BACKGROUND
Additive manufacturing (AM) is an emerging technology that promises a host of improvements over conventional manufacturing methods. These advantages, however, come with unique risks, especially where the fatigue performance of the component must be understood. For example, AM parts can have naturally occurring flaws that would affect the apparent material properties, such as porosity, lack of fusion, warping, and delamination. Additionally, the surface finish of additively manufactured components is significantly rougher than conventionally manufactured components, which can affect fatigue life. It is typical for fatigue-critical components to be inspected using nondestructive evaluation (NDE) techniques, but these same AM challenges (e.g., unique flaws and rougher surfaces) may produce unanticipated changes to the NDE performance. The objective of the proposed effort is to investigate two of the most pressing topics related to NDE of AM components: 1) predicting the performance of eddy current testing (ECT) as a function of surface roughness and 2) developing a process to qualify AM parts based on their microstructural properties using high-accuracy ultrasonic testing (UT) measurements.
APPROACH
There are two separate technical efforts associated with this proposed research program. First, to determine the influence of surface roughness on ECT inspections, test samples are being fabricated with slightly different build parameters to induce various surface conditions. After adding reference features (i.e., notches) to each specimen, signal-to-noise ratio (SNR) measurements will be collected using established ECT procedures and the results will be compared to surface profile measurements. This empirical data will be used to validate model-based relationships between AM surface character and ECT response that will be applicable on other structures. For the UT approach, additional test articles will be produced with AM build parameters adjusted to intentionally induce poor microstructural characteristics, such as high porosity, lack of fusion, etc. Studies have shown that damage of this type can influence the elastic properties of the material. SwRI previously developed an ultrasonic approach based on establishing acoustic resonances with a specialized sensor to measure acoustic wave speeds with a high-degree of precision. These test articles will be evaluated using this technique; the results will be compared to true elastic property values measured via destructive testing to determine the technique’s efficacy at detecting material degradations unique to AM.
ACCOMPLISHMENTS
The set of AM specimens has been designed and is now being fabricated. Furthermore, a custom UT transducer has been designed, built, and tested for evaluation of the specimens. All of the anticipated objectives of this program have yet to be achieved, as this is an ongoing program. The most significant accomplishment will be developing a knowledgebase at SwRI related to the NDE of AM materials. More specifically, relationships will be developed to predict ECT inspection performance as a function of AM surface conditions. This capability will be marketed to assist probability of detection (POD) studies and inspection performance modeling. This work will complement other work being done at SwRI to determine the effect of surface roughness on fatigue life and to determine the amount of post-production surface treatment required to reduce roughness needed to achieve the desired fatigue life and/or inspection interval. In addition, development of the ultrasonic measurement technique will result in intellectual property related to component qualification that positions the Institute to support the AM industry both by manufacturing NDE systems for production use as well as providing qualification services for AM service providers.