Development of high fidelity imaging procedures to identify the relationship between the material microstructure and mechanical behaviour of friction stir welds

Abstract

A procedure to evaluate the local post-yielding behaviour of similar and dissimilar weld combinations of copper (Cu) and stainless steel (SS) materials produced by the Friction Stir Welding (FSW) process is devised. Friction stir weld sub-regions such as the Heat Affected Zone (HAZ), Thermo-Mechanical Affected Zone (TMAZ), and Stir Zone (SZ)/weld nugget located on the advancing (AS) and retreating sides (RS) of the weld are characterised using a range of microscopy techniques. The microstructural characterisation is linked to the local stress-strain behaviour through the development of a novel High Resolution (HR) Digital Image Correlation (DIC) methodology. The overarching aim is to provide a holistic understanding of the weld mechanical behaviour linked directly to the material microstructure and composition generated by the FSW process. During FSW a thickness reduction occurs local to the weld zone as a result of the plastic deformation that is produced during the FSW process, hence a procedure is developed that accounts for the reduction in test specimen thickness and removes the need for post-weld machining to produce a specimen of uniform thickness. A high-resolution (HR) 2D-digital image correlation (DIC) methodology is developed to assess the local strain response across the weld surfaces and the crosssection in both elastic and plastic loading regimes. The HR-DIC methodology includes the stitching of multiple images, as it is only possible to partially cover the FSW region using a single camera with the high-resolution optical set-up. An image processing procedure is developed to stitch the strain maps as well as strain data sets that allows the full-field strain to be visualised and interrogated over the entire FSW region in the materials’ elastic range. As the loading was in the material elastic range it meant the load step necessary for the DIC could be applied repeatedly so that images could be gathered from different positions across the weld region from the same specimen. An alternative to the stereo-DIC method has been devised, which is particularly applicable where high magnification and small stand-off distances are used. The parasitic effect of the out-of-plane displacements evolved from the reduced thickness of the weld was eliminated using a correction procedure as well as the geometry of the weld nugget. The procedure is initially validated in the materials’ elastic loading range in FSW similar weld combinations by showing that the elastic modulus could be obtained accurately in the weld nugget region if the geometry of the weld nugget is known. This provides the important first step in enabling accurate HR-DIC strain measurements on dissimilar FSW welds in the elastic loading range. The microscopy results showed that the microstructure was not homogeneous through the thickness of the FSW region, hence the yielding would occur in a non-uniform fashion. Therefore, it was not possible to apply the methodology devised to account for the deformation resulting from thickness reduction post-yield. Instead, it was assumed that a welded material would yield homogenously in the through-thickness plane (i.e., across the width of the weld). Therefore, the 2D-DIC experimental methodology was applied to the weld cross-section so that the local strain gradients across the weld sub-regions were observed. Likewise, it was not possible to repeatedly load the specimen and collect several images across the weld. So instead of the image stitching, the procedure was modified so that the entire through-thickness view of the FSW region was captured by a sequence of single images during the plastic deformation. HR-DIC strain maps obtained during plastic deformation of the FSW weld cross-section are correlated with micrographs and microhardness measurements, and the local yielding characteristics of the different weld sub-regions are extracted. Finally, X-ray CT characterisation is used to study the influence of FSW tool wear on the local material response and also to trace the complex solid-state material flow across the FSW welds. In both similar and dissimilar FSW welds, materials characterisation results achieved from microscopy, HR-DIC, and X-ray CT techniques are spatially correlated to establish the microstructure-mechanical property relationships from the heterogeneous FSW weld sub-regions.

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