Densification, microstructure, mechanical properties, and thermal stability of high-strength Ti-modified Al–Si–Mg–Zr aluminum alloy fabricated by laser-powder bed fusion

Abstract

Micrometer-sized, irregularly shaped Ti particles (0.5wt% and 1.0wt%) were mixed with an Al–Si–Mg–Zr matrix powder, and a novel Ti-modified Al–Si–Mg–Zr aluminum alloy was subsequently fabricated via laser-powder bed fusion (L-PBF). The results demonstrated that the introduction of Ti particles promoted the formation of near-fully equiaxed grains in the alloy owing to the strong grain refinement of the primary (Al,Si)3(Ti,Zr) nanoparticles. Furthermore, the presence of (Al,Si)3(Ti,Zr) nanoparticles inhibited the decomposition of Si-rich cell boundaries and the precipitation of Si nanoparticles in the α-Al cells. The ultimate tensile strength (UTS), yield strength (YS), and elongation of the as-built 0.5wt% Ti (0.5Ti) alloy were (468 ± 11), (350 ± 1) MPa, and (10.0 ± 1.4)%, respectively, which are comparable to those of the L-PBF Al−Si−Mg−Zr matrix alloy and significantly higher than those of traditional L-PBF Al−Si−Mg alloys. After direct aging treatment at 150°C, the precipitation of secondary nanoparticles notably enhanced the strength of the 0.5Ti alloy. Specifically, the 0.5Ti alloy achieved a maximum UTS of (479 ± 11) MPa and YS of (376 ± 10) MPa. At 250°C, the YS of the L-PBF Ti/Al−Si−Mg−Zr alloy was higher than that of the L-PBF Al−Si−Mg−Zr matrix alloy due to the retention of Si-rich cell boundaries, indicating a higher thermal stability. As the aging temperature was increased to 300°C, the dissolution of Si-rich cell boundaries, desolvation of solid-solution elements, and coarsening of nanoprecipitates led to a decrease in the UTS and YS of the alloy to below 300 and 200 MPa, respectively. However, the elongation increased significantly.

1. Introduction

Al alloys are widely used in weight-sensitive applications owing to their advantageous characteristics, including exceptional specific strength, robust corrosion resistance, and competitive cost-effectiveness [1–2]. Currently, Al alloy components are predominantly produced using conventional techniques such as forging, casting, extrusion, and powder metallurgy [3]. When fabricating intricate structures, these established methods frequently require supplementary processes, such as welding [4], thereby increasing the complexity of the overall component manufacturing process. Laser-powder bed fusion (L-PBF) is an advanced manufacturing technology that enables the near-net-shape production of geometrically complex structures in a layer-wise manner. L-PBF can reduce the instability caused by solder joints, simplify production procedures, and produce extremely complex metal components that cannot be manufactured using traditional processing techniques [5–7].

High-strength Al alloys, such as those of the 2xxx and 7xxx series, are widely used in aeronautics and astronautics [3]. However, manufacturing these alloys by L-PBF is challenging owing to the formation of hot cracks [8–10]. Current applications of L-PBF Al alloys are predominantly restricted to Al−Si and Al−Si−Mg eutectic or hypoeutectic casting compositions [11]. Their low strength and thermal stability limit their extensive application [12]. The rapid cooling nature (~(105–107) K·s−1) of the melt material during L-PBF can effectively increase the solid solubility of alloying elements within the matrix. This promotes the precipitation of high-concentration strengthening nanoparticles during the subsequent heat treatment process, offering an ideal technical avenue for the composition optimization of high-performance Al alloys [13–14]. To increase the strength of the L-PBF Al−Si−Mg alloy, Geng et al. [15–17] designed a high-strength Al−Si−Mg aluminum alloy specifically for L-PBF by increasing the Mg content according to the rapid cooling nature of L-PBF technology. The novel L-PBF AlSiMg1.4 alloy exhibited exceptional processability and a broad processing window. The ultimate tensile strength (UTS), yield strength (YS), and elongation of the as-built alloy were (518 ± 6), (341 ± 14) MPa, and (7.1 ± 0.4)%, respectively [16]. After aging the alloy from 150 to 200°C, the Mg−Si clusters/Guinier–Preston zones in the as-built alloy promoted the nucleation of βʺ nanoparticles, thereby further increasing the strength of the alloy, with a maximum YS of 446 MPa and UTS of 546 MPa [17]. The strength of L-PBF AlSiMg1.4 alloy was significantly higher than that of conventional L-PBF Al−Si−Mg alloys [5]. Moreover, the addition of Zr has been demonstrated to be