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Title: Rare Earth Metals Effects on Machinability of Aluminium Based Hybrid Composites Using Magnetic Abrasive Flow Machining Process
Authors: Sharma, Vipin Kumar
Supervisor: Kumar, Vinod
Joshi, Ravinder Singh
Issue Date: 7-Dec-2020
Abstract: In the era of globalization, the demand of composite materials with some unique combination of properties as well as with improved surface finish rises continuously. The aluminium alloy metal matrix composite have tailored properties to fulfill the application-specific properties (like high stiffness, high strength, and low density) related to automobile and aerospace applications, etc. However, the inferior surface properties of the aluminium alloy based composites; such as poor corrosion resistance, poor hydrophobicity behaviour and low wear resistance have restricted the use of these aluminium alloys based composites in aerospace applications under hazardous environmental conditions. Moreover, the imperfections such as shrinkage and porosities observed in the as-cast alloys effect their properties and make them unacceptable for these application areas. Introduction of secondary phase in the form of reinforcement helps in improving the performance of the matrix material through changing its surface morphology, mechanical properties and microstructure. Rare Earth Oxides (REOs) as secondary phase reinforced hybrid composites are increasingly being used in aviation and automobile applications because of their tailored properties combinations of high temperature stability, satisfactorily level of wear resistance, online monitoring control ability, excellent corrosion resistance, good hydrophobicity behaviour, good sintering temperature and high specific strength. The aim of the present work is to develop aluminium stir cast hybrid composites reinforced with alumina and silicon carbide after incorporation of traces amount of REOs. Finally, magnetic abrasive flow machining process has been used for the internal finishing of hybrid composite that can be used in aircraft industry mainly for esthetic and architectural components. Earlier various types of ceramic particulate based composites like Al-SiC, Al-Al2O3, Al-B4C and Al-TiB2have been fabricated for mechanical and metallurgical characterization. Various researchers have been characterized various composite systems on the basis of mechanical and metallurgical testing based on TiC, MgO, TiO2,TiB2, CNTs and ZrO2 etc. However still there is need of exploration that has to be done on Al-6061 reinforced with mixture of (Al2O3+ SiC) and rare earth element CeO2(cerium oxide) as additive. The most commonly used particulate reinforcements of micro level are silicon carbide (SiC) and aluminium oxide (Al2O3). SiC reinforcement not only increases the tensile strength, hardness and density but also improves the wear resistance of aluminium alloy composites. The other micro level v particulate reinforcements Al2O3 have good compressive strength and wear resistance capability. Among all the available alloys of aluminium, Al-6061 was generally preferred for preparing MMCs for aircraft and tribological applications due better wear resistance characteristics, good corrosion resistance, low stiffness and high impact strength. Among all rare earth oxides, the cerium oxide possesses high temperature stability, excellent corrosion resistance capability, excellent water repellent properties and high specific strength. Hence, cerium oxide is selected as a potential candidate to be used in aluminium hybrid composites as reinforcement element. Keeping in mind the above facts, novel hybrid composites with traces amount of REOs have been synthesized via bottom pouring stir casting process. The weight percentage of CeO2 as REOs varied from 0.5 wt% to 2.5 wt% and SiC /Al2O3varied from 2.5 wt% to 7.5 wt%. Finally, the prepared hybrid composites are tested under mechanical, metallurgical and tribological testing. The finishing of the hybrid composites was done with the help of MAFM process in this thesis work. Mechanical testing like ultimate tensile strength (UTS), percentage elongation, hardness (VHN), impact strength and flexural strength of hybrid composite samples with standards ASTM (American Society for Testing and Materials) and IS (Indian Standards) are carried out to see the effect of cerium oxide. The ultimate tensile strength rises from 30 MPa to 123 MPa after incorporation of optimum amount of 2.5wt% of cerium oxide. Percentage elongation was increased from 2.0% to 11.5% with the increase in amount of cerium oxide. Hardness results shown that with the addition of cerium oxide from 0.5 wt% to 2.5 wt%, micro hardness of Al-6061 hybrid composites increased significantly. In Al-6061 hybrid composites with 7.5wt% of (SiC+Al2O3), the hardness is increased from 77 HV to 94 HV after addition of 2.5 wt% of cerium oxide. The Rockwell hardness of the base alloy Al-6061 increased from 61.73 HRB to 82.6 HRB with a percentage increase of 33.80% with the addition of 2.5wt% of cerium oxide. Whereas in case of 7.5 wt% of (SiC+Al2O3), the hardness value increased from 61.73 HRB to 71.8 HRB with a percentage increase of 16.32%. The impact strength of the hybrid composite increases 50 J to 56 J after addition of 0.5 wt% and 1.5 wt% of cerium oxide. But the trend of impact strength decreases after addition of 2.5 wt% of cerium oxide and the value of impact strength is observed as 46 J due to material properties tends to change from ductile to brittle which cause reduction in hardness and because of which ability of a material to absorb shock decreases and it break with less force as compared to other prepared composites. The flexural strength of the Al-6061 base alloy increased from 340 MPa to 450 MPa with the addition of 7.5 wt% ofv Si addition 2.5 wt% of cerium oxide. Wear tests were performed on hybrid composites with highest value of hardness and the base alloy. Optimal value of hardness was found in hybrid composite reinforced with 7.5 wt% of both Al2O3/SiC and 2.5 wt% CeO2. The results of wear test performed at sliding velocity of 0.5m/s and load value of 10N showed a significant improvement in wear rate of hybrid composites using cerium oxide particulates as reinforcement. The rate of wear in sample containing 7.5 wt% of both Al2O3 and SiC showed an improvement of around 38% in comparison to the base metal sample. Whereas, addition of 2.5 wt% of cerium oxides particulate in sample containing 7.5 wt% of Al2O3 and SiC led to 87.28% improvement in wear rate. Different wear mechanisms like oxidation, abrasion and delamination in samples were determined using SEM and EDS analysis of worn surfaces and wear debris for aerospace application. The microstructure analysis was done using SEM, XRD and EDS techniques. The SEM microstructure analysis shows a very significant refinement of the composites after addition of REOs. XRD and EDS analysis shows very good results in terms of fabrication of hybrid composite particles. The XRD study of CeO2 modified aluminium alloy showed that presence of α-Al ,SiO2, Mg5Si6, MgO, Al4Ce, Al3Ce and Al11Ce3 phases. The presence of rare earth particulate helps in grain refinement of the matrix with well defined grain boundaries is seen via Electron Backscatter Diffraction (EBSD) analysis. Wettability behaviour analysis between the aluminium liquid matrix and Al2O3/SiC reinforcement can be improved by the addition of REOs as measured by sessile drop method. The contact angle of composites with Al2O3/SiC reinforcement lies in the range of 87.940 to 98.520. But after incorporation of 2.5 wt % REO, the contact angle raised above 100.780. This result of increase in contact angle with addition of REOs helps to convert composites from hydrophilic to hydrophobic nature. Because of the hydrophobicity, the hybrid composites with REOs show better corrosion resistant properties. Magnetic abrasive flow machining process is generally used for finished intricate shapes of various parts used in aerospace applications. Current theses analyze the effect of a various process variable of MAFM process on the output response surface roughness. Response surface methodology has been applied to find out the effect of input parameters like magnetic flux density, number of cycles and extrusion pressure, size of abrasive particles and working gap on response parameter vii surface roughness. The experimental plan is based on Box-Behnken design. Response parameters have been optimized using the desirability approach in response surface methodology. The significance of different parameters is identified using analysis of variance. An optimum combination of parameters is designed for the process. Experimental results shows that magnetic flux density and extrusion pressure has a predominant effect on surface roughness while internal finishing of the aluminium hybrid composites as compared to the number of cycles. Furthermore, specimens were examined and analyzed using atomic force microscopy for seeing the topography of finished samples.
Appears in Collections:Doctoral Theses@MED

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