Standard Guide for Nondestructive Examination of Metal Additively Manufactured Aerospace Parts After Build
4.1 Metal parts made by additive manufacturing differ from their traditional metal counterparts made by forging, casting, or welding. Additive manufacturing produces layers melted or sintered on top of each other. The part’s shape is controlled by a computer as well as by the layers. The computer directs energy from a laser or electron beam onto a powder bed or wire input material. These processing approaches have the potential of creating flaws that are undesirable in the as-built or finished part. In general, processing parameter anomalies and disruptions during a build may induce such “flaws.” Flaws can also be introduced because of contaminants present in the input material.
4.2 Established NDT procedures such as those given in ASTM E07 standards are the basis for the NDT procedures discussed in this guide. These NDT procedures are used to inspect production parts before or after post-processing or finishing operations, or after receipt of finished parts by the end user prior to installation. The NDT procedures described in this guide are based on procedures developed for conventionally manufactured cast, wrought, or welded production parts.
4.3 Application of the NDT procedures discussed in this guide is intended to reduce the likelihood of material or component failure, thus mitigating or eliminating the attendant risks associated with loss of function, and possibly, the loss of ground support personnel, crew, or mission.
4.4 Input Materials—The input materials covered in this guide consist of, but are not limited to, ones made from aluminum alloys, titanium alloys, nickel-based alloys, cobalt-chromium alloys, and stainless steels. Input materials are either powders or wire.
Note 3: When electron beams are used, the beam couples effectively with any electrically conductive material, including aluminum and copper-based alloys.
4.4.1 Powders—High-quality powders required for AM process are produced by (1) plasma atomization, (2) inert gas atomization, or (3) centrifugal atomization using rotating electrodes (Fig. 1).
Note 15: There are longstanding NDT standard flaw classes for welds and castings. In general, the defect classes for welded and cast parts differ from the flaw classes for AM parts.
4.9 Process-Flaw Correlation—Given the range of materials and processes encountered in metal additive manufacturing, the process origins of flaws are still being characterized. However, examples exist. For example, when the energy input is insufficient, successive scan tracks do not properly fuse together and flaws appear along the scan line. In L-PBF parts, incomplete wetting and balling effects associated with insufficient energy input have been shown to lead to pores or voids. In addition, EB-PBF parts can show large voids or cavities extending across several layers when the process parameters are not carefully chosen. Smaller spherical pores can also develop in EBM parts due to entrapment of gases originally present gas-atomized metal powders.
4.10 Flaw-Property Correlation—Parts with flaws, for example, porosity, LOF, skipped layers, stop/start flaws, inclusions, or excessive surface roughness, can exhibit degraded strength and fatigue properties compared with parts with fewer flaws. Furthermore, it is accepted practice to identify regions experiencing principle stresses before NDT is performed to assess the potential effect of any detected flaws in those regions. In addition to flaw type, size, and location, other flaw characteristics may be relevant, such as number, total volume, flaw/length (aspect ratio), orientation, and average nearest neighbor distance, and proximity to surfaces.
1.1 This guide discusses the use of established and emerging nondestructive testing (NDT) procedures used to inspect metal parts made by additive manufacturing (AM).
1.2 The NDT procedures covered produce data related to and affected by microstructure, part geometry, part complexity, surface finish, and the different AM processes used.
1.3 The parts tested by the procedures covered in this guide are used in aerospace applications; therefore, the inspection requirements for discontinuities and inspection points in general are different and more stringent than for materials and components used in non-aerospace applications.
1.4 The metal materials under consideration include, but are not limited to, aluminum alloys, titanium alloys, nickel-based alloys, cobalt-chromium alloys, and stainless steels.
1.5 The manufacturing processes considered use powder and wire feedstock, and laser or electron energy sources. Specific powder bed fusion (PBF) and directed energy deposition (DED) processes are discussed.
1.6 This guide discusses NDT of parts after they have been fabricated. Parts will exist in one of three possible states: (1) raw, as-built parts before post-processing (heat treating, hot isostatic pressing, machining, etc.), (2) intermediately machined parts, or (3) finished parts after all post-processing is completed.
1.7 The NDT procedures discussed in this guide are used by cognizant engineering organizations to detect both surface and volumetric flaws in as-built (raw) and post-processed (finished) parts.
1.8 The NDT procedures discussed in this guide are computed tomography (CT, Section 7, including microfocus CT), eddy current testing (ET, Section 8), optical metrology (MET, Section 9), penetrant testing (PT, Section 10), process compensated resonance testing (PCRT, Section 11), radiographic testing (RT, Section 12), infrared thermography (IRT, Section 13), and ultrasonic testing (UT, Section 14). Other NDT procedures such as leak testing (LT) and magnetic particle testing (MT), which have known utility for inspection of AM parts, are not covered in this guide.
1.9 Practices and guidance for in-process monitoring during the build, including guidance on sensor selection and in-process quality assurance, are not covered in this guide.
1.10 This guide is based largely on established procedures under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of the appropriate subcommittee therein.
1.11 This guide does not recommend a specific course of action for application of NDT to AM parts. It is intended to increase the awareness of established NDT procedures from the NDT perspective.
1.12 Recommendations about the control of input materials, process equipment calibration, manufacturing processes, and post-processing are beyond the scope of this guide and are under the jurisdiction of ASTM Committee F42 on Additive Manufacturing Technologies. Standards under the jurisdiction of ASTM F42 or equivalent are followed whenever possible to ensure reproducible parts suitable for NDT are made.
1.13 Recommendations about the inspection requirements and management of fracture critical AM parts are beyond the scope of this guide. Recommendations on fatigue, fracture mechanics, and fracture control are found in appropriate end user requirements documents, and in standards under the jurisdiction of ASTM Committee E08 on Fatigue and Fracture.
Note 1: To determine the deformation and fatigue properties of metal parts made by additive manufacturing using destructive tests, consult Guide F3122.
Note 2: To quantify the risks associated with fracture critical AM parts, it is incumbent upon the structural assessment community, such as ASTM Committee E08 on Fatigue and Fracture, to define critical initial flaw sizes (CIFS) for the part to define the objectives of the NDT.
1.14 This guide does not specify accept-reject criteria used in procurement or as a means for approval of AM parts for service. Any accept-reject criteria are given solely for purposes of illustration and comparison.
1.15 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.
1.16 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.17 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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