INTRODUCTION

GENERAL

Owing to superior property combinations like specific strength, specific stiffness, seizure resistance, inhibited coefñcient of thermal expansión and enhanced damping characteristics, Aluminium (Al) alloys are becoming promising materials for industrial applications. Aluminum by itself has deprived tribological characteristics, as a result studying the behavior of Al based materials with respect to tribology has become necessity. Amongst all Al-alloys, Al-Si alloys are considered to be the materials for tribological applications due to several reasons. Al-Si alloys can be classified as Hypoeutectic, Eutectic and Hyper eutectic alloys. A356 (LM25) alloy being commercially available hypoeutectic Al-Si alloy find applications in the fields of automotive, marine, aerospace, aircraft pump parts, automotive transmission cases, aircraft fittings and control parts, water-cooled cylinder blocks and general engineering, where it is for blocks & heads of cylinder, bulk castings due to better fluidity, reduced Coefñcient of Thermal Expansión (CTE), specific strength and good resistance against corrosion.In this stüdy, the modified aluminiumA356 alloys with different iron contents were produced by stir casting process. The mechanical properties of the modified A356 alloys were evaluated and the microstructural changes were observed. CHEMICAL ANALYSIS

The chemical composition of the metal plays a main role on the characteristics of the material. The mechanical and thermal properties fully depend on chemical composition of metal. The chemical test was done by Optical Emission Spectrometer (OES) on A356 and modified A356 alloys to determine the material compositions. HARDNESS TEST

Hardness is resistance to surface indentation is an approximate guide to the condition of an alloy, and is used as an inspection measure. In the metáis industry, it may be thought of as resistance to permanent deformation. In the metallurgist, it means resistance to penetration. In the lubrication engineering, it means resistance to wear. In the design engineering, it is a measure of flow stress. In the mineralogist, it means resistance to scratching, and to the machinist, it means resistance to machining. Hardness may also be referred to as mean contact pressure.

TENSILE TEST

Mechanical testing plays an important role in evaluating fundamental properties of engineering materials as well as in developing new materials and in controlling the quality of materials for use in design and construction. If a material is to be used as part of an engineering structure that will be subjected to a load, it is important to know that the material is strong enough and rigid enough to withstand the loads that it will experience in service. The most common type of test used to measure the mechanical properties of a material is the Tensión Test. Tensión test is widely used to provide a basic design information on the strength of materials and is an acceptance test for the specification of materials. Tensile tests are performed for several reasons. The results of tensile tests were used in selecting materials for engineering applications. Tensile properties frequently were included in material specifications to ensure quality. Tensile properties often were measured during development of new materials and processes, so that different materials and processes can be compared. Finally, tensile properties often are used to predict the behavior of a material under forms of loading other than uniaxial tensión. WEAR TEST

Wear is erosión or sideways displacement of material from its derivative and original position on a solid surface performed by the action of another surface. Wear is related to interactions between surfaces and specifically the removal and deformation of material on a surface as a result of mechanical action of the opposite surface. The need for relative motion between two surfaces and initial mechanical contact between asperities is an important distinction between the mechanical wear compared to other process with similar outcomes. SCANNING ELECTRON MICROSCOPE
SEM stands for scanning electrón microscope. The SEM is a microscope that uses electrons instead of light to form an image. The SEM has allowed researchers to examine a much bigger variety of specimens. The scanning electrón microscope has many advantages over traditional microscopes. The SEM has a large depth of field, which allows more of a specimen to be in focus at one time. The SEM also has much higher resolution, so closely spaced specimens can be magnified at much higher levéis. Because the SEM uses electromagnets rather than lenses, the researcher has much more control in the degree of magnification. All of these advantages, as well as the actual strikingly clear images, make the scanning electrón microscope one of the most useful instruments in research today.

LITERATURE REVIEW

Nowadays A356 alloys are widely used material in automotive industries.The change in hardness, wear rate and tensile strengths of aluminium A356 and its modifíed alloys with effect of heat treatment were investigated in detail by authors in the previous works.Floweday et al. (2010) studied that new pistons exhibit a reduced coefficient of thermal expansión, increased thermal shock resistance, high temperature strength and improved fatigue resistance with smaller grains of primary silicon.

Haque et al. (1998) studied on wear properties of aluminium-silicon alloy. The heat treatment is carried out in a resistance heating muffle furnace at 520°C for 8 h and then the specimens are quenched in hot water at 60°C. After 15 min, they are removed and then after drying, they are kept in a freezer at-10°C for overnight. The ageing or precipitation treatment is carried out at 180°C for 8 h and then the test bars are removed from the furnace.

Annaiah et al. (2001) studied on study the effect of heat treatment on the mechanical properties and Tribological behavior of A2014aluminium-silicon pistón alloy. The hardness increases and wear rate decreases and thus improves the mechanical properties of aluminium alloy. Kaczmar et al. (2001) studied on the effect of heat treatment on aluminium alloys. After T6 heat treatment hardness of aluminium materials increased 10-20% to its máximum valué of 160HB. Simultaneously wear rate of composite materials considerably (almost three times) decreased.

Bayoumi et al. (2003) fabricated A356 alloy at three forming temperatures of 560, 570 and 580 °C by extrusión process. Smooth round bars were produced from conventional non-modified cast alloy A356. A clear modification of eutectic phase and an improvement in hardness and tensile properties were found compared with those of conventional casting. The tensile test specimens fracture surfaces were examined using SEM. Furthermore, wear resistance, fatigue strength and impact energy were also, improved significantly.

FadaviBoostani et al. (2009) studied the effect of a novel thixoforming process on the microstructure and fracture behavior of A356 aluminum alloy. In this study, a novel thixoforming process for semi-solid deformation of A356 aluminum alloy is introduced using a continuous hot deformation process to the temperature being lower than the eutectic temperature of the alloy. A new hypothesis was introduced and the deformation mechanism of the alloy was

investigated using the presented hypothesis. Microstructure and fracture surfaces of thixoformed samples were investigated using image analyzing technique and scanning electrón microscopy. Obtained results indicated that this novel thixoforming process produces fine and compact silicon particles, dispersed uniformly in the microstructure of the alloy, compared to those produced by conventional thixoforming and gravity-cast processes with large and integrated morphology for silicon particles. A new combination parameter, i.e. silicon density ratio (SDR) index was introduced. This parameter correlates the mechanical properties of samples to morphological properties of silicon particles and density ratio of them. Results of the study also indicated that samples with low SDR index have superior mechanical properties and consequently intergranular fracture mode.
FadaviBoostani et al. (2004) studied the effect of thixoforming process on morphologies of silicon particles that affect fracture mode of A356 alloy was investigated. Microstructure and fracture surfaces of thixoformed samples were investigated by image analyzing technique and scanning electrón microscopy. A new combination parameter, called silicon density ratio (SDR) index, was introduced. It is suggested that samples with lower SDR index have superior mechanical properties, especially elongation, and consequently intergranular fracture mode. On the contrary, samples with higher SDR index have inferior mechanical properties and fracture path tends to propágate along the cell boundaries leading to transgranular fracture.

Jiang Wen-ming et al. (1999) studied the microstructure, tensile properties and fractography of A356 alloy were studied under as-cast and T6 conditions obtained with expendable pattern shell casting, and the results were compared with lost foam casting (LFC). The results indícate that the eutectic silicon particles are spheroidized and uniformly distributed at the grain boundaries after T6 treatment. The average length, average width and aspect ratio of eutectic silicon particles after T6 condition decrease. The tensile strength, elongation and hardness of A356 alloy after T6 obviously increases. The fracture surfaces of expendable pattern shell casting with T6 heat treatment show a mixed quasi-cleavage and dimple fracture morphology as a transgranular fracture nature.

DETAILS OF ANALYSIS INTRODUCTION

In the present work, the effect of adding various percentage of iron contents on mechanical properties of A356 (LM25) were studied by using Chemical, SEM, Tensile, Wear, Hardness measurement and Heat treatment. These tests are explained in following passages. SPECIMEN PREPARATION

Samples of alloy compositions were prepared by stir casting route is shown in Figure 1. The melting was carried out in a resistance furnace. The Fe in various percentages was added along with A356.The aluminium and iron was melted by heating into 1550 °C. The mechanical mixing was carried out for about 10-15 min at an average mixing speed of 300 rpm. Casting were produced by pouring liquid aluminium in to mould having dimensions of lOOmm width and 600mm height. From these castings wear specimens of dimensions 10 mm diameter and 32 mm length (ASTM G99) were machined for wear test.

In this paper the tensile test was done on a round specimen which is prepared suitable for gripping into the jaws of the Hydraulic testing machine. Specimens of size having gauge length 25 mm and width of 6 mm with radius of fillet 6 mm is used for tensile testing shown in Figure 2.

PRINCIPLE OF OPTICAL EMISSION SPECTROMETR

Optical emission spectrometry involves applying electrical energy in the form of spark generated between an electrode and a metal sample (Figure 3), whereby the vaporized atoms are brought to a high energy state within a so-called "discharge plasma".

These excited atoms and ions in the discharge plasma créate a unique emission spectrum specific to each element, as shown at right.

Therefore, the light generated by the discharge can be said to be a collection of the spectral lines generated by the elements in the sample. This light is split by a diffraction grating to extract the emission spectrum for the target elements. The intensity of each emission spectrum depends on the concentration of the element in the sample. Detectors (photomultiplier tubes) measure the presence or absence or presence of the spectrum extracted for each element and the intensity of the spectrum to perform qualitative and quantitative analysis of the elements.

In the broader sense, optical emission spectrometry includes ICP optical emission spectrometry, which uses inductively coupled plasma (ICP) as the excitation source. The terms "optical emission spectrometry" and "photoelectric optical emission spectrometry," however, generally refer to optical emission spectrometry using spark discharge, direct-current are discharge, or glow discharge for generating the excitation discharge.

Sample preparation of metáis and materials have become more and more important because of the rapid development and improvement of software OES during the past few years that shifts the detection limit for trace analyses. The sample needs to be both representative, homogeneous and with an even surface in order to elimínate factors that can infiuence the results. This analysis technique requires a sample having mínimum 20x20x20 mm size. It's also have special attachments by which one can analyze a sample as thin as 0.1 mm in thickness. HEAT TREATMENT FURNACE

Furnace is an equipment used to heating and maintaining the metal or alloy to the required temperature in order to acquire designated mechanical properties. Heat-treating furnaces are divided according to mode of operation into batch furnaces (tank, compartment, and aerodynamic holding furnaces) and continuous-operation furnaces. Batch type furnace

Batch Furnaces refer to the method of heat treating - one "Batch" at a time. Vacuum and protective atmosphere furnaces are also most often batch furnaces. Batch furnaces may be stand-alone units, or combined with other processes such as Quench Systems (furnace to right) and atmosphere generation equipment. Continuous type furnace

A type of reheating furnace in which the charge introduces at one end moves continuously through the furnace and is discharged at other end. Continuous furnace are most widely used in the metallurgical industry for the heat treatment of rolled producís.

In the machine-building industry, batch furnaces are used for individual or small series production, whereas continuous-operation furnaces are used for large series production and mass production. Furnace can be heated by means on direct gas fired, gas fired radiant tubes, or electric elements. WEAR TESTING

The control of friction and wear is in movable pieces of machines is a critical element of mechanical property. Wear studies were performed using pin-on-disc wear testing machine attached with information acquirement system is shown in Figure 4.

Pin on disc wear tester:

The pin on disc wear tester serves for the investigation of friction and wear process under sliding conditions. It can be operated for solid friction without lubrication and for boundary lubrication with liquid lubricants. Thus both material and lubricant test can be executed.

According to the standard test (ASTM G99) principie a stationary test specimen (pin or ball) with a defined normal forcé is pressed against the surface of another test specimen placed on the rotary disc. The specification for pin on disc wear testing machine is shown in Table 3.1.

The normal forcé is applied over the pin or ball by means of a set of dead weight are applied. This way of application allows a stable forcé during the test. The friction coefficient is determined during the test by measuring the friction forcé by means of the deflection of the elastic arm.

Experimental Details:

During wear tests, specimen of standard test size (ASTM G99) were used as shown in Figure 5. They were positioned in collet of pin on disc machine which is clamped to the holder. Wear specimen is mounted on to the disc with ION, 20N, 30N weights with track diameter of 40

mm with a disc speed of 300 rpm. In order to maintain dry sliding conditions both wear specimens and steel disc were treated with emery papers, acetone before every test. Weight loss method is adopted to study the wear behavior and weight of the wear specimen before and after each test was measured using balance with an accuracy of 0.001 g. Wear behavior of A356 alloy before and after addition of iron was studied.

HARDNESS

Resistance to surface indentation is an approximate guide to the condition of an alloy, and is used as an inspection measure. Brinell (steel ball), Vickers (diamond) and Shore Scleroscope (diamond Hammer) testing machines are applied to aluminium alloys. The hardness testing of specimens are carried out in a Rockwell hardness testing machine with a standard thickness of 6.4 mm plates. The calibrated specimens are polished with the polishing grade paper before the test. A steel ball intender of diameter 1/16" is used in B scale with a load of 100 kg.

SCANNING ELECTRON MICROSCOPE

The SEM is an instrument that produces a largely magnified image by using electrons instead of light to form an image. A beam of electrons is produced at the top of the microscope by an electrón gun. The electrón beam follows a vertical path through the microscope, which is held within a vacuum. The beam travels through electromagnetic fields and lenses, which focus the beam down toward the sample. Once the beam shits the sample, electrons and X-rays are ejected from the samples.

Scanning Electron Microscopy uses a focused beam of high-energy electrons to genérate a variety of signáis at the surface of solid specimens. In most SEM microscopy applications, data is collected over a selected área of the surface of the sample and a two-dimensional image is generated that displays spatial variations in properties including chemical characterization,
texture and orientation of materials. The SEM is also capable of performing analyses of selected point locations on the sample. This approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions, crystalline structure and crystal orientations. The higher energy of the electrón beam permits viewing at much higher magnifications than are possible with light. Detectors collect these X-rays, backscattered electrons, and secondary electrons and convert them into a signal that is sent to a screen similar to a televisión screen. This produces the final image. Preparation of Sample

The SEM utilizes vacuum conditions and uses electrons to form an image, special preparations must be done to the sample shown in Figure 6. All water must be removed from the samples because the water would vaporize in the vacuum.

All metáis are conductive and require no preparation before being used. All non-metals need to be made conductive by covering the sample with a thin layer of conductive material. This is done by using a device called a "sputter coater."

The sputter coater uses an electric field and argón gas. The sample is placed in a small chamber that is at a vacuum. Argón gas and an electric field cause an electrón to be removed from the argón, making the atoms positively charged. The argón ions then become attracted to a negatively charged gold foil. The argón ions knock gold atoms from the surface of the gold foil. These gold atoms fall and settle onto the surface of the sample producing a thin gold coating.

RESULTS AND DISCUSSIONS

In this chapter, the experiment test results are taken by using the standard testing equipment. They are following as below, Chemical composition test result
The chemical composition test of alloys were done on by OESand the chemical composition results of A356 and modified A356 with iron contents of 0.48%, 0.79%, 0.86% and 1.09% are shown in Table 4.1.

Microstructural analysis by SEM

The effect of iron on the mechanical properties of aluminium alloys has been reviewed extensively by several authors. A cross section of the sample revealed that microstructure consists of inter-metallic P-AlsFeSi shown in Figure 4.1(e) phase that forms during solidification of aluminum alloy is detrimental to the mechanical properties because it is brittle and improves hardness and wéar properties [8].

A356 has a plain surface without intermetallic phases. As the iron content increases, the inter-metallic P-AlsFeSi phases begins to form and thus the P-AIsFeSi phase can be viewed clearly in specimen A356 with 1.09% iron content. The SEM images of the modified alloys are shown in Figures 7.

INFLUENCE OF IRON CONTENT ON HARDNESS

The effect of iron on the mechanical propérties of aluminium alloys showed that as iron levéis increase from 0.33% up to 1.09 wt. %, the hardness of Al-Si based alloys increases. The effect of iron on hardness can be described by the size and volume fraction of iron-containing intermetallic (particularly /?-AlsFeSi phase) which are increased with iron content. However, as iron level increases, porosity increases, and these defects also have a significant impact on hardness. Thus the hardness valué is improved by addition of iron content to the A356. A356 shows a hardness valué of 43.9 and the hardness valúes were increased up to 55.1 with addition of iron content upto 1.09% is shown in Table 4.2. Thus the hardness valué is increased about 25.51% in A356 with 1.09 % iron content.

INFLUENCE OF IRON CONTENT ON TENSILE STRENGTH

The influence of iron on mechanical propérties leads to decreasing tensile strength. A356 showed a tensile strength of 189.24 MPa and with increasing iron content the tensile strength of specimen's decreases. The modified A356 with 1.09 % iron content shows a tensile strength of 138.33 MPa. The tensile strength of modified A356 reduced about 27%.The tensile strengths of A356 and its modified alloys are shown in the Table 4.3.

The graph shows a decrease in ultímate tensile strength of A356 with increase in iron contents is shown in Figure 4.28.

INFLUENCE OF IRON CONTENT ON WEAR RESISTANCE

The effects of iron on wear properties of A356 were examined. It shows that wear rate decreases as the iron content increases from 0.33 % to 1.09 %, A356 shows a decrease in wear rate of 46.54 % under ION loading. A356 shows a decrease in wear rate of 47.65 % under 20N loading. A356 shows a decrease in wear rate of 44.41% under 30N loading. Further the wear rate can be reduced by T6 heat treatment (Table 1). The results of wear testing were shown in Table 4.4.

A graph is plotted to indicate change in wear rates of specimen before and after addition of iron. The reduction in wear rates of A356 and modified A356 with different Iron (Fe) contentsbefore heat treatment is shown in Figure 9. Further the table shows that wear rate increases as the load on specimen increases. By this result the wear resistance of the material is increase with increase of Iron content. The reduction in wear rates of modified A356 alloys after heat treatment is shown in Figure 10. It shows that modified A356 alloy have a better wear resistance when compare with A356.

Claims

1. Adding iron (Fe) 1.09% to the A356 aluminium alloy increases the hardness of the A356 alloy by 25.71%.

2. Addition of iron (Fe) content of 1.09% to the aluminium alloy improves wear resistance about 50% and further the wear resistance can be improved by heat treatment.

3. SEM experimentation shows the presence of p-AlsFeSi phase that forms during solidification of aluminium alloy which improves the hardness and wear properties.

4. Addition of iron (Fe) content of 1.09% reduces the tensile strength of A356 alloy by 27%.