Certification and Qualification in Additive Manufacturing Simplified
By Dr. Kevin T. Slattery
As highlighted by being the theme of America Makes’ recent TRX, the topics of Certification and Qualification will feature prominently in the future of Additive Manufacturing (AM). This is because AM is being developed in a wide range of industries to produce products with superior performance (weight, efficiency, cost, etc.), with the same reliability that conventional manufacturing has delivered for their customers. While the many regulatory bodies and certifying agencies have very detailed definitions unique to their field, in their simplest form the two terms are:
· Certification – A Component That Meets Design Intent is Fit for Service in a System
· Qualification – A Component Meets Design Intent, Including the Supplier, Machine, and Processing
Certification and qualification are not unique to highly regulated industries such as aerospace and medical. It’s just that they have some of the most codified requirements because of the severe consequences of unanticipated failure. In my interactions with engineers from a wide range of industries over the last 35 years, I have yet to come across a single one who was indifferent to the consequences of failure of a component or system they were responsible for. Below I will describe certification and qualification in terms of AM, and how it applies to both safety-critical and relatively mundane applications. The example of the former will be a topologically optimized hinge made from Ti-6Al-4V that holds a control surface onto an airplane wing (hinge fails, control surface falls off, and plane crashes), and the latter a car door handle with an internal lattice structure made from a thermoplastic polymer (handle fails, driver has to use the passenger door and crawl over the center console, auto company customer service gets an unpleasant e-mail, or if it happens too often, a recall involving thousands of vehicles).
We will use a modified version of the Systems Engineering V to illustrate the different aspects of Certification and Qualification, as detailed below.
Certification is best explained using a modified version of the general principles of the late Dr. John W. ‘Jack’ Lincoln’s pioneering work in aircraft structural integrity as an outline.
Requirements and Design Criteria– This includes the requirements for the individual part (volume, interfaces, loading conditions, number of cycles, temperature extremes, corrosion, etc), and for the system the part is in (generally what happens if the part fails). These criteria will often include factors for margin of safety and environmental factors that cannot be tested, such as long-term exposure of polymers to hot/wet conditions. While these are generally the same regardless of the method of manufacture, some unique considerations for AM could be load transfer in the event a topological element failed in the hinge or buckling of the lattice structure in the handle. Another example would be the impact on fatigue of the rough surface of the AM hinge or the potential for water entrapment in the lattice and freeze-thaw damage on the door handle.
At some point, the part and its method of manufacture will need to be successfully tested against its requirements by either the cognizant engineering authority or an outside body to certify that the part is fit for service in a system, or the system containing the part is fit for service. This can range from a full-up test of the component under service conditions (repeated cycling under loading, thermal, and moisture extremes) to coupon testing (extracting tensile, fatigue, and fracture coupons from the part) to the use of handbook data for the AM material and process combination and comparing it with the design analysis. In the case of parts made from conventional wrought manufacturing (sheet, plate, forging) with tens of years of production, billions of service hours in service, and well-understood material/process/property relationships, passing these tests and the underlying analyses are often sufficient for the authority to certify the part or the overall system the part belongs to. In the case of a new technology like AM, however, these need to be demonstrated. Demonstrating these requires the four other principles, which in some cases blurs the line between certification and qualification.
Stability, Robustness, and Repeatability– This is most simply described in the case of AM as showing that the first, last, and every part in between in production that is accepted by the production system meets the requirements demonstrated by the parts used for certification. Robustness generally refers to a process that does not have dramatic changes in properties (physical, mechanical, etc.) with relatively minor input parameter changes. This not only refers to the parameters over which the operator has a great deal of control, such as laser power, travel speed, and part geometry, but also parameters where there is little ability to control, such as local variations in powder density, bed temperature, and humidity. A robust process would have good repeatability throughout the full range of controlled parameters within the defined process window, even when the parameters that are not controlled vary widely. Finally, a stable process has minimal creep over time, which could be as short-term as bed temperature and property variation through the height of the build, or as long-term as both over a 20-year production span.
In the case of our two examples; stability, robustness, and repeatability are necessary, but the design criteria and length of production run can influence the degree of confidence and control needed. The door handle may only be in production for a year or two before a design refresh, so stability is only needed over that period of time. The hinge, on the other hand, could be in production for 20+ years, so stability is needed over that time period, noting that the rapid changes in AM will probably result in a process change (and probably re-qualification, and possibly re-certification) after 5 years.
This brings to the use of specifications to ensure Stability, Robustness, and Repeatability over the lifetime of a part, which can be a differentiator in the two examples. In the case of the hinge, a critical part being manufactured over a long period of time, the design authority will generally want to control more aspects of the process, even those which may not seem very critical to performance, just to ensure that no long-term drift occurs. This is in contrast with the door handle, where the primary criteria of static strength, stiffness, and environmental exposure over time, may lend themselves to simpler specifications for feedstock, processing, and post-processing.
Producibility– This basically means that the process is capable of producing the part without excessive scrappage and re-work. This is closely related to Stability, Robustness, and Repeatability, when applied to the actual part geometry. One can imagine that a process could be stable, robust, and repeatable for a relatively simple geometry (cylindrical test bars), but not for a more complicated one (3D shape with significant thickness changes). This requirement is the same for both examples, as high levels of scrap and rework for the door handle would result in much higher production cost; while the same in the hinge will result in a lack of confidence in its consistency of performance and freedom from rogue flaws that could compromise its integrity. Ensuring producibility necessitates that the development of a process for a given part considers its geometry and making sure that these features do not result in properties that differ significantly from expectations.
Characterized Mechanical and Physical Properties– This means that all of the relevant properties for the application in mind are known and understood, along with the influence of processing parameters and environment. In the case of the door handle, this may be as simple as characterizing the static, tensile, compressive, modulus, and bearing (at the attachments) strength of the polymer as a function of operating temperature (-30C to 50C). In the case of the hinge, it would be over a wider temperature range (-55C to 200C), and include fatigue and fracture properties, and other physical properties, such as thermal conductivity. This also includes characterizing the properties as a function of direction to account for any anisotropy that may exist.
Predictability of Performance– This essentially means that whenever a designer calls out using and AM process to make a part that they can be assured that the manufacturing process used to make the part will have the mechanical and physical properties that were used in performing the analysis of the part. Most commonly, this requires knowing not only the mean, median, or typical properties, but some type of minimum property that takes into account statistical variation. The most well-known published statistical mechanical properties are commonly referred to as A-Basis, B-Basis, and S-Basis design allowables in Metallic Materials Properties Development and Standardization (MMPDS https://www.mmpds.org/), which also contains typical physical properties, and some fatigue curves. These are values where 99% (A-Basis and S-Basis) or 90% (B-Basis) of a specific material will have properties above with 95% confidence. To generate such a value not only requires having a process that is fully stable, robust, and repeatable, but also requires following a rigorous path to generate these values, which has been accomplished for only a few AM processes. In some cases, the design values may be part or part family specific, and generated by making multiple parts of the configuration, and excising mechanical test coupons from them.
Note that these only refer to static properties. Fatigue and fracture properties that account for material scatter have their own methods of design value development, which tend to be very application specific, and so are generally not available in published documents. Finally, the generation of design values can require decrementing properties for non-typical variations, such as the presence of physical discontinuities; and process features, such as surface roughness. Returning to the the hinge, the methodologies are spelled out in MMPDS and the requirements of the certifying body and could include decrementing properties for surface roughness and potential pores that would not be detected by nondestructive testing (NDT).
There can also be a link between design margins (getting back to requirements) and the design values. An example is the door handle, where generation of the design values is up to the vehicle manufacturer. Here the handle design is driven by stiffness (to keep from having to pull the handle too far), with the maximum stress being only 10% of the typical material property. With such a high design margin, the manufacturer opts to not generate an MMPDS-style allowable, which would result in a negative minimum strength since the material scatter is high in relation to the typical value.
Summary – Once a component, along with its process and material has demonstrated it is capable of meeting design intent, it can be certified for use. In the case of the hinge, this may involve extensive analysis, part builds, and testing, while in the case of the door handle, it may be as simple as testing a first article the extremes of the temperature range. It should be noted that Certification usually requires completing Qualification requirements as well, as Qualification demonstrates that it is possible to repeatably produce components that conform to design requirements.
Repeating the simple definition of Qualification as: A Component Meets Design Intent, Including the Supplier, Machine, and Processing, the four (4) aspects of supplier qualification, machine/process qualification, part qualification, and finally lot acceptance will be discussed. AWS D20.1 contains a detailed set of requirements for qualification that represent a good starting point.
Facility Qualification– This generally refers to the supplier/production facility having the proper processes and controls (ISO-9000, AS-9100, Statistical Process Control, software configuration management, etc), but also has personnel training and machine maintenance programs in place. Finally, they generally need to have the actual AM process approved, along with any relevant pre and post-processes that are critical.
Machine/Process Qualification– In addition to demonstrating hardware and software control and stability, this generally refers to demonstrating via one or more builds that the machine is capable of producing conforming material. At one end (hinge), this could be making multiple builds, each with a mixture of parts and test (static, fatigue, fracture) coupons to demonstrate the ability to make material that meets the properties and scatter of the design data set. At the other end, this could mean making a build with multiple handles and testing one of more of them to show they meet service requirements.
Part Qualification– This generally entails making builds of the production configuration and completing them through all of the relevant post-processing, followed by testing against engineering requirements. This generally requires that the parts pass all of the standard test requirements, such as witness coupons (chemistry, microstructure, and mechanical properties), dimensional inspection, and NDT. In the case of the hinge, this could mean destructively testing several parts by excising mechanical test coupons. In the case of the door handle, it would generally be the same as the Machine/Process qualification test described above.
Lot Acceptance– Once a part has been Qualified and Certified, it is then put into production. Each production lot will generally require testing to accept the lot and simple dimensional testing using a gage. In the case of the door handle, it could be as simple as demonstrating that no anomalies occurred during the build process. In the case of the hinge, it will generally require testing of witness coupons from the build, along with passing dimensional, chemistry, microstructure, and nondestructive tests.
Summary– Like any other process, the proper application of AM requires feedstock and machines that are capable of producing parts that meet engineering requirements. While those requirements differ little from conventional processes, the newness of AM means that the approach for Qualification needs to uniquely consider the process and application until confidence in AM reaches the level of conventional processes.
Regardless of the industry and application, unless one is indifferent to the ability of an AM part to meet customer requirements, Certification and Qualification is an integral part of every AM implementation. The only difference between highly regulated industries such as aerospace and medical, and simple applications such as a drill template, is the consequences of failure and the amount of testing and analysis needed to ensure success. The key is striking the proper balance between requirements and scope.
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