Femtosecond Laser Ablation Regimes in a Single-Crystal Superalloy
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TRODUCTION
FEMTOSECOND (fs) lasers, with pulses in the 10 second range, offer new possibilities for micromachining, local spectroscopy, and nondestructive evaluation of structural materials.[1–7] This is possible due to the limited collateral damage that occurs with ultrashort fs pulses, compared to that experienced during ablation with conventional nanosecond and picosecond laser systems.[8–13] For example, it has been shown that high signal-to-noise spectroscopic signals can be obtained by ablating 100 ng of material, leaving no residual melted zone.[6] Thus, it may be possible to machine high precision micron-scale features while simultaneously gathering chemical information. For multiphase metallic materials, an improved understanding of the ablation and damage mechanisms due to fs ablation is needed to realize the characterization and fabrication benefits of these new laser systems. Nickel-base single crystals serve as good model metallic systems for ablation mechanism studies, due to the fact that they are two-phase materials containing no high-angle boundaries. Additionally, because singlecrystal superalloys with intermetallic or ceramic coatings are used in geometrically complex, internally cooled turbine airfoils in aircraft engine and power generation systems, the potential benefits of micromachining and local spectroscopy are readily apparent. -15
S. MA, B. TRYON, Research Fellows, S.M. YALISOVE, Associate Professor, and T.M. POLLOCK, Professor, are with the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109, USA. Contact e-mail: mashuwei@ umich.edu J.P. McDONALD, Postdoctoral Candidate, is with the Applied Physics Program, University of Michigan, Ann Arbor, MI 48109, USA. Manuscript submitted August 10, 2006. Article published online August 11, 2007.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Laser-material interactions during fs ablation in metallic materials are not well understood, especially for nickel-base alloys. Laser operating parameters expected to be important to material removal include laser fluence, pulse duration, wavelength, pulse repetition rate, and the focused beam size.[14–16] Of these parameters, the pulse duration has the most significant influence on local damage.[17] Preliminary studies of collateral damage produced by fs laser machining of superalloys have revealed very limited collateral damage following ablation from thin foils.[1] The present study provides a detailed analysis of fs laser ablation of bulk superalloys under a range of laser fluences. From this analysis, ablation rates are determined and ablationinduced damage morphologies are examined in detail.
II.
EXPERIMENTAL
The material investigated in this study is a secondgeneration single-crystal superalloy, CMSX-4, provided by Precision Castparts Corporations. The nominal composition of the alloy CMSX-4 is given in Table I. Samples were solution treated at 1321 C for 2 hours and subsequently air cooled. To achieve a more homogeneous microstructure and composition, the alloy
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