Inconel 718 (IN718) superalloy samples fabricated by selective laser melting (SLM) were submitted to different heating cycles and their microstructural characteristics were investigated. The selected heating rates, ranging from 10 °C/min to 400 °C/s, represent different regions in the heat-affected zone (HAZ) of welded additively manufactured specimens. A combination of differential thermal analysis (DTA), high-resolution dilatometry, as well as laser confocal and electron microscopy were used to study the precipitation and dissolution of the secondary phases and microstructural features. For this purpose, the microstructure of the additively manufactured specimen was investigated from the bottom, in contact with the support, to the top surface. The results showed that the dissolution of γ″ and δ phases were delayed under high heating rates and shifted to higher temperatures. Microstructural analysis revealed that the Laves phase at the interdendritic regions was decomposed in specific zones near the surface of the samples. It was determined that the thickness and area fraction of these zones were inversely related to the applied heating rate. A possible mechanism based on the influence of heating rate on Nb diffusion in the interdendritic regions and core of the dendrites is proposed to interpret the observed changes in the microstructure.

Selective Laser Melted (SLM) Inconel718 (IN718) superalloy was linear friction welded (LFWed) to forged AD730 TM Nickel-based superalloy. Successful joints free of micro-porosity, micro-cracking, and oxides were obtained. Microstructure variations across the weld line developed during LFW were examined using different techniques, including laser confocal microscopy, scanning electron microscopy, energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The microstructure was also evaluated, particularly in terms of Grain size and misorientation changes were determined and correlated with microhardness evolution in different regions of the weld joint. The characteristics of the microstructure on both sides of the weld joint was analyzed and related to the deformation and temperature paths imposed during the LFW process. Dynamic recrystallization (DRX) occurred on both sides of the dissimilar weld line and it was found that Discontinuous DRX (DDRX) and Continuous DRX (CDRX) took place in the WZ and in the TMAZ, respectively. In order to study the influence of the starting microstructure in the LFW experiments, LFW of a homogenized SLM IN718 sample was analyzed and compared with the non-homogenized sample. A clear change in the size and grain misorientation levels of the heat and thermomechanical affected zones were observed between the two conditions. The differences were related to a greater degree of strain induced in homogenized sample and the increasing effect of the solid solution strengthening mechanism caused by a partial dissolution of the second-phase strengthening particles in the matrix.

Hot ductility of the newly developed AD730 TM nickel-base superalloy was investigated in the temperature interval 1050-1240°C. The nil strength and nil ductility temperatures were determined by hot tensile testing using the Gleeble TM 3800 weld thermal simulation method. The influence of heating rate, representing the weld thermal cycle, on hot ductility behavior of the alloy was also investigated. The microstructure and the fracture mode of samples were examined by optical and scanning electron microscopy. The influence of heating rate on the extent of grain boundary liquation and void formation was determined and it is shown that the significant ductility loss near the NDT point could be related to the reduction of surface tension at the grain boundary-matrix interface. In addition, the contribution of hard precipitates, such as grain boundary MC carbides, voids, and cavities as other damage mechanisms responsible for ductility loss at high temperature, are discussed.

No abstract

Weld overlays of Ni-based superalloys provide high-temperature corrosion, oxidation, and wear resistance for industrial applications. The objective of this study was to evaluate the microstructure of Ni-Cr-Mo superalloy Inconel 686 overlays fabricated using automatic gas metal arc welding (GMAW) with three different levels of voltage, travel speed, and wire feed speed. The variation of these parameters and their effects on the weld quality, dilution, and microsegregation were analyzed. The weld surface quality was assessed by visual examination. The weld dilution was measured by both geometric and chemical composition definitions. The maximum dilution was measured at 13.63%, induced by the highest weld heat input (5.97 kJ.cm− 1) and lowest wire feed speed (6 m.min− 1). The overlay dilution and the heat-affected zone (HAZ) size decreased by increasing the wire feed speed. However, increasing the wire feed speed and the heat input reduced the weld quality and even resulted in noncomplete joints. Microsegregation of elements was evaluated using energy-dispersive X-ray spectroscopy (EDS) analysis and it was found that microsegregation of Mo and Cr and the volume fraction of secondary phases in the interdendritic areas increased with increasing the weld heat input. According to the visual examination results and macro- and microscopic evaluation, the weld overlay parameters were found to be optimized for the mid-level heat inputs (4–5 kJ.cm− 1) and wire feed speeds between 6 and 8 m.min− 1.