Influence of Mg and Si Content in Aluminium Alloys on Severe Plastic Deformation Behaviour during Solid-State Coating Deposition using Friction Surfacing
Final Report Abstract
Solid state joining techniques bring about many advantages, for example they do not require an open flame or arc, no auxiliary materials like gases and they are suitable for automating. In this project, the process Friction Surfacing (FS) was investigated, which is a solid state process for deposition of thick metal coatings. Solid state joining processes have in common that the heat required for joining is created partially from friction, but mainly from plastic deformation of the welding metals. The material undergoes severe plastic deformation, which in addition to heat results in a complex sequence of alterations in the microstructure, which in turn affect the material behaviour during joining. Dynamic recrystallization takes place, i.e. a dynamic balance of introduction of crystal defects by deformation and their healing out by recrystallization and recovery prevails during processing. Since strains, strain rates and the temperature are very high, the material behaviour under such conditions is not fully understood to date, and process development relies largely on trial-and-error approaches. In this project, 6 Al-base alloys differing only in their content of Mg or Si were processed by FS. It was investigated how the change in chemical composition affects the process behaviour and the resulting coating dimensions. The microstructure was studied in detail, from light- down to transmission electron microscopy, in order to understand the differences in microstructural mechanisms which may explain the differences in macroscopic process behaviour. Finally, sliding wear tests were conducted in order to clarify, whether the microstructural changes occurring during deposition are beneficial for the wear resistance of the materials. It was found that an increase in Mg in the alloy up to 3.5% leads to a pronounced reduction in deposit dimensions, and requires a significant reduction in process speeds and increase in process force. An increase in Si yields the same trends, but to a significantly lower extent, even for the alloy with the highest amount of Si (14.6%). When processing Mg-rich alloys more heat is generated than in case of the Si-rich alloys. A major microstructural difference between the two alloy groups is that Mg can be dissolved in the Al matrix at process temperatures, while Si has very low solubility and mainly remains present as hard phases. By increasing the Mg content in solid solution the stacking fault energy (SFE) is reduced, slowing down recovery processes in the deformed material. The dynamic balance of defect introduction and their healing out during processing is therefore shifted towards a higher number of defects present. The availability of a higher number of dislocations results in a smaller grain size and a more complete recrystallization in FS deposits from alloys with higher Mg content. At the same time, it can be assumed to rise the flow stresses during processing, which explains the need for higher forces and lower process speeds. Changing the dislocation behaviour and microstructural mechanisms of plastic flow and heat generation by solute foreign atoms (Mg) obviously has a significantly stronger effect on the FS process than the presence of hard phases (Si) in the microstructure. Al-base alloys with 10.6% and 6.6% Si in the form of FS coatings show significantly lower wear rates compared to the as-cast state. This is due to a spheroidisation of previously lamellar Si-phase. Such “bulky” Si-phase particles carry the contact load more efficiently and improve wear resistance. For the high (hypereutectic) Si content no improvement of wear resistance was found, because here “bulky” Si-phase particles are already present in the as-cast state, yielding a low wear rate in either microstructural state. Through processing by FS, cast Si-rich Al alloys are available as coatings providing locally enhanced wear resistance for components from wrought Al alloys.
Publications
- On the influence of heat treatment state on the processing behavior of aluminum alloys during Friction Surfacing. MSE 2018, Darmstadt, Germany, 26.09.2018
J. Ehrich, C. Feng, A. Roos, S. Hanke
- Effect of Mg and Si content in aluminum alloys on friction surfacing processing behavior. In: Chesonis C. (eds) Light Metals 2019. The Minerals, Metals & Materials Series. Springer, Cham
J. Ehrich, A. Roos, S. Hanke
(See online at https://doi.org/10.1007/978-3-030-05864-7_45) - Effect of Mg and Si Content in Aluminum Alloys on Friction Surfacing Processing Behavior. TMS 2019 Conference, San Antonio, Texas, USA, 13.03.2019
J. Ehrich, A. Roos, S. Hanke
- Precipitation distribution in the heat-affected zone and coating material of AA 2024 processed by Friction Surfacing: 17th International Conference on Aluminum Alloys (online). Grenoble, Frankreich, 26.10.2020
J. Ehrich, P. Staron, A. Roos, A. Karkar, S. Hanke
- Sliding wear of Al-base alloys in wrought state Friction Surfacing. MSE 2020 (online), Darmstadt, Germany, 22.09.2020
S. Hanke, M. Schütte, A. Roos, J. Ehrich, D. Linsler
