Theoretical Analysis Of Coefficient Of Thermal Expansion By Thermo Mechanical Analyzer Of LM13/Mgo Composites

Theoretical Analysis Of Coefficient Of Thermal Expansion By Thermo Mechanical Analyzer Of LM13/Mgo Composites

Ananda.G. K, Ramesha. H

Abstract: This paper describes a study of the thermal characteristics of LM13 Al based MMCs with vari- ous amounts of magnesium oxide (MgO) particulates.MMCs are having some excellent properties such as high hardness, high strength, low thermal expansion & high thermal conductivity at elevated tempera- tures.In the present investigation, the effects of thermal expansion & thermal conductivity of LM13/MgOp MMCs.The as-cast and heat treated LM13 alloy were fabricated using stir-casting method by varying the reinforcement. The specimens were prepared as per ASTME standards. These composites may be the alternate materials for aero-plane components, automobile parts like piston rod, connecting rod & cylinder head.

Key words: LM13-Light Metals, MgOP-Magnesium oxide particulate, MMCs- Metal Matrix Composites, ASTME-American Society Tools & Manufacturing Engineering.

1. INTRODUCTION

   The innovative materials open up unlimited possibilities for modern material science and development, the characteristics of MMCs can be designed into the materia, custom-made, dependent on the application. From this po- tential, MMCs fulfill all the desired concep- tions of the designer. This material group be- comes interesting for use as constructional and functional materials, if the property profile of conventional materials either does not reach the increased standards of specific demands. However, the technology of MMCs is in com- petition with other modern material technolo- gies, for example powder metallurgy. The ad- vantages of the composites materials are only realized when there is a reasonable cost- performance relationship in the component production. The use of a composite material is obligatory if a special property profile can on- ly be achieved by application of these mate- rials. MMCs are engineered combinations of two or more materials where tailored proper- ties are achieved by systematic combinations of different constituents. The nature and mor- phology of the composites are mainly depends on the shape & size of individual constituents, their structural arrangement & distribution, the relative amount of each contribute to the overall performance of the composites. The properties such as friction & wear resistance in lubrication

   condition are importance for the development of engine parts.Al alloy found widespread in transportation engineering applications be-

   cause of its high strength to weight ratio. Al based MMCs refer to the class of light weight Metals with high performance Al centric ma- terial systems. The reinforcement on Al MMCs could be in the form particulate, whiskers, continuous and discontinuous fibers in weight fraction ranging from 15% up to 70%. Properties of Al based MMCs can be tai- lored to the demands of different industrial applications of matrix, reinforcement and processing route.

2. LITERATURE REVIEW

   Nowdays, the world wide upsurge in MMCs research and development activities is focus- ing mainly on Al alloys. These Al Matrix Composites are unique combination of prop- erties like good wear resistances, lowdensity, & excellent mechanical properties.The unique thermal properties of Al Matrix Composites such as High metallic conductivity with lower coefficient of thermal expansion which can be tailored down to zero and its use in aerospace & aviniocs applications.The LM13/graphite, LM13/zricroium & LM13/quartz were re- spectively studied by A.K Gupta,Norman Tommis,& Joel Hemanth.They were explore the properties of LM13 Alloys and its compo- sites.

3. EXPERIMENTAL WORK

   The Matrix Material selected was LM13 and reinforcement material was MgOp and par- ticles were dispersed in the matrix material and chemical composition as shown in below Table1 . The size of the MgO particulates dis-

   persed varies from 5 to 8 µm.

   Elements	Al	Si	Mg	Mn	Ti	Cu	Fe
   %Wt	Bal	12	1.2	0.8	0.02	0.9	0.8

   Table1 Chemical Composition of LM13 alloy

   The MgO particulates were kindly provided by the chemical laboratory, Bangalore with an average particle size of 6 µm. the properties of MgO/LM13 was fabricated by stirrer die cast- ing method with maximum 10% weight frac- tion by step of 2%. A perform comprising MgO particulates was made firstly. Then the perform preheated MgO particulates on the crucible surfaces up to 4000c and molten metal of LM13 composites melted at 8000c then stir- rer is followed by 10-15 mins.in order to prop- er mixing of MgO particulates to the hot mol- ten metal LM13 alloy composites, So molten metal pouring to the molten dies, then fol- lowed by the solidification for few hrs.The CTE measurement and thermal cycling test were performed by thermal expansion mea- surement equipment(Model TMA-400, ASTME831-04) Thermo Mechanical Analyzer. This test was carried out in central power re- search institute, Bangalore. The specimen standard dimensions are 8mm &12mm lengths are respectively. All the specimens were tested from 500c to 5000c at 50c/mins temperature rise & supply nitrogen inert gas 50ml/mins for heated specimens instantly. The data were obtained in the form of PLC(percent linear change) vs temperature rise. The CTE of the composites testes & de- termined CTE value at every interval of 1000c.

   • Ananda.G.K is currently pursuing masters degree program in Thermal power engineering in Sri Siddartha higher acad- emy of education under UGC, India, PH-8050033180.E- mail: ananda_gk@rocketmail.com

   • Ramesha.H is currently pursuing master degree program in thermal power engineering in Siddartha higher academic

   education under UGC, india,, Ph-.9845718319, E-mail: hmtramesh@gmail.com

4. RESULT & DISCUSSION

   1. Composite characterizations

      Fig.1 Microstructure of polished LM13/MgO com- posite

      The representative SEM Micrograph of po- lished section of LM13/MgO particulates composites are as shown in below fig.1. It can be found that MgO particle cluster- ing.However,some coarse particles were ob- served. Due to the high temperature em- ployed during fabrication, the molten metal Al filled in the particle perform completely. So the composite material appeared to be free of porosity and macroscopically homogeneous.

   2. Coefficient of thermal expansion

      The thermal expansion results with the varia- tion of temperature for the composites & ma- trix are shown in below fig.2. Its obvious that the CTE of the composites was only a half of that of matrix due to the addition of rein- forcement with an increase in temperature. The LM13/MgO particulate composite is a two phase system composed of continuous LM13 Matrix & isolated MgO particulate each having different mechanical and thermal properties. So the thermal expansion beha- viour of the composites is the results of the interaction between LM13 Matrix and MgO particlate through interfaces. Because of the tightened restriction of the particles, the CTE of the composites is much lower that of Ma- trix. The Matrix expands with increasing tem-

      perature on the other hand; the rise of temper- ature would reduce the transferring abilities of interface which makes the constraints of par- ticles weaken. In the end, the CTE of the com- posites increases as the temperature goes up.

   3. Comparsion of CTE among experimental & theoretical results

      In order to understand the thermal expansion behaviour of the composite well, its important to compare quantitatively theoretical predic- tions with experimental results. But the CTE of Metal Matrix Composites is relatively difficult to the predict precisely because several factors. Such as volume fraction, morphology & dis- tribution of the reinforcement, Matrix plastici- ty, interfacial bondage & internal structure of the composites may influence the results. In the case of particulate composites many re- searchers have given expressions for the CTE of composites. Among these expressions, the typical ones are rule of Mixtures (ROM), Turners model & experimental work.

      1. According to ROM, the CTE of the Com- posites is expressed as below

         c= mvm+ p vp

      2. Turners model

         The turners model is based on the fact that only uniform hydrostatic stress exists in the phase & the CTE of a composites can be given below are

         4.3.4 Physical properties of matrix & reinforced phasse

         The numerical values of parameters needed in the computation of the CTE composite are ex- tracted from relative references and summa- rized in the below Table.2.

         Physical property	LM13 Alloy	MgOp
         Density (kg/m3)	2.71	3.21
         Thermal conductivity (w/m0c)	141-167	38-45
         Thermal expansion(µ/0c)	22.5	11.5
         Young modulus(Gpa)	72	249
         Bulk modulus (GPa)	68	165

         24

         CTE (10-6/oc)

         CTE (10-6/oc)

         22

         Turners model ROM Model

         c m.VmKm pVpKp 20

         VmKm VpKp

         Experimental

         K is the bulk modulus, which is calculated by the standard relationship is given by

         K E

         3(3 E )

         G

         4.3.3 Experimental work

         According to the thermal stress induced in the CTE of the composite bar is expressed as given below

         L .T Lo

         0 200 400 600

         Tempr. (oc)

         Fig.2.Comparison of CTEs among Experimental & Theoretical (ROM & Turners Model) results for MgO/LM13 Composites.

         A comparison of the experimental and theo- retical CTE values is presented in Fig.2. The experimental CTEs for entire higher tempera- ture range show significantly deviation from ROM and Turners model. This is not surpris- ing because Turners model is based on the fact that only uniform hydrostatic stresses ex- ist in the phases, while the stresses inside the composite are very complex. So Turners model does not describe the actual stress state

         in the composite. The result of the finite ele- ment method (FEM) indicated that particles with higher aspect ratio would give stronger constraint on the matrix .Therefore ,the predic- tions by Turners model is a better fit at high temperatures than those of other models. This may related to the facts that it takes into ac- count both the normal and shear stress in the matrix.

         1. Thermal cycling behvoiur

            The thermal strain response of the composite during cycling between 1000c to 6000c is shown in the Fig.2. The distinct features can be found from the Fig.2 as mentioned below

            0.006

            Therml Striain

            Therml Striain

            0.004

            Heating Curve Cooling Curve

            0.002

            perature, the tensile stress is progressively re- lived. After complete relief of the tensile stress, a compressive stress starts to build up, oppos- ing further expansion of the composite. This leads to a change in the heating curve, result- ing in the existence of inflexion. However, be- cause of the visco-plastic relaxation at the rela- tively high temperature, the build-up of a large compressive stress can be prevented. Upon cooling, the compressive stress, which assists in contraction, is relived quickly. Con- sequently, a tensile stress was engendered. When the tensile stress builds up sufficiently, tensile yielding occurs. So, the combined op- eration of the tensile and compressive stresses prevents superposition of the cycling curves, resulting in the observed strain hysteresis, and the effect of plastic and visco-plastic deforma- tion is a tensile plastic strain at the end of the cycle.

         2. UNITS FOR PHYSICAL QUANTITIES

         Symbol	Quantity		Units
         	Thermal expansion of matrix,composite,particulate		1/0c
         µ	Micron		10- 6mts
         V	Volume fraction of trix,composite, Particulate.	ma-	m3
         c,m,p	Composite,matrix,particulate		..
         T	Temperature		0c
         L0	Original length		mts
         L	Change in length		mts
         	Thermal strain		—-
         K	Bulks modulus		Gpa
         E	Young modulus		Gpa
         G	Modulus of rigidity		Gpa
         T	Change in tempaerature		0c

         Symbol	Quantity		Units
         	Thermal expansion of matrix,composite,particulate		1/0c
         µ	Micron		10- 6mts
         V	Volume fraction of trix,composite, Particulate.	ma-	m3
         c,m,p	Composite,matrix,particulate		..
         T	Temperature		0c
         L0	Original length		mts
         L	Change in length		mts
         	Thermal strain		—-
         K	Bulks modulus		Gpa
         E	Young modulus		Gpa
         G	Modulus of rigidity		Gpa
         T	Change in tempaerature		0c

         Table 3

         0 200 400 600

         Tempr. (oc)

         Fig.3.Thermal Strain response of the MgO/LM13 Composite during Cycling

         1. The existence of a distinct strain hysteresis can be found in the composite and the compo- site exhibits a large residual plastic strain after the cycle.

         2. There is a distinct inflexion in the heating curve of the composite. The strain hysteresis in the composites can be attributed to the stress state of the matrix. On cooling from the fabrication temperature, a tensile stresses ex- ists in the matrix before the cycle because of the contraction mismatch between the matrix and reinforcement.During heating, the tensile stress present in the matrix helps to the expan- sion of the composites. With increasing tem-

5. CONCLUSIONS

1. The CTE of LM13/MgO Composites is only a half of that of its Matrix.

2. The CTEs of LM13/MgO Composites lie

   within the elastic bounds derived by Turners

   Model Analysis. At higher temperature, the CTEs agree well with Turners Model, while the CTEs at elevated temperature are in the better agreement with the values predicted by (ROM) Rule of Mixtures.

3. The composite exhibited strain hysteresis during cycling, which was attributed primarily to the anelastic behaviour of the matrix in- duced by thermal stresses, and the residual plastic strain after the cycle resulted from the combined effect of plastic & visco-plastic de- formation.

   ACKNOWLEDGMENT

   The authors grateful thanks to the support of this work by Central Power Research Institute, and Indian Institute of Science & Management

   ,Bangalore.

   REFERENCES

   1. William.D.Callister,Material Science and Engineering an Introduction, Wiley and sons publication,7thedition,(2007).

   2. Evans, A.G., Lightweight materials and structures, MRS Bulletin, 26, No. 10, 790, 2001.

   3. Allison, J.E. and Cole, G.S., Metal-matrix composites in the automotive industry: oppor- tunities and challenges, J. Metals, 45, (1), 19, 1993.

   4. Lloyd, D.J., Particle reinforced aluminium and magnesium metal matrix composites, Int. Mater. Rev., 39, 1, 1994.

   5. T. W. Clyne, P. J. Withers, An Introduction to Metal Matrix Composites, Cambridge Uni- versity Press, Cambridge (1993). [6]R.Balasubramanian,callisters by materials science &engineering department of mate- rials and metallurgical engineering, indian in- stitute of technology, kanpur. copyright@2010 by weily india pv.ltd, new delhi. 4435-36/7.

1. K.srinivasan, composite materials for production, properties, testing & applications department of materials & metallurgical engi- neering, nit. copyright@2009 narosa publish- ing house pvt.ltd

2. Kauffman, chactersticization of composite materials wiely publications 2003 department of materials & metallurgical engineering ISBN 0471268828, pp. 337-365.

[10] I.j.polymear, light aluminum alloy book fourth edition department of materials engi- neering, Monash University, copy- right@2006,iccn-200-5932508, pp.205-235. [11]Qiang,Zhang,Study on the Thermal ex- pansion and Thermal cycling of AINp/Al composite school of material science and en- gineering technology, harbin, April 2001 pp.63-66