Microstructure Evolution of Selective Laser Melted Inconel 718: Influence of High Heating Rates

Abstract

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.

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

“…In the as-SLM IN718, in the narrow HAZ, between the TMAZ and the BM, the hardness increased as compared to the BM. This is due to the partial dissolution of Laves phase, the diffusion of Nb and Mo to the matrix (solid solution), and the remaining strengthening phases (γʺ and δ) in the blocky zones as also reported by [32].…”

“…In the as-SLM IN718 part between the TMAZ and the BM, the HAZ with a thickness ranging from 20-60 µm was observed (cannot be seen in Figure 3b), and contained some blocky zones in the middle of columnar grains as reported also by [32]. According to the latter's report, this zone was only affected by a high heating and cooling rate during the LFW process.…”

Dissimilar linear friction welding of selective laser melted Inconel 718 to forged Ni-based superalloy AD730™: Evolution of strengthening phases

“…In the HAZ ( Fig. 6 (d, e)), both spherical and long-striped shapes precipitates could be observed in the blocky zones which is in agreement with the investigation reported in [41] .…”

“…6 (d, e)). As reported by [41] , the thickness of HAZ was controlled by the heating and cooling rates and the shape of the Laves phase was changed from long-striped to spherical within the blocky zones. Hence, due to enough equivalent time ( t e ) and temperature ( T ) controlling the diffusion of elements from the core of the dendrite into interdendritic regions, no liquation and eutectic composition were found in the HAZ.…”

“…As a result, it is supposed that the welding temperature at the WZ, which can be affected because of a high heating rate (~350-400 °C s −1 [ 18 , 31 ]), which was achieved at least 1220 °C (1200-1250 °C). was investigated in both pre-weld (in HAZ and BM zones [41] ) and post-weld conditions. Fig.…”