Fundamental Study On Drilling Holes At TBC Coated Metal Part Using Composite Processing Of Abrasive Water Jet And Electrical Discharge Machining

Thermal barrier coating(TBC),which is commonly coated on the metal parts of aero engine,can significantly improve the temperature bearing capacity of the components,prolong the service life of components and reduce engine fuel consumption.However,it is difficulty to machine micro-holes on metal parts with TBC by electrical discharge machining(EDM)due to the non-conductivity of TBC.This thesis proposed a novel machining method combining abrasive water jet machining(AWJM)and EDM together to solve this problem.The goal of this research is to investigate the feasibility of this method.In the first stage,the AWJM experiments of TBCs machined by electro beam physical vapor deposition(EB-PVD)and air plasma spraying(APS)methods were carried out.The effects of jet pressure,target distance,abrasive concentration,impacting time,particle size and abrasive type on the pitting hole shape were analyzed,and the reasonable range of erosion processing parameters was searched.The results showed that TBCs by EB-PVD method had a “W” hole shape,while TBCs by APS had a "U" type.In addition,the erosion rate of TBCs by APS is higher than that by EB-PVD.No obvious micro cracks were found on the surface of both TBCs.Secondly,the TBCs processed by APS were studied by modeling the geometric characteristics of the erosion hole.The orthogonal tests were designed by selecting three parameters of abrasive concentration,target distance and impacting time.The power function model and quadratic polynomial regression model were used to model the aperture and hole depth.The results show that the prediction accuracy of quadratic polynomial model is higher than that of power function model.Aiming at the problem of uneven coating thickness of APS thermal barrier coating,the solution of increasing impacting time on the basis of model prediction was proposed.The tests found that the extra thermal barrier coating material at the bottom of the hole could be removed with the minimum impact on the pore diameter when the increase time was 5s.In the last,the experimental study of EDM on erosion hole by AWJM was carried out.The influence of the diameter of tube electrode on the machinability of EDM after impacting was analyzed,and the correlation between the metal substrate bare rate and the machinability of EDM after removing the TBCs was studied,and the influence of the parameters of EDM on the TBCs was explored.The results showed that the surface microcrack of TBCs can be reduced by using the tube electrode with smaller diameter,and the risk of secondary corrosion removal would be caused by excessive peak current,pulse width and flushing pressure,and the layered cracking phenomenon was easy to occur.