"Unsupportecl·Arch" "Overhanging·Surfaces" 35 degrus 30 degrus 10 degrees ,'fl'S LPBF) "Self-Supporting·Structures" 6/23/23 AC 33.15-3
7.4.2 Feature Related Surface Conditions.
Due to the layer-by-layer addition of m aterial that occurs with AM processes, varying surface cond itions can be created during the formation of features, such as unsupported arches, overhanging surfaces, and self-supporting structures (see Figure 4 in this AC).
Figure 4. Unique Feature Related Surface Conditions
7.4.3 Surface Condition Impact – Mechanical Properties.
Mechanical properties th at are sensitive to surface condition will require detailed characterization to establish mechanical property debits associated with the surface condition as described in paragraph 6.4.6.2.
7.4.4 Surface Condition Impact – Inspectability Limits.
The surface condition associated with AM processes can also im pact inspectability limits associated with conventional NDI processes. Depending on the surface condition resulting from selected processing, part surfaces may require NDI process development. The requirement is to demonstrate adequate resolution to detect the specified defect size (see paragraph 10.6).
7.4.5 Surface Finishing Considerations.
Traditional surface finishing approaches, includin g non-conventional methods, can be applied to both external and internal surfaces of AM parts. However, complex internal geometries associated with some AM parts may not be accessible. Parts designed to operate with the as-fabricated surface conditions should include associated mechanical property debits where such surface conditions exist on the part. See figure 3 in this AC.
dditi Part Upp rt 6/23/23 AC 33.15-3
7.4.6 Protective Coating Considerations.
Standard protective coatings, such as coatings used to address corrosion, oxidation, or erosion can typically be applied to both external and internal surfaces of AM parts that have been s ubjected to surface finish ing treatments. Application of coatings on asfabricated internal passages may be difficult due to as-fabricated surface conditions.
7.5 Support Structures.
Support structures can be used to mi nimize the occurrence of unsupported arches and overhanging surfaces. Additionally, support structures can be used to help transfer heat away from local areas as new layers are added and to help retain the part’s shape. For an example of the use of support structures during the AM PBF build process, see Figure 5.
Figure 5. Example of Support Structures Used with Additive Parts
7.6 Note: There are drawbacks to the use of support structures during AM. Supports form ed inside parts cannot be removed after part fabrication. Removal of supports from external surfaces can result in the creation of local stress concentrations that can have deleterious effects on mechanical properties.
Powder Removal.
When PBF AM processes are used to produce parts with internal features, unfused powder can become trapped inside the part during the build process. Part designs and part orientation during the build process should be defined such that removal of loose powder can be accomplished. Effective powder removal procedures should be identified. For cases where powder removal may not be accomplished, functional tests should be performed to verify that any potential residual powder does not interfere with intended function of the part.
Note: Failure to remove loose powder before application of post-processing therm al cycles can result in sintering of loose powder to the parts internal surfaces such that the loose powder can no longer be removed.
8 MATERI AL AND FABRICATION.
8.1 Material and Process Control.
Depending on the selected PBF AM process, more than 100 parameters may be used to control the fusion process. Examples of key process variables requiring control include directed energy source power, scan speeds, layer thickness, fill patterns, chamber atmosphere, chamber temperature, feedstock material, and reuse of feedstock material.
Process parameters that have a direct influence on the chemical, physical, metallurgical, dimensional, or mechanical properties of the final AM part should be identified, defined as key process variables, and carefully controlled. Process validation is the system that defines how to identify significant process parameters, identifies their limits, and determines if they have a deleterious effect on the defined requirements.
Specifications covering AM feedstock material, including reuse of feedstock material, material processing, and fabrication procedures should be established in conjunction with process validation to ensure a basis for fabricating reproducible and reliable m aterial.
8.1.1 Feedstock Material Specification.
8.1.1.1 For powder-based AM processes, control of the powder feedstock is essential to a quality fusion process. The key characteristics of the powder, such as powder chemistry, particle shape, particle size distribution (including size limits), cleanliness, and powder flow characteristics should be identified and defined in the feedstock specification along with acceptance testing requirements. The feedstock specification should also identify the powder production method.
8.1.1.2 Specifications for feedstock materials limit the variation in composition, trace elements, impurities, and other characteristics that are inherent to the feedstock material form based on material performance sensitivity studies.
These specifications are required to ensure consistent material is being procured. Feedstock material specifications should also define batch acceptance testing or statistical process controls that ensure material properties do not drift over time. Material requirements identified in the feedstock specification are derived from process validation activities used to identify the key process variables that strongly correlate with the attributes required in the final AM parts.
8.1.2 Reuse of Feedstock Material.
8.1.2.1 If reuse or recycling of powder – including closed loop powder flow processes – is allowed, the applicant should show that reused powder continues to conform to the requirements of the feedstock material specification.
8.1.2.2 If blending of reused and virgin powder is allowed, the applicant should show how blending of reused powder is accomplished. Regardless of whether 100 percent reused powder or a blend of virgin and reused powder is used – including close loop powder flow processes – the applicant should define any limitations on powder reuse (e.g., number of build cycles, number of build hours, hours in the machine, and powder oxygen level). Finally, the applicant should show that final part properties, including material properties, continue to conform to all design requirements at the limit of reuse (see paragraph 6 in this AC). Reuse of powder should be controlled by the specification.
8.1.3 Process Validation.
Process validation is used to ensure that key process variables which strongly correlate with the attributes required in the final AM parts, such as mechanical, physical, metallurgical, or chemical properties, are appropriately defined and frozen such that process requalification is required if one or more of these key process variables is changed. Before freezing these key process variables, parametric studies (i.e., process validation ) should be completed to define their critical tolerance values in the process specifications. Process specifications should address:
• Shop and equipment operating procedures.
• Power level verification.
• Software control.
• Build interruptions and machine restarts.
• Environmental controls.
• Contamination control plan.
• Powder handling and reuse requirements.
• Quality assurance requirements.
8.1.4 Part Material Specifications and Engineering Drawings.
Part material specifications and engineering drawings establish the acceptance requirem ents for parts fabricated by PBF AM. These documents include any required post-AM processing operations and should ensure that variation in strength and other properties meet design properties consistent with the design intent. Material anomalies, such as porosity and inclusions that are permitted by the material specifications, should be substantiated by test evidence at the specimen or part level. These specifications should include a process for confirming that material properties continue to meet assumptions reflected in the design data via periodic specimen or part level testing. Part material specifications, when combined with engineering drawings, should address chemistry, mechanical properties, microstructure, porosity, surface finish, thermal processing, and quality assurance. These documents should also address any required post-AM processing operations. Additionally, the part material specifications should address certification of conformance to the material specification for the feedstock material including such features as input powder chemistry, number of powder re-uses, process specifications, and attributes that link the AM part back to the material used to develop the design curves associated with the material specification.
9 POST BUILD PROCESSING.
9.1 Post Build Operations.
After compl etion of the build cycle, AM parts will be subjected to one or more post build operations. The sequence and specifics of these operations may affect the final part microstructure, material characteristics, residual stress, and dimensional control.
Post build operations, including the sequence of these operations, should be documented in a control plan. Examples of post build operations include removing AM parts from the build platform, removing support structure, removing residual powder from the AM parts, subjecting AM parts to one or more thermal cycles, and use of surface enhancement treatments to address surface condition.
9.2 Residual Powder Removal.
If applicable, residual powder should be removed from AM parts. Steps should also be included to confirm that all residual powder has been removed when required.
9.3 Residual Stress.
The mu ltiple localized melting and re-solidification cycles associated with AM processes can result in significant residual stress being developed in AM parts. As a result, part distortion can occur during the application of post build thermal processes and engine operation. The me thod(s) for addressing residual stress in AM parts associated with the build process should be accounted for in the design process.
9.4 Removal from the Build Platform.
The me thod for removing parts from the build platform should be defined. The sequence in which parts will be removed may also need to be defined.
9.5 Removal of Support Structures.
The me thod(s) for removing support structures from the AM part should be defined in the part m aterial specification or on the engineering drawing if removal is required. The sequence in which support structures will be removed may also need to be defined.
9.6 Thermal Processing.
It is expected that AM parts will require thermal processing operations to evolve the asbuilt microstructure into a final form, yielding proper and predictable material performance. If thermal cycles are used to improve material microstructure, such as to reduce anisotropy and/or improve homogeneity, each thermal cycle should be sufficiently controlled to ensure repeatable and reliable results. Any specific requirements for final microstructure attributes, such as an acceptable level of anisotropy and the m ethod used for confirming acceptable microstructure attributes, should be defined in the part material specification or on the engineering drawing.
9.7 Hot Isostatic Pressing (HIP).
If AM parts are subjected to HIP, the process param eters for this operation should be defined. Any specific requirements for final microstructure attributes and the method used for confirming acceptable microstructure attributes should also be defined.
9.8 Surface Enhancements.
To address surface rough ness associated with “as produced” AM parts surface enhancem ent treatments may be applied. The type of surface enhancement(s) applied to parts should be defined in the part material specification or on the engineering drawing.
9.9 Environmentally Protective Coatings.
Environm entally protective coatings may be applied to AM parts. The type of protective coating applied to the parts should be defined, and the potential effect on substrate m aterial properties should be understood.
10 INSPECTI ON METHODS.
10.1 Inspection Method Considerations.
While AM parts m ay present unique inspection challenges compared to cast and wrought products, from an NDI perspective they are finished parts that should be inspected. The NDI development and verification processes used for cast and wrought products can be applied to the parts produced by these new manufacturing processes.
Additionally, radiographic requirem ents specified in industry standard welding specifications may also be used.
10.2 Relationship to the NDI Development for Cast and Wrought Products.
The physics of the layered AM process produces different typ es of material anomalies than those produced in traditional cast and wrought products. The layer-by-layer deposition approach used in the AM processes may produce anomalies that do not possess significant height in the build direction. Planar anomalies, such as lack of fusion, tend to form along the build plane and can be only one to two layers thick. In addition, as-built surfaces that are not well supported during the build process can be rough and mask the presence of surface anomalies. While all the inspection processes typically used for cast and wrought products, such as radiography, dye penetrant, eddy current (EC), or ultrasonic testing (U T) are applicable to AM parts, they may need to be applied in new or combined ways to detect anomalies produced by AM processes.
10.3 Role of Inspection in AM Part Quality.
NDI should not be viewed in isolation when developing a plan to ensure the integrity of AM parts. N DI should be viewed as one option to ensure part integrity along with process validation, process monitoring, proof testing, periodic destructive evaluation, and other establish ed quality control methods. Inspection should be integrated with these other methods in a complementary way to ensure part quality.
10.4 Identification of NDI Criteria.
As with any trad itional part, the first step in developing an inspection technique is to understand the inspection requirements for the part. As part of the materials characterization process outlined in paragraph 6, material anomalies intrinsic to the selected AM processing parameters, material system, and part design should be identified. Examples of possible material anomalies include porosity, lack of fusion, excessive surface roughness, and inclusions. In addition to the type of anomalies produced by the AM process, the maximum size of those anomalies that the design can allow should be specified. Identification of areas with different acceptance criterion (for example, proximity of allowable defects) is acceptable similar to how castings are treated, provided the applicant can demonstrate that such an approach is consistent with engineering and design requirem ents. NDI experts can select and validate the appropriate NDI technique(s) with this information.
10.5 Placement of NDI within the Manufacturing Process.
Using the inspection criteria described in paragraph 10.4, it is im portant to locate the inspection correctly within the sequence of operations for the part. For example, if a part goes through a HIP operation to eliminate porosity, it may not be appropriate to inspect for porosity before the HIP operation. Inspection prior to HIP is acceptable but may produce unallowable indications that will be healed in a subsequent operation.
Sim ilarly, if machining damage may occur, the inspection for that damage should occur after all machining operations are complete.
10.6 Selection of NDI Techniques.
Once the type of anom alies, allowable sizes for those anomalies, and the placement of NDI within the manufacturing process sequence are established, NDI techniques can be evaluated and selected. To evaluate NDI techniques, components with examples of the anomalies, or artificial features that are considered representative of the anomalies, need to be manufactured. Due to the nature of the AM process, parameters used to produce these components should be as closely aligned to the final manufacturing process param eters as possible. Regardless of the NDI process(s) selected, acceptance criteria should be established and verified.
10.6.1 Radiographic Testing (RT) and Computed Tom ography (CT).
RT and CT process capabilities are dependent on part geom etry. One of the advantages of AM processes is rapid design iteration by eliminating the need to create dies or tooling. If CT capability is determ ined too early in the design process, changes in part geometry by the final design iteration may change capability in critical areas.
10.6.2 Ultrasonic T esting (UT) and Penetrant Testing (PT).
The capability of UT and PT are influenced by surface condition. The su rface condition of AM parts can change significantly based on build parameters and support structure placem ent. Inspection capability should be assessed on samples representative of the final manufacturing process.
10.7 Inspection of Rough Surfaces.
10.7.1 As discussed in paragraph 7.4, the as-built surface condition of AM parts differs from those produced by traditional manufacturing methods. The surfaces, in general, are rougher than those produced by casting or wrought processes and have more variation.
The condition of the as-built surfaces requiring inspection should be considered when selecting the NDI method(s) to be used to evaluate parts. An impact study should be conducted on specimens that are representative of the final as-built surfaces for the component to ensure adequate capability to detect conditions of concern.
10.7.2 For surfaces where NDI capability is reduced due to surface roughness, additional manufacturing steps should be considered to improve surface conditions. Surface finishing processes, typically used with cast and wrought products, may be used to improve the inspectability of surfaces. Improving surface finish is preferred to selecting an NDI method whose capability is not impacted by surface roughness but is not as capable of finding the specified anomaly types.
10.8 Inspection of Complex Geometries.
AM process can produce part geom etries that are not possible using traditional manufacturing methods. While these new geometries provide great benefits for cost and performance, they can create challenges for traditional NDI approaches. New complex geometries will require new inspection approaches, including the combination of multiple ND I methods and new or emerging NDI methods, such as laser ultrasonic testing (LUT) or CT.
10.9 Inspection of Internal Features.
AM parts often have internal features that need to be inspected, but access to these surfaces is pa rtially or completely obstructed. As a result, traditional dimensional and NDI methods may not be sufficient.
10.9.1 CT Inspection Systems.
CT is one inspection me thod currently approved for the inspection of internal features that c annot be directly accessed. Currently, there are four different categories of CT inspection systems:
• Nanofocus (140-200 KeV).
• Microfocus (200-450 KeV).
• Mini-focus (300-600 KeV).
Industrial (1000-9000 KeV).
The type of CT system to use depends on the size and density of the part to inspect, the size of the feature(s) to be measured, and anoma lies to detect. In general, higherpowered systems have more penetrating power for large or dense parts but produce images with lower resolution.
Note: Kiloelectron volt (KeV) is the power of the x-ray system measured in 1000s (K) of electron volts (eV).
11 PROCESS QUALIFICATION.
11.1 Process Qualification Considerations.
The funda mental goal of fusion process control is to consistently achieve consolidation of the feedstock material to the greatest extent possible without the presence of fusionrelated defects. The process parameters specified to control the fusion process are set to achieve the desired material density, geometric detail, microstructure, surface condition, and other fusion-related characteristics in the as-built structure. The AM process should be properly controlled to produce stable and reproducible dim ensions, properties, and quality.
11.2 Process Validation.
Process validation is the m ethodology used to verify that a component manufactured to a specific process, process sequence, and drawing requirements meets the design intent.
Process validation requirements include, but are not limited to, first article inspections, destructive evaluation of parts, NDI evaluations, metallurgical examinations, and chemistry and mechanical property testing. Process validation should also address potential feedstock variability, machine variability, and post processing variability.
Assessm ent of machine variability should include variability within the build volume, run-to-run variability, and variability between machines.
•
11.3 Qualifying a Fixed Process.
Process va lidation is necessary to assure that all physical, chemical, and metallurgical material characteristics are satisfied, and to ensure that manufacturing anomalies are not introduced by the process. Process characteristics that have a direct influence on the final AM part mechanical properties and physical attributes are identified as key process variables that should be defined and carefully controlled in a specification and/or technical plan. Review and approval of the overall process specification and quality control plan should involve engineering, manufacturing, and quality functions.
The process specifica tion and/or technical plan should require process requalification if one or more of the key process variables is changed.
11.4 Machine Qualification.
Qualificatio n runs should be accomplished in accordance with the approved specif ication and/or technical plan to verify that the fabricated parts have the material properties assumed in the design data for the part.
11.4.1 Part and Specim en Evaluation.
Both destructive and non-destructive evaluation of parts and specim ens can be used to support initial and ongoing process validation. Destructive evaluation is necessary for validation of certain engineering or quality characteristics, such as physical, chemical, and metallurgical aspects that are not inspectable by conventional NDI techniques. The selected combination of destructive and non-destructive evaluation of parts and specimens should demonstrate that all engineering requirements are achieved with the defined process parameters. Evaluation parts and specimens should be produced with feedstock material reflecting the approved feedstock material specification criteria including reused powder, if applicable, and be representative of the variability introduced by the m anufacturing process used to fabricate production parts. Parts selected for evaluation should also be representative of all build positions.
11.4.2 Part and Specim en Evaluation Plan.
The part and specim en evaluation plan should address ongoing process validation through a combination of destructive and non-destructive evaluation of parts and specimens. A periodic destructive evaluation sampling plan should be established that ensures confirmation that engineering requirements continue to be achieved with the defined process parameters. The sampling frequency and number of parts and specimens sampled should be established based on demonstrated statistical process control and should – over time – address all build positions. Periodic mechanical property validation through testing of parts or directly produced specimens should also be incorporated into the process validation plan.
11.4.3 Inspection Data.
NDI plus destructive evaluation of parts from process validation demonstration builds should be perform ed consistent with the inspection method considerations provided in paragraph 10. This evaluation should demonstrate that any anomalies present in parts are acceptable to design requirements. NDI tests for process validation demonstration should be performed at the sam e point in the overall part manufacturing sequence as defined in the process specification and/or technical plan.
11.4.4 Specime n Testing.
Specimen testing from process validation demonstration builds should be performed consistent with the test program approach described in paragraph 6. This testing should confirm that m aterial properties in the fabricated parts match the material properties assumed in the part design data including required post build processes.
11.4.5 Traceability.
Due to the batch nature of AM processes, development of a system for part traceability is highly recommended. Retention of records such as powder heat, lot and condition (for example, virgin or recycle count), machine serial num ber, build number, and part location in the build chamber are recommended for each part produced.
11.5 Qualificat ion of Multiple AM Machines.
When multiple AM machines will be used to produce parts using identical parameters, a separate process validation should be executed for each machine.
11.6 Changes to Key Process Variables.
Changes to key process variables should not be introduced to the fixed process unless additional qualification, including testing of differences, is completed. All changes to key process variables should be validated for each machine on which th ey are introduced.
12 SUGGESTIONS FOR IMPROVING THIS AC.
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