Nickel-based superalloys are precipitation hardened alloys with complex compositions. They are used in aircraft engines and land-based gas turbines in load-bearing structural components that are exposed to high temperatures. Failure mechanisms in this environment are high and low cycle fatigue, creep, and corrosion.
During manufacturing, residual stresses are often introduced into the material due to inhomogeneous plastic deformations, both intentionally and unintentionally. One such manufacturing process is metal cutting, which introduces residual stresses in the surface layer. The stress state in the near-surface zone of components is of special interest as the surface often experiences peak loads and cracks have their starting point there.
In this thesis, near-surface residual stress distributions and microstructural changes are studied in the nickel-based superalloy Inconel 718 for two different turning operations, face grooving and facing. Process variables are in both cases cutting speed and feed that have been varied between (10 and 1200) m/min and (0.01 and 0.5) mm, respectively.
The first turning technique face grooving, which gives cutting conditions similar to orthogonal cutting, showed a clear dependency of the residual stresses on the cutting speed. The tensile stress at the surface, the maximum compressive stress below the surface, and the thickness of the affected layer increase with increasing cutting speed. The tensile stresses are constrained to a thin surface layer and compressive residual stresses below the surface dominate the depth profile of the residual stresses. Only at low cutting speed, residual stresses were largely avoided.
The second turning technique facing confirmed the dependency of the residual stresses on the cutting speed and revealed a similar dependency on the feed. Microstructural investigations of near-surface cross-sections by means of transmission electron microscopy showed a zone where the grains had undergone plastic deformation, indicated by slip bands. On top of this layer, a surface layer exists where the grain size has radically decreased to between 50 nm and 130 nm. The grain size of the nanocrystalline layer is fairly constant for a certain cutting speed and feed but depends on these cutting parameters. An increase in cutting speed and feed leads to larger grains in the nanocrystalline layer.
Linköping: Linköpings universitet , 2003. , 34 p.