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Preform Design for Net Shape Sinter-forging of Powdered Magnets with Curved Shape (곡면 형상을 가지는 분말 자석의 정형 가공을 위한 예비성형체의 설계)
Since permanent magnets of Nd-Fe-B group were produced on a commercial scale in 1984, they have kept a key post in the permanent magnet market and they are expected to continue steady and remarkable growth. The magnets are currently used in electric machines, high energy motors , generators, sensors and so on. The permanent magnets of Nd-Fe-B group are manufactured by the sintering process, the GM Process ( Rapid Solidification and Hot Forming Process ) and the HDDR Process ( Hydrogenation Decomposition Desorption Recombination Process ). All processes are protected by a patent against its application without license. In order to overcome this obstacle of patent, the tube process was developed and patented. However, it has some problems to produce on a commercial scale. This process causes more flash due to large height reduction of a deformable tube, and non-uniformly densified magnets. The process also reduces the effective magnet area with flash and fails to produce efficient magnets with non-uniformly densifica-tion.In this study, a rigid-plastic finite element analysis considering the compressibility of the powder material has been carried out using 8-node hexahedral isoparametric elements. The analysis also deals with contact treatment using the displacement constraints at the rigid contact and adopt skew boundary conditions and arbitrarily curved die surface treatment. The finite element program developed is applied to a class of sinter-forging problems with powder materials. The problem includes sinter-forging of cylindrical green powder and tube processing of compac-tion of green powder. The tubed compaction is carried out on a flat die or a curved die with a predicted preform. The purpose of the analysis is to design an appropriate preform for net shape sinter-forging. In order to design a preform, a trial-and-error method is used considering the thickness of a tube, the friction factor, the curved shape of dies, and the slope of the tube wall. The application demonstrates the validity of the present algorithms and successfully suggests a appropriate preform to get uniform densification with reduced flash in the tube process.