Processing, Structure and Properties of Friction Stir Process Derived In-situ Nano Composites and Foams
High specific-strength materials with high toughness can reduce the weight of automobile and aerospace components. Lightweight components for these applications are essential, as they can significantly improve fuel efficiency. A promising area to produce high-specific strength materials is particulate Metal Matrix Composite (MMC). Other than high specific strength, MMC’s could also have higher modulus, wear, and corrosion resistance. Manufacturing methods and the particles used in MMC’s will dictate the resulting properties. MMC’s can be synthesized either by molten-state or solid-state methods. Molten state methods include gravity casting, squeeze casting, and liquid infiltration. Solid-state methods include powder metallurgy, accumulative roll bonding and diffusion bonding. MMCs synthesised by molten-state routes generally have low ductility and show large variability in properties. Lack of ductility in MMC’s is either due to lack of bonding or due to the formation of intermetallic phases between the particles and the matrix. The bonding between the matrix and the particles can be improved by secondary processes, such as a metallic coating on the particles. However, the intermetallic formation is difficult to control in molten-state methods. MMCs synthesized by molten methods also show large variability in properties, which is due to particle agglomeration and/or uneven distribution. The formation of intermetallic and agglomeration can be controlled up to an extent in solid-state methods. However, MMC’s synthesized by solid-state processes also face poor wettability and porosity issues. Solid-state processes, in general, tend to be expensive. Apart from the issues related to manufacturing, size and morphology of particles used also significantly influence the properties of MMC’s as strengthening mechanism changes with particle size. Micro-particles strengthen the composite by load transfer mechanism. In such composites, damage nucleation is either due to particle fracture or due to interfacial debonding, which creates voids. These voids consolidate to propagate cracks, due to which these composites exhibit a significant loss in ductility. If the particle size is in nanoscale, dominant strengthening mechanism changes from load transfer to Orowan strengthening mechanism, this leads to significant increase in the strength of the matrix. Further, due to the large surface area to volume ratio, the nanoparticles are expected to bond better with the matrix. However, nanoparticles are prone to agglomeration, especially when prepared by the melt-solidification route. Further, nanoparticles are more expensive and can be a health and environmental hazard. These issues can be mitigated if the nanoparticles are synthesized within a solid matrix. However, hard particles that are generally used as reinforcement cannot be refined to nanoscale within the matrix, due to their high fracture strength, which increases as the particle size is reduced. These limitations can be overcome if a soft material is distributed in a matrix and then converted to a hard particle in situ using a solid-state process. To refine soft particles within the matrix, the matrix should be subjected to stress higher than the fracture stress of the soft particles. Further, fractured particles should be uniformly dispersed attain optimum strength. To achieve these objectives, Friction Stir Processing (FSP) is well suited, as the processed material is subjected to severe plastic deformation under high stress. During FSP, a rotating tool with a pin is plunged into the workpiece and traversed along a path. Friction between the rotating tool and the workpiece heats the material and softens it. The softened material is stirred around the tool-pin, which severely deforms the material leading to grain refinement. If secondary particles were introduced along the tool path, these particles would be distributed in the stir-zone to create a composite. The use of a reinforcement that is soft when it is friction stir processed will also mitigate the tool wear associated when a hard particle is being distributed. This ability of the FSP to form nano-scale reaction products was used to synthesize composites. A groove on the plate being processed was filled with the reinforcement phase, sealed and mixed by FSP. Initially, porous, nanocrystalline titanium dioxide was used as reinforcement. Due to porosity, the titanium dioxide was expected to fracture with ease, thus generating nano-size particles that would have lower activation energy. These low activation energy particles could react with the matrix to yield hard reinforcements. Hence, six overlapping passes were done to distribute these particles in an aluminium matrix. It was found that the particles fractured to the nanoscale and reacted with the matrix. X-ray diffraction pattern confirmed titanium dioxide reaction with aluminium to result in Al3Ti. Al3Ti generally forms at above the melting point of aluminium but has formed at around 450°C, which is the temperature that is expected during FSP of Al alloys. The FSP has significantly changed the temperature at which the intermetallic form. Although six FSP passes were done to distribute the particles, the grain size of the composite did not grow beyond five microns, this was attributed to the Zener-pinning mechanism. Due to the combined effect of dispersion and grain boundary strengthening the mechanical properties of the composite showed significant improvement when compared to the base metal. The hardness of the composite increased by 70%. The yield and Ultimate Tensile Strength (UTS) of composite increased by 25% and 42% respectively. Ductility of the composite reduced to 34% from the base-metal ductility of 37%. However, despite the loss in ductility, the toughness of the composite improved by 30% when compared to the base metal. MMCs with carbides of titanium can be significantly stronger. However, these carbide systems form at temperatures much higher than the melting point of aluminium. In the previous section, it was seen that FSP could significantly reduce the reaction temperature. Given this possibility, if carbides of titanium can be synthesised in situ by FSP, they could yield high strength composites. To achieve this FSP of Al with titanium and graphite powders were done. Two blends of titanium and graphite particles were used to synthesize the composites. The first blend was an equiatomic mixture of titanium and graphite and the second blend was a titanium-rich mixture with an atomic ratio of 2:1. Six FSP passes were done to disperse the blends in the aluminium matrix. Titanium and graphite particles were significantly refined and uniformly dispersed in the matrix. In composites of both the blends, in situ reactions during FSP had resulted in the formation of Al3Ti intermetallic and Ti3AlC2 ternary carbide. Due to the faster reaction kinetics of titanium with the aluminium large number of Al3Ti particles were created, which significantly improved the mechanical properties compared to aluminium titanium dioxide composite. The yield and the UTS of the composites increased between 2 and 2.5 times. Although, the ductility of the composites was reduced to about 20 %, from the base-metal ductility of 37%, the toughness of the composites improved by 15 to 20%. Further, the composites exhibited an order of magnitude increase in wear resistance under certain conditions. Recently, a novel method to synthesize composites by FSP was reported, wherein a polymer that forms a Polymer Derived Ceramic (PDC) was dispersed in a copper matrix by FSP. PDC are polymers that convert to ceramics (can be porous) when heated above a particular temperature, even in the absence of air. As the fracture stress of polymers is low, the particles were refined to nanoscale during FSP. The polymer-dispersed sample was then pyrolyzed to convert the polymer to hard ceramic particles. The pyrolyzed sample was processed to further fracture and disperse the ceramic particles. The composites thus produced had a nano-size distribution of the in-situ ceramic phase. Due to Orowan mechanism, the hardness of the composite increased five times compared to the base metal without any loss in ductility. A similar result in aluminium could yield a high specific strength material without the limitations of MMCs mentioned earlier. To synthesize polymer-derived aluminium composite, a silicon-based polymer, Poly (methyl hydrogen siloxane) was used. After three FSP passes, polymer particles were significantly refined. Further, the influence of pyrolysis temperature and duration on mechanical properties of the samples was studied. Irrespective of pyrolysis time and temperature, the hardness of the as-pyrolyzed specimen reduced. This reduction in hardness was primarily due to porosity resulting from gasses evolved during pyrolysis. To eliminate this porosity and to refine and redistribute particles, as-pyrolyzed samples were frictions stir processed again. This led to a substantial increase in hardness and tensile properties of the composite. Under optimum processing conditions, yield, and UTS increased by 2 and 2.4 times, respectively. The ductility of the composite reduced to 23% compared to a base-metal ductility of 37%. However, the toughness of the composite increased by 50%. During the optimization of the process, a study on the effect of pyrolysis temperature on the strength of the composite was carried out. During these studies, samples with a significant increase in the volume were discovered. Essentially the processing had resulted in the formation of aluminium foam. Beyond 550°C, as the strength of the matrix was low the evolved gasses significantly increased the volume of the processed zone creating an aluminium foam. It was found that using this method the many limitations to foam the aluminium could be avoided. Currently foaming is not economical due to high equipment costs and handling cost of metal hydrides. As most of the foaming is done in molten state and porosity gradient is difficult to avoid. To reduce the porosity gradient, viscosity enhancers like calcium are added to the melt along with the blowing agents. These compounds change the chemistry of the melt and cause brittleness in the matrix. Due to change in chemistry, high strength aluminium alloys cannot be foamed. Further, as the bulk of metal is in molten state during foaming, a specific area in a part cannot be foamed. Due to the issues related to the chemistry of the foaming process, foams of high strength aluminium alloys like Al2024 have not been synthesised. Hence, further investigations on the synthesis of polymer-derived foams were carried out on Al2024. Initially, studies were done on Al2024. The effect of pyrolysis temperature and time on foaming behaviour was studied. The density and pore structure were highly dependent on the pyrolysis temperature and time. As Al2024 is a heat-treatable alloy, the compression behaviour was studied for as foamed as well as solutionized and aged sample. Although, all the three type of samples had a comparable compressive strength (plateau stress) of approximately 210 MPa, due to higher strain hardening in the aged samples, the energy absorbed, nearly doubled. As the plateau stress of the foam was high and large number of micro-pores were present, the foam was expected have better damping properties. Hence, damping behaviour of the foam was studied and compared with the monolithic samples. Influence of frequency, stress amplitude, and temperature were studied. It was seen that, with increasing frequency the damping capacity also increased by three-fold (from about 0.03 to 0.09), whereas for rest of the samples the damping capacity increased marginally (~0.02 to 0.03). In case of stress amplitude, at low stress amplitude the foam and the unparalysed composite showed similar damping behaviour. However, with increasing stress amplitude, the damping capacity of composite drastically reduced (0.075 to 0.03) whereas for the foam it reduced marginally (0.075 to 0.06). The temperature had a significant influence on all the samples, in case of foam and composite the damping capacity increased beyond 250°C (0.08), whereas for other monolithic samples it decreased to (0.015 ~ 0.03). The increase in damping capacity in foam and the composite is attributed to larger friction at the interface of particles and the matrix. In summary, high strength and tough composites were synthesised by FSP. It was seen that harder phases were synthesised from relatively softer reinforcements due to mechanical activation by FSP. Further, it was shown that PDC’s could be used to synthesize high strength and tough composites. Furthermore, it was seen polymers could also be used to locally foam aluminium alloys. These metallic foams not only retained heat-treatability but had significant damping capacity.