High-aspect-ratio microhole drilling in MAR-M247 superalloy using a quasi-CW fiber laser

Nickel-based alloys are of paramount importance for components operating at high temperatures within heavy-duty gas turbines. This study explores the potential of a quasi-continuous wave (QCW) fiber laser in the drilling of high-aspect-ratio holes in these materials. An anti-spatter coating composed of emery and silicone was developed to suppress molten material redeposition and improve hole quality. Furthermore, a polygonal percussion drilling method was introduced to address challenges associated with drilling thick, high-aspect-ratio holes. A systematic comparison was conducted among four drilling methods: percussion drilling, trepanning drilling, N-sided polygon drilling, and a hybrid method combining polygonal percussion drilling with trepanning drilling. The findings demonstrated that percussion drilling effectively controls heat accumulation, producing microholes with uniform profiles, minimal taper, low roughness, and a thin recast layer, capable of machining microholes with a diameter of less than 0.5 mm, an aspect ratio of 29, and a taper of 0.08°. In contrast, trepanning drilling generated straight hole walls but suffered from higher roughness and greater thermal damage. N-sided polygon drilling achieved clean surfaces with minimal taper but exhibited pronounced wall flaring and the thickest recast layer. The hybrid method offers a balanced performance profile and is particularly well-suited for larger-diameter holes characterized by a high aspect ratio of 11 and a very low taper angle of 0.27°. Microstructural analysis of deep holes produced by the hybrid method revealed depth-dependent variations in recast layer thickness and distinct evolutionary patterns in the peri-hole microstructure. EBSD analysis was performed on the upper, middle, and lower sections of the hole to map crystallographic changes. Combined with hardness testing, these analyses elucidated a strong correlation between grain morphology and localized hardness variations.