Investigations on machining behavior of Strontium Modified Hypereutectic Aluminum-Silicon Alloy produced by stir casting

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1 Investigations on machining behavior of Strontium Modified Hypereutectic Aluminum-Silicon Alloy produced by stir casting Om Prakash Tiwari Siddhartha P.K.Sood Abstract The present work focuses on the optimization of cutting parameters for machining of strontium modified hypereutectic aluminum-silicon alloy (Al-16Si-0.2Sr,Al-16Si-0.4Sr) by stir casting technique using single point coated carbide tool under dry conditioning, the controllable input factors namely cutting speed, feed rate and depth of cut are optimized at three levels, keeping approach angle constant. To identify the major output parameters such as cutting force and power consumption, the Taguchi parametric approach was selected. The experiments have been performed based on Taguchi L 9 orthogonal array. The results indicate that strontium modification on hypereutectic (Al- 16Si-0.4Sr) alloy reduces the cutting force and power consumption in comparison to modified hypereutectic (Al-16Si-0.2Sr) and unmodified hypereutectic (Al-16Si) alloy. It is also obtained feed rate is significant factor in comparison to others. Keywords-Al-Si alloy, Taguchi method, CNC turning center I. INTRODUCTION The automotive industry has seen a steady increase in the demand for improved fuel economy, enhanced safety and reduced exhaust emission. These features are predominately are available in aluminum alloys which meet most of these characteristics. Aluminum alloys applications in the automotive industry cover a wide variety of components such as power train components, aluminum wheels, radiators, heat exchanger, chassis, suspension parts, closure panels, pistons and other engine components as well as primary body structures [1]. These alloys usually contain about 2 % to 25 % of silicon, together with some amount of impurities of the order of 1.5 % to 2 %. The most widely used alloys are found in the range of 3 % 22 % of silicon. Based on the Si concentration in weight % (wt. %), the Al-Si alloy system can be classified into three major categories viz. hypoeutectic (<12 % Si), Eutectic (12 % 13 % Si) and Hypereutectic (14 % 25 % Si) [2]. The hypereutectic group of aluminum-silicon alloy enjoys common features like low thermal expansion, excellent castability, high corrosion resistance and high abrasive wear resistance. This has led to their use in automotive piston component but its poor machinability, reduces its applications range. The main reason for this is presence of coarser primary silicon particles and needle like eutectic silicon which generally degrade the mechanical properties of the alloy [4]. That s why strontium addition is done in order to achieve the refinement of primary silicon particles and eutectic silicon particles and these changes lead to decrease in the hardness, refine the microstructure and increase the ductility [10]. This paper investigates a possible route to improve machinability of this alloy, with the aim of reducing power consumption, cutting force and hence the overall cost, while improving the quality context. A. Development of alloy II. EXPERIMENTAL PROCEDURE Strontium modified hypereutectic aluminum silicon (Al-16Si-0.2Sr and Al-16Si-.4Sr) and unmodified hypereutectic aluminum silicon (Al-16Si) alloys were developed for experimental work under taken. The alloy was developed using stir casting technique, for this pure Aluminum (Al) selected which have some impurities. Aluminum was taken in the form of cylindrical rod and then cut into smaller pieces with the help of power hacksaw in order to keep the alloy inside the crucible properly. To induce silicon while developing required hypereutectic aluminum silicon alloy we used Al-Si substrate containing 30% silicon and for strontium we used aluminum strontium master alloy containing 10% strontium. Al Si alloys were prepared by melting 0.5kg of alloy at temperature about 800 C in a graphite crucible by a high frequency muffle furnace in order to attain homogeneous composition using stir casting. Complete preparation process of alloy is shown in Figure 2. Pure Al ingot is

2 cleaned and cut into small pieces and it is melted with Al-Si particle having 70%Al & 30% Si and Al-10%Sr master alloy in muffle furnace. In the mean while, the permanent metal mold is pre-heated and the mold halves are clamped together with the help of wire. Once the base with substrate is melted around 750 C to 800 C, the melt was stirred with help of a stirrer manually. After stirring the molten mixture, the molten metal is poured at a slow rate (0.1 kg/s) from a crucible into the mold through top of the preheated permanent mould mold. The molten metal is allowed to cool and solidify in the mold. Once the metal has solidifies the two mold halves are opened and the casting is removed. The cylindrical samples of dimension 30mm X 150mm are obtained C. CNC turning centre The machining experiments were carried out on CNC Turning centre Batliboi fitted with Siemens control system. Raw selected and cleaned Moulds are opened and samples are ready Charged into graphite crucible Stirred manually and poured into permanent mould Figure 1 Process diagram for development of alloy Placed in muffle furnace Superheated to C in muffle furnace D. Lathe Tool Dynamometer Figure 2:- CNC turning centre Lathe tool dynamometer was used for forces measurement. It was mounted on CNC lathe as shown in Fig.3. Then dynamometer was connected to a computer for data acquisition. Cutting force and power consumption was measured using Lathe tool dynamometer with digital indicator connected by data acquisition system.xkm software was used to collect the data. Lathe tool dynamometer is shown in the Figure 3. Figure 2:- Casting procedure (Muffle furnace, Molten Metal pour into Metal Mould and casted sample B. Tool The cutting tool selected for machining of strontium modified hypereutectic Al-Si alloy was coated carbide (CC) insert, it gives excellent wear and heat resistant properties. Table I shows the tool nomenclatures of cutting insert used in the present study. TABLE I. Nomenclature of Cutting Coated Carbide inserts Parameters Values Rake angle 60 o Clearance angle Inclination angle 0 0 Approach angle 90 0 Nose radius 0.4 Mm Figure 3:- Lathe Tool Dynamometer E. Selection of cutting parameters The control parameters used in this study are cutting speed, feed rate and depth of cut. Three levels were specified for each of the factors as indicated in TABLE II. Sr. no. TABLE II. Levels for various control factors Cutting Feed rate Depth of cut speed(m/min) Experimental design was done using Taguchi orthogonal array. As it facilitates to obtain more comprehensive results while doing fewer experiments. This drastically saves time and expenditure. In this

3 study, feed rate, cutting speed, depth of cut and different work were investigated to determine the effect of machining parameters on the cutting force and power consumption. This study aims to optimize the cutting forces (Fc) and power consumption (P). In Taguchi technique, result is obtained in the form of signal-to-noise (S/N) ratio. There are several types of S/N ratio available depending on the type of characteristic:- S/NL (the large-the-better) S/NS (the smaller-the-better) S/NN (the nominal-the better) Smaller-the-better characteristic is used for minimum cutting force and power consumption. Smaller-the-better characteristic is formulated as (1) Where ή is the S/N ratio, n is the number of experiment and Yi is the observed data. TABLE III. Orthogonal Array of Taguchi L 9 Runs Cutting speed(m/min) Feed rate Depth of cut(mm) Ma Um Ma Ma Ma Um Um Ma Ma1 Alloy designation:- Um= unmodified alloy (Al-16Si) Ma1=modified alloy 1(Al-16Si-0.2Sr) Ma2=modified alloy 2(Al-16Si-0.4Sr) B. Results for Cutting force Figure 4 Main effects plot for cutting force vs. cutting parameters The main objective of the experiment is to optimize the turning parameters in order to achieve low value of the cutting parameters. Figure 4 gives the main effects plot of cutting force vs. cutting parameters to determine the optimum value. Experiment number 8 gives minimum value of cutting force. This implies the selection of 120 m/min Cutting speed, 0.09 mm/rev feed rate, 0.5 mm depth of cut and modified alloy 2 for optimal cutting force. C. Results for power consumption IV.RESULT AND DISCUSSION A. Taguchi Analysis for strontium modified Al-16Si Alloy:- The experimental results of the machining characteristics obtained for the turning parameters mentioned in Table IV. TABLE IV. Experimental Results for Alloy Runs Cutting speed Feed Depth of cut Force (N) Power (W) (m/min) (mm) Ma Um Ma Ma Ma Um Um Ma Ma Fig. 5 Main effects plot for power consumption vs. cutting parameters From Figure 5 it can be concluded that experiment number 8 gives minimum value of power consumption. This implies the selection of 120 m/min cutting speed, 0.09 mm/rev feed rate, 0.5 mm depth of cut and modified alloy 2 for optimal power consumption. D. ANOVAand the effects of factors on machining behaviour of alloy 1.Effect of factors on Cutting Force Effect of various control factors like cutting speed, feed rate, depth of cut and work on the response of experimental data, it is necessary to develop the analysis of variance (ANOVA) to find the significant factors. ANOVA is used to analyze the impact of each variable on the variation of the results for strontium modified hypereutectic Al-Si alloy. In this table, the last column shows the percentage contribution of each variable in the total variation indicating the influence on cutting force.

4 TABLE V. ANOVA table for cutting force (means) Source DF Seq SS Adj MS P (%) Cutting speed (m/min) Feed Depth of cut (mm) Residual Error 0 Total From Table V, it is observed that work (P=38.33%), Feed rate (P=33.73%) and cutting speed (P=23.80%) have significant effect on the machining behavior of strontium modified hypereutectic aluminum silicon alloy while depth of cut (P=5.60%) has less significant effect. Rank of the control factors of machining behavior of strontium modified hypereutectic aluminum silicon alloy is shown in Table VI. TABLE VI. Rank table of control factors (means} Level Cutting Speed m/min Feed Rate mm/rev Depth of Cut mm Delta Rank Effect of factors on power consumption TABLE VII. ANOVA table for power consumption (means) Source DF Seq SS Adj MS P (%) Cutting speed (m/min) Feed Depth of cut (mm) Residual Error 0 Total From Table VIII, it is observed that work (P=40.30%), cutting speed (P=28.27%) and Feed rate (P=29.14%) have significant effect on the machining behavior of strontium modified hypereutectic aluminum silicon alloy while depth of cut (P=2.30%) has less significant effect. Rank of the control factors of machining behavior of strontium modified hypereutectic aluminum silicon alloy is shown in Table VIII. Level TABLE VIII. Rank table of control factors(means) Depth Cutting Speed Feed Rate of Cut m/min mm/rev mm Delta Rank V.CONCLUSION In this study machining variable such as cutting forces and power consumption are measured during machining of Al-16Si and strontium modified alloy. The effects of cutting parameter such as cutting speed, feed rate and depth of cut on machining variable are evaluated. Following conclusions can be drawn from the results:- 1. Feed rate is a significant factor as compared to cutting speed and depth of cut that influences the cutting force and power consumption during machining of Al-16Si-0.2Sr,Al- 16Si-0.4Sr and Al-16Si. 2. Strontium modified with 0.4Sr sample has shown improved machinability compared to Al-16Si-0.2Sr and Al-16Si these effects are due to change of morphology of eutectic silicon and the size of primary silicon decreases, which results in decrease of hardness and refinement of the alloy hence machinability of the alloy improves. 3. Cutting speed of 120 m/min, feed rate of 0.09 mm/rev, depth of cut of 0.5 mm are the preferable cutting parameter. 4. On the basics of result obtained it can be interpreted that Al-16Si-0.4Sr alloy is easy to machine and yield. References [1] H.R.Lashgari,Emamy,M. Razaghian & Najimi, The effect of strontium on the microstructure, porosity and tensile properties of A356 10%B4C cast composit, Materials Science and Engineering, 517(1-2), [2] Li.R. Pan, F.Jiang, B.Yin & Liu, Effects of yttrium and strontium additions on as-cast microstructure of Mg-14Li- 1Al alloys, Transactions of Nonferrous Metals Society of China, 21(4), [3] M. Timpel, N. Wanderka, G.S.V. Kumar,andJ. Banhart, Microstructural investiga- tion of Sr-modifed Al 15wt% Si alloys in the range from micrometer to atomic scale, Ultramicroscopy 111, [4] M. Timpel, N. Wanderka, R. Schlesiger, T. Yamamoto, N. Lazarev, D. Isheim, G. Schmitz, S. Matsumura, and J. Banhart, The role of strontium in modifying aluminium silicon alloys, Acta Materialia,2012. [5] M. M. Haque, Strontium Modification of Aluminum- Silicon Eutectic Alloy and Factors affecting it, Metals Forum, Vol. 6, No. 1, p. 54, [6] F.Yilmaz and O.A.Atasoy, Growth Structures in Aluminum Silicon Alloys. II: The Influenceof Strontium, J. Crystal Growth, Vol. 118, [7] M.Kim, Electron Back Scattering Diffraction (EBSD) Analysis of Hypereutectic Al-Si Alloys Modified by Sr and Sc, Met. Mater. Int., Vol. 13, No. 2, 2007.

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