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Grain refinement in laser powder bed fusion of in-situ metal matrix composite 6061 aluminum alloys
Johnson, Chloe
Johnson, Chloe
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2022
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Additive manufacturing (AM) presents a novel opportunity to generate complex geometries, including cooling channels, allowing for materials with typically low operating temperatures to be used for high temperature applications, inlcuding for light weighting. Aluminum alloys are of primary interests for these applications, but present difficulties for AM, such as a high thermal conductivity, coefficient of thermal expansion, and a large solidification range, leading to columnar growth and hot tearing during solidification. The addition of inoculants to currently unprintable Al alloys presents a method to refine grains and mitigate issues such as hot tearing, as well as reduce anisotropy in parts, by eliminating columnar grains. An in-situ inoculated Al alloy system, A6061-RAM alloys, consisting of a base 6061 alloy with Ti and/or B4C additive particles that react in the melt to form inoculants of Al3Ti, TiC, or TiB2, are explored in this work to understand grain refinement mechanisms in these alloys and the implications for design of inoculated Al alloys for AM.
The impact of solidification conditions, starting reactive particle content, and post-processing heat treatment on final microstructures were explored in this study, with special emphasis being placed on identification of grain refinement mechanisms in these alloys. In-situ imaging of the melt pool during single laser raster experiments was used to track the solid-liquid interface and extract solidification velocities, coupled with thermal gradient modeling using the Rosenthal model, to correlate solidification conditions to the microstructure and understand the impact on grain refinement. The microstructures for the laser conditions studied were found to be consistently refined down to ~2-3 µm throughout the melt pool, independent of solidification conditions. As-built samples, generated using laser powder bed fusion (LPB-F) and containing various amounts of Ti and B4C, were also studied to correlate the microstructure to grain refinement mechanisms. Samples containing only Ti were found to refine the same amount, if not more, than samples containing both Ti and B4C, achieving a grain size of ~2 µm throughout the microstructure. Alloys experiencing a columnar to equiaxed transition (CET), such as those containing only B4C, Al-Ti, or A6061-RAM1, revealed grain refinement determined by the distribution of reactive particles in the melt, and thus solute, determined primarily by flow in the melt pool. It was also found that a variety of particles could serve as effective inoculants, inlcuding TiB2, TiC, SiC, and Al3Ti with different starting type and amounts of reactive particles needed to achieve full refinement for a given chosen inoculant, implying a wide design space for these alloys and a need for consideration of particle types and amounts on final microstructural condition and performance. Finally, as-built A6061-RAM and 6061 alloys were heat treated according to a T6 heat treatment and compared to 6061 wrought to investigate the impact of inoculant and partially dissolved particle contents on precipitation and age hardening in A6061-RAM alloys. Vickers micro-hardness and scanning electron microscopy (SEM) investigation revealed A6061-RAM alloys achieved higher hardness values than AM built or wrought 6061 at shorter aging times (at an aging temperature of 165°C, A6061-RAM achieved a peak hardness of either144 HV or 123 HV depending on starting reactive particle content after 18 h, while AM and wrought 6061 achieved peak hardness values of 101 HV and 122 HV, respectively, after aging for 24 h). However, the presence of inoculant particles in RAM alloys lead to observation of precipitates larger than the expected strengthening precipitate β’’, typically on the scale of a few nm, which were up to ~4 µm. Precipitates were observed to nucleate off of inoculant particles, leading to faster growth and coarsening of these particles during aging and thus the observation of larger precipitates.
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