Friction surfacing is a relatively novel joining technology with a great potential to be applicable for deposition of various metallic materials. The processing of each new introduced material combination is rather challenging and significant; unexpected differences in material behaviour are regularly observed. Therefore, each new material combination can contribute to a more comprehensive understanding of the FS process.
In the current work, successful deposition of titanium alloy coatings by friction surfacing was demonstrated. During process development flash formation at the coatings, which has not been reported so far for any other materials, was investigated. Differences in the material behaviour in two rotational speed regimes were identified, which was only possible because an extensive parameter range was applied. Another important point of this work was the investigation of resultant microstructural and mechanical properties. The following conclusions are grouped into process development, characterisation of the coatings, and microstructural and mechanical examinations.
Process Development
The current study has demonstrated a thorough process development for the generation of Ti-6Al-4V and Ti-Gr.1 coatings by FS. The coatings have been deposited successfully in a wide range of process parameters. The use of the consumption rate control mode instead of conventional force control for depositing titanium coatings turned out to be a suitable mode due to the susceptibility to force and plastic flow instability of the material. The rotational speed has been varied in a broad spectrum. Two regimes of the rotational speed have been established.
In the first regime, flash generation at the coating may occur at certain combinations of the rotational and deposition speeds. Both parameters are responsible for the temperature development and strain rates that affect the material flow stresses while depositing, resulting either in domination of thermal softening (flash generated at the coating) or flow stability (flash formed at the rod).
In the high rotational speed regime, the temperature achieves its maximum. Here, the flash generation at the coating is precluded, and an alteration of rotational speed and deposition speed does not influence the temperature distribution significantly.
Characterisation of the Coatings
The geometry of the coatings varies with changing rotational speed and resultant axial force.
The employment of low rotational speeds requires high axial forces that lead to deposition of thin and wide coatings. In contrast, the employment of high rotational speeds results in low
axial forces that generate thick and narrow coatings. In this context, the deposition efficiency varies with changing rotational speed and resultant forces. The highest deposition efficiency was 38 % for Ti-6Al-4V and 52 % for Ti-Gr.1 when employing a high rotational speed.
Microstructure Evolution
The temperature history based on the thermocouples and thermal camera examinations have shown that the peak temperature exceeds the beta transformation field in all experiments. The temperature achieves its peak during the plastification phase, followed by a steady state. The temperature evolution, including the peak temperature and heating rate, is affected by the rotational speed.
Due to the fact that the process temperature exceeds the transformation temperature, the material is deformed in the beta phase. Here, during processing, dynamic recrystallisation takes place. While cooling, Ti-6Al-4V passes the temperature field of martensitic initiation.
Therefore, the phase transformation of the coatings occurs martensitically because of the high cooling rate, so no diffusion occurs. Furthermore, the influence of the combination of strain, strain rate and temperature on the recrystallised β grain size has been observed. It has been shown that by using a low rotational speed and the resultant low temperatures, the grains become refined relative to the base material. By raising the rotational speed, the temperature increases, and the grains become coarse. With an additional rise in rotational speed, the temperature does not increase any further, and the additional grain growth is precluded.
Moreover, the separation of the thermal cycles from the deformation induced on the coatings by FS have been realised via dilatometer tests. It has been demonstrated with the dilatometer results that the employment of such high temperatures in titanium samples leads to grain growth which is even more predominant when increasing the temperature. The grain growth is less pronounced when using identical temperatures with additional deformation applied by the FS process.
Mechanical Properties
The mechanical properties of the coatings have been examined by micro hardness analysis, micro flat tensile measurement and investigation of the fretting fatigue behaviour. The micro hardness behaviour of the pure titanium coatings has not been affected significantly by the process because no martensitic structure has been formed. Conversely, Ti-6Al-4V coatings have featured a hardness increase of approximately 15 % with respect to the martensitic structure formed by FS and high dislocation density, as observed by TEM analysis.
The micro flat tensile experiments have been conducted on coatings and the base material.
The tensile strength of the coatings has been increased along with a loss in ductility.
It could be seen that the fretting wear evolution can be divided into three regimes. First, the cracks are propagated, then the material above the cracks is deformed during the tests, and finally, the particles lost from the initiated cracks are used as filler material for the worn grooves. The filled grooves avoid further wear of the matrix, and these agglomerated particles can be seen as movable hills, protecting the matrix. The fretting wear behaviour of the coatings and base material are similar. The coating properties with regard to wear behaviour cannot be improved. However, the wear behaviour is still comparable so that, friction surfacing can be used as a repair method.
The mechanical properties of the coatings presented in the current study have demonstrated fretting wear behaviour that is at least similar to that of the base material. The micro tensile results of the coatings have shown higher strength and lower ductility than the base material.
Fusion-welded materials rarely achieve the mechanical properties of the base material [9].
The failure to deposit titanium coatings by friction surfacing reported in previous publications has been overcome in the current study. Furthermore, it was shown that, given the proper control mode, coatings of various dimensions can be produced at high quality and reproducibility. The coatings exhibit encouraging results regarding mechanical properties.
This work has opened up the new possibility and knowledge to apply friction surfacing for titanium alloys, which can be effectively used for repair of worn or damaged parts.