CHAPTER 3MATERIALS AND EQIUPMENT 3.
1INTRODUCTION A detailed discussion on materials forcomposites, production process as well as equipment for evaluating theproperties of the aluminium alloy metal matrix and hybrid composites is givenin this following section.3.2 MATERIALSThe materials selected for producingaluminium alloy metal matrix and hybrid composites are described in thefollowing section.3.2.1 Matrix MaterialIn this study, A16061T6 aluminium alloy(Table 3.1) with the density of 2700 Kg/m3 was used as the matrixmaterial. Al6061 alloy exhibits excellent resistance to corrosion under bothordinary atmospheric and marine conditions along with high strength and hardness.
Table 3.1 Chemical composition of Al6061 aluminiumalloy used (wt %) Al Mg Si Cu Fe Cr Mn Zn Ti Others 95.8-98.6 0.8-1.2 0.4-0.8 0.
15-0.40 0.70 0.04-0.35 0.15 0.25 0.15 0.
20 3.2.2 Reinforcements Generally, the following requirementsare applicable for selection of reinforcement material: low density,compatibility with matrix alloy, chemical compatibility, thermal stability,high compression and tensile strength and economics efficiency. In metal matrix composites,reinforcement like alumina, silicon carbide strengthens the metal matrix bothextrinsically through load transfer to the ceramic reinforcement, andintrinsically by increasing the dislocation density. The interaction betweenthe particular reinforcement and the metallic matrix is the basis for theenhanced physical and mechanical properties associated with metal matrixcomposites.Silicon carbide also known ascarborundum, is a compound of silicon and carbon with chemical formula (SiC).
Alumina (Al2O3) is one of the most cost effective andwidely used materials in the family of engineering ceramics. In this study, Silicon Carbide (SiC) andaluminium oxide (Al2O3) particles (Figure 3.1 (a)) wereused as reinforcement with an average particle size are 37 µm. Aluminium oxidepossesses very low reactivity in molten metal and is relatively cheap. Thedensity of silicon carbide (SiC) and alumina (Al2O3) are3.
2 gm/cc and 3.7 gm/cc respectively.Many ceramic particles Al2O3,SiC, TiC, SiO2 and graphite are added in the Al6061 composites toform composites. Among this SiC are commonly and commercially usedreinforcements. This is because SiC has advantages over other ceramic reinforcementssuch as thermal conductivity, density, relative cost and corrosion resistance.
In hybrid metalmatrix composites, a soft reinforcement like graphite contribute to low wearrate, friction and anti-seizing properties, graphite is a soft, slippery,greyish-black substance. It has a metallic luster and is opaque to light.Density of graphite is 2.
3 gm/cc. Special attention has been given to relativeamount of solid lubricants in the metal matrix composites, since it affects themechanical and tribological properties significantly. Figure 3.1 Reinforcement material 3.3 PRODUCTION OF AMMCBY STIR CASTINGIn the present investigation Al-6061T6alloy was chosen as the base matrix which is reinforced with silicon carbideand alumina having size of 37um. Silicon carbide and alumina being hard and brittlein nature, gets accommodated in soft ductile aluminium base matrix enhancingthe overall stiffness and strength of the metal matrix composites (MMCs). Inorder to achieve high level of mechanical properties in the composite, a goodinterfacial bonding between the dispersed phase and the liquid matrix has to beobtained. To increase the wettability of the liquid metal to 2 % by weight,cerium is added.
The silicon carbide and alumina is preheated at 500o Cfor one hour before mixing it to the molten metal. Care was taken to maintainan optimum casting parameter such as stirring speed (350 rpm), stirring time(5-10 min.) and pouring temperature (700o C). The molten metal waspoured into green silica sand mould of diameter 14 mm and length 120 mm. Andafter cooling the samples required for tribological testing and are prepared bydifferent machining processes. In the present work, Al6061T6 MMCs reinforced with 5 wt%, 10wt%, 15wt%, 20 wt%, 25wt%,30wt%, 35 wt% and 40wt%, silicon carbide were produced. The same procedure wasrepeated for hybrid metal matrix composite with 15 wt% of SiC and alumina each.The photograph of cast composites and machined were pin in shown in Fig 3.
2. Figure 3.2 Composite casting andwear test pins. 3.
4 EQUIPMENT USED FORCHARACTERISATION Density of composites is determinedusing top loading electronic balance. According to the Archimedean principle, asolid body immersed in a liquid apparently loses as much of its own weight ofthe liquid it has displaced. This makes it possible to determine the unknownvalue. The density of the solid body is determined by using a liquid of knowndensity.Microstructure of the compositespecimens was carried out using optical microscope.
The specimens weremetallographically polished to obtain an average roughness value of 0.8 m.The micrographs of the polishedspecimens were recorded with different magnifications.
Microhardness valueswere measured at various locations in composite specimen employing microhardnesstester which demand indenter at a load of 100 gm. The average of five readingswas taken as the hardness of composites.Morphology of worn surface of thecomposite specimen is carried out using optical micrograph. 3.5 DRY SLIDING WEARTEST The technique used for studying drysliding wear of composites is described in the following section.
3.5.1 Pin-on-discApparatusDry sliding wear behaviour of compositeswere studied using a pin-on-disc apparatus (DUCOM make), Figure3.3 shows thearrangement of pin-on-disc apparatus. The disc material was made of EN-32 steelwith a hardness of 65 HRC.
The pin specimen is pressed against discat a specified load usually by means of an arm and attached weights. Theapparatus has a friction force measuring system, for example, a load cell, thatallows the coefficient of friction to be determined. Figure 3.3 pin-on-disc apparatus3.5.
2 Dry Sliding WearTest procedure The dry sliding wear tests were carriedout at room temperature (30oC ± 3oC, RH 55 % ± 5 %) underdry sliding condition in accordance the ASTM G99-95 standard. Cylindrical pinsof 10 mm diameter and 50 mm long were machined from composite casting andmetallographically polished (Figure 3.2). Immediately prior to testing, it was cleanedand dried using acetone to remove all dirt and foreign matter from thespecimens.The following equations (3.1-3.
4) areused for calculating volume loss, wear rate, specific wear rate and coefficientof friction.Volume loss = ×1000 mm3 – (3.1)Wear rate = mm3/m – (3.2)Specific wear rate = mm3/ N-m – (3.
3)Coefficient of Friction = – (3.4) Where m1 is the mass of thespecimen before the wear test, m2 is the mass of the specimen afterthe wear test, ? is the density of the composite in gm/cm3, V is thevolume loss in mm3, L is the applied load in Newton, and D is thesliding distance in meter. The coefficient of friction is calculated by theratio between tangential forces (FT) and the normal force (FN).The tangential force is obtained from the load cell fitted in the pin-on-discapparatus. The measured tangential forces measured only during the steady statecondition. 3.6 HIGH STRESSABRASIVE WEAR TESTThe same pin-on-disc type apparatus wasemployed to evaluate the high stress abrasive wear characteristics ofcomposites.
The disc was covered with commercial SiC emery sheet was fastenedto a rotating disc. In order to encounter fresh abrasive material, the specimenwas also moved against the parallel surfaces of the rotational steel disc. Mass loss of the specimen were measuredbefore and after the wear test using electronic weighting balance (accuracy0.0001g) were repeated with additional specimens to obtain sufficient data forsignificant results. 3.
7 SUMMARY Continuous improvementsin testing and manufacturing process to obtain improvement of properties atlower cost remains at the forefront of efforts to expand the importance ofmetal matrix composites. During materials development and testing, every effortwas made to ensure quality of the composites as well as reliability andrepeatability of the test method adopted. The test methods were carried out inparticular wear test methods with reference to common occurrences of frictionand wear in machinery to understand and solve existing or expected wearproblems leading to significant cost reduction.