Selective laser melting (SLM) has proven itself as a promising method of additive manufacturing, allowing the creation of parts of complex geometry directly from metal powder. Aluminum-based alloys reinforced with dispersed particles are of particular interest to the aerospace and automotive industries due to their high specific strength. Alloying with scandium (Sc) and zirconium (Zr) in aluminum alloys promotes the formation of finely dispersed intermetallic particles Al3Sc and Al3Zr, providing effective strengthening. The study of the microstructure and mechanical properties of the Al-Fe-Sc-Zr alloy obtained by the SLM method is a topical task.
The SLM process has a significant effect on the alloy microstructure. The high cooling rate characteristic of SLM leads to the formation of a fine-grained structure and promotes the formation of supersaturated solid solutions. Dispersed Al3Sc and Al3Zr particles effectively inhibit grain growth during subsequent heat treatment, stabilizing the microstructure. The distribution of particles along the grain boundaries prevents their sliding, increasing the strength of the alloy.
The Al-5Fe-1Mg-0.8Sc-0.7Zr alloy (in weight percent) was specifically developed for additive manufacturing using selective laser melting (SLM) and is presented in the form of an atomized powder. The structure of this nonequilibrium alloy is based on the following principles: firstly, rapid laser solidification allows for a significant increase in the solubility limit of iron in the aluminum alloy; secondly, the addition of scandium and zirconium helps refine the microstructure, preventing the formation of microcracks and increasing the overall strength of the material. With optimal printing parameters, Al-Fe-Sc-Zr samples manufactured using SLM achieved a maximum density of 99.2%, with only rare intercrystalline cracks observed.
The microstructure was characterized by a bimodal grain structure consisting of an ultrafine equiaxed structure in the molten layer boundary region and a coarse columnar structure within the molten layer. At the weld pool boundary, numerous intermetallic particles (AlFe and Al13Fe4) were formed around the supersaturated α-Al grains, while a eutectic consisting of α-Al and Al6Fe/Al3(Sc, Zr) was present within the pool itself. Notably, a significant number of dislocations were found around the deposited particles using high-resolution transmission electron microscopy, which also contribute to the strengthening of the Al-Fe-Sc-Zr sample obtained by the SLM method.
As a result, at an optimized energy volume (OEV) of 70 J/mm3, an excellent tensile strength of 489 MPa was achieved for aluminum alloys. The results obtained have important practical implications for the development of compositions and microstructure control in the additive manufacturing process.
The mechanical properties of the Al-Fe-Sc-Zr alloy obtained by the SLM method demonstrate high strength and hardness values. The increase in strength is due to both the fine-grained structure and dispersion hardening. The presence of iron (Fe) in the alloy can lead to the formation of additional intermetallic phases that contribute to a further increase in strength. However, it should be taken into account that excess iron content can reduce the ductility of the alloy. Optimization of the chemical composition and parameters of the SLM allows us to obtain an Al-Fe-Sc-Zr alloy with an optimal combination of strength and ductility.
Author: Yueting Wang, Ruidi Li, Tiechui Yuan, Liang Zou, Minbo Wang, Haiou Yang
Institute: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, P.R. China, State Key Laboratory of Solidification Process, Northwestern Polytechnical University, Xi’an, 710072, P.R. China