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1. ABSTRACT
Advances in material development has been a significant breakthrough in technology throughout the history. Development of Composite materials is a major step in the optimization of materials. The aluminium metal matrix reinforced with Boron Carbide (B4C) particles, termed as Aluminum Metal matrix composites (AMMCs), have greater improvements in their physical, mechanical and tribological properties as compared to conventional MMC’s. The reinforcement is of fundamental importance in composite materials, owing to their responsibility to support stresses acting on the metal matrix.
The element AA2024, formed by stir casting process, as a matrix, to which B4C reinforcement material is added, is selected for experimental investigation. The fabricated AMMCs rods are tested for measurement of hardness, yield strength, ultimate tensile strength, percentage of of elongation and also wear properties. Mechanical properties (hardness, yield strength, ultimate tensile strength toughness and elongation) of fabricated composite AMMCs (%1, %2, %3, %4 and %5) are evaluated and compared with pure alloy.
2. INTRODUCTION
2.1 COMPOSITES
A composite material is a macroscopic combination of two or more distinct materials, having a recognizable interface between them. The resulting composite material has a balance of structural properties that is superior to either constituent material alone.
2.2 METAL - MATRIX COMPOSITES
A metal matrix composite (MMC) is composite material with at least two constituent parts, one being a metal necessarily, the other material may be a different metal or another material, such as a ceramic or organic compound. .
2.3 METAL MATRIX REINFORCEMENTS
Particulate reinforcements in MMCs typically use abrasive-grade ceramic grit. Silicon carbide, alumina, and boron carbide (B4C) are most often used. Titanium carbide (TiC) is also used for iron and titanium alloy matrices. While TiB is used as reinforcement in discontinuously reinforced titanium alloys, this reinforcement is typically obtained by in situ reaction with TiB2. Silicon carbide offers the best strength and stiffness for aluminium matrices, but is slightly more expensive than alumina.
2.4 Aluminium metal matrix composites
The composite series 2xxx Al-Cu-Mg aluminium alloys are precipitation hardenable alloys that rely on the precipitation of fine (metastable) precipitates for strengthening. Among the 2xxx alloys, the 2024-T3 alloy is widely used due to its good combination of specific strength and damage tolerance.
2.5 Boron Carbide (B4C)
Boron carbide (B4C) has many attractive properties, such as low specific gravity, high hardness value, high elastic modulus value and neutron absorption, which help B4C to be widely used as cermets and armour materials. From limited information of B4C reinforced aluminium matrix composites, there are several research works mainly focused on the wet ability and chemical reaction between aluminium and boron carbide.
3. Literature Review
The metal matrix composites are fabricated by various methods resulting in the improved mechanical properties. The various fabrication methods and properties are illustrated as below.
i) F. Muller, J. Monaghan [4] fabricated the particulate reinforced aluminum matrix composites that have good mechanical and wear properties. The various methods for fabrication of particulate reinforced metal matrix composites (MMCs) are powder metallurgy, squeeze casting, casting and so on. For the metal matrix composites, molten metal mixing is a cost effective method while powder metallurgy is costly, and squeeze casting provides good infiltration quality of chopped performs. In MMCs, it is obvious that the morphology, distribution and volume fraction of the reinforcement phase as well as the matrix properties are all factors that affect the overall mechanical and cutting properties.
ii) Y.Sahin. [5] used Al-2014 alloy as matrix material, produced Metal matrix composites with various volume fractions of SiC particles by liquid metallurgy method. MMCs consisting of 10 and 20 wt. % SiC particles with various sizes are produced successfully by molten metal mixing method and subsequently the pressure is applied.
iii) Jiwen Wang et al. [6] identified a process which combines fibre coating and particles hybridizing utilized for fabrication of fibre reinforced aluminum matrix composite. Fibre coating was done by Sol-gel method and SiC particle hybridizing by Sol-gel solution simultaneously.
iv) Ch. Vives et al.[7] fabricated metal matrix composites using helical induction stirrer. The AA2024 alloy was first melted using a direct fired furnace and then poured into the moulds, when the melt reached the superheat condition, SiC particles were added to the liquid surface at steady volumetric rate, while electromagnetic stirring was simultaneously started and maintained until the end of slurry motion and billets were casted. Results like homogeneity, crystal shape, grain size, fraction of primary solid, distribution of SiC particle.
v) Ding Min et al. [8] in their study applied furnace soldering with ultrasonic coating method to solder 6061 aluminium alloy and investigated the effects of both coating time and soldering temperature on its properties. The shear strength of joints changes with the coating temperature and time. The maximum joint strength was obtained under 260oc for 45s.
4. EXPERIMENTAL WORK
4.1 Aluminum metal matrix composites (AMMCs)
Aluminum metal matrix composites (AMMCs) are fabricated with AA2024 as matrix material by the addition of B4C particles of different weight percentages. These metals are tested for their physical properties. The fabricated AMMCs are tested for tensile strength, hardness, wear properties and micro structural characterization by optical microscope.
The matrix material used for stir casting is AA2024, particles of MoS2 Fact as reinforced particles and Magnesium ribbons acts as wetting agent. The chemical compositions of the matrix metal and reinforced particles are presented in Table 4.1 and 4.2 respectively.
4.2 Stir Casting Process
In a stir casting process, the reinforcing phases are distributed into molten matrix by mechanical stirring. When alumina particles introduced into an aluminium melt by stirring molten aluminum alloys containing the ceramic powders. The resultant molten alloy, with ceramic particles, can then be used for die casting, permanent mold casting, or sand casting. The final distribution of the particles in the solid depends on material properties and process parameters such as the wetting condition of the particles with the melt, strength of mixing, relative density, and rate of solidification .The distribution of the particles in the molten matrix depends on the geometry of the mechanical stirrer, stirring parameters, placement of the mechanical stirrer in the melt, melting temperature, and the characteristics of the particles added.
4.3 Electric Induction Furnace:
The electric induction furnace is used for melting of AMMCs and experimental setup is shown in Fig.4.1. Appropriately estimated amount of Aluminum alloy was fed into the electric furnace and was melted at 7000C. At this high temperature magnesium ribbons are added into the molten aluminium alloy. The magnesium ribbons are added to increase the wettability of aluminium so that the reinforcement added to the metal is evenly dispersed. An appropriate amount (1% of the wt. of base metal) of boron carbide (B4C) powder was then added slowly to the molten metal. The B4C powder added to the molten metal was pre-heated upto 5000C to remove the moisture (if any) in it. Simultaneously, the molten metal was stirred thoroughly at a constant speed of 300 rpm with a stirrer. The high temperature molten metal was poured into the pre-heated (3000C) cast iron moulds to get the required specimens. The same procedure was followed to get the AMMC’s of different weight percentages (wt. %) - 2%, 3%, 4% & 5%.
4.4 Heat Treatment of Base Metals
The matrix metal and AMMCs are subjected to heat treatment. All heat treatments are performed in an electrically heated muffle furnace (Fig.4.1) in air atmosphere. The matrix metal (AA2024) as well as AMMCs are solutionized at 520°C for one hour followed by water quenching and aged at 1800C for twelve hours followed by air quenching.
Wear Test
Wear test was conducted on the above specimens according to ASTM G99 specifications on a pin on disc Tribometer as shown in Fig.4.6.Values of time for which the test had to be conducted and also the revolutions that the disc is required to rotate is calculated and the controller is set at these values. The pins were cut according to the standard dimensions. For the evaluation of wear tracks under different heat treating conditions. Three set of pins were tested to arrive at a final reading for each condition. Average surface roughness value of the wear track is measured by Mitatayo SJ 210 surface roughness tester. All these specimens are tested by Ducom TR 20LE pin on disc wear testing machine.