EFFECT OF CRYO-FSP ON MICROSTRUCTURAL EVOLUTION AND MECHANICAL PROPERTIES OF STIR CAST Al7075-SiCP NANOCOMPOSITES

Abstract

Over the years, aluminum matrix composites (AMCs) have become the subject of extensive research because of possibility to achieve outstanding properties such as high strength to weight ratio, high stiffness, superior wear resistance and high thermal stability. The driving force for AMCs manufacturing is to tailor the desirable properties of the material by combining the light weight, tough and ductile aluminum (Al) alloy with a suitable ceramic reinforcement which have high wear resistance, high specific stiffness and superior high temperature mechanical properties. Therefore, AMCs could be highly useful as potential structural materials to substitute conventional monolithic Al alloys in automotive and aerospace industries. Because the AMCs not only possess excellent combination of mechanical properties, but also they are economically and environmentally sustainable. Presently, they are being used in some applications in aerospace and automobile sectors, such as cylinder blocks, drive shaft, rotor vanes etc. However, AMCs in as-cast states often show inferior mechanical properties due to its poor particle/matrix interface characteristics, dendritic structure of matrix phase, non-uniformity of reinforcement, lack of matrix continuity and presence of associated porosities, which limit their widespread adoption for the engineering applications to a large extent. To overcome this, present work investigates an effective two-steps method (stir casting followed by friction stir processing (FSP)) for development of cast Al7075-SiC nanocomposites and improvement of mechanical properties by FSP. To achieve the objectives, an attempt has been made to synthesize nanosize SiC particles (<100 nm) reinforced Al7075 composites through stir casting followed by modification of cast microstructures by FSP at two different cooling conditions, i.e. with cryocooling effect and normal air cooling (NAC) state. Aim of the FSP is to modify the cast microstructure thereby enhancing its mechanical strength, ductility, wear resistance and corrosion resistance simultaneously for its practical applicability. As the concept of the FSP is relatively new, first the Al 7075 alloy has been exploited to optimize the process parameters in details to obtain defect-free fine recrystallized microstructure after FSP. The alloy plates were friction stir processed (FSPed) using different combination of traverse speeds (25, 45, 65, 85, 100 and 150 mm/min) and rotational speeds (508 and 720 rpm) in order to investigate their influences on the microstructure, mechanical properties and corrosion resistance. The optimized result in terms of defect-free processed zone with refined microstructure was obtained only at a rotational speed of 720 rpm for a traverse speed of 25, 45, vi 65 and 85 mm/min. The grain size of the nugget zone was found to decrease with increase in the traverse speed from 25 to 85 mm/min at a constant rpm of 720. As a result, the yield strength (YS) and hardness of the FSPed samples increased gradually with an increase in the traverse speed, which effectively decrease the heat input during the FSP operation. Further, critical analysis was carried out thoroughly on the FSP parameters for the as-cast Al7075-2wt.% SiC micro- and nanocomposites. After the FSP, the nanoparticles reinforced composite showed better mechanical properties than that of the microparticles reinforced composite. Tensile strength (>3 times) and wear resistance were found to increase significantly with simultaneous enhancement of the ductility (10 times). The improvement is ascribed to the grain size reduction, distribution of SiC nanoparticles uniformly within the matrix, increase particle-matrix interface characteristics and elimination of casting defects such as porosity after the FSP. The corrosion potentials of the as-cast composites were found to shift towards noble direction after the FSP. Enhancement of corrosion resistance after the FSP is attributed to the decrease in the heterogeneity on the surface and uniform dispersion of the reinforced particles, which reduced the effective active surface area exposed to the corrosive solution. In the next set of experiments, FSP using in-process cryocooling (IPC) approach has been designed in the safe workable regions in order to control the precipitate kinetics besides the grain size refinement during FSP of Al7075 alloy. This study is aimed to understand external cryocooling effects during FSP on microstructural evolution, especially precipitate kinetics and mechanical properties. An indigenous cooling setup is designed to effectively draw out the generated heat from stir zone during FSP. The IPC during the FSP led to about 2 times more grain size refinement in the nugget zone (2.4 μm) with nanosize precipitates of η′ phase (20-30 nm) as compared to 4.7 μm average grain size after FSP at NAC state. This is due to the effective way of heat rejection from the nugget zone by circulating a chilled mixture (at −30 °C) of liquid nitrogen and methanol in the IPC condition. This controls the thermal boundaries of the material during FSP. The best combination of strength and ductility is achieved (UTS=535 MPa El.%= 22) for the IPC FSPed sample. The results have been analyzed in terms of working temperature, strain rate, Zener–Holloman parameter Z, precipitate interface characteristics & its strengthening effect, Hall–Petch strengthening due to grain size reduction and trade-off between them. vii Finally, similar kind of in-process cryocooling FSP schedule has been conducted for the stir cast Al7075-SiC nanocomposites with varying weight percentage of nanosize SiC particles (2%, 3% and 5%). It was found that the FSP with IPC approach offered a great potential in the development of fine grained Al7075-SiC nanocomposites with much improved mechanical properties through microstructural modification. The FSP at IPC state has resulted uniform distribution of the reinforced SiC nanoparticles, refined the matrix grain size, control the precipitation kinetics, enhanced the particle/matrix interface characteristics and eliminated the casting defects from the as-cast composites. Microstructural analysis through TEM and EBSD confirmed the formation of extremely fine matrix grains in which nanosize SiC particles and precipitates of η′ phase are dispersed uniformly in the nanocomposite samples after the FSP. These microstructural changes significantly improved the mechanical properties both strength (YS, hardness) and ductility. Overall, cryo-FSPed Al7075-3wt.% SiC nanocomposite sample showed superior mechanical properties (UTS=552 MPa, %El.=17) and excellent wear resistance. Moreover, the corrosion resistance of these samples (in aerated 3.5% NaCl solution) was also found to be highly significant as the corrosion potentials of the nanocomposites were found to shift towards noble direction after the FSP.