Influence of Mg and Si Content in Aluminium Alloys on Severe Plastic Deformation Behaviour during Solid-State Coating Deposition using Friction Surfacing

• Applicant: Dr.-Ing. Stefanie Hanke
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 323162991

Dynamic recrystallization has a major influence on process characteristics and material flow in friction-based solid state joining techniques. In addition to general material properties, a.o. heat capacity and high temperature strength, dynamic microstructural processes, e.g. dislocation movement, grain boundary migration, formation of substructures or precipitation of phases, have a strong effect on the acting flow stresses. The correlation of such microstructural mechanisms and the material behaviour during Friction Surfacing or similar solid state joining techniques has not been systematically investigated up to today. Small changes in the content of alloying elements, e.g. in Aluminium alloys, require significant adaptations of the process parameters, which are to date established by empirical or statistical approaches.During Friction Surfacing (FS), a stud made from the coating material, rotating around its longitudinal axis, is pressed onto the substrate. After a short heating phase (< 2 s), the stud material adheres to the substrate surface and the rotational relative motion is accommodated by shearing the softened stud material. When an additional translational motion is superimposed, the plastified stud material is sheared off the stud and deposited onto the substrate as a coating layer. The heat required for the process is solely generated from plastic deformation.For FS of Aluminium alloys, rotational speeds up to 4000 1/min are applied, process temperatures reach approximately 80% of the melting temperature and cooling rates range at 30 K/s. Although strain and strain rates can only be estimated from the dimensions of the shear layer, obviously the deformation conditions are extreme. The available knowledge of dynamic recrystallization and flow stresses under such severe conditions is very limited, and only few publications on Gleeble-tests and high-pressure-torsion experiments at high temperatures provide some clues.In the scope of this project 6 custom-made Aluminium alloys are processed by FS. Each of these alloys only differs in its content of Mg or Si, allowing a direct comparison and therewith the investigation of the effects of those alloying elements on the material behaviour. The Si content will be raised up to 17.5 wt%, providing undissolvable hard phases during processing, which will further influence the deformation and recrystallization mechanisms. Besides examining process forces and coating geometry, XRD, EBSD and TEM investigations of the microstructural mechanisms of plastic deformation will be carried out, and correlated with the material behaviour during FS.FS typically results in very low grain sizes and spheroidization of hard phases. The mechanical properties of the obtained coatings, which are relevant for a potential industrial application of the FS process to generate coatings via severe plastic deformation, will be evaluated through micromechanical tests in the scope of this project.

• DFG Programme: Research Grants