharge current value. Material removal rate decreases as the percent of titanium carbide particle in the composite increases due to the shielding effect of titanium carbide particle in the composites. As reported [2], the ceramic particles did not melt during the machining process and removal of material in the composite occurs as a result of matrix melting and vaporizing around the ceramic particles. The effect of flushing pressure on the material removal has been evaluated in the present investigation and is shown in Fig. 4(b). Increase in flushing pressure improves the machining process in the form of clearing the ceramic particles removed from the material thereby
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(a) Effect of current on MRR.
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(b) Effect of flushing pressure on MRR.
(c) Effect of pulse on time on MRR. Fig. 4.
(a) Effect of current on TWR.
(b) Effect of flushing pressure on TWR.
(c) Effect of pulse on time on TWR. Fig. 5.
setting the conductive path to continue the formation of ionized bridges. However, as the flushing pressure reaches very high values (0.75 kgf/cm2 ) the flow of dielectric blocks the formation of ionized bridges and reduces the material removal rate. The effect
of pulse on time on MRR is shown in Fig. 4(c). Metal removal rate increases with increasing pulse on time initially and after an optimum value is reached no visible improvement in the material removal rate has been noticed. Longer pulse duration creates
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(a) Effect of current on Electrode Wear Ratio.
(b) Effect of flushing pressure on Electrode Wear Ratio.
(c) Effect of pulse on time on Electrode Wear Ratio. Fig. 6.
larger removal of material in the form of craters which hinders the mechanism of creation of a conductive path between the electrodes and results in a drop in the material removal rate when pulse on time increases to higher value (500 µs). The amount of ceramic particles present in the crater produced by the longer duration of pulse on time is higher in the case of Al-5% TiC composites which exhibit lower MRR for any pulse on time values when compared to Al-2.5% TiC composites. 3.2. Effect of TiC addition on TWR The effect of discharge current on the tool wear rate is evaluated and a plot showing the trend is as shown in Fig. 5(a). Increase in discharge current has a moderate effect on TWR up to an optimum value and thereafter a significant rise in the wear rate is associated with higher thermal loading. The negligible tool wear in the initial period is due to the deposition of workpiece material on the tool electrode which inhibits the wear of electrode. However, as the current is further increased the tool electrode exhibits substantial wear. When compared to composites with low titanium carbide content (Al-2.5% TiC), composites with higher reinforcement content (Al-5% TiC) exhibit lower tool wear rate as the ceramic particles trapped in the conductive path require more flushing pressure for the machining process to continue. The effect of the flushing pressure on tool wear is shown in Fig. 5(b). The increase in flushing pressure clears the material particles trapped in the conductive path and facilitates the machining as evidenced in the present investigation has been in conformity with the works of earlier researchers [10]. Increase in flushing pressure also reduced tool wear rate substantially in the case of Al2.5% TiC when compared to Al-5% TiC suggests a careful selection of flushing pressure level for the effective machining of composites. Effect of pulse on time on the tool wear rate is shown in Fig. 5(c). Increasing the duration of pulse increases the tool wear during the
Fig. 7. SEM micrograph of machined surface in Al-5% TiC showing formation of different layers during EDM.
initial period. However, as the pulse on time reaches an optimum value a gradual reduction in the tool wear is noticed which is the combined effect of an increase in the flushing pressure particle crater formation due to longer pulse duration. A sudden drop in the tool wear rate has been noticed when machining Al-2.5% TiC composites at very high pulse on time value. 3.3. Effect of TiC addition on Electrode Wear Ratio (EWR) Electrode wear ratio of the composite specimens was evaluated in the present investigation. EDM input parameters such as current, flushing pressure and pulse on time were plotted against electrode wear ratio (MRR/TWR) as shown in Fig. 6(a)–(c). The EWR increases with increasing values of discharge current due to higher thermal loading. The discharge current requirement increasing with increasing levels of reinforcement in the composite materials is in good agreement with earlier reports [10]. A longer duration of pulse increases EWR during the initial period and
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Fig. 8. (a) Microvoids and cracks on the EDM surface of 5% TiC composites (b) Recast layer white layer on EDM machine surface 5% TiC composites.
Fig. 9. (a) Crater formation on the EDM surface on 2.5% TiC composites. (b) White recast layer in EDM for 2.5% Al/TiC.
thereafter it decreases due to formation of craters and resistance in the conductive path due to removed ceramic particles. A higher percentage of TiC content in the composite decreases EWR due to high electrical resistance. The effect of flushing pressure on EWR was evaluated as shown in Fig. 6(c). Increase in flushing pressure increases EWR continuously in the case of Al-2.5% TiC. However, the trend in Al-5% TiC is different in which during the initial period due to the formation of bigger size craters EWR is slightly reduced and when the flushing pressure is further increased EWR is increased drastically. 3.4. Effect of TiC addition on recast layer thickness Fig. 7 shows various surface layers produced during the electro discharge machining of composite samples. Three different distinct layers formed during electro discharge machining are a white or recast layer at the top of the surface, a heat affected zone in which the structure of the material is slightly altered and an unaffected zone with identical microstructure of the base material. A large number of micro cracks have been observed on the recast layer. Cracks appearing on the recast layer are due to high tensile stress developed due to imperfect joining of molten droplets inducing a lot of stress gradient exceeding the ultimate tensile strength of the composite material. The matrix material melts and recasts on the machining surface with lots of micro-cracks and voids formed, as shown in Figs. 8(a)–9(b), due to the formation of gas bubbles.
However, ceramic particles pulled out of the matrix during the process have not recast on the surface as evidenced from SEM pictures of 2.5% and 5% TiC composites thereby making the recast layer distinct with different surface integrity. The above theory has been further strengthened by the XRD studies carried out, the results of which have been provided in Fig. 10(a)–(b), on the machined surface of 2.5% and 5% TiC reinforced aluminium composites indicating only Aluminium peaks and no visible TiC peaks. It has also been found that the thickness of the recast layer increases with increasing percentage of TiC particle in the composite which is in good conformity with earlier research [10]. 4. Conclusions Al–TiC composites can be machined using electrodischarge machining and by selecting optimum levels for the EDM parameters, namely, discharge current, pulse on time and flushing pressure the effectiveness of process like metal removal rate and electrode wear rate can be improved. A careful investigation into the structure of the material after the machining suggests that ceramic particles (TiC) were not melted during the process and removal of material occurs as a result of matrix melting and ceramic particle pull out thereafter. The above phenomenon results in the reduced metal removal rate with the increased titanium carbide content in the composite material. Material removal rate and tool wear rates are
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(a) XRD pattern of the hole surface of Al-2.5% TiC composite.
(b) XRD pattern of the hole surface of Al-5% TiC composite. Fig. 10.
influenced by discharge current. Flushing pressure plays an important role in continuing the process and improving the material removal rate at higher discharge current and pulse duration levels. Acknowledgement This work has benefited from the use of the facilities from the Project funded by the Department of Science and Technology, Government of India, under Grant No. SR/FTP/ETA-69/07. References [1] Brazil D, Monaghan J, Aspinwall DK, Ng EG. Wear characterization of various diamond tooling when single point turning a particle reinforced metal matrix composite. In: Proceeding of the IMC-14 conference. 1997. p. 143–52. [2] Muller F, Monaghan J. Non conventional machining of particle reinforced metal matrix composites. J Mater Process Technol 2001;118:278–85. [3] Hung NP, Yang LJ, Leong KW. Electro discharge machining of cast metal matrix composites. J Mater Process Technol 1994;44:229–36. [4] Roux TLe, Wise MLH, Aspinwall DK. The effect of electro discharge machining on the surface integrity of an aluminium–silicon carbide metal matrix composite. J Process Adv Mater 1993;3:233–41.
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