Morphological evolution analysis and processing mechanism study of controlled tapered micro-holes by ultrashort pulse laser oblique incidence drilling

Ultrashort pulse laser oblique-incidence drilling controls hole taper by altering ablation profiles through beam inclination, yet the dynamic evolution mechanism of micro-hole morphology during this process remains unclear. This study reveals the formation of a unique beam overlap region during inclined drilling, wherein the hole morphology exhibits distinctly different evolutionary characteristics before and after this region. Consequently, this study divides the hole morphology evolution into two stages—Before Overlap Depth (BOD) and After Overlap Depth (AOD)—separated by the overlap region. During the BOD stage, the drilling depth reaches approximately 100–200 μm based on the selected processing parameters. Consequently, combined experimental and simulation approaches were utilized to investigate the micro-hole morphology evolution mechanism within this stage. What's more, a preliminary optimization of the processing parameters was conducted to establish a foundation for the subsequent AOD experiments. Experimental results demonstrate that inclined drilling is characterized primarily by a conical profile evolution during the BOD stage, while morphological changes in the AOD stage are dominated by the formation and development of “convex cone” structure. The emergence of this “convex cone” structure is essential for achieving deep holes with adjustable tapers. Thus, using the optimized BOD parameters, this study explores the impact of AOD parameters on the “convex cone” structure, establishing a dynamic hole evolution mechanism for inclined drilling. Applying this mechanism and optimized parameters enables controlled micro-hole machining with positive-to-negative tapers in a 3-mm-thick nickel alloy, providing comprehensive theoretical and experimental insights into inclined laser drilling.