Effect of Heat Treatment on the Fatigue Behavior of Additively Manufactured IN718

The Nickel superalloy IN718 has gained a lot of importance in aerospace, power generation, and petrochemical sectors owing to its outstanding ability to retain mechanical strength, corrosion resistance, and resistance to creep failure at temperatures up to 650 °C. Additive manufacturing (AM) is a more effective method of processing complex geometry IN718 parts because traditional manufacturing processes face difficulties due to the high strength and hardness of IN718. Parts produced through AM have residual stresses resulting from rapid cooling rates, making post-AM heat treatment essential for achieving enhanced microstructural and mechanical characteristics for both static and fatigue loads. Hence, in the present study, rotary bending fatigue characteristics of the additively manufactured IN718 specimens with and without heat-treatment states were studied. All the IN718 samples were fabricated as per the ISO 1143/ASTM E606 R.R. Moore completely reversed bending fatigue testing standard on a Renishaw RenAM 500e LPBF machine with a layer thickness of 60 m, laser power of 200 W, laser beam feed rate of rate of 1000 mm-s-1, and 0.1 mm of hatch spacing. The fatigue samples were tested on a rotary bending type TecQuipment SM1090 fatigue testing machine at a constant load of 75 kN, which resulted in a maximum bending stress of 338.9 MPa. The as-built specimens survived for a life of 100,096 cycles, whereas those specimens subjected to stress-relief heat treatment survived for 184,670 cycles, showing superior fatigue strength. The microstructural study was done using SEM to analyze the interstitial phases. SEM analysis reveals that the as-built sample contains the Laves phase, and in the heat-treated sample, this phase is dissolved. This Laves phase can prove to be detrimental to the fatigue life of the additively manufactured IN718 parts at high temperatures.