Two-step approach to improving the quality of laser micro-hole drilling on thermal barrier coated nickel base alloys

Abstract

To improve the quality and processing efficiency of laser micro-hole drilling on thermal barrier coated nickel base alloys, a two-step approach is proposed. Through-holes were initially fabricated by a millisecond laser and were subsequently refined by a femtosecond laser. In step one, the influence of different pulse shapes on hole characteristics, such as taper and delamination was investigated using a temporal pulse train modulation (TPTM). In step two, femtosecond laser trepanning with an axial feed method was used to remove the recast layer. The results showed that a ramp-down pulse shape with a short time interval could prevent the vapor pressure from suddenly dropping, which was conducive to the continuous upward flow of the melt. This pulse shape led to holes with a small taper. The delamination location was independent of pulse shape, and with the increase of maximum peak power in the pulse train, it would shift from the bond coat (BC)/substrate interface to the thermal barrier coating (TBC)/BC interface. The preset trepanning radius can be precisely determined based on the measurements of the prefabricated hole diameter and the thickness of the recast layer. Finally, the recast layer and delamination cracks could be completely eliminated by femtosecond laser trepanning. In total, about 0.4 s were spent for the prefabrication of a hole and 5 min for the refinement, which could be further reduced with higher power levels and higher trepanning speeds. The validity of this two-step approach was verified by experimental results, and finally, drilled holes of high quality were obtained.