Comparative study of metallurgical mechanical characteristics obtained through stir casting and centrifugal casting of Al-Si alloys

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1 Comparative study of metallurgical mechanical characteristics obtained through stir casting and centrifugal casting of Al-Si alloys Shrishail M B 1*, A Chaitanya 2*, Prabhu T B 3*, Gajendra S 4* 1,2,3,4UG Students, Department of Mechanical Engineering,AIET, Mangaluru,Karnataka, India shrishailbyakod1300@gmail.com, 2 achaitanya48@gmail.com 3prabhubyakod123@gmail.com, 4 gajendra.ss123@gmail.com Abstract---Al Si alloys are increasingly used in many applications due to its high tensile strength in relation to the density compared to other cast alloys, such as ductile cast iron or cast steel. As the properties of a specific Al Si alloys (hypoeutectic, eutectic or hypereutectic) can be attributed to the individual physical properties of its main phase constituents and to their volume fraction and morphology, different approaches have been used to control the microstructural features of Al Si alloys such as adding some alloying elements in order to refine the grain. However, the most common way to improve mechanical properties of cast Al Si alloys is by changing the casting technology. Each technology has particular aspects that have significant influence on microstructure and consequently on mechanical properties. The present work discusses on the mechanical and metallurgical characteristics of Al-Si hypereutectic alloy castings developed using stir casting and centrifugal casting. Mechanical characterization of the castings is carried out through UTM, Impact testing and Vickers hardness testing machines. Metallurgical characterization of the casting has been carried out through OM, SEM. Specimens produced through centrifugal casting process exhibited 20% higher tensile strength compared to the counter parts produced by stir casting, while stir cast specimens displayed 25% higher flexural strength than those produced by centrifugal casting. Centrifugal cast specimens showed higher hardness values and less impact resistance compared to stir cast specimens which is attributed to coarse polyhedral shaped morphology of the primary Silicon phase observed in the microstructure of the centrifugal cast alloy. Keywords: centrifugal casting, stir casting, Al-24Si alloy, microstructure, hypereutectic. I. INTRODUCTION In recent years the application of Al-Si alloys has been extended too many machine parts. Aluminium-Silicon (Al- Si) cast alloys are commonly used in automotive applications because of their low cost, low density, high temperature capability, and excellent cast ability. However, these alloys often suffer from low ductility, low tensile strength, and low fatigue resistance. Al-Si alloys are increasingly used in aerospace applications due to its high tensile strength in relation to density compared to other cast alloys, such as ductile cast iron or cast steel [5]. Hypereutectic aluminium-silicon (Al-Si) alloys are advanced engineering materials extensively used for electronic packaging and thermal management applications due to their low coefficient of thermal expansion (CTE), good electrical conductivity and lightweight. The microstructure of hypereutectic Al-Si alloy comprises of hard silicon particles dispersed in soft and ductile aluminium matrix. Al-Si composite materials are known for having poor machine ability due to the high tool wear and poor surface finish arising from the hard abrasive particles that act as small cutting surface thus resulting in poor surface finish [9]. These alloys are produced by several commercial casting processes. Conventional casting techniques result in coarse and segregated microstructures in these alloys leading to inferior mechanical and wear properties and hence they cannot be applied in critical service conditions. The microstructural features play a vital role on the mechanical properties of these alloys. These microstructures can be refined or modified by grain refinement and modification. 118

2 The alloy suffers from the defects such as casting porosities, accompanied by low strength, low toughness and poor machinability which is attributed to the presence of large primary silicon particles. Further it is also observed that, the increasing silicon content, also causes inherent brittleness, low formability and catastrophic failure of the components[3].as the properties of a specific Al-Si alloy (hypoeutectic, eutectic or hypereutectic) can be attributed to the individual physical properties of its main phase constituents and to their volume fraction and morphology, different approaches have been used to control the micro structural features of Al-Si alloys such as adding some alloying elements in order to refine the grain size. However, the most common way to improve mechanical properties of cast Al-Si alloys is by changing the casting technology. Each technology has particular aspects that interfere on microstructure and consequently on mechanical properties. Through the Al-Si composition to compare with the two type of casting technology stir and centrifugal casting, there are many parameters in this process which affect the final microstructure and mechanical properties of the composites. These two type of casting comparison is done with the conducting the several mechanical tests such as tensile, bending and impact test. And also we conclude by the SEM and XRD. Each casting technique has its own effect on the microstructure and consequently on the mechanical properties. Efforts have been taken to improve conventional casting techniques. Amongst them, centrifugal casting and stir such technique. The centrifugal cast alloy shows a fine to coarse microstructure from the inner mould surface to the centre [4]. The better microstructure at the outer surface in a centrifugal cast alloy is due to a high rate of heat extraction at mould wall and the metal interface. These castings have a high degree of metallurgical cleanliness and homogeneous microstructures and they do not exhibit anisotropy of mechanical properties evident in rolled / welded or forged parts [5]. Though centrifugal casting refines the microstructure to a certain extent, the results are not much appreciable. The properties of conventionally cast alloys can be improved by secondary processing but increases the cost of production II. EXPERIMENTAL DETAILS 2.1 MELTING AND POURING PROCESS FOR ALLOY PREPARATION The quantity of the alloy to be melted is calculated based on the samples and castings that are being poured. Based on the quantity, the alloy to be added is calculated. Keep all the alloys to be added ready to add to the furnace. The Electrical Resistance Furnace is a 36kW furnace with Kanthal heating elements all around the crucible. The crucible is a Silicon carbide crucible of superior quality and which has high refractoriness and good conductivity. The heating coils surround the crucible. When the furnace is switched ON, current passes in the heating elements and become red hot. This heat is transferred to the metallic charge inside the crucible by conduction. The metal heats inside the crucible and melts after continuous heating. The electrical furnace is switched ON. Calculated quantity of pure aluminium is added to the furnace. Allow the pure aluminium to melt completely. Add the required quantity of alloying elements except Magnesium. Allow it to melt. Continue heating till the metal attains the required pouring temperature. Measure the temperature using a dip type pyrometer. Once the temperature is attained, add the required quantity of pure Magnesium and immediately pour the metal to the required die/ingot mold which is heated to a temperature of C. Once the casting / ingot is cooled to around C, the casting/ingot is removed from the die. This is allowed to cool by keeping it in open air. Once it attains the room temperature, the alloy casting/specimen is taken for further processing like cutting, machining etc to prepare the test samples, prepared alloy elements are given in the table

3 Element Percentage of Compositio n Aluminiu m (Al) Silico n (Si) Fig 2.1 ingots prepared by die casting TABLE 2.1: composition of Al-Si alloy Coppe Ferrou Manganes Nicke Zin r (Cu) s (Fe) e (Mn) l (Ni) c (Zn ) Titaniu m (Ti) Tin (Sn ) Lea d (Pb) Chromiu m (Cr) STIR CASTING In this process 1.5 kg of Al-4928 B alloy was cut into small piece using hydraulic blade and filled in furnace initial temperature of the furnace is 23 0 C and start the furnace to increase the temperature up to C and stirring of the molten composite were accomplished for 10 minutes at 400 rpm and preheat the metal mould up to C to remove the moisture content from the metal mould and finally poured into metallic die at temperature of C.At the end a slab of 190mm 125mm 12mm cast composite of Al-4928 were obtained along with samples required for mechanical testing Fig 2.2 Stir casting product ( ) Fig 2.3 Metallic die The above fig 2.3 is indicate that metallic die which is used for the preparing the stir casting specimen in the size of length of 190 mm and thickens 12mm, width =

4 Fig 2.4 Pouring of molten metal Fig 2.5 String of the molten metal (100 rpm) 2.6 Schematic diagram of the stir casting 2.3 CENTRIFUGAL CASTING In this process, 1.5 kg of Al4928 B alloy was re-melted to a temperature of 800ºC in oil fired furnace and the molten metal was poured from the crucible in to a cast iron mould rotating at 1000 rpm. The mold has been pre-heated to 100 degree Celsius to remove the moisture.the molten metal was centrifugally thrown to the inside mold wall and then allowed to cool to get the required casting of length 106mm& 16mm(wall thickness). Fig.2.7 pouring of the molten metal into centrifugal die (1000RPM) Fig 2.8 Centrifugal casting product (l=106,t=16) 121

5 Fig 2.9 Schematic diagram of the centrifugal casting 2.4 Metallographic Samples were extracted from central regions of both the Al-24Si alloys for microstructural examination. The samples were prepared by polishing using standard metallographic techniques of grinding on water proof emery paper with grit size in the order of 100,500,1000,1500,2000, in specifications. Final polishing was done on a valuate cloth using diamond paste and kerosene. The polished samples were etched with Keller s reagent (1% vol. HF, 1.5% vol. HCl, 2.5%vol. HNO3 and rest water). We prepared for 200ml in which HF= 2ml, HCL=3ml, HNO 3=5ml distilled water=190ml.the microstructures of the sampleswere examined under ZEISS optical microscope. Fig 2.10 Stir cast alloy prepared for microstructure purpose Fig 2.11Centrifugal cast alloy prepared for microstructure purpose 2.5 Tensile testing Tensile samples of 25 mm gauge length and 100 mm gauge length were prepared from both Al 24Si alloys. Samples were extracted from the central regions of the castings. Tensile tests were conducted at room temperature on an INSTRON Universal Testing Machine at a cross speed of 0.05cm/min and a chart speed of 2 cm/min. Three samples have been considered for tensile testing. The average values of tensile strength for both the alloys have been reported in below table 122

6 TABLE 2.2 Types of casting Percentage of elongation Ultimate stress in MPa Ultimate force in N Stir casting Centrifugal casting Fig 2.12 Tensile test specimen (ASTM E10) Fig 2.13 Tensile specimen after conducting test 2.6 Hardness testing Hardness is the resistance of the martial to the indentation, penetration are called as the hardness. Hardness testing of Al 24Si alloys was carried out on Vickers hardness tester. All the samples were polished before conducting the test. A weight of 5 kg was considered for this study. At least five measurements were taken for each sample and the average values of hardness have been reported in Table 2.3 TABLE 2.3 Type of casting Load in kg HV in Kgf/mm2 Stir casting Centrifugal casting Three point bending testing 3 point bending test of 32 mm gauge length and 100 mm length and thickness is 4mm,width is 15 mm specimen were prepared from both Al 24Si alloys. Samples were extracted from the central in the stir casting and inner periphery from the centrifugal castings. Bending tests were conducted at room temperature on an INSTRON Universal Testing Machine at a cross speed of 0.05cm/min and a chart speed of 2 cm/min. Three samples have been considered for bending testing. The average values of bending strength for both the alloys have been reported in table 2.4 TABLE 2.4 Types of casting Elastic modulus in MPa Ultimate stress in MPa Ultimate strain in% Ultimate force in N Stir casting Centrifugal Fig 2.14 Bending specimen with ASTM E190-92Fig 2.15 Bending specimen after conducting test 123

7 2.8 Impact testing (Charpy Impact Test) Impact test is undoubtedly the most commonly used test that is done to characterize the ductile to brittle transition behaviour in materials. The impact test is done by placing a square shaped V-notched specimen in the machine. we prepared, the Charpy specimen has a square cross-section of dimensions L=56mm,t=5mm and contains a 45 0 V notch of 2 mm deep with root radius of 0.25 mm. Impact testing machine used for this experiment contains a heavy swing pendulum. This pendulum has the maximum capability of impacting energy of 264 ft pound force= m 9.8ms Kg = J. A heavy pendulum released from a known height strikes the sample on its downward swing and fractures it. After the test bar is broken, the pendulum rebounds to a height that decreases as the energy absorbed in fracture increases. by using this machine conduct experiment and take average of that the result is shown in the table 2.5 Fig 2.16 impact specimen with ASTM E23 Fig 2.17 Impact specimen after the test Type of casting Energy absorbed by pendulum for frictional resistance in joule Centrifugal Stir Energy absorbed by pendulum for fracture of the of the specimen in joule TABLE 2.5 Net energy (b-a) Impact strength =impact energy/area of cross section Mean value in j/mm III. RESULT AND DISCUSSION 3.1Micro structural features Scanning electron microscopy (SEM) and optical microscope (OM) was used to study the microstructures of Al 24Si alloy for stir cast and centrifugal cast alloys are shown in Fig.3.1 and 3.3 respectively. In microstructure the white region represent Al matrix and the globular shaped particles represent Si. Coarse acicular Si particle are distributed along the primary aluminium boundaries which indicated the non-uniform distribution of the Si particle throughout the aluminium matrix are shown in fig 3.1(a) and primary Si,aluminium dendrites are shown in fig 3.1 (b). In centrifugal cast alloy at the outer periphery we observed that refinement of the microstructure and homogeneous distribution of second Si phase particle and less porosity at the outer periphery shown fig 3.3 (a), and the centrifugal cast specimen has less α (Al) matrix and a less number of eutectic silicon and primary silicon toward the inner periphery shown fig 3.3 (b) 124

8 (a) (b) Fig 3.1: Optical microscopy micrograph of (a) coarse microstructure (b) primary silicon and aluminium dendrites Fig 3.2:SEM micrograph of stir cast alloy (a) (b) Fig 3.3 SEM microstructure (a) inner periphery (b) outer periphery 3.2 Mechanical properties Tensile characteristics The results of tensile testing of Al-24Si alloys were reported in Table 2.2. The results show that the centrifugal cast alloy gave considerable improvement in its tensile strength and % elongation compared to the stir cast alloy. The high strength and ductility observed in centrifugal cast alloy mainly due to the less porosities at the outer periphery, refinement of the microstructure and homogeneous distribution of primary Si phase particle [5].due to which centrifugal cast alloy specimen shows better tensile strength.dendrite structure more present in stir cast alloy specimen compare to centrifugal cast alloy, these dendrite structure reduce the tensile strength and percentage of elongation [4]. 125

9 Fig 3.4: Tensile graph of stress v/s stain From the above graph concluded that, Specimens produced through centrifugal casting process exhibited 20% higher tensile strength compared to the counter parts produced by stir casting Hardness characteristics The result of hardness testing were reported in the table 2.3.In this study,bulk hardness values have been considered because in centrifugal cast alloy it becomes difficult to indent a particular phase.it is seen form the table 2.3 that hardness value of the centrifugal casting is higher than that of the stir casting alloy.the higher the values of hardness in the centrifugal Cast alloy is due to the micro structural refinement owing to the rapid solidification effect [11] and lower values in the stir cast alloy 3, 2.3 Bending characteristics The results of three points bending testing of Al-24Si alloys were reported in Table 2.4.The bending test behaviour of the stir and centrifugal cast alloy specimen was tested as per ASTM E standards. The 3 point bending test is conducted for three specimens extracted from the toward inner periphery of the centrifugal cast alloy and for stir casting extracted from centre. To conducting test the variations in the bending properties for both casting. The graph is plotted with respect to stress v/s Strain is given below Fig 3.5: Graph of stress v/s stain From the above graph we concluded that stir cast specimens displayed 25% higher flexural strength than those produced by centrifugal casting this mainly due to the more porosities and less density at the inner periphery of the centrifugal cast specimens is shown in the figure 3.3 (b) and refinement of the microstructure and homogeneous distribution of primary Si phase is good in stir casting compare to inner periphery of the centrifugal cast alloy specimens. This leads to the stir casting gave better ultimate stress 126

10 3.2.4 Impact characteristics The results of impact testing of Al-24Si alloys were reported in Table 2.5.The impact test behaviour of the stir and centrifugal cast alloy specimen was tested as per ASTM E23 standards. The impact test is conducted for three specimens extracted from the toward inner periphery of the centrifugal cast alloy the and for stir casting specimen extracted from centre. By conducting impact test we concluded that stir casting specimen gave better impact resistance compare to centrifugal casting, this mainly due to the centrifugal cast specimen has less α (Al) matrix and a less number of eutectic silicon and primary silicon toward the centre at the speed range of 1000 to 1300 rpm [12], this cause reduce the impact resistance and hardness XRD RESULT Figure 3.6 shows the analytic results of crystal structure of the sample from the stir and centrifugal cast alloy, the material comprises aluminium, silicon, and silicon dioxide, AlCu,Al 20 3, the analytic results confirmed that formation of the, Al-Cu, this phase help to increase the hardness in both cast alloy and Al 20 3, SiO 2 reinforcement was also formed,these two reinforcement are help to increase the wear and creep resistance [6] Fig 3.6 XRD pattern of the sample from centrifugal and stir alloy IV CONCLUSIONS Outer periphery of centrifugal casting specimens has refined microstructure of Al 24Si alloy. The Si distribution is uniform in Al matrix. The centrifugal cast alloy showed higher strength, ductility and hardness compare to stir cast alloy. This clearly justified the effectiveness of centrifugal casting process. But when considered the inner periphery of centrifugal cast specimen result in less ultimate strength and impact resistance compare to stir casting specimen, we can clearly justify the effectiveness of stir casting process.so we can conclude that best structural components are produced by using the outer periphery of the centrifugal cast alloy. REFERENCES [1] Xiaoyu Huang, Changming Liu, XunjiaLv, Guanghui Liu, Fuqiang Li,Aluminum alloy pistons reinforced with SiC fabricated by centrifugal casting. Accepted 12 April 2011 [2] G. Chirita a, I. Stefanescu b,1, D. Cruz a, D. Soares a, F.S. Silva a,*, Sensitivity of different Al Si alloys to centrifugal casting effect. Accepted 26 December 2009 [3] P. Shailesh1*, S.Sundarrajan2, M.Komaraiah3. Optimization of process parameters of Al-Si alloy by centrifugal casting technique using Taguchi design of experiments. 127

11 [4] G. Chirita, D. Soares, F.S. Silva *.Advantages of the centrifugal casting technique for the production of structural components with Al Si alloys.accepted 12 December 2006 [5] P.R. Gurua, F. Khan MDa, S.K. Panigrahia,, G.D. JanakiRamb. Enhancing strength, ductility and machinability of a Al Si cast alloy by friction stir processing. [6] V.Balajia, N.Sateeshb, M.ManzoorHussainc. Manufacture of Aluminium Metal Matrix Composite (Al7075-SiC) by Stir Casting Technique. [7] A.G. Raoa,b,, V.P. Deshmukha, N. Prabhub, B.P. Kashyapb. Enhancing the machinability of hypereutectic Al-30Si alloy by friction stir processing, Accepted 13 June 2016 [8] Journal of Composite Materials : 1021 originally published online 15 August 2011, K Wang, JF Cheng, WJ Sun and HS Xue.An approach for increase of reinforcement content in particle rich zone of centrifugally cast SiCP/Al composits [9] A.G.Raoa,b,n, V.P.Deshmukh a, N.Prabhu b, B.P.Kashyap b. Ductilizingofabrittleas-casthypereutecticAl Si alloybyfrictionstir processing. [10]K. Raju1, A.P. Harsha1 and S.N. Ojha2 Effect of processing techniques on the mechanical and wearproperties of Al-20Si alloy [11] Archard J F and Hirst W, in ProcR SocLondSer A, 257 ( ) 51. [12] Zhao CHEN1,a, Effect of Centrifugal Casting on Microstructures and Properties of Hypereutectic Al-18wt.%Si Alloy 128