Development of high-strength aluminium alloys for laser powder bed fusion

Roscher, Moritz; Raabe, Dierk (Thesis advisor); Korte-Kerzel, Sandra (Thesis advisor); Jägle, Eric A. (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Laser Powder Bed Fusion (L-PBF) is an additive manufacturing technique in which three-dimensional components are produced from computer-aided design models by melting metallic powder in a layer-by-layer manner. Since L-PBF enables the production of geometrically-complex components that cannot be fabricated by conventional manufacturing methods, it offers the opportunity to produce metallic components with improved functionality and reduced weight. Due to their low mass density, aluminium alloys are well-suited for the production of weight-optimised structural parts. However, the use of aluminium alloys in L-PBF is associated with difficulties. On the one hand, conventional high-strength aluminium alloys cannot be used in L-PBF since solidification cracks form during the process, resulting in premature material failure. On the other hand, most of the aluminium alloys available for L-PBF do not exhibit attractive strength-ductility combinations. Therefore, this thesis aims at developing aluminium alloys that simultaneously exhibit high strength and good processability by L-PBF, and further do not rely on expensive alloying additions. For this purpose, three different alloy design strategies are developed and implemented. First, the feasibility of using L-PBF to process the commercial, high-strength aluminium alloys 2065 and 7034 is investigated in laser surface melting experiments. In both alloys, solidification cracks are detected, indicating their poor processability. The formation of cracks is rationalised in terms of grain refinement and a quantitative criterion for solidification cracking, relating the crack formation in both alloys to their chemical compositions and microstructural features. Using laser surface alloying, the chemical composition of alloy 2065 is modified with the goal of suppressing the crack formation and thus enabling processing. While the addition of copper has a negligible effect on the cracking tendency, a titanium-modified 2065 alloy is shown to have a high resistance to crack formation during laser remelting, suggesting its use in L-PBF. Secondly, a novel Al-Ti-Si alloy is developed that employs the L12-Al3X-forming element titanium as the main precipitation-hardening element. Initially, a range of Al-Ti-based compositions is fabricated from elemental powder mixtures and subsequently investigated, including binary Al-Ti and ternary Al-Ti-X alloys with additions of nickel and silicon. All alloys are shown to exhibit crack-free and highly supersaturated microstructures after L-PBF production. However, massive solid-state precipitation of the desired L12-Al3Ti phase, accompanied by an increase in hardness, exclusively occurs in a silicon-containing Al-Ti-Si alloy. This is due to the influence of silicon on the precipitation process, which is elucidated on the basis of thermodynamic and kinetic effects. Thirdly, a high-strength aluminium alloy based on the eutectic Al-Ni system is developed using a mechanism-based alloy design approach. With the goal of creating a microstructure in which several strengthening mechanisms contribute to the total yield strength, theoretical models are used to identify alloying additions capable of forming homogeneous and supersaturated solid solutions during L-PBF. By this method, an Al-Ni-Zr-Cr alloy is developed. An in-depth microstructure characterisation reveals that Zr and Cr are primarily in solid solution in the as-produced material. During heat treatment, however, a high precipitate number density of up to 10^24 L12-Al3Zr particles per m^3 is produced, while Cr largely remains in solid solution, providing solid solution strengthening. Tensile properties and temperature stability of the Al-Ni-Zr-Cr alloy are compared to other aluminium alloys and rationalised in terms of the underlying strengthening mechanisms. This thesis introduces novel aluminium alloy compositions for L-PBF, presents methods for efficient alloy screening, and discusses metallurgical mechanisms, the understanding of which can advance the alloy design for L-PBF.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Physics and Institute for Physical Metallurgy and Materials Physics [523110]