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Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
Understanding the melt flow behaviour of za alloys processed through centrifugal casting
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Understanding the melt flow behaviour of za alloys processed through centrifugal casting

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  • 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),INTERNATIONAL JOURNAL OFFebruary (2013) © IAEME ENGINEERING ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - MECHANICAL AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online)Volume 4, Issue 1, January- February (2013), pp. 163-171 IJMET© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2012): 3.8071 (Calculated by GISI)www.jifactor.com ©IAEME UNDERSTANDING THE MELT FLOW BEHAVIOUR OF ZA ALLOYS PROCESSED THROUGH CENTRIFUGAL CASTING Jyothi P.N1, A. Shailesh Rao1, M.C. Jagath2, K. Channakeshavalu3 1 K.S. School of Engineering and Management, Department of Mechanical Engineering Bangalore-062, Karnataka, India 2 Bangalore Institute of Technology, Department of Industrial Engineering and Management, Bangalore-004, Karnataka, India 3 East West Institute of Technology, Principal and Director, Bangalore-091 Karnataka, India ABSTRACT: Centrifugal casting is a process of producing casting by causing molten metal to solidify in rotating moulds. The study of melt flow in centrifugal casting is much more important as it determines the quality and properties of the final product. Understanding the flow of liquid metals in centrifugal casting is much more complex as there is a drop in temperature during the flow of molten metal. In the present work, ZA alloys (i.e. ZA 8, ZA 12, and ZA 27) with 6mm cast tube are prepared at 400,600 and 800 rotational speed of the mould. It was found that, for ZA8 and Z12 alloys, a uniform cast tube was observed for 600rpm, whereas for ZA 27 a uniform cast tube was not formed for various rotational speed of the mould due to the increased composition of aluminum. The observation made in the behaviour of molten metal during various rotational speeds of the moulds is explained. To know about mechanical properties of the alloys, the microstructure and hardness are discussed finally. KEYWORDS: Centrifugal Casting, ZA alloys, Microstructure, Hardness. 163
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEINTRODUCTION Centrifugal casting is a material processing technique in which the flow pattern of themolten metal during casting strongly affects the quality of the final product. Literature aboutfluid flow in centrifugal casting is very sparse. Theoretically, it should be possible to producea true cylinder even when the mould is rotated at low speeds. But practically, the moltenmetal has to be accelerated to a certain speed to form a uniform hollow cylinder. Dependingupon the conditions of the molten metal, there must be an optimum spinning speed, at which,the molten metal will be picked up to form a true cylinder. Jaluria [1] discussed theimportance of fluid flow in material processing. He points out several aspects of fluid flowwhich changes the properties in various processing techniques. Most of the studies of liquidmetal behavior are done on continuous casting, where cold modeling experiments werecompared with the final castings [2-6]. Janco [7] indicates several important parametersinvolved during the centrifugal casting process. He explains the design of gating, importanceof rotational speed, mold dimensions, etc. But he has not done much to explain theimportance of molten metal behavior during the process. Ping [8] has reported that nosystematic investigation of microstructure evolution in centrifugal casting has been done,although this information is important to know the mechanical properties of the material.Chang [9] studied the influence of process parameters on the microstructure formation invertical centrifugal casting, but not the effect of liquid metal during casting. From theexperiments, Shailesh [10] explained the optimum rotational speed for centrifugal casting ofaluminum silicon alloys, for a given diameter of the mould. Below, this speed, Couette,Taylor and Ekmann flows are seen in the final cast tube. The mechanical properties areevaluated to substantiate optimum rotational speed of the mould. Even though some attempts are tried to understand the nature of melt flow inaluminum silicon alloys [11], the comparison of various composition of alloy, inunderstanding the fluid behavior is not understood. To understand the nature of melt flow,Zinc based aluminum alloys commonly referred as ZA alloys are taken for our investigationas it is now increasing in various commercial usages. Moreover, they have better sliding,wear resistance, machinability and excellent corrosion resistance in various environments[12-15]. In the present work, an effort has been made to develop ZA alloys (i.e. ZA8, ZA12,and ZA27) through centrifugal casting process, at various rotational speeds (i.e.400, 600 and800 rpm) of the mould. The cast tube of a true cylinder had a dimension of φ 80x120mm and6mm thick. It is explained from the literature, that addition of aluminum into molten zincalloy improves the fluidity and cast ability under continuous casting [13]. In the case ofcentrifugal casting, it is understood that with the increase in the Aluminum content from 12%to 27% in zinc based alloy, fluidity decreases and do not form a true uniform cylinder underthe various rotational speed of the mould. The behaviour of the liquid metal for all the casttube is explained in this paper. The microstructure and micro hardness of the cast tube isfound out and explained finally.2. EXPERIMENTAL DETAILS The experimental alloys were prepared as per ASTM B86-11 Standard for Zinc-Aluminum (ZA) Alloy Foundry, by liquid metallurgy route. The alloy was melted and 200OCas super heat was maintained as teeming temperature for all the cast tubes. Horizontal type 164
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEcentrifugal casting machine as shown in Fig [1] is employed to cast ZA alloys samples in thisinvestigation. It is driven by a 2 HP DC motor for varying speed from 20 to 2000rpm. Fig. 1 Centrifugal Casting Machine Set upFor determining the microstructure, the samples were cut from the casting and werecharacterized using optical microscope. The samples were polished metallographically andetched suitably prior to their micro structural examination. Microstructures at the threeregions (inner, middle and outer surface of the specimen) were examined to know changes inthe mechanical properties along the radial direction of the cast tube. Finally, the Microhardness for the samples at three regions were determined and explained.3. RESULTS AND DISCUSSIONS3.1 APPEARANCE OF THE CAST TUBE The ZA8 alloy heated in a furnace to form 6 mm thick cast tube is poured into rotatingmould. An irregular pattern of the casting is formed at 400 rpm as shown in Fig. [2, a]. Themolten metal, which is poured, leads to its stickiness at particular locations on the innersurface of the mould due to the lifting of the melt. Some quantity of molten metal is alsofound to move in the axial direction and a lump of mass settles in on one side at lowrotational speed of the mould. Similar observations are also found in ZA12 alloy as seen fromfigure. With ZA27 alloy, an irregular pattern is seen in the final casting as observed from thefigure. This is possibly due to the quick lifting of the liquid metal and hence limiting its axialmovements. Further increase in the rotational speed of the mould to 600 rpm, a uniformly thick fullcylinder is observed at 600 rpm with ZA8 and ZA12 alloy. This is possibly due to the moltenmetal getting along the circumference of the inner mould and thus avoiding other types offlows (Couette, Taylor Ekmann flows). The driving force which is acting on the molten metalshould be sufficient, so that it is carried along the inner surface of the mould before it getssolidified. In case of ZA 27, the mould enables the rapid movement of the liquid metal incircumferential direction resulting in the formation of non-uniform thickness of the castingFig [2, b]. Further increase in rotational speed to 800 rpm, enables the molten metal of all thealloys to move along the inner surface of the mould due to larger driving force. Only smallamount of metal succeeds in moving along the axis getting solidified during its motion.Finally the casting which is formed has a varying thickness on its side Fig [2, c]. 165
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME From the above, it is understood that a uniform cast tube is formed at a rotationalspeed of 600rpm for ZA8 and ZA12 alloy for a given dimension of mould. Moreover,uniform cylinder is not formed in ZA27 alloy for different rotational speeds of the mould. Viscosity of the melt finds a major role in the formation of uniformly thick cylindercastings. A full cylinder is formed with irregular patterns inside the cast tube when rotated atspeed of 400rpm. Since the viscosity and the driving force of the melt is low, it moves axiallyand obtains a lift from the inner walls of the mould during teeming process. The meltconsumes more time for completing solidification which leads to formation of lumps in thefinal casting. Further with gradual increase in the magnitude of viscosity, the melt moveseasily along the circumference of the mould and impression of the bands is formed in thefinal casting. The solidification rate and driving force of the melt finds more dependent on theviscosity and rotational speed of the mould. The melt must rotate at larger rotational speedafter pouring, since it has low viscosity during teeming process. A uniform full cylinder isobserved, when the mould is rotated at a speed of 600 rpm. Further increase in the rotationalspeed to 800rpm, the driving force plays a predominant role; it guides the molten metal tomove along the circumference rather than moving along the axis. Finally, an irregular casttube is formed.3.2 MICROSTRUCTURES OF THE CAST TUBE The results of metallographic investigations of ZA alloys during centrifugal castingprocess are presented in Figure [3-5]. The microstructure of ZA8 alloy for various rotationalspeed of the mould is shown in Fig. [3].The structure is typically dendritic at the inner,middle and outer surface of the cast tube. Dendrites formed here are of complex shape and thegrowth of dendrite is moved from outer to inner surface of the mould. Here the liquid metalfinds difficult to move along the circumference of the mould during teeming. It probablyoscillates along the axis and when it becomes viscous, it moves along the circumference ofthe mould. This could be imagined and observed from the final cast tube. The process herehas a lower solidification rate of the liquid metal in cast tube and finally improper structuresare seen in the cast tube. Transformation of the dendritic structure into a fine structure is seen,when the rotational speed of the mould is increased to 600rpm.The solidification rate here iscomparatively more since the liquid metal moved along the axis and simultaneously risesalong the circumference of the mould forming a good cast tube. Increase in rotational speed to800rpm, the liquid metal moves along the circumference of the mould after teeming into it.The driving force is too high, so that the melt has limited axial movement. Since the lump ofmelt is accumulated in one portion of mould, the solidification is low and hence dendritestructure is formed at the middle and inner surface of the mould. Due to sudden quenching ofmelt into mould during pouring, fine structures are seen at the outer surface of the mould.Similar observations are also seen for ZA12 alloy Fig [4]. Here the solidification of meltbegins with the formation of aluminum rich dendrites. Further increase in aluminum forZA27 alloy, a poor microstructure is observed in all the cast tube. For 400rpm, the liquidmetal moves along the axial hitting the other side of the mould. It reverses back and tries tooscillate along the axis of the mould. Meanwhile the melt becomes viscous and then movesalong the circumference of the mould. Increase in rotational speed to 600 and 800rpm, themelt moves along the circumference of the mould. This is due to high centrifugal force andviscosity. The microstructure shown in the Fig [5] revealed the above explanation. A richprimary aluminum is seen for the cast tube rotated at 400rpm. Since the aluminum restricts 166
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEfluidity, a coarse grain structure is seen in the metal rotated at higher rotational speed of themould. L R (A) 400 RPM (B) 600 RPM (C) 800 RPMFig 2: ZA8, ZA12 and ZA27 alloys (L R) for Various Rotational Speeds (6mm Thick) Outer Middle Inner Fig 3: Microstructure of ZA8 alloy for 400rpm, 600rpm and 800 rpm (Top to bottom) with Magnification of 400µ 167
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME Outer Middle Inner Fig 4: Microstructure of ZA12 alloy for 400rpm, 600rpm and 800rpm (Top to bottom) with Magnification of 400µm 168
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME Outer Middle Inner Fig 5: Microstructure of ZA27 alloy for 400rpm, 600rpm and 800rpm (Top to bottom) with Magnification of 400µm.MICRO HARDNESS OF THE CAST TUBE Micro Hardness measurements of the test sample are made on Vickers Hardness testerof Model MMT-X7A with applied load of 1 kg, according to the standard testing protocols.The hardness is carried out on a piece cut radially about 10mm square. Since the sample isthin, the curvature is marginal and it is easily pressed flat. Averages of three readings aretaken as the hardness value for a given specimen. The hardness values on the outer, middle and inner surface of the samples as afunction of rotational speed is shown in Fig. [6]. Hardness of centrifugally cast specimendepends on flowability and melt filling Behaviour, which can be known by flow length andwall thickness. In the present work hardness of ZA8, ZA12, and ZA27 centrifugally cast tubeat different rotational speeds is found out. It is found that at 400 rpm and 800 rpm, hardness isnot uniform across the cross section of the cast tube. But at 600rpm uniform hardness at allthe three layers is recorded, as the flow of melt in the rotating mould is uniform. Moreover, atlower rotational speed of the mould, there will be no uniformity of melt over the inner surfaceof the mould. Due to this, the solidification rate of the cast tube is less and hence lowerhardness values are observed in the final cast tube. In the case of higher rotational speed, themould itself lifts the liquid metal over its inner surface making limited movement in axial 169
  • 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEdirection. Again, here, lower hardness values are found out due to lower solidification rate ofthe cast tube. For ZA27 alloy for three rotational speeds of the mould, hardness is varyingacross the section of the cast tube, as the flow of melt in to the mould is not uniform. This isdue to increase of Al content in the melt; which reduces fluidity of the molten metal formingirregular cast tube. (a) (b) (c) Fig 6: Vickers Hardness Value versus Rotational speed of the mould for (a) ZA 8 alloy (b) ZA 12 alloy and (c) ZA 27 alloy 170
  • 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMECONCLUSION ZA alloys are cast centrifugally with varying rotational speed of the mould. Thefollowing are the conclusions remarks from the experiments.1) A uniform cast tube was formed for ZA8, ZA12 alloy due to lower content of aluminum in the metal. When the aluminum content is increased, say ZA27 alloy a uniform cast tube was not formed.2) All the cast tube formed must be substantial with mechanical properties. A fine microstructure was formed for uniform cast tube (i.e. both ZA8, ZA12 alloy). With aluminum rich ZA27 alloy, a coarse grain structure was formed.3) Hardness test for all the cast tube showed an increased value for uniform cylinder. The hardness value is varied across the section for ZA27 alloy.ACKNOWLEDGEMENTThe authors acknowledge Management, Principal, Staff and Non-teaching Staff of K.S.Schoolof Engineering and Management, and NITTE Meenakshi Institute of technology, Bangalorefor their kind support for this project.REFERENCES[1] ogesh Jaluria, J. Fluid Engineering, 123, pp.173-210 (2001).[2] Z Zipnicki “Dynamical Solidification of metal filling a Cooled cylindrical channel”,Int. Comm. Heat and Mass Transfer, Vol 27, No5, pp. 689,(2000).[3] Wei Shyy, “Multi Scale computational heat transfer with moving solidification Boundaries”, Int. Journal of Heat and Fluid Flow, Vol 23, pp.278.( 2002).[4] S M H Mirbagheri, H Esmaeileian, S Serajzadeh, “Simulation of melt flow in coated mould cavity in the casting process”, Journal of Material Processing Technology, Vol 143, pp.493,( 2003).[5] Tatakani H, “EBSD characterization and modeling of columnar dendrite grains growing in the presence of fluid flow”, Acta Metallica”, vol 48, pp.675,( 2000).[6] S M H Mirbagheri, M Dadashzadeh, S Serajzadeh “Modeling the effect of mould wall roughness on the melt Flow simulation in casting process”,Applied Mathematical Modeling, Vol 28, pp.993,(2004).[7] Nathan Janco, Centrifugal casting, Schaumburg,IL: Americal Foundrymen’s Society, (1988).[8] Wu Shi Ping, LD Rong, GJ Jie, LC Yun, SY Qing, , “Numerical simulation of microstructure evolution of Ti-6Al-4V alloy in vertical centrifugal casting”, Material Science and Engineering A, vol 426, pp. 240,(2006).[9] S R Chang, JM Kim, CP Hong,“Numerical Simulation of microstructure evolution of Al alloys in centrifugal casting”, ISIJ International, Vol 41, No 7, pp.738, (2001).[10] Shailesh Rao A, P G Mukunda, Shrikantha S Rao,“Influence of Optimal Speed for Sound Centrifugal Casting of Al-12Si Alloys”, Journal of Materials. Vol 63, No.5, pp.25-29, (May 2011) 171
  • 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME[11] K. Keerthi Prasad, M. Murali, P.Mukunda,,“Analysis of fluid flow in centrifugal casting” Frontiers of Materials Science in China, Vol. 4, No. 1. Pp.103-110, (March 2010).[12] Yuanyuan Li,Junming Luo,Zongqiang Luo,Zhiyu Xiao, T. Leo Ngai,, “The microstructure and wear mechanism of a novel high-strength, wear-resistant Zinc alloy (ZMJ)”, Elsevier’s Journal of Materials Processing Technology Volume 55, Issues 3–4, pp154–161,(December 1995).[13] BALALAN and Mehmet KAPLAN,.”Investigation of the Microstructure and Wear Properties of a Cast ZA Alloy” International Journal of Science & Technology Volume 2, No 1, pp 75-81, (2007)[14] Pritha Choudhury, Karabi Das Siddhartha Das, “Evolution of as-cast and heat-treated microstructure of a Commercial bearing alloy”, Elsevier’s Materials Science and Engineering: A Volume 398, Issues 1–2, , pp 332–343, (May 2005).[15] B.K.Prasad, O.P. Modi, and H.K. Khaira1,“High-stress abrasive wear behaviour of a zinc-based alloy and its composite Compared with a cast iron under varying track radius and load conditions”, Elsevier’s Materials Science and Engineering: A Volume 381, Issues 1–2, pp 343–354, (September 2004).[16] Manjunatha L.H., P.Dinesh, “Development and Study On Microstructure, Hardness And Wear Properties Of As Cast, Heat Treated And Extruded Cnt- Reinforced With 6061al Metal Matrix Composites” International Journal of Mechanical Engineering & Technology (IJMET) Volume 3, Issue 3, 2012, pp. 583 - 598, Published by IAEME. 172

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