Computed tomography (CT) is getting applied to a growing number of applications involving additive manufacturing in aerospace product development and qualification. CT applications range from assisting in product design, multiphysics simulation models, and final product evaluation and failure analysis.

Brett Muehlhauser, R&D technical fellow at North Star Imaging, will present the MD&M Minneapolis session, Computed Tomography Applications: Helping Accelerate Aerospace & Additive Manufacturing Product Development, on Thursday, October 17,  at 12:15 pm. The presentation will include examples of applications as well as a report on the latest CT innovations and development.

CT for additive manufacturing

According to Muehlhauser, extensive studies have been performed using CT to help detect, classify, and better understand the types and cause of discontinuities found within various manufacturing processes including additive manufacturing. New discoveries have been made while performing 4D CT that provides three-dimensional observation of movement and breakdown of internal material structures over time. A CT scan can also provide metrology data for internal and external features. CT is rapidly becoming the "Go To" technology for evaluating many aspects of critical aerospace products.

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We caught up with Muehlhauser to get more details into the use of CT in additive manufacturing.

How is CT applied to additive manufacturing.

Brett Muehlhauser: CT is currently being applied in AM across areas spanning from R&D through failure analysis. The applications involve all AM methods and materials in the areas of product and material evaluations, printing process developments, and even the assessment and validation of AM in-situ monitoring methods. Even powder bed fusion materials are being evaluated by CT for single powder grain size, shape, voids, and foreign material.

Why it is particularly relevant to aerospace applications.

Muehlhauser: The aerospace industry is dealing with everything from replacing parts on aging aircraft to the development of new components to support the highest level of performance for the latest military aircraft. In both arenas, additive manufacturing and computed tomography are working hand-in-hand to meet even the most difficult demands.

Some aging aircraft today need parts replaced that simply are no longer available or their lead times or cost are out of range of usefulness. These parts were often produced in an era where only 2D paper drawings were used and, for example, the casting company that built the parts decades ago may no longer even be in business. In these cases, computed tomography can be used to scan a part or even a full assembly to provide a complete 3D model of the device.

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This model can then provide all internal and external dimensions needed for analysis and 3D printing. Understanding the material properties of the product helps the engineer best match the AM material properties selected for printing. The model of the part can then be placed into an FEA simulation program to help understand if any further design changes need to be made to assure optimal performance and reliability. The level of physical environmental testing of the part will be determined by its criticality level.

Even in cases where a casting replacement of the part is required, when molds are no longer available, the model generated from the CT scan can be sent to an AM wax or polymer printer to provide the required pattern for the investment casting process.

In new product development applications, additive manufacturing product designs can provide new levels of performance as they are no longer limited by the abilities of previous manufacturing methods. In many cases the design formerly consisting of numerous parts, can now be produced in a single part carrying with it a substantial increase in performance as well as a reduction in size and weight.

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I’m also interested in how CT helps to detect, classify, and better understand the types and cause of discontinuities found within various manufacturing processes including additive manufacturing. 

Muehlhauser: When performing typical 2D digital or film radiography of parts, all the features and discontinuities from one side of the part to the other are superimposed upon each other. The more complex the geometry and the thicker the part, the more difficult it becomes to interpret the image. If an indication is detected, without multiple views at varying angles, it can be nearly impossible to make a clear evaluation of an indication’s location and true size.

With computed tomography, images are typically captured 360 degrees around the product.  This provides a tremendous amount of new information, that when reconstructed, can be viewed as a 3-dimensional volume and sliced through any orientation.

This provides a significant improvement in image quality over 2D images, allowing for very accurate evaluation of an indication’s location, size and type. Whether the product is produced as a casting, weldment, or using additive manufacturing, when CT-scanned, the entire part can also be three dimensionally evaluated using automated software routines such as porosity & void analysis as well as metrology programs for comparison to a CAD model of the part.

Additionally, some of the AM processes are known for the potential of producing build plane fusion discontinuities. CT scan techniques can be established to take the build plane direction into account and significantly increase the probability of detection of these types of AM discontinuities.

Why is CT becoming the "Go To" technology for evaluating critical aerospace products.

Muehlhauser: Twenty years ago, industrial CT required a much more time-consuming acquisition process and it was not uncommon for the reconstruction times to take up to eight hours or more just to generate a volume. Due to the advancements in the technology and techniques, some production CT scans can be performed and reconstructed in just seconds. This provides the opportunities for at-line and in-line inspection processes.

With the thoroughness of the evaluation, a single CT scan of complex geometry parts can often replace multiple time-consuming 2D inspection views while yielding a higher probability of detection. There is also the potential of replacing the conventional metrology measurements with metrology data from the same CT scan. It is now often a requirement for flight critical metal AM Parts to be CT examined using ASTM E3375 while applying the acceptance standards of AWS D20.1.