Mechanical and Materials Engineering, Department ofhttp://hdl.handle.net/1974/7702018-01-17T03:04:31Z2018-01-17T03:04:31ZThe Inhomogeneous Microstructure and Deformation of Similar and Dissimilar Al-Zn Containing Mg Friction Stir WeldsHiscocks, Jessicahttp://hdl.handle.net/1974/238212018-01-09T13:46:22ZThe Inhomogeneous Microstructure and Deformation of Similar and Dissimilar Al-Zn Containing Mg Friction Stir Welds
Hiscocks, Jessica
The magnesium-based aluminum-zinc alloys have excellent stiffness to weight ratios, and may be combined by friction stir welding to expand the possible applications. The high aluminum alloy AZ80 in particular has the advantage of being relatively stiff but still extrudable. However limited friction stir welding research is available for this alloy and extrapolation from the extensive work in aluminum alloys is impractical due differences in precipitation behaviour, and magnesium's high plastic anisotropy and tendency to form strong textures during friction stir welding.
This work investigates the correlations between local friction stir welded microstructures, textures, residual strains, and the local deformation behaviour based on strain mapping during tensile tests. Covering bead-on-plate and butt configurations, joining of similar and dissimilar materials, and a range of processing conditions, many findings of interest for deformation modelling and industrial applications are presented.
Synchrotron x-ray diffraction study of an entire friction stir weld was used to determine texture, residual strain and dislocation density data from a single experiment. A number of unique findings were made, mainly related to the asymmetric distribution of properties both between sides of the weld and through the depth. Particularly in the case of strain measurements, features not detectable at coarser measurement spacing or by line scan are presented and compared for multiple processing conditions.
Investigation of the longitudinal material flow during welding showed that even when periodicity in grain size, precipitate distribution, or texture was not observed, periodic changes in texture intensity resulting from compaction of material behind the tool were present, providing evidence that movement of nugget material remained periodic.
Strain localisation and fracture behaviour were found to be completely different between good quality similar and dissimilar friction stir welds. For similar magnesium friction stir welds, higher heat input was shown to improve mechanical performance by reducing the residual strain, while for dissimilar friction stir welds, deformation behaviour was found to be more sensitive to the final material distribution in the friction stir weld nugget. For dissimilar welds, even minor changes to the material flow were shown to have a major impact on the tensile performance.
Topology Optimization for Cost and Time Minimization in Additive ManufacturingRyan, Lukehttp://hdl.handle.net/1974/237892017-12-21T08:19:15ZTopology Optimization for Cost and Time Minimization in Additive Manufacturing
Ryan, Luke
The ever-present drive for increasingly high-performance designs realized on shorter timelines has fostered the need for computational design generation tools such as topology optimization. However, topology optimization has always posed the challenge of generating difficult, if not impossible to manufacture designs. The recent proliferation of additive manufacturing technologies provides a solution to this challenge. The integration of these technologies undoubtedly has the potential for significant impact in the world of mechanical design and engineering.
This work presents a new methodology which mathematically considers additive manufacturing build time and cost alongside the structural performance of a component during the topology optimization procedure. Three geometric factors are found which have influence on the additive manufacturing build time and cost: total surface area, total overhung supported area, and total support structure volume. An innovative methodology to approximate each of these factors dynamically during the topology optimization procedure is presented. The methodology, based largely on the use of spatial density gradients, is developed in such a way that it does not leverage the finite element discretization scheme. This is done in order to overcome some of the shortcomings of the methods in the existing literature. Moreover, it investigates a problem which has not yet been explored in the literature: direct minimization of support material volume in density-based topology optimization. The entire methodology is formulated in a smooth and differentiable manner, and the sensitivity expressions required by gradient based optimization solvers are derived. In two numerical examples, minimization of compliance and total surface area was performed, and reduced build time by an average of 13% over minimization of compliance alone. For the same two numerical examples, minimization of compliance and support volume was performed. Support volume was reduced by an average of 40%, and build time by 25%, but came at the cost of increased compliance.
Multi-Material Multi-Joint Topology Optimization: A Unified ApproachWoischwill, Christopherhttp://hdl.handle.net/1974/237822018-01-02T18:18:37ZMulti-Material Multi-Joint Topology Optimization: A Unified Approach
Woischwill, Christopher
In this research, a methodology and computational tool that solves multi-material topology optimization problems while also optimizing the quantity and type of joints between dissimilar materials was developed. Multi-material topology optimization is a design optimization technique that can determine the optimal distribution of multiple materials within a domain and is typically used to create lightweight designs superior to those created by conventional single-material topology optimization. The usefulness of the technique is limited, however, since all current approaches for multi-material topology optimization assume that all materials are perfectly fused together as a single piece. Since the ideal geometry of a real-world, multi-material design is mutually dependent on the configuration of joints, it follows that current approaches are insufficient for creating practical multi-material designs.
The presented methodology uses a novel decomposition approach to determine both the optimal geometry of a multi-material design as well as the optimal joint design along the interfaces. By decomposing the problem into two simpler subproblems that are solved iteratively, gradient-based optimization techniques can be used, facilitating the solution of large problems that cannot be considered by combinatorial approaches including genetic algorithms. Since the joining interfaces are interpreted directly from multi-material topology optimization results, the shape of the interfaces and the quantity of joints connecting dissimilar materials do not need to be defined by the user a priori. By changing the design variable definition in each subproblem, the computational tool is able to solve both subproblems using the same finite element model provided by the user. Once optimization begins, all model preparation tasks are completed automatically by the tool and no further input is needed from the user.
The methodology and computational tool were verified with three numerical examples. In each example, the tool optimized the geometry of a multi-material design to maximize stiffness while also minimizing the cost of required joints. This study was the first of its kind to not only consider and optimize joints in multi-material topology optimization, but was also the first study to consider multiple types of joints in a joining optimization process.
Mechanical Testing and Characterization of Proton Irradiated 99.4% NI Using the Reactor Materials Testing LaboratoryMattucci, Mitchhttp://hdl.handle.net/1974/237612017-12-13T16:16:28ZMechanical Testing and Characterization of Proton Irradiated 99.4% NI Using the Reactor Materials Testing Laboratory
Mattucci, Mitch
The Queen’s Reactor Material Testing Laboratory uses proton irradiation to simulate the damage induced in a material from neutrons, within a nuclear reactor environment. Inconel X-750 is a 70wt% Ni superalloy, used for the spacer material in the CANDU® reactor. In this work, 99.4% Ni is irradiated to 0.1dpa, with 6MeV protons, at an average temperature of 120oC using the Proton Irradiation Sample Holder. The motivation for this work was to distinguish between damage mechanisms versus phase transformation effects that are exhibited in X-750 from γ’ precipitates and disordered phases. Cross-sectional nano-indentation showed an increase in hardness with increasing dpa from proton irradiation. The Nix-Gao (NG) model, a strain gradient plasticity model, was applied to Indentation Size Effect (ISE) experimentation. A bi-linear trend was observed in the NG model for both the irradiated and unirradiated material. The increase in hardness, in the micro-scale regime, was measured to be 558.3±138.1MPa and using the Busby relationship the increase in the shear yield strength was measured to be 171.4±42.2MPa. TEM characterization identified three types of irradiation induced defects: 1/3<111> Frank Loops, ½<110> Perfect loops and SFTs, with a mean loop size of 7.20, 11.50 and 3.02nm, respectively. The total defect density was measured to be 3.1∙1022m-3, with 24.3% consisting of SFTs, 49.5% and 26.2% consisting of Frank Loops and Perfect Loops respectively. Using the hardening contributions of various microstructural features proposed by G.E Lucas, obstacle barrier hardening models and the super-position principle, the increase in shear tensile strength was calculated. The Bacon-Kocks-Scattergood model yielded a value of 169.8±25.8MPa, which directly agrees with the experimentally measured value. It is proposed that irradiation induced defects result in a smaller ISE, bi-linear transition at a shallower depth and a larger density of geometrically necessary dislocations (GND) to accommodate an indentation of specific depth, shallower than 500nm. The Ma and Clarke model and independent SEM/EBSD analysis provide evidence for a larger GND density in an irradiated material compared to an unirradiated material. It is believed that irradiation induced defects reduced the size of plastic deformation from an indentation and produced larger elastic recovery.