Research PaperOn microstructure, crystallographic orientation, and corrosion properties of wire arc additive manufactured 420 martensitic stainless steel: Effect of the inter-layer temperature
Introduction
The emerging additive manufacturing (AM) technology, primarily invented by Charles W. Hull [1] and applied for the manufacturing of rapid prototypes (RP) in 1980s, has been widely adopted in the last decade for manufacturing of metallic components with intricate designs [2]. As compared to the conventional manufacturing technologies, AM implements a layer by layer deposition strategy, providing higher accuracy in the fabrication of near-net-shape products [2]. In wire arc additive manufacturing (WAAM) technology, which is a direct energy deposition (DED) process, an electric arc or plasma is used as the heat source and a wire as the feedstock material. WAAM hardware commonly includes a welding torch mounted on a multi-axis robotic arm or on a CNC system, a welding power source, and its associated wire feeding system. As the wire is fed into the electric arc at a controlled rate, it melts and is deposited on either the base plate, forming the first deposited layer, or on the previously solidified layers [3]. Compared to the other beam-based AM processes, the WAAM offers higher deposition rate (up to 8 kg/h), lower maintenance cost, accelerated lead time, and is capable of producing larger parts [4], [5], [6]. However, WAAM parts are exposed to a complex thermal treatment during the deposition process, altering localized metallurgical characteristics of the part ascribed to (i) frequent heating and cooling of the part, (ii) directional heat dissipation, (iii) high-temperature gradients, and (iv) re-melting of the previously deposited layers. Hence, the fabricated WAAM parts experience highly localized microstructural characteristics, affecting their mechanical performance and corrosion behavior [5], [7], [8], [9].
As the most frequently used materials in modern industry, ferrous alloys and in particular stainless steels were also adopted as the feedstock materials for the fabrication of parts through the WAAM. Hejripour et al. [10] used the WAAM process to build 2209 duplex stainless parts with different geometries, i.e., tube and wall, and developed a thermal model, describing the correlation between the calculated cooling rates and the formation of different phases in the fabricated parts.
In the case of martensitic stainless steels (MSSs), there are a few studies available in the literature that characterize the microstructure and mechanical properties of the WAAM fabricated parts. Ge et al. [11] studied the microstructural evolution and micro-indentation properties of a WAAM 2Cr13 part and reported the formation of random crystallographic orientation in the middle region of the part and a slight fiber texture within the top layers. Alam et al. [12] reported anisotropic mechanical properties in the longitudinal and transverse directions for the laser-cladded and additively manufactured 420 MSS and concluded that the anisotropic issue can be resolved by applying a post-cladding heat treatment for one-hour at 565 °C.
Despite the existing a few studies on the microstructure and mechanical properties of WAAM fabricated MSSs [11], [13], [14], there is no study in the literature investigating the corrosion response of the fabricated MSS components. A combination of electrochemical and microstructural factors govern the localized corrosion attack (pitting corrosion) in this family of stainless steels [15]. Therefore, it is vital to understand the process-induced microstructural features of the MSSs, in order to evaluate the corrosion response and be able to predict their electrochemical stability [15]. The conventional fabrication method of MSSs consists of annealing of wrought alloy, followed by applying warm or cold deformation steps, and subsequently exposing the parts to hardening and tempering heat treatments that generate carbide particles in a ferritic/martensitic matrix [16]. The origin of the formed precipitates is highly affected by the applied heat treatment cycle’s parameters. While the tempering leads to the formation of fine dissolved carbide precipitates, the annealing can form localized large spherical carbides in the microstructure [16]. Additionally, aforementioned fabrication method of the MSSs generally lead to the formation of residual delta ferrite and unwanted retained austenite due to the applied quenching cycle subsequently after the austenitization process [16], [17]. It is well stated that the high-volume fraction of the retained austenite can enhance the corrosion response and can lead to lower susceptibility to pitting corrosion for the MSSs [18]. On the other hand, the presence of delta ferrite in the microstructure with a high concentration of chromium can lead to the formation of chromium depleted zones, consequently causing susceptibility to localized corrosion [19].
Although corrosion performance of MSS is highly affected by the formation of micro-constituents during the manufacturing process, other microstructural features, such as crystallographic orientation and texture, can also play an important role in determining corrosion resistance. From the crystallographic perspective, the corrosion performance of the MSS materials is affected by two important factors. First, the formation of preferential texture during the manufacturing process, leading to the re-alignment of close-packed crystal planes with the exposed surfaces, can possibly enhance the electrochemical response by stabilizing the formed passive layer on the surface [20], [21]. Second, variation in the fraction of fabrication process-induced low angle grain boundaries can directly impact the corrosion properties. Low angle grain boundaries with low energy levels can balance the diffusivity inside and along the grain boundaries [22], further contributing to the formation of more stabilized passive layer and enhanced electrochemical response of the polycrystalline materials [23].
Overall, to the best of the authors’ knowledge, no study has been conducted to investigate the effects of microstructural evolution during fabrication and the crystallographic orientation on the corrosion behavior of WAAM fabricated 420 martensitic stainless steel parts. Attaining a homogenous microstructure in additively manufactured parts in the as-printed condition is essential to achieve isotropic properties. It is shown that the homogenous microstructure of WAAM parts can be achieved through controlling the inter-layer temperature (defined as the temperature of previously deposited layer upon the deposition of the new layer), affecting the grain growth orientation and their geometry during the solidification process [24]. However, the impacts of inter-layer temperature on the crystallographic orientation, secondary phase formation, and corrosion behavior of MSSs are not known. In the present study, ER420 wire, possessing a similar chemical composition to the AISI420 martensitic stainless steel is utilized to deposit wall-shaped parts at two different inter-layer temperatures. Focusing on the aforementioned gaps, this study aims to investigate the correlations between various interlayer temperatures, microstructure, and corrosion properties in the WAAM fabricated 420 martensitic stainless steel.
Section snippets
Materials and fabrication process
In this study, ER420 feedstock wire with a diameter of 1.14 mm was used to fabricate walls of 420 stainless steel on a 20 mm thick wrought plate of 420 stainless steel through WAAM process. The chemical composition of the utilized feedstock wire and the substrate plate (AISI 420) are reported in Table 1. A GMAW torch mounted on a six-axis robotic arm and an automated wire feeding system were employed to uniformly and consistently deposit the material layer-by-layer until the near-net-shape wall
Microstructural characterization
The three-dimensional SEM micrographs of the samples fabricated at the inter-layer temperatures of 25 °C (IT25) and 200 °C (IT200) are illustrated in Fig. 2a and b, respectively, showing a columnar dendritic structure, grown along the building direction (Z-axis) parallel to the heat dissipation direction. A relatively fine lath martensitic structure (M phase) is formed in the fusion zone at both inter-layer temperatures of 25 °C and 200 °C, due to the fast cooling rate during solidification
Effect of inter-layer temperature on the formation of the secondary phases in the microstructure
The dominant microstructure of all studied samples herein (Fig. 3a, e, f, and h) revealed the formation of a lath martensitic structure along with a significant content of interdendritic delta ferrite phase. In addition, the XRD spectra and analyzed EBSD results revealed a slight fraction of retained austenite in the IT25-XZ sample (~3%) and a noticeably higher fraction (~ 18%) of randomly distributed retained austenite in the IT200-XZ sample. Although the formation of a fully martensitic
Conclusions
In this study, the effects of the crystallographic orientation and the micro-constituents’ formation on the corrosion performance of a 420 martensitic stainless steel produced by the wire arc additive manufacturing at two different inter-layer temperatures of 25 °C (IT25) and 200 °C (IT200) and from top and side views (XY and XZ views, respectively) were investigated. The following conclusions were drawn from this study:
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The microstructure of all fabricated samples contained martensite laths
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.
Acknowledgments
The authors would like to acknowledge the contribution of the Memorial University of Newfoundland, Suncor Energy, Dalhousie University, Canada Research Chair (CRC) program, and Mitacs organization for sponsoring this work.
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Present/permanent address: Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, NL, A1B 3×5, Canada