To address the size limitation of the powder bed fusion system in additive manufacturing, the welding properties of 316L stainless steel manufactured by SLM 125HL are investigated by conducting hot ductility test and nil strength temperature (NST) test with a physical thermal mechanical simulator, Gleeble. In this study, the print orientations (Zdirection and XY-direction) and the laser patterns (stripe and checker board) are studied. In NST test, the orientation showed a statistical significance in NST: Z-direction was 1384°C and XY-direction was 1400°C. In hot ductility test, all of ductility curves show similar behaviors: hardening region, recrystallization region, and liquation region. The additively manufactured 316L shows poor ductility compared to wrought 316L stainless steel. Also, there is a noticeable difference in ductility between laser pattern. Finally, ductility after the thermal cycle shows higher than that before the thermal cycle. For the future recommendation, investigation on the interelayer temperatures and sigma phase determination should be conducted to confirm the hypotheses to explain the phenomena observed in this study.

Nickel based superalloys were developed to withstand the severe thermal and mechanical environment associated with rocket propulsion systems and jet engines. In many alloy systems the strength of a component rapidly deteriorates as the operating temperature increases. Nickel based superalloys, however, retain strength over a range of temperatures which includes the operating range for many propulsion systems. This improved performance is accomplished by a combination of solid-solution strengthening, precipitation strengthening and grain-boundary strengthening. Furthermore, super-alloy systems are designed for ease of fabrication, to include machining, welding and heat treating. Inconel 718 was developed to overcome problems with post-weld cracking that were common in precipitation hardened nickel based superalloys strengthened by γ’. Inconel 718 is strengthened by γ’’ and is less sensitive to cracking during post-weld thermal treatment. However, in some cases, compositional changes which improved the behavior of these alloys during stress relief actually led to greater difficulty during the joining process. Many approaches have been used to determine the hot-cracking sensitivity of Inconel 718. Historically, two approaches have been particularly valuable because of their repeatability, their ability to compare different alloy systems and their verisimilitude to actual fabrication. These are the Gleeble hot-ductility test and the Variable-Restraint (Varestraint) weld test. Varestraint samples were prepared as per standard preparation techniques and tested longitudinally with a GTAW. At a predetermined location a strain was applied perpendicular to weld direction. The applied strain varied from 0.25%, 0.5%, 1.0%, 2.0%, and 4.0%. The Inconel 718 yielded a maximum crack length of 0.6 mm with a saturation strain of 2.0%. Both the total crack length and the number of cracks did not have a saturation strain. Gleeble samples were prepared from rod stock and tested with standard methodology to determine the characteristic temperatures: nil ductility, nil strength, and ductility recovery temperature of Inconel 718. The samples were tested at various pull temperatures on-heating until the nil strength temperature then tested on-cooling with the nil strength temperature acting as the peak temperature. The nil strength temperature was 2273°F, nil ductility temperature was 2182°F, and the ductility recovery temperature was 1925°F. Both the Varestraint and Gleeble results were compared with relevant literature to determine the weldability of the Inconel 718. Four criteria were used to determine the weldability of Inconel 718 and in three of the four tests; the Inconel 718 had equal to or greater weldability than the compared materials. In the fourth test, the Inconel 718 demonstrated lower weldability than the compared alloy systems, however, Inconel 718 operates in different conditions specifically, the high temperature and pressure conditions mentioned above.