Thermodynamics formation of oxides in laser powder bed fusion processed aluminum-deoxidized maraging steel and their effects on toughness

Abstract

Maraging steels are ultrahigh-strength, low-carbon steels requiring strict control of impurity elements to ensure optimal strength and toughness. This study aims to elucidate the role of oxygen content in controlling oxide inclusions and cryogenic toughness in laser powder bed fusion (L-PBF) fabricated maraging steels. Two types of powders were used: vacuum induction gas atomization (VIGA) powder with 0.034wt% oxygen and plasma rotating electrode process (PREP) powder with 0.016wt% oxygen. The PREP deposit exhibited finer and more dispersed Al2O3 inclusions (average size: (37 ± 11) nm; number density: 7.6 × 1018 m−3) compared to the VIGA deposit ((59 ± 28) nm; 6.5 × 1018 m−3). As a result, the PREP specimens demonstrated significantly improved impact toughness—138 J at 23°C and 65 J at −196°C—representing 53.3% and 47.8% increase over the VIGA specimens, respectively. This difference is due to the lower oxygen content in PREP, leading to lower-temperature nucleation with a reduced nucleation barrier. In addition to quantitatively evaluating the oxygen inclusion–toughness relationship, a thermodynamic model was developed to capture the nucleation and evolution of nano-scale oxides under the rapid thermal cycles characteristic of the L-PBF molten pool, which enables prediction of the size and number density evolution of oxide inclusions under different oxygen levels. These findings offer insights for oxygen-level control and powder design strategies in additive manufacturing of maraging steels.

1. Introduction

Maraging steel is a class of special steels known for its superior strength and toughness, even at cryogenic temperature [1–2]. This makes it suitable for the applications that require both high strength and toughness, such as aerospace components [3], tooling [4], and high-performance machinery [5].

Laser powder bed fusion (L-PBF), as a representative metal additive manufacturing (AM) technique with the powerful ability to produce components with complex-geometry and high-performance, has shown an important potential for the manufacturing of complex maraging steel parts [6–8]. It is important to note that there usually requires strict control of the impurity element content in the maraging steel to ensure its long-term outstanding performance. For example, the oxygen content is typically controlled to be less than (3.0 × 10−5)wt% in the Ref. [9]. In L-PBF, micron-scale powder is used as the feedstock material. Due to the significantly high specific surface area of powder particles, the oxygen contents of the powder feedstock and the L-PBF-processed components are often an order of magnitude higher than those prepared by conventional manufacturing processes such as forging and casting [10]. Thus, the influence of oxygen content on the microstructure and performance of L-PBF-processed component is of significant concern [11–12].

Oxide is the main existing manner of oxygen element in steel due to its very low solubility in iron solid solution. The type, size, morphology, and distribution of oxide inclusions are significantly influenced by deoxidizing elements, e.g., Al, Mn, Si, and their contents, oxygen content, and L-PBF process. Sun et al. [13] found that there are nano- and micro-amorphous oxides containing Cr, Mn, Si, and/or Al elements in L-PBF-processed 17-4PH steel, and striped oxide slag was also found between the tracks on the top surface of the as-built samples. Yan et al. [14] revealed that there exist the dispersed ultra-fine MnSiO3 oxide inclusions in L-PBF-processed 316L stainless steel.

The higher oxygen content in powder can result in more oxide inclusions in L-PBF processed components, thereby affecting their mechanical properties [15]. On one hand, a large number of nano-oxides were demonstrated to improve the strength of L-PBF processed parts through oxide dispersion strengthening effect [16–17]. On the other hand, the larger oxide inclusions will become the stress concentration points to induce early initiation of cracks during plastic deformation process, thereby damaging the mechanical properties. In particular, the impact toughness is highly sensitive to the amount of oxide inclusions [18]. A study by Cooper et al. [19] suggested that the elevated oxygen levels in powder-metallurgy 316L stainless steel lead to a notable reduction in impact toughness relative to wrought steel. Anyway, the effect of oxide inclusions on either the strength or toughness depends on their quantity, size, and morphology.

In this study, a novel Fe–Cr–Ni–Co–Mo maraging stainless steel [20–22], which was specifically designed for applications with the requirement of both ultra-high strength and superior cryogenic toughness, was employed to investigat