Serious issues including solidification porosity, columnar grains, and large grain sizes are common during fusion-based metal additive manufacturing due to the inherent melting and solidification that occurs during printing. In recent years, a high-temperature, rapid plastic deformation technique called additive friction stir deposition (AFSD) has shown great promise in overcoming these issues. Because the deposited material stays in the solid state during printing, there are no melting and solidification events and the process can result in as-printed material that is fully-dense with equiaxed, fine grains. As AFSD is an emerging process, developing an understanding of the synergy between material deformation and the resultant microstructure evolution, especially the strain magnitude, its influence on dynamic microstructure evolution, and material flow details, is imperative for the full implementation of AFSD. Therefore, the purpose of this work is to investigate the severe plastic deformation in AFSD through complementary studies on the concurrent evolution of shape and microstructure during the deposition of dissimilar aluminum alloys. In this work, we systematically study (1) the entire deposition via dissimilar cladding along with (2) specific volumes within the deposited layer via embedded tracers printed at varied processing parameters. X-ray computed tomography and electron backscatter diffraction are employed to visualize the complex shape of the deposits and understand the microstructure progression.
Investigation of dissimilar cladding of homogeneous AA2024 feed-rods onto an AA6061 substrate establishes a working understanding of the mechanisms related to material flow and microstructure evolution across the whole deposit (macroscopic shape evolution) as well as at the interface between the deposit and the substrate. Variations in tooling and rotation rate affect the interfacial features, average grain size, and depth of microstructural influence. The non-planar and asymmetric nature of AFSD on the macro-scale is revealed and a maximum boundary of deposited material is established which gives a frame of reference for the next material flow study within the deposition zone.
An understanding of the mesoscopic morphological evolution and concurrent dynamic microstructure evolution of representative volumes within the deposition zone is determined by comparing depositions of hybrid feed-rods (AA6061 matrix containing an embedded tracer of AA2024). Samples were printed with and without an in-plane velocity to compare initial material feeding to steady-state deposition. Variations in initial tracer location and tool rotation rate/in-plane velocity pairs affect the final morphology, intensity of mixing, and microstructure of the deposited tracer material. The tracer material undergoes drastic mesoscopic shape evolution from millimeter-scale cylinders to long, curved micro-ribbons. There is simultaneous grain refinement in AA2024 via geometric dynamic recrystallization during initial material feeding, after which the grain size remains relatively constant at a steady-state size. The lower bound of strain is estimated based on extrusion, torsion, and shear-thinning factors.
The step-by-step mesoscopic deformation and microstructure evolution is further elucidated by characterizing depositions of hybrid feed-rods with a series of embedded tracers. The AFSD tooling is stopped quickly at the end of the deposition with a quench applied to "freeze" the sample. X-ray computed tomography reveals multiple intermediate morphologies including the progression from a cylinder to a tight spiral, to a flattened spiral shape, and to a thin disc. EBSD mapping shows that a refined microstructure is formed soon after the material leaves to tool head with areas off the centerline reaching a fully recrystallized state more quickly. The findings from this work summarize the current understanding of the link between material deformation and microstructure evolution in AFSD. Hopefully these first fundamental studies on the co-evolution of material flow and grain structure during AFSD can inspire future work, especially in the area of heterogeneous multi-material printing. / Doctor of Philosophy / Additive friction stir deposition (AFSD) is a new metal 3D printing process that uses friction to heat up and deposit materials rather than using a laser to melt the material into place. This is beneficial since it avoids problems that come from melting and solidification (e.g., porosity, hot cracking, residual stresses, columnar grains). Since AFSD is such a new technology, an understanding of some of the fundamental processing science is needed in order to predict and control the performance of the resultant parts. This is because the processing of a material affects its structure (at multiple scales, for example macro-, micro-, atomic) which then affects the properties a material will exhibit which, finally, dictates the performance of the overall part. Therefore, the purpose of this work is to explore how the feed material is transformed and deposited into the final layer after printing and to link the original processing conditions to the resultant structure. To investigate the interface between the deposited layer and the substrate, we use a simple feed-rod of one aluminum alloy (AA2024) and deposit it onto a substrate of another aluminum alloy (AA6061). To look at just one small volume within the deposited layer, we use a hybrid feed-rod that is mostly AA6061 except for small cylinders of AA2024 that are placed either in the center or on the edge of the feed-rod so that we can track the AA2024. Printing these feed-rods under different processing conditions will help us understand the connection between processing and structure. Using a characterization technique called X-ray computed tomography we can visualize a 3D representation of the final position for the AA2024 material. In order to evaluate the structure on the micro-scale, a characterization technique called electron backscatter diffraction is used to show the individual grains of our metal. The main contributions of this work are as follows: 1) a lower bound of strain is estimated for AFSD, 2) various intermediate deformation steps are captured for the tracer cylinders including a progression from cylinder to multiple spiral shapes to a thin disc to long ribbons, 3) these deformation steps are linked to different microstructures, and 4) changing the tool geometry and other processing parameters significantly alters the range of shapes and microstructures developed in the deposited material. These findings bring us closer to a fully controllable system as well as sparking some interesting areas for future research because of the complex shapes we observed. These results could lead to the customization and optimization of 3D spirals, ribbons, etc. designed for a certain application.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/113957 |
| Date | 02 September 2021 |
| Creators | Perry, Mackenzie Elizabeth Jones |
| Contributors | Materials Science and Engineering, Yu, Hang, Cai, Wenjun, Reynolds, William T., Mueller, Jennifer Elizabeth |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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