**4. Summary and future trends**

The speed difference between the viscoplastic material, which is rotating along with the rod at vxy, and the material effectively joined to the substrate (vxy = 0), causes the deposit to detach from the consumable. This viscous shearing friction between the deposit and the consumable

Since the deposited material at the lower end is pressed without lateral confinement, it flows outside the consumable diameter, resulting into a revolving flash attached to the tip of the consumable rod and side unbounded regions adjacent to the deposit. Flash and unbonded regions play an important role as boundary conditions of temperature and pressure for the

The process allows the deposition of various dissimilar material combinations as the deposi‐ tion of stainless steel, tool steel, copper or Inconel on mild steel substrates, as well as, stainless

The influence of processing parameters on the deposit characteristics and bonding strength has been studied [26,27] aiming to correlate the resulting coating geometrical characteristics (thickness and bonded width) and mechanical performance with forging force, spindle and travel speeds. The increase of forging force improves the bond strength and reduces the coating thickness. The undercut region decreased when the forging force increased and the travel speed decreased. Higher ratios between the consumable rod feeding rate and the travel speed resulted in superior bonding quality. The applied load on the consumable rod was found to be essential to improve joining efficiency and to increase the deposition rate. Higher rotation or travel speeds were detrimental for the joining efficiency. Tilting the consumable rod along the travel direction proved to improve the joining efficiency up to 5 %. The material loss in flashes represented about 40 to 60 % of the total rod consumed, while unbonded regions were reduced to 8 % of the effective coating section in mild steel deposition. Friction surfacing was seen to require mechanical work between 2.5 and 5 kJ/g of deposited coating with deposition

steel, mild steel and inconel consumables on aluminium substrates.

Fig 11 shows typical material combinations tested using FS with successful results.

is the most significant heat source in the process.

**Figure 11.** Different coatings/substrates combinations

joining process.

16 Modern Surface Engineering Treatments

Friction based processes comprise Friction Stir Processing (FSP) and Friction Surfacing (FS).

Friction stir processing is mostly used to locally eliminate casting defects and refine microstructures in selected locations, for property improvements and component perform‐ ance enhancement. Aluminium and steel castings are amongst the most common compo‐ nents improved by this technology aiming at eliminating porosities, destroy solidification structures with inhomogeneous segregated phases, refine grain structures improving nservice performance.

The recent advances in adding reinforcing particles to manufacture surface alloys and metal matrix composites is a breakthrough in this technology opening new possibilities to manu‐ facture composites nanostructured with tremendous properties.

Friction surfacing has been used in the production of long-life industrial blades, wear resistant components, anti-corrosion coatings and in the rehabilitation of worn or damaged parts such as, turbine blade tips and agricultural machinery. Other applications feature the hardfacing of valve seats with stellite and tools such as punches and drills.

Since the deposits result from severe viscoplastic deformation, friction surfacing presents some advantages over other coating technologies based on fusion welding or heat-spraying processes, that produce coarse microstructures and lead to intermetallics formation, thereby deteriorating the mechanical strength of the coatings. However, friction surfacing currently struggles with several technical and productivity issues which contribute to a limited range of engineering applications.
