Laser Powder-Bed Fusion of Elemental Powder Blends to Manufacture Precipitation and Oxide-dispersion Strengthened Aluminum Alloys

Glerum, Jennifer Anne

doi:https://doi.org/10.21985/n2-k0nf-gm36

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Laser Powder-Bed Fusion of Elemental Powder Blends to Manufacture Precipitation and Oxide-dispersion Strengthened Aluminum Alloys

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Elemental powder blends are an emerging alternative to prealloyed powders for high-throughput alloy design via additive manufacturing techniques due to their flexibility, low cost, and ease of customization. This dissertation investigates elemental alloying elements (Sc and Zr) which are high-melting and highly reactive, unlike previous work which focused on more concentrated elemental additions of lower-melting, lower-reactivity Cu and Si to aluminum. I am to verify the elemental blend approach as a viable alternative to prealloyed powders for developing novel precipitation- and oxide-dispersion-strengthened aluminum alloys by investigating the (i) processing, (ii) as-fabricated and aged microstructure, and (iii) mechanical properties.To begin, the aging behavior and creep resistance of eutectic AlSi10Mg manufactured by laser powder bed fusion (L-PBF) are studied at 260 and 300 °C. The Si phase, which forms a fine interconnected network of 100-200 nm filaments in the as-printed alloy, coarsens into blocky, 2-3 µm particles after 1 month exposure at 260 and 300 °C, with hardness decreasing following a power-law with exponent n=0.06-0.08. AlSi10Mg with ~ 1 µm Si particles (achieved by aging at 300 °C for 96 h) exhibits creep resistance at 260 and 300 °C, for test durations of a few days, comparable to those of (i) cast alloys: hypereutectic Al-Si alloys with coarse Si particles and eutectic Al-Ce, Al-Ce-Ni with much finer, and more coarsening-resistant eutectics phases (e.g., Al11Ce3, Al3Ni); and (ii) L-PBF Al-Mg-Zr alloys. L-PBF AlSi10Mg shows a power-law creep behavior with high apparent stress exponents (na=10-13, higher than n=4.0 for Al-Mg) for strain rates between ~10-8 and ~10-4 s-1 at 260 and 300 °C; also, a high apparent creep activation energy Qa = 256 kJ/mol is measured between 200 and 320 °C at 45 MPa, compared to Q = 142 kJ/mol for Al. These high apparent stress exponent and activation energy are consistent with power-law dislocation creep with a threshold stress, originating from load-transfer from the creeping Al(Mg) matrix to non-creeping Si particles. Subsequently, L-PBF is used to create Al-1.5Sc, Al-1.5Zr and Al-0.75Sc-0.75Zr (at.%) alloys from blends of elemental Al, Sc, and Zr powders. High-speed in situ synchrotron X-ray imaging and diffraction show that the 20-30 μm Al, Sc, and Zr powders fully melt and sufficiently mix in the molten state to create, on solidification, a homogeneous distribution of primary, micron-size L12 precipitates (Al3Sc, Al3Zr, and Al3(Sc,Zr), respectively), as confirmed by SEM imaging of cross-sections. These primary precipitates show the metastable L12, rather than the stable D023, structure and they nucleate micron-size Al matrix grains. After aging at 300-400 ºC, the alloys show large increases in hardness, consistent with an exceptionally high number density (1.4×1024 m-3) and volume fraction (2.5%) of secondary Al3(Sc,Zr) nano-precipitates with a Sc-rich core and Zr-rich shell, as measured via atom-probe tomography. Operando X-ray diffraction at the Swiss Light Source is performed to extract the structural and thermal history of the process. The pure Sc and Zr particles are found to react with the molten Al pool at 550-650 °C, well below their respective melting temperatures. Various scan areas (1×1, 2×2, 4×4, and 8×2 mm2) were studied to compare (i) the base plate “preheating” effect caused by prior laser scans, (ii) the return temperature reached after the melting scan and before the following scan, (iii) the initial cooling rate immediately after solidification, and (iv) the time spent in the “intrinsic heat treatment range”, defined as 300-650 °C, where secondary precipitation occurs. Microstructural analysis of the as-built samples shows 110-140 nm L12-Al3(Sc,Zr) primary precipitates at the bottom of the melt pool. The 1×1 mm2 samples exhibit the most elongated grains (long axis of 10 ± 5 µm), which correlates with the highest build plate temperature and the slowest initial cooling rate (3-5 × 105 K/s). In comparison, the 4×4 mm2 samples exhibit the smallest equiaxed grains (2 ± 0.6 µm), corresponding to the lowest build plate temperature and the fastest initial cooling rate (6-7 × 105 K/s). These results indicate the need for establishing a minimum feature size during part design or for modifying the laser parameters during processing to mitigate microstructure and performance differences across features of different sizes. Finite element modeling of coarse- and fine-grained alloys with different geometries is performed to evaluate the creep response of AM alloys as a function of (i) volume fraction, (ii) orientation, (iii) interconnectivity, (iv) shape/curvature, and (v) grain size. It is found that maintaining a continuous band in the loading direction of the reinforcing phase (coarse-grained regions in the low-stress regime, and fine-grained regions in the high-stress regime) is necessary to ensure good creep response. Finally, aluminum alloys fabricated via L-PBF and strengthened via L12 Al3Zr precipitates and Al2O3 dispersoids are investigated for their coarsening and creep resistance at 300-400 °C. The layerwise fabrication introduces nanoscale oxide dispersoids in both cases (nominally oxide-free and oxide-containing), likely due to the incorporation of the native oxide layer on the surface of the aluminum powders. The bimodal grain structure, with alternating bands of fine grains nucleated by primary Al3Zr precipitates and coarse grains growing in the direction of solidification, provides a good combination of hardness (~500 MPa) via grain boundary strengthening and creep resistance from the bands of coarse grains pinned by precipitates and oxide dispersoids. Aging the alloys at 400 °C results in a peak hardness of ~750-850 MPa after 1.5 h, and the alloys maintain the as-fabricated hardness up to 5,300 h. The compressive creep resistance at 300 and 400 °C of the Al-Zr and Al-Zr-Al2O3 alloys are comparable to each other and to cast hypereutectic Al-Si alloys despite the much smaller grain sizes, while being slightly less creep resistant than L-PBF AlSi10Mg. Oxygen content analysis is recommended for all Al-base alloys fabricated via L-PBF in a laboratory setting due to the potential for nanoscale oxide inclusions which can affect the mechanical properties of the tested alloys, but require TEM-EDS to identify. Future work is proposed for comparing the alloys manufactured from elemental powder blends to identical compositions manufactured from prealloyed powders to confirm the validity of the approach. Additionally, additions of Sc and Zr to AlSi10Mg is proposed to combine L12 and eutectic strengthening. Finally, development of an alpha-precipitate-strengthened Al-Mn-Si(-Sn) alloy is proposed via elemental powder blends and prealloyed powders.

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