| Abstract Scope |
Introduction:
The identification of metal parts is vital for the prevention of counterfeiting, tracing part history, and reducing the potential for future in-service failures, but is often performed via easy to replicate and remove surface modification (i.e. cosmetic markings). High energy density deposition processes, utilizing laser or electron beam, can provide the robust surface identification necessary for surviving counterfeiting operations and in-service conditions. This is accomplished by depositing small quantities of an alternate metal in unique patterns, with the goal to create a persistent and difficult to replicate tag that is detectable. Successful signatures were created using laser beam welding, demonstrating the ability to produce visible signatures in pre-defined patterns using titanium foil. Recent characterization has shown that these signatures, although detectable, could not withstand defacing operations. Additional signatures have been produced to improve persistence of the tag using both electron and laser beam methods, and to evaluate the detectability of different patterns. Finally, non-destructive techniques, specifically eddy current, will be evaluated to determine if these signatures can affect the magnetic properties of the material as a secondary means of detection.
Experimental Procedure:
<U>Laser Signatures:</U>
All laser beam signatures were generated using a 0.015mm titanium foil on a Type 304L or 316L Stainless Steel (SS) base plate in an EOS M290 system. Previously generated laser signatures were evaluated using electron backscatter diffraction (EBSD) to further characterize the mixing between the base plate and the titanium deposit; without full mixing, the titanium signature is less persistent. A destructive test was performed to validate this conclusion by removing a portion of the signature using 320 grit silicon carbide sandpaper; this technique was chosen to mimic realistic situations that may occur during counterfeiting. Handheld x-ray fluorescence (XRF) was performed after most of the signature had been visibly removed. Additional laser signatures were produced to improve the signature quality. The parameters of interest were laser power, speed, beam pattern, and number of passes.
<U>Electron Beam Signatures:</U>
Electron beam signatures are currently in process on a relatively new ProBeam electron beam system available at LANL. The new signatures are generated using the 0.015mm titanium foil on a 304L SS base plate, using a built-in pattern to evaluate if complicated and unique details can be generated on the system.
Results and Discussion:
Characterization of previously generated laser signatures showed adequate mixing between the foil and the base plate, indicating that this process was successful. The EBSD results indicated that mixing was not uniform throughout the solidified melt pool, though, as grain size was highly heterogeneous. The success of this deposition process was further complicated by the XRF results, which showed that titanium was no longer detectable after visually reducing the signature using sandpaper. The sandpaper exercise was not expected to completely eliminate the measurable titanium, as some of the signature was still visible. The resolution of the XRF may not be enough to detect the minute quantity of titanium that was left behind after visual removal.
The lack of persistence from the prior laser beam signatures demonstrates the need for homogeneous, deep mixing, such that enough titanium would be left behind after visual destruction for handheld surface characterization techniques (e.g., XRF) to detect. The beam parameters chosen for follow on signatures were selected to improve the depth of penetration and mixing quality, and characterization of the new signatures is ongoing. Additionally, a non-destructive eddy current inspection technique will be used to detect any base metal changes that may be occurring during the deposition process. Preliminary results indicate that eddy current inspection is capable of detecting the implantation of titanium in the SS base plate, even after visual removal of the signature. This may serve as a secondary means of detection in the event the titanium signature cannot survive destructive exams.
Conclusion:
Previously reported selective deposition laser beam results showed success, but further evaluation showed a lack of persistence in the signature. Prior characterization efforts did not fully capture the lack of homogenization of the melt pool (XRF of the surface prior to titanium removal), or the lack of signature robustness. Improvements in the beam parameters are ongoing, and electron beam selective deposition is being evaluated based on new system capabilities that are expected to improve upon prior EBAM studies. Selective deposition is considered a viable technique for imbuing the surface of a metal with a unique and detectable signature, but the presented work aims to improve the persistence and quality of this signature. Finally, eddy current studies are ongoing for detection purposes, as a secondary means to detect material changes beyond chemical signatures. The combination of surface deposition techniques, characterization, and detection strategies is expected to improve the quality of the signature.
Keywords:
EBAM, AM, electron beam, laser beam, surface alteration, deposition, eddy current.
Acknowledgements:
The author would like to acknowledge the support of Will Winter, Stephen Wiest, Andy Duffield, Rose Bloom, Kayla Molnar, and Cheryl Hawk.
This work was supported by the Office of Defense Nuclear Nonproliferation Research & Development (NA-22) of the National Nuclear Security Administration |