Studies of residual stress, microcracks, hardness and microstructure of cold compacted metallic green bodies
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Studies of residual stress, microcracks, hardness and microstructure of cold compacted metallic green bodies Torsten Ericsson, Cecilia Larsson and Ru Lin Peng, Linköping University, 581 83 Linköping, Sweden ABSTRACT The residual stresses have been measured by X-ray and neutron diffraction on PM green bodies manufactured by conventional and high speed compaction of iron powder with and without added copper and brass powder. Compressive residual stresses are present in a thin layer in both top and side surfaces. They are largest in the side surfaces due to plastic deformation of the surface material caused by the friction forces during ejection out of the die. In the interior of the green body residual stresses exist with certain region under compression (periferical regions) and other under tension (more central regions). It is unclear whether mixing iron powder with brass or copper powder leads to considerable phase stresses between the two phases. INTRODUCTION Cold compaction of metallic powder in a die results in a so called green body. The compaction process when a punch is pressed into the filled die cavity is made up of successive stages as it is usually described in text books. In the first stage a rearrangement of the grains occurs, in the second step the grains start to deform plastically and work harden and in the third stage the deformation is more homogenous. The density that is achieved for a given powder type or mixture of powders depends on the pressing force and the rate of compaction. The density expressed as the density relative to the theoretical density is normally in the range 0.7 to 0.9 for steel samples using normal press forces, 500 to 700 MPa. A relatively new technique using high speed hydraulic presses gives densities in the range 0.95 to 0.99 [1]. The green body must have enough strength to endure the release from the pressing die and the transport to the sintering furnace, the burning off of lubricants and the heating to the sintering temperature. This requires both a high enough green strength and freedom from defects such as cracks that are so large that they do not heal during sintering. The cold pressing leads to a nonhomogenous density in the green body due to friction between the powder and the die wall and to a lesser degree between the grains. All these aspects are well described in handbooks and textbooks, e.g. [2,3]. Two mechanisms are used to explain the origin of the green strength: cold welding and mechanical interlocking of particles. The former depends on the formation of intermetallic bonds between neighboring particles during pressing and the latter on the locking of irregularities on the surface of adjacent grains, a kind of clinching effect. In both cases irregular grain surfaces are beneficial. A good review of the testing and the theories for green strength has been given by Lampman [1]. In later years FE modeling of uniaxial coldpressing of powders have been carried out using finite element techniques, e.g. [4,5]. They can relatively well describe the densit
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