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Zach Murphree

Metrology

Calibrating Quality in Additive Manufacturing

Measuring before, during, and after a part build

Published: Tuesday, October 13, 2020 - 12:03

The metal additive manufacturing (AM, aka 3D printing) industry is in vigorous pursuit of repeatable part quality. Its aim is to match the reliability and performance found in traditional manufacturing industries such as machining or casting. Repeatable quality opens the door to wide-scale implementation in product development: Witness the explosive growth of molded plastics after the development of a well-defined shape-forming process for part consolidation and functionally creative geometries was established.

Industry analyst Smartech Publishing states that the AM market crossed $10 billion last year. Going forward, AM’s potential in series production and direct part replacement is huge. Growth now depends largely on quantitative quality measurement and establishing dependable machine-calibration methods. Even the improving cost-efficiencies of AM won’t advance the process much if component behavior can’t be fully understood and trusted in mission-critical applications or across mid- to high-volume orders.

What major manufacturing process has not gone through challenges in material science and pre-, post-, and in-process quality calibration and measurement? Metal AM is now moving fast to deliver designs of startling complexity and performance. The next step is products that are identical on the first and hundredth build.


Figure 1: This turbo impeller component, 3D printed support-free in a next-generation additive manufacturing (AM) system, has intricate, nearly impossible-to-make zero-degree overhanging surfaces that most current AM machines can’t reproduce without extensive support structures.  Image courtesy VELO3D.

Offline pre-build calibration

The standards and methods of pre-build calibration for metal AM are evolving. NASA, ASTM, SAE, AWS, and API, among others, continue to refine their requirements to reflect the increasing capabilities of the machines themselves to observe their own build performance. However, NASA guideline MFSC-SPEC-3717 (written in 2017) states that “calibration is effective only when maintained continuously. For pragmatic reasons, confirming calibration is not feasible on a pre-build basis. This time-based calibration interval [at least every 90 days] is set as a compromise between production efficiency and process assurance.”

This compromise is one that needs to be mitigated for AM to be considered mature. As machinists, welders, and casters know, many processes must be monitored in as close to real time as possible—not in days or weeks; too much is at stake. Yield rates alone will determine whether a bid price is met and if product can ship on time, profitably, and as a full, not partial, order. Consistent quality dictates product safety and affects customer value, brand identity, and warranty exposure.

Manufacturers have made efforts to place cameras and sensors inside AM equipment to gain insight into what exactly is going on within the build chamber, but generally the onus is then on the user to establish thresholds and control limits for each specific parameter. These metrics are often not immediately actionable, nor do they help correlate the chamber physics as an interrelated data set. In many current machines, devices have been added onto an existing AM architecture; trade-offs can abound in their placement and usefulness. Although this is an initial step toward greater insight into what makes a quality, repeatable build, AM must make a further leap and marry data science to material science. A modern AM system is capable of burying the user in data, but this has not had a significant effect on the user’s confidence in the part quality at the end of the day.

Feedstock, optics, powder bed, and recoater quality-control checks

Metal powder feedstock for 3D printing is widely produced and generally very well characterized before reaching the printer. There are a number of ways suppliers create powder. Atomization (generally via gas or plasma methods) provides high-purity, spherical material of predictable size and layering performance. Extracted from wire, bar, or ingot metal stock, many proven and certified metals have been developed for AM, along with custom superalloys designed to meet the requirements of a specific application—say, for oil and gas or aerospace—with select corrosion or strength characteristics. Keeping a diversified supply chain of reliable suppliers is always a sound idea. Powder suppliers provide a stable element in the AM quality movement that closely resembles traditional manufacturing.

Optics are critical to the metal AM micro-welding process. Most engineers are now familiar with the micron-scale of the powder layers involved, each literally welded onto the next. Systems of one to four lasers fuse metals with extreme precision; these must be perfectly aligned, directed to the right place, and at the proper speeds to create the precision thickness and consequent metallurgy necessary for a quality part.

Challenging part geometries and wall angles and heights make calibration even more important. Rough surfaces and porosity can be an outcome of poor process-design setups for the build, but also of poor optical settings. Again, the standard today is a 90-day calibration of AM machine optics, often done by placing an anodized aluminum plate (or thermal paper) in the build chamber set to a precise height to provide comparison metrics. Lines are burned into the plate and inspected via an optical coordinate measuring machine (CMM). This is a time-consuming, manual, and often-iterative process that can generate its own errors. This extends the scheduled downtime, something no shop floor or service bureau wants for its most expensive equipment.

As found in other manufacturing-process sectors, the AM industry is moving toward more real-time test solutions—with an advanced-entry example in operation today. This next-generation system automates the collection of a variety of metrics in the pre-build stage, ranging from beam stability, laser alignment, and focus to atmospherics and powder-bed statistics. This allows the establishment of control limits that guide each aspect of system performance and part creation. It also provides for the most advanced in-situ, real-time metrology environment in the industry.

 


Figure 2: Machine health in a VELO3D Sapphire system can be checked via a single click to ensure that powder-bed quality, sensors, and optics are calibrated and in optimal condition, and will not compromise part quality. Image courtesy VELO3D.

Powder-bed control is just as integral to part quality as laser-beam accuracy. Thickness and uniformity of the powder bed allow properly calibrated lasers to create melts that avoid porosity, excess vaporization, and mechanical properties that aren’t in spec.

The recoater systems that put down each successive layer of powder must be calibrated as well. Currently, the industry recommendation for these is every 180 days. If that timeline is followed, much can go wrong in the thousands of layers put down in even the smallest of parts. Incumbent AM systems do not yet have a quantitative means of actively monitoring the effects of recoater damage or improper powder dispensing.

Post-build inspection of parts has become more of a priority, either via dye-penetrant testing for potential crack initiation in fatigue-critical applications, CT scans for porosity and other flaws, or destructive testing. Full inspection is the norm with much of the current AM equipment, although the desire is to be able to move to batch inspections. The majority of data analysis is used to validate and determine parameter settings for lasers, powder beds, feeds, and other operations after the product is made, not during the critical manufacturing process.


Figure 3: Before and during a build, the Assure quality control system validates that critical parameters stay within control limits, ensuring high-quality parts. Clicking on individual squares reveals details on the underlying event. Image courtesy VELO3D.


Figure 4: The Assure system tracks whether the build is showing any signs of unwanted protrusions; crossing the warning threshold will alert the user. Image courtesy VELO3D.

The latest systems, however, include not only melt pool metrology, but also height mapping that surveys each layer of the build in progress against the position of the bed, as well as in-situ checks of calibrations and performance. All of these data are captured in a comprehensive build report for traceable quality.

Quality and additive manufacturing

There are many types of additive or 3D printing systems in the world. They use materials from metal to plastic to composites to concrete. All have performance objectives and tolerance limits that must be satisfied to meet quality requirements for safety or market acceptance. Advanced metal laser powder-bed fusion machines are arguably one of the most complex and dynamic of these systems due to the nature of the materials involved, the stresses induced by welding, the high operating temperatures, and certain metals’ sensitivities to environmental factors.

Laser powder-bed fusion is becoming key to expanding industrial applications in nearly every market that uses metals—and is particularly important in mission-critical applications such as space, defense and aviation, medical, and oil and gas. What’s more, on-demand, direct part replacement in small lots for maintenance, repair, and operations has the potential to determine the economic health of whole industries, as casting and tooling suffer from restrictive investment barriers outside of full production. All these industries are dependent on parts of certified quality—which is where opportunity and actuality intersect.

As AM breaks through with new paradigms in innovation and performance, provable quality of individual parts is critical. This can only be achieved through a vigorous combination of data acquisition and applied material science. This means developing sensors, cameras, and software for capturing, measuring, and monitoring quality metrics that can zero in on any moment and aspect of production, report those data, and synthesize the outputs for insights into continuous process improvement. Derived from lessons learned from the highly exacting semiconductor industry, those benchmarks for AM are now being met and refined by the most advanced 3D-printing systems. In terms of quality, function, and volume production, this should be encouraging news for those seeking certified AM product solutions.

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About The Author

Zach Murphree’s picture

Zach Murphree

Zach Murphree, Ph.D. is VELO3D's VP of Technical Partnerships. His background includes engineering roles for energy companies, where he was in charge of introducing metal additive manufacturing technology to a Fortune 500 energy company. He earned Bachelor of Science and Ph.D degrees in Aerospace Engineering from the University of Texas and has been granted more than 35 patents.