Additively manufactured (AM) metals require a modified heat treatment to accommodate for slight differences in composition caused by powder atomization and cover gas used in the manufacturing process. 17-4PH stainless steel (17-4PH) is a precipitation hardening steel which hardens through the formation of Cu precipitates in a martensitic matrix during aging treatment. The powders used in Laser Powder Bed Fusion (LPBF) fabrication of 17-4PH are typically spray atomized using N 2 cover gas, which is associated with a certain amount of nitrogen uptake. Nitrogen is a potent austenite stabilizer and will lower the martensite start temperature of the steel. To counteract the effect of nitrogen, a sub-zero heat treatment can be introduced to promote a more complete transformation into martensite. In this work, the effect of nitrogen on the heat treatment response of 17-4PH is investigated through comparing standard wrought, nitrogen loaded wrought, and LPBF 17-4PH. In particular, the effect of introducing a subzero treatment is addressed. After quenching from the solutionizing step (austenitization) LPBF fabricated 17-4PH was cold-treated in different combinations of dry ice (-78 °C) and boiling nitrogen (-196 °C). Subsequently, these conditions were aged in the conventional way. The sub-zero treatments were compared with the conventional heat treatment procedure, which does not entail a sub-zero step. In addition, phase transformations (above room temperature) were monitored in-situ using dilatometry. Finally, hardness tests and XRD analysis were performed to characterize the final microstructure. It is demonstrated that sub-zero treatment can be an effective route to address the problems associated with the additional nitrogen present in LPBF 17-4PH fabricated parts.

Mold repair is a viable strategy for saving energy and reducing CO 2 emissions. Papers in the literature show that repairing a limited damaged area of the mold instead of producing a new one is becoming increasingly attractive, especially considering the latest European and international regulations introduced with the green deal. In this paper, the authors are pleased to present some preliminary results related to the repair of AISI H13 tool steel molds by Laser-Directed Energy Deposition. Steel blocks (20 x 55 x 100 mm3), previously tempered at 435±10 HV, were machined to reproduce the material removal of the damaged part of the mold. Subsequently, the region was repaired by L-DED using commercial H13 powder. The process parameters were optimized to obtain a defect-free welded area. Since the microstructure of the deposited tool steel consists of hard (730±10 HV) and brittle (7 J Charpy impact toughness) martensite, a series of post-process heat treatments were performed at different temperatures to restore a hardness compatible with that of the base steel. However, this goal was only partially achieved due to the different tempering behavior of L-DED-deposited and bulk H13 steel. In particular, the tempering temperature had to be limited to avoid softening of the base steel. In the best case, double tempering at 620 °C resulted in a toughness recovery of up to 42 J. Thermal fatigue tests showed better resistance to crack propagation after tempering, as evidenced by the shallower penetration depth compared to the as-built material.

Additive manufacturing is increasingly used in a variety of applications. Directed Energy Deposition (DED) technology using powder feedstock enables the production of materials in combinations that would be very problematic using conventional technologies. DED is a technological process where the fed material is melted directly at the desired location using a laser beam. The research described here deals with the additive manufacturing and subsequent induction heat treatment of a functional deposited layer of M2 high-speed steel. Induction treatment has the advantage that only the functional layer of the component can be heat treated without affecting the base material. It is therefore possible to heat treat a combination of completely different materials with different properties without degrading the base material. Hardness values reached 950 HV (68 HRC) both after additive manufacturing and after additive manufacturing and induction treatment. Induction heat treatment of the deposited M2 layer ensured removal of traces of the original melt pools produced by the additive manufacturing. Investigation of the microstructure and mechanical properties of M2 tool steel after induction heat treatment produced by DED highlights its potential for high performance tooling and machining applications. The main objective of this research is to improve the final properties and tool life of forming tools when the tool is made of less expensive low-alloy steel and its functional layer is made of M2 high speed steel using additive manufacturing technology.

With the increasing amount of historical fatigue data for advanced manufacturing processes, such as additive manufacturing, it becomes increasingly feasible to use statistical and machine learning approaches to garner deeper insights into the contributions to fatigue performance in order to improve the design for fatigue failure or processing route parameters. Prior to model development, aggregated datasets, whether compiled through manual or automated processes, require extensive verification and profiling to eliminate systematic errors and identify insufficiently investigated parameter combinations. Without these steps, the veracity of any model, especially black-box models, is dubious. Once the structure and patterns of the dataset are established, proper implementation of random imputation can be used to expand the amount of usable data. This verified and augmented dataset can now be subjected to various statistical tools whose role in data exploration will be discussed, particularly regarding the role of distinguishing porosity- and microstructure-driven fatigue failure data.

As more industries look toward additively manufactured (AM) components to combat lead times, re-design, cost of complexity, etc., those industries are faced with re-evaluating the performance of AM-based materials as compared to their well-documented wrought or machined counterparts. A particular alloy of interest to many industries including aerospace and energy/power generation is Inconel 718 due to its resistance to oxidation and high temperature degradation [1]. Additively manufactured Inconel 718 parts typically receive a series of post-build heat treatments prior to deployment. If not properly controlled, these post-build treatments may introduce secondary precipitates and other inhomogeneities that will affect the parts’ mechanical properties and susceptibility to corrosion. This is specifically true of susceptibility to localized corrosion mechanisms that may lead to crack initiation, accelerated crack growth and ultimately premature failure. By utilizing electrochemical parameter testing to analyze for localized breakdown potentials, this work investigates the variation in tolerance to localized corrosion that results from common post-build heat treatment steps and the secondary phase precipitation that can ensue in Inconel 718 AM parts.

Selective laser melting (SLM) is an additive manufacturing technique that can be used to make the near-net-shape metal parts. M2 is a high-speed steel widely used in cutting tools, which is due to its high hardness of this steel. Conventionally, the hardening heat treatment process, including quenching and tempering, is conducted to achieve the high hardness for M2 wrought parts. It was debated if the hardening is needed for additively manufactured M2 parts. In the present work, the M2 steel part is fabricated by SLM. It is found that the hardness of as-fabricated M2 SLM parts is much lower than the hardened M2 wrought parts. The characterization was conducted including X-ray diffraction (XRD), optical microscopy, Scanning Electron Microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) to investigate the microstructure evolution of as-fabricated, quenched, and tempered M2 SLM part. The M2 wrought part was heat-treated simultaneously with the SLM part for comparison. It was found the hardness of M2 SLM part after heat treatment is increased and comparable to the wrought part. Both quenched and tempered M2 SLM and wrought parts have the same microstructure, while the size of the carbides in the wrought part is larger than that in the SLM part.

Powder metallurgy (PM) is the fabrication process of compacting metal powders to shape and sintering these compacts to yield the final material’s properties. The PM compaction process allows for complex geometries to be formed that would normally lead to long and expensive machining processes from wrought steels. Special alloy selection can allow for hardening of the microstructure during the sintering procedure. The sinter hardened (SH) alloys exhibit good mechanical properties along with good hardenability and dimensional stability and may be a suitable replacement for wrought steels where low distortion from heat treatment or microstructural control is required. In this study, it was found for a complex geometry coupler application, a SH alloy could successfully replace an austenitizing heat treatment process with a low carbon steel. The low carbon steel was found to have micro heterogeneities from heat treatment that lead to premature failure in the application. Dimensional distortion and production variance were also of concern with the low carbon steel. The SH material demonstrated acceptable physical properties, hardness and microstructural uniformity to solve the concerns associated with processing of the low carbon steel coupler. Post processing optimization also added to the life performance of the coupler by tailoring the final microstructure to mating components.

IN718 has good fabricability, high strength at elevated temperature, and corrosion resistance, and it is widely deployed in many aerospace and other high-performance applications. With the molten pool rapid solidification during laser powder bed fusion (L-PBF), the resulting microstructure is anisotropic and inhibits macro-segregation. The as-built condition usually exhibits lower mechanical properties. Four different heat treatment procedures were designed and tested to study the effect of different heat treatment parameters on the type of precipitates and grain size. The investigated heat treatment procedures showed the formation of equiaxed grain size and a significant amount of γ' and γ" particles at the grain boundary in addition to primary carbide types (MC). Three types of microstructure characteristics and grain size were achieved. Coarse grain size suitable for creep application was achieved by increasing the soaking time at the aging cycle. The formation of serrated grain boundaries suitable for good fatigue and creep properties was achieved by decreasing the stress relief cycle's soaking time and temperature. Fine-grain size, which is preferable for fatigue properties, was achieved by decreasing the soaking time at the solution annealing cycle.

Laser Powder Bed Fusion (L-PBF) processes are becoming more viable in place of traditional castings in a variety of industries. To compete, novel material grades are being considered with additive manufacturing (AM). In maximizing performance and manufacturing efficiency through AM, a novel approach to heat treatment and Hot Isostatic Pressing (HIP) processing needs to be considered. It has been shown that combining key heat treatment processes with (HIP) by utilizing fast cooling rates can benefit static properties as well as improve turn-around time for HIP processing [1,2]. Argon pressures up to 207 MPa with cooling rates above 170°C per minute are now available in production sized HIP systems to design ideal HIP cycles for high pressure heat treatment. Additive manufacturing with high pressure heat treatment is in need of further investigation for establishing new qualification standards. This study investigates designed High-Pressure Heat Treatment cycles to consider mechanical performance on LPBF CoCr. The combined cycles investigate possible alternatives to historically accepted two step HIP then heat treat processing by combining densification with homogenization treatment into one step. Tensile, fatigue, hardness, microstructure and Charpy impact performance are explored to seek optimal properties and with streamlined thermal processing. It was found that all trial conditions exceeded Electron Beam Melted (EBM) AM CoCr expectation, but traditional processing provided a slight advantage in ultimate tensile stress. One of the novel processes explored, “common” was found to provide a slight improvement on yield stress and direct hardness. Published fatigue data is rare for CoCr, however data generated from this study showed a slight advantage to the “common” HPHT process primarily for lower applied stress levels. Microstructures were comparable across all trial processes. It is recommended that each novel processing route be considered as viable alternatives to traditional processing, but that the “common” processing may prove advantageous for both mechanical properties and streamlined manufacturing.

Direct metal laser sintering (DMLS) is an established technology in metal additive manufacturing. This complex manufacturing process yields unique as-built material properties that influence mechanical performance and vary with different machine parameters. Part porosity and residual stresses, which lead to part failures, and grain structure, as it relates to mechanical properties and anisotropy of DMLS parts, require investigation for different print settings. This work presents results for density, residual stress, and microstructural inspections on designed test artifacts for the benchmarking of 3D metal printers. Results from printing artifacts on two separate DMLS printer models with default parameters show highly dense parts for both printers, with relative densities above 99.5%. Characterization of residual stress through cantilevered deflection specimens indicates similar resulting thermal stresses developed in both build processes, with deflection averages of 32.48% and 28.09% for the respective machines. Additionally, properties of the test artifact printed after adjusting default machine parameters for equal energy density are characterized.

Hastelloy X is used in turbomachinery and petrochemical applications as it is designed for excellent oxidation and stress corrosion cracking resistance, strength, and stress rupture behavior. This alloy is now being printed via powder bed fusion processes as many industries have developed interests in the benefits additive manufacturing (AM) offers. However as-printed Hastelloy X suffers from material defect formation such as hot cracking. Hot isostatic pressing (HIP) is often applied to improve performance and reliability. Although the conventional HIP process has been shown to eliminate defects, the equipment is unable to cool at desired rates allowing the formation of excessive carbide precipitation, negatively influencing corrosion resistance and toughness. In turn the product is solution treated at a similar temperature while applying rapid gas cooling for performance requirements. With use of uniform rapid cooling available in modern HIP equipment, a high-pressure heat treatment can be applied offering the ability to perform both HIP and heat treatment in one piece of equipment. Microstructure and tensile properties are evaluated and compared to the conventional processing routes. The results demonstrate that the novel high pressure heat treatment approach offers a processing route that is equivalent to or better than conventional methods.