Research On EDM-based Unitized Drilling Process And Recast Layer

The electrical discharge machining(EDM)has become the predominant processing technology for film-cooling holes.EDM has a unique thermal processing mechanism,where the processed surface is covered with a recast layer.These cracks,voids,and thermal stresses within the recast layer decrease the material resistance to fatigue and corrosion.Therefore,it is important to reduce or remove the recast layer and the cracks found in the EDM drilling holes of aeronautical material.To solve these problems,negative polarity EDM and electrical discharge machining/electrochemical machining hybrid machining(EDM/ECM)were carried out firstly.A novel unitized drilling process was proposed,which is based on the EDM drilling.This process fully utilized the advantage of the negative polarity EDM and the EDM/ECM.The same tool electrode was used in situ during the whole process.Combined work fluid and water were respectively used before and after breakthrough.EDM/ECM was initially used,and then converted to negative polarity EDM with external flushing water during the penetration period.Better machining results were achieved.The following work have been done in this paper:(1)An experimental platform for the unitized EDM drilling process was built based on the high speed EDM drilling machine.Using this platform,the EDM polarity changing and EDM/ECM can be carried out.According to the feature of negative polarity EDM and EDM/ECM,the paper proposed a novel unitized hole drilling process and designed the process flow.The breakthrough detection plan was designed in which the breakthrough moment was detected by the changing characteristics of open current.(2)To reduce the recast layer and cracks formed during the EDM of the TC4 alloy,negative polarity EDM and EDM/ECM were carried out.The machining efficiency,the recast layer,and the cracks were studied.The experimental results showed that when compared with common positive polarity EDM under the specific processing condition,the recast layer of the negative polarity EDM decreased by 65% in thickness but the material removal rate(MRR)decreased by 31%.In EDM/ECM,MMR was 18% higher than the positive polarity EDM and the majority of the recast layer and the cracks were removed.However,issues including the low efficiency occur during the penetration period.(3)The following paper proposed a novel unitized hole drilling process.In this unitized process,EDM/ECM was initially used,and then converted to negative polarity EDM with external flushing water during the penetration period.It is compared with the conventional EDM/EDM in several aspects including the general machining efficiency,the machining accuracy and the recast layer.The experimental results show that the general machining efficiency of a single hole increased by 73% over conventional EDM/EDM.The machining accuracy obviously increased.Furthermore,only a 7?m thick recast layer remained at the hole's outlet,with no obvious cracks found.