Study on the evolution process of recast layer for fast EDM drilling based on observation experiment and a novel thermal-fluid coupling model

The existence of a recast layer of film cooling holes on turbine blades imposes a crucial problem that affects the quality of fast electrical discharge machining (EDM) drilling. This paper focuses on an evolution process from molten material generation to recast layer formation. Single pulse discharge observation experiments with a high speed camera and a self-built observation platform were carried out to observe the formation and movement of the molten material. A novel thermal-fluid coupling model is proposed to combine fast EDM drilling methods with special materials and geometric properties of film cooling hole machining. A heat transfer module, a turbulent flow computational fluid dynamics module, a phase change module and a level set method are combined in this model. The complete process of generation, flow and solidification of molten materials is investigated by simulation with a single pulse discharging. The simulation results agree well with the experimental observation results. A thermal-mechanical coupling analysis of a molten material evolution process is carried out based on the temperature, velocity and pressure fields from the thermal-fluid model. Simulation results show that the molten material is generated at 1 mu s after the starting of a discharge, and solidifies within 20 mu s after the ending of the discharge. Under the influence of a high-speed flushing fluid, molten materials will not splash into a gap channel along the radial directions. Most of the molten material enters the gap channel along the workpiece surface with a velocity of over 25 m s(-1). The moving direction of the molten material is consistent with that of a flushing fluid. The thickness of a recast layer gradually increases from bottom to top in the fast EDM drilling.