Standards Related to Metals in Additive Manufacturing
A variety of standards organizations are working on AM standards.
In the last ten years metal additive manufacturing (AM) has transitioned from a rapid prototyping tool to a production tool. This exciting evolution is evidenced by production examples including GE’s Leap fuel nozzle, Stryker’s medical implants, and SSL’s satellite flight hardware. However, this success has been mostly limited to large companies that manufacture components in-house. Industry standards are required for the adoption of AM by small and medium-sized manufacturers and to support the growing AM supply chain. Standards are known to provide the common language and expectations that supply chains require, streamline certification by solidifying expectations, and reduce adoption costs by clarifying equipment capabilities, safety requirements, and workforce requirements.
A variety of standards organizations are working on AM standards, but the rapid development and adoption of AM technologies is challenging the traditional model of standards development. ASTM’s F42 committee was the first standard development organization to begin developing standards in 2009. Since then International Organization for Standardization (ISO), Society of Automotive Engineers (SAE), Aerospace Materials Society (AMS), American Society of Mechanical Engineer (ASME), and American Welding Society (AWS) have joined the cause. A variety of critical baseline standards have been published including AM process terminology and powder characterization specifications. However, as outlined in the Additive Manufacturing Standardization Collaborative (AMSC) mission roadmap developed by AmericaMakes and ANSI, there are a large number of critical gaps remaining.
Here, we discuss several standards gaps that span the AM material, process, post-process, inspection, and service value chain. For each, we concentrate on the importance of the gap, aspects unique to AM, and examples of work being done to address them.
The re-use of unmelted powder from one build to the next is often necessary for AM cost justification. To date, there are limited standards that define how powders are stored in between builds, how and if used powders are mixed with virgin powders before reuse, and what powder characteristics should be monitored to determine end of life. This question of powder recyclability is generally unique to AM. In traditional powder metallurgy, most powder is completely used during the process and powder recycling is not an option. With AM, all powder in the build has some level of change due to heating in build chamber, oxygen absorption, spatter debris, and other material affects. Recent progress includes AmericaMakes sponsoring a collaboration with Carnegie Mellon University (CMU), industrial and government sectors on various issues such as reusing of the powder, powder flow characteristics and broadening the size distribution.
The potential range of variability, as well as the relatively unique nature of the AM process, will require a continued rollout of new standards. To date, research and applications have focused most heavily on laser powder bed fusion within a limited set of materials including Ti-6Al-4V and Inconel 625 and 718. Even in these cases, techniques are continuously evolving with the addition of new lasers, software updates, and increased build envelopes. Additionally, other processes such as electron beam powder bed fusion and directed energy deposition techniques and new materials including high strength aluminum alloys and steels are getting more interest. To keep up with these developments, two classes of standards are being developed. First, process-agnostic standards that relate fundamental properties are being considered. These include the relationship between microstructure and mechanical properties. Second, process-specific standards are required to ensure that systems are reliably operated. These standards include calibration, process qualification or equivalence, and operator certification.
If no surface finishing steps are taken, the relatively high surface roughness of as-built AM parts can decrease fatigue life. The higher roughness can be due to the ejection of spatter from the melt pool, partially melted or semi-sintered powder particles, balling effects, variation in the powder size distribution across the build, and poor selection of processing parameters. To overcome this, new techniques are being developed and traditional techniques are being modified to smooth complex AM parts. Techniques include abrasive flow machining and electrochemical machining. New standards are also required to help with technique selection and to define inspection methods for quality assurance. As a first step towards addressing this gap, the Additive Manufacturing Consortium, operated by EWI, is currently working on a project to evaluate eight different AM surface finishing techniques.
Traditional NDT approaches are being challenged by AM parts due to their geometric complexity, their range of defect types, and their surface roughness. ASTM and ISO are both working on guides for NDT of AM parts covering traditional techniques, such as ultrasonic, eddy current, and visual inspection; relatively new techniques, such as process controlled resonant testing; and traditional techniques finding much wider application including X-ray CT. The adoption of these techniques in robust quality assurance programs requires the structural integrity community to work closely with the NDT community to define inspectable inspection requirements based on product requirements and industry standards. To bridge this gap, the recent ASTM Symposium on Structural Integrity of Additive Manufactured Parts included both the NDT and structural integrity communities.
In-process inspection, where data is collected on a layer-by-layer basis during the build, has promise to complement or, in some applications, replace post-process inspection. Inspection technologies are in the early stages of deployment, with equipment manufacturers including Concept Laser, Arcam, EOS, and SLM Solutions providing in-process monitoring on their newest systems, and companies like Sigma Labs and Plasmos providing third party solutions. The correlation of the in-process inspection data with the existence of defects in parts is the next critical step towards proving these techniques as viable quality assurance tools. This will require industry standards that specify the calibration of sensors, guide the development of specifications, and standardize terminology. The development of these standards will in turn require a higher level of transparency from equipment manufacturers.
The repeated rapid heating and cooling cycles experienced by the material during the AM process can result in the formation of unique non-equilibrium and metastable phases within the microstructure. These phases have the potential to affect the corrosion resistance of AM components. Early experiments have applied standard test methods such as pitting corrosion, crevice corrosion, erosion corrosion, galvanic corrosion, and stress corrosion cracking to study this effect. Further experiments are necessary to achieve the understanding and certainty required for industry standard specifications. In light of this, AmericaMakes is accepting proposals to study and characterize the various modes of corrosion. Possible topics of study including the effect of macro level microstructure, including porosity, lack of fusion, cracking, distortion, surface roughness, and residual stresses; micro-level microstructural features including grain size, morphology, precipitates, phase transformations; and compositional aspects/variations including loss of alloying elements and solute partitioning.
In response to these challenges, organizations are taking new approaches to standards development. ASTM International has established a global AM Center of Excellence (CoE) which brings together an international group of leading and world-class research organizations to streamline the collaboration between research and development and standards development. Founding members of the ASTM AM CoE include EWI, Auburn University, NASA and MTC (UK). Strategic partners include NIAR and NAMIC (Singapore). The first set of ASTM International funded research projects was announced in October targeting five focus areas including feedstock management, process qualification, post-processing, metallic, and non-metallic testing.