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Inspection Inside and Out: CT Scanning for Metal Parts

Category: Manufacturing Technology Jun 25, 2021

In the past five years, the market has seen a real acceleration of additive manufacturing (AM). While the medical device industry has had both polymer and metal applications in commercial production for the past decade, it has only been in the past few years that AM has moved from polymer-based prototypes to production-quality metals in other industries.

Challenges to AM Growth
Despite growing market demand, the AM industry is experiencing several important challenges to more rapid growth. The quality assurance (QA) of AM-produced metal parts is probably the greatest obstacle affecting the speed of AM technology adoption right now. Powders, print processes, and final parts are subject to a level of variability in terms of chemical and mechanical quality that cannot be tolerated in a production environment. Rigorous quality standards and inspections are necessary to ensure the consistency, repeatability, and reliability of final parts.

Metal powder chemistry must be consistent in every batch, including particle size distribution, composition, size, and shape. Additionally, for safety-critical applications, such as aircraft parts or medical devices, the material used must be 100-percent-free of contamination because impurities can alter the properties of the final part. Metal powders can become contaminated during the printing process, storage, transportation, and handling.

Complexity Abounds
“AM is turning out to be more complex than people realized,” said Dr. Marcin Bauza, Head of ZEISS Additive Manufacturing Process and Control. “There are so many variables that affect the production of a part. The same batch of powder from the same vendor can produce different physical and mechanical properties in different locations of the final product, depending on how the powder is processed and the heat treatment method used. For example, the size distribution of individual powder particles influences how the powder is compacted and the density of the build, possibly creating defects visible later in the process.”

Destructive testing of hundreds of copies of final parts to ensure quality is cost-prohibitive for any type of production and cancels many of AM’s operational advantages. Non-destructive testing is the only viable option, and these methods and standards are still evolving.

What Can CT Scanning See?
One of the most precise ways to qualify metal powders is by using non-destructive Computed Tomography (CT) scanning which can capture details down to microns. High-resolution micro-CT scanners can reach down to 0.5-micron voxel size. CT scanners provide in-depth data on powder microstructure and can measure material composition, particle size, shape distributions, porosity, density, substrate bonding issues, grain structures, and uneven material flow.

Maintaining the consistency and repeatability of the printing process to ensure build quality over time is another challenge. Process control must be established for printing equipment to ensure that end products are consistent from part to part. It is not uncommon to need to measure more than 100 initial samples before the tools are adjusted sufficiently to enable volume production to start. For example, a laser powder bed machine can drift from its optimal settings relatively quickly, and, during fused deposition modeling, vibration can occur on the print head.

Changes Can Happen During Processing Too
Next, final parts need to be examined for their material properties to ensure they have not experienced changes in microstructure during processing. This includes examination of chemical composition, density, porosity, mechanical strength, tensile strength, stiffness, internal part geometry, surface quality, and finish.

“An additional cost that needs to be factored into AM is the need for post-processing,” said Dr. Jason Jones, CEO, Hybrid Manufacturing Technologies. “3D printing was initially a design and prototype-oriented activity based on polymers where post-processing could be done pretty easily and cost-effectively. But, since metal is now being used for actual production parts, substantial post-production finishing is always a requirement. This takes time and requires inspection.”

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