Grain size and misorientation evolution in linear friction welding of additively manufactured IN718 to forged superalloy AD730™

Abstract

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.

Post-Weld Heat Treatment of Additively Manufactured Inconel 718 Welded to Forged Ni-Based Superalloy AD730 by Linear Friction Welding

“…As a result of deformation and dissolution (due to the higher temperature close to the weld line), the platelike d phase that had been formed during the SLM process (and before STA) in grain boundaries was transformed, as precursor, dissolved/transformed into short granular rods or semispherical d phase particles (Figures 4(b) and (c)). The other platelike particles could be formed during the ST. [49] As also reported previously, [17,50] there is an evolution in size, shape, and volume fraction of precipitates, from the WZ to the BM, on both sides. Figures 5(a) and (b) show the microstructure in the WZ in the W1 and W1-DA samples, on the AD730 side, before and after the PWHT, respectively.…”

“…It must be noted that the hardness data for as-linear friction welded conditions (W1 and W2) are reported from a recent publication by the present authors. [17] After PWHT, the hardness increased as compared to that of the as-linear friction welded samples (W1 and W2) in all zones. Table III provides the microhardness comparison values between the PWHT and as-linear friction welded conditions for both alloys in all zones.…”

“…For their part, the large Laves particles were partially dissolved and transformed to granular and finer particles since the cooling rate was high and, therefore, the time for the evolution was short. [12,17,57] After the PWHT (both DA and STA samples), very fine c¢ and c¢¢ precipitated and were observed in IN718 (in both SLM and H-SLM conditions) in all zones. Generally, the same fraction of c¢/c'' is expected close to the WZ of W1-DA and W2-DA.…”

“…Therefore, interactions between these fields can block the dislocation movements and increase the strength of the material. [7,40,64] Therefore, the precipitation of mainly c¢¢ and c¢ after PWHT and grain refinement [17] and solid-solution strengthening [12,16,17,65,66] could enhance the hardness of the IN718 alloy. Although grain refinement is a mechanism used to increase the hardness in as-linear friction welded conditions (W1 and W2), the grain sizes did not change after PWHT.…”

“…Sui et al [15] reported that the formation and presence of the Laves phase is accompanied by notable niobium microsegregation, which causes an inhomogeneous distribution and coarsening of c¢¢ phase. After homogenization on SLM IN718 (i.e., H-SLM in W2), as well as a significant reduction in the volume fraction of the Laves phase, [12,17] long-striped Laves phases are almost eliminated and transformed into spherical particles, the c'' phase distribution becomes uniform, and the residual stress accumulation could decrease. [15,16] Huang et al [14] and Marchese et al [67] reported that due to repeated rapid heating and cooling during the SLM process, dislocation density increases.…”