AMSC Gaps Portal

Post-Processing Section

Updates have been made to the following gaps:

No known standards have been issued that require consistent post-processing to be applied for qualification and production builds.

&D Needed: Yes.

Recommendation: Guideline standards should be issued that require consistent post-processing for the various AM processes to be applied for qualification and production builds. These standards should be process and material specific and should seek to define minimum best practices for qualification and production builds, along with reporting requirements.

Priority: Medium

Organization: AWS D20, ASTM F42/ISO TC 261 JG 55, SAE

Status of Progress: Green

Update: For metals, AWS D20.1 is in development and SAE AMS7000 was published in June 2018. For polymers, ASTM F42/ISO TC 261 JG 55 is in development for material extrusion.

4/4/2019, LY: The AWS recently published a new standard on AM, AWS D20.1/D20.1M:2019 Specification for Fabrication of Metal Components using Additive Manufacturing.

The existing and in-development ASTM standards for HT of metals built using PBF state the requirements for a specific metal within the standard, but not all metals have been addressed, and stress relief heat treatments in these standards may not be optimized for AM. In addition, differences between laser-based and electron beam-based PBF processes are insufficiently addressed in the existing standards. In this example, both processes are considered to be the same regarding HT requirements, when in reality PBF-EB is performed at much higher temperature and may not require residual stress relief and produce a more uniform microstructure. Heat treatment requirements for metals made with non-powder processes such as directed energy deposition using wire feedstock, sheet lamination, etc., are currently not addressed in any standards except for titanium-6Al-4V via DED. There are currently no standards on heat treatments designed to reduce anisotropy in properties. In cases where AM materials requires HIP processing, the process may be modified to meet HT requirements as well, negating the need for additional HT standards.

R&D Needed: Yes. R&D is needed to determine the optimized heat treatments for AM materials as a function of materials and process.

Recommendation: As the need arises for new metals, new standards will have to be written for each one, containing specific HT information. Also, as differences are found in required HT for laser versus electron beam processes, these differences should be added to the existing standard for that metal. Standards for metals made with non-powder processes need to be developed that contain HT requirements specific to that metal and optimized for the appropriate production process. As heat treatments are found to reduce anisotropy in properties for particular metals, these should be added to the existing standards for those metals.

Priority: Medium

Organization: R&D: universities, OEMs, government research labs, and others. Standards development: ASTM F42, SAE AMS-AM.

Status of Progress: Green

Update: SAE AMS7000, Laser-Powder Bed Fusion (L-PBF) Produced Parts, Nickel Alloy, Corrosion and Heat-Resistant, 62Ni – 21.5Cr – 9.0Mo – 3.65 Nb Stress Relieved, Hot Isostatic Pressed and Solution Annealed, states that several thermal processing steps (stress relief and solution annealing) need to be performed in accordance with SAE AMS2774E, Heat Treatment, Wrought Nickel Alloy and Cobalt Alloy Parts. ASTM F3301-18, Standard for Additive Manufacturing – Post Processing Methods – Standard Specification for Thermal Post-Processing Metal Parts Made Via Powder Bed Fusion (formerly WK58233) addresses this.

Just as for heat treatment and Gap P2, the existing HIP standards do not fully address AM material-related issues such as: slow cooling rate and its effect on formation of prior particle boundaries and carbide precipitation at grain boundaries, as well as the effect of thermal exposure on excessive grain growth, carbide size, incipient melting, and the effect of removing the part from the base plate before HIP. The HIP parameters in the existing AM standards are often developed for castings, forgings and sintered parts and may not be optimal for AM material since the thermal history, as-printed microstructure and property requirements often is a lot different from materials processed with the conventional manufacturing methods. Generally, the existing standards provide guidance for interpretation of processing parameters, tolerances, and conformance to industry accepted practices such as pyrometry, cleanliness, traceability, etc.

R&D Needed: Yes

Recommendation: Develop material specific standards based on R&D defined HIP parameters for AM with acceptance criteria for internal discontinuities. Some examples include the following:

  • Effect of max thermal exposure on microstructure evolution (X temperature for more than X hours)
  • Effect of cooling rate
  • Discontinuities extended to the surface
  • Incipient melting with and without voids
  • Discontinuities larger than X inches depending on location
  • Lack of fusion
  • Interconnected porosity
  • Nonmetallic contamination
  • Cross contamination due to processing of different customer parts in commercial HIP vessels
  • Grain morphology
  • Material dependent microstructure (e.g., in 718 laves phase, delta phase morphology, etc.)
  • Number of discontinuities larger than X in per certain view area (e.g., within 1 sq. inch)
  • Number of discontinuities in subsurface area (X microns from the surface) larger than X inch
  • Linear formation of discontinuities (other than interconnected porosity) and minimum distance of X inches between adjacent discontinuities.

Priority: Medium

Organization: R&D: various entities. Standards: ASTM F42, SAE AMS-AM, possibly SAE AMEC

Status of Progress: Green

Update: Some R&D is taking place in the commercial sector and at the university level. In terms of standards development, the referenced ASTM F42 work items may address the gap. SAE AMS7000 was published in June 2018 and SAE AMS AMEC is working on a HIP spec.

5/8/2019, LY: SAE AMS-AMEC is resolving comments from fourth ballot of the Hot Isostatic Pressing specification.

Unique features, such as helixes, spirals, lattice structures, and internal surfaces and cavities, can be manufactured using AM versus subtractive machining. However, the applicability of current measurement methods to the surface of these features is not clear or captured in standards. For example, features such as helixes or lattices may produce wire-like structures that are not as easily measured using stylus instruments as flat surfaces. Also, the suitability of current specification methods must be investigated for AM.

  • ANSI/ASME Y14.36M-1996 (R2008), Surface Texture Symbols may be sufficient, but further investigation is required to determine if AM-specific symbols are necessary (e.g., to control stair-stepping or allowable surface porosity).
  • Furthermore, although there are methods available for finishing AM materials, many lack standard practices. Some methods require material removal, such as micro-machining or abrasive techniques, and it is not known at this time how to accommodate this in AM product specifications in a standard form. Other methods require the addition of material, such as electroplating and coatings but it is also unknown how to accommodate these into AM standards.
  • Lastly, as the effects of surface finish on performance become more apparent, material specification recommendations must go beyond “supplier and purchaser agreement,” specifically for as-built, non-machined surfaces.

R&D Needed: Yes

  • Standards for reliable NDT, such as XCT, for evaluation of internal passages
  • Guidance for validation of surface finish on complex features (such as wires or non-planar surfaces)
  • Investigation of mechanical techniques such as shot peening or media blasting and their effect on fatigue life for AM materials

Recommendation: Verify if there are certain measurement methods more appropriate to AM-unique features than a stylus approach such as laser or white light 3D scanning. If so, they should be reviewed for their use on AM materials and appropriate standards written.

  • The applicability of existing surface texture symbols to AM materials should be investigated.
  • Available finishing methods should be reviewed for their effects on final material properties, and improved with standardized practices or guidelines where none exist.

Priority: Medium

Organization: ISO/ASTM; ASME (B46 new project team 53 on surface finish), IEEE-ISTO PWG, NIST

Status of Progress: Green for R&D (metals). Unknown for Standards (metals and polymers).

Update: In terms of R&D for metals, NIST is currently investigating several research topics related to surface texture of parts produced via laser powder bed fusion. Current research is focused on process-structure relationships and the identification of complex structures that result from the AM process in anticipation that better identification and definition of as-built surfaces will lead to stronger functional correlations for AM parts. To this end, current topic areas include: investigation of surface texture parameters beyond Ra (including both areal and profile parameters) to better define AM parts, variability of as-built surface texture (i.e., methods for describing changes in the as-built surface texture as position and orientation within the build chamber change), and use of XCT for determining surface texture.

An evaluation of the toxic gases resulting from uncured reagents in liquid resins used during processes such as Vat Photopolymerization (e.g., SLA) would be warranted to ensure product and environmental safety during and after production.

R&D Needed: No

Recommendation: Augment existing standards with AM-specific recommendations for processes that utilize liquid resins. Evolved gas analysis, an analytical method by which the amount and characteristics of the volatile products released by an AM-built part under controlled temperature variation, is recommended for finished product safety and toxicity. To analyze evolved gas quantitatively, parameters such as sample chamber volume, thermal/vacuum conditions for releasing/analyzing the volatiles and the techniques for the analysis need to be specified.

Priority: Low

Organization: ASTM D20, ISO/TC 261/ASTM F42

Status of Progress: Not Started, or Unknown

Update: None provided

Guidelines for evaluating the outgassing and offgassing properties and the effects of post-polymerization treatments have not been evaluated, specifically for AM materials. The voids and entrapments that can form in this case warrant some method of evaluating AM plastics over traditional methods.

R&D Needed: Yes, R&D may be needed to look at environmental conditions and health and safety aspects. Outgassing (thermal vacuum stability) and offgassing (toxicity) performance data for some materials may be archived in NASA’s Materials and Processes Technical Information System (MAPTIS). In space systems, materials typically undergo outgassing testing for use in external environments and offgassing testing for use in crewed environments.

Recommendation: Extend existing methods with AM-specific recommendations.

Priority: Low

Organization: ASTM E21.05, ASTM D20, ISO/TC 138, ISO/TC 261/ASTM F42

Status of Progress: Not Started, or Unknown

Update: None provided

Heat treatment is an effective method to modify the properties of AM built polymer parts. Presence of fillers, as in the case of composites, can alter the nucleation rate causing significant increase in tensile strength and hardness of the finished part. It also becomes essential to consider the variation of morphology of the polymer parts and layers because of the difference in the cooling rate from the surface to the center. The outer surface could end up less crystalline due to a rapid solidification rate and result in less resistance to wear. The contraction of volume due to crystallization in the bulk could increase the residual stresses at the interface. There are currently no standards on specific heat treatments (heating and cooling rates, anneal conditions) which could guide the AM practitioners to arrive at an optimum anisotropic structure and properties for the polymer parts.

R&D Needed: Yes. R&D is needed to determine the conditions for optimized heat treatments of AM built parts as a function of materials (semi-crystalline polymers, composites, etc.) and AM post process parameters.

Recommendation: As AM expands to include new and high performance semi-crystalline polymers, polymer nanocomposites and thermosets, advanced machine design and processing, the standards for the measurement of mechanical properties will have to describe specific HT information on the test samples. These HT requirements (slow cooled vs. quenched vs. gradient cooled) will be specific to the polymer and the production process. A guideline on HT treatment procedures followed by sampling for testing would enable achieving optimum polymer microstructure and properties.

Priority: Low

Organization: R&D: NIST, universities, OEMs, government research labs, and others. Standards development: ASTM F42, SAE AMS-AM.

Status of Progress: New