Quality control the next frontier in metal 3D printing

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From a material perspective, metal additive manufacturing (AM) is an imperfect science. This isn’t meant to imply the alloys are unreliable; they’re clearly reliable, as evidenced by the increasing number of flight and safety-critical components 3D-printed each year for the aerospace, medical, and energy industries. What it does mean is that, unlike metal billet, rod, and sheet, additively manufactured metals are produced “on the fly.” They are in a constant state of flux during the build process, their properties indeterminate until well after the part is complete. Again, no disparagement intended towards metal AM. I’m one of its biggest fans.

One author’s views
So is Professor Ehsan Toyserkani, director of the Multi-Scale Additive Manufacturing (MSAM) Lab at the University of Waterloo, one of the top five such laboratories in the world. Since 2003, he has worked to advance knowledge across all seven of the ASTM-recognized AM technologies, but has placed particular emphasis on 3D metal printing over the last decade.

This emphasis (and no small amount of persistence) has paid off. Assuming another construction-related delay doesn’t interfere, he and his team will soon be moving MSAM’s fleet of Renishaw, EOS, and GE Additive metal powder bed printers to a new 2,300 square metre (25,000 sq. ft.) facility at the Catalyst137 “IoT makerspace” in Kitchener.

Toyserkani has published many papers on 3D printing and recently collaborated on a textbook titled Metal Additive Manufacturing. It describes the technology’s many benefits along with its not insignificant challenges, among them the shortage of qualified materials. For instance, the book explains that, compared to the 1000 or so steel alloys available for use in castings, “just a handful” exist for metal AM, and are five to ten times more expensive besides. The figures are similar for aluminum and other alloys.

Perhaps more importantly, Toyserkani’s textbook notes that “although the technology has already produced impressive results, it is also true that reliability and repeatability are still significant AM problems, particularly for mass production. Failure rates for many applications remain in a range where using the technology simply is not economically justifiable due to the number of failed parts and the need for post quality checking by an expensive setup such as CT [computed tomography scanning]. The underlying problem is that AM is so sensitive to both environmental and process disturbances, from fluctuating temperature and humidity levels to nonuniform powder sizes.”

A peek through the windows of the ZEISS Industrial Quality Solutions lab at Oak Ridge National Laboratories, part of a $25 million cooperative research and development agreement. image: ZEISS Industrial Quality Solutions
A peek through the windows of the ZEISS Industrial Quality Solutions lab at Oak Ridge National Laboratories, part of a $25 million cooperative R&D agreement. Photo courtesy ZEISS Industrial Quality Solutions

Strengthening the chain
These are profound problems. And like Toyserkani, Paul Brackman, additive manufacturing manager for ZEISS Industrial Quality Solutions, intends to do something about them. He and a team of people are part of a $25 million cooperative research and development agreement with Oak Ridge National Labs (ORNL) in Knoxville, Tenn., where they spend their days studying the AM process chain and attempting to make it stronger.

The links in this chain include qualification of raw powder, evaluation of different material chemistries for various applications, 3D modeling of laser scan strategies and part placement within the build chamber, determining the effects of heat-treatment and other postprocessing methods, and—of course—part inspection using CT scanning, blue light systems, and coordinate measuring machines. All play an integral role in finished part integrity and predictability.

“There are a lot of different steps to additive manufacturing, and ZEISS, recognizing its importance to the industry, developed what we call a blue line of hardware and software tools that touches all of them,” says Brackman.

In one example, ZEISS collaborated with AM service and software provider Materialise and Senvol, a developer of AM databases and analysis tools, to help Rosswag Engineering of Germany improve the qualification process for its laser powder bed fusion (LPBF) feedstocks. Here again, there were many steps to the process, but ZEISS leveraged its AM parameter workflow tool to help evaluate and then certify parameter sets for the company’s different printers. This not only saved the forging company significant time and money but served to improve part quality as well.

Assigning responsibility
But shouldn’t print parameter development be the responsibility of 3D printer manufacturers, rather than big-name metrology providers? It certainly is, but just asking this question indicates the tremendous complexity of metal AM. “We work with a number of AM equipment providers, but there’s no one-size-fits-all parameter set,” Brackman says. “Someone making replacement parts for fifty-year-old machinery, for instance, will have a completely different set of requirements than an aerospace or medical manufacturer, even though the material used in each case is identical.”

As noted, ZEISS has developed and continues to develop tools to support numerous processes within the broad spectrum of additive manufacturing. Metal 3D printer manufacturer Renishaw Inc., West Dundee, Ill., offers additional tools, some of which can be used to improve the quality of parts made on competing equipment. As industrial metrologist Denis Vasilescu notes, whatever the brand of 3D printer, it’s critical that those using them must strive for the highest yield possible and avoid a “passive approach” to part inspection if they’re to be successful.

“Manufacturers with low scrap rates or low piece prices might be satisfied with simply screening out bad parts using mass inspection,” says Vasilescu. “With additive, however, this old-fashioned approach is unacceptable. The capital equipment and materials costs are quite high and cycle times are relatively long, which is why it’s especially important to assure maximum part quality throughout all stages of the process, even after the build is complete.”

Because practically all metal AM parts require machining after heat treatment—most often on a five-axis machining centre—Vasilescu strongly suggests shops implement a robust preventive maintenance regimen using one of Renishaw’s machine calibration and optimisation tools (which is good advice for all manufacturers, AM or otherwise). Doing so eliminates the positional and orientation-related errors that can lead to the scrapping of very expensive workpieces.

Monitoring the pool
Renishaw’s AMG Americas business manager John Laureto seconds this but suggests 3D printer manufacturers must also do their part to assure high quality. He points to the company’s InfiniAM Spectral technology as one element of this, a subscription-based software tool that, when combined with Renishaw’s Laserview and Meltview hardware (both part of its RenAM-series printers), provides feedback on energy input and emissions during metal 3D printing.

“InfiniAM Spectral leverages a series of photodiodes within the machine that measures the feedback intensity from the melt pool and maps it to positional coordinates on the build platform,” he says. “From this, we can construct a 3D historical model of the build, which can then be used to analyze and understand everything that occurred during those hours or days it took to make the part. It’s just one piece of an overall solution meant to assure quality throughout the manufacturing process.” SMT

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