Investigation of microstructure changes in Al2O3-YSZ coatings and YSZ coatings and their effect on thermal cycle life

Yttria-stabilized zirconia (YSZ) coatings and Al2O3-YSZ coatings were prepared by atmospheric plasma spraying (APS). Their microstructural changes during thermal cycling were investigated via scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). It was found that the microstructure and microstructure changes of the two coatings were different, including crystallinity, grain orientation, phase, and phase transition. These differences are closely related to the thermal cycle life of the coatings. There is a relationship between crystallinity and crack size. Changes in grain orientation are related to microscopic strain and cracks. Phase transition is the direct cause of coating failure. In this study, the relationship between the changes in the coating microstructure and the thermal cycle life is discussed in detail. The failure mechanism of the coating was comprehensively analyzed from a microscopic perspective.

The Effect of Grain Orientation of β-Sn on Copper Pillar Solder Joints during Electromigration

The grain orientation of Sn-based solder joints on copper pillars under the combined action of electron wind force and temperature gradient greatly affects their electromigration damage. The copper pillars with Sn-1.8Ag lead-free solder on the top was subjected to a current density of 1.5 × 104 A/cm2 at 125 °C to study the electromigration behaviors. The grain orientation was characterized by scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD) detector. Metal dissolution and voids formation in the cathode as well as massive intermetallic compounds(IMC) accumulation in the anode were observed after electromigration. Closer examination of solder joints revealed that the Sn grain whose c-axis perpendicular to electric current may have retarded Cu diffusion to anode and IMC accumulation. In addition, the newly formed Cu6Sn5 exhibited preferred orientation related to the electric current direction.

PRODUÇÃO DE MÓVEIS RÚSTICOS COM MADEIRA RESIDUAL DA FLORESTA AMAZÔNICA

A exploração madeireira e as conversões de áreas de florestas para uso alternativo do solo na região Amazônica, em especial no estado do Amapá, resultam em grandes quantidades de madeira residual que em geral são abandonadas em campo. Essa problemática da geração de resíduos florestais e principalmente do desperdício e da falta de alternativas para o seu uso apropriado, resultou nesta proposta de pesquisa que teve como objetivo a produção de móveis rústicos artesanais como uma alternativa para o uso da madeira residual da floresta Amazônica. Foram coletados resíduos de madeira deixados no campo após a colheita de floresta Amazônica manejada, destinando-os à fabricação de mobiliário rústico, obtendo como resultado final os seguintes protótipos: banco peça de tora única, balcão para barzinho, aparador de madeira, pia de madeira, mesinha de centro com tampo de madeira, mesinha de centro com tampo de vidro. Foi testada a viabilidade técnica para fabricação artesanal das peças de mobiliário utilizando máquinas e ferramentas portáteis. Os resíduos de madeira se mostraram excelente fonte de matéria-prima para movelaria rústica possibilitando múltiplas alternativas de designs, apesar de certas dificuldades terem sido evidenciadas no decorrer do processo como: peso, dureza, orientação da grã da madeira, entre outras que, no entanto, não impossibilitaram a produção dos móveis. O uso da madeira residual para fabricação de móveis rústicos pode se constituir num novo e empreendedor negócio para a região Amazônica, contribuindo para a geração de renda local. Verificou-se que o método empregado proporcionou a produção de móveis originais, o que agrega valor à sua comercialização. Palavras-chave: trabalho artesanal; empreendedorismo; projeto. Rustic furniture production with wood residues from the Amazon forest ABSTRACT: Harvesting and conversions of forest areas to alternative land use in the Amazon region, especially in the Amapá state, result in large amounts of wood residues that are generally abandoned. This problem about the forest residues and mainly of its loss and the lack of alternatives for its proper use, resulted in this research proposal that aimed to produce handcrafted rustic furniture as an alternative to the use of wood residues from the Amazon forest. Wood residues left in the field after harvesting the managed Amazon forest were collected and destined for the manufacture of rustic furniture, obtaining the following prototypes as a final result: log bench, bar counter, wood dresser, wood sink, coffee table with wood top, coffee table with glass top. The technical feasibility of handcrafting furniture pieces using portable machines and tools were tested. Wood residues proved to be an excellent source of raw material for rustic furniture, enabling multiple design alternatives, although certain difficulties were evidenced during the process such as: weight, hardness, wood grain orientation, among others that did not prevent the furniture production. The use of wood residues to rustic furniture manufacture can constitute a new and entrepreneurial business for the Amazon region, contributing to the generation of local income. It was found that the method used provided the production of original furniture, which adds value to its marketing. Keywords: artisan work; entrepreneurship; design.

Crystal plasticity model of residual stress in additive manufacturing using the element elimination and reactivation method

Selective laser melting is receiving increasing interest as an additive manufacturing technique. Residual stresses induced by the large temperature gradients and inhomogeneous cooling process can favour the generation of cracks. In this work, a crystal plasticity finite element model is developed to simulate the formation of residual stresses and to understand the correlation between plastic deformation, grain orientation and residual stresses in the additive manufacturing process. The temperature profile and grain structure from thermal-fluid flow and grain growth simulations are implemented into the crystal plasticity model. An element elimination and reactivation method is proposed to model the melting and solidification and to reinitialize state variables, such as the plastic deformation, in the reactivated elements. The accuracy of this method is judged against previous method based on the stiffness degradation of liquid regions by comparing the plastic deformation as a function of time induced by thermal stresses. The method is used to investigate residual stresses parallel and perpendicular to the laser scan direction, and the correlation with the maximum Schmid factor of the grains along those directions. The magnitude of the residual stress can be predicted as a function of the depth, grain orientation and position with respect to the molten pool. The simulation results are directly comparable to X-ray diffraction experiments and stress–strain curves.

Molecular Dynamics Simulations of the Tensile Mechanical Responses of Selective Laser-Melted Aluminum with Different Crystalline Forms

The mechanical deformation of cellular structures in the selective laser melting (SLM) of aluminum was investigated by performing a series of molecular dynamics (MD) simulations of uniaxial tension tests. The effects of crystalline form, temperature, and grain orientation of columnar grains on the mechanical properties of SLM aluminum were examined. The MD results showed that the tensile strength of SLM aluminum with columnar grains at different temperatures was lower than that of single-crystal aluminum, but greater than that of aluminum with equiaxed grains. The tensile strength and Young’s modulus both decreased approximately linearly upon increasing the temperature. The deformation mechanisms of equiaxed and columnar grains included dislocation slip, grain boundary migration, and torsion, while the deformation mechanisms of single crystals included stacking fault formation and amorphization. Finally, the influence of the columnar grain orientation on the mechanical properties was studied, and it was found that the Young’s modulus was almost independent of the grain orientation. The tensile strength was greatly affected by the columnar grain orientation. Reasonable control of the grain orientation can improve the tensile strength of SLM aluminum.

Microstructural differences between naturally-deposited and laboratory beach sands

The orientation of, and contacts between, grains of sand reflect the processes that deposit the sands. Grain orientation and contact geometry also influence mechanical properties. Quantifying and understanding sand microstructure thus provide an opportunity to understand depositional processes better and connect microstructure and macroscopic properties. Using x-ray computed microtomography, we compare the microstructure of naturally-deposited beach sands and laboratory sands created by air pluviation in which samples are formed by raining sand grains into a container. We find that naturally-deposited sands have a narrower distribution of coordination number (i.e., the number of grains in contact) and a broader distribution of grain orientations than pluviated sands. The naturally-deposited sand grains orient inclined to the horizontal, and the pluviated sand grains orient horizontally. We explain the microstructural differences between the two different depositional methods by flowing water at beaches that re-positions and reorients grains initially deposited in unstable grain configurations.