Wide cracks in land-based Ni- or Co-base superalloy turbine components are difficult to repair successfully using conventional welding or brazing techniques. This project examined the feasibility of liquid phase diffusion brazing using novel Ni- and Co-base braze alloys containing Hf or Zr as melt point depressant for the repair of wide cracks in turbine components. An optimized braze cycle was developed and the joints were evaluated using various metallographic techniques and mechanical tests (elevated temperature tensile tests, creep rupture tests and low cycle fatigue tests). Microstructural examination revealed the presence of Hf- or Zr-rich intermetallic phases (most likely Ni7Hf2 or Ni5Zr) in Ni-base braze joints. These intermetallic compounds were, however, observed to be significantly softer than the boride phases routinely found in commercially available braze alloys with boron as melt point depressant. As a result, the novel wide gap brazed joints displayed excellent mechanical properties (ranging from 80% to 100% of the base metal’s properties). The low cycle fatigue properties of wide gap braze joints performed using a combination of MarM247 superalloy powder and Ni-Cr-Hf or Ni-Cr-Zr braze filler metals were found to be superior to those of the widely used Ni-Cr-B braze filler metals. Wide gap braze repair of FSX-414 Co-base superalloy using novel MarM509/MarM509B and MarM509/Co-Hf braze alloys resulted in high temperature tensile properties equivalent to those of weld repairs in the same parent material (using Nozzalloy filler metal). The creep rupture and low cycle fatigue (LCF) properties of the braze joints were superior to those of welds performed using MarM918 filler metal. / Thesis (PhD)--University of Pretoria, 2008. / Materials Science and Metallurgical Engineering / unrestricted

Scanning Laser Epitaxy (SLE) is a new laser-based layer-by-layer generative manufacturing technology being developed in the Direct Digital Manufacturing Laboratory at Georgia Tech. SLE allows creation of geometrically complex three-dimensional components with as-desired microstructure through controlled melting and solidification of stationary metal-alloy powder placed on top of like-chemistry substrates. The proposed research seeks to garner knowledge about the fundamental physics of SLE through simulation-based studies and apply this knowledge for hot section turbine component repair and ultimately extend the process capability to enable one-step manufacture of complex gas turbine components. Prior methods of repair specifically for hot-section Ni-base superalloys have shown limited success, failed to consistently maintain epitaxy in the repaired part and suffered from several mechanical and metallurgical defects. The use of a fine focused laser beam, close thermal control and overlapping raster scan pattern allows SLE to perform significantly better on a range of so-called “non-weldable” Ni-base superalloys. The process capability is expanded further through closed-loop feedback control of melt pool temperature using an infra-red thermal camera. The process produces dense, crack-free and epitaxial deposit for single-crystal (SX) (CMSX4), equiaxed (René-80, IN 100) and directionally solidified (DS) (René-142) Ni-based superalloys. However, to enable consistent and repeatable production of defect-free parts and future commercial implementation of the technology several concerns related to process capabilities and fundamental physics need to be addressed. To explore the process capability, the fabricated components are characterized in terms of several geometrical, mechanical and metallurgical parameters. An active-contour based image analysis technique has been developed to obtain several microstructural responses from the optical metallography of sample cross-sections and the process goes through continuous improvement through optimization of the process parameters through subsequent design of experiments. The simulation-based study is aimed at developing a multiphysics model that captures the fundamental physics of the fabrication process and allows the generation of constitutive equations for microstructural transitions and properties. For this purpose, a computational fluid dynamics (CFD) finite-volume solver is used to model the melting and solidification process. The development work also focuses on studying process response to different superalloy materials and implementing a multivariate statistical process control that allows efficient management and optimization of the design parameter space. In contrast to the prior work on single-bead laser scan, the model incorporates the raster scan pattern in SLE and the temperature dependent local property variations. The model is validated through thermal imaging data. The flow-thermal model is further tied to an empirical microstructural model through the active-contour based optical image analysis technique, which enables the identification of several microstructural transitions for laser beam describing a raster scan pattern. The CFD model can effectively be coupled with finite element solver to assess the stress and deformation and can be coupled with meso-scale models (Cellular Automata) to predict different microstructural evolutions. The research thus allows extending the SLE process to different superalloy materials, performs statistical monitoring of the process, and studies the fundamental physics of the process to enable formulation of constitutive relations for use in closed-loop feedback control; thus imparting ground breaking capability to SLE to fabricate superalloy components with as-desired microstructures.

Wind power is a renewable energy source that is making great strides in the global energy sector. While wind power is a renewable energy source, it is not entirely free from carbon emissions. This is because the production of wind turbines is dependent on the use of energy, and as a result can emit large amounts of carbon dioxide. This is because the production of wind turbines is dependent on the use of energy and as a result can emit large amounts of carbon dioxide. The emissions come from two sources, the materials used in the wind turbine and the energy used in the manufacturing process. Because wind turbine production is global, the geographical location also affects the climate impact. The purpose of this study is therefore to evaluate the climate impact from material and energy use for the different turbine components. Furthermore, it aims to evaluate the total climate impact between on-and offshore wind power as well as evaluate the climate impact between production in Sweden, Germany and China. The climate impact is based on 13 Vestas LCA reports, together with a model developed in excel. The results show that the location of production plays a significant role in the total emissions, due to the large variation in the electricity mix between different countries. Generally, the steel components are the largest contributors to the total CO2 emissions. Consequently, offshore wind has a higher climate impact than its onshore counterpart because the offshore foundation is made of steel. The result is, however, limited due to the lack of standardisation and since specific information regarding wind power is hard to acquire. / Vindkraft är en förnyelsebar energikälla, som gör stora framsteg inom den globala energisektorn. Samtidigt som vindkraften är förnyelsebar, är den inte helt fri från koldioxidutsläpp. Detta beror på att produktionen av vindkraftverk kräver energi och kan därför släppa ut stora mängder koldioxid. Utsläppen kommer från två källor, de material som används i vindkraftverket och energin som behövs vid tillverkningen. Eftersom produktion av vindkraftverk sker på ett globalt plan, har även den geografiska platsen där tillverkningen sker en påverkan på klimatpåverkan. Syftet med denna studie är att undersöka klimatpåverkan från material och energianvändningen fördelat över vindkraftverks huvudkomponenter. Utöver detta, syftar den även till att undersöka den totala klimatpåverkan mellan land- och havsbaserad vindkraft samt hur klimatpåverkan skiljer sig åt mellan produktion i Sverige, Tyskland och Kina. Studien utgår från 13 Vestas LCA rapporter och använde en excelmodell för att utvärdera utsläppen av koldioxid. Resultatet visar att den geografiska platsen där produktionen sker har stor betydelse för de totala utsläppen, eftersom det är stor variation i energimix mellan olika länder. Överlag är det de stora stålkomponenterna som har störst bidrag till klimatpåverkan. Till följd av detta har havsbaserad vindkraft större klimatpåverkan än landbaserad, eftersom fundamentet primärt består av stål. Resultatet är dock begränsat, på grund av bristen av standardisering i rapportering och eftersom det är svårt att tillhandahålla specifika data gällande vindkraft.