INFLUENCE OF Ti, B AND Sr ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A356 ALLOY

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1 International Journal of Mechanical Engineering and Technology (IJMET), ISSN (Print) ISSN (Online) Volume 1 Number 1, July - Aug (2010), pp IAEME, IJMET I A E M E INFLUENCE OF Ti, B AND Sr ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES ABSTRACT OF A356 ALLOY D. G. Mallapur Department of Metallurgical and Materials Engineering National Institute of Technology, Surathkal Karnataka, drmallapur@yahoo.com K. Rajendra Udupa Department of Metallurgical and Materials Engineering National Institute of Technology, Karnataka S. A. Kori Department of Mechanical Engineering Basaveshwar Engineering College Bagalkot, Karnataka, India. In the present investigation, the microstructural and mechanical properties study of A356 alloy, have been discussed. The microstructural aspect of cast A356 alloy employed in the present study is strongly dependent on the grain refinement (Ti and B) and modification (Sr). The mechanical properties such as PS, UTS, %E, %R, Young s modulus and VHN have been investigated. This paper deals with the combined effect of grain refinement and modification, which improves the overall mechanical properties of the alloy. The quality of castings and their properties can be achieved by refining of α-al dendrites in A356 alloy by means of the addition of elements such as Ti and B which reduces the size of α-al dendrites, which otherwise solidifies with coarse columnar α-al dendritic structure. In addition modification is normally adopted to achieve improved mechanical properties. Metallographic studies reveal that the structure changes from coarse columnar dendrites to fine equiaxed ones on the addition of grain refiner and further, plate like eutectic silicon to fine particles on addition of 0.20% of Al-10Sr modifier. The present result shows that a reduction in the size of α-al dendrites, 49

2 modification of eutectic Si and improvement in the mechanical properties were observed with the addition of grain refiner Al-3Ti, Al-3B and modifier Al-10Sr either individual addition or in combination. Keywords: A356 alloy, grain refinement, modification, mechanical properties, microstructure. 1. INTRODUCTION Aluminium silicon casting alloys are essential to the automotive, aerospace and engineering sectors. Al-Si alloys allow complex shapes to be cast; however the silicon forms brittle needle-like particles that reduce impact strength in cast structures. As an additive to Al-Si casting alloys, strontium improves strength, enhances mechanical properties and disperses porosity as it modifies the eutectic structure. The modified alloy displays a finer, less needle-like microstructure. Al Si alloys, which comprise 85% to 90% of the total aluminum-cast parts produced, exhibit excellent cast ability, mechanical and physical properties Ejiofor J.U et al, The microstructure and alloy constituents are necessitated to achieve optimum mechanical properties. A356 aluminum alloys are mostly used for cast hypoeutectic Al-Si alloys to improve flow ability of the melt and interfacial properties Caceres C.H et al, 1996 and Jeng S.C et al, However, eutectic Si particles present along solidification cells of the A356 aluminum alloy deteriorate strength, ductility, and fracture toughness, and thus researches to develop processes for enhanced distribution of eutectic Si particles have been actively pursued Benzerga A.A et al, 2001 and Gokhale A.M., A356 alloy belongs to group of hypoeutectic Al-Si alloys and has a wide field of application in the automotive and avionics industries. It is used in the heat-treated condition in which an optimal ratio of physical and mechanical properties is obtained Conley J. G et al, The alloy solidifies in a broad temperature interval (43 0 C) and is amenable to treatment in semi solid state as well as castings Pacs M et al, For this reason it is the subject of rheological investigations Yang X et al, 2002, as well as methods of treatment in the semi solid state Pacs M et al, 1963 and Freitas de E.D et al, By these methods it is possible to obtain castings with reduced porosity of a non-dendritic structure and with good mechanical properties. Besides this A356 alloy is used as a matrix for obtaining composites Natarajan N et al, 2006, which have an enhanced wear resistance, favourable mechanical properties at room temperature 50

3 and enhanced mechanical properties at elevated temperatures. Cast aluminium A356 alloy is one of the most well developed aluminium alloys due to its outstanding properties. It is widely employed in numerous automotive and industrial weight sensitive applications, such as aeronautics and space flight. A356 alloy contains about 50 vol. % eutectic phases and finds wide application in the marine, electrical, automobile and aircraft industries. It is well known that on solidification of hypoeutectic Al-Si alloys the primary α-al solidifies with coarse columnar or twinned columnar Murty B.S, et al, Actually, in most cases high-level mechanical properties are needed for industrial applications, so the performance of this alloy has been the subject of many micromechanical investigations Gokhale A.M., 2005, L opez, V.H et al, 2003, Yu Y.B et al 1999 and Yang Z et al, Since the strength and hardness of alloys mainly depend on their microstructure, a lot of efforts have been made to refine the microstructure of the castings in order to enhance the mechanical properties of aluminium-a356 alloy. Adding modifier and refiner Wang J et al, 2003 and Liao, H.C et al 2002 to the melt is a common way of doing this, and has been adopted by many researchers. The microstructure of alloy A356 comprises of an aluminium matrix, which is strengthened by MgSi precipitates and, to a far lesser extent, by Si precipitates, and a dispersion of eutectic silicon particles and Fe-rich intermetallics. The variables affecting the microstructure mainly include composition, solidification conditions, and heat treatment. The physics governing the formation of aluminium dendrites and the eutectic structure are reasonably well understood Flemings M.C et al 2004 and Spear R.E et al Mechanical properties of Al-Si alloys are related to the morphology of silicon particles (size, shape and distribution), grain size, shape and dendrite parameters, Aluminum Foundry Products, Metals Handbook, 9th ed., Vol. 2, 145, Ganger D.A et al Solidification of Eutectic Alloys, Metals Handbook, 9th ed., Vol. 15, 159, Closset B et al 1982 and Liao H. et at Modification changes silicon morphology and is achieved by rapid solidification, chemical modification and thermal modification in the solid state. The refinement of grain structure is achieved by controlling casting process parameters and / or melt chemistry (i.e. grain refinement, eutectic modification). Each control methodology refines a certain aspect of the microstructure. 51

4 Thus, keeping in view, an attempt has been made to study the effect of minor additions of Ti, B and Sr in the form of master alloys on the microstructure and mechanical properties of A356 alloy either individually or in combinations. 2. EXPERIMENTAL DETAILS Specimens for microstructure and mechanical properties were prepared by melting A356 alloy in a resistance furnace (Kanthal heating elements, M/s Cera Therm International, India) under a cover flux (45%NaCl+45%KCl+10%NaF) and the melt was held at 720 C. After degassing with solid hexachloroethane (C 2 Cl 6 ), master alloy chips (Al-3Ti, Al-3B and Al-10Sr as the case may be) duly packed in an aluminium foil were added to the melt. The melt was stirred for 30 seconds with zircon coated steel rod after the addition of master alloys, after which no further stirring was carried out. Melt were poured at 0 min. and 5 min. into cylindrical graphite mould (25mm φ and 100mm height) surrounded by fire clay brick with it s top open for pouring and also the melt was poured into split type graphite mould (12.5mm φ and 125mm height) for preparing tensile (10mm φ x 50mm length) specimens. The 0 min. refers to the melt without the addition master alloys. The cast alloys are characterized by microscopy, macroscopy and SEM microanalysis. Figure. 1a shows the cylindrical graphite mould surrounded by fireclay bricks and Figure.1b shows castings obtained from cylindrical graphite mould showing the sections selected for characterization. Figure 1(a) Figure 1(b) Figure 1a Cylindrical graphite mould surrounded by fireclay bricks and 1b Castings obtained from cylindrical graphite mould showing the section selected for characterization. 52

5 2.1. SPECIMEN PREPARATION FOR MICROSTRUCTURE STUDIES Specimens of 5mm height from the section that was left after macroscopic study were taken for optical microscopic studies. The specimens so obtained were initially polished using belt grinder and then on a series of SiC water proof emery papers with increasing fineness, to remove any of the scratches present. Then the samples were polished on a disc polisher using 75µm Al 2 O 3 powder, until the mirror finish and scratch free surface is obtained. Final polishing was carried out using elctropolishing machine (Model: Electropol, METATECH, Pune) with electrolyte having composition of 72.4% Methanol, 7.8% perchloric acid, 9.8% Butylcellosolve and 10% distilled water by volume. Polished samples were cleaned with soap solution, distilled water and ethyl alcohol followed with drying. The polished samples were etched using Keller s reagent (2.5%HNO %HCl+1%HF+95%H 2 O by volume) for about 75-90s in order to develop microstructure with grain boundaries. The samples so prepared and polished were taken for optical microscopy, SEM/EDX analysis MECHANICAL PROPERTY STUDIES For the mechanical property studies, the tensile specimen shown in Figure 2 (10mm dia. x 50mm length) were prepared and properties like (0.2% PS, UTS, % E, % R, YM and VHN) of A356 alloy were evaluated before and after grain refinement and modification. The tensile tests were carried out using the Precision Controlled Computerized Tensile Testing Machine (UNITEK 9450PC, Blue Star India). A part of split type graphite mould (12.5 mm diameter and 125 mm height) for preparing tensile specimens (12.5 mm diameter x 50 mm length) is shown in the Figure 2b. Figure 2(a) Figure 2(b) 53

6 Figure 2a Tensile specimen (10mm x 50mm Gauge length) and 2b Part of split type graphite mould (12.5 mm diameter and 125 mm height). 3. RESULTS AND DISCUSSIONS 3.1. MICROSTRUCTURAL STUDIES Figure 3a shows the SEM photomicrograph of A356 alloy in the absence of grain refiner. From figure it is clear that in the absence of Al-3Ti master alloy, A356 alloy shows coarse columnar α-al dendritic structure and unmodified needle/plate like eutectic silicon. With the addition of 0.65% of Al-3Ti the master alloy, A356 alloy shows response towards grain refinement with structural transition from coarse columnar dendritic structure to fine equiaxed structure as shown in figure 3b. With the addition of 0.60% of Al-3B master alloy, the structure of A356 alloy changes from columnar to finer equiaxed α-al dendrites compared to the addition of Al-3Ti grain refiner as clearly observed in figure 3c, while eutectic silicon remains unmodified as expected. This could be due to the presence of AlB 2 particles present in the Al-3B master alloy and these particles are act as heterogeneous nucleating sites during solidification of α-al. While addition of A356 alloy to 0.20% of Al-10Sr master alloy, the plate like eutectic Si is converted in to fine particles and α-al dendrites remain as columnar dendritic structure only as clearly seen in figure3d. However, figure 3e shows the simultaneous refinement (α-al dendrites) and modification (eutectic Si) of Al356 alloy due to the combined action of AlB 2 and Al 4 Sr particles present in Al-3B grain refiner and Al-10Sr modifier respectively. Figure 3(a) Figure 3(b) 54

7 Figure 3(c) Figure 3(d) Figure 3(e) Figure 3 SEM photomicrographs of A356 alloy (3a) as cast alloy; (3b) with 0.65% of Al-3Ti grain refiner; (3c) with 0.60% of Al-3B grain refiner; (3d) with 0.20% of Al-10Sr modifier and (3e) combined addition of 0.65% Al-3Ti, 0.60 % of Al-3B grain refiner and 0.20% of Al-10Sr modifier MECHANICAL PROPERTIES Figure 4 shows the influence of the Ti, B and Sr on the mechanical properties of A356 alloy. From the figure, it is clearly observed that the improvement in the mechanical properties such as PS, UTS), %E, %R, YM and VHN increases with the addition of master alloys containing Ti, B and Sr due to change in the microstructure. It is also clear that the combined addition of grain refiner and modifier to A356 alloy has resulted in maximum improvement in mechanical properties as compared to the individual addition of grain refiners, modifier and in an untreated as cast condition. Addition of the grain refiners to A356 alloy predominanantly converts columnar grain/dendritic structure to fine equiaxed grain/dendritic structure thereby by enhances the mechanical properties. The effect of silicon on the mechanical properties of Al-Si 55

8 alloys is a well-known fact. The mechanical properties depend on the shape, size and distribution of eutectic silicon and α-al grains/dendrites in case of Al-Si alloys. It is also clear from the experimental results that the combined addition of 0.65% Al-3Ti, 0.60 % of Al-3B grain refiner and 0.20% of Al-10Sr modifier to A356 alloy resulted in maximum UTS, when compared to the individual addition of grain refiner and modifier in an untreated conditions. The effect of grain refinement and modification on the mechanical properties is shown in Table 1. In the absence of grain refiner and modifier, A356 alloy shows 131MPa 0.2% PS, 185MPa UTS, 3.25 %E, 3.32%R, 87YM and 245VHN, while with the combined addition of grain refiner and modifier, 142MPa 0.2% PS, 202MPa UTS, 4.92 %E, 7.55%R, 100YM and 298VHN were obtained. Alloy No. Alloy Composition 0.2% PS (MPa) UTS (MPa) % E % R YM VHN 1 A A % of Al-3Ti A % of Al-3B A % of Al-10Sr A % of Al-3Ti % of Al-3B % of Al-10Sr Table 1 Mechanical property studies on A356 alloy 56

9 Mechanical Properties % P.S ; U T S ; % E ; % R ; Y M ; V H N A lloy 1 A lloy 2 A lloy 3 A lloy 4 A lloy 5 A llo y C om p o sition Figure 4 Mechanical properties of A356 alloy before and after the addition of grain refiner and or modifier 4. CONCLUSIONS Addition of grain refiner (Al-3Ti and Al-3B) and modifier Al-10Sr master alloys to A356 alloys leads to change in microstructure. Improved mechanical properties were obtained due to the change in the microstructure of A356 alloy due to the addition of grain refiner/modifier. 5. REFERENCES [1] Aluminum Foundry Products, Metals Handbook, 9th ed., Vol. 2, 145. [2] Benzerga A.A, Hong S.S., Kim K.S., Needleman A and Van E. der Giessen., (2001), Smaller is Softer: An inverse size effect in a cast aluminum alloy, Acta Materialia, Vol.49, No.15, pp [3] Caceres C.H. and Selling B.I., (1996), Casting defects and the tensile properties of an Al-Si-Mg alloy Materials Science Engineering: A, Vol. 220, No.1, December 15, pp (8). [4] Closset B.M. and Gruzleski J., (1982), AFS Trans., Vol.90, pp

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