Minimization and Mitigation of Wire EDM Cutting Errors in the Application of the Contour Method of Residual Stress Measu
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THE contour method of residual stress measurement is a relatively new method that, while destructive, has significant advantages over many other techniques.[1–7] A stressed component is cut into two parts using wire electro-discharge machining (WEDM). This causes relaxation of residual stress on the cut surfaces. The resulting surface profiles are then measured to determine the displacements caused by the relaxed stresses. The measured displacements are applied as displacement boundary conditions to a model surface using the finite element method, and allowing the model to relax elastically provides a calculation of the pre-existing residual stress field. Wire EDM is currently the best method available for application of the contour method. In wire EDM, an electrically charged wire, typically copper or brass, travels between wire guides on the top and bottom sides of the workpiece and material removal takes place by a spark erosion process. The component to be cut is placed in a temperature-controlled chamber filled with BILAL AHMAD, Research Associate, formerly with the Department of Engineering and Innovation, The Open University, Walton Hall, Milton Keynes MK7 6AA, U.K., is now with the Centre for Manufacturing and Materials Engineering, Coventry University, Priory Street, Coventry CV1 5FB, U.K. MICHAEL E. FITZPATRICK, Professor, Lloyd’s Register Foundation Chair in Structural Integrity and Systems Performance, formerly with the Department of Engineering and Innovation, The Open University, is now with the Centre for Manufacturing and Materials Engineering, Coventry University, Priory Street, Coventry CV1 5FB, U.K. Contact e-mail: michael.fi[email protected] Manuscript submitted June 5, 2015. Article published online November 9, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
de-ionized water. The amount of material removed during WEDM is equivalent to the wire diameter and the overcut.[7] Tension is applied to the wire between the guides to keep it straight. Injection nozzles are used for flushing of cut workpiece material. The WEDM cutting process is depicted in Figure 1. There are a number of potential types and location of wire EDM cutting errors and artifacts:[8,9] wire entry and exit artifacts; artifacts at the start of cut; wire breakage; and bowing and waviness of the cut surface. Errors can be symmetric and anti-symmetric with respect to the two cut surfaces. Cutting artifacts have significant impact on the quality of contour residual stress results. They can potentially be reduced by bonding sacrificial layers composed of a similar material at the affected locations, such as the EDM wire entry and exit points, and the start and the end of the cut.[10] The use of sacrificial layers to avoid errors and artifacts is further explored in this present study. Reducing the conductivity of the de-ionized water used in the EDM chamber gives a slight improvement to the roughness of the cut surfaces.[11] However, reducing the conductivity of the water means that the machine filtration system may need additional main
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