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Laser powder bed fusion (LPBF) is an emerging additive manufacturing technique that is currently adopted by a number of industries for its ability to directly fabricate complex near-net-shaped components with minimal material wastage. Two major limitations of LPBF, however, are that the process inherently produces components containing some amount of porosity and that fabricated components tend to suffer from poor repeatability. While recent advances have allowed the porosity level to be reduced to a minimum, consistent porosity-free fabrication remains elusive. Therefore, it is important to understand how porosity affects mechanical properties in alloys fabricated this way in order to inform the safe design and application of components. To this aim, this article will review recent literature on the effects of porosity on tensile properties, fatigue life, impact and fracture toughness, creep response, and wear behavior. As the number of alloys that can be fabricated by this technology continues to grow, this overview will mainly focus on four alloys that are commonly fabricated by LPBF—Ti-6Al-4 V, Inconel 718, AISI 316L, and AlSi10Mg.

Pure titanium (Ti) ERTi-2 was accumulated on an aluminum (Al) alloy ER5356 component via wire and arc additive manufacturing. The effect of processing parameters, mainly the input heat per unit length, on Ti/Al components was investigated. The microstructure of the Ti deposited layer and the Ti/Al reaction layer was analyzed using optical microscopy, scanning electron microscope, energy-dispersive spectroscopy, and an X-ray diffractometer. The fabrication of the surface layer equivalent to pure Ti as the used wire or Ti-Al alloy on the Al alloy components was achieved under low and high input heat conditions, respectively, although the Ti/Al components had low joinability and cracks at the reaction layer. Finally, the potential of additive-manufactured Ti/Al components with reference to our results and previous reports was discussed.

In recent years, Additive Manufacturing (AM), also called 3D printing, has been expanding into several industrial sectors due to the technology providing opportunities in terms of improved functionality, productivity, and competitiveness. While metal AM technologies have almost unlimited potential, and the range of applications has increased in recent years, industries have faced challenges in the adoption of these technologies and coping with a turbulent market. Despite the extensive work that has been completed on the properties of metal AM materials, there is still a need of a robust understanding of processes, challenges, application-specific needs, and considerations associated with these technologies. Therefore, the goal of this study is to present a comprehensive review of the most common metal AM technologies, an exploration of metal AM advancements, and industrial applications for the different AM technologies across various industry sectors. This study also outlines current limitations and challenges, which prevent industries to fully benefit from the metal AM opportunities, including production volume, standards compliance, post processing, product quality, maintenance, and materials range. Overall, this paper provides a survey as the benchmark for future industrial applications and research and development projects, in order to assist industries in selecting a suitable AM technology for their application.

Wire + arc additive manufacturing (WAAM) is a versatile, low-cost, energy-efficient technology used in metal additive manufacturing. This WAAM process uses arc welding to melt a wire and form a three-dimensional (3D) object using a layer-by-layer stacking mechanism. In the present study, a Ni-based superalloy wire, i.e., Inconel 625, is melted and deposited additively through a cold metal transfer (CMT)-based WAAM process. The deposited specimens were heat-treated at 980 °C (the recommended temperature for stress-relief annealing) for 30, 60, and 120 min and then water quenched to investigate the effect of heat treatment on microstructure and phase transformation and to identify the optimum heat treatment condition. Microstructural results show that the heat treatment, in general, eliminates the brittle Laves phases regardless of the time without changing the grain morphology. However, an increment in the amount of the delta phase is observed with the longer heat treatment periods. Additionally, the size of MC (metal carbide) of Nb is also observed to increase with heat treatment time. This study provides an in-depth understanding of the correlation between heat treatment time and microstructure in additively manufactured Inconel 625, which can facilitate determining the optimum heat treatment condition in a later study.

Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.

Invar 36 has gained considerable popularity in many industries, including the aerospace industry, because of its low coefficient of thermal expansion. In this paper, a brief overview for the research needs in metal additive manufacturing is presented. A thorough study for the influence of process parameters on the quality of the parts produced is presented. This study is beneficial for the long-term growth of the additive manufacturing industry. The paper aims to select the process parameters that can be used to fabricate dense parts from Invar 36 ( UNS K93600 ) using the selective laser melting process. In this research, a group of cubes was fabricated using different process parameters from Invar 36 powder using a selective laser melting machine. The density, microstructures, and surface features of these cubes were measured. Experimental observations were drawn from the results of the preliminary analyses. The influence of the process parameters on the density of the parts produced is discussed in this paper.

Alloys processed by laser powder-bed fusion show distinct microstructures composed of dislocation cells, dispersed nanoparticles, and columnar grains. Upon post-build annealing, such alloys show sluggish recrystallization kinetics compared to the conventionally processed counterpart. To understand this behavior, AISI 316L stainless steel samples were constructed using the island scan strategy. Rhodonite-like (MnSiO3) nanoparticles and dislocation cells are found within weakly-textured grains in the as-built condition. Upon isothermal annealing at 1150 °C (up to 2880 min), the nucleation of recrystallization occurs along the center of the melt pool, where nuclei sites, high stored elastic energy, and local large misorientation are found in the as-built condition. The low value of the Avrami coefficient (n = 1.16) can be explained based on the non-random distribution of nucleation sites. The local interaction of the recrystallization front with nanoparticles speeds up their coarsening causing the decrease of the Zener-Smith pinning force. This allows the progression of recrystallization in LPBF alloys, although sluggish. These results allow us to understand the progress of recrystallization in LPBF 316L stainless steel, shedding light on the nucleation mechanisms and on the competition between driving and dragging pressures in non-conventional microstructures. They also help to understand the most relevant microstructural aspects applicable for tuning microstructures and designing new LPBF alloys.

Metal additive manufacturing (AM) presents advantages such as increased complexity for a lower part cost and part consolidation compared to traditional manufacturing. The multiscale, multiphase AM processes have been shown to produce parts with non-homogeneous microstructures, leading to variability in the mechanical properties based on complex process–structure–property (p-s-p) relationships. However, the wide range of processing parameters in additive machines presents a challenge in solely experimentally understanding these relationships and calls for the use of digital twins that allow to survey a larger set of parameters using physics-driven methods. Even though physics-driven methods advance the understanding of the p-s-p relationships, they still face challenges of high computing cost and the need for calibration of input parameters. Therefore, data-driven methods have emerged as a new paradigm in the exploration of the p-s-p relationships in metal AM. Data-driven methods are capable of predicting complex phenomena without the need for traditional calibration but also present drawbacks of lack of interpretability and complicated validation. This review article presents a collection of physics- and data-driven methods and examples of their application for understanding the linkages in the p-s-p relationships (in any of the links) in widely used metal AM techniques. The review also contains a discussion of the advantages and disadvantages of the use of each type of model, as well as a vision for the future role of both physics-driven and data-driven models in metal AM.

Specimens were additively manufactured in 316L stainless steel (SS316L) with a technology that combines the extruding method of fused filament fabrication (FFF) with the strengthening stages of metal injection moulding (MIM). A thorough metallographic analysis and tensile testing were carried out to investigate the effect of sintering in the final microstructures, mechanical properties, and fracture modes of the manufactured material. SS316L wrought specimens were also characterised and tested for comparison. Results showed that the sinter-based technology produced a near-fully dense material with a porosity of 1.27% v/v, and a microstructure and mechanical properties comparable to the standard requirements of the UNS S31603 grade. The sintered specimens were characterised at as annealed condition, with fully austenitic microstructures, annealing twins, and sintering defects such as (1) scattered round microporosity, (2) elongated macroporosity, (3) spherical inclusions rich in Si, Mn and O —also found in the precursor powder— and (4) irregular inclusions rich in Cr, Mn and O. The average mechanical properties of the printed SS316L were Young’s modulus (E) 196 GPa, 0.2% offset yield strength (Sy) 166 MPa, tensile strength (Su) 524 MPa, elongation after fracture 85% and reduction of area 51%. Based on the findings, a mechanism is outlined explaining the departure from the typical cup-and-cone ductile fracture in the necked region observed in the printed samples.

This paper investigates the influence of the main process parameters on the geometry of the bead deposited during WAAM using MIG welding technology. A campaign of experimental tests was conducted using a design of experiments approach. The campaign was conducted under a wide range of processing conditions up to a deposition rate of 22 kg/h. Geometrical characterization of the weld bead was performed by optical microscopy and 3D reconstruction techniques. The key geometrical features of the beads and suitable processing windows were identified. The experimental measurements of the cross-sectional profile of the weld bead were compared with common approximation models. A new model based on a circular approximation was proposed. The results demonstrated that the circular approximation showed better agreement with the experimentally measured profiles than the commonly adopted parabolic approximation. This commonly adopted model was fully unable to describe the weld bead profile under medium–large deposition rates. Under these conditions, the parabolic approximation predicted taller and larger weld bead profiles as compared to the experimental measurements. On the other hand, the circular profile showed much better agreement with the experimental profiles within the entire experimental window. A semi-empirical model capable of predicting the bead cross section given the deposition parameters was developed. The model showed good reliability and agreement with the experimental measurement. Consequently, this model would represent a compelling tool to select the process parameters to achieve more precise geometries during WAAM processes.