Investigation of Mechanical Properties of AA 7075 / B4C Composite Fabricated by Friction Stir Processing

Abstract

In last few years, Friction stir processing (FSP) has been evolved as a solid state surface modification process, being widely used for fabrication of surface composites these days. Multiple passes of FSP homogenizes the microstructure in the friction stir zone (FSZ) due to the material momentum generated by intense plastic deformation and heat input while stirring. In the course of surface composite fabrication, the reinforcement particles are packed into the cavities prepared on the substrate plate and distributed into the substrate matrix by such thermo-mechanical plastic material flow generated by the FSP tool. Surface composites offer rare combinations of the properties like hardness and toughness, wear resistance and yield strength, etc. which promises a broad application base in several engineering industries. The greater hardness, wear resistance, hightemperature strength and neutron absorptivity clubbed with substantial weight saving to be achieved from Al 7075-B4C surface composites substantiate exploring their suitability for use in recent and incipient applications. Though, AA 7075 being one of the strongest alloy among aluminum grades, the properties such as high yield strength and high flow stress, make it extremely difficult to rupture substrate material and mix reinforcement during FSP. Consequently, a limited number of studies have been reported in the area of AA 7075 substrate surface composites manufacturing through FSP. Also, very limited literature are available that reports effect of process parameters like alteration in tool travel direction between passes, reinforcement particle size, tool traverse speed and tool rotation speed on microstructure, particle distribution pattern, microhardness and wear properties of AA 7075 substrate based composites. x On the basis of research gap, above mentioned FSP conditions were studied independently and in combinations through the experimentations followed by characterization & testing. The experimental methodology has been described, which includes an outline of the experiments, experimental setup, metallurgical and mechanical examination techniques. Fabrication of immaculate surface composite using FSP becomes challenging as, it is required to maintain a proper balance between processing parameters like tool rotation speed, tool traverse speed, the number of FSP passes, tilt angle, reinforcement powder particle size and much more. Influences of the parameters like alteration in tool travel direction between passes (ATTD), tool rotation speed (TRS), traverse speed (TS) and particle size were investigated individually and in combinations, on B4C particle distribution, microstructure, grain size, and mechanical properties. Processing trials conducted employing ATTD displayed more homogeneous B4C particle distribution in the FSZ as compared to the trials conducted employing same direction (SD), attributing to inverted material flow encountered between passes. Additionally, higher peak temperature (Tpeak) and higher grain refinement were witnessed attributed to higher heat dissipation caused by plastic deformation energy and grain breakdown. Superior hardness and wear resistance were also realized in these samples attributed to more uniform particle distribution and several strengthening mechanisms. Furthermore, TRS and TS predominantly contributes to the heat input in the substrate material and plastic deformation encountered while processing. Processing trials conducted employing lowest TRS & TS exhibited more homogeneous B4C particle xi distribution in the FSZ with minimal particle agglomeration/alignment. Hardness and the wear resistance was also found to be higher for these samples in line with the particle distribution results. Superior mechanical properties (viz., hardness and wear) in these samples were also elicited by microstructural homogeneity and finer grains. The microstructural inhomogeneity was more significant in the specimens produced at higher TRS and TS. Additionally, processing trials conducted employing fine B4C particles exhibited more uniform distribution comparatively, attributed to the strong particle/substrate bond, aiding particle flow in the trace left by the FSP tool. The grain size in the FSZ was also influenced by powder particle size. Fine particles offer a larger number of preferred nucleation sites which intensified pinning to grain boundaries and led to finer grains. Additionally, greater breakage of the established grain was perceived to be instigated by a higher number of fine B4C particles comparatively and which resulted in the fine grains. Superior properties like hardness and wear resistance were witnessed in these samples. Almost three times higher life may be expected from the best-processed composites as compared to untreated substrate owing to these superior mechanical properties. Keywords: Composite, surface, particles, MMC, aluminium, 7075, boron, carbide, friction, stir, processing, FSP, grains, precipitates, direction, temperature, hardness, wear.