Introduction: The goal of this efforts was to determine a method to imbue a surface with a unique pattern, such that the material can be identified upon future inspection. Engraving, a common subtractive technique for marking surfaces, is easily identifiable, replicable, and removable. Surface alternations using selective material deposition, conversely, can provide a persistent marker that may only be detectable or readable through the use of compositional characterization. This work was focused on the production of embedded signatures in the material, in the literal sense, utilizing deposited material that was different than the base material. Successful signatures were produced using a laser beam to embed a predetermined pattern into a base plate, and are currently undergoing characterization to determine the quality of the deposit and the feasibility of detection.
Experimental Procedure: Two processes, electron beam additive manufacturing (EBAM) and laser surface deposition were utilized to produce the surface variations in the form of visible words. A 30kV beam energy was used to deposit Ti-6Al-4V wire, 1.575 mm in diameter, on a Ti-6Al-4V plate using a Sciaky EBAM; the bead size produced was determined to be too large, and optimization would not minimize the bead size enough to meet the objectives of this project. Due to the improved control and deposition size, a laser process was implemented using an EOS M290 powder bed additive manufacturing machine. As a safer alternative to powder, a 0.015 mm Ti foil was used as the deposition material, and the laser was used to selectively implant the titanium on to a stainless steel plate in a specific pattern; the remaining foil was easily removed by hand upon completion. Characterization of the surface, and of the plate cross-section, is currently ongoing to determine the persistence of the deposition, the level of mixing between the foil and base plate, and the ability to decipher the pattern. In addition to laboratory characterization methods such as scanning electron microscopy and energy dispersive spectroscopy, handheld X-Ray fluorescence (XRF) is also being performed to determine the viability of field techniques. Pending these results, additional laser trials will be performed to implant patterns that are only machine readable (e.g., a QR code).
Results and Discussion: The depositions produced via the EBAM and the EOS M290 are shown in Figure 1 (Figure available upon request). The EBAM deposition was determined to be poor due to the size of the bead and the lack of directionality control of the beam, as only a linear bead was possible. Both of these limitations were overcome by the EOS M290, as shown in Figure 1b.
Figure 1: (a) The bead produced in the Sciaky EBAM, approximately 3 mm in height and 11 mm in width and (b), the signature produced in the EOS M290, approximately 64 mm in width across the entire signature. Full characterization of the laser deposition is still ongoing to determine the overall quality, but the control offered by laser deposition is promising for intricate patterns such as QR codes. Utilizing machine readable codes, rather than patterns visible and readable by the human eye, can be useful for marking parts in a way that is not easily detectable or replicated. Furthermore, the use of discernibly different metals in the deposited foil and the base plate provides additional means for protection of the marker. First, the type of material deposited may provide further information about where the material or part was manufactured. Second, the need for additional technology, such as a handheld XRF, provides an additional level of security to the marker. Finally, even if a marker is removed via a process such as grinding, it is possible that some of the deposited material may remain behind; this assumption will be tested as part of the current characterization effort. Conclusion: Although characterization efforts are still ongoing, preliminary results using the laser deposition process show promise for producing a persistent, discernible surface alteration. To date, EBAM has not shown the level of detail and control that can be obtained using the laser process, but improvements in electron beam welders may close this gap. Follow-on efforts will focus on the feasibility of using a machine-readable pattern to mark parts, and the ability to detect a marker after visible removal, as well as parametric trials to evaluate laser beam parameters and base or foil material changes.
Keywords: EBAM, electron beam, laser beam, powder bed additive manufacturing, surface alterations, deposition
Acknowledgments: This work was supported by the Office of Defense Nuclear Nonproliferation Research & Development (NA-22) of the National Nuclear Security Administration.