In-situ technique for measuring the absorption during laser surface remelting
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The authors wish to express their appreciation to Noranda Technology Centre, Pointe Claire, PQ, Canada for performing the chemical analyses of the specimens.
REFERENCES 1. R.R. Irving: Iron Age, Dec. 1982, pp. 44-45. 2. M. Nageswararao, C.J. McMahon, Jr., and H. Herman: Metall. Trans., 1974, vol. 5, pp. 1061-68. 3. J. Kameda and C.J. McMahon, Jr.: Metall. Trans. A, 1980, vol. IlA, pp. 91-101. 4. J. Kameda: Metall. Trans. A, 1981, vol. 12A, pp. 2039-48. 5. A.H. Ucisik, C.J. McMahon, Jr., and H.C. Feng: Metall. Trans. A, 1978, vol. 9A, pp. 604-06.
In-Situ Technique for
Measuring the Absorption during Laser Surface Remelting A. FRENK, A.F.A. HOADLEY, and J.-D. WAGNII~RE Considering a metal irradiated at time t with the 10.6 /~m light of a CO2 laser beam of incident energy E0(t), the transmitted energy at a depth z, E(z, t), is given by Lambert-Beer's law: E(z, t) = Eo(t) (1 - R) exp (-t~z)
[1]
where R is the reflectivity and a the absorption coefficient. As a is very high for metals ( - 1 0 s m-l), the absorption depth (1/t~) is very small. Eq Therefore, the efficiency of optical coupling is determined by (1 - R) which is called, somewhat confusingly, the absorptivity or the absorption ft. For a given wavelength and for intensities lower than the critical intensity Ic, where plasma formation occurs, fl is an optical property of the material, tEj Theoretical approaches, such as the Drude's model, TM or semiempirical ones, for example, that of Bramson, t41 can be used to predict the absorption in the solid state. Different methods have been developed for an experimental determination of/3, using either calorimetry [5,6,71 or ellipsometry, t81 Only one of these techniques tSj yields data for the liquid state but under conditions very different (i.e., ultrahigh vacuum and using a stationary low-power laser beam (3 to 5 W) irradiating a sample heated with an oven) from usual laser processing conditions. However, the absorption value for the liquid state is certainly the main parameter in laser remelting, alloying, and cladding, where the beam lands essentially on the liquid phase. The aim of this communication is to present a calorimetric method which has been used to determine the mean absorption during the dynamic process of remelting. Under specific processing conditions where most of the laser beam impinges on the liquid pool, this technique yields the absorption value in the liquid. It can also be extended to multitrack experiments or to other laser processes, such as laser surface alloying or cladding, which is not possible with stationary methods, such as ellipsometry. The laser-treated sample is used as a calorimeter, and the determination of the energy input is obtained through its temperature rise. This is possible due to the high thermal conductivity of metallic samples relative to their external cooling by exchange with the atmosphere. Thus, shortly after the laser has passed over the specimen, the absorbed energy conducted from the laser-heated zone is redistributed to give a uniform temperature in the sp
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