The Effect of Additional Shear Strains Induced by Die Rotation on the Radial Pressing of Metal Powder Billets

The axial pressing of a tubular metal powder billet accompanied by simultaneous rotation of the billet synchronously with the inner rod relative to the outer die wall has been considered. Such a compaction pattern for nonporous compact materials is known as ‘high-pressure tube twisting’ and used to improve the structure and design features of the tubular product by strain accumulation. Unlike a compact material, the behavior of a powder billet is determined both by hardening its solid phase and compaction with a gradual decrease in pores volume. These factors specify the peculiar behavior of porous materials under the formulated above conditions, which requires the plasticity theory for porous bodies to be applied. Additional assumptions concerning kinematic restrictions enabled finding explicit expressions for hardening and compaction pressure in analytical form. The derived analytical solution is multi-parametric and describes the influence exerted on the compaction pressure by hardening constants of the material, initial and final density of the compacts, and degree of mutual die walls rotation. The performed analysis allowed concluding that the short-term application of shear strains induced by the mutual rotation of the die elements reduces, in any case, the value of the current operating pressure during compaction. On the other hand, in the long-term compaction process, the hardening of the solid phase in a porous body caused by additional shear strains can lead to an increase in radial pressure at the same final density. The change from the pressure decrease stage to pressure increase stage significantly depends on how hard the powder material is. Therefore, the die is advised to be rotated for powders produced of a nearly ideal plastic material. In most cases, the rotation of a die throughout the entire 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 rotated only at the end of the compaction process.