Inconel 718 is a nickel-based precipitation-hardening alloy designed for very high yield strength, tensile strength and creep rupture properties even at very high temperatures. As such, Inconel 718 has been used in the fabrication of gas turbine disks, rocket motors, space crafts, nuclear reactors, pumps and tooling for its excellent hot corrosion resistance, remarkable high static and fatigue strengths at elevated temperatures, wear resistance and good weldability. Unfortunately, machining of Inconel 718 at room temperature is difficult due to extensive tool wear and poor material removal rates. Advances in the 3D printing technology has borne its most advanced yet technique; the powder bed fusion by a laser beam (L-PBF), whose full exploitation often yield parts that are lighter in weight, cheaper to produce and have complex geometries that are difficult or impossible to produce conventionally. To date, fatigue strength (i.e. durability) of L-PBF Inconel 718 parts is one of the most important challenges to the qualification of structural parts and, therefore, to the widespread application of L-PBF technology in industry. Noteworthy publications have demonstrated that post fabrication HIP treatment of Inconel 718 would heal internal defects but for a significantly increased part cost.
Generally, the surface quality of L-PBF parts with as-built surfaces negatively affects their fatigue performance and post fabrication surface finishing may be cost-wisely unacceptable in most applications and often impossible because of surface inaccessibility. To address this, Professor Gianni Nicoletto from the Department of Engineering and Architecture at University of Parma in Italy conducted an in-depth assessment on both notched and unnotched L-PBF Inconel 718 and provide additional valuable knowledge on the notch fatigue behavior of the L-PBF Inconel 718 compared with the findings of recent publications. His work is currently published in the research journal, International Journal of Fatigue.
In his approach, two aspects significantly affecting the fatigue performance of L-PBF Inconel 718 parts were investigated; i.e. the directional knock-down factor of the as-built surface state with respect to the surface machined condition, and the role of a geometrical notch in the as-built surface state as commonly found in complex L-PBF parts. Generally, the researcher tested eight batches of un-notched and notched miniature specimens of heat treated L-PBF Inconel 718 with as-built surfaces, in a cyclic plane bending.
The author reported that the unnotched fatigue strength was sensitive to direction of the applied stress with respect to build direction. Additionally, the as-built notch fatigue strength of heat treated L-PBF Inconel 718 was seen to be also highly directional in nature. Remarkably, the un-notched and notched fatigue behavior of L-PBF Inconel 718 with as-built surfaces obtained with the novel test method using miniature specimens were seen to compare satisfactorily with recently published data for the same material and L-PBF technology but different specimen geometry and test method.
In summary, the original evidence on the unnotched and notched fatigue behavior of L-PBF Inconel 718 with as-built surfaces presented in the study provided new knowledge about the structural design of metal additive manufacturing parts. Overall, results in consensus with existing literature were achieved and novel knowledge on the subject matters introduced. In a statement to Advances in Engineering, Professor Gianni Nicoletto stressed that the presented experimental methodology based on the use of miniature specimens in cyclic bending is a valuable tool for the efficient evaluation of the fatigue response of L-PBF metals as proved by on-going industry-sponsored programs aimed at the L-PBF fabrication of advanced structural metal parts.
Gianni Nicoletto. Smooth and notch fatigue behavior of selectively laser melted Inconel 718 with as-built surfaces. International Journal of Fatigue, volume 128 (2019) 105211