Effects of Extrusion Parameters by KoBo Method on the Mechanical Properties and Microstructure of Aluminum

Abstract

Commercial purity aluminum was extruded by means of KoBo method at varied processing parameters. Received extrudates, with different mechanical and electrical properties and work hardening behavior, were obtained. It was found, that some conditions of KoBo extrusion process such as low initial billet temperature, low extrusion rate and low frequency of oscillating die lead to extremely high strength and high electrical resistivity of the material. The absence of work hardening (up to 40% strain) during subsequent groove rolling is also a specific feature of received materials. It was suggested that mentioned features are related to the development of overbalance concentration of point defects (clusters) generated during the extrusion process. During following cold rolling of the extrudate, mentioned defects annihilate at gliding dislocations and make the dislocation climbing and their rearrangement easier. Therefore, until the exhaustion of this mechanism, the hardening of material during cold rolling is very limited. Following increase of the material strengthening at higher rolling strains point to the return of the material to its typical behavior observed for cold deformed aluminum produced by conventional hot extrusion.Keywords: aluminum, KoBo extrusion, electrical resistivity, work hardening Aluminium o czystości handlowej poddano wyciskaniu metodą KoBo przy zastosowaniu zmiennych parametrów procesu, w wyniku czego uzyskano prasówki o zróżnicowanych własnościach mechanicznych i elektrycznych oraz odmiennych charakterystykach umocnieniowych. Stwierdzono, że w przypadku procesu wyciskania wsadu o niskiej temperaturze początkowej, prowadzonego z małą prędkością i przy niskiej częstotliwości oscylacji matrycy, prasówka wykazuje nadzwyczaj wysokie wła-sności wytrzymałościowe, wysoki opór elektryczny oraz brak umocnienia odkształceniowego podczas walcowania (w zakresie do 40%), co związano z obecnością w materiale ponadrównowagowej koncentracji defektów punktowych (klasterów), wygenerowanych w procesie wyciskania. Podczas walcowania defekty te ulegają anihilacji na przemieszczających się dyslokacjach, ułatwiając ich wspinanie oraz przegrupowanie. W konsekwencji, dopóki mechanizm ten nie ulegnie wyczerpaniu, umocnienie odkształceniowe takiego materiału podczas walcowania ma ograniczony charakter, a typowe efekty umocnieniowe, podobne do tych dla aluminium wyciskanego konwencjonalnie na gorąco, występują przy większych odkształceniach.

Microstructure and Mechanical Properties of Spark Plasma Sintered and Severely Deformed AA7075 Alloy

“…A similar behaviour in the compressive strength (a decrease from 618 to 464 MPa) is observed. These results demonstrate that softening processes such as dynamic recrystallization and dynamic recovery are the main microstructure formation mechanisms during KOBO extrusion due to the high content of point defects generated by severe deformation and significant enhancement of diffusion [12][13][14]. Nonetheless, reduction in the grain size and dynamic strain aging are hardening processes which seem to increase the hardness.…”

“…The complex twisting-extrusion method (KOBO) seems optimal for Al alloys [12,13] because of combining monotonic extrusion, responsible for the Al profile shape formation, with excessive cyclically changing deformation (reverse twisting) [12]. Generally, the main advantage of the KOBO method is due to the specific die oscillation that results in an effective reduction in the processing energy and increases the processing efficiency [14]. The lack of studies deeply analysing the microstructures and properties reached in SPSed 7xxx series alloys after severe deformation by the KOBO extrusion method hinders the efforts to find ways to effectively strengthen the alloy without compromising its tension ductility.…”

Strength properties and structure of CuCrZr alloy subjected to low-temperature KOBO extrusion and heat treatment

“…As shown [40,70], pure zinc and aluminum reach high strength properties that are thermally and mechanically stable as a result of low-temperature KOBO extrusion. Their level depends on process parameters, particularly on the extrusion force, which reacts strongly to the frequency of die oscillation [39].…”

Mechanical Properties of Product From 7075 Aluminum Chips After Consolidation by Kobo Method

“…In particular, cyclic torsion leads to deformation in the sheared layers, and thus "exposure" of new, non-oxidized surface elements of adjacent chips. High compressive stresses provide them with good mutual adhesion and in effect their stable joint at atomic level [33][34][35]. The KoBo method is an efficient, low energetic method for consolidation of chips.…”