A combined experimental, characterization, and simulation study has been conducted to develop a quantitative description for microstructural evolution during Laser-Directed Energy Deposition (L-DED) of Grade 91 steel. Initial L-DED deposits of Grade 91 made without preheat exhibiting much greater volume fractions of delta-ferrite and much less martensite than anticipated, with non-uniform hardness values and lower average hardness levels. Based on our hypothesis for this result, preheating was applied to subsequent deposits, but not for the conventional reasons related to control of austenite decomposition. Rather, the results were consistent with the concept that greater times in the higher temperature range for the delta to gamma transformation permitted the transformation to progress further toward completion during L-DED of Grade 91 owing to slower cooling rates resulting from increased preheating. Increasing preheat temperatures up to 350 degrees C resulted in deposits with progressively lower volume fractions of retained delta-ferrite with greater fractions of martensite and higher average hardness values. The final phase fractions and resulting hardness levels are dependent on the delta to gamma transformation kinetics that are dictated by the cooling rates for transformation, which in turn are governed by the preheat temperatures. The results of our work have broader implications toward rationalizing the microstructures and properties for AM with rapid cooling rates of many hardenable alloy steels that solidify as delta-ferrite. Finally, the model results were used to develop a novel revision to the continuous cooling transformation diagram for Grade 91 that captures the kinetics of the delta to gamma phase transformation.

Multi-material fabrication between steel and aluminum is challenging because of the formation of several intermetallic phases. Embrittling B2 ordered intermetallics form in the steel rich side and are stable till about 60% of Al dilution in steel. In this work by using multi-length scale characterization coupled with integrated computational process and thermokinetic modeling, we show that the ordered B2 intermetallics in the steel rich side of the Fe-Al system forms via a nucleation and growth mechanism. The extent of B2 ordered intermetallics can be controlled by modifying the directed energy deposition-additive manufacturing (DED-AM) process parameters. Our findings lay the foundation for enabling fabrication of crack-free functionally graded compositions between the two alloys.

Type 420 martensitic stainless-steel powder was clad onto a CF3M austenitic stainless steel substrate using laser direct-energy-deposition (L-DED). Through comprehensive characterization and numerical simulation, the evolution of microstructures, particularly the retention of δ-ferrite and austenite, was investigated. In the clad fusion zone, the primary phase to solidify was δ-ferrite, followed by austenite resulting from the peritectic reaction. The segregation patterns developed during rapid solidification from the L-DED process exerted a significant effect on the stability of the austenite in the room temperature microstructures. Due to the fast cooling rate in the solid state, the time available for transformation of the δ-ferrite to austenite regarding the segregation patterns was limited, and the transformation was incomplete. The room temperature microstructure was therefore comprised of δ-ferrite distributed in the dendrite core regions, surrounded by martensite, which was further surrounded by a small fraction of retained austenite in the interdendritic regions. The retained austenite region was enriched with segregated alloying elements, which pushed the martensite start temperature Ms below room temperature. The retention of both δ-ferrite and interdendritic austenite was proved to have caused a softened fusion zone with a lowered martensite fraction.

AISI 420 stainless steel (420 SS) is a martensitic type known for its high strength and abrasion resistance, and it is extensively used in hard-facing applications (Refs. 1, 2). However, Type 420 SS contains untempered martensite in the as-welded condition and is generally considered to have poor weldability. Successful arc welding of 420 SS requires proper preheating, post-weld heat treatment, and strict adherence to low hydrogen practices. Direct energy deposition (DED) is a fusion-based additive manufacturing (AM) process that melts metal powders to produce complex shapes and can be used for surfacing or hard-facing operations. However, as small-volume AM deposits undergo self-quenching, the cooling rate can be much faster than in welding. Consequently, the phase transformations in the AM deposit deviate further from equilibrium relative to traditional welding processes, resulting in heterogeneous microstructures and mechanical properties, potentially limiting the direct use of AM components without post-AM heat treatment (Ref. 3). Our prior work with laser-DED (L-DED) hard-facing 420 SS powder without preheat combined experiments, characterization, and kinetic modeling to rationalize the presence of remnant δ-ferrite and austenite quantitatively in room temperature microstructures. This study aimed to quantify the effects of preheating above the Ms temperature on the extent of solidification segregation and microstructures for a wall build for L-DED processing of the same heat of 420 powder. The results of the current work are contrasted with our prior study, which allows for a greater understanding of L-DED opportunities with 420 SS.

The grade 91 ferritic/martensitic steel is considered a promising structural or cladding material for various nuclear reactor applications. Here, grade 91 was fabricated via the Directed Energy Deposition Laser technique. This alternative manufacturing process potentially enables tailoring of the mechanical properties through increased control of the product's microstructure. Aimed at linking fabrication to performance via defining the process-structure-property relationships, the current research includes macro and up to nano-scale mechanical testing using microhardness, tensile, and in-situ nanoindentation hardness, coupled with electron diffraction-based microstructure characterization. Mapping of the product structure and properties was conducted by testing miniature-sized samples, parallel to the built direction ('Z' direction) and perpendicular ('X' direction) at constant distances. We found the majority of the microstructure consists of fine and coarsened-size lath-type martensite grains, with up to 15% d-phase, preferentially observed at the melt pool boundaries. Most intriguing was the gradual decrease found in observed metallurgical pores alongside softening at farthest distances from the cold build platform. Here, these changes were successfully explained in terms of phase composition, 'grain-like' size effects of the lath-type martensite , geometry necessary dislocation density. In final-izing this work, several competing strengthening mechanisms were addressed, , their activity was considered owing to fabrication-related mechanisms.& COPY; 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Although Cu-Cr-Nb alloys / superalloys bimetallic components for liquid rocket engine applications have been fabricated using additive manufacturing, a detailed understanding of their process-structure relation is currently lacking. To bridge this gap, GRCop-42 / Alloy 718 bimetallic structures were fabricated with two different depo-sition sequences, using powder feedstock with a laser-based directed energy deposition process. The deposition sequence resulted in significant variations in precipitate morphology, composition, and crystal structure within the GRCop-42, particularly at locations near the interface. When Alloy 718 was deposited first, dilution and convective mixing resulted in elevated Ni and Fe levels within the GRCop-42 deposit, which contributed to the formation of C14 (Cr, Ni, Fe)2Nb Laves and ������-Cr phases. The anticipated C15 Cr2Nb precipitates were observed within the GRCop-42 material when the deposition sequence was reversed.

The properties and serviceability of 3D-printed metal parts depend on a variety of attributes. These include the chemical composition, phases, morphology, spatial distributions of grain size and shape, crystallographic texture, and various defects. Control of these attributes remains an exciting opportunity and a major challenge because of the many process variants and parameters that need to be optimized. The desired attributes of industrially relevant common additive manufacturing alloys such as steels, nickel, titanium, aluminum, and copper alloys, and func-tionally graded materials vary widely and require alloy-specific strategies for their control. The recent reviews address the valuable processing-microstructure-property relations but do not focus on their control strategies. Here we seek to unify the disjointed literature and critically review recent advances in controlling grain structure, phases, and defects. The emerging use of digital tools such as mechanistic models and data-driven techniques such as machine learning, dimen-sional analysis, and statistical methods in controlling part attributes is emphasized. Finally, we identify opportunities for high-impact research in metal printing and present an outlook for the future based on existing evidence.

This work aims to adapt nanoindentation mapping combined with a k-means algorithm as a high-throughput technique to study the nano-scale spatial changes in mechanical properties for a heterogeneous material. This technique can also classify the individual data points based on their properties. Hundreds to thousands of indents were performed on additively manufactured T91 at room temperature, 300°C, 400°C, and 500°C across a square area with a side length of 120 μm to 400 μm. From this data, the hardness and reduced modulus at each point could be calculated and mapped. Using k-means clustering, we were able to arrange the data into three or four clusters corresponding roughly to the ferritic and martensitic phases as well as one or two intermediate clusters sampling both the phases. The hardness of these two phases appears to be quite stable as a function of temperature. Nanoindentation mapping and the k-means algorithm can therefore be used to rapidly assess the feasibility of heterogeneous materials under extreme conditions, such as nuclear reactor steels.

Grade 91 steel forms martensite during additive manufacturing and the extent of tempering of martensite significantly affects the mechanical properties of parts. Currently, there is a lack of quantitative understanding of the tempering kinetics for Grade 91 steel, and as a result, the effects of repeated thermal cycles on properties for different processing conditions cannot be determined. Here we evaluate the tempering kinetics by determining the constant terms in the Johnson Mehl Avrami kinetic equation from the tempering data available in the literature and the thermal cycles computed using a rigorously-tested heat and fluid flow model of multi-layer additive manufacturing. The raw tempering data are cleaned using a neural network to enhance accuracy. The lower layers experience repeating cycles of heating and cooling when the upper layers are added. As a result, the hardness is reduced owing to the tempering of martensite. In contrast, martensite formed in the upper layers is not tempered to the same extent and the hardness remains high. Therefore, the hardness of the part increases with the distance from the substrate. Variations in the heat input at different laser powers and scanning speeds significantly affect the extent of tempering. Since the method used here can provide a quantitative understanding of the tempering of martensite and the spatial variation of hardness, it can be used to tailor the microstructure and hardness of heat treatable printed metallic parts.

Efforts to advance structural materials with improved properties and service life in support of next generation designs for nuclear reactor components have recently led to development of nano-ferritic alloys (NFAs) containing nano-oxides such as 14YWT. A key enabling technology to realizing the useful properties of NFAs during service involves preservation of the oxide dispersions during joining. Solid-state welding processes, such as projection-capacitor discharge resistance welding (P-CDRW) used here, are well suited for joining NFAs while retaining the oxides. Due to limitations in the supply of 14YWT NFA material, initial experiments were conducted using 430 stainless steel as an inexpensive surrogate material. The goal of the surrogate experiments was to scale suitable parameters from 430 welds to 14YWT using ratios of key properties for the two materials including flow stress at temperatures and strain rates relevant to hot working. Results indicated that weld displacement increased with increasing weld force and increasing weld energy for all other variables held constant. Weld energy appeared to have a larger effect on displacement than weld force for the sample geometry used here. Appropriate process parameters (no melting) were established for the two materials. The process window for the 430 material extended from 350 J to 600 J of energy for weld forces of 2.2 kN and 3.1 kN. Suitable parameters for 14YWT were similar in terms of energy but for force levels of 3.1 kN and 4.0 kN. Displacement for both materials ranged from 150 mu m to 300 mu m for welds that did not experience melting. Simple heat flow analysis confirmed that the extent of displacement was limited by the characteristic thermal distance determined from thermo-physical properties and the weld current rise time. The higher flow stresses of 14YWT relative to 430 were apparently offset by greater heating due to higher electrical resistivity near the projection tip and lesser heat conduction from the projection tip owing to lower thermal conductivity. Based on the results presented here and in our companion paper. The P-CDRW process appears capable of successfully joining the 14YWT NFA while retaining the microstructures and properties of the original material.

Joining nanostructured ferritic alloys (NFAs) has proved challenging, as the nano-oxides that provide superior strength, creep resistance, and radiation tolerance at high temperatures tend to agglomerate, redistribute, and coarsen during conventional fusion welding. In this study, capacitive discharge resistance welding (CDRW)—a solid-state variant of resistance welding—was used to join end caps and thin-walled cladding tubes of the NFA 14YWT. The resulting solid-state joints were found to be hermetically sealed and were characterized across the weld region using electron microscopy (macroscopic, microscopic, and nanometer scales) and nanoindentation. Microstructural evolution near the weld line was limited to narrow (~50–200 μm) thermo-mechanically affected zones (TMAZs) and to a reduction in pre-existing component textures. Dispersoid populations (i.e., nano-oxides and larger oxide particles) appeared unchanged by all but the highest energy and power CDRW condition, with this extreme producing only minor nano-oxide coarsening (~2 nm → ~5 nm Ø). Despite a minimal microstructural change, the TMAZs were found to be ~10% softer than the surrounding base material. These findings are considered in terms of past solid-state welding (SSW) efforts—cladding applications and NFA-like materials in particular—and in terms of strengthening mechanisms in NFAs and the potential impacts of localized temperature–strain conditions during SSW.

Titanium/Titanium Carbide (Ti/TiC) composites with 20, 40 and 60 vol% of TiC powders were deposited on Ti-6Al-4V substrates using a laser-directed energy deposition method. The bulk relative densities exceeding 99% were achieved in the deposits. The evolution of microstructures and local chemical compositions in the deposits under rapid melting and solidification were analyzed using X-ray diffraction, electron probe micro-analyzer and scanning electron microscopy. Assessment of mechanical integrity of the deposits involved microhardness, tensile testing and fractography. The deposition process resulted in defect-free deposits with 20% TiC. However, cracks were observed originating from the substrate/deposit interface in the 40% and 60% TiC deposit. The L-DED process caused only partial dissolution of the initial TiC particles and the amount of undissolved particles in the deposited matrix increased with increasing TiC volume fraction in the initial powder feedstock mixture. A non-stoichiometric TiC0.55 compound was found to form during solidification. The solidified product in the deposits included dendritic and equiaxed TiC0.55 precipitates homogeneously distributed in the matrix. Micro-hardness measurements indicated that hardness values increased monotonically with TiC content in the deposit. On the other hand, a gradient Ti/TiC composite with composition ranging between 20% and 60% TiC did not develop any growth cracks suggesting an efficient processing route for synthesizing MMCs with high volume fraction of brittle ceramic reinforcements. It was found that pre-existing cracks in the TiC used in the starting feedstock played a key role in the mechanical integrity of the deposits. This observation suggests that the mechanical performance of the Ti/TiC composite deposits can be improved using techniques that promote complete dissolution of the original TiC.

Several key industries routinely make complex parts using metal printing, but its continued growth will require the ability to control the microstructure and properties of parts. Many process variables affect the spatially variable thermal cycles that affect the microstructure and properties of parts. Here we show that the evolution of hardness of a tool steel part at various locations can be calculated using computed thermal cycles and a Johnson-Mehl-Avrami kinetic relation. The calculated hardness values agreed well with the independent experimental data for various processing conditions. At a given location, the hardness continued to decrease with progressive thermal cycles. Lower layers of the part experienced continued thermal cycles during the deposition of upper layers and the hardness decreased with distance from the top of the deposit. High heat input due to high laser power and slow scanning speed resulted in low cooling rate, high temperature, more pronounced tempering of martensite, and low hardness. Since the model can predict the spatial variation of hardness as a function of process variables, the work can serve as a basis for tailoring the hardness of some additively manufactured parts.

Laser powder bed additively manufactured Grade 91 composition steel was investigated in comparison to wrought Grade 91 steel in terms of microstructure and mechanical properties. As-deposited additively manufactured Grade 91 steel had a microstructure of lower bainitic regions surrounded by martensite. This is significantly different from the typical tempered martensitic microstructure of conventionally produced Grade 91 steel. The as-deposited additively manufactured material had excellent tensile mechanical properties with greater strength than the wrought material at room temperature, 300 and 600°C showing excellent promise for nuclear applications. Retention of strength at 300 and 600°C for the as-deposited additively manufactured material was attributed to transitional carbides in the lower bainitic regions. The additively manufactured material was also investigated in the tempered as well as normalized and tempered conditions, each showing decreased strength at elevated temperature than the as-deposited material.

The traditional manufacturing approach to produce engineering components can have a high energy cost, high material waste, longer delivery times, and specific geometries that may be unattainable. The recent developments in additive manufacturing might provide the opportunity to produce complex engineering components, reduced manufacturing costs, and reduced delivery times. Direct Energy Deposition (DED) offers excellent possibilities such as fabrication of metal components with complex geometries, repair of high-value equipment, development of functionally graded materials, and large-scale additive fabrication or repair. This work focuses on the fabrication of AISI 420 martensitic steel using DED for the application of the aerospace, automotive, and medical industries. AISI 420 martensitic steel (12.7%Cr, 0.4%C in wt.%) was successfully deposited onto 316L substrate by a Laser Engineered Net Shaping (LENS®) process carried out in an open atmosphere. The cross-sectional examination by electron microscopy and XRD confirms the dual-phase microstructure of martensitic needles in random orientation and approximately 22 wt. % of austenite lamellar phase by Rietveld refinement and quantitative phase analysis. There are no cracks observed throughout the materials. However, the area fraction of porosity was found to be 0.4%, with the max size of 2μm. Preliminary mechanical characterization by micro-Vickers hardness tests shows uniform hardness about 725 (HV) trend across the build. The microstructure, the chemical composition of the phases, and the mechanical properties of the steel could be affected by the post-heat treatment, which is very sensitive. The team investigates to optimize the heat-treating method to improve the microstructure and mechanical properties.

manufactured material was attributed to transitional carbides in the lower bainitic regions. The additively manufactured material was also investigated in the tempered as well as normalized and tempered conditions, each showing decreased strength at elevated temperature than the as-deposited material.

Additive manufacturing addresses all aspects of manufacturing. Aerospace, machining, mining, or even medical applications make use of metal based additive manufacturing. Naturally metal additive manufacturing is also used in nuclear applications. The efforts of deploying additive manufactured components in nuclear power applications have been increased and large testing campaigns of additive manufactured parts are exposed to neutron irradiation today through various DOE sponsored programs. properties of stainless to manufacture martensitic the microstructural characterization of powder bed laser fusion additively that the is largely ferritic and subsequent heat treatment is