Grit Blasting for Removal of Recast Layer from EDM Process on Inconel 718 Shaft: An Evaluation of Surface Integrity

Abstract

The heat generated during EDM melts the work material and thereby allows large amounts to be removed, but an unfavorable surface of a recast layer (RCL) will also be created. This layer has entirely different properties compared to the bulk. Hence, it is of great interest to efficiently remove this layer and to verify that it has been removed. The main objective of this work has been to study the efficiency of grit blasting for removal of RCL on an EDM aero space shaft. Additionally, x-ray fluorescence (XRF) has been evaluated as a nondestructive measurement to determine RCL presence. The results show that the grit-blasting processing parameters have strong influence on the ability to remove RCL and at the same time introduce beneficial compressive stresses even after short exposure time. Longer exposure will remove the RCL from the surface but also increase the risk that a larger amount of the blasting medium will get stuck into the surface. This investigation shows that a short exposure time in combination with a short grit-blasting nozzle distance is the most preferable process setting. It was further found that handheld XRF equipment can be used as a nondestructive measurement in order to evaluate the amount of RCL present on an EDM surface. This was realized by analyzing the residual elements from the EDM wire.

Surface integrity after post processing of EDM processed Inconel 718 shaft

“…The only available results concerning grit blasting of Inconel 718 was reported by the authors in the present investigation from tests performed evaluating grit-blasting parameters on a forged EDM shaft Inconel 718. These results showed the influence of stand-off distance and exposure time on the surface integrity; the erosion increased with exposure time resulting in an increased removal of RCL [23].…”

“…Prior work performed on the Fig. 8 Microstructures of the different samples from SEM images in ×2000 magnification same sample evaluated the influence of exposure time and is presented in [23]. This showed an almost linear increase of the erosion with exposure time.…”

“…This method has been shown to describe the amount of residue in the surface from the EDM process very accurately [23]. The results show that the HPWJ process is the most capable process that completely removes the RCL.…”

“…In prior investigations conducted by the authors, the grit-blasting process' ability to remove RCL was evaluated by testing different exposure times and nozzle standoff distances [23]. The grit-blasting parameters resulting in the lowest amount of RCL residue and highest compressive residual stress level were selected for the present study.…”

A Review on Porous EDM Electrodes

“…Holmberg et al [11] the results show that grit blasting is a possible method to be used in order to remove RCL after EDM processing. The grit-blasting parameters blasting time and nozzle distance have great influence on the resulting surface integrity generating a RCL-free surface with high compressive residual stresses after exposure times greater than 4s.…”