Improving 3D printing with laser ultrasound

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3D printing technologies have made possible the rapid prototyping of previously unfeasible structures. There are several types of 3D printing methodologies that are compatible with a range of materials, including everything from soft polymers to metals and foods.1

Image Credit: Alex_Traksel/Shutterstock.com

One approach to 3D printing is laser powder bed fusion. Laser power bed fusion printing, like all 3D printing methods, involves first taking a CAD model and cutting it into thin layers. The size of these slices or layers dictates the amount of powder that will be deposited at each stage of the additive manufacturing process. Thinner layers allow better resolution of structures but take much longer to print.

Once the layers are set, each step in the printing process involves laying down a layer of powder, then scanning the area with a powerful laser beam to fuse the powder together to form a continuous region of solid metal.

Laser powder bed fusion is one of the most common additive manufacturing methods for metals because a good degree of precision can be achieved and it is one of the most affordable printing methods.

However, while very good results can be achieved with the laser powder bed fusion method and the technique is particularly well suited to printing very complex structures, the method can leave sub-surface cracks in the material. .

One of the biggest obstacles to the adoption of additive manufacturing methods in industry is uncertainty in product control.3 and one of the main challenges for metal powder bed fusion approaches, in particular, is a more rigorous understanding of the physics involved in fusion processes so that more accurate computer models can be used to optimize fusion conditions. treatment.2

Quality control

The formation of cracks below the surface is problematic because they reduce the strength and stability of the structure. Such cracks are not always observable by visual inspection, so different types of imaging methods should be used as part of post-manufacturing quality control.

Methods such as X-ray tomography are commonly used to inspect the internal structures and voids of 3D printed objects.4 Most 3D prints use internal voids in the structure to reduce the mass of the final object, saving material and reducing print times. As high energy X-rays can have good penetration depths even for metals, they can pass through the object to render a complete 3D image of the internal structure.

Although X-ray tomography is an excellent high-resolution, non-destructive method and can provide detailed structural information on any defects present, it is expensive and means parts must be small enough to fit in the scanner. The use of X-radiation also involves additional safety considerations and makes it difficult to make the technique more portable for in situ measurements.

Recent work from Lawrence Berkeley National Laboratories has explored how the use of surface acoustic waves generated during ultrasonic measurements could be used for non-destructive flaw analysis.5 The team was able to demonstrate that surface acoustic waves could successfully detect surface and internal defects which could also be detected by optical microscopy and X-ray tomography respectively.

Acoustic waves

Images are created from ultrasound measurements by reconstructing the intensity of reflected ultrasound waves relative to incident waves. Depending on the density of the material, different amounts of absorption will occur, creating regions of contrast in the image.

Some of the advantages of ultrasound imaging methods include their robustness, affordability, and ability to deliver 3D data sets at much faster rates than X-ray tomography methods.

Ultrasound machines are also more portable and easily scalable to handle larger sample sizes. The improved data acquisition rate is important for use in additive manufacturing processes, as it would potentially mean that defect analysis could be integrated into the manufacturing process itself.

The team generated their surface acoustic waves to perform the ultrasonic measurements using a high-powered Nd:YAG laser that acts as a thermoelastic source. Laser sources for ultrasound are ideal because the characteristics of the source tend to be highly controllable, and by using different optical configurations the team could control the shape of the output light for imaging.

Using these new surface acoustic wave measurements, the team was able to detect different types of defects such as spatter and breaks between lines – common surface defects – as well as voids in the substructure where the material had not been successfully filed.

Diagnostic power

Realizing the full potential of additive manufacturing as a fully automated process requires diagnostic methods that are fast enough and easy to analyze. They can work “on the fly” with little or no human guidance.

One of the limitations of surface acoustic wave methods is that the penetration depth of the wave is much lower than that of X-ray radiation and only voids 200 μm below the surface can be detected.

However, the ability to detect problems around melt lines – the region where each layer is deposited during the additive manufacturing process – and improved measurement speed make ultrasound an attractive prospect for future developments in process inspection. on line.

References and further reading

  1. Nachal, N., Moses, JA, Karthik, P., & Anandharamakrishnan, C. (2019). Applications of 3D printing in food processing. Food Engineering Reviews, 11(3), 123–141. https://doi.org/10.1007/s12393-019-09199-8

  2. King, WE, Anderson, AT, Ferencz, RM, Hodge, NE, Kamath, C., Khairallah, SA and Rubenchik, AM (2015). additive manufacturing of metals by laser powder bed fusion; the challenges of physics, calculation and materials. Applied physics exams2(4), 041304. https://doi.org/10.1063/1.4937809

  3. PWC, 3D Printing and the New Shape of Industrial Manufacturing (PricewaterhouseCoopers LLP, Delaware, 2014) https://www.pwc.se/sv/pdf-reports/3d-printing-and-the-new-shape-of- industrial -fabrication.pdf
  4. Khosravani, MR, & Reinicke, T. (2020). On the use of X-ray computed tomography in the evaluation of 3D printed components. Nondestructive Evaluation Log, 39(4). https://doi.org/10.1007/s10921-020-00721-1
  5. Harke, KJ, Calta, N., Tringe, J. & Stobbe, D. (2022). Laser-based ultrasonic interrogation of surface and subsurface features in advanced manufacturing materials. Scientific reports, 12(1), 1–11. https://doi.org/10.1038/s41598-022-07261-w

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