Laser Powder Bed Fusion of Stainless Steel Grades: A Review

Corrosion response of steels fabricated through arc directed energy deposition additive manufacturing: a review

Steels exhibit distinct properties that underscore their pivotal role in critical industries, such as maritime, aerospace, automotive, petrochemical, and biomedicine. In recent times, there has been an increasing trend towards manufacturing near-net-shape steel components through various additive manufacturing (AM) modalities, utilizing intricate 3D model data. Initially, powder bed fusion (PBF) technology garnered significant attention for the fabrication of steel components. Nonetheless, arc-directed energy deposition (arc-DED), also known as wire arc additive manufacturing (WAAM) technology, is progressively gaining prominence in the AM enterprise due to its high production rate, the ability to print large-scale components, and notably, reduced capital investment. While early research on WAAM-fabricated steels primarily focused on microstructural and mechanical characteristics, there is an increasing emphasis on the corrosion performance of WAAM steel components. These components often encounter exposure to corrosive environments in their intended applications. The existing literature lacks a comprehensive review that delves into the nuanced factors influencing the corrosion behavior of WAAM-fabricated steels and the primary corrosion mechanisms governing their degradation. Therefore, this review is dedicated to exploring the corrosion properties of WAAM-fabricated steels, identifying key parameters influencing their degradation behavior. Moreover, it offers an in-depth examination and discussion of the underlying mechanisms governing corrosion-induced deterioration. Furthermore, this review meticulously scrutinizes the microstructural features and WAAM technologies, providing clarity and organization regarding details relevant to the corrosion of WAAM steel components. To conclude, the paper highlights the existing research gaps related to the corrosion of WAAM steel, delineating potential avenues for future research.

Artifact suppression and improved signal-to-noise ratio by phase-locked multiplexed coherent imaging

Laser additive manufacturing (AM) promises direct metal 3D printing, but is held back by defects and process instabilities, giving rise to a need for in situ process monitoring. Inline coherent imaging (ICI) has proven effective for in situ, direct measurements of vapor depression depth and shape in AM and laser welding but struggles to track turbulent interfaces due to poor coupling back into a single-mode fiber and the presence of artifacts. By z-domain multiplexing, we achieve phase-sensitive image consolidation, automatically attenuating autocorrelation artifacts and improving interface tracking rates by 58% in signal-starved applications.

State of the Art in Development of Heat Exchanger Geometry Optimization and Different Storage Bed Designs of a Metal Hydride Reactor

The efficient operation of a metal hydride reactor depends on the hydrogen sorption and desorption reaction rate. In this regard, special attention is paid to heat management solutions when designing metal hydride hydrogen storage systems. One of the effective solutions for improving the heat and mass transfer effect in metal hydride beds is the use of heat exchangers. The design of modern cylindrical-shaped reactors makes it possible to optimize the number of heat exchange elements, design of fins and cooling tubes, filter arrangement and geometrical distribution of metal hydride bed elements. Thus, the development of a metal hydride reactor design with optimal weight and size characteristics, taking into account the efficiency of heat transfer and metal hydride bed design, is the relevant task. This paper discusses the influence of different configurations of heat exchangers and metal hydride bed for modern solid-state hydrogen storage systems. The main advantages and disadvantages of various configurations are considered in terms of heat transfer as well as weight and size characteristics. A comparative analysis of the heat exchangers, fins and other solutions efficiency has been performed, which makes it possible to summarize and facilitate the choice of the reactor configuration in the future.

Influence of PBF-LB Process Atmosphere on the Fatigue Strength of Hot Isostatically Post-Densified Duplex Steel Parts Produced via the Shell Core Approach

Laser-based additive manufacturing is a great manufacturing technology for producing parts of any geometry. To also increase the strength and reliability of parts produced via powder bed fusion with laser beam (PBF-LB), hot isostatic pressing (HIP) is often used to densify residual porosity or lack-of-fusion defects. When components are post-densified via HIP, they do not require a high density beforehand, only a closed porosity or a dense shell. By building up samples with increased porosity, the PBF-LB process can be accelerated and productivity increased. HIP post-treatment gives the material its full density and good mechanical properties. However, with this approach, the influence of the process gases becomes important. Either argon or nitrogen is used in the PBF-LB process. It is assumed that these process gases are trapped in the pores and thus have an influence on the HIP process and also the mechanical properties after HIP. In this study, the influence of argon and nitrogen as process gases on the properties of duplex AISI 318LN steel after powder bed fusion with laser beam and hot isostatic pressing is investigated for the case of very high initial porosities.

Metallic Coatings through Additive Manufacturing: A Review

Metallic additive manufacturing is expeditiously gaining attention in advanced industries for manufacturing intricate structures for customized applications. However, the inadequate surface quality has inspired the inception of metallic coatings through additive manufacturing methods. This work presents a brief review of the different genres of metallic coatings adapted by industries through additive manufacturing technologies. The methodologies are classified according to the type of allied energies used in the process, such as direct energy deposition, binder jetting, powder bed fusion, hot spray coatings, sheet lamination, etc. Each method is described in detail and supported by relevant literature. The paper also includes the needs, applications, and challenges involved in each process.

Fatigue Behavior of Additively Manufactured Stainless Steel 316L

316L stainless steel is the material of choice for several critical applications in which a combination of mechanical strength and resistance to corrosion is required, as in the biomedical field. Additive Manufacturing (AM) technologies can pave the way to new design solutions, but microstructure, defect types, and surface characteristics are substantially different in comparison to traditional processing routes, making the assessment of the long-term durability of AM materials and components a crucial aspect. In this paper a thorough review is presented of the relatively large body of recent literature devoted to investigations on fatigue of AM 316L, focusing on the comparison between different AM technologies and conventional processes and on the influence of processing and post-processing aspects in terms of fatigue strength and lifetime. Overall fatigue data are quite scattered, but the dependency of fatigue performances on surface finish, building orientation, and type of heat treatment can be clearly appreciated, as well as the influence of different printing processes. A critical discussion on the different testing approaches presented in the literature is also provided, highlighting the need for shared experimental test protocols and data presentation in order to better understand the complex correlations between fatigue behavior and processing parameters.

Selective Laser Melting of Pure Ag and 925Ag Alloy and Their Thermal Conductivity

Due to the high reflectivity of Ag to infrared lasers, there is little research focused on the manufacturing of Ag and Ag alloys by selective laser melting (SLM) technique. In this paper, the manufacturing characteristics, microstructure, and thermal conductivity of SLMed Ag, 925Ag, and their heat-treated parts were studied. With the suitable processing parameters, Ag and 925Ag samples with relative densities of 91.06% and 96.56%, respectively, were obtained. Due to the existence of non-molten particles inside the samples and local high energy density of the laser during the processing, a large number of irregular pores and micropores were formed in the microstructures. XRD analysis shows that no phase transition occurred in the annealed Ag and solution-treated 925Ag parts, as compared to their as-built conditions. The SLMed Ag exhibited fine equiaxed grains, while both columnar grains and elongated lath grains existed in the SLMed 925Ag parts. The annealed Ag and solution-treated 925Ag exhibited large equiaxed grains. Due to the grain growth that occurred in the microstructure, the thermal conductivity of Ag increased by 11.35% after completing the annealing treatment. However, that of 925Ag decreased by 17.14% after completing the solid solution treatment, due to the precipitation of the strengthening phase at grain boundaries. A comparison of the thermal conductivities of Ag and 925Ag shows that the influence of the materials on the obtained thermal conductivities was more pronounced than that of the porosity.

Influence of Deposition Plane Angle and Saline Corrosion on Fatigue Crack Growth in Maraging Steel Components Produced by Laser Powder Bed Fusion

Maraging steels are used in several industries, namely in the molds industry. The determination of fatigue crack propagation resistance in 18Ni300 maraging steel at the Paris regime is a vital issue for safety-relevant components, which are designed to work for a large number of loading cycles before periodic inspections. The main goal of this work is to analyze the influence of the deposition plane angle and saline corrosion on fatigue crack growth in maraging steel samples produced by Laser Powder Bed Fusion (LPBF). The crack closure parameter was used in order to analyze the different fatigue crack growth behaviors, as well as the metallographic, hardness, fractography and corrosion/oxidation analysis. From this work, the main achievement was that the deposition plane angle did not reveal a notable influence in the fatigue crack growth behavior for the fatigue tests unsubmitted to saline corrosion. On the other hand, the fatigue crack growth behavior for the tests under saline corrosion showed an increase in the crack closure parameter due to the appearance of the crack closure induced by oxides, which reduced the fatigue crack growth speed. This phenomenon depends on the deposition plane angle, which controls the martensite amount and consequently controls the level of corrosion/oxidation.

Tensile Properties of 21-6-9 Austenitic Stainless Steel Built Using Laser Powder-Bed Fusion

Alloy 21-6-9 is an austenitic stainless steel with high strength, thermal stability at high temperatures, and retained toughness at cryogenic temperatures. This type of steel has been used for aerospace applications for decades, using traditional manufacturing processes. However, limited research has been conducted on this alloy manufactured using laser powder-bed fusion (LPBF). Therefore, in this work, a design of experiment (DOE) was performed to obtain optimized process parameters with regard to low porosity. Once the optimized parameters were established, horizontal and vertical blanks were built to investigate the mechanical properties and potential anisotropic behavior. As this alloy is exposed to elevated temperatures in industrial applications, the effect of elevated temperatures (room temperature and 750 °C) on the tensile properties was investigated. In this work, it was shown that alloy 21-6-9 could be built successfully using LPBF, with good properties and a density of 99.7%, having an ultimate tensile strength of 825 MPa, with an elongation of 41%, and without any significant anisotropic behavior.

Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts

Undetected and undesired microstructural variations in components produced by laser powder bed fusion are a major challenge, especially for safety-critical components. In this study, an in-depth analysis of the microstructural features of 316L specimens produced by laser powder bed fusion at different levels of volumetric energy density and different levels of inter layer time is reported. The study has been conducted on specimens with an application relevant build height (>100 mm). Furthermore, the evolution of the intrinsic preheating temperature during the build-up of specimens was monitored using a thermographic in-situ monitoring set-up. By applying recently determined emissivity values of 316L powder layers, real temperatures could be quantified. Heat accumulation led to preheating temperatures of up to about 600 °C. Significant differences in the preheating temperatures were discussed with respect to the individual process parameter combinations, including the build height. A strong effect of the inter layer time on the heat accumulation was observed. A shorter inter layer time resulted in an increase of the preheating temperature by more than a factor of 2 in the upper part of the specimens compared to longer inter layer times. This, in turn, resulted in heterogeneity of the microstructure and differences in material properties within individual specimens. The resulting differences in the microstructure were analyzed using electron back scatter diffraction and scanning electron microscopy. Results from chemical analysis as well as electron back scatter diffraction measurements indicated stable conditions in terms of chemical alloy composition and austenite phase content for the used set of parameter combinations. However, an increase of the average grain size by more than a factor of 2.5 could be revealed within individual specimens. Additionally, differences in feature size of the solidification cellular substructure were examined and a trend of increasing cell sizes was observed. This trend was attributed to differences in solidification rate and thermal gradients induced by differences in scanning velocity and preheating temperature. A change of the thermal history due to intrinsic preheating could be identified as the main cause of this heterogeneity. It was induced by critical combinations of the energy input and differences in heat transfer conditions by variations of the inter layer time. The microstructural variations were directly correlated to differences in hardness.

The effects of particle size distribution on the rheological properties of the powder and the mechanical properties of additively manufactured 17-4 PH stainless steel

It is well known that changes in the starting powder can have a significant impact on the laser powder bed fusion process and subsequent part performance. Relationships between the powder particle size distribution and powder performance such as flowability and spreadability are generally known; however, links to part performance are not fully established. This study attempts to more precisely isolate the effect of particle size by using three customized batches of 17-4 PH stainless steel powders with small shifts in particle size distributions having non-intersecting cumulative size distributions, designated as Fine, Medium, and Coarse. It is found that the Fine powder has the worst overall powder performance with poor flow and raking during spreading while the Coarse powder has the best overall flow. Despite these differences in powder performance, the microstructures (i.e., porosity, grain size, phase, and crystallographic texture) of the built parts using the same process parameters are largely the same. Furthermore, the Medium powder produced parts with the highest mechanical properties (i.e., hardness and tensile strength) while the Fine and Coarse powders produced parts with effectively identical mechanical properties. Parts with good static mechanical properties can be produced from powders with a wide range of powder performance.

Optimization of Open Die Ironing Process through Artificial Neural Network for Rapid Process Simulation

The open die forging sequence design and optimization are usually performed by simulating many different configurations corresponding to different forging strategies. Finite element analysis (FEM) is a tool able to simulate the open die forging process. However, FEM is relatively slow and therefore it is not suitable for the rapid design of online forging processes. A new approach is proposed in this work in order to describe the plastic strain at the core of the piece. FEM takes into account the plastic deformation at the core of the forged pieces. At the first stage, a thermomechanical FEM model was implemented in the MSC.Marc commercial code in order to simulate the open die forging process. Starting from the results obtained through FEM simulations, a set of equations describing the plastic strain at the core of the piece have been identified depending on forging parameters (such as length of the contact surface between tools and ingot, tool’s connection radius, and reduction of the piece height after the forging pass). An Artificial Neural Network (ANN) was trained and tested in order to correlate the equation coefficients with the forging to obtain the behavior of plastic strain at the core of the piece.

Heat Treatment Effect on Microstructure Evolution in a 7% Cr Steel for Forging

Well-defined heat-treatment guidelines are required to achieve the target mechanical properties in high-chromium steels for forgings. Moreover, for this class of materials, the microstructure evolution during heat treatment is not clearly understood. Thus, it is particularly important to assess the steel microstructure evolution during heat treatment, in order to promote the best microstructure. This will ascertain the safe use for long-term service. In this paper, different heat treatments are considered, and their effect on a 7% Cr steel for forging is reported. Results show that, following the high intrinsic steel hardenability, significative differences were not found versus the cooling-step treatment, although prior austenite grain size was significantly different. Moreover, retained austenite (RA) content is lower in double-tempered specimens after heat treatments at higher temperatures.

Corrosion Behavior and Mechanical Properties of AISI 316 Stainless Steel Clad Q235 Plate

This paper deals with carbon steel and stainless steel clad-plate properties. Cladding is performed by the submerged-arc welding (SAW) overlay process. Due to element diffusion (Fe, Cr, Ni, and Mn), a 1.5 mm wide diffusion layer is formed between the stainless steel and carbon steel interface of the cladded plate affecting corrosion resistance. Pitting resistance is evaluated by measuring the critical-pitting temperature (CPT), as described in the American Society for Testing and Materials (ASTM) G-48 standard test. Additionally, Huey immersion tests, in accordance with ASTM A262, Type C, are carried out to evaluate the intergranular corrosion resistance. Some hardness peaks are detected in microalloyed steel close to the molten interface line in the coarse-grained heat-affected zone (CGHAZ). Results show that stress-relieving treatments are not sufficient to avoid hardness peaks. The hardness peaks in the CGHAZ of the microalloyed steel disappear after quenching and tempering (Q and T).

Laser Operating Windows Prediction in Selective Laser-Melting Processing of Metallic Powders: Development and Validation of a Computational Fluid Dynamics-Based Model

The rapidly ascending trend of additive manufacturing techniques requires a tailoring of existing solidification models and the development of new numerical tools. User-friendly numerical models can be a valid aid in order to optimize operating parameter ranges with the scope to extend the modelling tools to already existing or innovative alloys. In this paper a modelling approach is described simulating the generation of single tracks on a powder bed system in a selective laser melting process. The approach we report attains track geometry as a function of: alloy thermo-physical properties, laser speed and power, powder bed thickness. Aim of the research is to generate a numerical tool able to predict laser power and speed ranges in manufacturing porosity-free printed parts without lack of fusion and keyhole pores. The approach is based on a simplified description of the physical aspects. Main simplifications concern: the laser energy input, the formation of the pool cavity, and the powder bed thermo-physical properties. The model has been adjusted based on literature data providing the track’s geometry (width and depth) and relative density. Such data refer to different alloys. In particular, Ti6Al4V, Inconel625, Al7050, 316L and pure copper are considered. We show that the printing process presents features common to all alloys. This allows the model to predict the printing behavior of an alloy from its physical properties, avoiding the need to perform specific experimental activities.

Manufacturing and Applications of Stainless Steels

Stainless steels represent quite an interesting material family, both from a scientific and commercial point of view, owing to their excellent qualities in terms of strength and ductility, combined with corrosion resistance [...]

Defect Reduction and Quality Optimization by Modeling Plastic Deformation and Metallurgical Evolution in Ferritic Stainless Steels

Manufacturing of ferritic stainless steels flat bars is an important industrial topic and the steel 1.4512 is one of the most commonly used grades for producing this component. In this paper, the origin of some edge defects occurring during hot rolling of flat bars of this grade is analyzed and thermomechanical and microstructural calculations have been carried out to enhance the quality of the finished products by reducing the jagged borders defect on hot rolled bars. An accurate investigation has been carried out by analyzing the defects on the final product from both the macroscopic and microstructural point of view through the implementation of thermomechanical and metallurgical models in a finite element (FE) MSC Marc commercial code. Coupled metallurgical and damage models have been implemented to investigate the microstructural evolution of ferritic grain size and material damaging. Three levels of prior ferritic grain size (PFGS) and three furnace discharge temperatures have been considered in the thermo-mechanical simulations of the roughing passes. Rheological laws for modeling the evolution of ferritic grain have been modified to describe the specific cases simulated. Results have shown that the defect is caused by processing conditions that trigger an anomalous heating which, in turn, induces an uncontrolled grain growth on the edges. The work-hardened and elongated grains do not recrystallize during hot deformation. Consequently, they tend to squeeze out the surrounding softer and recrystallized matrix towards the edges of the bar where the fractures that characterizes the surface defect occur.

Austenitic Stainless Steel Powders with Increased Nitrogen Content for Laser Additive Manufacturing

Nitrogen is used as an alloying element, substituting the expensive and allergenic element nickel, in austenitic stainless steels to improve their mechanical properties and corrosion resistance. The development of austenitic stainless steel powders with increased nitrogen content for laser additive manufacturing has recently received great interest. To increase nitrogen content in the austenitic steel powders (for example AISI 316L), two measures are taken in this study: (1) melting the steel under a nitrogen atmosphere, and (2) adding manganese to increase the solubility of nitrogen in the steel. The steel melt is then atomized by means of gas atomization (with either nitrogen or argon). The resulting powders are examined and characterized with regard to nitrogen content, particle size distribution, particle shape, microstructure, and flowability. It shows that about 0.2–0.3 mass % nitrogen can be added to the austenitic stainless steel 316L by adding manganese and melting the steel under nitrogen atmosphere. The particles are spherical in shape and very few satellite particles are observed. The steel powders show good flowability and packing density, therefore they can be successfully processed by means of laser powder bed fusion (L-PBF).

Effect of Uncertainty in Localized Imperfection on the Ultimate Compressive Strength of Cold-Formed Stainless Steel Hollow Sections

Stainless steel has excellent corrosion resistance properties, considerable long-term durability, and good mechanical strength. Hollow sections are a versatile and efficient form for construction applications. The use of cold-formed stainless steel rectangular hollow section (RHS) and square hollow section (SHS) in construction industry grasps the attention of designers conceiving long-term, cost-effective structures. For cold-formed RHS and SHS, localized imperfection (ω) resulting from rolling and fabrication process is inevitable. ω has inherent variability and has no definitive characterization. In this paper, statistical analysis of the maximum value of ω collected from available experimental data is conducted. A new approach utilizing Fourier series to generate the three-dimensional (3D) models of members with random ω is proposed. Probabilistic studies based on the proposed 3D models are then carried out to evaluate the effect of uncertainty in ω on the ultimate compressive strength of stainless steel columns with cold-formed RHS and SHS. A total of 21 columns that are prone to local buckling reduction are studied. The results show that uncertainty in ω has a considerable influence on the columns with relatively higher cross-sectional slenderness.