CrCoNi medium-entropy alloy thin-walled parts manufactured by laser metal deposition: Microstructure evolution and mechanical anisotropy

Highlights

• The microstructure, texture and porosity of CrCoNi thin-walled parts manufactured by LMD are analyzed.

• The influence of microstructure, texture and porosity on the mechanical anisotropy of CrCoNi thin-walled parts are discussed.

• The difference in fracture morphology and microcrack characteristics in the deposition and scanning direction is explored.

• The strengthening mechanism of CrCoNi thin-walled parts manufactured by the LMD is studied.

Abstract

Laser metal deposition (LMD) shows outstanding advantages in manufacturing complex parts. In practice, some formed parts usually require directional properties in order to obtain better service performance in its use. This study has comprehensively analyzed the microstructure, texture and porosity of CrCoNi medium-entropy alloy thin-walled parts manufactured by LMD process, as well as the effect of these factors on its mechanical anisotropy. The results show that the yield strength in the building direction is slightly higher than the scanning direction and the uniform elongation is about 1.5 times. Meanwhile, the fracture morphology and microcrack characteristics of the tensile samples in these two directions have also been studied. The results reveal that cross-slip microcracks and microcrack deflection appeared in the scanning direction and building direction, respectively. In addition, it has also been made clear that initial dislocations can contribute to the yield strength of thin-walled samples. The work provides guidance for the design freedom and scanning strategy of engineering parts with directional performance by the LMD process.

Introduction

High-entropy alloys (HEAs), also called multi-component alloys, are composed of at least five elements in nearly equiatomic proportions, which is different from the traditional alloys design concept with one main element [1]. This new alloy design strategy has produced many types of HEAs with good strength, ductility, corrosion resistance and radiation resistance [2]. Among many possible alloy systems, CrMnFeCoNi HEA have attracted great attention from industry and academia due to their excellent strength and ductility in recent years [3,4]. It is encouraging that the CrCoNi medium-entropy alloy (MEA), which contain only three elements compared to at least five elements in HEAs, exceed CrMnFeCoNi HEA in terms of strength and ductility especially at cryogenic temperatures [[5], [6], [7]]. At present, there have been some studies on the microstructure and properties of CrCoNi MEA [8,9]. Feng et al. [10] found that forged CrCoNi has excellent strength and crack resistance, which is due to the strong resistance of highly distorted deformed twins to crack propagation and the adaptability of activated dislocation motion to plastic deformation. Xie et al. [11] found that CrCoNi after casting and cold rolling has excellent creep resistance at high temperatures, which is attributed to local chemical ordering and low stacking fault energy (SFE). Agustianingrum et al. [12] studied the high temperature oxidation behavior of CrCoNi and found that it has better oxidation resistance than CrMnFeCoNi and SS304, which is related to the chromium-based oxides such as CoCr₂O₄ and Cr₂O₃ formed on the surface of the alloy.

Recently, laser-based additive manufacturing (LBAM) by means of layer-by-layer accumulation strategy has brought revolutionary changes to the manufacturing industry [13], which is mainly divided into selective laser melting (SLM) and laser metal deposition (LMD). As one of LBAM process, LMD brings great flexibility to the manufacture of complex parts and is gradually applied in the aerospace, defense and biomedical fields [14,15]. Compared with other forming processes, LMD does not require molds and requires less subsequent machining. In addition, it may also produce fine microstructures due to the rapid cooling rate, and unique advantage of manufacturing gradient functional components [16]. Generally, textures often appear in the parts manufactured by LBAM due to the large temperature gradient along the building direction, which usually leads to anisotropy of mechanical properties [17,18]. Liu et al. [19] found that the formation of texture is greatly affected by laser energy density in Inconel 718 fabricated by SLM. The <001> fiber texture becomes weaker with the decrease of laser energy density, and the difference in tensile properties of samples in different directions comes from the change of Taylor factor caused by strong texture. Ni et al. [20] also studied the anisotropic tensile properties of Inconel 718 fabricated by SLM and found that the tensile samples along the building direction showed lower ultimate tensile strength compared with horizontal samples, but with a better elongation. In addition to nickel-based alloys, the anisotropy of mechanical properties of titanium alloys and 316L stainless steel manufactured by LBAM has also been studied by some scholars [21]. Carroll et al. [22] found that the average ultimate tensile strength of samples in the longitudinal and transverse directions on Ti–6Al–4V components manufactured by LMD is almost same, but the elongation are 11% and 14%, respectively. Jeon et al. [23] conducted mechanical properties tests of samples with different directions on 316L stainless steel fabricated by SLM. The study found that the difference in mechanical properties of the sample in two directions is significant in the tensile test but not obvious in the compression, which due to the combined effect of the microstructure and internal defects.

As mentioned above, the current research mainly focuses on the relationship between the microstructure and mechanical anisotropy of traditional alloys manufactured by LBAM process. However, for HEAs/MEAs, the current research mainly focuses on the influence of the process parameters and material composition on its the microstructure and mechanical properties [[24], [25], [26], [27]]. In addition, the parts manufactured by the LBAM process usually have forming defects such as porosity, and whether this affects the anisotropy of the mechanical properties remains to be studied. Since thin-walled parts manufactured by the LMD process have important applications in aerospace, petrochemical and other fields [28,29], and the anisotropy of its microstructure has an important impact on mechanical properties. Therefore, this work has two main purposes: the first is to study the microstructure, texture and porosity of CrCoNi MEA thin-walled parts manufactured by LMD process. The second is to analyze the influence of these factors on its mechanical anisotropy and the strengthening mechanism.