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The EDM process leaves a recast (white) layer on the surface. This layer is detrimental to tool life especially in highly stressed situations such as those in die casting and forging. The high speed milling (HSM) process was investigated as an alternative to EDM with respect to surface integrity issues (i.e. residual stresses and hardness variations). Machining processes are also capable of leaving a white layer on the surface as illustrated by the hard turning example below
The figure on the left shows a clear surface created with a sharp tool. The figure on the right exhibits a white layer created by a worn tool. Because of tool wear, the energy input to the process was about four times that of a relatively sharp tool (note Fy on both figures). The energy level was high enough to cause melting on the surface.
All of our HSM experiments were performed on H13 tool steel at a hardness of 46 Rc. No white layer was observed; however, we observed a "Deformed Layer" as shown on the figure below:
The black region at the center of the figure is the Bakelite used to mount the two pieces face to face. The piece...