Effect of beam angle on heat affected zone, recast and oxide layer in laser drilling of thermal barrier coated nickel alloys

Industrial applications of laser drilling include the production of cooling holes at acute angles in certain parts of the aero-engine components. These parts are usually covered with Thermal Barrier Coatings (TBC) to protect them from reaching excessive temperatures in hot engine environments. The acute angle TBC drilling brings three major simultaneous complications to the process. These are (i) multi-layer drilling, (ii) non-symmetrical geometry and melt ejection, and (iii) increased depth of drilling. In a previous investigation by the authors, delamination of TBC was addressed as a main problem of acute angled hole drilling and mechanisms involved were studied. In the present study implications of these difficulties on the hole quality is investigated through a comparative study of vertical and acute angle drilled holes. Characteristics of recast layer, heat-affected zone (HAZ), oxide layer and TBC delamination are investigated. Variation of recast layer and HAZ with depth of the hole is evaluated. Results for vertical and inclined holes are compared. Both recast layer and HAZ extent was found increasing with decreasing drilling angle to surface. Numerical simulation of single pulse laser heating of TBC Nimonic 263 was carried out at acute angles with assist gas consideration. Results from the simulation suggested that total heat transfer rate is higher on leading edge side of the heated region which can be accounted for the increased HAZ. On the other hand, the larger recast layer on trailing edge was attributed to the higher temperature achieved on trailing edge side due to skewed heating.