ML20351A204

NRC Technical Assessment of Additive Manufacturing - Laser Powder Bed Fusion

ML20351A204
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Issue date:	01/08/2021
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NRC Technical Assessment of Additive Manufacturing - Laser Powder Bed Fusion

1. Introduction and Purpose This document provides a Nuclear Regulatory Commission (NRC) technical assessment of the safety significance of the identified differences between additive manufacturing - laser powder bed fusion (LPBF) and traditional manufacturing methods and the aspects of LPBF not addressed by codes and standards or regulations. This assessment is primarily based upon the technical information and gap analysis developed by Oak Ridge National Laboratory (ORNL) in technical letter report (TLR), Review of Advanced Manufacturing Techniques and Qualification Processes for Light Water Reactors - Laser Powder Bed Fusion Additive Manufacturing, (Agencywide Documents Access & Management System (ADAMS) Accession No. ML20351A217) [hereafter referred to as the ORNL TLR]. This assessment, combined with the ORNL TLR, highlights key technical information related to LPBF-fabricated components in nuclear power plants, and fulfills the deliverable for LPBF under Subtask 1A of the Revision 1 Advanced Manufacturing Technologies (AMT) Action Plan (ADAMS Accession No. ML19333B973).

2. NRC Identification and Assessment of Differences This section describes the differences between an LPBF-fabricated component and a traditionally-manufactured component, assesses the safety significance of the identified differences, and identifies specific technical considerations related to LPBF-fabricated components. The safety significance of each identified difference in the context of this assessment refers to the impact on component performance, not overall plant safety. The overall impact to plant safety is a function of component performance and the specific component application, e.g., its intended safety function. The impact on plant safety is not included in this report as such an assessment would not be possible without considering a specific component application.

Staff identified the differences between LPBF fabrication and traditional manufacturing processes by reviewing the information and gap analysis rankings from the ORNL TLR, as well as other relevant technical information (e.g., NRC regulatory and research experience, technical meetings and conferences, codes and standards activities, Electric Power Research Institute and Department of Energy products and activities). The identified differences originated either as important aspects or gaps of the LPBF process or product performance as defined here:

• Important aspect: part of the AMT fabrication process or product performance that needs to be considered and carefully controlled during manufacturing (e.g. powder quality for LPBF)

• Gap: part of the AMT fabrication process or product performance that either lacks knowledge or understanding due to limited information/data The results of this technical assessment are provided in two tables. Table 1 includes the material-generic differences for LPBF process and product performance compared to traditional manufacturing. Table 2 includes additional material-specific differences for 316L stainless steel, which is the alloy relevant to LPBF-fabricated nuclear applications with the greatest quantity of information currently available in the open literature. While Table 2 is based on the available

information in the open literature for 316L, the differences identified in Table 2 involving material-specific properties and performance would likely need to be considered for any new material to be fabricated using LPBF. In general, an important need for any nuclear LPBF-fabricated component is material-specific data for the proposed processing and post-processing parameters to ensure adequate component performance in environment, including various properties (e.g., fracture toughness, tensile strength) and aging mechanisms (e.g., thermal aging, irradiation effects, and stress corrosion cracking [SCC]).

Tables 1 and 2 below identify and provide technical information for the LPBF process and product performance through the following columns:

• Difference:

o Corresponding ORNL Gaps: Identification of corresponding gaps from Section 3.4 of the ORNL TLR.

• Definition: Brief description of LPBF process difference.

• NRC Ranking of Significance:

o Importance: Impact on final component performance.

A High ranking would signify that the difference has a significant impact on component performance.

A Medium ranking would signify that the difference has a moderate impact on component performance.

A Low ranking would signify that the difference has a minimal impact on component performance.

o Knowledge / Manageability: Description of how well-understood and manageable the difference is.

• Technical Information: Technical information for consideration of LPBF-fabricated components for use in nuclear power plants.

3. Codes and Standards Section 3.5 of the ORNL TLR provides a comprehensive overview of the existing standards relevant to LBPF as well as a detailed analysis of standards identified as highly relevant to nuclear power plants (NPPs). The ORNL TLR indicates that three specific standards (AWS D20.1M, MSFC-STD-3717, and MSFC-STD-3716) provide a reasonable basis for machine, process, and component inspection qualification procedures. Although NRC staff has not reviewed these standards, we recommend close consideration of the approaches in those standards when developing codes and standards for LPBF for nuclear applications.

In addition, the ORNL TLR provides recommendations for areas where standards development is most needed to support LPBF-fabricated components for NPP applications. NRC staff generally agree with these recommendations. In particular, codes and standards development, and the corresponding research to support their development, would be most valuable in the following areas to support LPBF use in nuclear applications:

• Material-specific criteria for powder recycling and sieving should be developed to prevent powder degradation from significantly impacting final component performance.

• Adequately assessing microstructural and material property heterogeneity and developing statistically driven requirements for the number, location and orientation of

witness specimens required to quantify the effects of heterogeneity is an important area for codes and standards development. This should also consider the positive impact of post-processing, such as hot-isostatic pressing (HIP), on heterogeneity.

• Weld integrity and weldability, including pre- and post-weld heat treatments: additional data and codes and standards development are needed.

4. Summary and Conclusion In this report, the staff has identified and assessed the material-generic differences for LPBF process and product performance as well as the material-specific differences for 316L stainless steel compared to conventional manufacturing. The staff has also identified gaps in existing codes and standards that should be addressed to support LPBF use in nuclear applications.

Table 1 -Technical Information - LPBF Generic Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability Machine process Medium Machine Machine process

• Control of LPBF file preparation is needed to ensure LPBF Machine control includes the process control control is very process control. Improper file control can significantly Process Control software controlling could have a manageable with impact final product properties and performance and the scan strategy of moderate impact quality assurance affect fabrication replication.

(Software and the LPBF machine on final product (QA) including

• Machine calibration is vital for fabrication replication, File Control & and the machine performance. appropriate particularly contamination minimization when recycling Machine calibration to calibration. powder, ensuring correct laser power and beam shape, Calibration) reliably fabricate and atmospheric quality control in addition to geometric components. tolerances.

Powder quality High Powder Powder quality can

• Powder contamination is a critical issue that may covers the quality can have a be challenging to adversely affect material properties and process by important significant impact manage and the introducing oxides and changing chemical composition.

characteristics of on the final effects on final

• Powder should always be used after sieving, because the powder, such product product performance unsieved powder may not be representative of as composition and performance. are material specific. composition as elemental composition and phases may Powder Quality size distribution, Powder quality is an not be uniformly distributed across the powder size and how it is area of active range.

(Contamination managed in the research to

• Powder re-use acceptance/rejection depends on Management, production process understand what the routinely sampling and characterizing powder after Powder prior to the build critical powder sieving. The LPBF system, sieving system and Characterization, process (e.g., characteristics are for maintenance of inert environment are all important Sieving System) sieving, reuse, a given alloy and factors that influence the amount of powder re-use that storage, their impacts on can be done safely.

contamination). product performance.

• For example, in 316L Si and Mn content in the powder can create oxides that have adverse effects on SCC growth rates. Consideration should be given on oxide content in powder acceptance (virgin and recycled) criteria.

LPBF Build Build process Medium Build This issue is

• Build interruptions (planned and unplanned) can have a Process management and interruptions and manageable with QA very significant impact on quality of the component and Management and control includes loss of process and the use of in situ should be avoided.

Control monitoring control can monitoring and

• In situ monitoring without feedback control can be used parameters during adversely impact environmental sensor to identify issues in the build process in real time and (Environmental fabrication using product data. may be used alone or in conjunction with other Sensor Data, In- environmental performance by approaches to demonstrate process control.

Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability Situ Monitoring sensors, in-situ creating defects, Regarding the use of

• In situ monitoring with feedback control is still a and Feedback, monitoring, and altering local in situ monitoring with developing area of research and should be carefully Planned and evaluating the material feedback control managed and strongly demonstrated if proposed for use Unplanned effects of build microstructure designed to correct during production.

Interruptions) interruptions. and properties, defects automatically

• While artificial intelligence (AI) is commonly used to flag and creating during the build defects for human review; lack of AI-flagged defects warping and process, knowledge should not be interpreted as no existing defects.

distortion due to is relatively limited

• One limitation of all build chamber surface monitoring changing the and still maturing. methods is that only the top surface is observed.

thermal distribution by cooling.

Witness specimens Medium Witness Witness specimens

• The most highly representative test specimens are are test specimens specimens offer are well-established obtained from end use component geometries that are fabricated one approach to for use to provide o Geometry impacts, particularly thickness, on witness concurrently with demonstrating empirical evidence of specimen microstructure and properties should be end-use process control by incomplete considered and addressed components and measuring spreading,

• Optimal witness specimen parameters (geometry, size, used to provide properties from delamination, or location, spatial orientation, and frequency) depends confirmation of parts built other events that may highly on the end use component geometry and the goal Witness build quality and coincidentally with result in component of the witness testing approach (e.g. monitoring build Specimens product service product. rejection. However, issues as part of process control or generating performance. the use of witness representative material properties data as part of (Witness specimens for process qualification).

Specimens) optimization and

• When sectioning end-use geometries is not feasible, generating functional evaluations of end-use geometries such as quantitative data for burst tests are recommended in conjunction with qualification is less simplified witness specimen geometries.

well-established and could involve demonstration for the material and geometry of the final product.

Post-processing High Post- Post-processing heat

• Post-processing heat treatments without HIP generally Post -

includes methods processing should treatments are are designed to provide two benefits: stress relief and/or Processing used after the initial make material commonly done for

Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability product build, such properties and LPBF and annealing, but likely have little impact on porosity or as hot isostatic performance more conventional flaws.

pressing and heat homogeneous materials and are o Stress relief heat treatments will primarily reduce treatments, to and similar to fairly well- residual stresses from the as-built part without improve material conventional understood. Hot- otherwise affecting the microstructure or properties.

properties and forged materials isostatic pressing o Annealing heat treatments should greatly reduce or performance by and may (HIP) is also a well- eliminate residual stress as well as coarsen the increasing density significantly established method, microstructure (to improve toughness) and reduce and reducing impact but less commonly heterogeneity in microstructure and properties.

porosity. considerations used for conventional

• HIP may be beneficial for reducing residual stress, related to the materials where porosity, heterogeneity, and internal cracks, while also other LPBF- porosity is not a coarsening the microstructure (to improve toughness).

specific topics significant issue.

• For all post-processing approaches, material-specific identified in lower demonstration is important to identify adequate heat rows. treatment or HIP parameters to achieve desired improvements in microstructure, properties, heterogeneity, porosity and fabrication flaws.

• Post-processing may significantly impact considerations related to the other LPBF-specific topics identified in lower rows.

The geometry of High Local Local geometry

• The role of geometry on local microstructure and the component and geometry impacts impacts are highly properties is one of the key differences between LPBF Local Geometry the heat transfer can have a dependent on the produced components and conventionally produced ones.

Impacts on characteristics from significant impact material and

• Local geometry significantly impacts thermal profiles Product the product build on product geometry of the final during fabrication, which affects the local microstructure Properties and directly affect local performance if not product. They can be and properties.

Performance microstructure managed or managed through o For example, a thin section with relatively rapid cooling (e.g., grain size addressed. post-processing and rates will likely have a much finer microstructure than a (LPBF Design and orientation), sampling / witness thicker section with a slower cooling rate due to more Considerations, which can affect specimens to surrounding material being melted.

Geometry-Scan material properties measure the impacts. o As a result, local material properties such as strength, Strategy and performance, ductility and toughness will be affected by the variation Interactions) including SCC in microstructure as a function of geometry.

susceptibility.

• Witness specimens can be used to assess local geometry impacts but should be carefully demonstrated to be applicable to the end-use geometry.

Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability

• Post-processing and/or scan strategy refinement have the potential to minimize the local geometry impacts, however, they can vary significantly based on the geometry and materials used

• Varying processing parameters to be nonconstant and nonlinear is potentially another method to compensate for the effect of geometry and minimize local geometry impacts. This is a less mature approach and could benefit from additional research and demonstration.

Heterogeneity and High This effect is

• Heterogeneity generally manifests with different properties anisotropy Heterogeneity and generally well- in the build direction relative to the other two directions generally manifest anisotropy in understood but due to the nature of the layer-by-layer build process. This as different LPBF fabricated requires specific impacts the microstructure and fabrication defect structure properties in the components are a measures to and generally creates poorer properties between build Heterogeneity build direction significant manage, whether layers.

and Anisotropy in relative to the other difference from through an

• Post-processing with appropriate parameters would be Properties two directions due conventional appropriate sampling expected to make material properties and performance to the nature of the materials, which methodology or post- more homogeneous and similar to conventional forged (Material layer-by-layer build are largely processing to help materials.

Property process. This isotropic, and can minimize this effect.

• For example, in as-fabricated and stress-relieved 316L, Sampling impacts the have a significant the variation in microstructure due to geometry also Methodology and microstructure and impact on product causes fatigue and SCC cracks to preferentially travel in Heterogeneity) fabrication defect performance if not the build direction should they initiate.

structure and addressed in the generally creates design and poorer properties fabrication between build process.

layers.

Residual stresses Medium Residual There is significant

• High residual stress may result in warping, cracking, and Residual Stress form during the stress and knowledge on delamination; however, these events typically can be LPBF build process associated residual stress, visually detected.

(Residual Stress and can lead to defects can have including how to

• In addition, residual stress can make the product

- Warping, warping, cracking, an impact on manage it through susceptible to future degradation such as SCC or fatigue Cracking, and and delamination if product post-processing or from the presence of high tensile residual stress on the Delamination) not properly performance. NDE. surface.

managed.

• Post-processing with appropriate parameters would be expected to relieve residual stress.

Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability Porosity includes High Porosity can There is knowledge

• Porosity is known to adversely affect fatigue life, SCC, the size, have a significant on how to manage and IASCC, though the precise quantitative impact distribution, and impact on product porosity both in the depends on the material and porosity characteristics total volume of performance. By build process and (pore frequency, pore size, pore morphology, and total Porosity voids and pores in the nature of through post- void fraction).

the LPBF LPBF, the porosity processing.

• Machine parameters and scan strategy refinement have (Porosity component. may have smaller the potential to address porosity concerns; however, they Measurement) size and higher may vary significantly based on the geometry and density than materials used.

forged materials.

• For post-processing, HIP with appropriate parameters has been demonstrated to reduce porosity and produce properties more similar to conventionally forged materials.

Note 1: Discussion of the corresponding ORNL gaps can be found in Section 3.4 of the ORNL TLR (ADAMS Accession No. ML20351A217).

Table 2 -Technical Information - LPBF 316L Material-Specific Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability Tensile properties Low Failure due to 316L LPBF materials

• High porosity would likely degrade tensile tensile overload is not a have generally sufficient performance but would have a greater impact on Tensile common failure mode data showing similar or other material properties.

Properties in nuclear components superior tensile and no more likely in properties compared to (Tensile LPBF materials due to similar forged materials.

Properties) their similar or superior tensile properties.

Initial fracture High Low initial There is limited data on

• Limited data on 316L LPBF materials have toughness refers to fracture toughness can fracture toughness for shown significantly lower initial fracture the materials lead to brittle 316L LPBF materials. toughness depending on post-processing than starting fracture component failure if not Post-processing should Initial Fracture similar forged materials.

toughness upon adequately managed. improve fracture Toughness entering service toughness and minimize

• Data in representative environments is important after fabrication. any difference. to demonstrate that fracture toughness will be (Fracture adequate to meet component design Toughness) assumptions.

• Post-processing with appropriate parameters would be expected to improve fracture toughness.

Thermal aging High Thermal aging NRC is not aware of any

• Data in representative environments is important refers to the can lead to brittle significant data on to demonstrate that fracture toughness does not reduction in component failure if not thermal aging behavior degrade excessively due to thermal aging and fracture toughness adequately managed. of 316L LPBF materials.

will be adequate to meet component design Thermal Aging after significant However, this issue can time at elevated be managed by assumptions.

(Aging and temperature, which assuming a very

• Post-processing with appropriate parameters Irradiation is a known aging conservative toughness would be expected to make material properties Degradation) mechanism for value for design and and performance more similar to conventional stainless steels flaw evaluation forged materials.

containing purposes until sufficient significant levels of data can be generated.

ferrite.

Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability SCC refers to High SCC can lead to Very limited data exists

• Data in representative environments is important stress corrosion component failure if not on SCC behavior of to demonstrate that changes in material crack initiation and adequately managed. 316L LPBF materials, performance due to SCC will not be degraded to growth of which is a known SCC a greater degree in LPBF materials than forged susceptible degradation mode in materials under LWRs. materials.

(SCC and

• Post-processing with appropriate parameters roughly constant IASCC) would be expected to make material properties stress operating conditions due to and performance more similar to conventional the corrosive forged materials.

environment.

Fatigue refers to Medium While fatigue Limited data is available

• Surface roughness is known to be a greater issue the initiation and can be a concern and in the literature on the with LPBF materials and can reduce fatigue life.

propagation of lead to component fatigue life of a LPBF

• Fatigue properties are strongly dependent on cracks due to cyclic failure, it is generally materials compared to Fatigue post-processing heat treatment and component loading with or addressed conventionally without conservatively through manufactured materials. porosity.

(Fatigue)

• Data in representative environments is important environmental design standards and effects playing a has not generally led to to support fatigue calculations including significant role in many safety-significant environmentally-assisted fatigue (EAF) in LPBF the process. failures or flaws. materials.

Irradiation effects High Irradiation effects Very limited data exists

• Data in representative environments is important refers to the impact are a highly relevant on irradiation effects, to demonstrate that irradiation effects will not be of neutron issue to address for particularly neutron significantly greater in LPBF materials than Irradiation Effects irradiation on irradiated reactor irradiation, on the forged materials.

various aspects of internals components behavior of 316L LPBF (SCC and material properties in LWRs, which can materials.

• Post-processing with appropriate parameters IASCC, Aging and performance, lead to premature would be expected to make material properties and Irradiation including, but not component failures. and performance more similar to conventional Degradation) limited to, loss of forged materials.

fracture toughness, IASCC, and void swelling.

Weld integrity High Welds can be a NRC is not aware of any

• Data in representative environments is important refers to the location of degradation significant data on weld to demonstrate that welds with LPBF base Weld Integrity properties and and may behave integrity for of 316L performance of the LPBF materials.

Difference NRC Ranking of Significance (Corresponding Definition Knowledge / Key Technical Information Importance ORNL Gaps)1 Manageability weld and significantly differently materials will perform similarly to those with surrounding heat- with LPBF materials. conventionally manufactured base materials.

affected zone.

Weldability refers Medium Weldability is NRC is not aware of any

• Limited data shows a narrower weld parameter to the ability to a concern for the significant data on range may be appropriate for LPBF 316L.

successfully weld a licensee but should not weldability of 316L Weldability / material to another greatly impact LPBF materials.

Joining component without component unacceptable performance as long as (Weldability) defects. satisfactory welds passing Code requirements can be made.

Note 1: Discussion of the corresponding ORNL gaps can be found in Section 3.4 of the ORNL TLR (ADAMS Accession No. ML20351A217).