THE EFFECT OF ADDITIONAL SHEAR STRAINS INDUCED BY DIE ROTATION ON THE RADIAL PRESSING OF METAL POWDER BILLETS

Abstract

The axial pressing of a tubular metal powder billet combined with the simultaneous rotation of the billet synchronously with the inner rod relative to the outer die wall is considered. A similar compaction pattern for nonporous compact materials known as 'high-pressure tube twisting' is used to improve the structure and design features of tubular products by strain accumulation. Unlike a compact material, the behavior of a powder billet is determined by both the hardening of its solid phase and compaction with a gradual decrease in pore volume. These factors govern the peculiar behavior of porous materials under the above conditions, requiring the plasticity theory for porous bodies to be applied. Additional assumptions regarding kinematic constraints made it possible to find explicit expressions for the hardening and the compaction pressure in an analytical form. The derived analytical solution is multiparametric and describes the effect exerted on the compaction pressure by hardening constants of the material, initial and final densities of the compacts, and degree of mutual rotation of the die walls. The solution established that the short-term application of shear strains induced by the mutual rotation of die elements reduced, in any case, the current operating pressure in the compaction process. On the other hand, in the long-term compaction process, hardening of the solid phase in a porous body resulting from additional shear strains can increase the radial pressure at the same final density. The instant the pressure decrease stage changes to the pressure increase stage significantly depends on how hard the powder material is. For this reason, the die should be rotated for powders produced of a nearly ideal plastic material. In most cases, the die rotating throughout the pressing process is advisable only for sufficiently porous billets subjected to very insignificant compaction. In the case of billets compacted to small porosities, the die walls should be better rotated only at the end of the compaction process.