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Characterization of Stir Cast Aluminium Silicon Carbide Metal Matrix Composite
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 23844–23852
www.materialstoday.com/proceedings
IConAMMA_2017
Characterization of Stir Cast Aluminium Silicon Carbide Metal Matrix Composite Sijo M T a,*, K R Jayadevan b a b
Research scholar, Government Engineering College Thrissur, Affiliated to Calicut University, Kerala, 680009, India Professor, Department of Mechanical Engineering,Government Engineering College,Thrissur, Kerala, 680009, India
Abstract Aluminium silicon carbide metal matrix composites (Al-SiC MMC) are gaining wider acceptance in various fields like aerospace, aircrafts, automobile, substrate in electronics, turbine blades, etc, due to their high strength, stiffness, high corrosion and wear resistance, good thermal and electrical properties and good damping capability. In this work Al-SiC MMCs are fabricated by mixing molten aluminium with Silicon carbide by the aid of mechanical stirring, called stir casting method. Also in this work, the stir casting process parameters are optimized within the range to obtain good mechanical characteristics and uniform mixing of reinforcement, which is a major concern in stir casting. The processing parameters that were investigated include stirrer speed, volume fraction of reinforcement, number of blades on stirrer and diameter ratio. Microstructure observed through optical microscope was correlated to clustering tendency of reinforcement particles. The results of mechanical characterization study and microstructure study show that the stirrer speed, volume fraction of reinforcement, diameter ratio, and number of blades on stirrer appears to have significant effect on mechanical properties and clustering tendency of reinforcement particles. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: Aluminium silicon carbide metal matrix composites; Stir casting; Microstructure; Mechanical characterisation;
1. Introduction The various fabrication methods used to produce Al-SiC MMCs are Powder metallurgy, Melt infiltration, Stir casting and Squeeze casting etc. Among these methods stir casting method is most preferred, due to its unique advantages like simplicity, high productivity, complex shaped and size components can be easily produced etc. The * Corresponding author. Tel.:+91- 9847520278 E-mail address:[email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017].
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main limitation of stir casting process is the clustering of reinforcement during solidification process, which in turn degrades mechanical properties. From literature review it has been found that the distribution of reinforcement particles depends upon various process parameters like stirrer speed, volume fraction, geometry of stirrer blades etc[1-3]. Moreover, the challenge for foundry person behind the melt stir casting is that rapid cooling of the molten metal in the mold. The rapid cooling of molten metal produces internal cavities in the solidified composite. The properties of stir cast MMCs are influenced by rate of solidification of molten metal [4]. Various researchers have conducted real time experiments to evaluate characteristics of solidification phenomena and they also investigated various process parameters affecting solidification phenomena for Al SiC MMC [4-6]. Behra et al. [4] conducted experiments for fluidity test and found that fluidity is decreased with increase in reinforcement percentage. Additionally, the formation of Al4C3 at the interface causes a local solidification of the metal matrix. The presence of reinforcement in the form of particles can affect the final microstructure of the composite. This in turn affects the mechanical properties of the composites. Microstructure study is used to analyse reinforcement particle distribution inside the matrix material. The information that can be gathered from microstructure study is particle clustering, internal defects, formation of reactive products etc. Among various equipments optical microscopy is used to view upto 2000X magnified image of the specimens prepared. If further magnification is needed, then it is recommended to use SEM or TEM. Various researchers successfully used optical microscope to study the microstructure of aluminium silicon carbide metal matrix composite [7-9]. The microstructure of such materials consists of a major phase, aluminium or silicon and the mixture of these two elements. These elements make a direct impact on final MMC characteristics. The existing methods for mixing reinforcement in matrix results in segregation and hence all the material properties are reduced. Thus, it requires further exploration towards developing advanced and improvised methods towards the same that will not result in segregation. Moreover, research work on mechanical characterization of AlSiC MMCs consider variation of one process parameter at a time, which does not account interaction between process parameters. In this work simultaneous variation of process parameters provides interaction effects between process parameters, which will be useful result for further research in this field. The present paper investigates microstructure examination and mechanical characterization of Al-SiC MMC produced by stir casting technique for various combinations of process parameters such as stirrer speed, volume of fraction of reinforcement, number of blades on stirrer and diameter ratio. To find the effect of these parameters on solidification and microstructure formation, microstructure examination on prepared MMC is carried out using optical microscopy of 100X magnifications. To find the effect of these process parameters on mechanical behavior of composite, the mechanical properties like density, hardness and wear resistance are measured. These result show a strong influence of process parameters on clustering of particles and, mechanical characterization of the MMC. 2. Experimental Work The specimens for mechanical characterization study are prepared by stir casting method. Stir casting is carried out in a closed crucible, therefore as a first step of experiment a cylindrical crucible with diameter 120 mm is selected to conduct the experiment. The next step in experimental work consists of preparation of stirrer blade. Blade is made of cast steel strip of 25 mm width and 2 mm thickness and length of the blade is varied from 30 mm, 20 mm and 14 mm respectively. All the blades are welded around a rotor of cast steel of 20mm diameter. The previous work in this field have used 4 blade stirrer for stirring and volume fraction is varied from a minimum value of 5% to a maximum possible value of 30% and stirrer speed is varied from 300rpm to 2000rpm. In this experiment three different types of stirrer blade geometries are prepared, which consists of three blade geometry, four blade geometry and five blade geometry (Figure 1). The other process parameters varied for experimental work are speed of the stirrer (SS) which is changed from 50rad/sec, 100rad/sec to 150rad/sec, volume fraction (VF) of reinforcement SiC is varied from 10%, 20% to 30% and diameter ratio (DR), which is a ratio between diameter of crucible to diameter of stirrer blade is varied from 1.5, 2 to 2.5.
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Fig. 1. Different blade geometries
The composite is prepared from LM6 aluminium alloy as the matrix material and silicon carbide of 5µm average size as reinforcement material. A schematic of stir casting setup is shown in Figure 2. The experimental setup consists of three phase electrical heating furnace, graphite crucible, motor and a mechanical stirrer. Prior to mechanical stirring, the surfaces of both aluminium and SiC are cleaned. The cleaned aluminium scarps were preheated to 450oC and SiC were preheated to 1000oC. Preheating of component prevents the reaction between matrix and reinforcement. During stir casting process aluminium scraps were placed in the graphite crucible and its temperature was increased to 750oC so that aluminium scraps were completely melted. The preheated SiC particles were added in the matrix melt. To improve the wettability of SiC, magnesium was added in small quantities to the mixture [10]. Finally, the mixture is poured into the cylindrical moulds prepared. The melt was allowed to solidify in the moulds and after solidification the specimens prepared are cleaned for further study (Figure 3). To study the significance of process parameters such as stirrer speed, volume of fraction of reinforcement, number of blades on stirrer and diameter ratio, the entire process is repeated for different combination of process parameters and specimens are prepared for the microscopic study and for different mechanical characterisation tests.
Fig. 2. Schematic of stir casting setup
Fig. 3. Specimens prepared by stir casting
Fig. 4. Specimens prepared for microscopic study
Sijo M T and K R Jayadevan/ Materials Today: Proceedings 5 (2018) 23844–23852
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Composite samples for microscopic study were cut from solidified Al-SiC MMC, and then it is passed for grinding and fine polishing using diamond paste. After Polishing the specimens were etched in Kellers reagent (2ml HF + 3ml HCl + 5ml HNO3 + 190ml H2O) at 70°C for 40 seconds followed by quenching in water. The microstructures of the polished and etched specimens were observed using optical microscopy (Figure 4). Theoretically, rule of mixture relation is used to calculate the density of MMC and actual density value is found by dividing mass and volume of the produced composite. The composite weight is measured using Shimadzu AUY 220 Analytic balancer with an error of 1mg. Hardness is measured using Brinell hardness testing machine, for which cylindrical specimens of 10mm diameter and 10mm height are prepared (Figure 5). The Brinell hardness values of polished specimens of MMCs in three different locations are measured to get an average value using a steel ball indenter of 5 mm diameter with load of 250kgf. The specimens prepared for wear resistance calculation are shown in Figure 6. The wear resistance is calculated using pin disc Tribometer with following specification TR-20-CH 600 at room temperature (Figure 7). The wear disc material is EN 31 hardened to 60HRC with a diameter of 100mm is used for the operation. The wear disc is rotated at 80rpm and a fixed load of 7 kg is applied on the test samples. The weight of the specimen before and after the wear is measured which is a direct indication of the wear property of the material.
Fig. 5. Brinell hardness testing machine
Fig. 6. Specimens prepared for wear resistance study
Fig. 7. Pin disc Tribometer
3. Results and Discussions In this section, results obtained from microstructural study and mechanical characterisation study are presented and discussed in detail. The microstructure shows three different regions (as marked in Figure 8a) which are as follows: aluminium matrix (pirate gold colour), SiC (olive colour), and internal defects (black spots). The variation of mechanical properties like density, hardness and wear resistance with variation in process parameters are explained in detail in this session.
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3.1. Effect of stirrer speed Microstructure corresponding to different stirrer speed, ω =50, 100, and 150rad/sec are shown in Figures 8(a) – 8(c), respectively. These microstructures correspond to the case of diameter ratio 1.5, number of blades 4, and volume fraction 10%. From Figures 8a-c it is observed that when stirrer speed increases from 50rad/sec to 100rad/sec the clustering of olive colour area is reduced but there is a slight increase in clustering tendency for 150rad/s which may due to over swirl produced by increase in speed beyond certain limit. Moreover, this may be due to the difference in centrifugal force from the inner to outer part of the crucible, the particles were pushed towards circumference of the crucible. The pores or internal defects in MMC are observed to be low for the range of stirrer speeds varied. When speed increases the shear rate and turbulence will increase which in turn accelerates the solidification rate. Faster the solidification lesser will be the internal defects. Comparatively higher number of defects are seen in Figure 8a corresponds to lower speed 50rad/sec, which is due to the low shear rate, which in turn produce lower solidification rate and cause higher number of internal defects. It is also inferred from Figures 8a-c that all internal defects (black spots) are seen at the interface of Al matrix and SiC reinforcement. This shows that interface regions are more prone to internal defects. Long needle type formation of SiC is seen in Figure 8c, which shows at higher speeds there is tendency for clustering. In Figure 8b less number of defects and uniform distribution of reinforcement particles are seen, which shows there exist an optimum speed which will yield better production results for MMC. These results show that stirrer speed has a significant effect on final microstructure of the MMC. a
b
c
Fig. 8. Microstructure for 4 blade, VF 10%, DR 1.5 and stirrer speeds (a) 50rad/sec (b) 100rad/sec (c) 150rad/sec
The Various Mechanical characteristics corresponding to stirrer speed variations are depicted in Table 1. The difference between theoretical and actual value of density is observed to be low for 100rad/sec stirrer speed. This may be due to the observed low internal cavities during 100rad/sec stirrer speed. For 150rad/sec stirrer speed a slight increase in difference is seen between theoretical and actual value. This shows stirrer speed is a significant process parameter that affects solidification rate which in turn affects formation of internal defects. Hardness value is found to be more or less uniform with respect to variation in stirrer speed. A slight increase in hardness value is observed for 100 rad/sec and 150rad/sec compared with 50rad/sec stirrer speed. This may be due to the fact that uniform distribution of reinforcement is observed for these stirrer speeds. At lower stirrer speed, due to lower solidification rate, clustering of particles are observed, which in turn degrade the mechanical properties. The wear resistance is found to be more or less uniform with respect to variation in stirrer speed. This shows stirrer speed is having less effect on wear resistance property. A lower value of mass loss is observed for 100rad/sec stirrer speed. Table 1 Mechanical characteristics for 4blade, 10%SiC, diameter ratio 1.5 and different stirrer speeds Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
50
0.155
70.86
0.0080
100
0.042
72.67
0.0070
150
0.070
71.23
0.0075
Stirrer speed, ω (rad/sec)
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3.2. Effect of volume fraction
Microstructure corresponding to different volume fraction of reinforcement, VF =10%, 20%, and 30% are shown in Figures 9(a) – 9(c), respectively. These microstructures correspond to the case of diameter ratio 1.5, number of blades 4, and stirrer speed, ω = 50rad/sec. From Figures 9a-c it is clear that when volume fraction is increased from 10% to 30%, there is clustering of olive colour area. Moreover, when volume fraction is increased more defects are seen as black spots in the microstructure. In figure 9b and 9c clustering of reinforcement particle is seen together with black patches of defects surrounding the SiC particles. Lesser number of defects and uniform distribution of reinforcement particles are seen in Figure 9a. But in Figure 9b and 9c lesser number of defects and non uniform distribution of particles are seen. This is because at higher volume fractions and small size particles, the particle-particle interaction develops clustering in the composite. As mentioned in the previous section, more defects are seen at the interface region. It is also observed that these defects are in contact with the reinforcement particles. This may be due to the fact that increase in the weight percentage of reinforcement particles decreases the fluidity and rate of solidification, which in turn enhance solidification time. If solidification time is increased the chance for formation of internal defects will increases due to uneven solidification of MMC. From figures 9a-c it is inferred that volume fraction is a predominant factor which depends upon formation of internal defects and clustering tendency. These experimental results are in line with previously conducted experimental study in this field [11]. a
c
b
Fig. 9. Microstructure for 4 blade, Stirrer speed 50rad/sec, DR 1.5 and VF (a) 10% (b) 20% (c) 30%
The Various Mechanical characteristics corresponding to volume fraction variations are depicted in Table 2. The difference between theoretical and actual value of density is observed to be low for 10% volume fraction of reinforcement. This may be due to the observed low internal cavities during 10% volume fraction. For 20% and 30% volume fraction, increase in difference is seen between theoretical and actual value. This shows volume fraction is a significant process parameter that affects solidification rate which in turn affects formation of internal defects. Hardness value is found to increase with increase in percentage of volume fraction of reinforcement. This may be due to the percentage increase in high hardness SiC particles. The wear resistance is found to be increase with increase in percentage of volume fraction. This shows volume fraction is having significant effect on wear resistance property. Table 2 Mechanical characteristics for 4blade, 50rad/sec, diameter ratio 1.5 and different volume fractions Volume fraction VF(%)
Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
10
0.155
70.86
0.0080
20
0.167
72.14
0.0070
30
0.166
72.60
0.0070
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3.3. Effect of number of blades Microstructure corresponding to the cases with different number of blades 3, 4, and 5 are shown in Figures 10(a) – 10(c), respectively. These microstructures correspond to the case of diameter ratio 1.5, and volume fraction 10% and stirrer speed, ω = 100rad/sec. It is clear from Figures 10a-c that there is occurrence of clustering of SiC particles in molten aluminium corresponds to 3 blade and 5 blade geometry stirrer. But for 4 blade geometry better distribution of SiC particles are seen together with less casting defects. In 3 blade geometry the force exerted by stirrer is low compared with other two blade geometries, which in turn induces low swirl inside the crucible, thus a low solidification rate is observed in 3 blade stirrer geometry. In 5 blade geometry stirrer induces very high swirl inside the crucible, which in turn produce non uniform mixing of reinforcement and results in clustering of particles. Further, figures 10a and 10c it is observed that reinforcement particles form like a needle type structure due to clustering of reinforcement particles. Lesser number of defects are also seen in Figures 10a-c which shows number of blades on stirrer have less impact on formation internal defect. As in the previous sections internal defects are seen at the interface regions. From the microstructure examination, it is clear that number of blades on stirrer is a significant process parameter that will affect final microstructure of Al-SiC MMC. It is evident from experiment that there should have an optimum number of blades on stirrer for uniform mixing of reinforcement. It can also be inferred from the study that the shear rate exerted by the stirrer should be optimum to yield better mixing of particles. a
b
c
Fig. 10. Microstructure for diameter ratio 1.5, VF 10%, Stirrer speed 100rad/sec and (a) 5 blade (b) 4 blade (c) 3 blade
The Various Mechanical characteristics corresponding to number of blade variations are depicted in Table 3. The difference between theoretical and actual value of density is observed to be low for 4 blade stirrer geometry. This may be due to the formation of low internal cavities with 4 blade geometry. For 3 blade stirrer geometry a slight increase in difference is seen between theoretical and actual value. This may be due to low swirl produced inside the crucible. Hardness value is found to be more or less uniform with respect to variation in number of blades. A slight increase in hardness value is observed for 4 blade stirrer geometry compared with 3 blade and 5 blade stirrer geometry. The wear resistance is found to increase with increase in number of blades on stirrer. This shows number of blades is having significant effect on wear resistance property Table 3 Mechanical characteristics for diameter ratio 1.5, 10%SiC, 100rad/sec and different number of blades Number of blades (NB)
Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
3
0.049
71.12
0.0110
4
0.042
72.67
0.0070
5
0.042
72.63
0.0070
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3.4. Effect of diameter ratio Microstructure corresponding to different diameter ratio 1.5, 2, and 2.5 cases are shown in Figures 11(a) – 11(c), respectively. These microstructures correspond to the case of number of blades 4, volume fraction 10% and stirrer speed, ω = 100rad/sec. From Figures 11a-c it is evident that when diameter ratio increases there is chance for particle clustering. In Figures 11b and 11c when diameter ratio is increased casting defects are also increased. The gap between stirrer tip and crucible increases the force exerted by stirrer on fluid inside the crucible reduce, which in turn reduce the shear rate. When force exerted by stirrer is small the swirl produced will be small, which in turn reduces solidification rate. From Figures 11a-c lesser number of defects is seen and also defects are seen at the interface region. Needle type structure of reinforcement particles is seen in Figures 11b and 11c. This is due to the fact that reduction in solidification rate will reduce heat transfer inside the crucible, which in turn increase clustering tendency. From Figures 11a-c it is further inferred that increase in diameter ratio will accelerate the clustering tendency but is has less impact on formation of internal defects. The solidification rate theoretically depends upon diameter ratio by the following formula r =πn(D/δ-2), where n and D are the rotation speed and the outer diameter of the blade, respectively, and δ is the gap between the blade and the inner surface of the crucible [12]. The above equation validates the results obtained in this experimental study. From the microstructure examination, it is clear that diameter ratio is a significant process parameter that will affect microstructure of Al-SiC MMC. b
a
c
Fig. 11. Microstructure for 4 blade, VF 10%, Stirrer speed 100rad/sec and diameter ratio (a) 1.5 (b) 2 (c) 2.5
The Various Mechanical characteristics corresponding to diameter ratio variations are depicted in Table 4. The difference between theoretical and actual value of density is observed to increase with increase in diameter ratio. This may be due to low shear rate which in turn produce internal defects. This shows diameter ratio is a significant process parameter that affects solidification rate. Hardness value is found to decrease with increase in diameter ratio. A high value of hardness is observed for diameter ratio 1.5. This may be due to the fact that uniform distribution of reinforcement is observed for diameter ratio 1.5. The wear resistance is found to be more or less is uniform with respect to variation in diameter ratio. This shows diameter ratio is having less effect on wear resistance property. Table 4 Mechanical characteristics for 4blade, 10%SiC, 100rad/sec and for different diameter ratio Diameter ratio (DR)
Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
1.5
0.042
72.67
0.0070
2
0.079
69.14
0.0070
2.5
0.088
69.10
0.0072
Among various process parameters, stirrer speed and volume fraction are the predominant factors which affect clustering tendency. The diameter ratio and number of blades on stirrer are having lesser effect on clustering tendency. This work suggests that there should be an optimum value for stirrer speed and number of blades on stirrer
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to have uniform distribution of reinforcement particles. The interactions between process parameters may also affect final microstructure and clustering tendency. It is inferred from the study that stirrer speeds, number of blades on stirrer and diameter ratio have interaction effect between them. But volume fraction of reinforcement is an independent process parameter, which does not depend upon variation of other process parameters. The proper selection of process parameters are necessary to yield better results in MMC production. The mechanical characterisation study reveals that variation in stirrer speed, diameter ratio significantly affect the density of the MMC. The hardness and wear resistance property are less affected by variation in stirrer speed and diameter ratio. The variation in volume fraction of reinforcement affects hardness value of the MMC. The variation of number of blades on stirrer affects the wear resistance property of the MMC. This work will guide researchers to do further extended works in this field by incorporating more process parameters. 4. Conclusion In this work the microstructure study of Al- SiC MMC are conducted to identify the major factors which affects particle clustering in stir casting. The microstructure study for different combination of process parameters such as stirrer speed, volume fraction of reinforcement, number blades on stirrer and diameter ratio clearly show the influence of these parameters on particle clustering tendency. Among different process parameters stirrer speed and volume fraction have significant effect on particle segregation. It is also seen from the study that number of blades on stirrer and diameter ratio have less impact on segregation phenomena. Among various process parameters investigated volume fraction is the major factor which will affect the formation of internal defects. Stirrer speed promotes faster solidification rate. But when stirrer speed is increased beyond certain limit there is a chance for increase in segregation of reinforcement particles. When volume fraction is increased clustering tendency will increase. Moreover, it will also increase formation of internal defects in the materials. Increase in number of blades increases the solidification rate but an uneven distribution of the reinforcement is observed. Increase in diameter ratio decreases turbulence and solidification time which in turn increases clustering phenomena. Moreover, mechanical characterisation study also shows the same results of microstructure study. It is also seen from the experimental study that four blade stirrer gives a better distribution of reinforcement particles compared with other two types of blade geometry. Comparing all cases, it is seen that a combination of 4 blade stirrer, 10% SiC, diameter ratio 1.5 and stirrer speed 100rad/sec will produce better properties to the MMC with less number of internal defects and uniform mixing of reinforcement particles. References [1] M K Surappa, Aluminium matrix composites: Challenges and Opportunities, Sadhana Vol. 28, Parts 1 & 2, 2003, pp 319–334. [2] Naher, S, Brabazon D and Looney L, Development and assessment of a new quick quench stir caster design for the production of metal matrix composites, Journal of Material Processing Technology, Vol. 166, 2004, pp 430-439. [3] Hashim J, Looney L and Hashmi M.S.J, “Metal Matrix Composites: Production by the Stir Casting Method, Journal of Material Processing and Technology, 92, 1999, pp 1-7. [4] Rabindra Behera, S Kayal, D Chatterjee and G Sutradhar, Solidification Behavior and Forgeability of Stir-Cast Aluminum Alloy Metal Matrix Composites, Canadian journal of pure and applied sciences, Vol. 5, No. 2, 2011, pp. 1541-1546. [5] P egizabal, A. garc´ia romero, A. torregaray, Analysis of the solidification and properties of plaster cast Al based composites, Archives of metallurgy and materials, Volume 57, issue 1, 2012, pp 119-125. [6] Shusen Wu, Ya You, Ping An, T Kanno, H Nakae, Effect of modification and ceramic particles on solidification behavior of aluminummatrix composites, Journal of Materials Science, Volume 37, Issue 9, 2002, pp 1855-1860. [7] J.P Pathik, J K Singh and S Mohan, Synthesis and characterization of aluminum –silicon, silicon carbide composite, Indian journal of engineering and materials sciences, Vol 13, 2006, pp238-246. [8] Prashant Kumar Suragimath, and G. K. Purohit, A study on mechanical properties of aluminium alloy (Lm6) reinforced with sic and fly ash, IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) Volume 8, Issue 5, 2013, pp 13-18. [9] Dunia abdul sahib, Aluminum silicon carbide and aluminum graphite particulate composites, ARPN Journal of Engineering and Applied Sciences Vol. 6, NO. 10, 2011, pp 41-46. [10] A. C Reddy and E Zitoun, Matrix Al-alloys for silicon carbide particle reinforced metal matrix composites, Indian Journal of Science and Technology, Vol. 3 No. 12, 2010, pp 1184-1187. [11] Di Zhang, Kenjiro Sugio, Kazuyuki Sakai, Hiroshi Fukushima and Osamu Yanagisawa, Effect of Volume Fraction on the Flow Behavior of Al-SiC Composites Considering the Spatial Distribution of Delaminated Particles, Materials Transactions, Vol. 49, No. 3, 2008, pp 661 – 670. [12] Z G Lei, XU Jun, Z Z Feng, L G Jun and Z. Fan, Effect of high shear rate on solidification microstructure of semisolid AZ91D alloy, Transactions Nonferrous Metals Society of China 20, 2010, pp 868-872.
ScienceDirect Materials Today: Proceedings 5 (2018) 23844–23852
www.materialstoday.com/proceedings
IConAMMA_2017
Characterization of Stir Cast Aluminium Silicon Carbide Metal Matrix Composite Sijo M T a,*, K R Jayadevan b a b
Research scholar, Government Engineering College Thrissur, Affiliated to Calicut University, Kerala, 680009, India Professor, Department of Mechanical Engineering,Government Engineering College,Thrissur, Kerala, 680009, India
Abstract Aluminium silicon carbide metal matrix composites (Al-SiC MMC) are gaining wider acceptance in various fields like aerospace, aircrafts, automobile, substrate in electronics, turbine blades, etc, due to their high strength, stiffness, high corrosion and wear resistance, good thermal and electrical properties and good damping capability. In this work Al-SiC MMCs are fabricated by mixing molten aluminium with Silicon carbide by the aid of mechanical stirring, called stir casting method. Also in this work, the stir casting process parameters are optimized within the range to obtain good mechanical characteristics and uniform mixing of reinforcement, which is a major concern in stir casting. The processing parameters that were investigated include stirrer speed, volume fraction of reinforcement, number of blades on stirrer and diameter ratio. Microstructure observed through optical microscope was correlated to clustering tendency of reinforcement particles. The results of mechanical characterization study and microstructure study show that the stirrer speed, volume fraction of reinforcement, diameter ratio, and number of blades on stirrer appears to have significant effect on mechanical properties and clustering tendency of reinforcement particles. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: Aluminium silicon carbide metal matrix composites; Stir casting; Microstructure; Mechanical characterisation;
1. Introduction The various fabrication methods used to produce Al-SiC MMCs are Powder metallurgy, Melt infiltration, Stir casting and Squeeze casting etc. Among these methods stir casting method is most preferred, due to its unique advantages like simplicity, high productivity, complex shaped and size components can be easily produced etc. The * Corresponding author. Tel.:+91- 9847520278 E-mail address:[email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017].
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main limitation of stir casting process is the clustering of reinforcement during solidification process, which in turn degrades mechanical properties. From literature review it has been found that the distribution of reinforcement particles depends upon various process parameters like stirrer speed, volume fraction, geometry of stirrer blades etc[1-3]. Moreover, the challenge for foundry person behind the melt stir casting is that rapid cooling of the molten metal in the mold. The rapid cooling of molten metal produces internal cavities in the solidified composite. The properties of stir cast MMCs are influenced by rate of solidification of molten metal [4]. Various researchers have conducted real time experiments to evaluate characteristics of solidification phenomena and they also investigated various process parameters affecting solidification phenomena for Al SiC MMC [4-6]. Behra et al. [4] conducted experiments for fluidity test and found that fluidity is decreased with increase in reinforcement percentage. Additionally, the formation of Al4C3 at the interface causes a local solidification of the metal matrix. The presence of reinforcement in the form of particles can affect the final microstructure of the composite. This in turn affects the mechanical properties of the composites. Microstructure study is used to analyse reinforcement particle distribution inside the matrix material. The information that can be gathered from microstructure study is particle clustering, internal defects, formation of reactive products etc. Among various equipments optical microscopy is used to view upto 2000X magnified image of the specimens prepared. If further magnification is needed, then it is recommended to use SEM or TEM. Various researchers successfully used optical microscope to study the microstructure of aluminium silicon carbide metal matrix composite [7-9]. The microstructure of such materials consists of a major phase, aluminium or silicon and the mixture of these two elements. These elements make a direct impact on final MMC characteristics. The existing methods for mixing reinforcement in matrix results in segregation and hence all the material properties are reduced. Thus, it requires further exploration towards developing advanced and improvised methods towards the same that will not result in segregation. Moreover, research work on mechanical characterization of AlSiC MMCs consider variation of one process parameter at a time, which does not account interaction between process parameters. In this work simultaneous variation of process parameters provides interaction effects between process parameters, which will be useful result for further research in this field. The present paper investigates microstructure examination and mechanical characterization of Al-SiC MMC produced by stir casting technique for various combinations of process parameters such as stirrer speed, volume of fraction of reinforcement, number of blades on stirrer and diameter ratio. To find the effect of these parameters on solidification and microstructure formation, microstructure examination on prepared MMC is carried out using optical microscopy of 100X magnifications. To find the effect of these process parameters on mechanical behavior of composite, the mechanical properties like density, hardness and wear resistance are measured. These result show a strong influence of process parameters on clustering of particles and, mechanical characterization of the MMC. 2. Experimental Work The specimens for mechanical characterization study are prepared by stir casting method. Stir casting is carried out in a closed crucible, therefore as a first step of experiment a cylindrical crucible with diameter 120 mm is selected to conduct the experiment. The next step in experimental work consists of preparation of stirrer blade. Blade is made of cast steel strip of 25 mm width and 2 mm thickness and length of the blade is varied from 30 mm, 20 mm and 14 mm respectively. All the blades are welded around a rotor of cast steel of 20mm diameter. The previous work in this field have used 4 blade stirrer for stirring and volume fraction is varied from a minimum value of 5% to a maximum possible value of 30% and stirrer speed is varied from 300rpm to 2000rpm. In this experiment three different types of stirrer blade geometries are prepared, which consists of three blade geometry, four blade geometry and five blade geometry (Figure 1). The other process parameters varied for experimental work are speed of the stirrer (SS) which is changed from 50rad/sec, 100rad/sec to 150rad/sec, volume fraction (VF) of reinforcement SiC is varied from 10%, 20% to 30% and diameter ratio (DR), which is a ratio between diameter of crucible to diameter of stirrer blade is varied from 1.5, 2 to 2.5.
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Fig. 1. Different blade geometries
The composite is prepared from LM6 aluminium alloy as the matrix material and silicon carbide of 5µm average size as reinforcement material. A schematic of stir casting setup is shown in Figure 2. The experimental setup consists of three phase electrical heating furnace, graphite crucible, motor and a mechanical stirrer. Prior to mechanical stirring, the surfaces of both aluminium and SiC are cleaned. The cleaned aluminium scarps were preheated to 450oC and SiC were preheated to 1000oC. Preheating of component prevents the reaction between matrix and reinforcement. During stir casting process aluminium scraps were placed in the graphite crucible and its temperature was increased to 750oC so that aluminium scraps were completely melted. The preheated SiC particles were added in the matrix melt. To improve the wettability of SiC, magnesium was added in small quantities to the mixture [10]. Finally, the mixture is poured into the cylindrical moulds prepared. The melt was allowed to solidify in the moulds and after solidification the specimens prepared are cleaned for further study (Figure 3). To study the significance of process parameters such as stirrer speed, volume of fraction of reinforcement, number of blades on stirrer and diameter ratio, the entire process is repeated for different combination of process parameters and specimens are prepared for the microscopic study and for different mechanical characterisation tests.
Fig. 2. Schematic of stir casting setup
Fig. 3. Specimens prepared by stir casting
Fig. 4. Specimens prepared for microscopic study
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Composite samples for microscopic study were cut from solidified Al-SiC MMC, and then it is passed for grinding and fine polishing using diamond paste. After Polishing the specimens were etched in Kellers reagent (2ml HF + 3ml HCl + 5ml HNO3 + 190ml H2O) at 70°C for 40 seconds followed by quenching in water. The microstructures of the polished and etched specimens were observed using optical microscopy (Figure 4). Theoretically, rule of mixture relation is used to calculate the density of MMC and actual density value is found by dividing mass and volume of the produced composite. The composite weight is measured using Shimadzu AUY 220 Analytic balancer with an error of 1mg. Hardness is measured using Brinell hardness testing machine, for which cylindrical specimens of 10mm diameter and 10mm height are prepared (Figure 5). The Brinell hardness values of polished specimens of MMCs in three different locations are measured to get an average value using a steel ball indenter of 5 mm diameter with load of 250kgf. The specimens prepared for wear resistance calculation are shown in Figure 6. The wear resistance is calculated using pin disc Tribometer with following specification TR-20-CH 600 at room temperature (Figure 7). The wear disc material is EN 31 hardened to 60HRC with a diameter of 100mm is used for the operation. The wear disc is rotated at 80rpm and a fixed load of 7 kg is applied on the test samples. The weight of the specimen before and after the wear is measured which is a direct indication of the wear property of the material.
Fig. 5. Brinell hardness testing machine
Fig. 6. Specimens prepared for wear resistance study
Fig. 7. Pin disc Tribometer
3. Results and Discussions In this section, results obtained from microstructural study and mechanical characterisation study are presented and discussed in detail. The microstructure shows three different regions (as marked in Figure 8a) which are as follows: aluminium matrix (pirate gold colour), SiC (olive colour), and internal defects (black spots). The variation of mechanical properties like density, hardness and wear resistance with variation in process parameters are explained in detail in this session.
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3.1. Effect of stirrer speed Microstructure corresponding to different stirrer speed, ω =50, 100, and 150rad/sec are shown in Figures 8(a) – 8(c), respectively. These microstructures correspond to the case of diameter ratio 1.5, number of blades 4, and volume fraction 10%. From Figures 8a-c it is observed that when stirrer speed increases from 50rad/sec to 100rad/sec the clustering of olive colour area is reduced but there is a slight increase in clustering tendency for 150rad/s which may due to over swirl produced by increase in speed beyond certain limit. Moreover, this may be due to the difference in centrifugal force from the inner to outer part of the crucible, the particles were pushed towards circumference of the crucible. The pores or internal defects in MMC are observed to be low for the range of stirrer speeds varied. When speed increases the shear rate and turbulence will increase which in turn accelerates the solidification rate. Faster the solidification lesser will be the internal defects. Comparatively higher number of defects are seen in Figure 8a corresponds to lower speed 50rad/sec, which is due to the low shear rate, which in turn produce lower solidification rate and cause higher number of internal defects. It is also inferred from Figures 8a-c that all internal defects (black spots) are seen at the interface of Al matrix and SiC reinforcement. This shows that interface regions are more prone to internal defects. Long needle type formation of SiC is seen in Figure 8c, which shows at higher speeds there is tendency for clustering. In Figure 8b less number of defects and uniform distribution of reinforcement particles are seen, which shows there exist an optimum speed which will yield better production results for MMC. These results show that stirrer speed has a significant effect on final microstructure of the MMC. a
b
c
Fig. 8. Microstructure for 4 blade, VF 10%, DR 1.5 and stirrer speeds (a) 50rad/sec (b) 100rad/sec (c) 150rad/sec
The Various Mechanical characteristics corresponding to stirrer speed variations are depicted in Table 1. The difference between theoretical and actual value of density is observed to be low for 100rad/sec stirrer speed. This may be due to the observed low internal cavities during 100rad/sec stirrer speed. For 150rad/sec stirrer speed a slight increase in difference is seen between theoretical and actual value. This shows stirrer speed is a significant process parameter that affects solidification rate which in turn affects formation of internal defects. Hardness value is found to be more or less uniform with respect to variation in stirrer speed. A slight increase in hardness value is observed for 100 rad/sec and 150rad/sec compared with 50rad/sec stirrer speed. This may be due to the fact that uniform distribution of reinforcement is observed for these stirrer speeds. At lower stirrer speed, due to lower solidification rate, clustering of particles are observed, which in turn degrade the mechanical properties. The wear resistance is found to be more or less uniform with respect to variation in stirrer speed. This shows stirrer speed is having less effect on wear resistance property. A lower value of mass loss is observed for 100rad/sec stirrer speed. Table 1 Mechanical characteristics for 4blade, 10%SiC, diameter ratio 1.5 and different stirrer speeds Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
50
0.155
70.86
0.0080
100
0.042
72.67
0.0070
150
0.070
71.23
0.0075
Stirrer speed, ω (rad/sec)
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3.2. Effect of volume fraction
Microstructure corresponding to different volume fraction of reinforcement, VF =10%, 20%, and 30% are shown in Figures 9(a) – 9(c), respectively. These microstructures correspond to the case of diameter ratio 1.5, number of blades 4, and stirrer speed, ω = 50rad/sec. From Figures 9a-c it is clear that when volume fraction is increased from 10% to 30%, there is clustering of olive colour area. Moreover, when volume fraction is increased more defects are seen as black spots in the microstructure. In figure 9b and 9c clustering of reinforcement particle is seen together with black patches of defects surrounding the SiC particles. Lesser number of defects and uniform distribution of reinforcement particles are seen in Figure 9a. But in Figure 9b and 9c lesser number of defects and non uniform distribution of particles are seen. This is because at higher volume fractions and small size particles, the particle-particle interaction develops clustering in the composite. As mentioned in the previous section, more defects are seen at the interface region. It is also observed that these defects are in contact with the reinforcement particles. This may be due to the fact that increase in the weight percentage of reinforcement particles decreases the fluidity and rate of solidification, which in turn enhance solidification time. If solidification time is increased the chance for formation of internal defects will increases due to uneven solidification of MMC. From figures 9a-c it is inferred that volume fraction is a predominant factor which depends upon formation of internal defects and clustering tendency. These experimental results are in line with previously conducted experimental study in this field [11]. a
c
b
Fig. 9. Microstructure for 4 blade, Stirrer speed 50rad/sec, DR 1.5 and VF (a) 10% (b) 20% (c) 30%
The Various Mechanical characteristics corresponding to volume fraction variations are depicted in Table 2. The difference between theoretical and actual value of density is observed to be low for 10% volume fraction of reinforcement. This may be due to the observed low internal cavities during 10% volume fraction. For 20% and 30% volume fraction, increase in difference is seen between theoretical and actual value. This shows volume fraction is a significant process parameter that affects solidification rate which in turn affects formation of internal defects. Hardness value is found to increase with increase in percentage of volume fraction of reinforcement. This may be due to the percentage increase in high hardness SiC particles. The wear resistance is found to be increase with increase in percentage of volume fraction. This shows volume fraction is having significant effect on wear resistance property. Table 2 Mechanical characteristics for 4blade, 50rad/sec, diameter ratio 1.5 and different volume fractions Volume fraction VF(%)
Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
10
0.155
70.86
0.0080
20
0.167
72.14
0.0070
30
0.166
72.60
0.0070
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3.3. Effect of number of blades Microstructure corresponding to the cases with different number of blades 3, 4, and 5 are shown in Figures 10(a) – 10(c), respectively. These microstructures correspond to the case of diameter ratio 1.5, and volume fraction 10% and stirrer speed, ω = 100rad/sec. It is clear from Figures 10a-c that there is occurrence of clustering of SiC particles in molten aluminium corresponds to 3 blade and 5 blade geometry stirrer. But for 4 blade geometry better distribution of SiC particles are seen together with less casting defects. In 3 blade geometry the force exerted by stirrer is low compared with other two blade geometries, which in turn induces low swirl inside the crucible, thus a low solidification rate is observed in 3 blade stirrer geometry. In 5 blade geometry stirrer induces very high swirl inside the crucible, which in turn produce non uniform mixing of reinforcement and results in clustering of particles. Further, figures 10a and 10c it is observed that reinforcement particles form like a needle type structure due to clustering of reinforcement particles. Lesser number of defects are also seen in Figures 10a-c which shows number of blades on stirrer have less impact on formation internal defect. As in the previous sections internal defects are seen at the interface regions. From the microstructure examination, it is clear that number of blades on stirrer is a significant process parameter that will affect final microstructure of Al-SiC MMC. It is evident from experiment that there should have an optimum number of blades on stirrer for uniform mixing of reinforcement. It can also be inferred from the study that the shear rate exerted by the stirrer should be optimum to yield better mixing of particles. a
b
c
Fig. 10. Microstructure for diameter ratio 1.5, VF 10%, Stirrer speed 100rad/sec and (a) 5 blade (b) 4 blade (c) 3 blade
The Various Mechanical characteristics corresponding to number of blade variations are depicted in Table 3. The difference between theoretical and actual value of density is observed to be low for 4 blade stirrer geometry. This may be due to the formation of low internal cavities with 4 blade geometry. For 3 blade stirrer geometry a slight increase in difference is seen between theoretical and actual value. This may be due to low swirl produced inside the crucible. Hardness value is found to be more or less uniform with respect to variation in number of blades. A slight increase in hardness value is observed for 4 blade stirrer geometry compared with 3 blade and 5 blade stirrer geometry. The wear resistance is found to increase with increase in number of blades on stirrer. This shows number of blades is having significant effect on wear resistance property Table 3 Mechanical characteristics for diameter ratio 1.5, 10%SiC, 100rad/sec and different number of blades Number of blades (NB)
Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
3
0.049
71.12
0.0110
4
0.042
72.67
0.0070
5
0.042
72.63
0.0070
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3.4. Effect of diameter ratio Microstructure corresponding to different diameter ratio 1.5, 2, and 2.5 cases are shown in Figures 11(a) – 11(c), respectively. These microstructures correspond to the case of number of blades 4, volume fraction 10% and stirrer speed, ω = 100rad/sec. From Figures 11a-c it is evident that when diameter ratio increases there is chance for particle clustering. In Figures 11b and 11c when diameter ratio is increased casting defects are also increased. The gap between stirrer tip and crucible increases the force exerted by stirrer on fluid inside the crucible reduce, which in turn reduce the shear rate. When force exerted by stirrer is small the swirl produced will be small, which in turn reduces solidification rate. From Figures 11a-c lesser number of defects is seen and also defects are seen at the interface region. Needle type structure of reinforcement particles is seen in Figures 11b and 11c. This is due to the fact that reduction in solidification rate will reduce heat transfer inside the crucible, which in turn increase clustering tendency. From Figures 11a-c it is further inferred that increase in diameter ratio will accelerate the clustering tendency but is has less impact on formation of internal defects. The solidification rate theoretically depends upon diameter ratio by the following formula r =πn(D/δ-2), where n and D are the rotation speed and the outer diameter of the blade, respectively, and δ is the gap between the blade and the inner surface of the crucible [12]. The above equation validates the results obtained in this experimental study. From the microstructure examination, it is clear that diameter ratio is a significant process parameter that will affect microstructure of Al-SiC MMC. b
a
c
Fig. 11. Microstructure for 4 blade, VF 10%, Stirrer speed 100rad/sec and diameter ratio (a) 1.5 (b) 2 (c) 2.5
The Various Mechanical characteristics corresponding to diameter ratio variations are depicted in Table 4. The difference between theoretical and actual value of density is observed to increase with increase in diameter ratio. This may be due to low shear rate which in turn produce internal defects. This shows diameter ratio is a significant process parameter that affects solidification rate. Hardness value is found to decrease with increase in diameter ratio. A high value of hardness is observed for diameter ratio 1.5. This may be due to the fact that uniform distribution of reinforcement is observed for diameter ratio 1.5. The wear resistance is found to be more or less is uniform with respect to variation in diameter ratio. This shows diameter ratio is having less effect on wear resistance property. Table 4 Mechanical characteristics for 4blade, 10%SiC, 100rad/sec and for different diameter ratio Diameter ratio (DR)
Difference between theoretical and actual value of density (103 kg/m3)
Hardness(BHN)
Difference between mass of the specimen before and after wear test (g)
1.5
0.042
72.67
0.0070
2
0.079
69.14
0.0070
2.5
0.088
69.10
0.0072
Among various process parameters, stirrer speed and volume fraction are the predominant factors which affect clustering tendency. The diameter ratio and number of blades on stirrer are having lesser effect on clustering tendency. This work suggests that there should be an optimum value for stirrer speed and number of blades on stirrer
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to have uniform distribution of reinforcement particles. The interactions between process parameters may also affect final microstructure and clustering tendency. It is inferred from the study that stirrer speeds, number of blades on stirrer and diameter ratio have interaction effect between them. But volume fraction of reinforcement is an independent process parameter, which does not depend upon variation of other process parameters. The proper selection of process parameters are necessary to yield better results in MMC production. The mechanical characterisation study reveals that variation in stirrer speed, diameter ratio significantly affect the density of the MMC. The hardness and wear resistance property are less affected by variation in stirrer speed and diameter ratio. The variation in volume fraction of reinforcement affects hardness value of the MMC. The variation of number of blades on stirrer affects the wear resistance property of the MMC. This work will guide researchers to do further extended works in this field by incorporating more process parameters. 4. Conclusion In this work the microstructure study of Al- SiC MMC are conducted to identify the major factors which affects particle clustering in stir casting. The microstructure study for different combination of process parameters such as stirrer speed, volume fraction of reinforcement, number blades on stirrer and diameter ratio clearly show the influence of these parameters on particle clustering tendency. Among different process parameters stirrer speed and volume fraction have significant effect on particle segregation. It is also seen from the study that number of blades on stirrer and diameter ratio have less impact on segregation phenomena. Among various process parameters investigated volume fraction is the major factor which will affect the formation of internal defects. Stirrer speed promotes faster solidification rate. But when stirrer speed is increased beyond certain limit there is a chance for increase in segregation of reinforcement particles. When volume fraction is increased clustering tendency will increase. Moreover, it will also increase formation of internal defects in the materials. Increase in number of blades increases the solidification rate but an uneven distribution of the reinforcement is observed. Increase in diameter ratio decreases turbulence and solidification time which in turn increases clustering phenomena. Moreover, mechanical characterisation study also shows the same results of microstructure study. It is also seen from the experimental study that four blade stirrer gives a better distribution of reinforcement particles compared with other two types of blade geometry. Comparing all cases, it is seen that a combination of 4 blade stirrer, 10% SiC, diameter ratio 1.5 and stirrer speed 100rad/sec will produce better properties to the MMC with less number of internal defects and uniform mixing of reinforcement particles. References [1] M K Surappa, Aluminium matrix composites: Challenges and Opportunities, Sadhana Vol. 28, Parts 1 & 2, 2003, pp 319–334. [2] Naher, S, Brabazon D and Looney L, Development and assessment of a new quick quench stir caster design for the production of metal matrix composites, Journal of Material Processing Technology, Vol. 166, 2004, pp 430-439. [3] Hashim J, Looney L and Hashmi M.S.J, “Metal Matrix Composites: Production by the Stir Casting Method, Journal of Material Processing and Technology, 92, 1999, pp 1-7. 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