Evaluation of microstructure and tensile properties of grain-refined, Ti-alloyed ferritic stainless steel fabricated by laser powder bed fusion

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Abstract

Many alloys produced by Laser Powder Bed Fusion (LPBF) suffer from coarse grains and anisotropic mechanical properties. Here, we investigate how the addition of Ti can cause significant strengthening via grain refinement in a model ferritic stainless steel. We perform LPBF experiments using elemental Fe, Cr, and Ti powders (with significant O impurity in the Fe powder) and perform microstructural analysis by SEM, EDS and EBSD as well as tensile tests on the alloys Fe-19at.%Cr and Fe-19at.%Cr-5at.%Ti. The Fe-Cr alloy displays very large grains after LPBF and a strong cube texture, rendering its mechanical properties similar to a single crystal. In the Fe-Cr-Ti alloy, TiO particle formation in the melt causes strong grain refinement, leading to a texture-free microstructure consisting of equiaxed grains ~1.6 μm in diameter. The 0.2% proof strength of the ternary alloy is more than double that of the binary alloy (281 MPa → 591 MPa), and the work hardening rate is also increased. While the elongation at fracture is reduced for the Fe-Cr-Ti alloy, at ~15 %, it remains sufficient, and samples show ductile fracture surfaces. We estimate that the grain refinement accounts for the majority of the strengthening, however, solid solution strengthening and the effect of the texture are also significant.

Introduction

Additive manufacturing (AM) processes realize complicated and custom-shaped parts by the fabrication layer by layer. AM processes have specifically been used to produce shape-optimized parts via topological optimization [[1], [2], [3], [4]]. The optimization achieved lightweight, high strength, high stiffness parts, which could not be formed by conventional processes. However, in reducing the volume of parts in the shape optimization procedure, the material in use is put under a higher stress. Thus, an improvement of the mechanical properties of the materials in use is required.

The Laser powder bed fusion (LPBF) process is one of the metal AM processes and can achieve comparable or even superior strength to conventionally-processed alloys when optimized process parameters are used. However, higher strength combined with ductility and fracture toughness is still required. The poor ductility and fracture toughness are usually caused by defects in the samples and by coarse grains. Of the many mechanisms applicable to improve the mechanical properties, grain refinement of the matrix is an important one.

Several studies have been conducted to control the microstructure of samples fabricated via LPBF process. For example, the effects of laser scanning strategies of LPBF on the microstructure have been investigated by many researchers (see, e.g. Refs. [[5], [6], [7]]). In these cases, control was usually exerted via the columnar to equiaxed transition during solidification. Heterogeneous nucleation by forming nucleation sites is another mechanism for grain refinement. For example, we recently reported that the formation of primary Ti(N,O) particles by the addition of Ti significantly reduced average grain diameters [8]. This grain refinement mechanism is widely applicable to ferrite steels and does not require modification of the laser scanning strategy.

The effects of microstructure control during LPBF fabrication on tensile properties have been investigated using many alloys. Using a nickel-base superalloy (Inconel 718) [5,6], Wan et al. revealed that a scanning strategy without a 90° rotation showed higher tensile strength than a strategy with a 90° rotation. The improvement resulted from refined microstructures [6]. Park et al. investigated the effects of scanning speeds on the tensile properties of a carbon-containing high entropy alloy and confirmed an improvement of tensile strength by grain refinement [7]. AlMangour et al. revealed that a lower laser energy density realized grain refinement and improved tensile properties for austenitic stainless steel reinforced by TiC particles [9]. Additionally, the directional relationship between the tensile axis and the building direction is essential. For example, Kimura et al. reported that a specimen taken parallel to the building direction showed a higher fracture elongation than a specimen taken perpendicular to the building direction [10]. This tendency was resulted from columnar grains elongated along the building direction.

Several new alloys have been developed to promote grain refinement and confirmed its enhancement of mechanical properties. Montero-Sistiaga et al. added Si to Al7075 alloy and realized fine grains and less susceptibility to hot cracking [11]. The addition of Zr to an Al-Cu-Mg alloy also promoted grain refinement and increased the tensile strength [12]. The compositional optimization of Al-Mg-Si-Sc-Zr alloys also realized grain refinement and suppressed hot cracking [13].

Although many researchers have investigated the mechanical properties of samples fabricated via the LPBF process, studies of ferritic steels are limited. These steels are characterized as cost-effective and possess a high corrosion resistance. One of the problems of ferritic steels is the lack of solid-state phase transformations, which can be used to refine the matrix grains by heat treatment. Several studies have been conducted for ferritic oxide dispersion strengthening (ODS) steel. Boegelein et al. investigated the tensile properties of the ODS steel PM2000 fabricated by LPBF [14]. They revealed that the grains were highly columnar and showed strong 001 textures along growth directions. These samples tended to show lower yield strength and ultimate tensile strength than conventional, extruded samples. Additionally, coarse elongated grains have poor resistance against hot cracking. Thus, microstructural control is crucial to improving mechanical properties for ferritic steels.

In this paper, we focus on the relationship between microstructure and tensile properties of ferritic stainless steel. The AISI 430 stainless steel was chosen as base ferritic steel. The effects of adding 5 at.% Ti on microstructure and tensile properties were investigated. The grain distributions of samples built by LPBF were analyzed.

Section snippets

Powder preparations

For LPBF experiments, gas-atomized, spherical, pure Fe, Cr, and Ti powders were purchased (powder supplier: TLS Technik GmbH, Germany). The mean powder sizes are 20.83 μm, 22.11 μm, and 23.05 μm, respectively. AISI430-based Fe-19at.%Cr (18 mass% Cr) and Fe-19at.%Cr-5at.%Ti (18 mass%Cr-4.3mass%Ti) alloys were prepared by mixing the raw powders for 1 h by a tumble mixing machine (WAB AG, Switzerland) at a rate of 30 rpm. The alloys were named 19Cr and 19Cr5Ti, respectively. All powder mixtures

Microstructure of the raw powder

Fig. 2 shows the backscattered electron (BSE) images of the powder microstructures used for the experiments (cross sectional images). All powders showed an almost spherical shape. As shown in Fig. 2 (a), the pure Fe powder contains many fine, dark particles. From the EDS analysis, it can be concluded that these particles are iron oxides. They are formed due to the high oxygen content of the powder (0.27 mass%). In contrast, only a few visible particles were confirmed in the Cr and Ti powders,

Mechanism of grain refinement

As described in section 3.2, the microstructure of 19Cr consists of huge primary grains and island grains. The checkerboard-like pattern was confirmed on the XY plane. The primary grains show an intense <001> texture along the building direction and the scanning directions. On the other hand, the 19Cr5Ti showed fine equiaxed grains without a texture. The mechanism of the grain refinement by the addition of Ti will be discussed in this section.

Several mechanisms have been proposed for grain

Summary and conclusions

We investigated the microstructures of Fe-19at.%Cr (19Cr) and Fe-19at.%Cr-5at.%Ti (19Cr5Ti) steels fabricated by the laser powder bed fusion (LPBF) process. Tensile tests were carried out for both samples, and the strengthening mechanisms were evaluated.

  • (1)

    The microstructure of the 19Cr sample consisted of the grains (primary grains), which epitaxially grew with a strong <001> texture along the building and scanning directions (cube texture). There were some relatively small grains (island grains)

CRediT authorship contribution statement

Hideaki Ikehata: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft. Eric Jägle: Conceptualization, Methodology, Writing – review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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