The effect of workpiece grain size on surface integrity in orthogonal cutting of Inconel 718

Inconel 718 represents a large portion of high-temperature metals in key industries, where the machining-induced surface integrity is quite important. As the different grain size results in various densities of grain boundaries in the deformed area, the surface integrity can be significantly affected. This paper studies the effect of workpiece grain size on machining-induced surface integrity when cutting Inconel 718. Workpiece materials with fine or coarse grain features are prepared with various heat treatment methods. Orthogonal cutting experiments are conducted with uncoated carbide tools under different cutting speeds (20 - 100 m/min). The cutting force results indicate that Inconel 718 with larger grain size will cause lower cutting forces and the percentage reduction approximately ranges from 20 % to 33 %. An interesting phenomenon is found that the workpiece with coarse grains exhibits large-scale geometrical defects due to tearing within the grains at low cutting speed (v_c = 20 m/min). This phenomenon can be diminished if the cutting speed v_c is increased to 60 m/min. In addition, the subsurface deformation layer is reduced with decreasing workpiece grain size due to the barrier effect of grain boundary on plastic deformation.