Quantification of the Effect of Tool Geometric Features on Aspects of Friction Stir Welding

In the friction stir welding (FSW) process, tool stirring and synchronized movement of the weld materials along a pre-existing seam line causes thermal gradients and severe plastic deformation resulting in the bonding of the adjacent materials. For a given set of welding parameters, tool pin geometries (dimensions, shape) and vertical/helical features that dictate the material motion also have significant effects on process response variables during FSW. Among the primary process controlling parameters in FSW, tool pin geometry and feature vary multifariously in terms of shape, dimensions, feature insertion technique depending on the weld material and application of joint. The current state-of-the-art of FSW tool design is evolved with instinctive perceptions which are typically based on empirical knowledge. It is important to understand the behavior of FSW process response variables such as, in-plane reaction forces, torque, weld power, stir zone temperature and material transport phenomena with the variation of pin features and geometries in order for the process to flourish over a range of manufacturing applications. This dissertation seeks to systematically quantify and establish the relationships among the tool geometric parameters, welding parameters and FSW process response variables to a reasonable extent. In this work, the friction stir weldability of different aluminum alloys in similar/dissimilar joints as well as other aspects of the process including: condition of defect free welds, material flow, process temperature, forces, etc. are examined. The feasibility of using different pin features (thread forms/flats/flutes) coupled with a conventional scrolled shoulder configuration was investigated. Results revealed that joint quality, tool reaction forces, temperature and weld power are highly affected by complex geometric features of the pin. Thread form was found to be an essential component in pin design criteria for effective downward material movement during welding. Completely defect free welds as well as lower in-plane forces were produced in both similar and dissimilar butt joint arrangement while using a mildly tapered conical coarse threaded pin having three shallow flats. The placement of the stronger alloy on the advancing side during bi-material welds also resulted in an effective material flow to produce defect free welds as well as involved with less in-plane forces. In order to prevent premature failure of the pin during the welding process, the stress condition and stress concentration of the pin were also estimated using finite element method (FEM). Moreover, the structural analysis using FEM also provided an optimum dimension of pin features. Finally, welding was performed using a stationary shoulder configuration to demonstrate the effectiveness of a coarse threaded conical pin with three flats in producing defect free welds. Taken together, the studies encompassed in this dissertation provided the basis for a systematic evaluation of tool design criteria as well as the basis to optimize processing window as FSW technology continues to evolve

Structural Analysis of Polyhedral Oligomeric Silsesquioxane Coated SiC Nanoparticles and Their Applications in Thermoset Polymers

The SiC nanoparticles (NPs) were sonochemically coated with OctaIsobutyl (OI) polyhedral oligomeric silsesquioxane (POSS) to create a compatible interface between particle and thermoset polymer. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) techniques were used to analyze the structure of OI-POSS coated SiC nanoparticles. These results revealed the formation of a covalent bonding between SiC and OI-POSS. The transmission electron microscopy (TEM) analysis of OI-POSS coated SiC nanoparticles has also shown the indication of attachment between these two nanoparticles. The OI-POSS coated SiC nanoparticles were further reinforced into a thermoset resin system in order to evaluate mechanical and thermal properties of nanocomposites. The flexural strength, modulus, and glass transition temperature were found to be enhanced while SiC and OI-POSS coated SiC were infused into epoxy system compared to those properties of neat epoxy resin

Unprecedented electrical performance of friction-extruded copper-graphene composites

Copper-graphene composites show remarkable electrical performance surpassing traditional copper conductors albeit at a micron scale; there are several challenges in demonstrating similar performance at the bulk scale. In this study, we used shear assisted processing and extrusion (ShAPE) to synthesize macro-scale copper-graphene composites with a simultaneously lower temperature coefficient of resistance (TCR) and improved electrical conductivity over copper-only samples. We showed that the addition of 18 ppm of graphene decreased the TCR of C11000 alloy by nearly 11 %. A suite of characterization tools involving scanning and transmission electron microscopy along with atom probe tomography were used to characterize the grain size, crystallographic orientation, structure, and composition of copper grains and graphene additives in the feedstock and processed samples. We posit that the shear extrusion process may have transformed some of the feedstock graphene additives into higher defect-density agglomerates while retaining the structure of others as mono-to-tri-layer flakes with lower defect density. The combination of these additives with heterogeneous structures may have been responsible for the simultaneous decrease in TCR and enhanced electrical conductivity of the copper-graphene ShAPE composites