Effect of pulse duration and vibration tumbling on re-cast layer thickness and surface roughness of super alloy undergoing EDM

Abstract

Many geometrically complex functional parts of turbo-pumps of Liquid Oxygen (LOX)-Hydrocarbon based rocket engines (Semi-Cryogenic Engines) which are difficult to be realized through conventional machining processes, are realized through Electric Discharge Machining (EDM). Every EDM process, deposits a re-cast layer on the spark eroded surface due to melting and subsequent quenching of the eroded surface. Studies show that structure and hardness of the re cast layer, differs from the parent material. Energy Dispersive Spectroscopic (EDS) analysis reveal that the chemical composition of the re-cast layer matches very closely to that of the parent metal itself. Re-cast layer has inherent micro cracks due to both thermal and tensile stresses during cooling. Even though re-cast layer is a superficial phenomenon, cracks once initiated on the re-cast layer, penetrate deep down to the parent metal as well, under fatigue loads causing fractures. In order to reduce the micro cracks on such spark eroded surfaces, thickness of the re-cast layer is to be reduced to the extent possible by fine tuning the EDM parameters. Failure analysis carried out on the fractured turbine blades of a LOX-Hydrocarbon based engine, realized through 4-axis EDM, which failed during the functional tests, indicated the propagation of such micro cracks from spark eroded surface into parent material causing fatigue fracture at turbine blade tips. Amongst the significant parameters governing the formation of re-cast layer in the EDM process, the effect of pulse duration is studied for its effect on re-cast layer thickness and surface roughness on super Nickel alloy turbine blades. Additionally, a vibration tumbling process is introduced and its effect on re-cast layer thickness and surface roughness is demonstrated. Turbine blades realized from the fine-tuned pulse duration, followed by vibration tumbling, show great improvements in surface finish, reduction of micro cracks, surface porosities and thickness of recast layer. This turbine successfully passes through the functional tests without any fractures.