Surface integrity after post processing of EDM processed Inconel 718 shaft

Electrical discharge machining (EDM) is considered as an efficient alternative to conventional material removal concepts that allows for much higher material removal rates. However, EDM generates unwanted features such as re-cast layer (RCL), tensile residual stresses and a rough surface. In order to recover the surface integrity, different post processes has been compared: high-pressure water jet (HPWJ), grit blasting (GB) and shot peening (SP). Surface integrity has been evaluated regarding microstructure, residual stresses, chemical content and surface roughness. The results showed that a combination of two post processes is required in order to restore an EDM processed surface of discontinuous islands of RCL. HPWJ was superior for removing RCL closely followed by grit blasting. However, grit blasting showed embedded grit blasting abrasive into the surface. Regarding surface roughness, it was shown that both grit blasting and HPWJ caused a roughening of the surface topography while shot peening generates a comparably smoother surface. All three post processes showed compressive residual stresses in the surface where shot peening generated the highest amplitude and penetration depths. However, the microstructure close to the surface revealed that shot peening had generated cracks parallel to the surface. The results strongly state how important it is to evaluate the surface at each of the different subsequent process steps in order to avoid initiation of cracks.First Online: 22 November 2017</p

The model of product distortion in AWJ cutting

V článku uvedeno Libor M. HlaváčThe abrasive water jet (AWJ) retardation inside the cut material, the characteristic phenomenon of the AWJ cutting, causes declination of the kerf sidewalls especially in corners and curvatures. This paper is aimed at a description of the origin of these negative consequences of jet retardation. The model for calculation of the limit traverse speed from both the jet parameters and material properties has been derived using laws of conservation. The equation expressing dependence of the angle between the tangent to the striation curve and the impinging jet axis on the depth of jet penetration into material has been used for evaluation of the product distortion in the cutting process. Proposed model has been applied for setting up the tilting angle of the cutting head during the AWJ cutting process to reduce the product shape distortion. The model was supplemented by geometrical analysis of curved parts of cut trajectories. The resulting equation makes possible to calculate the shift of the jet trajectory at the outlet side of the workpiece from its regular position determined by projection of the trajectory at the inlet side of the workpiece along the jet axis. The model is capable to determine the appropriate tilting angles of the cutting head for compensation of the jet retardation and the taper. The experimental data measured on metal samples seem to be in a good accordance with the proposed model.Web of Science621-416615