Using Ultrasonic Waves to Analyze Residual Stress in 3D Printed Metal Parts

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Researchers from the Czech Republic and Brazil have come together to highlight ultrasonic testing for stress analysis in ‘Residual stress analysis of additive manufacturing of metallic parts using ultrasonic waves: State of the art review.’

Residual stresses (RS) are still a notorious problem in metal 3D printing, with the rapid heating and cooling resulting in potentially numerous defects, part failure and even damage to an additive manufacturing (AM) machine. Other factors also come into play such as grain size, porosity, voids, shape or structure, supports, and processing parameters. In turn, RS can cause the following issues in printed parts:

  • Deformation
  • Poor fatigue resistance
  • Critical failure during operation
  • Lower chemical resistance
  • Lower magnetization
  • Decreased strength

Direct consequences of residual stresses in AM parts – (a) distortion and separation from the base plate; (b) crack formation [10].

For these reasons, a number of methods are being used to avoid or compensate for RS during printing, ranging from simulation of print scenarios in order to optimize print setup and process parameters to the use of sensors to attempt closed-loop process control, thus ensuring proper printing throughout the build with a system that self-corrects. Then, once a print is completed, post-processing treatments are used to reduce the effects of RS in a finished part.

Process variables (PBF methods) influencing RS (adapted from Ref. [30]).

Even with all of the aforementioned methods to address RS in metal parts, there remains a need to be able to examine components in a non-destructive once they’ve been printed to ensure they meed specifications. Researchers Acevedo et. al, highlight the potential for ultrasonic testing (UT) for measuring RS both during and after a build. The use of sound in testing and characterizing materials is age-old and can be extremely valuable in locating issues like distortion, delamination, or structural failure. As a non-destructive testing method, UT involves sending short pulses of ultrasonic waves into the material being tested to detect internal flaws.

Residual Stress measurement techniques.

The authors suggest a number of benefits to the technique, including accuracy, speed, repeatability, affordability, unlimited types of materials that can be tested, minimal influence from temperature and the fact that it is not destructive, so that it can even be incorporated into monitoring systems built into 3D printers. Its drawbacks, however, include limited spatial resolution, issues with differentiated multi-axial stresses. It is more suited to measuring RS in the entire part, rather than specific areas.

Typical configuration for a UT method using Spatially Resolved Acoustic Spectroscopy.

This compares to other testing techniques, such as hole drilling (HD) and X-ray diffraction. While HD and X-ray diffraction are still the most common methods for measurement of RS—offering precision and reliability for industrial users—there are still constrictions in terms of small sample size, rough surfaces rather than the desired polish for measuring, and limitations with X-rays overall. HD measurements may also be destructive, as well as posing numerous errors.

The authors highlight a number of research developments currently underway dedicated to the use of UT in testing for RS, including Spatially Resolved Acoustic Spectroscopy (SRAS), which uses two lasers to inspect surface and near subsurface features, and a variety of other laser-based methods. They suggest that—while most machines today rely on X-rays, infrared cameras, and high resolution cameras—these UT techniques could be incorporated into metal AM systems to perform in-situ monitoring of parts, stating:

“This method has great potential to be employed in the next generation of metal-AM machines, focusing on the measurement of RS, voids, roughness, and defects. The most remarkable challenges remain in the field of data exchange, surface effects and spatial resolution. Namely, the optimization of the link between UT apparatus and the AM hardware is required.”

The biggest challenge for UT as a quality control mechanism is the complex geometries of AM parts. As users continue to 3D print parts that are expected to be strong and highly functional, they must also be conscious of the need to be aware of the effects of printing parameters and update accordingly. The connection between material properties must be analyzed also for improving quality, precision, and efficiency in production.

[Source / Applications: ‘Residual stress analysis of additive manufacturing of metallic parts using ultrasonic waves: State of the art review’. Feature image: Olympus.]

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