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

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.]

The post Using Ultrasonic Waves to Analyze Residual Stress in 3D-Printed Metal Parts appeared first on 3DPrint.com | The Voice of 3D Printing / Additive Manufacturing.

University of Cincinnati say ice could help the NDT of metal 3D printed parts

Francesco Simonetti, an aerospace engineering professor at the University of Cincinnati,  and his undergraduate student Michael Fox have developed a new method of non-destructive testing (NDT) for inspecting metal 3D printed components. Detailed in a study for NDT & E International the technique, termed cryoultrasonics, unusually combines ice and ultrasound to overcome inadequacies associated with other […]

CT Scanning Proves to be an Effective Method for Qualifying 3D Printed Parts

Many things can go wrong with an additively manufactured part, and those things are not always visible. Particularly in powder-based 3D printing, there are several things that can happen during the print that cause issues such as cracking and voids, which may be inside the part and invisible to the eye. There are ways, however, of discovering these defects without having to break apart the part and look inside. In a new paper entitled “The Role of Computed Tomography in Additive Manufacturing,” which you can access here, a group of researchers argue that computed tomography, or CT scanning, is the most effective way of performing quality checks on 3D printed parts.

Complex parts are especially challenging to inspect for quality purposes, as they can contain internal channels or structures that are prone to voids or inclusions, which are unmelted particles or powder residues. These flaws are difficult for traditional non-destructive testing, or NDT, techniques to fully assess. These techniques include ultrasonic, infrared, eddy current, radiographic inspection, and light-based technologies. Optical methods of defect detection can only detect flaws at the surface or through a surface opening.

“Eddy-current testing and ultrasonic techniques can detect defects within the volume if they are not located very deep inside the testing sample, but the one drawback is the limited spatial resolution of detection, which is in the millimeter range or some fraction of millimeters in the most optimal situations and for even more limited depths into the surface,” the researchers explain.

The best method for nondestructive inspection of complex geometries inside a part, they argue, is X-ray CT, which has a resolution from millimeter to micrometer ranges, and even sub-micron levels in some cases. In fact, they continue, in many cases it is the only viable option. It can detect cracks, porosity, dimensional deviations from CAD models, and powder residues or inclusions.

“In general, tactile CMMs (coordinate measuring machines) or optical measuring instruments like laser scanners are limited to the measurement of the external surface of an AM part and can provide additional measurements for partial qualification of CT measurements,” the researchers add. “In addition, tactile CMMs can produce compressive stresses and friction during sliding that could produce wear at the surface. In contrast, X-ray CT eliminates the above difficulties because it is a non-contact technique that can access internal features.”

X-ray CT analysis of a 3D printed turbine blade

The importance of qualification for additively manufactured parts cannot be overstated. If a part is being used for an aerospace application, for example, it’s absolutely critical that that part is perfect, with no hidden flaws. There are many methods for checking the quality of parts, but most of them come up short in terms of detecting flaws that are hidden deep inside a part. The paper goes on to highlight a case study in which X-ray CT was able to detect minute deviations in dimension from the CAD model to the final part, as well as material inclusions in the internal cavities.

It can be challenging to use CT technology with metal parts, as metal parts can scatter X-rays, disrupting CT reconstructions and producing unwanted artifacts in the data. The solution, the researchers say, is to use a 2D fan beam of X-rays and a linear detector, which can reduce the scattering. Overall, they conclude, CT technology is an effective method of non-destructive testing.

Authors of the paper include Herminso Villaraga-Gómez, Christopher M. Peitsch, Andrew Ramsey and Stuart T. Smith.

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Motorbikes, ships, scandium and NDT: inside Additive Manufacturing journal Oct. 2018

As seen from the highly efficient 3D printed batteries at Carnegie Mellon University (CMU) and Missouri University of Science and Technology, the October 2018 edition of ScienceDirect’s Additive Manufacturing journal is now in progress. Volume 23 currently includes the work of 9 innovative research groups from leading institutions all around the world, including projects from the Fraunhofer Institute for Laser Technology (ILT), the University of Cádiz and […]