Objective  In the machining process of film cooling holes on turbine blades using ultrafast lasers, it is crucial to protect the opposite wall from laser damage. Typically, protective materials are filled into the blade cavity for this purpose. However, researchers have found that the use of these materials leads to a longer time for complete formation after hole penetration. If the elapsed time is insufficient, the hole exit will not be circular, and the quality of the hole wall will deteriorate. Conversely, if the processing time exceeds the optimal duration, the ultrafast laser may penetrate the protective materials, resulting in damage to the opposite wall. Additionally, the hole depth, aperture, and inclined hole angle of the film cooling hole vary in different areas of the blade, making it challenging to determine the optimal processing time accurately. Therefore, precisely controlling the processing time is crucial to ensure the quality of the film cooling hole and to prevent any damage to the walls.

  Methods  In this study, superalloy samples with a sandwich structure and protective materials in the middle were utilized to simulate turbine blades with protective materials inside. The experiment involved drilling to examine the relationship between various factors, including hole depth, filling radius (dependent on hole aperture), inclined hole angle, and the penetration forming time of micro-holes machined by ultrafast lasers in filling mode (refer to Fig.3). The penetration forming time was automatically measured using online image recognition technology (see Fig.4). The criterion for image recognition was determined by the state in which the hole was almost formed without causing any damage to the wall. The curve of the test data was fitted using the nonlinear least square method.

  Results and Discussions   In the 1-3 mm hole depth test, it was observed that the time required for small holes to penetrate increased exponentially rather than linearly with the depth of the circular hole (refer to Fig.7). The functional relationship between the penetration forming time of small holes and the hole depth was represented by the Gaussian function (see Fig.8). In the filling radius test, it was noticed that as the filling radius increased, the amount of material removed also increased, leading to a corresponding increase in penetration forming time (Fig.11). After fitting, it was determined that the Belehradek function (Fig.12) can be used to express the penetration forming time and filling radius of small holes in 2 mm and 2.5 mm test pieces. The drilling experiment at an inclined hole angle of 18.19°-60.00° revealed that the functional relationship between the penetration forming time of small holes and the inclined hole angle can be expressed using the Boltzmann function (Fig.16). Within this range, the larger the inclination angle, the less time it takes for the small hole to penetrate and form. The analysis suggests that tilting the test piece will cause certain areas of the small holes to undergo negative defocusing during processing, and higher laser ablation removal rates will expedite the formation of stable slag discharge channels. In response to the issue of non-circularity in forming small hole exits (Fig.6) without causing wall damage in filling mode, a drilling method was proposed. Initially, the hole was made in filling mode, with strict time control based on the hole depth, aperture, and inclined hole angle of the small holes. Subsequently, trepanning mode was employed to process and expand the small holes. This method successfully produces well-formed holes without damaging the opposite wall (Fig.17).

  Conclusions  This article investigates the influence of hole depth, filling radius, and inclined hole angle on the penetration forming of small holes using ultrafast laser in filling mode, with the objective of achieving optimal results without causing any damage to the walls. Within the typical range of hole depth, aperture, and inclination angle for turbine blade film cooling holes, the relationships between hole depth, filling radius, and inclined hole angle with the time it takes for small hole penetration forming can be accurately described by the Gaussian function, Belehradek function, and Boltzmann function, respectively. Drawing from the research results, a drilling method was proposed to control the processing time in the filling mode and subsequently slightly enlarge the hole in the trepanning mode. This method ensures the creation of small holes that remain undamaged to the wall and have a rounded exit shape. The proposed approach provides a practical solution for achieving precise and high-quality results in the machining of film cooling holes.