Flow drilling and flow tapping as a mean to improve the fatigue life of specimens representing plate-type orthopedic implants

• Flow processing induce ultrafine grain structure and work hardening. • Crest folds are created by flow tapping. • The fatigue life of flow formed holes is shorter than its conventional counterpart. • Folds are preferential fatigue crack initiation sites. • Flow forming increase the amount of sigma phase and delta ferrite. This study investigates the impact of flow drilling and flow tapping on the microstructure, microhardness, and fatigue life of 316L stainless steel specimens, which are designed to simulate plate-type orthopedic implants. In this novel approach, threaded holes are created by combining both cutting and deformation techniques. Three methods are examined: cut drilling with cut tapping, flow drilling with cut tapping, and cut drilling with flow tapping. The resulting specimens are analyzed through microscopic observations, microhardness testing, and four-point bending fatigue tests, conducted in accordance with ASTM standards. The S-N curves and fractographic analyses reveal that cutting methods generally offer superior fatigue performance. At the lowest stress level, the average fatigue life of the flow tapped specimens is 4.10 5 cycles, whereas specimens with holes formed with cutting method exhibit an average fatigue life of 1.5x10 6 . Specifically, cut threads are free of discontinuities, whereas flow threads systematically had crest folds. These folds are sites for crack initiation that contributes to the reduction of fatigue lives. Furthermore, the presence of the sigma phase, related to flow drilling and flow tapping, contribute to material embrittlement, which adversely affects fatigue life. Although deformation methods did not yield encouraging results for the 316L specimens, the characteristics they impart, such as the formation of ultrafine grains, may prove beneficial for other materials and applications. This study is the first to evaluate the fatigue life of threaded holes produced through a combination of cutting and deforming manufacturing methods.