EXPERIMENTAL STUDIES ON REINFORCED ALUMINUM ALLOYS (MMCs)-ABSTRACT

The energy crisis and global inclination to reduce green house gas emissions have been catalytic in directing the attention of researchers/scientists to look for good physical and mechanical properties light weight materials. Aluminum (Al) is an important engineering material being used in a number of engineering applications. It is the most abundant metal in nature around 8% by weight of the earth's crust. Al is a good electrical and thermal conductor with Face-Centered Cubic structure. Al/Al alloys have been attracting much attention as light weight materials due to their high specific strength, good castability, good machinability, high damping capacity, and its availability as a natural mineral. Al/Al alloys are also 35% lighter than iron based materials. The main focus currently is to develop new generation of composite materials capable of exhibiting good combination of thermal, mechanical and other properties. New materials such as Metal Matrix Composites are necessary to fulfill the requirements in applications for instance in sports, aerospace and automobile sectors.

The most commonly used and economically viable techniques for fabrication of MMC's are solidification processing and stir casting. Another important technique used for the purpose, is infiltration of liquid metal through narrow crevices between fibers or particulate reinforcements that are arranged in a perform. In solidification process, liquid metal is combined with the reinforcement phase and solidified in a mold. However in stir casting, the molten metal is stirred with the help of either a mechanical stirrer or high intensity ultrasonic waves. This action disperses the reinforcing phase, which is added to the surface of the melt in the molten metal and solidifies the composite melt, containing reinforcements suspended in it. Stir Casting route is now used for large-scale production of Metal Matrix Particulate Composites. Various metals such as Al, Mg, Ni, and Cu have been used as the matrix and a wide variety of reinforcements like ZrSiO4, SiC, graphite, SiO2, Si3O4 and Al2O3 have been used in the available literature using the aforesaid techniques. In this work, new composites based on SiC, MgO and Al2O3 are developed to address the industries problems.

In the first phase of the present work, the reinforced MMC's of Al/Al alloy-Al2O3 system, with nominal composition (A384.1)(1-x)[(Al2O3)p]x were fabricated under extruded and peak aged conditions by using A384.1 Al Alloy as matrix and Al2O3 with 0.220, 0.106 and 0.053 microns particle sizes as reinforcement in varying amounts. The modified stir casting method is employed to produce twenty four composite samples from 0.0 to 0.20 and then characterized along with the unreinforced alloy.

The results revealed reasonably uniform distribution of Al2O3 particulate, good adherence of reinforcement with the matrix, and presence of minimal porosity suggesting the suitability of processing methodology adopted in the present study. The microstructure of the reinforced samples is found to have lower amount of overall porosity as compared to any other similar composite system. Along with reduction in porosity, a better distribution of particles was achieved in all alumina composites.

No interaction layer or any other reaction product was found at the interface that appeared clean in the as-cast Al-Al2O3 materials. The EDX analysis, however, revealed the presence of the elements Al and O at the interface layer. It is believed that this is probably coming due to an oxide layer formed during sample preparation. With increasing amount of alumina, the MMCs showed a significant increase in tensile failure strain (from 5.6% to 29.5%), compressive strengths (0.2% proof stress and compressive strength) and tensile strengths (0.2% proof stress and ultimate tensile strength) and hardness reduced during ageing. Moreover, the composite of (A384.1)(1-x)[(Al2O3)p]x exhibited a good combination of porosity, density, tensile strength, compressive strength and age hardening properties.

In the second stage of this work, the (A384.1)(1-x)[(MgO)p]x composite system has been investigated by taking MgO, with grain sizes of 0.220, 0.106 and 0.053 microns, as reinforcement and Al alloy (A384.1) as matrix and composite samples with x=0.0 to 0.20. In general densification is obtained in the base matrix (alloy) and all composites samples (having reinforcements) are fabricated under extruded and peak aged conditions.

But the composites having reinforcement MgO, in the extruded condition, observed some amount of porosity, probably due to dissolved gases i.e. hydrogen, nitrogen etc. A characteristic change is observed in porosity and density of the Al alloy and MMCs from extrusion to peak aged as a function of reinforcement from x=0 to x=0.20. It noticed that the value of porosity from unreinfroced to reinforced conditions increases with the increase in reinforcement. The change in porosity observed from unreinfroced to reinforced conditions is 5.47% for x=0.0 sample, but this is found to be 25.6%, 29% and 32% for x=0.10, 0.15 and 0.20 samples respectively. It is further noticed that density exhibits an increase of 13.2% from from unreinfroced to reinforced conditions, which increases to 18.2% as x varies from 0.0 to 0.20. The micro-hardness and macro-hardness measurements on unreinforced alloy and the MMCs in extruded and peak aged conditions showed that the composite with x=0.10 has higher macro-hardness than the unreinforced alloy (x=0.0) in both extruded conditions and peak aged conditions. On the other hand, composites x=0.15 and 0.020 show lower hardness than the alloy in the extruded condition. This is due to the porosity present in these two composites. Due to extrusion the pores are closed and as a result these two composites exhibit higher hardness than the unreinforced alloy in the extruded condition. The results show that increasing the content of MgO led to a decrease in amount of MgAl2O4 phase, an increase in the amount of Mg,Al phase and the significant reinforcement of matrix grain size. The increasing percentage of reinforcement with change in particle size also led to a noticeable improvement in ageing properties, tensile and compressive strengths.

In the third stage of this work, the (A384.1)(1 - x)[(SiC)p]x composite system has been investigated by taking SiC, with grain sizes of 0.220, 0.106 and 0.053 microns, as reinforcement and Al alloy (A384.1) as matrix. The study revealed that SiC can be used for production of in-situ reinforced composites. Depending upon the particle size, SiC up to 10-20% by weight can be successfully added to Al alloy matrix. Twenty four composite samples with 0x0 from 0.0 to 0.20 have been fabricated. The micro-structural characterization of MMCs and unreinforced alloy exhibited lower amount of porosity as compared to any other similar composite system. SiC particles are found to get wetted by Al Alloys melt in a better way as compared to other ceramic reinforcements and thus the interfacial structure bonding is comparatively strong in this system as evidenced by the improved mechanical properties of these materials. The observed strengthening of the composite can be explained in terms of dispersion strengthening due to SiC particle reinforcement. A change in density from as-cast to extruded conditions is found to be 5.546%, 6.444%, 8.086% and 9.304% for x values of 0.0, 0.10, 0.15 and 0.20 respectively. Accordingly, the reduction in porosity from unreinforced to reinforced samples was also observed for the different values of 0x0 (94.4% for x = 0 and 67.05% for x=0.20). Moreover, the values of macro- and micro-hardness of the samples exhibited a trend of variation. The higher values of micro-hardness observed in case of peak-aged composites can be explained in terms of higher dislocation density near the particle-matrix interface due to the large different in thermal expansion coefficient between the matrix and reinforcement as compared to unreinforced alloys. The mechanical testing of the fabricated composites was also done under compressive and tensile stresses.

To evaluate the performance of the fabricated MMC samples under actual service conditions and hence to elucidate the application potential of the fabricated Al/Al Alloys matrix-ceramics particulate composite in technological applications, the Diffraction Scanning Calorimetric (DSC) was performed on the different composites of Al2O3, MgO and SiC at 2_C/min to determine the phase stability and transformations and to study precipitation in aluminum alloys. Quantitative interpretation of DSC experiments on aluminum based alloys had been done. All the materials were found to have phase stability and no thermal evidence of formation of any undesirable phase. Overall, the developed composites show a different range of physical and mechanical properties and thus show a great potential in diverse engineering applications such as in sports industry, aerospace, automobile , military applications and transportation. Fuzzy model of the system is also developed for density and porosity of the reinforced MMCs, using adaptive neuro-fuzzy inference system (ANFIS) and performance of experimental values is evaluated by comparing it with fuzzy model and good correlation is achieved between them.