Study of the surface integrity and mechanical properties of turbine blade fir trees manufactured in Inconel 939 using laser powder bed fusion

In this study, Inconel 939 turbine blade fir tree samples were fabricated by the additive manufacturing method using a direct metal laser sintering (DMLS) machine. This critical gas turbine connection part requires high geometric accuracy and excellent mechanical properties. The fir tree profiles of turbine blades were machined using creep feed grinding (CFG) and wire electrical discharge machining (EDM) methods. The effects of CFG and wire EDM on the recast formation, heat affected zone (HAZ), geometrical accuracy, microhardness, and tensile strength were compared. The experimental results revealed that the CFG process was superior in recast formation, HAZ, and microhardness, whereas better geometrical accuracy and ultimate tensile strength values were measured for the wire EDM method. The ultimate tensile strengths of turbine blades shaped using wire EDM were higher than the CFG method and measured 769 MPa and 748 MPa, respectively. Profile deviation and recast formation were the most influential parameters on the ultimate tensile strength of the fir tree geometry. The specimens were fractured from the pressure faces of the fir tree root during the tensile tests. Microscopic voids, cracks, and chloride impurities that would facilitate the propagation of fatigue cracks were observed on the fractured surface.