Surface integrity of moulds for microcomponents manufactured by micromilling and electro-discharge machining

Abstract

Milling is one of the most important processes to manufacture dies and moulds. However, it cannot machine regions with small sizes and difficult access to the cutting tool. Such regions must be machined by electro-discharge machining (EDM). It is known that EDM can damage the integrity of the machined surface, and also requires long processing time, due to both, the necessity to manufacture the electrode and its low material removal rate. The micromilling process, using high-frequency spindle together with cutting tools smaller than 1 mm of diameter has been emerging as an option for machining small regions in dies and moulds. In this context, this paper aims to help the understanding of the cutting phenomenon to manufacture small areas using both machining techniques, in order to identify the adequacy to replace EDM for micromilling in such circumstances. Machining experiments were carried out on AISI P20 (29HRC) and AISI H13 (45HRC) steels. These materials are commonly used in the mould and die industry. Residual stress on machined surface, surface finishing (2D and 3D), SEM images, microstructure and microhardness were accessed. The residual stress was tensile for the EDM pieces and compressive for the milled parts. The material had more influence on the residual stresses values than the process and H13 had higher values than P20. The surface roughness from the EDM machining pieces was not influenced by the material. The EDM caused white layer and microcracks on both materials, but much more intensely on H13. These occurrences were not found on the milled workpieces. Plastic deformation occurred on the micromilled surfaces, but without phase transformation of the material’s microstructure. Unexpectedly, the roughness on the hardest material (H13) was worse than P20 for the milling experiments. It was attributed to more intense tool deflection when milling H13. In general, roughness obtained in micromilling was about six times lower than that obtained using EDM and it presented a regular surface topography, unlike the EDM specimens.