Texture based formability enhancement of Mg AZ31 using high speed friction stir processing
Creators: name: Hütsch, Leon Leander
Publisher: name: TUHH Universitätsbibliothek
Year: 2014
Subjects: subject: Rührreibbearbeiten
subject: Magnesium
subject: AZ31
subject: Umformbarkeit
subject: Textur
subject: Friction Stir Processing
subject: Formability
subject: Texture
classificationCode: 620
subject: Ingenieurwissenschaften
classificationCode: 620
subject: Engineering and allied operations
subject: Magnesium
subject: AZ31

description: The widespread industrial use of magnesium alloys as a structural material is often limited by their poor room temperature formability, resulting from the low symmetry of readily available slip systems. Controlled microstructural and textural modifications, imposed by severe plastic deformation processes, have the ability to reduce the anisotropy and fur- thermore increase the ease of activating basal slip. Such modifications have been realized by friction stir processing (FSP), a derivative of friction stir welding, which can be used to impose the necessary amounts of massive local deformation without altering the shape of a structural component. In the present study, FSP has been conducted at industri- ally viable speeds of up to 20 m/min. A numerical thermal model, calibrated by thermo couple measurements, has been adapted to predict the thermal evolution in the center of the process zone in order to gain insight into the occurring recrystallization processes. The model predicts the resulting temperatures and corresponding cooling rates to be suf- ficiently high for dynamic recrystallization processes to take place as well as to prevent annealing after processing, respectively. In combination with metallurgical investigations, these results provided the foundation for a knowledge-based selection of suitable process- ing parameters within the investigated parameter range. Micromechanical and textural investigations revealed the stir zone to be particularly suited for enhanced formability. The effct of different processing conditions on the mechanical response of the stir zone has been evaluated by micro flat tensile tests showing an increase in ductility with rising processing speed, surpassing the base material values by over 80%. Additional texture investigations revealed that the observed increase can be ascribed to the continuous align- ment of the basal planes with the macroscopic shear plane, represented by the shear layer, which is surrounding the tool. It has further been shown that the geometry of the shear layer can be controlled using suitable processing parameters resulting in the ability to tailor the texture of the material, and thus its ductility. Finally, this knowledge has been transferred to process larger areas out of which formability specimens have been prepared. The evaluation of the processed as well as base material specimens has been conducted at ambient temperature, in which information obtained from a digital image correlation system has been used to establish forming limit curves. The results show a significant improvement in formability and a decrease in anisotropy of the processed over the base material.