Investigation of bonding mechanisms of friction surfacing coatings by scanning- and transmission-electron-microscopy
Ahmet Karkar, University Duisburg-Essen, Duisburg, GermanyJonas Ehrich, University Duisburg-Essen, , Arne Roos, Helmholtz-Zentrum Geesthacht, , Stefanie Hanke, University Duisburg-Essen, Duisburg, Germany
Friction Surfacing is a solid-state joining process, by which metal coatings can be deposited without obvious dilution. The coating material is plasticized by friction and severe plastic deformation, and undergoes dynamic recrystallization during deposition. Depending on process parameters and material combination, in the substrate material a heat affected zone but hardly any deformation is observed. The bonding mechanisms are to date not fully understood. Theories as to the reasons for the good joint strengths are based on mechanical interlocking or diffusion, but no detailed investigations have been carried out. In this study, microstructural features in the bonding zone of coatings from Al alloys 5083 and 6082 on substrate from Al alloy 2024, as well as from Ni base alloy 625 on carbon steel were investigated by Scanning- and Transmission-Electron-Microscopy.
The results show that processing times were too short for significant diffusion to take place between the Ni and Fe based materials. All material combinations showed a sharp interface between coating and substrate by SEM. Nevertheless, for the Al alloys TEM investigations showed that a transition region exists between the fine grained coatings and the coarse microstructure of the substrate. Within this region, oxide particles from the sheet surface are distributed along grain boundaries, and irregular grain sizes and precipitate distributions were observed.
Further studies are required to investigate the microstructure in the bonding region in more detail, and to clarify effects of different joining parameters (forces, speeds…) and variations between different material combinations. Still, the results of this study imply that joining in FS is mainly achieved by the plastic deformation at high temperatures and process pressures, which leads to a direct metal-metal contact across large areas of the contacting surfaces and in some cases the mechanical mixing of a thin layer of the materials.