Additive manufacturing of ultra-strong precipitation-hardened aluminum alloys for high and ambient temperature applications
Abstract
Aluminum alloys have attractive properties, such as low density, high specific strength and good corrosion resistance. They have, however, experienced a slower adoption in the additive manufacturing (AM) field, compared to steels, titanium alloys and superalloys, mainly due to the specific difficulties associated with laser melting aluminum powders, including high laser reflectivity, a tenacious surface oxide, poor spreadability of low-density aluminum powders, and the high thermal conductivity of aluminum. Additionally, most of the aluminum alloys have poor weldability and relatively large freezing ranges, which makes the layer-by-layer laser manufacturing a challenging task. Thus, the commercially-available aluminum powders for AM are to date limited to the alloy systems with good castability and weldability, such as Al-12Si or Al-Mg based alloys. Different studies have investigated the mechanical properties and heattreatability of Al-Si-based alloys fabricated by selective laser melting (SLM), with particular focus on Al-Si-10Mg. The tensile strength of SLM AlSi10Mg was found to be weakly anisotropic and higher than die-cast A360 (Al-9Si-0.3Mg). Post-processing solutionizing, however, weakened Al- Si-10Mg, and the ultimate tensile strength increased dramatically from ~ 430 MPa in the as-built state to ~ 170 MPa after 2 h at 550 oC. Thus, current research has focused on improving the strength of these alloys by creating thermally stable precipitates within the aluminum matrix. Al-Mn based alloys (AA3000 series) are another class of alloys that exhibit excellent weldability and castability. Thus, our working hypothesis is that Al-Mn-based alloys can be processed with SLM to produce dense and crack-free parts. This hypothesis is strengthened by a recent article where researchers demonstrated that the introduction of Mn and Sc significantly improved the SLM processability of aluminum. The developed alloy, Al-Mn-Sc, demonstrated a high thermal stability, a yield strength of ~ 560 MPa and a ductility of ~ 18% after a simple postprocess heat treatment at 300 oC. An APT study of this alloy demonstrated that Mn remains within the aluminum matrix in solid-solution after SLM fabrication. Alternatively, Sc clusters resulting from intrinsic heat treatment effects of the SLM process serve as precursors of the Al3Sc(L12)- nanoprecipitates for further precipitate growth during subsequent heat treatments. The precipitation kinetics of the a-precipitates in the Al-Mn based alloys is extremely sluggish and the precipitates are extremely coarse and form at low number densities; thus, the strength imparted is negligible. Even in a highly supersaturated SLM processed alloy, mentioned above, Mn tends to stay in the matrix if the heat treatment temperature is not sufficiently high. Our truly breakthrough discovery of Sn-assisted heterogeneous nucleation of a-precipitates in cast alloys, which leads to significant precipitation-hardening, is applicable for AM alloys. We are proposing to extend these extremely encouraging results, to other alloys processed by SLM. An order of magnitude higher supersaturation of the aluminum matrix, achieved through SLM processing, will lead to a much higher volume fraction of smaller a-precipitates, and thereby ultrahigh strengths. Employing this alloy design concept, we will perform a two-Phase study commencing with the relatively simple Al-Mn-Si-Sn alloys, strengthened with Sn-modified a- precipitates, Phase I, and proceed with multicomponent alloys, Al-Mn-Zr-(Y,Er)-Si-Sn, Phase II, utilizing a combination of the a-precipitates and L12-nanoprecipiates to create an ultra-strong aluminum alloy for ambient and high-temperature applications. We do not anticipate any deleterious interactions between the solute atoms in the multicomponent alloys proposed above, which may affect the aging response of the alloys.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 20, 2021
- Source ID
- N000142112782
Entities
People
- David N. Seidman
Organizations
- Northwestern University
- Office of Naval Research
- United States Navy