Nd: YAG laser beam welding of UNS N07718 superalloy and UNS S32304 duplex stainless steel: Phase transformations and mechanical properties of dissimilar joints

Abstract

This article implies the phase transformations and mechanical behavior of dissimilar laser welded UNS N07718 superalloy/UNS S32304 duplex stainless steel. Laser power, welding speed, and focal point were the variables of the laser beam welding (LBW) approach. Optical microscopy and electron backscattered diffraction (EBSD) analysis were utilized. They demonstrated that the weld metal (WM) mainly had an austenitic microstructure with a face-centered cubic structure in the form of columnar and equiaxed dendrites. Also, the occurrence of directional solidification in WM was verified by EBSD. The heat-affected zone (HAZ) microstructure of Inconel 718 superalloy included austenite grains with normal grain growth, annealing twins, and precipitates. There was a ferritic-austenitic microstructure in the HAZ of 2304 duplex stainless. In this area, the volume fraction of the ferrite phase was excessively higher than that of the austenite phase and the austenite was characterized as Widmanstätten plates and grain boundary austenite. Furthermore, abnormal ferrite grain growth was identified in this. Welding defects e.g., molten spatter, solidification crack, and lack of penetration were observed, as well. Based on the uniaxial tensile test, it was realized that the highest failure load (9.7 $\pm$ 0.4 kN) was achieved in the laser power of 1900 W, welding speed of 3 mm/s, and focal point of 0 mm. Therefore, these variables were known as the optimum variables of the LBW process. All laser weldments failed from the WM and fractography via scanning electron microscopy (SEM) showed a mixture of dimple and cleavage features in the fracture surfaces of the laser welds.