C AdvisoryU.S. Department of Transportation Federal Aviation Circular Administration Subject: Powder Bed Fusion Additive Date: 6/23/23 AC No: 33.15-3 Manufacturing Process for Aircraft Engine Initiated By: AIR-621 Parts 1 PURPOSE.
This advisory circular (AC) describes an acceptable means for demonstrating compliance with Title 14, Code of Federal Regulations (14 CFR) 33.15 for aircraft engine parts with materials produced by the powder bed fusion (PBF) additive manufacturing (AM) process. Guidance is also presented on closely related design and manufacturing aspects associated with AM.
2 APPLICABILITY.
2.1 The guidance in this AC is for aircraft engine manufacturers, modifiers, Federal Aviation Administration (FAA) aircraft engine type-certification engineers, and FAA designees. The contents of this AC do not have the force and effect of law and are not meant to bind the public in any way, and this AC is intended only to provide information to the public regarding existing requirements under the law or agency policies.
2.2 This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, to demonstrate compliance to the material requirements of 14 CFR 33.15. However, if you use the means described in the AC, you must follow it in all important respects. When the method of compliance in this AC is used, terms such as “should,” “may,” and “must” are used in the sense of ensuring applicability to this particular method of compliance. The FAA will consider other means of showing compliance that an applicant may elect to present. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. If, however, the FAA becomes aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation as a basis for finding compliance.
2.3 The material in this AC does not change or create any additional regulatory requirements, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.
RELATED READING MATERIAL.
The following materials are related to the guidance in this document. Unless otherwise indicated, you should use the current edition if following the method of compliance set forth in this AC.
Title 14, Code of Federal Regulations.
• Part 21, Certification Procedures for Products and Parts.
• Section 33.15, Materials.
Industry Documents.
Many industry standards are available (and under development) that provide additional information, guidance and standards for AM material. The following listing of industry publications from the American Society for Testing and Materials (ASTM), Battelle, International Organization for Standardization (ISO), and the Society for Automotive Engineers (SAE) represents a sample of available industry publications supporting the adoption of AM.
3.1
3.2
• ASTM F2924, Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with Powder Bed Fusion.
• ASTM F2971, Standard Practice for Reporting Data for Test Specimens Prepared by Additive Manufacturing.
• ASTM F3001, Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium ELI (Extra Low Interstitial) with Powder Bed Fusion.
• ASTM F3049, Standard Guide for Characterizing Properties of Metal Powders Used for Additive Manufacturing Processes.
• ASTM F3055, Standard Specification for Additive Manufacturing Nickel Alloy (UNS N07718) with Powder Bed Fusion.
• ASTM F3056, Standard Specification for Additive Manufacturing Nickel Alloy (UNS N06625) with Powder Bed Fusion.
• ASTM F3122, Standard Guide for Evaluating Mechanical Properties of Metal Materials Made via Additive Manufacturing Processes.
• ASTM F3184, Standard Specification for Additive Manufacturing Stainless Steel Alloy (UNS S31603) with Powder Bed Fusion.
• Battelle, Metallic Materials Properties Development and Standardization (MMPDS) Handbook.
• ISO/ASTM52900, Additive manufacturing — General principles — Terminology.
• ISO/ASTM52915, Standard Specification for Additive Manufacturing File Format (AMF) Version 1.2.
• ISO/ASTM52921, Standard Terminology for Additive Manufacturing-Coordinate Systems and Test Methodologies.
• ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories.
• SAE AMS7000, Laser-Powder Bed Fusion (L-PBF) Produced Parts, Nickel Alloy, Corrosion and Heat-Resistant, 62Ni - 21.5Cr - 9.0Mo - 3.65Nb Stress Relieved, Hot Isostatic Pressed and Solution Annealed.
• SAE AMS7001, Nickel Alloy, Corrosion and Heat-Resistant, Powder for Additive Manufacturing, 62Ni - 21.5Cr - 9.0Mo - 3.65Nb.
• SAE AMS7002, Process Requirements for Production of Metal Powder Feedstock for Use in Additive Manufacturing of Aerospace Parts.
• SAE AMS7003, Laser Powder Bed Fusion Process.
DEFINITIONS.
• Additive Manufacturing (AM) – A process of joining materials to make objects from three-dimensional (3D) computer model data, usually a layer upon layer technique or method, as opposed to subtractive manufacturing methodologies.
• Anisotropy – Exhibiting properties with different values when measured in different directions.
• Build Cycle – Single process cycle in which one or more components are built up in layers in the process chamber of the additive manufacturing system.
• Build Chamber – Enclosed location with the additive manufacturing system where the parts are fabricated.
• Build Platform – A substrate on which the parts are built.
• Debit – A reduction applied to material strength properties to account for unique thermal exposures, design features (e.g., surface finish), operational environment, and such.
Design Values – Material strength properties that are established on a statistical basis and include the effects of debits related to part featur es and the operational environment in which the material operates.
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• Frozen Process – A set of identified significant process parameters required to control the fusion process to achieve the desired physical and chemical properties, material density, geometric detail, microstructure, surface condition, and other fusion-related characteristics to meet design intent in the as-built structure. Once established, these significant process parameters cannot be changed unless additional qualification is completed, which may include testing of differences.
• Hot Isostatic Pressing (HIP) – The simultaneous application of high temperature and pressure to metals for a specified amount of time to improve mechanical properties.
• Key Process Variable – An element of the AM process (e.g., build plate configuration, build layout, energy level, layer thickness, interpass time, melt pool environment, etc.) that, if changed, could affect chemical, physical, metallurgical, dimensional, or mechanical properties.
• Mechanical Properties – Physical properties that the material exhibits, including tensile properties, elasticity, creep and stress rupture properties, high-cycle fatigue, and low-cycle fatigue.
• Metallurgical Properties – Microstructural characteristics including grain size and orientation.
• Nondestructive Inspection (NDI) – A means of evaluating the properties of a material, component, or system without causing damage.
• Powder Bed Fusion (PBF) – An AM process that uses a high-energy source, such as a laser or electron beam, to selectively fuse, layer-by-layer, portions of a powder bed.
• Powder Blend – Quantity of powder made by thoroughly intermingling powders originating from one or several powder lots of the same nominal composition.
• Powder Heat – Powder from batch processes made from one melt and atomization, or powder produced during an individual production cycle from non-batch (semicontinuously melted) processes.
• Powder Lot – A single heat or a blend of two or more individual powder heats.
• Powder Recycling/Powder Reuse – The use of powder in a build that has been exposed to, one or more, previous builds in a powder bed fusion machine.
• Process Specification – The set of files and drawings necessary to define, run, and control all aspects of an additive manufacturing build such that the as-built parts with post processing consistently meet type design. The required process specification information may be contained in several documents including, but not limited to, part or layout drawings, process specification, process control documentation (PCD), deposition process specification, the machine build file, feedstock handling and storage plan, contamination plan, and the digital file and software configuration control plan.
• Process Validation – A manufacturing, quality, or engineering system for process control of a complex manufacturing process that ensures fabricated parts meet the design intent.
• Support Structure – An auxiliary element built concurrently with the part to provide dimensional stability during the build and/or to transfer heat away from the part as new layers are added. The support structure may be subsequently removed when required.
5 BACKGROUND.
5.1 Additive manufacturing, also known as 3D printing, refers to a range of manufacturing methods where feedstock m aterial is consolidated by a machine using a 3D computer model into a near-finished part condition. This AC focuses on PBF AM for metallic materials, where the feedstock material in the form of alloyed or elemental powder is fused by lasers or electron beams into a near-finished final shape.
5.2 PBF AM materials are very process dependent. It is important that design values used for PBF AM materials reflect not only the variability of the feedstock materials as purchased by manufacturers, but also the variability introduced by the manufacturing process used to fabricate production parts. While PBF AM methods have many process parameters (more than 100) identified by AM experts, studies have shown that the actual number of key process variables may be much smaller. Key process variables include elements of the AM process that could affect the chemical, physical, metallurgical, dimensional, or mechanical properties of the part. Defining the key process variables for a specific AM process application, including the level of control required to yield capability for producing parts in a stable and repeatable manner, is key to successful implementation of additive manufacturing.
For life-limited parts, additional information beyond the scope of this AC may be required to demonstrate compliance with § 33.14, Stop-start cyclic stress (low-cycle fatigue) at amendment 33-10 (49 FR 6836, February 23, 1984) and § 33.70, Engine lifelimited parts.
6 MATERIAL DESIGN VALUES.
6.1 Fundamentals.
6.1.1 Compliance with § 33.15 requires that suitability and durability of materials “be established on the basis of experience or tests; and conform to approved specifications.”
For cases where sufficient experience and/or approved specifications do not exist, material design values for AM parts should be established through test. This testing may be conducted using specimens produced with feedstock materials and process parameters representative of parameters used to produce AM parts, including postprocessing operations, or with a combination of component level testing and specimen testing. Test specimens should represent multiple builds, build locations, specimen orientations (including height in the build chamber), specimen thickness, and feedstock powder lots. Resultant test specimens should conform to the material and process specifications defined for AM parts.
6.1.2 Alternately, applicants may rely on component testing supported by statistical analysis to demonstrate mechanical properties of AM parts provided that this approach fully demonstrates that final part mechanical properties meet design requirements.
6.2 Specimen Testing Considerations.
After establishing sufficient process and quality controls to consistently produce AM parts, substantiation of material design values can be achieved by testing specimens extracted from representative parts or directly produced specimens. In either case, the applicant should demonstrate that these specimens appropriately represent the variability and corresponding properties in parts fabricated by the AM production process.
6.2.1 Validating Process Windows.
Specimens used to establish material design values should address the range (i.e., window) of process variation allowed in the frozen process specification including, but not limited to the following:
• Laser or electron beams parameter windows.
• Purge gas purity possible ranges.
• Energy source performance possible ranges.
• Powder reuse possible ranges (see paragraph 8.1.2. in this AC).
6.2.2 Test and Laboratory Procedures.
The metallurgical, mechanical, and physical property test and laboratory procedures used to demonstrate the capabilities of AM metallic materials produced should conform to approved specifications and meet the following requirements:
• Laboratory and test procedures used for this purpose should be performed by a qualified laboratory that conforms to accepted standards provided by technical societies, such as the ASTM, SAE, the National Aerospace and Defense Contractors Accreditation Program (NADCAP) or equivalent.
• The test facility should demonstrate competence through International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) 17025 accreditation, or equivalent, or by a recognized accreditation body.
The applicant should identify the standards and specifications used to conduct each test procedure, including AM specific considerations when appropriate, and provide evidence of accreditation for the laboratory that performed the test .
Note: Documented company internal test standards may be used if these standards have been revi ewed and accepted by the FAA. When company test standards are used, the company should have an internal process for ensuring that the testing is conducted in accordance with the prescribed test procedure.
6.3 Building Block Approach.
Design values should be developed as part of an overall program to establish the capability of a component to meet design objectives. Design values should be reliably established through a program of statistical analysis and a series of tests conducted using specimens of varying levels of complexity. Often referred to in industry as the “building block” approach, these tests and analyses at the coupon, details, and component levels can be used to quantify variations that could be present in the final AM part in an economically feasible manner. The lessons learned from initial tests help avoid early failures in more complex part level tests, which are costlier to conduct and often occur later in a certification program schedule. Figure 1 provides a conceptual schematic of tests included in a building block approach for a PBF AM engine component.
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6.4 Material Design Value Considerations.
6.4.1 Material strength and design values used for AM parts need to account for variability due to the feedstock materials and production methods used to manufacture a part. In general, the variability of the material in the final part is accounted for by using statistical tools to analyze data obtained from testing of specimens extracted from actual parts or purpose-built specimens.
6.4.2 Data used to derive design values should be obtained from stable and repeatable feedstock material procured per a controlled material specification. The material should be processed with a stable and repeatable process per a representative manufacturing process specification and with process validation that establishes significant key process variables that are defined by a frozen process window. This approach ensures that the variability permitted in manufactured materials is captured in the statistical analysis used to derive the design values. Design values derived too early in the material’s development stage, before feedstock material and AM part production processes have matured and are demonstrated to be stable, may not satisfy the requirements of § 33.15. As with traditional manufacturing processes, actual part design features and processing, including thermal effects, environmental effects, and geometric features that impact material properties should be addressed.
6.4.3 Characterizing Material Anisotropy.
Depending on process param eters, resulting parts may exhibit anisotropic material behavior. Specimen testing should be performed with specimens representing XY and Z orientations, as well as the full build height used to produce parts (see Figure 2 in this AC). If the results of this testing indicate a directional variation in properties, design values should be developed for each orientation or the orientation yielding the lowest properties should be conser vatively considered when establishing the minimum baseline design values.
Figure 2. Orientation of XY and Z Directions 9
6.4.4 Establishing Design Values.
Design values are established using industry accepted solid test specimens with machined external surfaces. Material properties generated from these specimens represent material properties that can be produced with the selected feedstock material using the frozen AM process parameters including (if required) post-fabrication operations, such as stress relief cycles and other thermal cycles including HIP. Design minimums for AM materials should be established using industry accepted standards, such as the Metallic Materials Properties Development and Standardization (MMPDS) Handbook with the same rigor and requirem ents as minimum design values for conventionally processed materials.
Note: Design valu es are only applicable for AM parts that are solid, fully machined on all surfaces, and do not have geometric features that require the use of additional debits to baseline material design values.
6.4.5 Establishing Thermal Exposure Debits.
6.4.5.1 Many AM parts will have elevated temperature exposures during postfabrication processing and/or during exposure to engine operating conditions. Examples of post-fabrication thermal exposures include thermal exposures associated with coating application and brazing cycles, HIP cycles, etc. These elevated temperature exposures can reduce actual material baseline values below minimum baseline design values.
6.4.5.2 Specimen testing should be performed with specimens processed through all required post-fabrication thermal exposures and expected operating conditions. If the results of this testing indicate a difference between design values for specimens subjected to thermal exposures and the minimum baseline design values, the results of thermal exposure specimen testing should be used to establish a thermal exposure specific debit that should be applied to the minimum baseline design values.
6.4.6 Establishing Part Feature Debits.
6.4.6.1 Depending on part complexity, AM parts may include unique features that can result in material values that are lower than the minimum baseline design values established by the data obtained from testing described in paragraph 6.4.1. Part features that can impact material values include features such as holes, internal passages, and overhanging surfaces as well as thin wall sections, stress concentrations associated with removal of supports, and as-deposited surfaces.
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6.4.6.2 Specimen testing should be performed with features representative of actual features present in final parts. For example, if the final part includes as-fabricated surfaces, test specimens should be created with as-fabricated surfaces such that testing evaluates the effect of the actual part surface condition on overall material properties. If the results of this testing indicate a property debit for specimens with representative features as compared to the minimum baseline design values, the results of featurespecific specimen testing should be used to establish a feature-specific debit that should be applied to the minimum baseline design values.
6.4.7 Establishing Final Design Values.
Figure 3 depicts how the final design values are to be used with AM parts. The final design values are established by applying the debits determined by the testing described in paragraphs 6.4.5 and 6.4.6 to the m inimum baseline material design values established by the data obtained from testing described in paragraph 6.4.1.
Figure 3. Example of Establishing Final Design Values 11
7 DESIGN CONSIDERATIONS.
7.1 Part Design Considerations.
Parts manufactured using AM require unique considerations not typically pertinent when using traditional manufacturing techniques. The resulting microstructures for AM material may vary significantly from cast or wrought microstructures for the same material. Layer-by-layer processing may generate directionally oriented microstructure that can result in both local and directional anisotropy. As with other melt processes, anomalies such as porosity, unmelted particles, and lack of fusion can occur with the AM process and should be understood and controlled. Furthermore, the physics of the layered AM process is such that anomalies tend to for m along the build plane and may not have significant height in the build direction making detection difficult.
7.2 Part Orient ation.
A part’s orientation relative to the build platform can have a significant impact on the material properties of the final part (e.g., anisotropy, residual stress, or unsupported/ overhanging surfaces). Part orientation should be considered as part of the design process to ensure final material properties meet part requirements.
7.3 Dimensional Control.
Different AM processes/parame ter combinations provide different dimensional control capabilities. The capability of the selected AM process/parameter combination to create essential part features, such as minimum and maximum wall thickness and fillet radii, should be considered during the design process. Localized heat input associated with AM processes used with metallic materials can result in the creation of significant residual stress in the fabricated part. These stresses and potential post processing warpage should be accounted for in the design process and associated mechanical property analysis. If post-processing thermal cycles, such as stress relief thermal cycles or HIP cycles are employed, dim ensional control for the final part should be demonstrated after all post-processing thermal cycles have been completed.
7.4 Surface Condition.
The surface conditions associated with AM processes can be significantly different from surface conditions produced by traditional subtractive manufacturing approaches.
7.4.1 Surface Finish.
The surface finish of an AM part can vary significantly depending on the selected AM modality, machine, machine parameters, feedstock material, and orientation of a given surface. For this reason, the surface finish can vary significantly as a function of location on a part. Surface finish values should be determ ined during the part development process and reflected in the design values.