David J. Rowenhorst

Material Flow in Friction Stir Welds

Friction stir welding generates periodic features within the weld. These “onion ring” features are associated with variations in both texture and the orientation of that texture along the length of the weld. Analysis of an AA2195 friction stir weld reveals the presence of periodic oscillations between the dominant B and

Consistent representations of and conversions between 3D rotations

\overline{\text{B}}

Collection, Processing, and Analysis of Three-Dimensional EBSD Data Sets

components of the ideal shear texture, suggesting a periodic reversal in the predominant shear orientation during welding that is inconsistent with current understandings of the friction stir welding process. Microstructural features present in the weld and machine force variations during welding indicate that these textures may arise from the oscillation of an off-centered tool. Such a tool oscillation can generate a periodic extrusion of material around the tool, giving rise to the observed flow features, machine force variations, and reversals of the local shear texture orientations. A new model of material flow during friction stir welding is proposed to explain the observed features.

Misorientation Aspects of Growth during Recrystallization

Fluid flow within the dendritic structure at the solid–liquid interface in nickel-based superalloys has been studied in two directionally solidified alloy systems. Millimeter-scale, three-dimensional (3D) datasets of dendritic structure have been collected by serial sectioning, and the reconstructed mushy zones have been used as domains for fluid-flow modeling. Flow permeability and the influence of dendritic structure on flow patterns have been investigated. Permeability analyses indicate that the cross flow normal to the withdrawal direction limits the development of flow instabilities. Local Rayleigh numbers calculated using the permeabilities extracted from the 3D dataset are higher than predicted by conventional empirical calculations in the regions of the mushy zone that are prone to the onset of convective instabilities. The ability to measure dendrite surface area in 3D volumes permit improved prediction of permeability as well.

Material Flow in Aluminum Friction Stir Welds

In materials science the orientation of a crystal lattice is described by means of a rotation relative to an external reference frame. A number of rotation representations are in use, including Euler angles, rotation matrices, unit quaternions, Rodrigues–Frank vectors and homochoric vectors. Each representation has distinct advantages and disadvantages with respect to the ease of use for calculations and data visualization. It is therefore convenient to be able to easily convert from one representation to another. However, historically, each representation has been implemented using a set of often tacit conventions; separate research groups would implement different sets of conventions, thereby making the comparison of methods and results difficult and confusing. This tutorial article aims to resolve these ambiguities and provide a consistent set of conventions and conversions between common rotational representations, complete with worked examples and a discussion of the trade-offs necessary to resolve all ambiguities. Additionally, an open source Fortran-90 library of conversion routines for the different representations is made available to the community.

Direct parameter estimation for generalised balanced power diagrams

We present an archival format for electron back-scatter diffraction (EBSD) data based on the HDF5 scientific file format. We discuss the differences between archival and data work flow file formats, and present details of the archival file layout for the implementation of h5ebsd, a vendor-neutral EBSD-HDF5 format. Information on sample and external reference frames can be included in the archival file, so that the data is internally consistent and complete. We describe how the format can be extended to include additional experimental modalities, and present some thoughts on the interactions between working files and archival files. The complete file specification as well as an example h5ebsd formatted data set are made available to the reader.

Automated Image Alignment and Distortion Removal for 3-D Serial Sectioning with Electron Backscatter Diffraction

Three-dimensional (3D) characterization methods are required to completely determine microstructural descriptors such as the true shape and size of features, the number of features, and the connectivity between these features. Experimental methods to characterize microstructure in 3D have undergone dramatic improvements in the past decade, and there now exists a host of methodologies that are capable of determining 3D microstructural information, ranging from counting individual atoms to imaging macro-scale volumes. The state of the art for this field has been reviewed recently in a Viewpoint Set for Scripta Materialia (Spanos 2006). This chapter focuses on the specific topic of experimental interrogation and analysis of microstructural features such as grains or precipitates in 3D that includes crystallographic orientation data. Currently there are two main experimental pathways to collect such information. Serial sectioning experiments are more commonplace, but consume the sample as part of the experiment; while X-ray methods are nondestructive, but typically require the use of high-intensity X-ray sources. For the X-ray methods, there are a handful of groups world-wide that are working towards spatially-resolved crystallographic analyses of grain structures in 3D using high-intensity X-ray systems (Schmidt et al. 2004; Lauridsen et al. 2006; Budai et al. 2004, 2008; Lienert et al. 2007). These meth-

EBSD Texture Analysis as a Measure of Local Flow in Friction Stir Welding

Effects of the crystallographic misorientation across boundaries between recrystallising grains and the neighbouring deformed matrices are discussed and exemplified by recrystallisation investigations of fcc metals. Classic misorientation observations are reviewed in the introduction, whereas the main parts of the paper focuses on two special boundary migration phenomena observed by in-situ recrystallisation experiments; namely protrusions and facets.