Femtosecond Laser Micromachining of Single-Crystal Superalloys

Abstract

Investigations on femtosecond laser micromachining of single crystal superalloys with and without plasma-sprayed thermal barrier coatings were conducted under laser fluences ranging from 0.1 J/cm 2 up to 160 J/cm 2 . Micromachining was carried out in air using a titanium:sapphire laser system ( = 780 nm) operating at a repetition rate of 1 kHz and delivering individual pulses of ~150 fs duration. The ablation threshold of the single crystal superalloy was determined as 203 ± 20 mJ/cm 2 . Laser-induced damage was examined by means of scanning electron microscopy and transmission electron microscopy. These studies indicate a complete absence of any melting, recast layers, heat-affected zones or microcracks in the vicinity of the machining area. The only form of damage observed in the single crystal superalloy machined near or above the ablation threshold was a laser-induced plastically deformed layer with a maximum extent of ~5 m. Machining through ceramic thermal barrier coatings on a superalloy produced no delamination along the superalloy/coating interfaces or cracks within the TBC or bond coat. The residual roughness of the machined surface was in the sub-micron range. The present study suggests that femtosecond laser micromachining is a very promising technique for production of finescale features in multi-layer material systems for aerospace and power generation components.

Femtosecond Laser Ablation Regimes in a Single-Crystal Superalloy

“…Electron diffraction analyses in the vicinity of the laser-machined hole indicated no loss of single-crystal structure or any presence of an amorphous layer. [26] This provides direct evidence that the material removal process was mainly accomplished by vaporization. The use of an fs laser at low fluence for micromachining of small-scale features is clearly desirable from the point of view of minimizing collateral damage.…”

“…In the low fluence regime (/ th1 to 18/ th1, i.e., 0.3 to 5.3 J/cm 2 ), material ablation is dominated by optical absorption, and material ablation occurs with no detectable melting of the sample surface and no microcrack formation. The extent of microstructural degradation in this regime has recently been investigated [26] by TEM on pre-jet-polished TEM foils micromachined by a fs laser. Electron diffraction analyses in the vicinity of the laser-machined hole indicated no loss of single-crystal structure or any presence of an amorphous layer.…”

Development of Ultrasonic Fatigue for Rapid, High Temperature Fatigue Studies in Turbine Engine Materials

“…Notches of depth < 30 m and width 10 m could be made using this procedure. Details of the laser notching procedure and its effects in aluminum alloys and nickel-base superalloys could be found elsewhere [27,28]. An important feature of the femtosecond laser micromachining approach is the elimination of any melting or heat affected zones typically encountered with conventional laser drilling techniques [28].…”