Structure and Composition of the White Layer in the Wire-EDM Process

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In components under bending load the top layer of the workpiece is strained the most. The properties of this layer are therefore very crucial for the mechanical behavior of the component as a whole. The Wire-EDM process, like every machining process, interacts with this surface layer by thermal material load and modification. The thermal material removal principle in Wire-EDM creates a characteristic rim zone alteration. The process generates especially an outer shell that is known as white layer. Although this layer is generally reducing the mechanical endurance, the precise structure and composition of this layer is unknown. The characteristics of the white layer should affect the mechanical behavior significantly. This motivates a comprehensive analysis of this layer within this paper. Chemical analyses of the rim zone together with TEM-images allow identifying the structural constitution of the microstructure. The influence of local defects and transformations on the workpiece functionality is investigated. For determining the chemical composition of the white layer EDX-maps and conventional EDX-analyses were generated. In addition nanoindentation-based mechanical testing methods measure the local hardness and modulus of elasticity in the white layer. By comparing the measured local hardness and modulus of elasticity of the altered rim zone with an unaffected surface layer the influence of the edge geometry is quantified. The addition of the mentioned analyses allows a complete characterization of the chemical composition and metallurgical constitution in the white layer. These characteristics define mainly the mechanical properties of the white layer. This helps predicting the mechanical behavior of workpieces machined with Wire-EDM, especially for components with high aspect ratios and filigree geometries.