Sintering schedule for warm formed iron powder compacts

Introduction Warm compaction process has shown to provide increased density in ferrous powder metallurgy parts. This improvement in density contributes significantly to mechanical properties and thus the overall performance of the part. The combination of increased density with high performance material selections provides parts that can exceed the performance of forged or cast material counterparts while taking advantage of powder metallurgy's net shape forming capabilities [1-3]. During the forming of powder mass to a predesigned shape, a significant friction force is generated which is the major cause of density variation and also affects the component deformation. Inter-particle and particle to die wall friction also hinder pressure transmission and therefore produce density gradients inside the compact. The friction also generates significant amount of heat that affects the compaction process generally, and material properties specifically. Therefore, there is a need to use lubricant as friction reduction agent [4-6]. Sintering is a processing technique which is performed to produce density-controlled materials and components from metal or ceramic powder by applying thermal energy [7-10]. It also acts as a thermal treatment process in powder metallurgy. Small particles of a material are bonded together by solid-state diffusion during the sintering process. As material synthesis and processing become crucial in recent years for materials development, the importance of sintering is increasing as a part of metal processing technology through powder route. Unlike other processing technologies, various processing steps and variables are required to be considered for the production of high quality sintered parts. For example in the shaping step, one may use simple die compaction, isostatic pressing, slip casting or injection molding, according to the shape and properties required for the end products [11]. The sintering conditions and also the sintered properties may vary considerably depend on the shaping techniques used [12]. Various techniques and processing variables in sintering step can give variations in sintered microstructure and properties. In general, sintering process aims to produce sintered parts with designed microstructure through the control of sintering variables. Microstructural control means the control of grain size, sintered density, size and distribution of other phases including pores. In most cases, the final goal of microstructural control is to prepare a fully dense body with a fine grain structure [13]. Review in the existing literature in powder metallurgy indicates that the mechanical properties as well as the microstructure of the final product can be affected by the sintering schedule i.e., sintering temperature, heating and cooling rate, and holding time. Although some researchers have gone through this area, there is no published information on the proper sintering schedule for the production of high quality, near-net shape components from iron powder mixed with lubricant and formed at above ambient temperature. Therefore, the objective of this paper is to investigate the effect of sintering temperature, holding time and heating/cooling rate together with zinc stearate addition to the mechanical properties and microstructures of sintered iron-base components formed through warm compaction. Materials and Method A lab scale warm compaction rig was designed and fabricated (Figure 1) which enabled the generation of green compacts at the designated temperature, i.e., 180oC. The main powder used in this project is iron powder ASC100.29 and zinc stearate (C36H70O4Zn) powder was selected as admixed lubricant. The as-received iron powder has the particle size range of 20-180 μm. Two types of green compacts were prepared, i.e., green compacts without and with zinc stearate. In the case of green compacts with lubricant, the feedstock was prepared by mechanically mixing iron powder ASC 100.29 with 0.4 wt% of zinc stearate for 30 minutes. The whole experiment is divided into three sections which are green compact generation, sintering of green compacts in argon gas fired sintering furnace, and characterization of final products through mechanical testing and microstructure evaluation. Sintering schedule for warm formed iron powder compacts M. M. Rahman, S. S. M. Nor and H. Y. Rahman Department of Mechanical Engineering, Universiti Tenaga Nasional Putrajaya Campus, Jalan IKRAM-UNITEN43000 Kajang, Selangor, Malaysia.