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Measurement Science for Additive Manufacturing

INTRODUCTION

The Intelligent Systems Division within the Engineering Laboratory (EL) focuses on the key areas of smart manufacturing. The smart manufacturing program areas within the Intelligent Systems Division include Measurement Science for Additive Manufacturing (MSAM), Robotic Systems for Smart Manufacturing (RSSM), and Smart Manufacturing Operations Planning and Control (SMOPAC).

Additive manufacturing (AM), also known as three-dimensional (3D) printing, continues to be dubbed as the “next best thing” in advanced manufacturing owing to its promise to fundamentally revolutionize part designs and manufacturing methods. By printing parts directly from a 3D computer-aided design (CAD) digital file, the technology enables unique organic shapes that cannot be manufactured through conventional manufacturing methods. Additionally, additive technologies can reduce the manufacturing lead time, part cost, and materials use. In spite of these benefits, the technology has not had widespread adoption within the U.S. manufacturing industry—except for niche applications. The key barriers for adoption are the following: material characterization, including types of defects and impact on mechanical properties; lack of design tools and allowables; investment costs of additive equipment; added cost and time for making components; qualification and certification requirements; process controls and variability; and lack of industry standards.

The AM work in the MSAM program is aimed at developing and deploying measurement science that will enable rapid design-to-product transformation through advances in four projects: (1) Characterization of Additive Manufacturing Materials; (2) Qualification for Additive Manufacturing Materials, Processes, and Parts; (3) Real-Time Monitoring and Control of Additive Manufacturing Processes; and (4) Systems Integration for Additive Manufacturing. The MSAM program is approximately three and one half years into a five-year program duration. The current total budget is $7.05 million, with a total staff of 21 federal employees, 4 NIST associates, and 4 students.

ASSESSMENT OF TECHNICAL PROGRAMS

Accomplishments

The individual projects within the MSAM program are well aligned with the overall program objective to

The MSAM projects, which are discussed below, are designed to address some of these barriers for metal laser powder bed machines. The MSAM staff have been leading and driving the development of additive standards that are required to establish the metal additive manufacturing industry.

The Characterization of Additive Manufacturing Materials project addresses current needs and challenges for AM practitioners in the areas of powder characterization and handling. These two areas apply not only to powder bed AM technologies but also to other AM techniques. The strategy pursued in this project is to determine the limits and errors of major measurement techniques for AM powders in order to quantify uncertainties in the characterization of powders and along the processing chain. The project has resulted in excellent contributions that are helping to advance the state of the art in powder and powder bed characterization.

The project excellence is displayed, for example, in the approach to powder characterization where several different characterization techniques have been set up, including powder size distribution and shape analysis, a custom powder spreading device, different flow and density measurement devices, as well as a suite of X-ray tomography instruments for defect analysis in additively manufactured parts.

The project has generated, and continues to generate, new standards on the use of this equipment to characterize powders and parts. These new standards will help in comparing results from different groups on powder characterization. Another factor contributing to the excellence in the Characterization of Additive Manufacturing Materials project is the cross-project availability of the AM testbed. This testbed system is crucial to creating variables in the hardware and software of additive production systems. The experiments this testbed enables have great possibilities to advance system capabilities to produce higher quality parts.

Another example of the excellence in the Characterization of Additive Manufacturing Materials project is the work on powder recycling. Powder recycling is of great value to industry, and a round robin study has been initiated on the effects of powder recycling that will lead to guidelines on measurement techniques and uncertainties, and information on how these effects influence the results of recycling studies.

The Characterization of Additive Manufacturing Materials project aims at developing an AM database. Features of the web-based AM database include a viewable and navigable AM schema and currently accessible data and attributes (which include microstructure information on properties and dimensional information). The database is currently composed of NIST-funded round robin data collected in 2012 and 2014. Progress, so far, has been made to develop the framework of the database, and, if successful, this AM open database effort could significantly help U.S. industry.

The Qualification for Additive Manufacturing Materials, Processes, and Parts project includes three MSAM research areas. The first is generation and interpretation of reference data to serve model-based qualification efforts. The second is the development of preproduction run and post-processing test methods to assist in equivalence efforts. The third is the establishment of minimum requirements for testing to reduce the cost for empirical approaches. Through research in qualification, the EL will reduce the cost of empirical approaches and provide foundational information, which will support equivalence and model-based qualification approaches.

Work in the MSAM program contributes to model-based qualification through high-fidelity temperature measurements for metal laser-based powder bed fusion, which are used to validate multi-

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¹ National Institute of Standards and Technology (NIST), “Measurement Science for Additive Manufacturing Program,” updated December 08, 2016, https://www.nist.gov/programs-projects/measurement-science-additivemanufacturing-program.

physics models simulating individual melt tracks in an additive manufacturing process and low-fidelity temperature measurements typically made with an infrared (IR) camera. For example, reflections on the powder bed surface from spatter may be erroneously interpreted to be hot spots on the surface. Machine qualification studies have also led to the development of AM XY scanner test patterns. Additionally, an MSAM AM test artifact has been developed and is being distributed to assist with equivalence-based qualification and system performance characterization. The EL developed a CAD file for a metal plate with specific geometrical features, the test artifact. This plate and its features are designed for additive manufacturing, and different laboratories could use this particular CAD design to build the part and compare geometrical features or microstructures. The test artifact thus represents a key standard for additive manufacturing, and similar standards need to be developed for other aspects of additive manufacturing. The artifact is, essentially, a square plate containing features or assessing process accuracy and repeatability. From surface roughness measurement studies, MSAM researchers have concluded that standard R_A roughness measures do not fully capture important surface morphology. Use of R_pc and R_Sm has given better surface discrimination than the R_A measurement. Research is under way to explore relationships found between surface texture and microstructure. X-ray computed tomography is a powerful tool for assessing internal defects. MSAM researchers are analyzing the process to establish detection limits and to properly interpret images.

To support empirically based qualification, a methodology for round robin testing has been articulated by MSAM staff. The plan is to publish the information as an International Organization for Standardization (ISO) American Society for Testing and Materials (ASTM) guide standard. If properly conducted, round robin studies distribute the cost burden of testing and provide statistically valuable information.

The Real-Time Monitoring and Control of Additive Manufacturing Processes project team is working on visual monitoring of the melt pool in metal laser powder bed machines. They are motivated by the need to monitor the process so that correlations between the data and visually detectable flaws in parts can be investigated, to use the data as a quality control signature for the part, and to use the data for real-time control.

Selective laser sintering (SLS) is the dominant method for AM of metal parts, and control of this process is a significant issue. The intent of the MSAM work—to improve the process control of selective laser melting (SLM)—is sound. Visual monitoring of the melt pool is a good target, and, in fact, visual monitoring of the melt pool in SLS has been under investigation by various groups for approximately 20 years. In principle, NIST’s prowess in measurement technology can form the basis of useful contributions to this field.

This work has been pursued on two hardware platforms: the testbed and the EOS M270 machine. A considerable portion of the testbed effort is allocated to an in situ measurement of the emissivity of the powder bed. The expectation is that the increased accuracy of the emissivity measurement will allow for better process monitoring and, hopefully, better real-time control. A thermal imaging camera has been adapted to an EOS M270 machine by fabricating a custom door and a camera mounting system. This camera is not concentric with the beam, but views the powder bed at an angle, perhaps in the range of 30 to 45 degrees from normal to the surface of the powder bed. In an effort to measure emissivity the team has done experiments with powder on hot plates at temperatures up to approximately 500ºC. The team reported experiments on emissivity measurements during the 27^th International Solid Freeform Fabrication Symposium at the University of Texas, Austin, in 2016. They have explained that measurements at higher temperatures are exceedingly difficult—due to the fact that there is a challenge in uniformly heating.

Another MSAM program focus is the Systems Integration for Additive Manufacturing project, the objective of which is to deliver an information systems architecture, including metrics, information

models, and validation methods, to shorten the design-to-product cycle time in AM.² The major deliverables for this project are separated into six thrust areas: (1) Additive Manufacturing Product Life Cycle Management and Digital Support in what is commonly called the “digital thread;” (2) Product Definition for Additive Manufacturing; (3) Design Allowable Database for Additive Manufacturing Materials (which focuses on statistically based mechanical property data and represents a subset of the Characterization of Additive Materials AM database); (4) Design Rules for Additive Manufacturing Parts; (5) Characterization and Uncertainty Quantification of Physics-Based Models for Additive Manufacturing; and (6) Process Planning Guidelines for Additive Manufacturing. The Systems Integration for Additive Manufacturing team has designed the architecture of each of these databases using state-of-the-art database methods and software project design tools. They have initially focused on just AM metal laser powder bed methods. In particular, they have planned for product definition and tolerance representations; AM design rules for metal parts; the characterization of powder bed fusion metal physics-based models; data structures for AM metal parts, processes, and materials; and path and process planning related to their home-grown powder bed fusion metal machine.

Opportunities and Challenges

A key challenge and opportunity for the MSAM program is to identify the limit of NIST’s territory within the U.S. science and technology landscape of AM. Some of the activities in the Characterization of Additive Manufacturing Materials project—for example, the database development or the powder recycling study—overlap with similar activities in the private sector and in academia. The MSAM projects seem to be on a good track; however, there is a need to hone in on the measurement and standards aspect throughout the diverse range of AM topics currently pursued in the program, and the Characterization of Additive Manufacturing Materials project in particular.

The round robin studies in the Characterizations of Additive Manufacturing Materials project were conducted with outside partners. The many challenges in controlling AM machines render round robin studies rather difficult, and a gradual approach might be better than the full-scale study that was conducted. The in-house real-time monitoring capabilities could be used even more than they currently are to start with a study of repeatability and variations for parts built on the same machine, as well as post-processing. The round robin study may also be too ambitious and could benefit from narrowing the parameter space or even from using polymers and polymer machines first. The primary goal of the round robin study might be modified to focus on the development of round robin testing procedures, and applying these testing procedures over multiple runs to understand the variance on a machine (with the constant parameter settings as a baseline), rather than determining differences in the properties from different sources per se. There are many different materials, machine brands, and build themes—and a quantification of variations for all combinations seems to be too broad.

In connection with powder recycling, more information is needed on material discontinuities or consistency within new powders. The Characterization of Additive Manufacturing Materials project team has the opportunity to work with material manufacturers (powder producers) on better consistency of powders, and to identify critical powder properties for the AM process. This could also result in better density in the mechanical powder spreading process. There are many variables: geography, systems, calibrations, powders, method of powder shipment, shelf life of the powder, operator or technician experience, and the system surroundings or atmosphere. Findings cannot be trusted unless all of these variables can be controlled. The testing could possibly involve the same parts, build orientation, and

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² See NIST, “Systems Integration for Additive Manufacturing,” updated July 13, 2017, https://www.nist.gov/programs-projects/systems-integration-additive-manufacturing.

support structure. Recycling could have many variables involving differences within the new powder added, including physical chemical properties, chemical composition, and the size and shape of powder, and these variables could play into the testing if they vary throughout different orders or lot numbers from the manufacturer. These factors need to be studied before recycling testing.

The MSAM AM database competes with commercial codes and it is not clear what the pathway is to populate the database. It may be better to focus this endeavor on a database structure and format, which could be very useful as a framework to be made openly available to the AM community—specifically, schema.

Another opportunity would be to broaden the manufacturing focus within AM, or consider closed loop manufacturing for additive technologies. In almost every manufacturing process or method there are challenges to produce high integrity or quality parts or components that do not need secondary processes or mechanical treatment. Within the limitations of AM to produce high-quality parts, secondary processes will be needed to produce suitable parts or components for many industries. Broadening the focus into post- or secondary processing will help researchers understand what limitations within the technologies cannot be overcome with post-processing methods. Expanding research veins into support structures and post-processing, and defining new procedures in this field, would create an opportunity to develop new standards, which could help industry find common ground.

The use of software to analyze images utilizing algorithms to understand the laser effects within the powder bed would help to improve the technology itself. Another opportunity within the Characterization of Additive Manufacturing Materials project is to examine more closely (with the inclusion of a measurement of the degree of planarity in powder spreading) the mechanical powder spreading methods, being blade or rollers.

The project could advance AM if software, in situ measurement data and images, and hardware could be correlated. While progress in that direction is under way for laser effects on powder, the integration of software and hardware needs to be continued and expanded into additive printing systems.

For the Qualification for Additive Manufacturing Materials, Processes, and Parts project, the team is addressing key elements of precompetitive AM part and process qualification. They are overall doing a very good job of advancing the primary research tasks. The results will be helpful to industry in qualifying parts for service; however, for other areas within AM, it would be helpful to expand the study to include AM processes other than metal laser-based powder bed fusion.

Real-time monitoring imposes a significant computational burden and for the Real-Time Monitoring and Control of Additive Manufacturing Processes project, the overriding concern is that there is not enough evidence of having laid the proper groundwork before embarking on a costly and time-consuming research path. It would be good to see more evidence of an informed effort to measure emissivity off-line. The team might also want to consider experiments designed to bracket the possible impact of uncertainty in emissivity values on the SLS process. For the temperature measurements a case could be made for why two-color pyrometry—a standard approach to simultaneous measurement of temperature and emissivity—could not be used.

On a more fundamental level it is noted that the measurement of emissivity is actually a proxy for a measurement of absorptivity. That is, the optical property of most interest in SLS is the absorptivity of the powder bed. At steady state it is known from instantaneous power balance that absorptivity = emissivity + transmissivity. For an opaque body, transmissivity = 0, leading to the familiar result that absorptivity = emissivity. In this case measuring emissivity will also give absorptivity. A powder bed, however, is opaque only over some depth. Measuring emissivity indicates only what the absorptivity is through a depth of powder. It is possible for two powder beds to have the same emissivity (and absorptivity), one absorbing essentially all energy within the first 100 microns of powder, and the other, in an extreme case, requiring 5 millimeter (mm), or 50 times the depth to fully absorb the power. The latter will require far more power to achieve sintering and melting than the former. And so, knowledge of emissivity alone is not sufficient. The proposition that it is difficult to uniformly heat a small target of metal powder up to temperatures above 500ºC is simply not correct. In fact, there are several types of relatively low-cost electric furnaces that will do this job admirably. Up to temperatures of 1200ºC, Kanthal wire may be used

as a heating element. Silicon carbide heating elements can be used to temperatures of 1500ºC. Moly disilicide elements (a bit more expensive) can be used up to 1800ºC. All three types are readily available as off-the-shelf units, and there are many vendors who will build custom units. The combination of a well-insulated furnace—which would act like an integrating sphere—and a light chopping wheel for the incident light- and phase-sensitive, wavelength-dependent detection could provide accurate data.

It is highly desirable to see a series of experiments where the powder is deliberately modified to vary the emissivity. These powders used in an SLS machine, in order to get a sense of the magnitude of emissivity difference, need to have an impact on process control. For example, the emissivity of the powder could be increased by etching the powder so as to create surface texture, and the impact on the process could then be assessed.

It is also desirable to see results where the robustness of SLS against changes in laser power or scanning speed are assessed for impact on part quality. For example, a 5 percent increase in laser power could be used to simulate a 5 percent relative increase in emissivity. If this makes a noticeable difference in the sintering process, then that is an indication that measurement of emissivity differences of 5 percent is useful. Several such experiments would bracket the accuracy of emissivity measurement that is desired.

At the temperatures of a typical melt pool (1400ºC and higher), two-color pyrometry is very widely used. In this application, the light emitted at two wavelengths is measured. Under the assumption that the emissivity is the same at both wavelengths, both temperature and emissivity can be extracted from the data. At these temperatures the two wavelengths can be relatively close together, making emissivity dependence on wavelength less likely. For example, silicon detectors with filters have been used to select approximately 550 nanometers (nm) and 950 nm. A comparison of emissivity measurement using an off-the-shelf two-color pyrometer with off-line measurements would be a good starting point.

Furthermore, it would be useful to see experiments on light penetration into powder beds as a function of depth—even at room temperature. The depth of penetration would be compared with the depth of a melt pool. If the depth of penetration is larger than the thickness of a melt pool, this factor would need to be taken into account.

For the Systems Integration for Additive Manufacturing project, the focus on metal laser-based powder bed fusion systems is a good starting point; however, planning needs to include processes that are used more frequently today for manufacturing, such as powder bed fusion polymer systems and extrusion systems. Archiving better design rules and methods for AM is a very useful and important mission for NIST. Also, developing new and improved dimensioning and tolerance methods for this new technology is a task that is very appropriate for this organization. Both of these project elements could lead to new standards in the field.

Although the architecture of these databases is very comprehensive, populating these databases with useful data represents a real challenge. These data are often held very close by commercial entities and they will be reluctant to share this information with others. The EL might want to consider licensing or giving out the architecture as open source and letting commercial database companies use it. These companies could buy the data as required and sell the results.

Although the desire to focus on carefully selected areas to start is understood, one item that seems glaringly absent is the design and use of support structures in metal parts. The design of these supports and their removal represents a substantial amount of the manufacturing time to deliver a metal AM part. Support structures may already be addressed in the project; however, they were not clearly discussed during the review.

The vast majority of National Laboratories currently pursue AM, and the significant progress that has been made within the MSAM program could benefit other laboratories that, in turn, could likely help the MSAM program. Additionally, a dialogue could be intensified with MML and its Polymers Processing Group in order to exploit common strengths and overlapping research interests to evolve toward common research projects, such as feedstock characterization or the design of additive manufacturing machines. Such a dialogue could build on existing meetings with the MML and the Polymers Processing Group.

Several highly industry relevant topics within AM are not currently pursued with sufficient emphasis. These topics include support structures and their design, the effects of different build orientations, design

limitations for AM, and, as previously mentioned, the post-processing side of AM. Expanding research veins into support structures and post-processing, and defining new procedures in this field, would create an opportunity to develop new standards, which could help industry find common ground.

Last, for the MSAM program, the dynamic and fast-changing nature of metal laser powder bed machines requires continual project reviews, but also decisions to end projects early if project goals and elements become obsolete.

PORTFOLIO OF SCIENTIFIC EXPERTISE

Accomplishments

MSAM staff are very qualified with good trajectories for achieving broad internationally recognized technical leadership. They are a new group that was formed about 3 years ago. The MSAM is composed of 21 Federal employees, 4 NIST associates, and 4 students—62% of their staff have Ph.Ds, and 17% have masters degrees. The group is technically qualified to perform the experimental investigations and accomplish the project objectives. The overall team is comprised of diverse engineering backgrounds that undoubtedly help with the overall program goals.

The MSAM has published 14 journal papers since the last review, and the key personnel involved with the Qualification for Additive Manufacturing Materials, Processes, and Parts project efforts have contributed to this number through the publishing of archival journal articles as well as standards. Several project researchers have been recognized for outstanding contributions and collaborations, including the 2016 America Makes Distinguished Collaborator Award. Additionally, staff within the Characterization of Additive Manufacturing Materials project have won the 2015 Best Paper/Presentation Award at the 26th International Solid Freeform Fabrication Symposium and the 2016 Editor’s Choice Article in the Journal of Materials Engineering and Performance. Overall, these teams are developing into a strong talent pool of AM expertise.

The Real-Time Monitoring and Control of Additive Manufacturing Processes project brings together several NIST researchers with different backgrounds. For the tasks the team has set out to achieve, this broad array of scientific expertise is particularly well-suited.

The two key personnel involved with the Systems Integration for Additive Manufacturing project efforts have been recognized for outstanding contributions and collaboration and are both publishing archival journal articles and standards at an acceptable rate. Although the team is small, its size appears to be appropriate with respect to the early stage of many elements of the design rules, commercial processes, and machine development, as well as other aspects of metal laser powder bed machines.

The team is quite capable and the architecture it has developed is very comprehensive.

Opportunities and Challenges

The Characterization of Additive Manufacturing Materials project currently uses several different commercial equipment items for powder characterization and also for mechanical testing and for post-processing. Given the goal not only to use the equipment but also to understand its limitations and measurement uncertainties, it is quite a challenge for the team to master all equipment items and techniques at the necessary level. The MSAM could consider convening a meeting to obtain user input on the round robin study in order to define ways to narrow the scope to focus activities on NIST’s mission in supporting industry.

Like other areas within the EL AM effort, the Qualification for Additive Manufacturing Materials, Processes, and Parts team consists of junior researchers who are, for the most part, recent graduates. Their trajectory to outstanding experienced researchers is impressive, and as they continue doing excellent, relevant research, they are likely to become better known and will be suitably recognized.

The Real-Time Monitoring and Control of Additive Manufacturing Processes project appears to touch upon several different scientific and engineering topics, and the current and future monitoring projects might benefit from additional expertise—for example, in control theory or solid state physics—within NIST. If the MSAM was to move into support structures, it could benefit from additional expertise in component designs.

ADEQUACY OF FACILITIES, EQUIPMENT, AND HUMAN RESOURCES

Accomplishments

State-of-the-art powder characterization equipment was set up to support the Characterization of Additive Manufacturing Materials project and the cross-project availability of the AM testbed. The equipment includes a custom raking device to study the motion of powder particles during powder spreading, as well as a range of commercial equipment. As previously mentioned, several different characterization techniques have been set up. These techniques include the following: powder size distribution and shape analysis; a custom powder spreading device; different flow and density measurement devices; and a suite of X-ray tomography instruments for defect analysis in additively manufactured parts. Collectively, the equipment is highly adequate to meet the project objectives. The database part of the project draws from experts on database development within NIST.

For the Real-Time Monitoring and Control of Additive Manufacturing Processes project, the AM testbed facility is a major investment both for the actual testbed and for the added equipment—for example, the EOS M270 machine and testbed and the temperature measurement equipment that is coupled to the beam parameter values. Enabling full control over the laser and galvo system is invaluable for the team in setting up the testbed for calibrations of secondary equipment.

The Qualification for Additive Manufacturing Materials, Processes, and Parts project and the Systems Integration for Additive Manufacturing project both appear to have adequate facilities, equipment, and human resources.

Opportunities and Challenges

For the Qualification for Additive Manufacturing Materials, Processes, and Parts project expansion to other metal-based AM processes will necessitate purchase of new equipment.

A major challenge is that the Real-Time Monitoring and Control of Additive Manufacturing Processes project testbed represents one particular AM technology—powder bed AM. Other metal AM technologies exist, and new technologies could emerge very quickly that could render the powder-bed technology less relevant than it currently is. The testbed and other real-time monitoring equipment need to be devised with the greatest possible flexibility in mind. A larger vacuum chamber in the testbed, for example, could possibly accommodate a small robot for electron-beam or plasma gun AM.

The Systems Integration for Additive Manufacturing project needs to develop a plan for population of its database that can bring in statistically significant information.