Research On Machining Characteristics Of Blasting Erosion Arc Machining

Nicke l-based high temperature alloy is generally used as the material of critical compo nents in turbine engines. However, the unique properties of this material lead to a very poor machinability by means of cutting: high-temperature stability, severe cutting hardening, drastic tool wear, and greater cutting forces which induce chartering, resulting in very low machining efficiency, too short tool life, longer processing cycle and high machining cost. Thus, how to efficiently machine the nickel-based high temperature alloy has always been a bottleneck in turbine engi nes manufacturing.In this study, a novel high-efficiency arc discharge machining process namely Blasting Erosion Arc Machining(BEAM) based on hydrodynamic arc-breaking mechanism is implemented in order to solve above mentioned problem. This technology composes of several key elements, such as high-de nsity energy input carried by arcing, multi-hole electrode with three-dimensional endface, high veloc ity flow field in the discharge gap, and multi-axis feed control. By combining them altogether, the BEAM process substantially differs from the conventional electrical discharge machining(EDM) in terms of machining mechanism and characteristics.According to the machining mechanism, this study designs out a special machining system to investigate the machining characteristics of BEAM. In order to attain the basic machining characteristics of BEAM, prelimina ry experiments are carried out with an opt imized bundled electrode to machine the nickel-based high temperature alloy GH4169(similar to Inconel718). An arcing crater mode l is the n simulated out to verify the thermal eroding mechanism on large volume workp iece material. Investigations are also carried out to illustrate the influence of main machining parameters on the machining performance, and to analyse the BEAMed workpiece surface. Experimental results show that in BEAM process with ne gative too l po larity, the material remova l rate is up to 14,000mm3/min(500 A discharge current) with sink ing mode. Meanwhile, the minimum too l wear ratio is less than 1%. The thickness of the recast layer and heat affected zone is less than 100 μm.It is discovered that the stung polarity effects also exist in BEAM process. That means the machining performances between positive and negative tool polarities are substantially different. For further disclosing the polarity effects, a comparison of negative and positive BEAM is carried out. It shows that the surface roughness of positive BEAM can be significantly improved, but the material removal rate is decreased. On the basis of comparative experiment, a 3- factor, 3-level performance test is subsequently carried out to investigate the effect of machining parameters(peak current, flushing inlet pressure, pulse duration) on the machining performance(material removal rate, tool wear ratio, surface roughness) in order to opt imize the machining process parameters to attain a better surface roughness. The positive BEAMed surface integrity includ ing of recast layer and heat affected zone, surface micro-cracks, microhardness and residual stress is also investigated. The results show that an effective combined process can be achieved by conducting a high current negative BEAM for roughing followed by a low current positive BEAM for relatively smoother surface by only skimming the rough surface layer itself. Therefore, it is possible to machine a part with high efficiency and a better controlled surface quality by combining negative and pos itive(N-P) BEAM processes together, which is beneficial for reducing the machining a llowance of the consecutive machining processes such as cutting, gridd ing etc.In depth investigations into the aspects such as flow field in the discharge gap and the debris particles are also carried out. The influence of the geometry of flushing holes on the dielectric fluid field distribution is analised by numerical simulation follow ed by the experimental verification to optimize the geometric design of the electrode. The morphology of the debris particles is observed by us ing SEM. The structure and chemical composition of the debris particles are also examined. The results are helpful to better understand the material removal mechanism of BEAM.For verifying the feasibility of BEAM process, typical 3D cabities are machined with a sinking mode which never invo lves the rotation of the tool electrode. Finally, an inducer in rocket engine is machined by the N-P BEAM method, which demonstrates the capability of high efficiency rough machining of complex 3D part.