An Investigation of the Sequential Micro-Laser Drilling and Conventional Re-Drilling of Angled Holes in an Inconel 625 Ni-Based Alloy

Abstract

The conventional (mechanical) micro-drilling of Inconel 625 alloys suffers from premature breakage of the drill bit due to its brittle nature and limited cutting tool life. Even greater problems are encountered when micro-drilling holes at an acute angle to the machining plane. In such a process, there are great difficulties associated with the low stiffness of the tool, which leads to the frequent breakage of the drill during machining. Therefore, in this type of mechanical drilling operation, the hole surface is usually milled with an end mill to provide a flat surface on the entry side of the drill bit. The aim of this article is to recognise the process of sequential micro-drilling and to assess the possibility of its use as an effective and efficient method of micro-drilling in hard-to-cut metals. The paper describes the process of initial laser drilling followed by final mechanical micro-drilling. Inconel 625 Ni-based alloy sheets were used as the test material. The shape and microstructure of pre-holes made with a laser, the volumetric efficiency of laser processing, the energy in the mechanical drilling process, and tool wear were analysed. The research results show that in the sequential drilling process, mechanical re-drilling eliminates the geometrical discrepancies resulting from the laser pre-drilling. In addition, it was found that, compared to mechanical micro-drilling, the use of sequential micro-drilling resulted in a two-fold increase in drill life. It has been also observed that sequential machining reduces the energy demand by 60% compared to mechanical micro-drilling. In addition, it was found that the edge of the drill bit is a key factor in deciding the target diameter of the laser-drilled pilot hole, and thus in selecting the micro-drilling parameters.