IMPRESSION CREEP AND HIGH CYCLE FATIGUE BEHAVIOUR OF ULTRASONICALLY PROCESSED INSITU Al6061-Al3Ti/Al3Zr COMPOSITES

Abstract

Engineering sectors demand materials with stringent combination of properties unattainable by conventional alloys. Particulate reinforced aluminium matrix composites (AMCs) are attractive materials for aerospace, marine and automobile applications. Components such as cylinder blocks, pistons, brake, drums/rotors, cylinder liners, connecting rods, gears etc. are made by AMCs due to their high specific strength and elastic modulus, good high temperature properties and damping capabilities, high wear resistance, good electrical and thermal properties. In the present work, AMCs are processed by in-situ salt reaction in aluminium melt. It is well known that in-situ process allows finer precipitates and better interfaces to be obtained. By additionally incorporating ultrasonication, better distribution of precipitates is achieved which have a bearing on the mechanical properties of the composite. This improvement is verified by comparing with the mechanical properties of the base aluminium alloy. To develop AMCs for engine components, creep studies are very important as the components are subjected to high temperature. In recent decades, impression creep method has been used as an alternative of conventional creep test by several researchers to analyze creep property of many materials such as metal matrix composites, superalloys, intermetallics and steel at high as well as low temperature. Unlike conventional creep test, in this test cylindrical indenter with flat end is used to provide constant strain rate under constant stress leading to steady state penetration. High cycle fatigue resistance is often required for automotive components such as power trains, brake discs or piston, where excellent stiffness or damping properties are required. Therefore, it is of great interest to investigate damage behaviour of AMCs under cyclic loading conditions besides mechanical and physical properties of these materials. AMCs reinforced with particulate are expected to show superior high cycle fatigue property ii as compared to the unreinforced matrix alloys. Fatigue life depend on particle size and volume fraction. When the particle size is decreased and the volume fraction is increased, fatigue life is improved due to combination of direct and indirect strengthening. Uniform distribution of the particles in the matrix also increase high cycle fatigue life of the AMCs. There is very less work reported on the fabrication of in-situ Al3Ti and Al3Zr reinforced AMCs by using ultrasonication assisted casting method. Their creep behaviour and fatigue behaviour investigations have not been done so far to the best of our knowledge. It is well established that aluminides can withstand at high temperature and therefore their addition to the base Al alloy can improves its high temperature property. Also, their uniform distribution in the base Al alloy can improve its high cycle fatigue. Ultrasonication has potential to not only disperse reinforced particles uniformly throughout the Al matrix but also eliminate casting defects such as porosity in the AMCs which will lead to superior mechanical properties. The organization of the thesis is as follows: Chapter 1 contains a brief introduction to in-situ AMCs, their fabrication techniques, effect of ultrasonication on the microstructure and the mechanical properties of the composite and their creep and high cycle fatigue behaviour. Chapter 2 gives comprehensive literature review on AMCs and their in-situ fabrication routes, ultrasonic processing, creep behaviour and high cycle fatigue behaviour of AMCs. It also defines the objectives of the present work based on the literature review. Chapter 3 deals with the details of experimental procedure and equipments used for the characterization of the composites and investigation of their creep and high cycle fatigue behaviour. The procedures of specimen preparation for microstructural evolution (scanning electron microscopy, optical microscopy and transmission electron microscopy), mechanical testing (hardness and tensile test), creep test and high cycle fatigue test are explained. iii Chapter 4 deals with processing of Al-Al3Ti and Al-Al3Zr composites with different amounts of Al3Ti and Al3Zr particles by in-situ reaction of aluminium alloy with potassium hexafluorotitanate (K2TiF6) and hexafluorozirconate (K2ZrF6), respectively. Ultrasonication of the aluminium melt during salt reaction was carried out to refine the cast microstructure and achieve better dispersion of in-situ formed particles. The in-situ composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particles generated in the melt promoted heterogeneous nucleation, which was responsible for grain refinement of the cast microstructure. The well dispersed in-situ formed particles significantly improved the mechanical properties including ductility, yield strength (YS), ultimate tensile strength (UTS) and hardness. The dominant strengthening mechanism in the composite was the thermal mismatch strengthening followed by Hall-Petch strengthening. It was observed that the mechanical properties of the in-situ Al3Zr reinforced composite was better than the in-situ Al3Ti reinforced composite. This was explained as due to the average size of Al3Zr particles was 1.8 ± 0.8 μm which was around 50% smaller than the in-situ Al3Ti particles whose average size was 3.4 ± 1.2 μm. Creep analysis was carried out under stresses between 113 and 170 MPa and temperatures ranging from 543 to 603 K. The results obtained from creep analysis revealed that the composites had higher activation energy and stress exponent compared to the base aluminium alloy. The improved creep behaviour is attributed to the presence of homogeneously distributed particles in the aluminium matrix. The obtained stress exponent and activation energy values suggest that the main creep mechanism in the base Al alloy and the developed composites was lattice diffusion-controlled dislocation climb. However, activation energy values of in-situ Al3Zr reinforced composite was higher than that of in-situ Al3Ti reinforced composite which suggested that Al3Zr reinforced composite was more creep resistant than Al3Ti reinforced composite. iv The high cycle fatigue behaviour of the developed composites was investigated for stress ratio R = 0.1. The results obtained from high cycle fatigue analysis revealed that both the composites had higher fatigue life as compared to the base Al alloy due to uniform dispersion of reinforced particles, refinement of the matrix and clear interface between the particles and the matrix. It was observed from the comparison of high cycle fatigue between the two developed composites that the Al3Zr reinforced Al composites had higher fatigue life as compared to Al3Ti reinforced composites under comparable stress amplitude. This improvement was mainly due to presence of fine in-situ Al3Zr particles. Chapter 5 summarizes the main conclusions on processing, creep and high cycle fatigue behaviour two intermetallic reinforced aluminium matrix composites, Al-Al3Ti and Al-Al3Zr. Additionally, scope for future work is included.