Surface Preparation of Powder Metallurgical Tool Steels by Means of Wire Electrical Discharge Machining
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TRODUCTION
POWDER metallurgical (PM) tool steels are a new and evolving category of tool steels, exhibiting excellent tribological properties and superior mechanical properties as compared to their cast counterparts. In contrast to conventional ingot cast tool steels (such as AISI D2), PM tool steels contain homogeneously distributed fine carbides and nitrides. This microstructure is achieved by hot isostatic pressing prealloyed gas atomized powder to full density.[1] Due to their good wear resistance and reasonable toughness, PM tool steels are increasingly finding applications in the cold forming industries (e.g., sheet metal forming, deep drawing, and powder metallurgy) and are typically used as die and punch material.[2–6] Whether it is sheet metal forming, deep drawing, or metal powder compaction, a low friction die surface is invariably a necessity as it will reduce the risk of premature failure. Furthermore, the surface condition of the final component (i.e., work material) is directly related to that of the die. Therefore, in order to enhance the performance of the die and the surface quality of the final product, correct surface preparation of the forming SEPEHR HATAMI, formerly Postdoctoral Researcher, Department of Materials and Manufacturing Technology, Chalmers University of Technology, 412 96 Gothenburg, Sweden, is now R&D Engineer, AB SKF, 415 50 Gothenburg, Sweden. Contact e-mail: [email protected] MEHRDAD SHAHABI-NAVID, formerly Master’s Student, Department of Materials and Manufacturing Technology, Chalmers University of Technology, is now PhD Student, Department of Environmental Inorganic Chemistry, Chalmers University of Technology. LARS NYBORG, Professor and Head of the Department, is with the Department of Materials and Manufacturing Technology, Chalmers University of Technology. Manuscript submitted June 23, 2011. Article published online May 26, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
die is essential and it is usually performed differently by different manufacturers. In metal forming operations where high loads are applied and components with intricate geometries are to be manufactured (e.g., powder metallurgy), high strength and hardness die materials are required. A means of creating intricate cavities in high strength forming dies is by application of electrical discharge machining (EDM). In EDM, the material is removed by electric sparks (i.e., electrical discharge) that takes place between the electrode and the work material, inside a dielectric liquid. The energy within the spark is enormous, 1017 W/m2, and the local temperature is reported to reach up to 20,000 K.[7] Therefore, the spark can melt and even vaporize the work material and the electrode. When the current stops and the spark ceases (the socalled ‘‘off-time’’), the machined debris and the molten material are flushed away by new deionized dielectric fluid. Despite the fact that the machined surfaces are flushed by the dielectric medium, it has been reported that only 15 pct (or less) of the molten material is removed
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