Investigation on Fluidity of Aluminum Alloy in Semisolid Metal Processing

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International Journal on Mechanical Engineering and Robotics (IJMER)

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ISSN (Print) : 2321-5747, Volume-4, Issue-3,2016 51

Investigation on Fluidity of Aluminum Alloy in Semisolid Metal Processing

1M.V. Kishore, ²D. Hanumantha Rao, ³M. Manzoor Hussain

1AssistantProfessor, Dept. of Mechanical Engineering, Matrusri Engineering College, Saidabad, Hyderabad, India.

2Principal, Matrusri Engineering College, Saidabad, Hyderabad, India

3 Principal, Jawaharlal Nehru Technological University Hyderabad College of Engineering, Sultanpur, India Abstract : The benefits of combining both the traditional

methods of casting and forging are obtained with the development of new technology called semisolid metal processing. Fluidity is the ability of the material to flow before it completely solidifies. The present study comprises of evaluating an important parameter known as mold filling ability – a measure for fluidity. An attempt is made to understand the applicability of mold filling ability parameter to die casting. Simulation studies were carried out to optimize the process parameters such as temperature, billet size and ram speed. Experiments were carried out to determine the mould filling ability and validate the simulation results. Simulation and experimental study were conducted on A356 material.

Keywords: semi-solid slurry, fluidity, A356, finite element simulation, mould filling ability

I. INTRODUCTION

Semi-solid metal casting (SSM) or die casting in the semi-solid state is a near net shape manufacturing method that offers the favorable costs of die casting with much improved part quality [1]. The process is used with non-ferrous metals, such as aluminum, copper, and magnesium. The process combines the advantages of casting and forging. The process is named after the fluid property thixotrophy, which is the phenomenon that allows this process to work. Simply, thixotropic fluids shear when the material flows, but thicken when standing [2, 3].

Semisolid metal processing is carried out when the metal is in the semisolid state. The temperatures are in the freezing range i.e. in between the solidus and liquidus range. Typical solid fraction suitable for this process is about 35 percent. Modeling of the thixoforming process has been attempted earlier for aluminum alloys [4]. The three major steps associated with this emerging technology are billet production, reheating to semi-solid condition and forming operation [5]. By optimizing the forming parameters dendritic microstructure changes to globular and mechanical properties improve [6]. Enhancing the fluid flow properties in the mushy zone by stirring is one way to suppress the dendritic growth.

The emerging technology consisted of three major steps.

1) Billet production: Proprietary technologies that are developed to cast billets include direct chill casting with active (mechanical or electromagnetic) or passive stirring and thermo-mechanical processing of solid feedstock.

2) Reheating to the semi-solid condition: The necessity to heat billets accurately and homogeneously to the semi-solid condition required specific heating strategies and the development of adequate equipment and control systems. Inductive heating was not only preferred but also conventional type radiation or convection-furnaces were applied in series production.

3) Forming operations and applications: After reheating, the semi-solid billet is usually formed into the final shape in just one forming operation

For a metal to be thixotropic it must be heated to the semisolid region of temperatures have a globular microstructure at the processing temperature instead a dendritic one.

Fig. 1(a) Dendritic and (b) Globular microstructure Fluidity

The term fluidity is normally used in foundry to designate the casting materials ability to fill the mould cavity. Fluidity, therefore, is a complex property and there are a number of variables, which affect it. Fluidity depends on the casting material as well as the mould.

Numerical simulations were carried out on aluminum alloy to determine the fluidity in aluminum alloy [7].

It was also analyzed that under vigorous stirring conditions, semisolid metallic slurries undergo a

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structural change wherein primary dendrites are replaced with nearly spheroidal particles [8]. It was shown consequently that semisolid metallic slurries have higher viscosities than fully liquid melts which brought about such advantages as laminar filling of die cavity rendering higher integrity of semisolid cast components.

At the same time, lower fluidity of the slurries makes their gravity casting impossible. Therefore use of external mechanical force is required to push such viscous slurries along the running system and into the die cavity.High-pressure die-casting has been conveniently used for this purpose. However mass production is more economically viable due to high manufacturing cost of dies.

The casting materials properties which affect the fluidity to a great extent are viscosity of the melt, heat content of the melt, surface tension, freezing range and specific weight of the liquid metal [9].The lower the coefficient of viscosity of molten metal, the higher would be the fluidity since the melt will be able to flow freely. Since the heat content and super heat of the molten metal decreases the coefficient of viscosity, it would also be responsible for an increase in the fluidity. Similarly, lower surface tension, which promotes wetting of the mould by the melt would make the mould quickly filled, particularly, the narrow sections. In general, the alloys, which have a narrow freezing range, have a higher fluidity compared to the wide freezing range ones. In wide freezing range alloys, during the process of solidification the dendrites are spread over a much larger part of the mould and thus reduce the flow of the metal, decreasing the fluidity.

The mould properties that affect the fluidity are thermal characteristics, permeability and the mould cavity surface. The way in which heat is transferred by the mould from the melt affects the ability of the melt to fill the mould cavity. For example, in a green sand mould the fluidity would be lower since more heat is extracted by it then in a dry sand mould. The fluidity of a mould, which is coated with refractory washes, would be high compared to the one with no coatings. Similarly a higher permeable mould is conducive to better fluidity.

Mould filling ability

Mould filling ability is influencing considerably the heat transfer and solidification of the metal. Mould filling is a critical parameter in the production of quality castings, especially in the case of complex shaped castings where section thickness is varies considerably [10]. The die used for the present study of mould filling ability was developed based on the model designed by Engler and Ellerbrok [11].

Methodology adopted for calculating mould filling ability

The hot metal poured into the diefills the curved cavity between the two cylindrical cores having a line contact at the center, but solidifies before filling up the complete casting.

The inverse of the diameter of curvature of the edge tip of the fin gives the value of the mould filling ability. The diameter at the tip of the fin gives the meniscus diameter of the liquid metal at the time of solidification as represented in the figure 2.It is difficult to measure the diameter of the tip of the edge and hence an indirect way of calculation has been used.

From Figure 2

R² + (r+x)²= (r+R)² ………Equation (1) By solving Equation (1) we get,

1 / 2r = (R-x) / x² (since 2r = d)

So, 1 / d = (R-x) / x²……… Equation (2) Where R = Radius of the sand core in mm,

r = Radius of the meniscus (2r = d) in mm, 2x = Distance between edges in mm, 1 / d = Mould filling ability, 1 / mm or mm^-1

Fig 2 Measurement of mould filling ability For this design of die, the value of mould filling ability can be calculated by applying simulation process using numerical simulation software. The values obtained by simulation are to be compared with the experimental values. Simulation of die filling for calculating mould filling ability is explained in the following section.

Numerical Simulation

The following simulations were carried out in process simulation software for Aluminum alloy A356 to determine the effect of temperature of the billet on the final shape of the billet. The simulations were carried out at different temperatures between the solidus and liquidus temperature range of the alloy.

The position of the billet on the bottom die was determined by placing it in different positions and also for various temperatures. After a few trials the position of the billet was finalized after studying the aspect of under filling of die cavity and also dimensional accuracy of the component. The top die, bottom die and the work piece were modeled in Solid Works and exported into the process simulation software Deform 3D in stl format.

The following are some of the conditions that were chosen to simulate the process. The material chosen for

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the work piece is A356 and for the dies AISI H – 26. A top die speed of 10mm/sec and a friction coefficient of 0.3 between the work piece and billet were chosen. The dies and billet were modeled to exchange heat with the ambient surroundings. The initial size of the billet was taken as 80L X 45 H X 35W. The final shape of the component obtained at the end of simulation is shown in figure 5. The simulations were carried out by varying the temperature of the billet and keeping all the other parameters constant.

II. EXPERIMENTAL STUDIES

The commercial grade aluminium alloy A356 was used for the present study. The chemical composition of the alloy is given in Table 1.

The freezing range of this alloy is between 555 ^oC and 616 ^oC.

Table1 Chemical composition of A356 Alloy

Component Si Cu Mg Ti Fe Mn Zn Al

Wt % 6.5-7.5 Max. 0.2 0.25 - 0.45 Max 0.2 Max. 0.2 Max. 0.1 Max. 0.1 93 The schematic representation of die used for this study is

represented in Figure 3. The actual die used for this study is shown in figure 4. The die was made of MS plates of thickness 12.5mm to form the cavity. The diameters of the cylindrical cores are 25mm and are fixed to the bottom plate. Initially raw material of AL 356 was heated in an electric arc furnace to a completely molten state.The temperature inside the furnace was measured by inserting a thermocouple into the furnace.

The thermocouple was connected to a digital display unit. The molten metal was vigorously stirred by inserting an impeller into the molten metal for about five minutes. The melt was poured into the sand mould to obtain the billet. The billet was reheated to 580 ^oCand was cast in the die. The component was allowed to cool and attain room temperature.The shape of the cast component is shown in Figure 6. The average distance x between the two edges was calculated and using equation 2 mould filling ability value was obtained. The value of 1/d was found as 1.6025 mm^-1

Fig 3 Schematic representation of die used for experimentation

Fig 4 Manufactured die

Fig 5 Billet at the end of simulation

Fig 6 Formed component

III. RESULTS AND DISCUSSIONS

Mould – filling ability value can be calculated by knowing the values of radius of core and distance between two edges. After substituting all the measured values in below equation mould filling ability (1 / d) is obtained.

Mould filling ability, 1 / d = (R-x) / x²

The mould filling ability values at various initial billet temperatures and for the three simulation run orders are given at table 2. Of the three simulations that were carried out, the simulation result when the billet temperature was 580 ⁰C provided a better shape of the component. Experiment was carried out by reheating the billet to this temperature and for the cast component the distance between the edges is measured at three different locations and the average value was calculated. The

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mould filling ability was calculated for the three simulation runs and the results are tabulated in Table 2.

Comparison of the experimental and simulation resultsis tabulated in Table 3.

IV. CONCLUSIONS

Mould filling ability is a very important measure which determines the fluidity of the material in forming operations. It was observed from the simulations that when the temperature of the billet was about 580 ⁰C the

material was having better fluidity and the final shape achieved by the billet was closer to the shape of die cavity. The amount of under fill was also very less.

These results were also confirmed from experimental values. The difference in mould filling ability value between simulation and experiment result was observed to be around 13 percent. The study may further be extended to determine the applicability of mould filling ability parameter for different materials.

Table 2 Mould filling ability values for A356 for different simulation run orders Simulation

run order

Billet Temperature (⁰C)

Top die movement (mm/sec)

Distance between two edges 2x (mm)

x (mm) Mould – filling ability 1 / d (1 / mm)

1 560 10 19.98 10 0.15

2 580 10 7.7911 3.9 1.38

3 600 10 23.576 11.788 0.095

Table 3 Comparison of mould filling ability value

Process Parameter Experimental result Simulation result Mould filling ability value, 1/d

mm ^-1

1.6025 mm ^-1 1.38 mm ^-1

REFERENCES

[1] Thixoforming, A textbook on Semi-solid Metal Processing Edited by Gerhard Hirt and Reiner Kopp Wiley VCH Verlag GMBH & Co KGaa.

[2] Flemings, M.C.” Behavior of Metal Alloys in the Semisolid State”, Metallurgical Transactions, Volume 22A, pages 957–981, May 1991

[3] Spencer, B., Mehrabian, R. and Flemings, M.C.

“Rheological behavior of Sn-15 pctPb in the crystallization range”, Metallurgical Transactions, Vol. 3, Issue 7,1925-1932.

[4] Gap -Yong Kim, Muammer Koc, Rhet Mayor, Jun Ni “ Modeling of the Semi-solid Material Behaviour and Analysis of Micro-Meso Scale Feature Forming” Journal of Manufacturing Science and Engineering, Vol. 129, April 2007, pages 237 -245

[5] Pradip Dutta and K.S.S. Murthy, “ Semisolid Manufacturing: Recent Advances”, Indian Foundry Journal Vol. 55 No. 7 pages 33-39 [6] Salman Nourouzi, Amin Kolahdooz and

Mohammad Botkan “Behaviour of A356 Alloy in Semi-Solid State Produced By Mechanical Stirring” Advanced Materials Research Vol. 402, pages 331-336 , 2012.

[7] Du Zhi-ming, Liu Jin, Chen Gang , “ Investigation of semi-solid aluminum alloy filling-plastic flowing in thixoforging”, Transactions of Nonferrous Metals Society of China, Vol. 20, pages 893-897, 2010.

[8] Flemings, M.C.“Semi-solid forming: the process and the path forward”, Metallurgical Science and Technology, Volume 18, pages3–4,2000

[9] H. Mirzadeh and B. Niroumand, “Fluidity of Al- Si semi-solid slurries during rheocasting by a novel process” Journal of Materials Processing Technology, Vol. 209, pages 4977-4982, 2012.

[10] Samavedam Santhi, S.B.Sakri, Dharwada Hanumantha Rao and Srinivasan Sundarrajan “ Studies on Mould Filling Ability of US 413 and US A356 Cast Aluminium Alloys” Indian Foundry Journal Vol. 56, pages 28-34 August 2010.

[11] Samavedam Santhi, S.B.Sakri, Dharwada Hanumantha Rao and Srinivasan Sundarrajan

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