Laser Powder Bed Fusion of a New Inconel Superalloy Via In-Situ Alloying

Laser Powder Bed Fusion (L-PBF) technology was used to manufacture a new alloy by means of the new approach of in-situ alloying. Rather than use a pre-alloyed powder, the research involved the creation of a blend of Ti and Inconel 718 powders mixed in a 1:99 ratio (1%wt. Ti and 99%wt. Inconel 718). The aim of the study was first, to increase the mechanical properties of the new alloy compared to the reference Inconel 718 in its range of application temperatures (up to 600°C). Second, to study the feasibility of the situ-alloying as an alternative method to achieve desired alloy compositions, contrary to the more common use of pre-alloyed powders. The former relied on the Ti tendency to enhance γ’ precipitation, while the latter exploited the metallographic characterization as a powerful means to study how differences in chemical composition, mass fraction, and size distribution of the powders affected the outcomes of the printing process. After the first step of printing parameters optimization, a metallographic characterization was carried out on both pure Inconel 718 and the in-situ alloyed version, namely Inconel 718 + Ti. Two different heat-treated conditions were analyzed in addition to the as-built (ab) condition - solution treated (ST), and solution treated and double aged (SDAG). The thermal treatment followed the AMS 5667, a common process usually performed on Inconel 718. Investigations on the microstructure were carried out by scanning electron microscopy (SEM) and optical microscopy (OM) along with microhardness measurements. As a result, Ti-segregations were discovered in Inconel 718 + Ti in both the as-built and heat-treated conditions. These segregations caused a significant effect on the microstructure evolution under heat treatments. It caused the precipitation of (Nb, Ti)-rich needle-like secondary phases, mainly in the regions with the higher concentration gradient. The latter caused the microhardness of the new alloy to be higher than Inconel 718 in the ST-condition, while lower when in the SDAG-condition. Through a cross-analysis of morphology, mechanical data, and literature review these needle-like precipitates seemed to be to the η-phase. However, by and large, the in-situ alloying did not affect the weldability of the powder, allowing it to achieve densities comparable to the reference Inconel 718. Although this process still requires further studies, it seems a significantly faster and cost-effective alternative to the conventional pre-alloyed routes.