Development of Structural Steels for Powder Bed Fusion - Laser Beam

Over the past decade, powder bed fusion – laser beam (PBF-LB) has attracted noticeable attention from both academia and industry. However, there remains a scarcity of approved material for the process, as fewer than 40 alloys are commercially available. Although structural steels are some of the most commonly used materials in traditional manufacturing, they have yet to be developed for PBF-LB as their high carbon content makes them susceptible to cracking. The objective of this thesis was to develop structural steels for PBF-LB by determining the impact of various process parameters on part quality, microstructure and mechanical properties. This involved the production and analysis of various carbon (0.06 to 1.1 wt.% C) and low-alloy steels (AISI 4130, 4140, 4340 and 8620).In terms of part quality, specimen density was related to the volumetric energy density (VED) and the carbon content of the alloy. Regarding the VED, specimens produced at low VED formed lack of fusion porosity, while specimens produced at high VED formed keyhole porosity. As for the carbon content, increasing the carbon content would reduce lack of fusion porosity at low VED, while lowering the required VED to form keyhole porosity. As for cold cracking, this occurred in structural steels with ≥ 0.38 wt.% C as elevated carbon contents would increase specimen hardness. However, cracking could be mitigated by increasing the VED, laser power or build plate preheating temperature, as each enhanced the level of in situ tempering during PBF-LB. From these findings, process windows were established for each structural steel that produced defect-free and high-density specimens (> 99.8%).In terms of the microstructure, the as-built specimens were primarily composed of tempered martensite, with retained austenite also observed in alloys with ≥ 0.75 wt.% C. During PBF-LB, martensite formed during layer melting and was initially in a quenched-like state, with carbon atoms segregating to dislocations and martensite lath boundaries. Subsequent tempering of this martensite was due to micro-tempering within the heat affected zone and macro-tempering within the previously solidified material. Although both influenced martensite tempering, micro-tempering had the most significant effect as it reduced martensite hardness by up to ~380 HV. This noticeable reduction in hardness was due to the precipitation of nano-sized carbides at the previously carbon enriched regions of martensite.Lastly, mechanical testing found that structural steels produced by PBF-LB achieved a high ultimate tensile strength (4140: ∼1400 MPa, 4340: ∼1500 MPa, 8620: ∼1100 MPa), impact toughness (4140:∼90–100 J, 4340:∼60–70 J, 8620:∼150–175 J) and elongation (4140:∼14%, 4340:∼14%, 8620:∼14–15%) that met or exceeded the ASTM standards. Additionally, these specimens displayed limited directional anisotropy due to small grains with weak crystallographic texture, a homogenous microstructure and low levels of internal defects. These findings are meant to highlight that these alloys are not only suitable but actively take advantage of PBF-LB to achieve properties that meet or exceed those of conventionally produced alloys

Laser Based Powder Bed Fusion of Plain Carbon and Low-Alloy Steels: Microstructure and Processability

Despite the prominence of laser based powder bed fusion (LB-PBF) as an additive manufacturing technique, the number of alloys that have been approved for the process remains limited. In traditional manufacturing, ferrous alloys are the most common alloy group, consisting primarily of plain carbon and low-alloy steels. However, in LB-PBF, the production of ferrous alloys is limited to a small number of austenitic/precipitation-hardened stainless steels and tool steels. The lack of plain carbon and low-alloy steels stems from the negative impacts of carbon during processing, which promotes the formation of cracking defects within the as-built material. Hence, to expand the opportunities of LB-PBF, an understanding of how to process these carbon-containing ferrous alloys must be established.This work addresses the LB-PBF processability and microstructure of various plain carbon (0.06 to 1.1 wt.% C) and low-alloy steels (4130, 4140, 4340 and 8620). Microstructural analysis found the as-built specimens to consist of tempered martensite that formed due to the initial rapid cooling and subsequent intrinsic heat treatment that takes place during LB-PBF. Additionally, the presence of retained austenite was observed in alloys with ≥0.75 wt.% C and was caused by the depression of the martensite transformation temperatures, which left some austenite untransformed when cooling to room temperature.In terms of defects, porosity within the as-built specimens could be related to the chosen volumetric energy density (VED) and the carbon content of the alloy. At low VEDs, specimens contained large, irregular pores that related to lack of fusion porosity, while at high VEDs, specimens contained rounded, medium-sized pores that related to keyhole porosity. In terms of carbon content, increasing the amount of carbon was found to reduce the amount lack of fusion porosity at low VEDs while increasing the amount of keyhole porosity at higher VEDs. The decrease in lack of fusion porosity was caused by the improved wettability and flowability of the melt pool, while the increase in keyhole porosity was caused by increase in melt pool depth with higher carbon contents. Besides porosity, cold cracking was observed in some plain carbon and low-alloy steels, forming in specimens with a hardness above certain thresholds: ≥425 HV for Fe-C alloys, >460 HV for 4140 alloys and >500 HV for 4340 alloys. Increasing the VED or the laser power decreased the specimen hardness as both factors enhanced the intrinsic heat treatment of LB-PBF. This meant that cracking (in some alloys) could be avoided if a large enough VED or laser power was used. The carbon content also affected the as-built specimen hardness and thus the cracking susceptibility, a finding that explains why low carbon alloys (<0.43 wt.% C) did not display cracking at any of the tested VEDs, whereas high carbon alloys (≥0.75 wt.% C) displayed cracking at every tested VED. Using these findings, processing windows were established that produced high-density (>99.8%), defect-free plain carbon and low-alloy steel specimens without the requirement of build plate preheating

Effect of Carbon Content on the Processability of Fe-C Alloys Produced by Laser Based Powder Bed Fusion

The present study examines the processability of Fe-C alloys, with carbon contents up to 1.1\ua0wt%, when using laser based powder bed fusion (LB-PBF). Analysis of specimen cross-sections revealed that lack of fusion porosity was prominent in specimens produced at low volumetric energy density (VED), while keyhole porosity was prominent in specimens produced at high VED. The formation of porosity was also influenced by the carbon content, where increasing the carbon content reduced lack of fusion porosity, while simultaneously increasing the susceptibility to form keyhole porosity. These trends were related to an improved wettability, viscosity, and flow of the melt pool as well an increased melt pool depth as the carbon content increased. Cold cracking defects were also observed in Fe-C alloys that had an as-built hardness ≥425 HV. Reducing the carbon content below 0.75\ua0wt% and increasing the VED, which improved the intrinsic heat treatment during LB-PBF, were found to be effective mitigation strategies to avoid cold cracking defects. Based upon these results, a process window for the Fe-C system was established that produces high density (>99.8%), defect-free specimens via LB-PBF without the requirement of build plate preheating

Estimating long-term growth-rate changes of southern bluefin tuna (Thunnus maccoyii) from two periods of tag-return data

Southern bluefin tuna (SBT) (Thunnus maccoyii) growth rates are estimated from tag-return data associated with two time periods, the 1960s and 1980s. The traditional von Bertalanffy growth model (VBG) and a two-phase VBG model were fitted to the data by maximum likelihood. The traditional VBG model did not provide an adequate representation of growth in SBT, and the two-phase VBG yielded a significantly better fit. The results indicated that significant change occurs in the pattern of growth in relation to a VBG curve during the juvenile stages of the SBT life cycle, which may be related to the transition from a tightly schooling fish that spends substantial time in near and surface shore waters to one that is found primarily in more offshore and deeper waters. The results suggest that more complex growth models should be considered for other tunas and for other species that show a marked change in habitat use with age. The likelihood surface for the two-phase VBG model was found to be bimodal and some implications of this are investigated. Significant and substantial differences were found in the growth for fish spawned in the 1960s and in the 1980s, such that after age four there is a difference of about one year in the expected age of a fish of similar length which persists over the size range for which meaningful recapture data are available. This difference may be a density-dependent response as a consequence of the marked reduction in the SBT population. Given the key role that estimates of growth have in most stock assessments, the results indicate that there is a need both for the regular monitoring of growth rates and for provisions for changes in growth over time (possibly related to changes in abundance) in the stock assessment models used for SBT and other species

Development of powder bed fusion–laser beam process for AISI 4140, 4340 and 8620 low-alloy steel

This study focuses on process development and mechanical property evaluation of AISI 4140, 4340 and 8620 low-alloy steel produced by powder bed fusion–laser beam (PBF-LB). Process development found that increasing the build plate preheating temperature to 180\ub0C improved processability, as it mitigated lack of fusion and cold cracking defects. Subsequent mechanical testing found that the low-alloy steels achieved a high ultimate tensile strength (4140:∼1400 MPa, 4340:∼1500 MPa, 8620:∼1100 MPa), impact toughness (4140:∼90–100 J, 4340:∼60–70 J, 8620:∼150–175 J) and elongation (4140:∼14%, 4340:∼14%, 8620:∼14–15%) that met or exceeded the ASTM standards. Mechanical testing also revealed limited directional anisotropy that was attributed to low levels of internal defects (< 0.1%), small grains with weak crystallographic texture and improved tempering due to build plate preheating and post PBF-LB stress relief. This indicates that with adequate process development, low-alloy steels produced by PBF-LB can meet or exceed the performance of conventionally produced alloys

Laser-based powder bed fusion of non-weldable low-alloy steels

This study focuses on the processability of four low-alloy steels (AISI 4130, 4140, 4340 and 8620) via laser-based powder bed fusion (LB-PBF). In the as-built condition, the alloys consisted of tempered martensite that was the result of an intrinsic heat treatment (IHT) during LB-PBF. In terms of defects, a distinct transition in porosity was observed that correlated to the volumetric energy density (VED). At low VED, specimens contained a lack of fusion porosity, while at high VED, they contained keyhole porosity. Additionally, cold cracking was observed in 4140 and 4340 specimens produced at low/intermediate VEDs. This cracking could be mitigated by increasing the VED or laser power, as both enhance the IHT. This enhanced IHT lowered the material hardness below specific thresholds (<500HV 4340 and <460 4140), increasing ductility and allowing the specimens to avoid cracking. From these findings, crack-free, high-density (>99.8%) low-alloy steel specimens were produced without the requirement of build plate preheating

In situ tempering of martensite during laser powder bed fusion of Fe-0.45C steel

During laser powder bed fusion (L-PBF), materials experience cyclic re-heating as new layers are deposited, inducing an in situ tempering effect. In this study, the effect of this phenomenon on the tempering of martensite during L-PBF was examined for Fe-0.45C steel. Detailed scanning electron microscopy, transmission electron microscopy, atom probe tomography, and hardness measurements indicated that martensite was initially in a quenched-like state after layer solidification, with carbon atoms segregating to dislocations and to martensite lath boundaries. Subsequent tempering of this quenched-like martensite was the result of two in situ phenomena: (i) micro-tempering within the heat affected zone and (ii) macro-tempering due to heat conduction and subsequent heat accumulation. Hardness measurements showed that although both influenced martensite tempering, micro-tempering had the most significant effect, as it reduced martensite hardness by up to ∼380 HV. This reduction was due to the precipitation of nano-sized Fe3C carbides at the previously carbon-enriched boundaries. Lastly, the magnitude of in situ tempering was found to be related to the energy input, where increasing the volumetric energy density from 60 to 190 J/mm3 reduced martensite hardness by ∼100 HV. These findings outline the stages of martensite tempering during L-PBF and indicate that the level of tempering can be adjusted by tailoring the processing parameters

SAFETY INSTRUMENTED FUNCTIONS AS CRITICALITY DEFENSES

The objective of this paper is to share the SRS methodology for identifying the reliability requirements and documenting the expected performance of Safety Instrumented Functions (SIFs) used as criticality defenses. Nuclear Criticality SIFs are comprised of sensors, logic solvers, and final control elements, which may be either automatic or manual, to detect a process hazard and respond to prevent a criticality. The Savannah River Site (SRS) has invoked the chemical process industry safety standard (ANSI/ISA 84.00.01) for the design of safety significant instrumented systems. The ISA standard provides a graded approach to design based on the amount of risk reduction that is required of an SIF. SRS is embarking on application of this standard to nuclear criticality defenses, thus integrating criticality safety requirements with verifiable design methodology. Per the DOE G 421.1-1 discussion of the double contingency principle, guidance for a single contingency barrier includes, ''The estimated probability that the control will fail (when called upon for protection) is not greater than 1 in 100 demands''. The application of this standard to nuclear criticality SIFs will provide clear requirements in terms of safety availability and testing to assure that the instrumented criticality system as designed, installed, and maintained will meet is performance requirements. The paper identifies the numerous challenges presented by this initiative and the benefits of this approach

First Record of the Southern Red-Backed Vole, Clethrionomys gapperi, in Newfoundland: Implications for the Endangered Newfoundland Marten, Martes americana atrata

We report on the first capture of the Southern Red-backed Vole (Clethrionomys gapperi), the eleventh non-native terrestrial mammal established on the island of Newfoundland over the last 150 years. Red-backed Voles may have been accidentally introduced by unknown sources in pulpwood imports or may have been deliberately introduced in an attempt to augment the depauperate small mammal fauna as a vigilante recovery effort for the endangered Newfoundland Marten (Martes americana atrata). We anticipate significant utilization of the Red-backed Vole as prey by both Newfoundland Marten and Red Fox (Vulpes vulpes) with associated demographic responses within and between these species. Red-backed Voles will likely change habitat utilization patterns for the endemic subspecies of Meadow Vole, Microtus pennsylvanicus terraenovae

Charge-tagging liquid chromatography–mass spectrometry methodology targeting oxysterol diastereoisomers

The introduction of a hydroxy group to the cholesterol skeleton introduces not only the possibility for positional isomers but also diastereoisomers, where two or more isomers have different configurations at one or more of the stereocentres but are not mirror images. The differentiation of diastereoisomers is important as differing isomers can have differing biochemical properties and are formed via different biochemical pathways. Separation of diasterioisomers is not always easy by chromatographic methods. Here we demonstrate, by application of charge-tagging and derivatisation with the Girard P reagent, the separation and detection of biologically relevant diastereoisomers using liquid chromatography – mass spectrometry with multistage fragmentation