INTERNATIONAL JOURNAL OF APPLIED ENGINEERING … · INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 2, 201 1 © Copyright 2010 All rights reserved Integrated

INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 2, 2011

© Copyright 2010 All rights reserved Integrated Publishing Association

REVIEW ARTICLE ISSN 09764259

350

Solidification behavior of stir cast Al alloy metal matrix composites Sourav Kayal 1 , Behera. R 2 , Nandi. T 3 , Sutradhar. G 4

1 S.R.F under DSTPURSE Scheme, Department of Mechanical Engineering, Jadavpur University, Kolkata, West Bengal ,India

2 Asst. Professor, Department of Mechanical Engineering, Seemanta Engg. College, Orissa, India

3 Workshop Superintendent, Jadavpur University, Kolkata, West Bengal, India 4 Professor, Department of Mechanical Engineering, Jadavpur University, Kolkata, West

Bengal, India [email protected]

ABSTRACT

In recent times, aluminum and its alloy based cast metal matrix composites have gained lot of popularity in all the emerging fields of engineering and technology owing to their superior mechanical properties compared to monolithic materials. The desired properties of the cast metal matrix composites have influenced by the solidification behavior of the cast metal matrix composites, which imposed to study the solidification behavior of the cast metal matrix at different weight fraction of reinforcement particles. The present paper aims to investigate the solidification behavior of Aluminum alloy (LM6)SiCp composites at different section of fivestepped component (composite castings) which has produced by using stir cast technique. The temperature of the cast composites during solidification has measured by putting Ktype thermocouples at the centre of the each step/section, from which the solidification curves have obtained (constructed.) Experiments were carried out over range of particle weight percentage of 2.5 to 15 wt% in steps of 2.5 wt%. The solidification curves of Aluminum alloy (LM6)SiCp composites reveal that significant increase in solidification time with the addition of SiC particles. The curves also show that the rate of cooling and the solidification time are different at different section of the castings.

Key words: LM6, SiCp, Metal matrix composites (MMCs), Solidification, Cooling rate.

1. Introduction

Metal matrix composite (MMC) is engineered combination of the metal (Matrix) and hard particle/ceramic (Reinforcement) to get tailored properties. Composites have developed with great success by the use of fiber reinforcement in metallic materials (ASM Metals Hand Book, 2008). Metal–matrix composites (MMCs) have been one of the key research subjects in materials science during the past few decades (Lindroos, V.K., Talvite, 1998). MMCs are either in use or in prototyping for the space shuttle, commercial airliners, electronic substrates, bicycles, automobiles and a variety of other applications.

Like all composites, aluminummatrix composites are not a single material but a family of materials whose stiffness, strength, density, thermal and electrical properties can be tailored. The matrix alloy, reinforcement material, volume and shape of the reinforcement, location of the reinforcement and fabrication method has all varied to achieve the required properties. The aim involved in designing metal matrix composite materials is to combine the desirable attributes of metals and ceramics. The addition of high strength, high modulus refractory




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particles to a ductile metal matrix produce a material whose mechanical properties are intermediate between the matrix alloy and the ceramic reinforcement. These properties have further improved by carefully controlling the relative amount and distribution of the ingredients of a composite as well as the processing conditions.

Among discontinuous metal matrix composites, stir casting has generally accepted as a particularly promising route because of its simplicity, flexibility and applicability to large quantity production. This liquid metallurgy technique is the most economical of all the available routes for metal matrix composite production (Surappa M.K., Mater J, 1997) and allows fabrication of very large sized components. The cost of preparing composites material using a casting method is about onethird to half that of competitive methods, and for high volume production, it is projected that the cost will fall to onetenth (Skibo D.M., Schuster D.M., Jolla L, 1998). In general, the solidification synthesis of metal matrix composites involves producing a melt of the selected matrix material followed by the introduction of a reinforcement material into the melt, obtaining a suitable dispersion.

The next step is the solidification of the melt containing suspended dispersoids under selected conditions to obtain the desired distribution of the dispersed phase in the cast matrix. There are several factors that need considerable attention during preparing metal matrix composites by the stir casting method, including the difficulty of achieving a uniform distribution of the reinforcement material, wettability between the two main substances, porosity in the cast metal matrix composites, and chemical reactions between the reinforcement material and the matrix alloy. In order to achieve the optimum properties of the metal matrix composite, the distribution of the reinforcement material in the matrix alloy must be uniform, and the wettability or bonding between these substances should be optimized. The literature review reveals that the major problem was to get homogenous dispersion of the ceramic particles by using low cost conventional equipment for commercial applications.

The rate of solidification has a significant effect on the microstructure of cast composites, which in turn affects their mechanical properties. T.P.D. Rajan et al. studied on solidification and casting / mould interfacial heat transfer characteristics of aluminum matrix composite. They have shown that, addition of ceramic reinforcement particles with the aluminum alloy reduces the total solidification time in all the moulds (i.e. sand, graphite and metal mould) studied at lower volume fractions and increases at higher volume fractions. Arda Cetin et al. [6] studied on effect of solidification rate on spatial distribution of SiC particles in A356 alloy composites. They concluded that distribution of SiC particles in A356 alloy composites is highly dependent on secondary dendrite arm spacing. With decreasing arm spacing, particles exhibit fewer tendencies towards clustering. At higher cooling rates, however, when the arm spacing were smaller than the average particle size, this tendency was reversed, resulting in formation of larger clusters. Depending on SDAS (secondary dendrite arm spacing) and particle content clustering was most pronounced at distance scales ranging approximately between 150 500µm which is much larger than average nearest neighbor distances.

The objective of the present investigation is to study the effect of varying weight percentage of SiCp on the solidification curves of aluminumsilicon alloy (LM6) matrix composites solidifying in a five step sand mould. The rate of solidification at section of the casting,




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hardness and the metallographic properties has studied at different weight percentage of silicon carbide particles.

2. Experimentation

The aluminum–silicon alloy i.e.LM6, which is a well known alloy of aluminum, is used as the base/matrix metal in the experiments for the fabrication of the composites that has been reinforced with 2.5 wt%, 5 wt%, 7.5 wt%, 10 wt% 12.5 wt % and 15 wt. % of SiCp of average 400 mesh size. The chemical composition of the matrix material (LM6) and the thermo physical properties of SiCp & LM6 have given in the table1 & table2. The composites have fabricated by the liquid metal stir casting technique. The melting has carried in a tilting electric resistance furnace in a range of 760 ± 10 0 C. A schematic view of the furnace has shown in Figure 1.

Figure 1: Schematic view of the furnace

1. Motor 2. Shaft 3. Molten aluminium 4. Thermocouple 5. Particle injection chamber 6.Insulation hard board 7. Furnace 8. Graphite crucible

The melt has mechanically stirred by using an impeller after addition of the preheated silicon carbide particle (about 1000 0 C.). The processing of the composite has carried out at a temperature of 750 0 C with a stirring speed of 600 rpm. The melt has poured at a temperature of 730 0 C into a stepped silica sand mould. Five (i.e.T1, T2, T3, T4 & T5) Ktype thermocouples of 3.0 mm size has used at the centre of the different section (step) of the mould to measure the temperature variation in the casting during solidification has shown in Figure 2. One more Ktype thermocouple has inserted into the sand to measure the temperature variation of the molding sand after pouring of molten metal and during solidification of the castings. The solidification curves of the castings and the variation of temperatures at different sections in the mould are recorded with the help of a computer aided data acquisition system, the schematic sketch of the computer aided data acquisition set up has shown in Figure 3. Experiments carried out for a wide range of particle weight percentage




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varying from 2.5% to 15% in steps of 2.5%. Finally, the solidification curves of LM6SiCp composites have compared with the unreinforced LM6 matrix alloy at different section of the casting. The micro structural characteristics and hardness of the composites have evaluated.

Table 1: Chemical Composition (LM6)

Elements Si Cu Mg Fe Mn Ni Zn Pb Sb Ti Al Percentage(%) 1013.0 0.1 0.1 0.6 0.5 0.1 0.1 0.1 0.05 0.2 Remaining

Table 2: Thermo physical properties of reinforcement particle and LM6

Properties SiC particulates LM6

Density(gm/cm 3 ) 3.2 2.66 Average particle size( mesh) 400 Thermal conductivity(W/m/K) 100 155

Specific heat (j/Kg/K) 1300 960

Figure 2: The geometry of mould cavity with Ktype thermocouples (T1, T2, T3 ,T4 & T5). (All dimensions are in mm)

Figure 3: Schematic sketch of the computer aided temperature data acquisition set up. (T1, T2, T3 ,T4 & T5 Thermocouples connected with different section of casting and T6

inserted into the sand)

3. Results and discussion

3.1. Analysis of cooling curves




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Addition of an alloying element or second phase particles into a matrix alloy usually affects the various time and temperature parameters of its solidification curve. The variation in the nature of the cooling curve always has a significant impact on the microstructure and mechanical behavior of the material. Figure 410 shows the cooling curve of the Al alloy (LM6) metal matrix composites reinforced with 2.5wt%, 5wt%, 7.5wt%, 10wt%, 12.5wt% and 15 wt% of SiCp. From the cooling curves it has observed that the start of eutectic solidification of the matrix alloy (LM6) at a temperature of 572 o C with the solidification ending at 570 o C. After addition of reinforcement particles i.e. SiCp in matrix alloy, the start and end temperature of eutectic solidification changes.

Figure 11 shows the variation in eutectic solidification time with respect to different weight percentage of SiCp and at different section of the castings. It has observed that on increasing the weight percentage of SiCp in the cast Al alloy (LM6) metal matrix composites the eutectic solidification time increases at different section of the castings. That means the total solidification time (i.e. the time interval between the start of primary aluminum phase nucleation and the end of the eutectic phase solidification) increases on increasing weight percentage of SiCp. This trend may be attributed to the fact that the amount of heat extraction reduced on increasing the weight percentage of SiC particles in the liquid matrix metal as the presence of SiC particles in the matrix metal reduced the thermal conductivity and thermal diffusivity [7].

Figure 4: Cooling curves of Al (LM6) Figure 5: Cooling curvesof Al (LM6) 2.5wt%SiC composites at different 5wt%SiC composites at different

section of casting section of casting






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Figure 10: Cooling curves of Al (LM6) at different section of casting

Figure 11: Effect of wt% SiCp on Figure 12: Cooling rates from Eutectic Solidification time at superheat to liquidus different section of castings. temperature.

The cooling rates from superheat to nearliquidus are plotted in Figure 12. The data show that the cooling rates of reinforced materials are faster than that achieved in the original LM6 alloys. Similar results of increasing cooling rate with reinforced of Al alloy are also reported by previous researchers under differential thermal analysis (in other research) [5,6]. The phenomenon of decreasing cooling rate (due to longer freezing time) accompanied by increasing casting modulus has observed in all castings. This validates that the Chvorinov’s rule still applies to the solidification process, irrespective of what additives are added to the molten metal [8,9]. The eutectic solidification time has also observed to be longer in composite castings.




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3.2. Hardness

Hardness test has conducted on each specimen using a load of 250 N and a steel ball of diameter 5 mm as indenter. Diameter of impression made by indenter has predicted by Brinnel microscope. The corresponding values of hardness (BHN) were calculated from the standard formula. The results as indicated in Table 3 show the increasing trend of hardness with increase in weight percentage of SiC particles.

Table 3: Hardness (BHN) of the MMCs at different wt% of SiCp

Material LM6 LM6 2.5wt%

LM6 5wt%

LM6 7.5wt%

LM6 10wt%

LM6 12.5wt%

LM6 15wt%

Hardness(BHN) 53.5 63.8 64.6 65.1 66.7 67.4 68.2

3.3. Microstructural Analysis

Samples of as cast MMCs for metallographic examination were prepared by grinding through different size of grit papers followed by polishing with 6 µm diamond paste. Then the samples were etched with the etchant i.e. Keller’s reagent (2.5 ml Nitric acid, 1.5 ml HCl , 1.0 ml HF,95.0 ml Water) . The etched samples were dried by using electric drier and then the microstructure observed by using Scanning Electron microscope (JEOL, JSM 6360). The microstructure of the as cast LM6/SiCp MMCs are shown in Fig.13 (af) at different weight fraction of SiCp.

(a) (b)

(c) (d)




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(e) (f) Figure 13: Microstructure of LM6/ SiCp as cast MMCs at different Weight fraction of SiCp.

(a) 2.5%wt% of SiCp.(b) 5%wt% of SiCp (c)7.5%wt% of SiCp (d) 10%wt% of SiCp (e) 12.5%wt% of SiCp (f) 15%wt% of SiCp

Figure13 predicts that the main micro structural features in cast composites have been in the form of dendrites. The SiC dispersiods in the crystal structure of aluminum were in solid solution and distributed among the aluminum atoms in an atomic dispersion. The composites does not freeze at a single temperature but instead over a temperature range (temperature at which the freezing begins was called the liquidius, and the temperature at which freezing was complete is called the solidus) [10].

The phenomenon of dendrite structure formation may be due to supercooling effect in which certain preferred regions protrude as spikes into the supercooled regions and once started, grow more rapidly then neighboring regions. This had happened because the driving force for freezing was greater in the super cooled regions and the spikes reject the solute at their sides, thus delaying freezing of the side regions. These spikes consequently tend to form side arms producing a dendritic structure.

Nucleation of α Al phase starts in the liquid at a distance away from the particles where the temperature is lower. The growth of α Al nuclei lead to enrichment of Si in the melt. The enrichment of Si in the melt around the particles leads to heterogeneous nucleation. Another effect of thermal lag was that the melt around the particles would solidify in the last stage. This would make the particles located between dendrites. In other words, the interdendritic clusters of SiC particles have partly inherited from inhomogeneous distribution of particles in the original slurries. In commercially pure aluminium (without SiC particles), dendrites were observed to be distinctively columnar and almost randomly distributed. However in cast composites, dendrites were found to be equiaxed and in the regions with clusters of SiC particles in the primary α Al seemed to be finer.

4. Conclusions

Solidification curves have recorded experimentally for Al alloy (LM6) with varied percentage of SiC particles from 2.5 wt% to 15 wt% in steps of 2.5 wt% and studied. The following lines have concluded

1. The cooling rate decreases with the introduction of SiCp with increasing SiCp content due to lower heat transfer rates within the solidifying melt owing to the reduced




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effective thermal diffusivity of the composite system. That indicates the cooling rate is faster in case of unreinforced matrix alloy or containing low fraction of SiCp in the matrix.

2. Introduction of silicon carbide reinforcements into the Al (LM6) matrix alloy reduces the liquidus temperature; this may be results as the presence of silicon carbide particles in matrix alloy reduces the superheat temperature range.

3. Addition of ceramic reinforcement to alloy enhances the total solidification time, as the presence of insulating dispersoids i.e. SiCp plays a dominant role in reducing the cooling rates. The solidification time has also varied with the change in thickness of castings. The solidification time is less in case of thinner section in comparison with thicker section, due to rapid cooling of thinner section. This trend is similar to the monolithic metal and its alloys.

4. The results of study suggest that with increase in fraction of SiCp, an increase in hardness has observed.

5. The microstructural results reveal that the silicon carbide particles have uniformly distributed through out the MMC castings.

Acknowledgements

The authors would like to acknowledge the financial support of DST PURSE Project, New Delhi,Ref.:SR/S9/Z23/2008 dated 10.6.2009 in carrying out this research, without which this work could not be attempted.

5. References

1. ASM Metals Hand Book, (1998), casting, 15, Ninth edition, pp 323–327.

2. Lindroos, V.K., Talvite, M.J (1995), Recent advances in metal matrix composites, Journal of Materials Processing Technology., 53, pp 273–284.

3. Surappa M.K., Mater J, (1997), Journal of Materials Processing Technology, 63, pp 325–333.

4. Skibo D.M., Schuster D.M., Jolla L (1988), process for preparation of composite materials containing nonmetallic particles in a metallic matrix, and composite materials made by, US Patent No. 4 786 467,

5. Rajan T.P.D., Prabhu Narayana, Pillai K., Pai B.C (2007), solidification and casting/mould interfacial heat transfer characteristics of aluminium matrix composites, Compos. Sci. Technol. 67, pp 70–78.

6. Cetin Arda, Ali Kalkanli (2007), effect of solidification rate on spatial distribution of SiC particles in A356 alloy composites. Journal of Materials Processing Technology.




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7. Kobashi M., Choh T., The production of Al Si alloySiCp composites via compocasting: some microstructural aspects, (1993), Journal of Materials Science, (28) pp 684.

8. D.M. Stefanescu (2002), science and Engineering of Casting Solidification, Kluwer Academic/Plenum Publishers, New York, 311319.

9. Campbell J (1991), Castings, pp 130131.

10. Richard A. Flinn, “Fundamentals of Metal Casting” (2010), Addisson Wesley Series in Metallurgy and Materials, pp 1132.

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INTERNATIONAL JOURNAL OF APPLIED ENGINEERING … · INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 2, 201 1 © Copyright 2010 All rights reserved Integrated