| Sweep blasting erosion arc machining(Sweep＿BEAM)is a novel technique for machining open channels mainly used in blisk manufacturing as well as many other aviation and turbomachinery components.These parts are mostly made of hard to cut materials such as titanium and nickel based superalloys.Because of sever condition of application in turbojet engines or land turbines,difficult to cut materials have special characteristics that make them withstand hot and harsh working environment.However,resistance to tough condition cause such materials unsuitable for conventional machining and leads to poor machining efficiency.Notwithstanding,Sweep＿BEAM is founded on blasting erosion arc machining(BEAM)which utilizes consecutive arc discharges for material erosion and as a result eliminates machining toughness of hard to machining materials.Nonetheless,controlling plasma arc channel is critical for it could damage machined surface because of tremendous plasma temperature and exploding consequences.To overcome this,a specific multi hole tooling system is designed to lead high pressure water liquid to the gap distance and prevents disadvantages of plasma arc channel by stretching plasma root and expelling molten materials from crater.Key elements in Sweep＿BEAM are high pressure inner flushing unit to provide effective fluid flow in gap distance for hydrodynamic arc controlling,high pulsed power supply unit to generate high intermittent electric current with elongated duration in order to initiate and maintain plasma arc channel,a CNC machine for tool path generation and a tooling unit.In sweep blasting erosion arc machining,a simple and rectangle graphite tool electrode was devised for further investigation Concept of sweep blasting erosion arc machining is the formed tool electrode which slides over the workpiece surface without rotation and erodes the scanned area by thermal effect of plasma arc discharge.Meanwhile,high pressure inner flushing assists plasma arc channel behaviour and improves process stability.To exploit the flushing effect,a tubular tool was designed to handle a uniform fluid distribution in gap distance.In this condition,effect of fluid velocity on plasma arc channel is the same along the tubular tool bottom surface.Therefore,effect of different fluid velocity on plasma arc channel in high input energy level was investigated using a tubular tool.Machining performance and discharge waveform condition were observed closely to establish principle of fluid velocity on plasma arc channel.According to the findings,when fluid velocity reaches10.5m/s plasma arc channel root slides on workpiece surface and remains a stretched crater.However,high fluid velocity near 21m/s leads to early plasma arc channel break off and deteriorates material removal mechanism.Therefore,effective fluid velocity is considered to be between 10.5 m/s and 21m/s for the best hydrodynamic effect which are stretching plasma root and debris removal.Effective flushing velocity in the gap distance is further implemented to develop a unique model for flushing hole distribution in tool design procedure for sweep blasting erosion arc machining.Proposed model is based on the highest effective fluid velocity coverage in gap distance.Therefore,for different flushing hole size and location pattern fluid velocity distribution is different.Simulation of fluid flow distribution in the gap distance in a variety of condition eventuated to the flushing hole pattern principle.Based on this rule,a row of d=3mm flushing holes should be arranged along the tool bottom centreline and d=2mm holes considered adjacent to main holes and symmetrically.In order to find the best tool path for machining a groove by sweep blasting erosion arc machining,a procedure introduced for path planning by considering uniform tool wear on the end face of tool electrode.For specific tool shape in this process and absence of tool rotation,wear mechanism is different from other discharge methods.For this,material erosion starts from the edge facing the feeding direction and then expands to other regions in the bottom surface of the tool electrode.Therefore,to achieve a uniform tool wear and a smooth surface,tool feeding direction should change in specific moments to transfer tool erosion to other edges.This rule arose from tool wear prediction mechanism which was carried out theoretically and then investigated for different tool paths.To verify theoretical findings,some experiments were carried out.Initially,effect of electric parameters such as discharge current and pulse duration was studied for the best roughing and finishing results.Based on basic experiment,MRR(material removal rate)in sweep blasting erosion arc machining can reach up to 8000 mm~3/min with TWR(tool wear ratio)of 2.9%.Moreover,experimental findings for different tool paths demonstrated the higher MRR,lower TWR and better surface quality in machining with proposed tool path method.To benefit from high material removal rate in Sweep＿BEAM and achieve a favourable machined surface quality,three steps combined machining process was introduced.In this method,sweep blasting erosion arc machining with low input energy level was added to roughing with high input energy proceeding further milling machining for a perfect surface integrity.Combined sweep blasting erosion arc machining and CNC milling was successfully carried out and resulted in high material removal rate and great surface quality without heat affection to the sublayers of Ti6Al4V and mold steel. |
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