This thesis investigates the effect of microstructural strengthening mechanisms on the overall mechanical performance of micro-truss periodic cellular metals (PCMs). Prior to the author’s work, the primary design considerations of micro-truss PCMs had been topological issues, i.e. the architectural arrangement of the load-supporting ligaments. Very little attention had been given to investigate the influence of microstructural effects within the cellular ligaments. Of the four broad categories of strengthening mechanisms in metals, only solute and second phase strengthening had previously been used in micro-trusses; the potential for strengthening micro-truss materials by work-hardening or grain size reduction had not been addressed. In order to utilize these strengthening mechanisms in micro-truss PCMs, two issues needed to be addressed. First, the deformation-forming method used to produce the micro-trusses was analyzed in order to map the fabrication-induced (in-situ) strain as well as the range of architectures that could be reached. Second, a new compression testing method was developed to simulate the properties of the micro-truss as part of a common functional form, i.e. as the core of a light-weight sandwich panel, and test the effectiveness of microstructural strengthening mechanisms without the influence of typical high-temperature sandwich panel joining processes, such as brazing. The first strengthening mechanism was achieved by controlling the distribution of plastic strain imparted to the micro-truss struts during fabrication. It was shown that this strain energy can lead to a factor of three increase in compressive strength without an associated weight penalty. An analytical model for the critical inelastic buckling stress of the micro-truss struts during uniaxial compression was developed in terms of the axial flow stress during stretch forming fabrication. The second mechanism was achieved by electrodeposition of a high-strength nanocrystalline metal sleeve around the cellular ligaments, producing new types of hybrid nanocrystalline cellular metals. It was shown that despite the added mass, the nanocrystalline sleeves could increase the weight-specific strength of micro-truss hybrids. An isostrain model was developed based on the theoretical behaviour of a nanocrystalline metal tube network in order to predict the compressive strength of the hybrid materials.

This thesis investigates the effect of microstructural strengthening mechanisms on the overall mechanical performance of micro-truss periodic cellular metals (PCMs). Prior to the author’s work, the primary design considerations of micro-truss PCMs had been topological issues, i.e. the architectural arrangement of the load-supporting ligaments. Very little attention had been given to investigate the influence of microstructural effects within the cellular ligaments. Of the four broad categories of strengthening mechanisms in metals, only solute and second phase strengthening had previously been used in micro-trusses; the potential for strengthening micro-truss materials by work-hardening or grain size reduction had not been addressed. In order to utilize these strengthening mechanisms in micro-truss PCMs, two issues needed to be addressed. First, the deformation-forming method used to produce the micro-trusses was analyzed in order to map the fabrication-induced (in-situ) strain as well as the range of architectures that could be reached. Second, a new compression testing method was developed to simulate the properties of the micro-truss as part of a common functional form, i.e. as the core of a light-weight sandwich panel, and test the effectiveness of microstructural strengthening mechanisms without the influence of typical high-temperature sandwich panel joining processes, such as brazing. The first strengthening mechanism was achieved by controlling the distribution of plastic strain imparted to the micro-truss struts during fabrication. It was shown that this strain energy can lead to a factor of three increase in compressive strength without an associated weight penalty. An analytical model for the critical inelastic buckling stress of the micro-truss struts during uniaxial compression was developed in terms of the axial flow stress during stretch forming fabrication. The second mechanism was achieved by electrodeposition of a high-strength nanocrystalline metal sleeve around the cellular ligaments, producing new types of hybrid nanocrystalline cellular metals. It was shown that despite the added mass, the nanocrystalline sleeves could increase the weight-specific strength of micro-truss hybrids. An isostrain model was developed based on the theoretical behaviour of a nanocrystalline metal tube network in order to predict the compressive strength of the hybrid materials.

Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-11-08T18:39:47Z (GMT). No. of bitstreams: 1 Silva_RenatoRafaelda_M.pdf: 15646556 bytes, checksum: 19ff8cd2fd9081ac7985bc44c94795f5 (MD5) Previous issue date: 2008 / Resumo: Este trabalho teve como objetivo estudar o processo de produção de esponjas metalicas por tixoconformacao em pré-formas removiveis e caracterizacao mecanico-metalurgica do produto, visando a compreensão da influencia de parametros de processo nas suas caracteristicas estruturais e propriedades. A liga empregada, AA2011, foi infiltrada sobre pre-formas de particulas de NaCl de granulometrias distintas, sinterizadas ou soltas. As esponjas metalicas produzidas foram caracterizadas quanto a sua arquitetura, propriedades fisicas (densidade real e relativa, condutividade e difusividade termicas) e mecanicas (tensão de compressão, modulo de Young, tensão no plato, deformação e energia de impacto absorvida); foi analisada a influencia do tipo de pre-forma utilizada para produção das esponjas, nestas caracteristicas e propriedades. Os resultados mostraram boa reprodutibilidade do processo, principalmente quando da utilização de pre-formas de partículas medias e grosseiras nao sinterizadas. A densidade da esponja e a espessura da parede celular apresentam tendencia a aumentar com o aumento das dimensões das celulas produzidas em preformas de particulas mais grosseiras. Todas as amostras produzidas apresentaram condutividade termica da ordem de 10x inferior e difusividade termica da ordem de 4x superior as do metal maciço. O comportamento das esponjas em compressão estatica ou dinamica se mostrou tipico de materiais celulares, com reduzidos valores de modulo de Young e de tensão de compressão, grande plato de deformação plástica sem acréscimo de tensão, e elevados valores de deformação total e energia absorvida no impacto / Abstract: The aim of this work was the analysis of the thixoforming process to produce metallic sponges and the mechanical and metallurgical characterization of the product, searching for better understanding the influence of processing parameters in the material structural characteristics and properties. The aluminium alloy AA2011 was infiltrated in the semi-solid state into performs of NaCl particles with different sizes and in sintered and non sintered conditions. Cellular products were characterized concerning internal architecture, physical properties (actual and relative densities, thermal conductivity and diffusivity) and mechanical properties (Young's modulus, compressive strength, plateau stress and absorbed impact energy); it was analyzed the influence of processing parameters on the properties and characteristics of the produced sponges. Results showed good reproducibility of the process, mainly when medium and coarse non sintered space holder particles were employed. Density of the product as well as cell wall thickness increases as space holder particles sizes increase. All the sponges produced showed low thermal conductivity (~10x inferior compared to the bulk material) and high thermal diffusivity (~3x superior compared to the bulk alloy). Results of static and dynamic compression tests showed typical cellular material behaviour in all cases, presenting low values for Young's modulus and compressive strength, a well defined plateau of plastic deformation, high plastic deformation and high capacity of energy absorption in impact events / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica