In the present study, Inconel 718 (IN718) superalloy fabricated by laser powder bed fusion (LPBF) has been characterized focusing on the effect of both homogenization and solution treatment time on grains structure, crystallographic texture, precipitates formation/dissolution and material hardness. For this purpose, a heat-treatment time window with a wide range of soaking times for both treatments was established aiming to develop the optimal post-treatment conditions for laser powder bed fused IN718. It was found that the as-printed IN718 is characterized by very fine columnar/cellular dendrites with Laves phase precipitating at the grain boundaries as well as inter-dendritic regions, which differs from the microstructure of wrought and cast materials and requires special heat-treatment conditions different from the standard treatments. The results reveal that the relatively short homogenization treatment at 1080 °C for 1 h was not enough to significantly change the as-printed grain structure and completely dissolve the segregates and Laves phase. However, a completely recrystallized IN718 material and more Laves phase dissolution were obtained after homogenization treatment for 4 h. A further increase in time of the homogenization treatment (7 h) resulted in grain growth and coarsening of carbides precipitates. The solution treatment time at 980 °C did not cause noticeable changes in the crystallographic texture and grain structure. Nevertheless, the amount of δ-phase precipitation was significantly affected by the solution treatment time. After applying the heat-treatment time window, the hardness increased by 51–72% of the as-printed condition depending on the treatment time due to the formation of γ′ and γ″ in the γ-matrix. The highest material hardness was obtained after 1 h homogenization, whereas the prolonged time treatments reduced the hardness. This study provides a comprehensive investigation of the post heat-treatments of the laser powder bed fused IN718 that can result in an optimized microstructure and mechanical behavior for particular applications.

In the present study, multi-objective optimization is employed to develop the optimum heat treatments that can achieve both high-mechanical performance and non-distinctive crystallographic texture of 3D printed Inconel 718 (IN718) fabricated by laser powder bed fusion (LPBF). Heat treatments including homogenization at different soaking times (2, 2.5, 3, 3.5 and 4 h) at 1080 °C, followed by a 1 h solution treatment at 980 °C and the standard aging have been employed. 2.5 h is found to be the homogenization treatment threshold after which there is a depletion of hardening precipitate constituents (Nb and Ti) from the γ-matrix. However, a significant number of columnar grains with a high fraction (37.8%) of low-angle grain boundaries (LAGBs) have still been retained after the 2.5 h homogenization treatment. After a 4 h homogenization treatment, a fully recrystallized IN718 with a high fraction of annealing twins (87.1%) is obtained. 2.5 and 4 h homogenization treatments result in tensile properties exceeding those of the wrought IN718 at both RT and 650 °C. However, considering the texture requirements, it is found that the 4 h homogenization treatment offers the optimum treatment, which can be used to produce IN718 components offering a balanced combination of high mechanical properties and adequate microstructural isotropy.

We extensively studied serigraphic screen-printed commercial silver flake inks loaded with silicon inclusions in order to achieve pinning at the grain boundaries. Based on grain size measurements using electron backscattered diffraction (EBSD), commercial silver ink with silicon microparticle content of 5 wt.% shows significant grain growth retardation compared to pristine silver ink, which stabilizes electrical conductivity up to 700 °C via a Zener pinning mechanism. The modified silicon-loaded silver ink experiences a two-times increase in grain size when heated up to 700 °C, compared to a seven-times increase for pristine silver ink. In turn, this enables operation temperatures significantly higher than the conventional operational window of microparticle-based silver inks, which are usually limited to 400 °C. Using isothermal exposures of 10 min up to 4 h, this phenomenon is observed at temperatures ranging from 250 °C to 900 °C. The electrical conductivity stability, grain size evolution and oxide contents were studied up to 4 h. The activation energy of silver ink with silicon inclusions is 54% lower than for pristine silver ink due to the pining effect, which retards grain growth via the Zener mechanism. Most importantly, the electrical resistivity remains stable up to 700 °C, which is more than twice the operation limit for off-the-shelf screen-printable silver flake inks. Hence, we demonstrate that adding controlled amounts of silicon particles to silver inks for grain structure engineering can open new vistas of possibilities for screen-printed metallic inks.

One major challenge of friction stir processing (FSP) is its sensitivity to parameters like advancing and rotational speeds. This study examined the effect of tool travel speed on the microstructural evolution and mechanical properties of a new-generation Al-6Mg alloy. Optical and electron microscopy, EBSD, and shear-punch testing (SPT) were used. Two travel speeds, 50 and 120 mm/min, revealed significant differences in microstructure and properties at ambient temperature. EBSD provided misorientation maps and boundary fraction data. Microstructure analysis showed continuous dynamic recrystallization in the nugget zone, with finer grains observed at the higher speed. Microhardness was greater on both sides at 120 mm/min. The TMAZ showed elongated grains at 120 mm/min, while recrystallized grains were more prominent at 50 mm/min. In the HAZ, partial recrystallization occurred at 120 mm/min, whereas extensive recrystallization was observed at 50 mm/min. The SPT results indicated variations in stiffness between advancing and retreating sides, especially 2 mm from the nugget center. At 10 and 20 mm from the center, higher stiffness and strength were recorded at 120 mm/min. This study established correlations between joint stiffness, grain misorientation, and travel speed.