Influence of Grinding Depth on the Surface Integrity and Fatigue Property of -TiAl Alloy

gamma-TiAl is a family of promising structural materials with low density, high stiffness, and good oxidation and creep resistances at elevated temperatures. They can replace heavier nickel-based alloys at 600-800 degrees C; thus, they are used in the construction of low-pressure turbine blades for aeroengines, i.e., General Electric next-generation and leading edge aviation propulsion. Grinding is an important processing step in blade production to ensure the accuracy of assembling. However, the limited ductility and fracture toughness at room temperature and low thermal conductivity of gamma-TiAl alloys narrow the parameter windows of the grinding process. Cracks often form on the surface when the processing parameters are not well controlled. Additionally, grinding greatly influences the surface integrity (i.e., roughness, microstructure, and hardness), which influences the mechanical properties, especially those of brittle gamma-TiAl alloys that are sensitive to notch. Grinding depth is a major parameter in blade production because it influences quality and efficiency. Investigating the effect of grinding depth on the surface integrity and fatigue properties of gamma-TiAl samples is necessary to optimize the grinding process and identify the major factors of surface integrity that guarantee optimal mechanical properties. In this work, cast gamma-TiAl alloy (Ti-45Al-2Nb-2Mn-1B, atomic fraction, %) samples were ground with different depths. The surface integrity (surface roughness, microstructure, and microhardness) and fatigue properties of the samples were compared. Cracks were detected in samples ground to 0.5 and 1 mm depths, while no cracks were detected in samples ground to 0.2 mm or less depths, this is related to the tensile stress induced by temperature increase caused by deformation heat. With the increased grinding depth, the number and depth of grooves increased and for the surface roughness parameters, arithmetic mean deviation and 10-point mean roughness (R z) increased, while skewness decreased. The gamma + alpha(2) lamellae bended in the surface layer and layer thickness increased with the increased grinding depth. The microhardness initially decreased and then increased from the surface to the interior. The rotating bending fatigue life at 650 degrees C under a load of 440 MPa decreased with the increased grinding depth: it was > 10(6) cyc at 0.05 mm grinding depth but dropped to similar to 10(4) cyc at 0.2 mm grinding depth. Fracture surface analysis showed that the cracks mainly nucleated at the surface grooves caused by grinding, which resulted in stress concentration and reduced the fatigue life of samples ground to 0.2 mm depth. The fatigue life decreased with increasing R-z, but remained above 10(6) cyc when R-z was less than 4 mu m. A nonlinear relationship between fatigue life and R-z was shown.