ABSTRACT

Discontinuously reinforced aluminium matrix composites are fast emerging as engineering materials and competing with common metals and alloys. They are gaining significant acceptance because of higher specific strength, specific modulus and good wear resistance as compared to ordinary unreinforced alloys. Reinforcing particles or short fibers normally used are silicon carbide and alumina which are added externally. Recently it has been shown that alumina particles can be produced in-situ by reaction with metallic oxides reduced by aluminium. Alumina particles thus produced are dispersed in the aluminium matrix and the elementary metal gives solid solution strengthening of the matrix. In-situ particulate composites in comparison with conventional cast particulate composites produced by external addition promote cleaner interface, eliminates interface incompati bility of the matrices with the reinforcements, help to achieve greater thermodynamic stability of reinforcement particles in the matrix at elevated temperature and also increase the possibility of developing coherency between the matrix and particles formed in-situ. The morphology and the distribution of particles strongly influence the physical and mechanical properties of composites. In the present investigation, iron ore was added to molten aluminium, aluminium-magnesium and aluminium-silicon alloys by vortex method. The iron oxides present in the iron ore are observed to react with aluminium, magnesium resulting in production of Al₂O₃, MgO and metallic iron which dissolved in liquid aluminium. The composites thus produced were cast into cast iron die. The mechanical properties of the composites were evaluated. The dry sliding wear behavior of the cast composites was studied at different loads and different sliding velocities using Pin-On-Disk configuration wear testing machine. The worn surfaces and the wear debris were also analyzed using optical microscope and scanning electron microscope.

1. Introduction

Metals such as aluminium and its alloys are one of the most used material systems in the automotive and aerospace industry due to their high strength to weight ratio as well as because of their high thermal conductivity. It is mostly used in high temperature applications such as in automobile engines and in other rotating and reciprocating parts such as piston, drive shafts, brake rotors and in other structural parts which require light weight but high strength material [1,2]. One of the main drawbacks of this material system is that they exhibit poor tribological properties. Hence the desire in the engineering community to develop a new material with greater wear resistance and better tribological properties, without much compromising on the strength to weight ratio led to the development of metal matrix composites [3,4].

A composite may be defined as a material system comprising of two or more constituent materials that remain separate and distinct while forming a single component. The bulk material forms the continuous phase that is the matrix (e.g. metals, polymers, etc) while the other acts as the discontinuous phase that is the reinforcements (e.g. fibers, whiskers, particulates, etc). While the reinforcing material usually carries the major amount of load, the matrix enables the load transfer by holding them together [5,6].

The properties of ceramic reinforced metal matrix composites have been reported much better to that of its unreinforced counterpart [7-9]. The addition of reinforcing phase significantly improves the tribological properties of aluminium and its alloy system [10]. The thinking behind the development of metal matrix composites is to combine the desirable properties of metals and ceramics. The combination of refractory particles with metallic matrix produces a material system with properties intermediate to that of matrix alloy and ceramic reinforcements. Aluminium have useful properties such as high strength, ductility, high thermal and electrical conductivity but have low stiffness whereas ceramic reinforcements are stiffer and stronger and have excellent high temperature resistance but they are brittle in nature [11,12].

There are number of processing techniques which have been developed in recent years for processing metal matrix composites. According to the type of reinforcements, the fabrication techniques also vary considerably. The different techniques employed for metal matrix composites are powder metallurgy, spray deposition, liquid metal infiltration, squeeze casting, stir casting, etc. [13,14]. All of them have their own advantages and disadvantages.

At the early stage of development of metal matrix composites, emphasis was on preparation of fiber reinforced composites. But the high cost of reinforcement fibers, restricted the commercial exploitation of this class except for some high technology applications. The particulate reinforced metal matrix composites are gaining importance nowadays because of their low cost with advantages like isotropic properties and the possibility of secondary processing [15,16].

Among the various processing techniques available for particulate or discontinuous reinforced metal matrix composites, stir casting is the technique which is in use for large quantity commercial production. This technique is most suitable due to its simplicity, flexibility and ease of production for large sized components. It is also the most economical among all the available processing techniques [17].

The present investigation has been focused on in-situ particulate [18] composite formation by utilization of low grade powdered iron ore by its dispersion into aluminum matrix by stir casting method. The objective is to form the reinforcing phase within the metallic matrix by reaction of iron ore with aluminium in the metallic melt. For increasing the wettability, silicon and magnesium were also added. The composites were characterized with the help of optical, x-ray diffraction and scanning electron microscopy. Its tensile strength, hardness and wear behavior were also evaluated.

2. Experimental Procedure

The aluminum-iron ore metal matrix composite was prepared by stir casting route. For this we took 500 gm of commercially pure aluminum and desired amount of iron ore in powder form. The iron ore particles were preheated to 300˚C for three hours to remove moisture. Commercially pure aluminum was melted in a resistance furnace. The melt temperature was raised up to 720˚C and by purging hexachloroethane tablets degassed it. Then the melt was stirred with the help of a mild steel turbine stirrer.

The stirring was maintained between 5 to 7 min at an impeller speed of 200 rpm. The melt temperature was maintained 700˚C during addition of Si or Mg and iron ore particles. The dispersion of iron ore particles were achieved by the vortex method. The melt with reinforced particulates were poured into the preheated permanent metallic mold. The pouring temperature was maintained at 680˚C. The melt was then allowed to solidify in the mould.

Particle size of the iron ore powder was measured by Malvern particle size analyzer (Model Micro-P, range 0.05 - 550 micron). First, the liquid dispersant containing 500 ml of distilled water was kept in the sample holder. Then the instrument was run keeping ultrasonic displacement at 10.00 micron and pump speed 1800 rpm. The dispersant used was Sodium hexametaphosphate (15 ml).

2.1. Hardness

Bulk hardness measurements were carried out on the base metal and composite samples by using standard Vickers hardness test machine. Vickers hardness measurements were carried out in order to investigate the influence of particulate weight fraction on the matrix hardness. Load applied was 10 kg and indenter used was square based diamond pyramid.

2.2. Tensile Behavior

The tensile testing of composites was done on series IX automated materials testing system 1.26 manufactured by Instron Corporation. The sample rate was 9.103 pts/sec and cross-head speed 5.0 mm/min. Standard specimens with 28 mm gauge length were used to evaluate ultimate tensile strength, yield strength and percent elongation.

2.3. Sliding Wear Behavior

Wear has been defined as the displacement of material caused by hard particles or hard protuberances where these hard particles are forced against and moving along a solid surface. Two body sliding wear tests were carried out on prepared composite specimens. A Ducom, Bangalore made computerized pin ondisc wear test machine was used for these tests. The wear testing was carried out at different sliding velocities with normal loads of 10 N, 15 N, 20 N. A cylindrical pin of size 1.1 cm diameter and 2.1 cm length, prepared from composite casting, was loaded through a vertical specimen holder against horizontal rotating disc. Before testing, the flat surface of the specimens was abraded by using 2000 grit paper. The rotating disc was made of carbon steel of diameter 50 mm and hardness of 64 HRC. Wear tests were carried out at room temperature without lubrication for 30 min. The principal objective of investigation was to study the coefficient of friction and wear. The weights were measured before and after each test segment to determine the wear loss of each sample. Scanning electron microscopy was used to analyze the morphology of the worn surfaces of sample.

2.4. Worn Surface and Debris Analysis

The wear debris and worn out surfaces from wear tests was analyzed with the help of optical and scanning electron microscopy to study different wear mechanisms operating on the surface.

3. Results and Discussion

Iron ore analysis: Besides iron oxides, it also contains compounds of Si and Al with other elements in traces. The SEM micro-graph of the iron ore particles is shown in Figure 1.

The chemical compositions of iron ore was determined by X-ray fluorescence technique at the Rourkela steel plant. The result obtained is shown in Table 1.

Iron ore had a wide particle size distribution (Figure 2). The particle size of the ore as received condition, lies in the range from 0.1 - 100 μm.. The average particle size is about 4.78 µm.

3.1. Distribution of Particles in Particulate Composites

The micro structure of the samples, cut from the plate casting at different locations, was observed to study the particle distribution. In the case of Al-10% iron ore, particles were not uniformly distributed throughout the casting. The particles were segregated at the top, bottom, and sides of the plates. The interior of the casting contained very few particles. This is due to the poor wettability and gravity regulated segregation of the particles. Whereas in the case of Al-2% Mg-10% iron ore and Al-2% Si-10% iron ore, particles were present more or less throughout the casting. The particle distribution strongly influences the physical and mechanical properties of the composites. The result shows that volume percentage of reinforcement increases with the addition of magnesium and silicon to the melt. It is probably due to better wetting conditions.

3.2. Mechanical Properties Hardness

The Table 2 shows that there is significant improvement in the hardness with addition of iron ore. The increase in hardness value is more in case of Al-2%Mg alloys with reinforcement as compared to 2% Si addition. This is may be due to the higher volume percentage of the reinforcement. In the case of alloys added with Mg there is a possibility of MgO and MgAl₂O₄ formation and possibly these are much finer and give greater strengthening in the composite.

3.3. Tensile Properties of Composites

From the Table 3, it is clear that addition of iron ore leads to improvement in the ultimate tensile strength of the aluminium alloy. The addition of Magnesium improves the strength of the composites significantly.

The composites prepared in the present study are reinforced with particles of variety of simple and complex oxides as seen from the XRD analysis given above (Figure 3). The size range of the particles is also very wide as compared to the composites investigated earlier. The size varies from the very fine ones of 0.1 µm to the coarse one of 100 µm. The size range of the iron ore particles present indicates that the composite prepared can be considered as dispersion strengthened as well as particle reinforced composite. Thus the strengthening of composite can be due to dispersion strengthening as well as due to particle reinforcement. Dispersion strengthening is due to the incorporation of very fine particles, which help to restrict the movement of dislocations, whereas in particle strengthening, load sharing is the mechanism. The improvement in strength may also result from the dissolution of iron released on aluminothermic reduction of iron oxides present in iron ore. Though, it is very difficult to quantify the contribution of these factors in improvement of strength.

3.4. Dry Sliding Wear Behavior

Wear behavior of different composites were studied with different parameter like sliding velocity and applied loads. There result and discussion are given in the following subsections.

3.5. Effect of Sliding Distance

Figure shows the relationship between weight loss and sliding distance.

Figures 4 and 5 shows wear curves of MMCs specimen with 10% iron ore, 10% iron ore with 2% Si and 10% iron ore with 2% Mg content with normal load of 10N and 20 N. All the MMCs showed a very small initial nonlinear wear regime. After a certain sliding distance, the wear has increased linearly with time indicating steady state wear regime. The transition from initial wear regime to steady state regime has taken place within few minutes (2 - 3 min) of commencement of the test. From the graph (Figure 4) it is evident that the wear resistance of composite is much greater than the commercially pure aluminium. Bulk wear decrease with addition of magnesium.

Incorporation of iron ore content significantly reduces wear. This evidence from the amount of wear observed for commercially pure aluminum and composite with 2% Mg 10% iron ore content. This is because of the presence of hard iron ore particle which will increase the overall bulk hardness.

In the initial wear regime, the reinforced particles act as load carrying elements and as inhibitors against plastic deformation and adhesion of matrix material. In the later stages of wear regime, the worn particles get dislodged from their positions in the matrix and get mixed with the wear debris. The wear debris containing matrix material, worn particles and iron from the disc get pushed into the craters formed by dislodging of particles and act as load bearing elements.

3.6. Effect of Variation in Sliding Velocity

Variation in sliding velocity was achieved by varying rotational speed of the disc with different rpm keeping track diameter constant. The wear of the composites are significantly increased with increase in sliding velocity (Figure 6).

3.7. Effect of Variation in Normal Load

Figure 7 shows the relation between applied load and wear of the MMC (Al-2% Mg-10% Iron ore) for a particular velocity of 1.25 m/sec. The amount of wear has been increased with increase in normal load. Like other Al composites, Al-iron ore composites also have an increasing trend of wear with applied load due to deformation and generation of cracks within oxide films that might acts as a three body wear on removal of the particles, thereby increase the wear rate at higher loads.