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Microstructure development and weldability of inoculated 6061 processed with gas metal arc directed energy deposition
Kleindienst, Joseph
Kleindienst, Joseph
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2024
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2026-04-04
Abstract
High strength aluminum alloys have been largely inaccessible to fusion based additive manufacturing (AM) due to extensive solidification cracking. Recently, it has been shown that adding nucleation sites, also called inoculants, to high strength aluminum alloys results in grain refinement and elimination of solidification cracking during additive manufacturing. These inoculated alloys processed with various AM technologies also possess unique microstructural features such as nearly equiaxed grains and an absence of dendritic microsegregation. Although cracking is often eliminated in inoculated alloys, very little work has been done to predict the unique microstructural features or assess weldability of processes that result in crack free AM builds. In this work, the solidification microstructure and weldability of plain and inoculated 6061 aluminum produced with gas metal arc directed energy deposition (GMA-DED) was studied with a combination of models and experiments. Single bead wide walls were manufactured with GMA-DED using plain and inoculated wires, and the as-solidified microstructure was characterized using electron back-scatter diffraction (EBSD). The results showed large, columnar grains with intergranular solidification cracking in the plain 6061 build, while a crack free, fine grain, near-equiaxed microstructure was seen in the inoculated build. By combining EBSD and energy dispersive spectrometry (EDS), the inoculated build was shown to have exhibited globular growth while the non-inoculated build displayed a dendritic microstructure. A modification is proposed to the criterion marking the transition from globular to dendritic growth that better matches experimental results in this work. To evaluate weldability, plates of plain and inoculated 6061 processed with GMA-DED and wrought 6061 were autogenously gas tungsten arc welded (GTAW) with varying heat inputs while using the SigmaJig weldability test, and the degree of cracking was evaluated. It was found that the degree of cracking in the inoculated 6061 material was lower than that of plain GMA-DED and wrought 6061 materials. Microstructure characterization revealed that the autogenous weld on the inoculated 6061 material showed fine equiaxed grains whereas the plain 6061 showed coarse columnar grains, consistent with the microstructures of the GMA-DED materials themselves. A combination of heat transfer and modified grain morphology models were employed to predict the solidification morphology of the 6061 builds and welds, which closely match experimental results in all cases. The results of this work provide improved solidification modeling methods which can guide the design of inoculated alloys for printing and welding applications, as well as inform the development of printing and welding processes for inoculated wire feedstocks.
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