Paper Title
Constitutional Liquation in Solid-State Welding of SLM IN718/AD730 Super Alloy

Abstract
Selective Laser Melting (SLM) is an advanced manufacturing technique that enables the production of complex geometries using IN718. In advanced aerospace applications, SLM IN718® is expected to be utilized for blade manufacturing, while solid-state welding offers a promising alternative for joining these blades to disks. However, the rapid solidification inherent to SLM results in the formation of a high fraction of Laves phase, which can influence the microstructural stability during post-processing and solid-state welding operations. The present study investigates the thermal behavior of SLM IN718® under rapid heating conditions relevant to solid-state welding processes, focusing on constitutional liquation. High-temperature experiments were conducted using high-resolution dilatometry to determine the liquation onset and analyze phase transformations. The dilatometry technique was employed to apply rapid heating cycles (400°C/s) up to 1200°C, experimentally simulating the thermal cycle of solid-state welding. The results confirmed the occurrence of a distinct phase transformation at 1090°C, which was identified as the onset of constitutional liquation, where Nb-rich phases undergo localized melting. Microscopic analysis of the samples revealed that during heating, the gradual dissolution of the Laves phase led to a localized increase in Ni, Nb, Mo, and Ti concentrations. Thermodynamic calculations further demonstrated that an increased Ti concentration lowers the local melting temperature of Ni-Nb-Mo from 1134°C to 1113°C, ultimately promoting localized melting. Consequently, constitutional liquation occurs between 1090–1200°C due to this localized compositional evolution in the matrix. Moreover, microstructural observations revealed the presence of solidification cracks in the fast-heated samples. However, no evidence of solidification cracks was found in the weld zone of the solid-state welded dissimilar joint of SLM IN718® and AD730® Ni-base superalloy. In the solid-state welding process, compressive stress significantly enhances diffusion, reducing liquation susceptibility and preventing cracking in the weld zone, despite experiencing peak temperatures of 1200°C. Microstructural analysis confirmed that optimized solid-state welding parameters can effectively minimize liquation-related defects, ensuring reliable blade-to-disk joining. Keywords - SLM IN718, Laves phase, Constitutional liquation, Phase transformation, Solid-state welding.