Feasibility and Process Development of Mechanical Micro Drilling for Nickel Based Super Alloys

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Abstract

Mechanical micro machining is an emerging material removal process in precision manufacturing industries. There are challenges involved in micro drilling of difficult to cut alloys. These relate to the development of a feasible and reliable manufacturing process given the fragile nature of the micro drill and the poor machinability of difficult to cut materials. Moreover, the established knowledge of macro scale machining may not be directly transferable into micro machining domain. Therefore, mechanical micro machining needs to be adapted to a specific application. Currently, electrical discharge machining (EDM) is an established industrial process for making micro holes in nickel alloys. The mechanical micro drilling process is at present being considered for improved surface integrity, better hole definition and high productivity. Considering the potential of mechanical micro drilling process in nickel based super alloys, the research presented in this thesis focused on developing a novel micro drilling strategy and a process window. Having developed the process window and selected optimum tool geometry, workpiece surface integrity was evaluated at various cutting conditions. Mechanical and microstructural characterization of the modified layers was conducted using electron backscatter diffraction (EBSD), focused ion beam (FIB), backscatter election (BSE), transmission electron microscopy (TEM) and nano-indentation techniques. The mechanisms behind the generation of these modified layers were revealed. The effects of various feedrates, cutting speeds and tool edge radius were analyzed under dry and wet cutting conditions. A new and novel contribution to modified material microstructure analysis was presented in dry and wet drilling conditions. Furthermore, important findings were presented on the tool-chip and tool-workpiece cutting zones. This research provides a comprehensive picture of the surface integrity definition of the micro hole features in drilling nickel based super alloys.Since nickel based super alloys are known for their poor machinability, tool life becomes an important economic variable. For this purpose, tool wear was studied in the micro machining domain. A new tool wear map was developed on a feed-speed plane, identifying low tool wear zones at high productivity. Wear mechanisms were identified which contributed to better understanding of tool-workpiece interactions. A range of different heat resistant and wear resistant coatings were tested which helped identifying the critical material requirements of machining these alloys.Finally, after having developed a complete set of requirements for the mechanical micro drilling process in terms of process window, suitable tool geometry, workpiece surface integrity, tool wear evaluation and selection of suitable coatings for the micro drilling process; the surface integrity produced by mechanical drilling was compared with EDM and laser drilling processes. Mechanical and microstructural character of surface and subsurface layers was assessed. Comparison of surface integrity parameters showed that the mechanical micro drilling process has the potential to benefit industry making micro size holes with better hole definition and surface integrity. This work is an important contribution to industry in that it presents process feasibility assessment and characterization and is regarded by the industrial partners as having achieved Manufacturing Capability Readiness Level (MCRL) 3.

Keyword(s)

machining; micro drilling

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