Additive Manufacturing Of Steel Research Articles

Review of Opportunities and Challenges for Additive Manufacturing of Steels in the Construction Industry

Abstract Additive manufacturing (AM), or 3-D printing, encompasses a range of technologies that “print” material layer by layer to create the final part. Though there is significant interest in the AM of concrete in the construction sector, opportunities for the AM of steel still need to be explored. This review focuses on the AM of low-alloy steels, stainless steels, duplex stainless steels (DSSs), precipitation-hardened (PH) stainless steels, and tool steels, highlighting the challenges and opportunities of employing AM technology for construction applications. Fusion-based AM technologies, such as wire arc additive manufacturing (WAAM), laser powder bed fusion (LPBF), and laser-directed energy deposition (LDED), are the core technologies that have been tested in the industry so far. WAAM has seen the most exploration for construction applications because of its higher deposition rate, larger build volume, and lower cost than other AM technologies. The mechanical performance of low-alloy steel, stainless steel, and tool steel shows increased tensile strengths after AM processing compared with wrought counterparts. Although AM is not economical for geometrically simple metal components or geometries, there is potential for AM to fabricate unique structural connections or joints, optimized load-bearing columns, and even entire bridges, as highlighted in this paper. AM’s digital nature (i.e., using computer-aided design (CAD) to create G-code paths for printing) can increase structural efficiency if coupled with topology optimization methods and high-strength alloys. Currently, however, general applications of AM in the industry are limited because of barriers with structural codes and standards not incorporating AM parts and AM technology barriers (i.e., limited build volumes).

Unravelling the roles of TiN-nanoparticle inoculant in additively manufactured 316 stainless steel

As a potent grain refiner for steel casting, TiN is now widely used to refine γ-austenite in steel additive manufacturing (AM). However, the refining mechanism of TiN during AM remains unclear despite intensive research in recent years. This work aims to boost our understanding on the mechanism of TiN in refining the γ-austenite in AM-fabricated 316 stainless steel and its corresponding effect on the mechanical behaviour. Experimental results show that addition of 1 wt.% TiN nanoparticles led to complete columnar-to-equiaxed transition and significant refinement of the austenite grains to ∼2 µm in the 316 steel. Thermodynamic and kinetic simulations confirmed that, despite the rapid AM solidification, δ-ferrite is the primary solid phase during AM of the 316 steel and γ-austenite forms through subsequent peritectic reaction or direct transformation from the δ-ferrite. This implies that the TiN nanoparticles actually refined the δ-ferrite through promoting its heterogenous nucleation, which in turn refined the γ-austenite. This assumption is verified by the high grain refining efficiency of TiN nanoparticles in an AM-fabricated Fe-4 wt.%Si δ-ferrite alloy, in which δ-ferrite forms directly from the melt and is retained at room temperature. The grain refinement is attributed to the good atomic matching between δ-ferrite and TiN. Grain refinement in the 316 steel through 1 wt.% TiN inoculation not only eliminated the property anisotropy but also led to a high strain-hardening rate upon plastic deformation and thereby a superior strength-ductility synergy with yield strength of 561 MPa, tensile strength of 860 MPa and elongation of 48%.

Additive Manufacturing of an IH-Type Linac Structure from Stainless Steel and Pure Copper

Additive manufacturing (AM) of metals has the potential to provide significant benefits for the construction of future particle accelerators. The combination of low cost manufacturing of complex geometries in combination with efficiency gains from improved linac design enabled by AM may be one way towards future cost-effective green accelerator facilities. As a proof of concept, we present a high-efficiency Zeff=280 MΩ/m, 433.632 MHz IH-DTL cavity based on an AM design. In this case, the complex internal drift tube structures with internal cooling channels have been produced from 1.4404 stainless steel and from pure copper using AM. The prototype cavity, as well as stainless steel AM parts have been electroplated with copper. We present results from successful vacuum tests, low level RF measurements of the cavity, as well as the status of preparations for high-power RF tests with a 30 kW pulsed power amplifier.

Atomistic characterization of multi nano-crystal formation process in Fe–Cr–Ni alloy during directional solidification: Perspective to the additive manufacturing

The directional solidification phenomenon has been observed in conventional castings and state-of-the-art manufacturing processes like additive manufacturing of steel. Precise control of temperature gradient during directional solidification in the additive manufactured components is essential to get growth and evolution of various crystalline structures, which enhance the properties. In this study, we have used molecular dynamics simulation to investigate the directional solidification of Fe–Cr–Ni steel under a wide range of temperature gradients. It is observed that multiple crystal morphology such as single crystal, nano-grains, multiple stacking faults, twins etc. Form during directional solidification depending on the temperature gradient. Evolution of various crystalline structures characterized by adaptive common neighbour analysis, surface area, volume and corresponding dimensionless aspect ratio. The current investigation adds to the pedagogical understanding of the effects of thermal gradients on the evolution characteristics of crystal morphology evolved during modern additive manufacturing processes.

Qualification pathways for additively manufactured components for nuclear applications

This research paper evaluated three pathways for qualification of 316 L stainless steel components made by laser powder bed fusion additive manufacturing (AM). Comprehensive and consistent process flows with computational modeling, in-situ measurements, ex-situ characterization and mechanical testing with simple- and complex- geometries were explored. The role of post-process hot isostatic pressing (HIP), and solution anneal treatment were evaluated. By using HIP, the scatter in 316 L steel AM properties within single and complex components was minimized to meet the requirement of existing industry standards. For applications where HIP may not be feasible and with some extent of defect tolerance, alternative qualification methodologies of deploying l-PBF AM parts were also explored with samples made with and without engineered porosities. The data generated in this research will be relevant to deployment of AM components for emerging nuclear energy applications.

Microstructure, Mechanical and Corrosion Behavior of Additively Manufactured Steel: A Review (Part 1)

Abstract Flexible and digital manufacturing technologies like additive manufacturing (AM) have evolved as the future of modern manufacturing with the capability of obtaining multi-dimensional components and material functionality improvement. In the past decade, the additive manufacturing of steel has advanced into an effective approach for controlling local microstructure and fabricating hybrid build with tailored performance. As an emerging technology, there are still some challenges in the additive manufacturing of steel that need to be circumvented in order to attain the full potentials of this novel technology. This review paper examines the current state of additively manufactured steel as well as the associated microstructure, mechanical properties, and corrosion of as-built steel. An insight into further and future research directions is provided.

Additive texturing of metallic surfaces for wetting control

Abstract Additive manufacturing (AM) has enabled the realization of custom products with intricate geometric features that are either too complex or even intractable for subtractive manufacturing processes. On the other hand, owing to their relatively poor roughness, functional surfaces generated in AM have to be often finish machined. This research explored the prospect of turning this limitation around by pursuing a novel concept of additive texturing (AT) wherein the topography of an AM part is tailored in-process to entail textures that further enhance the functionality of an AM product. In this context, the limits of the selective laser melting process in printing metallic surface microfeatures were investigated as a precursor to realizing texture patterns comprising pillars, channels, and re-entrant structures. Following on from this, the efficacy of such textures in controlling the wetting behaviour of stainless steel AM surfaces was examined. The notion of AT is demonstrated in terms of its capability to generate hydrophobic AM surfaces with water contact angles exceeding 140°.

Additive manufacturing by means of parametric robot programming

3D printing or additive manufacturing (AM) is now becoming a common technology in industry. The research activities in this area are constantly increasing, because with the high level of automation and the possibility to produce individual and complex structures, the advantages of additive manufacturing are promising. Most materials used in the construction industry can be used for additive manufacturing, for example steel and concrete. The print head (for example, a welding torch in the AM of steel) is mainly led by industrial robots, whose movements must be transferred from the 3D geometry files to be manufactured. In contrast to all-in-one systems, where hardware, software and printed material are coordinated, most robot-based AM systems are made of components from different manufacturers and branches. The objects to be manufactured are complex and the manufacturing parameters, which significantly influence the geometry and quality of the manufactured part, are manifold. This makes the workflow from the 3D model to the finished object difficult, especially because it is almost impossible to predict the exact manufactured structure geometry or layer height (which would be indispensable for accurate slicing). During the manufacturing process, deviations between the target and actual geometry can occur. In this paper, parametric robot programming (PRP) is presented, which allows flexible motion programming, and a quick and easy reaction to deviations between target and actual geometry during the manufacturing process. Complex geometries are divided into iso-curves whose mathematical functions are determined by means of polynomial regression. The robot can calculate the coordinates to be approached from these functions itself. This allows a simple adjustment of the manufacturing coordinates during the process as soon as target–actual deviations occur. The workflow from the file to the manufactured object is explained. The principle of PRP is transferable and applicable to all robot manufacturers and all conceivable printing processes. In the following article, it will be presented using wire + arc additive manufacturing, in which welding robots or portals can be used to produce steel structures with high deposition rates. Furthermore, the project “AM Bridge 2019” is presented, in which a steel bridge was manufactured in situ over a little creek and the presented PRP was applied.

Investigating relationship between surface topography and emissivity of metallic additively manufactured parts

Due to the direct relationship between thermal history and mechanical behavior, in situ thermal monitoring is key in gauging quality of parts produced with additive manufacturing (AM). Accurate monitoring of temperatures in an AM process requires knowledge of environment and object parameters including object emissivity. The emissivity is dependent on several variables, including: wavelength, material composition, temperature, and surface topography. Researchers have been concerned with the thermal emissivity dependence on temperature since large ranges are seen in metal powder bed processes, but there is also an extensive range of surfaces produced by AM. This work focused on discovering what roughness characteristics control thermal emissivity through investigation of prototypic 316 stainless steel AM samples produced with a range of build conditions on a laser powder bed fusion machine. Through experimental measurements of emissivity using hemispherical directional reflectance (HDR), guided by simulations using a finite-difference time-domain (FDTD) Maxwell solver, it was found that combinations of existing roughness parameters describing both height and slope of the surface correlate well with emissivity changes. These parameters work well due to their apt description of surface features encouraging internal reflection, which is the phenomenon that increases emissivity when a surface falls under the geometric optical region conditions.

Microstructure, Mechanical and Corrosion Behavior of Additively Manufactured Steel: A Review (Part 1)

Abstract Flexible and digital manufacturing technologies like additive manufacturing (AM) have evolved as the future of modern manufacturing with the capability of obtaining multi-dimensional components and material functionality improvement. In the past decade, the additive manufacturing of steel has advanced into an effective approach for controlling local microstructure and fabricating hybrid build with tailored performance. As an emerging technology, there are still some challenges in the additive manufacturing of steel that need to be circumvented in order to attain the full potentials of this novel technology. This review paper examines the current state of additively manufactured steel as well as the associated microstructure, mechanical properties, and corrosion of as-built steel. An insight into further and future research directions is provided.

In situ fatigue monitoring investigation of additively manufactured maraging steel

This work describes an experimental validation set for assessing the real-time fatigue behavior of metallic additive manufacturing (AM) maraging steel structures. Maraging steel AM beams were fabricated with laser powder bed fusion (LPBF) and characterized with ex situ studies of porosity through X-ray computed tomography (CT), nano-indentation, and atomic force microscopy, as well as quasi-static testing to evaluate the as-printed state. Microscale evaluation showed void content of 0.34–0.36% with hardness and stiffness variation through the build direction on the order of 5.1–5.6 GPa and 139–154 GPa, respectively. The microscale inhomogeneities created an as-printed state where the compression and tension plasticity behavior at the macroscale was unequal in quasi-static loading, leading to greater yielding in tension. Specimens were subjected to cyclic loads, while the structural behavior was characterized through in situ magnetic permeability, digital image correlation (DIC) strain, and structural compliance measurements. In the range of 3 × 103 to 3 × 104 cycles to failure, magnetic permeability measurements were able to capture the mechanical state as early as 60% of life depending on failure location. Results are discussed with an emphasis on material-property-structure relationships in terms of the multi-scale material state and fatigue validation data for improving the durability of AM parts.

Additive Manufacturing of Steel and Copper Using Fused Layer Modelling: Material and Process Development

AbstractFused Layer Modelling (FLM) is one out of several material extrusion (ME) additive manufacturing (AM) methods. FLM usually deals with processing of polymeric materials but can also be used to process metal-filled polymeric systems to produce metallic parts. Using FLM for this purpose helps to save costs since the FLM hardware is cheap compared to e.g. direct metal laser processing hardware, and FLM offers an alternative route to the production of metallic components.To produce metallic parts by FLM, the methodology is different from direct metal processing technologies, and several processing steps are required: First, filaments consisting of a special polymer-metal composition are produced. The filament is then transformed into shaped parts by using FLM process technology. Subsequently the polymeric binder is removed (”debinding”) and finally the metallic powder body is sintered. Depending on the metal powder used, the binder composition, the FLM production parameters and also the debinding and sintering processes must be carefully adapted and optimized.The focal points of this study are as following:1. To confirm that metallic parts can be produced by using FLM plus debinding and sintering as an alternative route to direct metal additive manufacturing.2. Determination of process parameters, depending on the used metal powders (steel and copper) and optimization of each process step.3. Comparison of the production paths for the different metal powders and their debinding and sintering behavior as well as the final properties of the produced parts.The results showed that both materials were printable after adjusting the FLM parameters, metallic parts being produced for both metal powder systems. The production method and the sintering process worked out well for both powders. However there are specific challenges in the sintering process that have to be overcome to produce high quality metal parts. This study serves as a fundamental basis for understanding when it comes to the processing of steel and copper powder into metallic parts using FLM processing technology.

A Metallurgical Evaluation of the Powder-Bed Laser Additive Manufactured 4140 Steel Material

Using laser powder bed fusion (PBF-L) additive manufacturing (AM) process for steel or iron powder has been attempted for decades. This work used a medium carbon steel (AISI 4140) powder to explore the feasibility of AM. The high carbon equivalent of 4140 steel (CEIIW ≈ 0.83) has a strong tendency toward cold cracking. As such, the process parameters must be carefully controlled to ensure the AM build quality. Through an orthogonally designed experimental matrix, a laser-welding procedure was successfully developed to produce 4140 steel AM builds with no welding defects. In addition, the microstructure and micro-cleanliness of the as-welded PBF-L AM builds were also examined. The results showed an ultra-fine martensite lath structure and an ultra-clean internal quality with minimal oxide inclusion distribution. After optimizing the PBF-L AM process parameters, including the laser power and scan speed, the as-welded AM builds yielded an average tensile strength higher than 1482 MPa and an average 33 J Charpy V-notch impact toughness at −18°C. The surface quality, tensile strength, and Charpy V-notch impact toughness of AM builds were comparable to the wrought 4140 steel. The excellent mechanical properties of 4140 steel builds created by the PBF-L AM AM process make industrial production more feasible, which shows great potential for application in the aerospace, automobile, and machinery industries.