Effect of Magnesium Powder Application on the Microstructure and Properties of Rods Extruded by the Forward-Backward Rotating Die Extrusion Method

Abstract

The effects of severe plastic deformation (SPD) with a forward-backward rotating die (KOBO extrusion) on pure magnesium, in the form of cold-compacted powder, sintered powder, or cast ingots as reference, were examined. This method is known to reinforce metals, but the role of the initial form of magnesium applied in the fabrication of metal-based rods, as well as related phenomena, has not been characterized until now. The problem is important in the potential processing of commercial metal powders, the recycling of metal shavings, and the fabrication of metal matrix composites with discontinuous reinforcing phases. In the presented experiments, rods of 8 mm in diameter and 400 mm in length were obtained, and the structural effects induced by KOBO that occurred on a macro- and microscale on the surface and cross sections were characterized. Changes in the size and orientation of α-Mg crystallites were determined by XRD. The porosity, hardness, tensile strength, and compressive strength were measured, and the mechanisms of decohesion dependent on starting metal form were analyzed. After KOBO extrusion, significant differences were observed in the microstructure and properties between the materials derived from cold-compacted powder, sintered powder, and reference cast magnesium. Due to the application of KOBO, apart from α-Mg grain refinement, the MgO derived from the initial powder’s surface was refined to fine regular particles surrounded by magnesium. Their bands curved in the perpendicular plane and were oriented with the extrusion direction of the formed network, which augmented some mechanical properties and changed the decohesion mechanism. The conducted experiments revealed that before extrusion by KOBO, the magnesium powder required sintering under pressure.

Analysis of the Effects of an Unconventional Rolling Process under Cumulative Plastic Deformation Conditions

“…The research focused on determining the causes and understanding the effects of self-excitation and forced change of the deformation path, regarding the possibility of controlling the structure of the deformed metal, and thus its mechanical properties. The concept allowed the development of an original method of extrusion of KoBo metals with a cyclic torsion matrix [23][24][25][26].…”

Processability of Mg-Gd Powder via Friction Extrusion

“…Next to FE, similar solid state extrusion processes like CoreFlow [13] or KOBOextrusion with the forward-backward rotating die method [14] have been used to process Mg, confirming the main challenges, which are achieving a controlled microstructure and homogeneity in the extrudate as well as ensuring the stability of thermal and time-dependent conditions during processing. Overall, these investigations are highlighting the interest in the studied processing technique FE.…”