Optical Emission Spectroscopy for Metallic Bond Quality
The industrial components used in aviation engines, ships, commercial vehicles, offshore drilling and tool and mold construction see a tremendous amount of wear and tear in their lifetimes. Critical components are expected to last, despite extreme operating conditions and regular mechanical abrasion. Laser Metal Deposition (LMD) is gaining ground as a method to extend the life of these parts through specialized coatings and repair, thereby saving money, reducing maintenance and improving safety. Optical emission spectroscopy using modular components offers the data needed to provide real-time feedback to LMD systems, optimizing the coating process and quality.
What is LMD and How Does it Work?
In Laser Metal Deposition (LMD), a laser facilitates the addition of metal layers to the surface of an object. This is useful for repair or restoration of damaged areas, or to adjust the dimensions of an object already fabricated. Also, LMD can be used to add coatings that improve performance of a component by increasing hardness, changing conductivity characteristics, improving chemical resistance or reducing corrosion.
LMD uses a laser to melt a small pool on the surface of the part under treatment. Metal powder is simultaneously sprayed into the laser focus, melting and bonding with the part’s surface to add a new layer of material to the part. The metal(s) added may be the same as the base object material or different, depending on the intended purpose. One benefit of LMD is that it creates a metallurgical bond far superior to that of spray welding or plating techniques, making its expense well justified when compared against long term manufacturing cost savings.
Precise control of the laser focus allows layers of metal to be built up only in the regions needed and to the required thickness, but how can control of the deposition quality be automated? A team at the University of Twente in the Netherlands has been looking at exactly this problem. They studied LMD of stainless steel using a 1064 nm Nd:YAG laser, varying the laser power and studying the light emitted during deposition. It is believed that a very thin plasma is generated at the surface during LMD. Optical emission spectroscopy can be used to measure the properties of that plasma, providing information that can be used to optimize the deposition process to minimize the melting of the substrate and ensure good metallic bonding.
Making Sense of the Spectra
The researchers collected optical emission spectra with a collimator attached to the laser head, focusing the light into an optical fiber for delivery to a high resolution HR series spectrometer. To achieve a resolution of 0.15 nm (FWHM) over the range 400-600 nm, the spectrometer was configured with a 5 µm slit and 1200 line/mm H9 grating. Spectra were acquired every 5 ms, displaying both emission and absorption lines on a broad background spectrum resulting from blackbody radiation. Information about the onset of metallic bonding was obtained using emission lines to calculate the electron temperature (Te), and absorption lines to calculate the intensity ratio (IR).
The electron temperature of a plasma, Te, is often useful to characterize laser material interaction, and can be calculated using the ratios of several emission lines. In this work, the region from 520-540 nm was particularly rich with chromium and iron lines, as can be seen from a spectrum in which the blackbody background has been subtracted. Four well-resolved chromium (Cr I) lines of similar intensity were chosen to create a Boltzmann plot, the slope of which was used to determine Te.
Another plasma characterization calculation known as the intensity ratio (IR) compares the intensity of an absorption line to the background level at a nearby wavelength. For this measurement, an iron line at 588.91 nm was used, yielding IR values as laser power was varied.
Both Te and IR were recorded as laser power was varied, from low power through stable deposition. Results were compared to cross sections taken of the material at each stage to look for correlations between changes in Te and IR and the metallic bond formation. The researchers found that the onset of metallic bonding was clearly related to a sudden decrease in the intensity ratio, providing one important metric for automating LMD. Achieving stable deposition with minimal melt depth of the base material is also important for high-quality LMD to minimize mixing of the coating material and substrate. While both Te and IR values correlated with melt depth, the variation of Te with melt depth was larger in magnitude, and thus would be recommended as the automated control indicator for melt depth.
Laser metal deposition allows for repair and enhanced coating of critical parts operating in harsh environments, increasing their lifetime and reducing long term costs substantially. Optical emission spectroscopy using high-resolution fiber-coupled spectrometers offers a quantitative method that can be automated for use in online monitoring and optimization of the metallic bonding process. By controlling the onset and extent of metallic bonding, the highest quality coatings can be applied, thus maximizing their benefits.
- Ya, Wei, et al. “Spectroscopic monitoring of metallic bonding in laser metal deposition.” Journal of Materials Processing Technology 220 (2015): 276-284.
- Locke, David. “Laser Metal Deposition Defined.” Industrial Laser Solutions. PenWell Corporation, 1 Nov. 2010. Web. <http://www.industrial-lasers.com/articles/print/volume-250/issue-6/features/laser-metal-deposition.html>.