Abstract
In Friction Surfacing, material is deposited onto a substrate in plasticised state, using frictional heat and shear stresses. The coating material remains in solid state and undergoes severe plastic deformation at high process temperatures (≈0.8Tmelt), followed by high cooling rates in the range of 30 K/s. Dynamic recrystallization and the thermal cycle determine the resulting microstructure. In this study, Ni-based alloy 625 was deposited onto 42CrMo4 substrate, suitable e.g. for repair welding of corrosion protection layers. Alloy 625 is known to undergo discontinuous dynamic recrystallization under severe plastic deformation, and the resulting grain size depends on the strain rate.
The coating microstructure was studied by microscopy and electron backscatter diffraction. The coatings exhibit a fully recrystallized microstructure with equiaxed grains (0.5 to 12 μm) and a low degree of grain average misorientation. Flow lines caused by a localized decrease in grain size and linear alignment of grain boundaries are visible. Grain nucleation and growth were found to be strongly affected by localized shear and non-uniform material flow, resulting in varying amounts of residual strain, twins and low angle grain boundaries in different regions within a single coating layers’ cross section.
Friction Surfacing can be used to study dynamic recrystallization at high temperatures, strains and strain rates, while at the same time materials with a recrystallization grain size sensitive to the strain rate can be used to study the material flow during the process.
