1 Experimental Study of the Effects of Wire EDM on the Characteristics of Ferritic Steel, At a Micro-Scale on the Contour Cut Surface N. Naveed 1,2 1 University of Sunderland, The David Goldman, St Peter's Way, SR6 0DD 2 The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom Nida.Naveed@sunderland.ac.uk Abstract This study, on a micro-scale, of the WEDM cut surfaces of specimens to which the Contour Method of residual stress measurement is being applied provides detailed information about the effects of the cutting process on the surface quality. This is defined by a combination of several parameters: variation in surface contour profile, sub-surface damage and surface texture. Measurements were taken at the start, the middle and at the end of the cut. This study shows that during WEDM cutting, a thin layer, extending to a depth of a few micrometres below the surface of the cut, is transformed. This layer is known as the recast layer. Using controlled-depth etching and X-ray diffraction, it is show that this induces an additional tensile residual stress, parallel to the plane of the cut surface. The WEDM cut surface and sub-surface characteristics are also shown to vary along the length of the cut. Moreover, these micro-scale changes were compared with macro scale residual stress results and provides an indication of the point at which the changes occurred by cutting process can be significantly relative to the macro-scale residual stress in a specimen. Keywords: Wire Electrical Discharge Machine (WEDM), Recast layer, Electro-chemical polishing, Surface roughness, Contour method. Introduction The contour method is a destructive technique for measuring residual stress. It was first proposed in 2000 by Mike Prime [1]. The theory of the contour method is mainly based on Bueckner’s elastic superposition principle [2]. The first step is to cut the test components in two along the plane of interest. The cut surfaces deform (deviate from perfect flatness) owing to the relaxation of residual stresses. This deformation is measured and input as the boundary conditions in a finite element analysis to back-calculate a 2-dimensional map of the original residual stresses normal to the plane of the cut. Thus, the contour method mainly involves four steps: specimen cutting, surface contour measurement, data reduction and finite element modelling. The cutting step is the fundamental part of the contour method, because all the raw data from which the results are calculated are, directly obtained from the surfaces so created. Therefore, as a destructive procedure that by definition cannot be repeated, cutting must be conducted with special care, as the quality of the cut surface directly affects all the subsequent steps. In general, destructive techniques – including the contour method – for measuring residual stresses are based on material removal. However, other material removal techniques such as incremental central hole drilling, deep hole drilling [3], [4], slitting (crack compliance) [5], layer removal, ring coring [4] Sachs boring [6] are not as sensitive as the contour method to the cutting step. In most destructive residual stress measurement techniques, some of the stressed material is removed from the specimen and the resulting deformation in the adjacent material is measured, as a strain. This is often done by monitoring strain gauges attached before any material has been removed. Often, the geometry of the surface where the deformations are to be measured is different from that of the removed material. In these cases there is not a direct relationship between the measured relieved strain and inferred residual stress. The contour method is not subject to this limitation. The purpose of the present work is to investigate the effects of the cutting process on the characteristics of ferritic steel at a micro-scale on the contour cut surface. The surface form, depth of recast layer, roughness and the depth of residual stresses are measured. The Contour Method Cutting Assumptions It is assumed that any deviation of the newly created cut surfaces has occurred solely because of the elastic relaxation of residual stresses acting normal to the plane of interest prior to the cutting. Therefore, for the cutting processes the following criteria must be fulfilled: a) cutting should be flat at the plane of interest and a minimal quantity of material should be removed; b) the width of any material removed by the cutting process (the kerf) should be constant; c) the cutting process must not induce any stress on the cut surface and does not modify the original residual stresses; d) the cutting process should not induce plastic deformation. In summary, the ideal cut would have zero width, induce no stresses and allow no plasticity at the tip of the cut [7]. The cut must be done in a single pass. This is different from conventional cutting, where multiple cuts can be made in order to achieve the best surface finish. Wire Electric Discharge Machining (WEDM) has previously been identified as the best choice for the cutting step in the contour method [8], [9] as it is capable of producing a cut surface with high precision, low roughness, and high wear resistance in a single pass [10]. It can be used for all electrically conductive materials irrespective of their hardness, material strength, shape and toughness. WEDM is a non-contact machining process; there is no direct contact between the electrode and the work piece during cutting. Consequently, it generates no cutting force, in contrast to other conventional cutting methods. Because very little heat is generated either, it does not deform the cut surface, further reducing mechanical stresses and entirely eliminating vibration problems that can occur with other methods (such as fine wire abrasive cutting) [11]. The Wire EDM Process and Its Effects on the Contour Cut Surface WEDM cutting is a thermo-electric process and it is performed by generating a series of electrical sparks between the WEDM wire and the component [12]. Because of the high temperature of the spark, the surface and the sub-surface (up to a few microns of the specimen) are heat-affected and there is a potential for consequent changes in texture and other materials characteristics. The extent to which these could affect residual stress measurements have not yet been fully investigated, although some progress has been made in relating WEDM parameters to the effect on the material condition [13]. When residual stress normal to the plane of the cut is relieved, both cut surfaces will deform either towards or away from each other, depending on whether the stress was compressive or tensile. There is a potential for geometric changes to occur on the cut surface contours in the result of any violation of the cutting assumptions. These introduce artefacts and consequently errors and uncertainties in the residual stress measurements [7], [14]. These artefacts can be classed as either symmetric (since ideally the surfaces are mirror images of one another) or anti-symmetric [7]. In general, not all of the distortion of a cut surface, even in the absence of