Study On Surface Properties And Heat Affected Zone Of Nickel Alloy Machined By EDM

In electrical discharge machining(EDM),the high temperature of the plasma channel between the tool electrode and the workpiece is used remove the material of the workpiece surface in the way of melting and vaporizing.After one electrical discharge,a modified layer composed of two different layers of metals forms on the surface of the workpiece.The modified layer metal in the outer layer is called recast layer.The recased layer is a loose layer of material which is melted by high temperature in the process of machining and resolidified on the surface of the workpiece after one discharge.The modified layer metal in the inner layer is called heat affected zone(HAZ).The HAZ is the surface metal which is not melted or vaporized but whose material properties change under the influence of high temperature.The existence of the modified layer affects the performance of the workpiece,so it is necessary to explore the thickness of the modified layer in order to remove it in the following process.In the previous work,the experiments of processing nickel by EDM under the condition of different electrode materials and different electrical parameters were carried out.While the main content of this paper is to explore the thickness of the modified layer including the recased layer and the HAZ.At the same time,the relationship between workpiece surface characteristics and electrical parameters after EDM is explored.The thickness of the melting solidification layer on the workpiece surface after EDM is measured.The results show that when processing nickel alloy GH4169 by EDM with brass tool electrode,under the condition of rough processing,the maximum thickness of the melting solidification layer on the workpiece surface is 31.17 ?m and under the condition of finish processing,the maximum thickness of melting solidification layer is less than 5 ?m.In addition,because the surface characteristics of the recased layer represent the surface characteristics of the workpiece after EDM,the law of the influence of different processing conditions on the surface roughness and surface appearance of the melting solidification layer is also studied.In order to explore the thickness of the HAZ on the surface of the workpiece after EDM,the metallographic observation of the sub surface of the nickel alloy after EDM was carried out.The observation results show that only after EDM under the condition of long-term abnormal discharges,the obvious grain refinement phenomenon will appear on the surface of the nickel alloy workpiece.After EDM under normal conditions even the maximum machining energy condition in this paper,there is no obvious difference between the sub surface grain size and that of base metal of nickel alloy.In order to estimate the thickness of the HAZ of nickel alloy after EDM,a simulation model of sub surface temperature distribution of nickel alloy GH4169 after EDM is established according to heat conduction theory.Based on simulation results,the possible HAZ thicknesses in rough and finish processing are less than 8 ?m and 2.5 ?m respectively.