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Gas flow rates in Directed Energy Deposition (DED) Additive Manufacturing (AM) can significantly affect the quality of built parts by altering melt pool geometry. Using a DED process replicator and in situ synchrotron radiography, together with analogous experiments in an industrial DED machine, we investigate the impact of carrier gas and shield gas flow rates on build quality. The results reveal that there is a critical shield gas flow rate above which melt pools are flattened, tracks widen, and thus layer thickness decreases. The reduction in layer thickness is most prominent in conditions with low carrier gas flow rate, as the highly turbulent shield gas flow may divert slow moving powder particles away from the melt pool, decreasing capture efficiency. Very high flow rates increase internal porosity, as fast-moving particles impacting the melt pool surface can entrain chamber gas behind them. High gas flow rates also cool the melt pool, creating shallower melt pools with increased thermal gradients near the solidification front, increasing pore entrapment in the solidified track.

Wire arc directed energy deposition (Arc–DED) of metal components has gained considerable attention in different industries, including construction and landscaping. However, the deposition of single metal materials and associated constrained fabrication environments limit their potential in industrial applications. Furthermore, inherent solidification defects in the melting-based Arc–DED processes must be addressed. In this study, a flux cored wire (FCW)-based Arc–DED, which enables the manufacturing of components in outdoor environments, was presented. In addition to the basic Arc–DED equipment, FCW material and an ultrasonic needle peening (UNP) device for slag crushing were incorporated in the essential work tools. By investigating the flow behavior of molten slag and examining the internal solidification defects, the mechanism of slag inclusion based on physical phenomena was determined. Additionally, the microstructure and mechanical properties resulting from the droplet transfer mode were examined based on the variations in the transfer conditions of the molten slag. Further, the microstructure of the deposited layer induced by the slag crushing process of the UNP was characterized by equiaxed particles, which effectively mitigated the degradation of the mechanical properties caused by slag.

The tensile properties, fracture toughness, and fatigue crack growth (FCG) characteristics of a directed energy deposited precipitation hardened stainless steel (grade: PH15-5; age hardening heat treatment condition: H900) were examined. In the as-fabricated condition, the alloy contains microfissures that are oriented parallel to the build direction, whose appearance was difficult to be detected using optical microscopy. Due to their relative orientation w.r.t. the loading direction, significant anisotropy in tensile and FCG behavior was noted, with the properties being particularly lower when the loading direction is perpendicular to the crack orientation. Despite the presence of microfissures, the alloy’s fracture initiation toughness is comparable to (or in some cases exceeds) those manufactured using either conventionally techniques or laser powder bed fusion. Activation of the extrinsic toughening mechanisms, such as crack deflection when its mode I direction is perpendicular to the microfissures and a combination of crack bridging and deflection when it is parallel, are the micromechanical reasons for the observed high toughness. The efficacy of such mechanisms is observed to depend on the plastic zone size relative to the microfissure spacing. The understanding developed in this study enables the development of strategies for enhancing the damage tolerance of additively manufactured alloys.

Directed energy deposition (DED) has a widely application prospect, but dendrite and component segregation are not easily controlled and predicted. In this study, the method of applying ultrasonic assisted rolling was introduced to the DED to eliminate above phenomenon. A multiple process microstructure evolution simulation (involved melting, solidification, dynamitic recrystallization and grain growth) was also carried out, which introduced ultrasonic effect in the form of energy. The influence of ultrasonic amplitude (A), distance between laser focus point and ultrasonic roller (d0) on microstructure of Pb-Sn alloy deposition layer were explored. There is a good consistency between experiment and simulation with relative error of 4.33%. The results show that the roundness of grains is improved significantly after rolling, and the grain are refined with the increase of A and d0. In all tests of this study, comprehensive effect of ultrasonic rolling achieves the best when d0 is taken as 24 mm and A is taken as 20 μm, the average grain diameter (D0) in grain size counted zone is 18.8 μm which is 23.23% lower than that without rolling. This study laid a foundation for the development of ultrasonic assisted rolling DED and provide a multiple process microstructure simulation method.

Here we present results on the fabrication of steel-aluminum bi-metallic structures using directed energy deposition additive manufacturing. The challenges associated with the fabrication of a sharp transition from steel to aluminum are uncovered using ex-situ characterization techniques and thermo-mechanical modeling of the deposition process. It was found that the fabrication of a sharp steel-aluminum transition is challenging with extensive cracking observed at the interface. The cracking was attributed to the combined effect of residual stress development due to thermal expansion coefficient mismatch and the presence of ordered intermetallics with low ductility at the interface. Using a coupled thermodynamic and thermo-mechanical modeling approach, potential pathways to enable the fabrication of steel-aluminum bi-metallic structures using additive manufacturing are proposed. The results presented here can lay the foundation for future work on the fabrication of bi-metallic steel-aluminum structures using directed energy deposition.

While the fabrication of graded materials by directed energy deposition (DED) has led to accelerated materials discovery, the ability to rapidly explore sufficiently large material composition spaces is limited due to the time-intensive nature of conventional materials characterization techniques. The present study investigates the viability of small punch test (SPT) protocols for rapidly evaluating DED-fabricated alloy mixtures of stainless steel 316L (316L) and Inconel 625 (IN625). The SPT protocols evaluated in this study include both the recently established two-step Bayesian estimation framework as well as the empirical relationships established in prior literature. It is shown that these protocols are capable of reliably and quantitatively tracking the changes in the mechanical properties of the alloy mixtures studied. Enhancement of mechanical properties was observed with the addition of IN625 to 316L, which is attributed to the austenite stabilization in the matrix and the formation of fine δ - Ni3Nb precipitates. It is shown that CALPHAD-based Scheil model simulations predicted the formation of different precipitate phases for each composition. The novel protocols presented in this paper open new avenues for high throughput material explorations for additive manufacturing.

In Laser-based Directed Energy Deposition (L-DED) the laser-induced spatial and temporal thermal cycles strongly determine the microstructure of deposited layers. The effect of three different laser intensity profiles (beam shapes) on the shape of the melt pool and the resulting microstructure was studied. To this end, thermal gradients and growth rates, derived from measured melt pool emissions, are compared to characteristics of the microstructure in the deposited tracks. These characteristics are obtained using Electron Back Scatter Diffraction (EBSD). It was found that the shape of the laser beam strongly affects the melt pool morphology. Therefore it affects also the solidification characteristics and thus the resulting microstucture. Correlations are found between the thermal gradient - growth rate ratios and the grain shapes and amount of texture. Hence, the beam profile is a tool to steer the microstructure of deposited parts during L-DED.

In this study, the additive manufacturing of a functionally graded material (FGM) via directed energy deposition was investigated as an alternative to joining dissimilar metals. The metal powder composition of the FGM was gradually changed from fully low-carbon steel to austenite steel along the building direction. A convolutional neural network model was employed to classify the austenite, martensite, and ferrite phases in the FGM. The volume fraction of the phases was calculated using X-ray diffraction Rietveld refinement and compared with that predicted by the thermodynamic model and that determined from electron-backscattered-diffraction maps. The volume fraction of the bcc phase gradually increased, and the grain size decreased from top to bottom. Nanostructural investigations confirmed the absence of carbide and twin structures due to the relatively low carbon concentration in the upper layers and the presence of a hexagonal ω-Fe phase with twin structures in the interlayers. Furthermore, electron channeling contrast images and kernel average misorientation maps revealed the activation of the deformation twinning and strain-induced transformation of the retained austenite to martensite, which increased the strain-hardening rate. This study can guide the selection of a tailored manufacturing strategy and process parameters to obtain the required material distribution.

To study the mechanical behavior of the interface region that appears when dissimilar metals are deposited by additive manufacturing process, maraging steel was deposited on S45C carbon steel via directed energy deposition. A 3.3 mm-thick plate obtained from the deposition containing four different regions in the vicinity of the interface (substrate/HAZ/dilution layers/feed material) was subjected to uniaxial tensile deformation parallel to the interface. The strength of the layered structure exceeded the theoretical strength calculated through the rule of mixture, while elongation to failure was almost the same as the carbon steel and yield point phenomena disappeared. The synergetic heterostructure effect of the layered structure was attributed to the formation of strain gradient and back stress strengthening effect due to the mechanical incompatibility between each region in the layered structure. Applying 220 °C preheating reduced the layered structure's elongation by assigning different strengths and residual stresses to each layer. The current result suggested that the mechanical incompatibility between domains in the interface region, including the residual stress field distribution, should be carefully controlled to improve the mechanical behavior of the interface region.

The effect of intrinsic heat treatment (IHT) induced by cyclic heat input in laser additive manufacturing (LAM) has been receiving increasing attention in recent years. Herein, the feasibility of tailoring the microstructure and mechanical properties of Inconel 625 alloy to improve material properties and help achieve part quality consistency by controlling IHT in directed energy deposition (DED) is demonstrated. In this study, thermal monitoring was used to obtain thermal histories of as-built specimens to reveal the differences in thermal characteristics under different IHT. The results show that the variation of IHT has an important effect on the primary dendrite arm spacing (PDAS), the temperature and cooling rate of the molten pool, the morphology and content of Laves, and can initiate unusual δ precipitation in the interdendritic region which may be related to the dissolution of Laves. And this time-position-dependent IHT effect results in a hardness fluctuation of about 23.6 % in the bottom and top of the as-built specimen with a dwell time of 1 s, while the hardness is relatively uniform in the specimen with a dwell time of 10 s. Furthermore, by simulating the IHT variation process, its effect on precipitate and hardness was further confirmed.

The dynamic thermal cycle of the directed energy deposition process inevitably leads to considerable residual stress within the as-deposited NiTi components. The high residual stresses in the as-deposited NiTi components were mostly removed by stress-relief annealing at 200 °C for 3 h without detectable change of its chemical composition and microstructure. The incremental hole drilling method was used to measure the residual stresses in samples of both the as-deposited and stress-relief annealed variants. Meanwhile, the changes in residual martensite and dislocation density during cycling were examined by quasi-in-situ EBSD observations. The results indicate that the stability of the cyclic process is jointly influenced by the residual stress and dislocation. When the residual stress was 1.6 %, the superelasticity of the specimens increased from 69.81 % to 95.72 %; After releasing the residual stress at a strain of 2.5 %, the superelasticity of the specimens increased from 63.09 % to 73.70 %. This study provides useful guidance for improving the superelasticity of NiTi alloys fabricated by directed energy deposition.

The adaptation of additive manufacturing (AM) for Ni-based superalloys has gained significance in aerospace and power-generation industries due to the ability to fabricate complex, near-net-shape components on-demand and with minimal material waste. Besides its advantages, challenges remain in metal AM, especially for printing complex alloys such as superalloys. These challenges are often linked to heterogeneity in the as-fabricated parts and continue to limit the practical applications of AM products. A thorough understanding of the relationship between the complex AM process and the resulting microstructure heterogeneity needs to be established before mitigation strategies can be developed. The ability to fabricate more homogeneous Ni-based superalloy parts is expected to unlock not only better mechanical properties but also additional fields of applications.This review aims to summarize the current understanding of heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys. Microstructure heterogeneities discussed include heterogeneity in the chemical composition, phase constitution, porosity, grain and dendrite morphology, and solid-state precipitates. Related heterogeneities in hardness, tensile, creep, fatigue, and residual stress are discussed to represent mechanical properties, and mitigation strategies are summarized. The origins of heterogeneity in the as-fabricated parts are linked to the variations in AM thermal conditions caused by the complex thermal histories.

Hybrid process of wire arc additive manufacturing (WAAM) and high-pressure rolling can build large-scale components with low detrimental residual stress (RS) and distortion. We developed an efficient coupled process model for a steel wall to simulate the interaction between WAAM deposition and rolling. The predicted RS distributions and wall dimensions agree well with experimental results. Cyclic variation of longitudinal tensile RS occurs during WAAM deposition and inter-layer rolling in clamped condition. The influence depth of deposition and rolling is characterised by the number of the underlying layers that are plastically deformed after each process cycle. For the inter-layer rolling with a flat roller, the rolling has smaller influence depth than the deposition; consequently, the rolling does not eliminate but rather contains the regeneration of WAAM tensile RS after thermal cycles. Rolling with a slotted roller introduces more tensile plastic strain and thereby more effectively reduces WAAM tensile RS and unclamping distortion. Compared to the inter-layer rolling, stacked-four-layer rolling has larger influence depth and hence achieves similar RS mitigation efficacy with fewer rolling operations, while post-build rolling has lower efficacy due to insufficient penetration. Therefore, stacked-layers rolling with slotted roller is recommended for an optimal hybrid process of WAAM and rolling.

In order to address the high throughput data generation challenges in the directed energy deposition (DED) process development, a design of experiments (DOE) informed deep learning (DL) model is developed for modeling of laser powder-based DED process. A small-size experimental dataset is obtained according to DOE, by which a large-size dataset is augmented via the DOE regression model and then used to pre-train the DL model. A subset of experimental data is employed to fine-tune the DL model. The presently developed DOE-informed DL model is validated via single-track deposition of stainless steel 316L, in which the cross-section dilution shape (including depth) and the geometrical characteristics of beads, including the width, height, area, and wetting angle are predicted accurately. The prediction of the porosity and hardness are acceptable for single-track deposition, since variations of both the experimental porosity (from 0.04 % to 0.30 %) and hardness (from 168 HV to 182 HV) are quite small for the single-track deposition, especially predicted ranges for the porosity (from 0.05 % to 0.25 %) and hardness (from 170 HV to 180 HV) are the subset of the experimental results.The DOE-informed DL model developed in this study is based on a single-track deposition dataset; in future work, the DOE-informed DL model will be extended to multi-layer deposition.

Additive Manufacturing (AM) is becoming an important technology for manufacturing of metallic materials. Laser-Powder Bed Fusion (L-PBF), Electron beam-Powder Bed Fusion (E-PBF) and Directed Energy Deposition (DED) have attracted significant interest from both the scientific community and the industry since these technologies offer great manufacturing opportunities for niche applications and complex geometries. Understanding the physics behind the complex and dynamic phenomena occurring during these processes is essential for overcoming the barriers that constrain the metal AM development. In-situ synchrotron X-ray characterization is suitable for investigating the microstructure evolution during processing and provides new profound insights. Here, we provide an overview of the research on metal PBF and DED using in-situ synchrotron X-ray imaging, diffraction and small-angle scattering, highlighting the state of the art, the instrumentation, the challenges and the gaps in knowledge that need to be filled. We aim at presenting a scientific roadmap for in-situ synchrotron analysis of metal PBF and DED where future challenges in instrumentation such as the development of experimental stations, sample environments and detectors as well as the need for further application oriented research are included.

[Display omitted] • Gas flowrates in DED can significantly affect the build quality and track morphology. • Melt pool features affected include the width, depth, and deposited layer thickness. • Careful selection of gas parameters is essential to mitigate internal porosity. Gas flow rates in Directed Energy Deposition (DED) Additive Manufacturing (AM) can significantly affect the quality of built parts by altering melt pool geometry. Using a DED process replicator and in situ synchrotron radiography, together with analogous experiments in an industrial DED machine, we investigate the impact of carrier gas and shield gas flow rates on build quality. The results reveal that there is a critical shield gas flow rate above which melt pools are flattened, tracks widen, and thus layer thickness decreases. The reduction in layer thickness is most prominent in conditions with low carrier gas flow rate, as the highly turbulent shield gas flow may divert slow moving powder particles away from the melt pool, decreasing capture efficiency. Very high flow rates increase internal porosity, as fast-moving particles impacting the melt pool surface can entrain chamber gas behind them. High gas flow rates also cool the melt pool, creating shallower melt pools with increased thermal gradients near the solidification front, increasing pore entrapment in the solidified track. [ABSTRACT FROM AUTHOR]

Bimetallic structures of nickel (Ni) and commercially pure titanium (CP Ti) were manufactured in three different configurations via directed energy deposition (DED)-based metal additive manufacturing (AM). To understand whether the bulk properties of these three composites are dominated by phase formation at the interface, their directional dependence on mechanical properties was tested. X-ray diffraction (XRD) pattern confirmed the intermetallic NiTi phase formation at the interface. Microstructural gradient observed at the heat-affected zone (HAZ) areas. The longitudinal samples showed about 12% elongation, while the same was 36% for the transverse samples. During compressive deformation, strain hardening from dislocation accumulation was observed in the CP Ti and transverse samples, but longitudinal samples demonstrated failures similar to a brittle fracture at the interface. Transverse samples also showed shear band formation indicative of ductile failures. Our results demonstrate that AM can design innovative bimetallic structures with unique directional mechanical properties.

The objective of this work is to mitigate flaw formation in powder and laser-based directed energy deposition (DED) additive manufacturing process through close-loop control of the meltpool temperature. In this work, the meltpool temperature was controlled by modulating the laser power based on feedback signals from a coaxial two-wavelength imaging pyrometer. The utility of closed-loop control in DED is demonstrated in the context of practically inspired trapezoid-shaped stainlesssteel parts (SS 316L). We demonstrate that parts built under closed-loop control have reduced variation in porosity and uniform microstructure compared to parts built under open-loop conditions. For example, post-process characterization showed that closed-loop processed parts had a volume percent porosity ranging from 0.036% to 0.043%. In comparison, open-loop processed parts had a larger variation in volume percent porosity ranging from 0.032% to 0.068%. Further, parts built with closed-loop processing depicted consistent dendritic microstructure. By contrast, parts built with open-loop processing showed microstructure heterogeneity with the presence of both dendritic and planar grains, which in turn translated to large variation in microhardness.

Post-build rolling can mitigate residual stress (RS) and distortion in large-scale components built by wire arc additive manufacturing (WAAM). In this study, based on numerical simulations that considered both WAAM deposition and vertical rolling, the mechanisms of rolling-enabled mitigation of RS and distortion in a WAAM-built steel wall are elucidated. The influences of process configuration and condition, such as roller design (flat, profiled and slotted rollers), rolling load (25–75 kN) and roller-to-wall friction coefficient (0–0.8) on the distributions of plastic strain (PS) and RS were investigated. It is found that the slotted roller is most effective to introduce tensile PS for counteracting the compressive PS generated by the WAAM deposition, thereby reducing the tensile RS in the clamped condition and the final distortion after removal of clamps. Higher rolling load increases the rolling-induced tensile PS, which leads to more extensive mitigation of the WAAM-generated tensile RS. The simulations also demonstrate that the friction coefficient significantly affects the PS and RS when the slotted roller is employed. However, the efficacy of the flat/profiled roller is insensitive to friction coefficient. This study could underpin the development of an optimal post-build rolling process for efficient mitigation of RS and distortion in WAAM components.

Owing to the relatively lower dimensional accuracy and poorer surface finish compared to other additive manufacturing (AM) technologies, directed energy deposition (DED) yields parts that often require extensive post-processing. Thus, it is well suited to be combined with subtractive or deformation processes into hybrid manufacturing strategies, which may enable microstructure engineering of near-net-shape parts directly upon production. In this work, we use a custom-made machine that combines DED and single point incremental forming (SPIF) processes to produce samples of stainless steel 316L which are amenable to undergo recrystallization upon heat treatment. After recrystallization, the microstructure exhibits a high density of twin boundaries, which are known to enhance the physical and mechanical properties of alloys. We investigate how different SPIF parameters affect the extent of recrystallization and find that our strategy may be used to produce both gradient and “sandwich” microstructures, which integrate dissimilar grain boundary character distributions and grain structures. We assess the corrosion resistance and mechanical properties of such samples and discuss the resulting performance enhancement. Our results showcase new opportunities for microstructure engineering of DED components and lay the groundwork for the design of AM processes that enable grain boundary engineering of metal alloys.

Composite material development via laser-based additive manufacturing offers many exciting advantages to manufacturers; however, a significant challenge exists in our understanding of process-property relationships for these novel materials. Herein we investigate the effect of input processing parameters towards designing an oxidation-resistant titanium matrix composite. By adjusting the linear input energy density, a composite feedstock of titanium-boron carbide-boron nitride (5 wt% overall reinforcement) resulted in a highly reinforced microstructure composed of borides and carbides and nitrides, with variable properties depending on the overall input energy. Crack-free titanium-matrix composites with hardness as high as 700 ± 17 HV0.2/15 and 99.1% relative density were achieved, with as high as a 33% decrease in oxidation mass gain in the air relative to commercially pure titanium at 700 °C for 50 h. Single-tracks and bulk samples were fabricated to understand the processing characteristics and in situ reactions during processing. Our results indicate that input processing parameters can play a significant role in the oxidation resistance of titanium matrix composites and can be exploited by manufacturers for improving component performance and high temperature designs.

Metal additive manufacturing involves manufacturing techniques that add material to produce metallic components, typically layer by layer. The substantial growth in this technology is partly driven by its opportunity for commercial and performance benefits in the aerospace industry. The fundamental opportunities for metal additive manufacturing in aerospace applications include: significant cost and lead-time reductions, novel materials and unique design solutions, mass reduction of components through highly efficient and lightweight designs, and consolidation of multiple components for performance enhancement or risk management, e.g. through internal cooling features in thermally loaded components or by eliminating traditional joining processes. These opportunities are being commercially applied in a range of high-profile aerospace applications including liquid-fuel rocket engines, propellant tanks, satellite components, heat exchangers, turbomachinery, valves, and sustainment of legacy systems. This paper provides a comprehensive review of metal additive manufacturing in the aerospace industry (from industrial/popular as well as technical literature). This provides a current state of the art, while also summarizing the primary application scenarios and the associated commercial and technical benefits of additive manufacturing in these applications. Based on these observations, challenges and potential opportunities are highlighted for metal additive manufacturing for each application scenario.

Bimetallic structures processed via laser-based additive manufacturing (AM) efficiently combine multiple materials in single components, which are difficult to achieve using traditional methods. However, limited understanding exists on how processing parameters influence the resulting microstructures and thermal profiles of base material during processing, affecting component performance. To this end, we present a combined numerical and experimental study on the effects of input laser power and scanning speed on the heat-affected-zone (HAZ) and fusion-zone (FZ) of a tantalum-titanium bimetallic structure, two compatible materials that are challenging to process traditionally and are desirable in a bimetallic system. We demonstrate that a wide array of track characteristics can be achieved by varying the laser power and scanning speed. The HAZ and FZ microstructures' width and depth within the underlying Ti6Al4V substrate can be estimated within 6–19% of the experimentally determined values using a 3D transient-thermal finite element analysis (FEA) model, with the most significant errors occurring at low scanning speeds. Adjusting the absorptivity input to the model improves prediction to within 1–11% difference of the HAZ/FZ dimensions and corresponding temperatures relative to the experimental dimensions across a broad range of laser power and scanning speeds. Our results indicate that the underlying temperatures achieved in bimetallic components can be predicted accurately using a detailed FEA approach, aiding larger-scale approaches towards improved process modeling and manufacturing of bimetallic structures using laser-based AM.

Directed energy deposition (DED) of Ni-based superalloys has wide applications in the fields of aviation, energy and power. However, for the non-weldable superalloys like Inconel 738, cracking frequently occurs during DED and cannot be thoroughly controlled up to now. We propose a novel method to prevent the cracking during Inconel 738 DED, in which a small amount of Inconel 718 is in-situ doped between the deposited layers of Inconel 738. The obtained layered-gradient-material is found to be free of both macro- and micro-cracks. The microstructure shows that doping Inconel 718 cannot interrupt the epitaxial growth of grains, but can modify the precipitation of γ′. In Inconel 718 layers, nano γ′ particles are intensively precipitated only in the inter-dendrites, while in Inconel 738 layers, they are precipitated in both the inter- and inner-dendrites. This modification on γ′ precipitation can effectively decrease the inner stress and alleviate the stress concentration at the grain boundaries, thus the cracking is prevented. The tensile tests, which were conducted at room temperature, 600 °C and 800 °C respectively, demonstrate that the composite deposited workpieces possess promising strength and plasticity. The proposed method has great potential to improve the printability of un-weldable superalloys in additive manufacturing.

In this work, we accomplished the crack-free directed energy deposition (DED) of a multi-layer Cobalt-Chromium alloy coating (Stellite 21) on Inconel 718 substrate. Stellite alloys are used as coating materials given their resistance to wear, corrosion, and high temperature. The main challenge in DED of Stellite coatings is the proclivity for crack formation during printing. The objective of this work is to characterize the effect of the input energy density and localized laser-based preheating on the characteristics of the deposited coating, namely, crack formation, microstructural evolution, dilution of the coating composition due to diffusion of iron and nickel from the substrate, and microhardness. It is observed that cracking is alleviated on preheating the substrate and depositing the coating at a moderate energy density (~200 J·mm−3). The main finding is that cracking of DED-processed Stellite 21 coating at higher levels of energy density is linked to the elemental segregation of chromium and molybdenum, which form hard and brittle phases in the inter-dendritic regions. Cracking in the inter-dendritic regions is caused by residual stresses resulting from the steep thermal gradients at higher input energy. Localized laser-based preheating and moderate energy density mitigate steep temperature gradients and thereby avoid thermally induced cracking of the Stellite coating along the inter-dendritic regions.

The application of Directed Energy Deposition (DED) when using new materials or new instruments, requires significant empirical testing to define a suitable or optimum process operation window. Determining the ideal DED parameters is challenging due to the complexity of the deposition process being dynamic in nature, with a multitude of parameters being highly influential on the resultant melt pool dimensions and the subsequent evolution of solidification. The present study seeks to rationalise the notion of a processing window by using artificial neural networks (ANN) to elucidate the complex interaction between the input parameters - specifically the relationship between the energy density of the laser and material deposition rate on the shape of single-track deposits. Herein, cross-sectional data was collected from single tracks of Inconel 625, Hastelloy X and stainless steel 316 L deposited onto mild steel substrates; whilst using a matrix of process parameters. The ANN was used to model the interplay between laser power, scan speed, laser beam diameter, material deposition rate and material type. The network was then used to visualize a theoretical relationship between the volumetric energy density and the energy required to melt a specific amount of the supplied powder.

Additive manufacturing of Al alloys presents an opportunity to produce complex parts that leverage their high specific strength. This paper aims to study the effect of processing parameters on the fabrication of single tracks and thin walls using AlSi10Mg powder in L-DED. A processing window was identified to enable continuous single track deposition with a regular shape. Deposition of thin walls and single tracks were used as a guideline to deposit blocks. Blocks were deposited using L-DED, and the physical, microstructural, and mechanical properties of the samples were investigated. Results demonstrated that the morphology of the thin walls were a better indicator of the morphology of the block samples than single tracks were, and care must be taken when transitioning from depositing single tracks to bulk (thin walls and blocks) samples. Results from mechanical testing show slight inhomogeneity as a function of height within the build for elongation and ultimate tensile strength. Microhardness and yield strength, however, were mostly uniform throughout the build. Overall, the results of this study demonstrate that L-DED is suitable for depositing AlSi10Mg and obtaining mechanical properties superior to those achieved through casting.

[Display omitted] • The cold metal transfer (CMT) process hinders the escape of slag from the molten pool, leading to slag inclusion. • In the application of ultrasonic needle peening (UNP), particles are refined simultaneously with the crushing of slag. • Recrystallization of grains through UNP resulted in a reduction of anisotropy. Wire arc directed energy deposition (Arc–DED) of metal components has gained considerable attention in different industries, including construction and landscaping. However, the deposition of single metal materials and associated constrained fabrication environments limit their potential in industrial applications. Furthermore, inherent solidification defects in the melting-based Arc–DED processes must be addressed. In this study, a flux cored wire (FCW)-based Arc–DED, which enables the manufacturing of components in outdoor environments, was presented. In addition to the basic Arc–DED equipment, FCW material and an ultrasonic needle peening (UNP) device for slag crushing were incorporated in the essential work tools. By investigating the flow behavior of molten slag and examining the internal solidification defects, the mechanism of slag inclusion based on physical phenomena was determined. Additionally, the microstructure and mechanical properties resulting from the droplet transfer mode were examined based on the variations in the transfer conditions of the molten slag. Further, the microstructure of the deposited layer induced by the slag crushing process of the UNP was characterized by equiaxed particles, which effectively mitigated the degradation of the mechanical properties caused by slag. [ABSTRACT FROM AUTHOR]

Abstract In Laser Directed Energy Deposition (L-DED), closed loop control systems can be used to enhance system reliability; however, modulating controlled parameters can have unintended secondary morphological and microstructural effects. To enable development of control systems more sensitive to the complicated interplay between powder flow, thermal transfer, and long-term stability in the machine, the L-DED process, in an open loop configuration, was studied both experimentally and theoretically. A fully physics based semi-analytical model was created that incorporates descriptions of the powder spray pattern, laser attenuation through the powder cloud, and a thermal equilibrium model to predict melt dimensions. The model was validated against an experimental matrix of 258 single track deposition experiments with stainless steel 316 L. It was found that the powder flow field causes working distance (WD) to converge to an equilibrium value, and that this equilibrium position is strongly influenced by many effects, such as thermal energy accumulation in the part and powder flow dispersion. Several metrics to quantify the stability of this equilibrium working distance are proposed and discussed. Graphical abstract Unlabelled Image Highlights • A passive, natural feedback loop that stabilizes working distance in Directed Energy Deposition was discovered • Stabilization depends on powder capture efficiency of the melt pool as a function of its position in the powder spray cloud • A physics-based model was built and experimentally validated to explore this stabilization effect • Effects of accumulating thermal energy and modifying powder spray patterns are quantified and explained [ABSTRACT FROM AUTHOR]

[Display omitted] • Microfissures in directed energy deposited PH15-5 stainless steel are visually elusive and poses quality control issues. • The highly aligned microfissures introduces significant anisotropy in fatigue crack growth and tensile behavior. • Remarkably, the material demonstrates a high fracture initiation toughness, irrespective of the crack propagation direction. The tensile properties, fracture toughness, and fatigue crack growth (FCG) characteristics of a directed energy deposited precipitation hardened stainless steel (grade: PH15-5; age hardening heat treatment condition: H900) were examined. In the as-fabricated condition, the alloy contains microfissures that are oriented parallel to the build direction, whose appearance was difficult to be detected using optical microscopy. Due to their relative orientation w.r.t. the loading direction, significant anisotropy in tensile and FCG behavior was noted, with the properties being particularly lower when the loading direction is perpendicular to the crack orientation. Despite the presence of microfissures, the alloy's fracture initiation toughness is comparable to (or in some cases exceeds) those manufactured using either conventionally techniques or laser powder bed fusion. Activation of the extrinsic toughening mechanisms, such as crack deflection when its mode I direction is perpendicular to the microfissures and a combination of crack bridging and deflection when it is parallel, are the micromechanical reasons for the observed high toughness. The efficacy of such mechanisms is observed to depend on the plastic zone size relative to the microfissure spacing. The understanding developed in this study enables the development of strategies for enhancing the damage tolerance of additively manufactured alloys. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ultrasonic assisted rolling was introduced to laser metal wire deposition to eliminate dendrite and grain coarsening. • A multiple process microstructure evolution simulation for laser metal wire deposition was carried out. • The influence of ultrasonic amplitude, distance between laser focus point and ultrasonic roller on microstructure of deposition layer were explored. Directed energy deposition (DED) has a widely application prospect, but dendrite and component segregation are not easily controlled and predicted. In this study, the method of applying ultrasonic assisted rolling was introduced to the DED to eliminate above phenomenon. A multiple process microstructure evolution simulation (involved melting, solidification, dynamitic recrystallization and grain growth) was also carried out, which introduced ultrasonic effect in the form of energy. The influence of ultrasonic amplitude (A), distance between laser focus point and ultrasonic roller (d 0) on microstructure of Pb-Sn alloy deposition layer were explored. There is a good consistency between experiment and simulation with relative error of 4.33%. The results show that the roundness of grains is improved significantly after rolling, and the grain are refined with the increase of A and d 0. In all tests of this study, comprehensive effect of ultrasonic rolling achieves the best when d 0 is taken as 24 mm and A is taken as 20 μm, the average grain diameter (D 0) in grain size counted zone is 18.8 μm which is 23.23% lower than that without rolling. This study laid a foundation for the development of ultrasonic assisted rolling DED and provide a multiple process microstructure simulation method. [ABSTRACT FROM AUTHOR]

[Display omitted] • For the first time, additive manufacturing was explored as a processing route for the fabrication of steel-aluminum bi-metallic structures. • Challenges associated with fabrication unraveled using coupled thermodynamics, thermo-mechanical modeling, and advanced characterization techniques. • Potential pathways which can enable the fabrication of steel and aluminum structures by additive manufacturing proposed. Here we present results on the fabrication of steel-aluminum bi-metallic structures using directed energy deposition additive manufacturing. The challenges associated with the fabrication of a sharp transition from steel to aluminum are uncovered using ex-situ characterization techniques and thermo-mechanical modeling of the deposition process. It was found that the fabrication of a sharp steel-aluminum transition is challenging with extensive cracking observed at the interface. The cracking was attributed to the combined effect of residual stress development due to thermal expansion coefficient mismatch and the presence of ordered intermetallics with low ductility at the interface. Using a coupled thermodynamic and thermo-mechanical modeling approach, potential pathways to enable the fabrication of steel-aluminum bi-metallic structures using additive manufacturing are proposed. The results presented here can lay the foundation for future work on the fabrication of bi-metallic steel-aluminum structures using directed energy deposition. [ABSTRACT FROM AUTHOR]

[Display omitted] • Structure-property linkages are evaluated on the DED-processed 316L – IN625 graded alloy mixtures through high throughput material characterization methods. The constitutive material response with varied compositions is studied through small punch tests by employing novel analysis protocols for extracting tensile properties from SPT load–displacement curves. • On validating SPT analysis protocols such as empirical relations (ER) and two-step Bayesian estimation (BE) framework methods against the standard uniaxial tensile test, BE framework model provided better estimates of σ ys and σ ut than the ER approaches and was consistent with both 316L wrought material and 316L – IN625 graded alloy mixtures. • Microstructural characterization reveals that all material compositions exhibited γ – austenite as a primary phase; however, secondary precipitates (Laves, δ, and σ phases) rich in Nb and Mo have been observed along interdendritic regions and originated only in the blends of IN625 with 316L. • On adding IN625 to 316L, the average hardness increased from 207 HV to 268 HV, and average tensile strength, comprising σ ys and σ ut, increased from 310 MPa to 396 MPa and 577 to 754 MPa, respectively. This enhancement in the mechanical properties is majorly attributed to the increase in austenitic stabilization and the formation of secondary precipitates (δ phase in fine globular morphology) that are primarily observed in electron microscopy and CALPHAD simulations. While the fabrication of graded materials by directed energy deposition (DED) has led to accelerated materials discovery, the ability to rapidly explore sufficiently large material composition spaces is limited due to the time-intensive nature of conventional materials characterization techniques. The present study investigates the viability of small punch test (SPT) protocols for rapidly evaluating DED-fabricated alloy mixtures of stainless steel 316L (316L) and Inconel 625 (IN625). The SPT protocols evaluated in this study include both the recently established two-step Bayesian estimation framework as well as the empirical relationships established in prior literature. It is shown that these protocols are capable of reliably and quantitatively tracking the changes in the mechanical properties of the alloy mixtures studied. Enhancement of mechanical properties was observed with the addition of IN625 to 316L, which is attributed to the austenite stabilization in the matrix and the formation of fine δ - Ni 3 Nb precipitates. It is shown that CALPHAD-based Scheil model simulations predicted the formation of different precipitate phases for each composition. The novel protocols presented in this paper open new avenues for high throughput material explorations for additive manufacturing. [ABSTRACT FROM AUTHOR]

[Display omitted] • The joining of functionally graded low carbon-austenite steels was successfully accomplished. • Although C and Cr coexisted in the middle layers, Cr-carbide and other intermetallic phases were not observed. • Phase transformation, residual stress, and discontinuous thermal expansion coefficient of each layer was investigated. • Hexagonal ω phase with twin structures found in the interlayers. In this study, the additive manufacturing of a functionally graded material (FGM) via directed energy deposition was investigated as an alternative to joining dissimilar metals. The metal powder composition of the FGM was gradually changed from fully low-carbon steel to austenite steel along the building direction. A convolutional neural network model was employed to classify the austenite, martensite, and ferrite phases in the FGM. The volume fraction of the phases was calculated using X-ray diffraction Rietveld refinement and compared with that predicted by the thermodynamic model and that determined from electron-backscattered-diffraction maps. The volume fraction of the bcc phase gradually increased, and the grain size decreased from top to bottom. Nanostructural investigations confirmed the absence of carbide and twin structures due to the relatively low carbon concentration in the upper layers and the presence of a hexagonal ω-Fe phase with twin structures in the interlayers. Furthermore, electron channeling contrast images and kernel average misorientation maps revealed the activation of the deformation twinning and strain-induced transformation of the retained austenite to martensite, which increased the strain-hardening rate. This study can guide the selection of a tailored manufacturing strategy and process parameters to obtain the required material distribution. [ABSTRACT FROM AUTHOR]

[Display omitted] • Heterostructure effect was found at the interface region of a maraging steel (M789) deposition upon a carbon steel (S45C). • The titanium carbide precipitated at the dilution layers and decreased the mechanical properties. • 220 °C preheating reduced the ductility of the layered structure (S45C/HAZ/dilution layers/M789) in the interface region. • The preheating induced compressive residual stress field at the dilution layers and increased mechanical incompatibility with HAZ. To study the mechanical behavior of the interface region that appears when dissimilar metals are deposited by additive manufacturing process, maraging steel was deposited on S45C carbon steel via directed energy deposition. A 3.3 mm-thick plate obtained from the deposition containing four different regions in the vicinity of the interface (substrate/HAZ/dilution layers/feed material) was subjected to uniaxial tensile deformation parallel to the interface. The strength of the layered structure exceeded the theoretical strength calculated through the rule of mixture, while elongation to failure was almost the same as the carbon steel and yield point phenomena disappeared. The synergetic heterostructure effect of the layered structure was attributed to the formation of strain gradient and back stress strengthening effect due to the mechanical incompatibility between each region in the layered structure. Applying 220 °C preheating reduced the layered structure's elongation by assigning different strengths and residual stresses to each layer. The current result suggested that the mechanical incompatibility between domains in the interface region, including the residual stress field distribution, should be carefully controlled to improve the mechanical behavior of the interface region. [ABSTRACT FROM AUTHOR]

[Display omitted] • Designing experiments to eliminate residual stresses and keep the microstructure essentially unchanged. • The evolution of residual stresses and dislocations during cycling is examined. • The role of residual stresses and dislocations on superelastic stability is discussed. The dynamic thermal cycle of the directed energy deposition process inevitably leads to considerable residual stress within the as-deposited NiTi components. The high residual stresses in the as-deposited NiTi components were mostly removed by stress-relief annealing at 200 °C for 3 h without detectable change of its chemical composition and microstructure. The incremental hole drilling method was used to measure the residual stresses in samples of both the as-deposited and stress-relief annealed variants. Meanwhile, the changes in residual martensite and dislocation density during cycling were examined by quasi-in-situ EBSD observations. The results indicate that the stability of the cyclic process is jointly influenced by the residual stress and dislocation. When the residual stress was 1.6 %, the superelasticity of the specimens increased from 69.81 % to 95.72 %; After releasing the residual stress at a strain of 2.5 %, the superelasticity of the specimens increased from 63.09 % to 73.70 %. This study provides useful guidance for improving the superelasticity of NiTi alloys fabricated by directed energy deposition. [ABSTRACT FROM AUTHOR]

[Display omitted] • The intensified intrinsic heat treatment promotes the abnormal δ phase precipitation. • Laves and δ are heterogeneously distributed within the as-built specimens. • Longer interlayer dwell time was introduced to uniform the distribution of precipitation. • Hardness fluctuation within the as-built specimens was decreased by 47%. The effect of intrinsic heat treatment (IHT) induced by cyclic heat input in laser additive manufacturing (LAM) has been receiving increasing attention in recent years. Herein, the feasibility of tailoring the microstructure and mechanical properties of Inconel 625 alloy to improve material properties and help achieve part quality consistency by controlling IHT in directed energy deposition (DED) is demonstrated. In this study, thermal monitoring was used to obtain thermal histories of as-built specimens to reveal the differences in thermal characteristics under different IHT. The results show that the variation of IHT has an important effect on the primary dendrite arm spacing (PDAS), the temperature and cooling rate of the molten pool, the morphology and content of Laves, and can initiate unusual δ precipitation in the interdendritic region which may be related to the dissolution of Laves. And this time-position-dependent IHT effect results in a hardness fluctuation of about 23.6 % in the bottom and top of the as-built specimen with a dwell time of 1 s, while the hardness is relatively uniform in the specimen with a dwell time of 10 s. Furthermore, by simulating the IHT variation process, its effect on precipitate and hardness was further confirmed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys are reviewed. • The origins of heterogeneities are linked to the variations in thermal conditions throughout the build. • A short case study is presented. • Strategies to minimize microstructure heterogeneity are discussed. The adaptation of additive manufacturing (AM) for Ni-based superalloys has gained significance in aerospace and power-generation industries due to the ability to fabricate complex, near- net -shape components on-demand and with minimal material waste. Besides its advantages, challenges remain in metal AM, especially for printing complex alloys such as superalloys. These challenges are often linked to heterogeneity in the as-fabricated parts and continue to limit the practical applications of AM products. A thorough understanding of the relationship between the complex AM process and the resulting microstructure heterogeneity needs to be established before mitigation strategies can be developed. The ability to fabricate more homogeneous Ni-based superalloy parts is expected to unlock not only better mechanical properties but also additional fields of applications. This review aims to summarize the current understanding of heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys. Microstructure heterogeneities discussed include heterogeneity in the chemical composition, phase constitution, porosity, grain and dendrite morphology, and solid-state precipitates. Related heterogeneities in hardness, tensile, creep, fatigue, and residual stress are discussed to represent mechanical properties, and mitigation strategies are summarized. The origins of heterogeneity in the as-fabricated parts are linked to the variations in AM thermal conditions caused by the complex thermal histories. [ABSTRACT FROM AUTHOR]

[Display omitted] • Inter-layer, stacked-layers and post-build rolling is modelled to investigate their interaction with wire arc additive manufacturing (WAAM) for mitigating WAAM residual stress and distortion. • Difference in mitigation efficacy of rolling using flat roller and slotted roller is demonstrated and explained. • Influence depth of cyclic WAAM deposition and rolling to alter plastic strain that causes residual stress is determined and analysed for different process conditions. • Hybrid process of WAAM and rolling can be optimised through changing the number of deposition layers for each rolling cycle and modifying the roller geometry. Hybrid process of wire arc additive manufacturing (WAAM) and high-pressure rolling can build large-scale components with low detrimental residual stress (RS) and distortion. We developed an efficient coupled process model for a steel wall to simulate the interaction between WAAM deposition and rolling. The predicted RS distributions and wall dimensions agree well with experimental results. Cyclic variation of longitudinal tensile RS occurs during WAAM deposition and inter-layer rolling in clamped condition. The influence depth of deposition and rolling is characterised by the number of the underlying layers that are plastically deformed after each process cycle. For the inter-layer rolling with a flat roller, the rolling has smaller influence depth than the deposition; consequently, the rolling does not eliminate but rather contains the regeneration of WAAM tensile RS after thermal cycles. Rolling with a slotted roller introduces more tensile plastic strain and thereby more effectively reduces WAAM tensile RS and unclamping distortion. Compared to the inter-layer rolling, stacked-four-layer rolling has larger influence depth and hence achieves similar RS mitigation efficacy with fewer rolling operations, while post-build rolling has lower efficacy due to insufficient penetration. Therefore, stacked-layers rolling with slotted roller is recommended for an optimal hybrid process of WAAM and rolling. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new framework of design of experiments (DOE) informed deep learning (DL) model is developed for modeling of directed energy deposition (DED) process with a small-size experimental dataset. • The developed DOE-informed DL model can accurately predict the cross-section dilution shape (including depth) and the bead geometrical characteristics (including width, height, area, and wetting angle), porosity, and hardness at any position of single-track deposition. • Even though only the single-track deposition dataset is applied in this study, the DOE-informed DL model can be extended to multi-layer deposition seamlessly to solve data generation challenges in DED process development in future work. In order to address the high throughput data generation challenges in the directed energy deposition (DED) process development, a design of experiments (DOE) informed deep learning (DL) model is developed for modeling of laser powder-based DED process. A small-size experimental dataset is obtained according to DOE, by which a large-size dataset is augmented via the DOE regression model and then used to pre-train the DL model. A subset of experimental data is employed to fine-tune the DL model. The presently developed DOE-informed DL model is validated via single-track deposition of stainless steel 316L, in which the cross-section dilution shape (including depth) and the geometrical characteristics of beads, including the width, height, area, and wetting angle are predicted accurately. The prediction of the porosity and hardness are acceptable for single-track deposition, since variations of both the experimental porosity (from 0.04 % to 0.30 %) and hardness (from 168 HV to 182 HV) are quite small for the single-track deposition, especially predicted ranges for the porosity (from 0.05 % to 0.25 %) and hardness (from 170 HV to 180 HV) are the subset of the experimental results. The DOE-informed DL model developed in this study is based on a single-track deposition dataset; in future work, the DOE-informed DL model will be extended to multi-layer deposition. [ABSTRACT FROM AUTHOR]

[Display omitted] • An overview on laser-based metal powder bed fusion and directed energy deposition using in-situ synchrotron X-ray characterization is given. • The existing instrumentation and the relevant synchrotron measurement capabilities are described. • The future challenges on new equipment development and real-time data collection during additive manufacturing are highlighted. • Motivation for in-situ synchrotron characterization of metal electron powder bed fusion is provided. Additive Manufacturing (AM) is becoming an important technology for manufacturing of metallic materials. Laser-Powder Bed Fusion (L-PBF), Electron beam-Powder Bed Fusion (E-PBF) and Directed Energy Deposition (DED) have attracted significant interest from both the scientific community and the industry since these technologies offer great manufacturing opportunities for niche applications and complex geometries. Understanding the physics behind the complex and dynamic phenomena occurring during these processes is essential for overcoming the barriers that constrain the metal AM development. In-situ synchrotron X-ray characterization is suitable for investigating the microstructure evolution during processing and provides new profound insights. Here, we provide an overview of the research on metal PBF and DED using in-situ synchrotron X-ray imaging, diffraction and small-angle scattering, highlighting the state of the art, the instrumentation, the challenges and the gaps in knowledge that need to be filled. We aim at presenting a scientific roadmap for in-situ synchrotron analysis of metal PBF and DED where future challenges in instrumentation such as the development of experimental stations, sample environments and detectors as well as the need for further application oriented research are included. [ABSTRACT FROM AUTHOR]

[Display omitted] The objective of this work is to mitigate flaw formation in powder and laser-based directed energy deposition (DED) additive manufacturing process through close-loop control of the meltpool temperature. In this work, the meltpool temperature was controlled by modulating the laser power based on feedback signals from a coaxial two-wavelength imaging pyrometer. The utility of closed-loop control in DED is demonstrated in the context of practically inspired trapezoid-shaped stainlesssteel parts (SS 316L). We demonstrate that parts built under closed-loop control have reduced variation in porosity and uniform microstructure compared to parts built under open-loop conditions. For example, post-process characterization showed that closed-loop processed parts had a volume percent porosity ranging from 0.036% to 0.043%. In comparison, open-loop processed parts had a larger variation in volume percent porosity ranging from 0.032% to 0.068%. Further, parts built with closed-loop processing depicted consistent dendritic microstructure. By contrast, parts built with open-loop processing showed microstructure heterogeneity with the presence of both dendritic and planar grains, which in turn translated to large variation in microhardness. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ni-CP Ti reactive bimetallic structures processed using DED-based metal additive manufacturing. • In situ Ni-CP Ti reaction improved mechanical properties of bimetallic structures than pure metals. • Longitudinal samples showed a brittle fracture, but shear samples showed ductile failure. • Transverse samples also showed shear band formation indicative of ductile failures. Bimetallic structures of nickel (Ni) and commercially pure titanium (CP Ti) were manufactured in three different configurations via directed energy deposition (DED)-based metal additive manufacturing (AM). To understand whether the bulk properties of these three composites are dominated by phase formation at the interface, their directional dependence on mechanical properties was tested. X-ray diffraction (XRD) pattern confirmed the intermetallic NiTi phase formation at the interface. Microstructural gradient observed at the heat-affected zone (HAZ) areas. The longitudinal samples showed about 12% elongation, while the same was 36% for the transverse samples. During compressive deformation, strain hardening from dislocation accumulation was observed in the CP Ti and transverse samples, but longitudinal samples demonstrated failures similar to a brittle fracture at the interface. Transverse samples also showed shear band formation indicative of ductile failures. Our results demonstrate that AM can design innovative bimetallic structures with unique directional mechanical properties. [ABSTRACT FROM AUTHOR]

[Display omitted] • Directed energy deposition and single point incremental forming are combined to develop a novel hybrid process. • The hybrid process produces alloys with gradient or "sandwich" microstructures and improved properties. • Our method may open the path to grain boundary engineering of neat-net-shape parts. Owing to the relatively lower dimensional accuracy and poorer surface finish compared to other additive manufacturing (AM) technologies, directed energy deposition (DED) yields parts that often require extensive post-processing. Thus, it is well suited to be combined with subtractive or deformation processes into hybrid manufacturing strategies, which may enable microstructure engineering of near-net-shape parts directly upon production. In this work, we use a custom-made machine that combines DED and single point incremental forming (SPIF) processes to produce samples of stainless steel 316L which are amenable to undergo recrystallization upon heat treatment. After recrystallization, the microstructure exhibits a high density of twin boundaries, which are known to enhance the physical and mechanical properties of alloys. We investigate how different SPIF parameters affect the extent of recrystallization and find that our strategy may be used to produce both gradient and "sandwich" microstructures, which integrate dissimilar grain boundary character distributions and grain structures. We assess the corrosion resistance and mechanical properties of such samples and discuss the resulting performance enhancement. Our results showcase new opportunities for microstructure engineering of DED components and lay the groundwork for the design of AM processes that enable grain boundary engineering of metal alloys. [ABSTRACT FROM AUTHOR]

[Display omitted] • Post-build rolling can efficiently mitigate residual stress and distortion in wire arc additive manufacturing (WAAM). • Influences of roller design, rolling load and friction coefficient on plastic strain and residual stress distributions are analysed and elucidated. • Slotted roller can more effectively introduce tensile plastic strain and reduce the WAAM tensile residual stress than the flat and profiled rollers. • Increase of rolling load improves efficacy of rolling process to reduce WAAM residual stress and distortion. Post-build rolling can mitigate residual stress (RS) and distortion in large-scale components built by wire arc additive manufacturing (WAAM). In this study, based on numerical simulations that considered both WAAM deposition and vertical rolling, the mechanisms of rolling-enabled mitigation of RS and distortion in a WAAM-built steel wall are elucidated. The influences of process configuration and condition, such as roller design (flat, profiled and slotted rollers), rolling load (25–75 kN) and roller-to-wall friction coefficient (0–0.8) on the distributions of plastic strain (PS) and RS were investigated. It is found that the slotted roller is most effective to introduce tensile PS for counteracting the compressive PS generated by the WAAM deposition, thereby reducing the tensile RS in the clamped condition and the final distortion after removal of clamps. Higher rolling load increases the rolling-induced tensile PS, which leads to more extensive mitigation of the WAAM-generated tensile RS. The simulations also demonstrate that the friction coefficient significantly affects the PS and RS when the slotted roller is employed. However, the efficacy of the flat/profiled roller is insensitive to friction coefficient. This study could underpin the development of an optimal post-build rolling process for efficient mitigation of RS and distortion in WAAM components. [ABSTRACT FROM AUTHOR]