CT scans for 3D printing

WENZEL America and NIDEC are continuing their partnership to explore the potential of the exaCT U computed tomography system from WENZEL in additive manufacturing research with directed energy deposition (DED). This system, integrated with NIDEC Machine Tool America's LAMDA system, utilizes state-of-the-art technology to process even the hardest metals.

Since the introduction of additive manufacturing in Japan in 1987, numerous new applications have developed using various technologies to transform CAD files into physical 3D objects. Today, even highly complex objects and shapes are recognized and widely used in many industries. In this article, we look at directed energy deposition (DED) and how this technology can be used to ensure product quality.

LP-DED (Laser Powder-Directed Energy Deposition) is a powerful additive manufacturing (AM) process in which a focused laser beam melts and bonds metal powder layer by layer to create a desired 3D object. The metal powder is introduced into the molten pool created by the laser via a nozzle, enabling precise material placement and the production of complex design features.

Compared to other AM processes such as powder bed fusion, LP-DED offers greater flexibility as it can work directly on existing components. This makes the process ideal for repairs, adding features to existing parts and producing functionally graded structures where material properties vary within the object. LP-DED can also process a wider range of materials, including metals that are difficult to process with other methods.

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A big advantage

A key advantage of NIDEC's LAMDA LP-DED system is the ability to perform large-scale metal additive manufacturing without the use of a full environmental chamber. This is achieved through localized shielding - a gas enclosure that surrounds the deposition area. This shielding minimizes the interaction of the laser and metal powder with the environment, reducing the risk of fumes, spatter and oxidation. This not only simplifies set-up, but also reduces costs and energy consumption compared to chamber-based AM systems.
A significant advance is the use of monitoring and real-time feedback by NIDEC to control the process. Combined with artificial intelligence and machine learning, the LAMDA systems can detect anomalies early and automatically stop the process before the component is damaged.

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The combination of material versatility, repairability and large-scale manufacturing capability makes LP-DED a valuable tool for various industries such as aerospace, automotive and energy. As research and development continues to improve process control and material understanding, LP-DED is expected to play an even more significant role in the future of additive manufacturing.

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Industrial computed tomography (CT) is an advanced, non-destructive testing method that enables detailed internal views of components, even penetrating materials such as metal and plastic. In combination with suitable software, industrial CT becomes a powerful tool for engineering and metrology. CT technology has been around for decades and enables fast inspections, revealing internal structures that remain hidden with conventional measurement methods, improving cost efficiency and productivity. Unlike other inspection machines, CT offers in-depth analysis of internal structures, material properties and potential defects.

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CT systems are invaluable in materials testing and offer a unique opportunity to reveal hidden features in metals. With Directed Energy Deposition (DED) technology in particular, it is crucial to know the quality of the material when adding new features to existing parts or creating functionally graded structures where material properties vary within the object. CT systems precisely measure material density, which allows conclusions to be drawn about strength and durability. They can also detect pores that could affect the performance of the material, as well as cracks that are invisible to the naked eye but could lead to catastrophic failure of the product.

Another important application of CT systems is checking the dimensional accuracy of a component to ensure that it meets the specified dimensions and tolerances. This capability is essential in precision industries. This can be achieved through a nominal/actual comparison, where the CAD drawing of the part is compared to the actual CT scan of the same part. In cases where CAD data is not available, scan data of a reference part can also be compared with the scan data of the part to be inspected.

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The most common defects that occur with DED (Directed Energy Deposition) are porosity and cracks. These can be caused by impurities that are trapped in the component during the additive manufacturing process. When analyzing the DED process, potential defects such as burr formation, voids, cracks, porosity, surface lines and increased surface roughness can be identified. Such deposition defects pose a significant challenge in both the PBF and DED processes, and addressing them is a complex and challenging task. Fortunately, by measuring and detecting these defects, the latest CT software provides valuable insight into the necessary corrections to ensure the highest quality products.

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Porosity and cavities

Porosity and voids are common problems in additively manufactured castings and molded parts. They are often caused by air or gas trapped in the metal during solidification, or by the shrinkage of the metal leaving voids inside it, known as shrinkage porosity. Since porosity consists of trapped air, it can be recognized as an area of lower density during CT analysis, which simplifies detection.

Suitable software is crucial for the precise determination of porosity due to density fluctuations. WM | PointMaster from WENZEL is a CT analysis tool that identifies porosities with a simple click. It enables the quality assurance engineer to easily measure and visualize the size, shape and possible clusters of porosities. The operator can define a range for porosity sizes and color-code them for easy identification to prevent the detection of porosities that are too small. CT is particularly effective in detecting trapped porosity in printed parts. Types of porosity include through-porosity, which extends across the entire part, and blind porosity, which typically occurs on one surface of the part. Porosity detection should focus on machined areas and other critical, high-stress sections.

The CT system has certain limitations in terms of resolution and penetration performance. The selection of the X-ray tube, the detector and the positioning of the object in the scan area significantly influence the maximum magnification and resolution. Some CT systems offer scan field extensions that make it possible to combine several fields in order to capture a larger scan area. The resolution is also determined by the precision of the turntable, which determines the slice thickness of the scan.

The voxel size (v) of a tomographic reconstruction can be calculated using the formula v = p M (1), where p is the detector pixel distance and M is the ratio of SOD (Source-to-Object Distance) and SDD (Source-to-Detector Distance). However, the actual value of v is also determined by factors such as the drift of the X-ray source, thermal expansion of the CT components, the inclination of the detector and the object slide as well as other influences.

With optimum settings, we should be able to detect and measure cavities, blockages and cracks in the 21µ to 26µ range with a high degree of certainty. With a precise angle, we can detect these even better. When measuring edges, the density transition should not be more than three pixels and the sharpness of an edge should ideally be around 3 to 4 pixels.

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Cracks and internal fractures

The search for the causes of cracking and the exact phase in the manufacturing process can be extremely complex. Finding the crack and observing its propagation through the object can provide crucial clues to solving the problem.

In many cases, high-resolution CT technology such as the exaCT system is required to precisely detect cracks in printed parts. Cracks are often irregular and can run in different directions through a component. It is particularly important to identify cracks caused by uneven cooling during the manufacturing process. These cracks, similar to porosities, can be visualized and colored using WM | PointMaster software to analyze material properties and the manufacturing process. CT technology is particularly useful for investigating crack migration in parts that have undergone tensile testing.

A notable example of the application of CT in the study of crack migration is the analysis of ballistic tests on body armor. Here it can be shown how polyurethane layers separate after a ballistic test while the overall integrity of the material is maintained and it is able to withstand projectiles such as bullets or shrapnel splinters.

WM | PointMaster software can provide sub-voxel measurements of CT scans to evaluate shape, strength and the effects of cracks on the material.

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Internal geometry deviations

CT scanning provides detailed data on the inside and outside of the most complex parts. Plastic parts often suffer deformation after demolding due to shrinkage and warping. To counteract these effects, compensated shaping is usually carried out during the injection molding process. The plastic part is first given a "wrong" shape so that after cooling, it can shrink and warp into the desired final shape and come as close as possible to the target shape.

Traditionally, the tool geometry is adapted by iterative reworking (milling, grinding or eroding). However, this process is time-consuming and can mean that the mold can no longer be reused.

With virtual deformation, the deformation specifications can be derived from simulation systems or measurement results of actually scanned components. This enables WM | PointMaster to automatically calculate the deformation result, taking into account factors such as local volumes, shrinkage and the experience of the toolmaker. The automatically calculated, deformation-compensated geometry is then converted into CAD surface models using the powerful reverse engineering functions of WM | PointMaster and WM | Quartis, into which the existing tool data is integrated.

For critical additively manufactured components, the WENZEL exaCT series offers precise measurements of internal and external geometry and reliable defect detection. Watch this short video to get a graphical overview of this valuable tool.

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