Plasma optical emission spectroscopy of laser cladding for process control and oxidation behavior of laser clad NbAl alloys
Tewari, Sudhir Kumar
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Permalink
https://hdl.handle.net/2142/23044
Description
Title
Plasma optical emission spectroscopy of laser cladding for process control and oxidation behavior of laser clad NbAl alloys
Author(s)
Tewari, Sudhir Kumar
Issue Date
1995
Doctoral Committee Chair(s)
Mazumder, Jyotirmoy
Department of Study
Materials Science Engineering
Discipline
Materials Science Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
Spectroscopic diagnostics based on the plasma optical emission spectroscopy has been employed in this investigation to infer composition, thickness, and temperature of the clad during laser cladding of Nb-Al-Hf alloys on Nb substrate. Isothermal oxidation behavior of the laser clad NbAl$\sb3$ based alloys containing: up to 1.0 at. % B; 1.5 at. % Hf; and 3 or 6 at. % Ti has been investigated at 800$\sp\circ$C, 1200$\sp\circ$C, and 1400$\sp\circ$C.
The results indicate that the intensity of the optical emission from plasma during laser cladding is affected by the variation in the thickness of the clad. Temperature of the clad is inversely proportional to clad thickness. Composition of the clad is affected by the local variation in the temperature of the clad. Both continuum and characteristic line emission may be used to monitor variation in clad thickness and laser power. Intensities of Nb-I and Hf-I lines are directly proportional and that of Al-II is inversely proportional to the thickness of the clad. The Nb-I/Al-II ratio and continuum intensities are directly proportional to laser power and to clad thickness. Nb-I/Al-II and Nb-I/Hf-I ratios increase, and Al-II/Hf-I ratio decreases, approximately linearly with increase in the concentration of Nb in the clad. The physical and chemical states of the alloying elements in the initial powder mixture and the constitution of the phase in the clad affect the correlation between the intensity ratio and composition.
The results indicate that the alloys containing Al-rich interdendritic phase form an outer layer exclusively of $\alpha$-Al$\rm\sb2O\sb3$ at all three test conditions irrespective of the alloying additions. Addition of Ti and/or presence of Al-rich interdendritic phase suppress the disintegration of NbAl$\sb3$ at 800$\sp\circ$C caused by rapid oxidation along the grain boundaries known as 'pest phenomenon.' The oxidation resistance degrades as the fraction of Nb-rich phases (Al $<$ 75 at. %) in the alloys increases and a mixture of $\alpha$-Al$\rm\sb2O\sb3$ and NbAlO$\sb4$ forms upon oxidation. Alternating layers of $\alpha$-Al$\rm\sb2O\sb3$ and NbAlO$\sb4$ form at 1200$\sp\circ$C and 1400$\sp\circ$C only in NbAl$\sb3$ + 3 at. % Ti alloy containing Nb-rich phases.
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