Synthesis of Fe-Al-Ti Based Intermetallics with the Use of Laser Engineered Net Shaping (LENS)
The Laser Engineered Net Shaping (LENS) technique was combined with direct synthesis to fabricate L21-ordered Fe-Al-Ti based intermetallic alloys. It was found that ternary Fe-Al-Ti alloys can be synthesized using the LENS technique from a feedstock composed of a pre-alloyed Fe-Al powder and elemental Ti powder. The obtained average compositions of the ternary alloys after the laser deposition and subsequent annealing were quite close to the nominal compositions, but the distributions of the elements in the annealed samples recorded over a large area were inhomogeneous. No traces of pure Ti were observed in the deposited alloys. Macroscopic cracking and porosity were observed in all investigated alloys. The amount of porosity in the samples was less than 1.2 vol. %. It seems that the porosity originates from the porous pre-alloyed Fe-Al powders. Single-phase (L21), two-phase (L21-C14) and multiphase (L21-A2-C14) Fe-Al-Ti intermetallic alloys were obtained from the direct laser synthesis and annealing process. The most prominent feature of the ternary Fe-Al-Ti intermetallics synthesized by the LENS method is their fine-grained structure. The grain size is in the range of 3–5 μm, indicating grain refinement effect through the highly rapid cooling of the LENS process. The Fe-Al-Ti alloys synthesized by LENS and annealed at 1000 °C in the single-phase B2 region were prone to an essential grain growth. In contrast, the alloys annealed at 1000 °C in the two-phase L21-C14 region exhibited almost constant grain size values after the high-temperature annealing.
Laser additive manufacturing of iron aluminides strengthened by ordering, borides or coherent Heusler phase
Abstract Laser Additive Manufacturing (LAM) is a near-net-shape processing technology well-suited for the production of parts from intermetallic alloys. Three different iron aluminide alloys have been processed by Selective Laser Melting (SLM) and Laser Metal Deposition (LMD). The alloys selected for LAM represent three different strategies for strengthening iron aluminides at high temperatures: Increase of the ordering temperature D0 3  ↔ B2 (Fe–30Al–10Ti; all compositions in at.%), precipitation of borides (Fe–30Al–5Ti–0.7B) and generation of coherent A2 + L2 1 microstructures (Fe–22Al–5Ti). It is shown that all three alloying strategies established from as-cast alloys can also be realised through both LAM processes, generating defect free and dense (> 99.5%) samples. Yield stress and compressive creep strength also match that of the as-cast alloys above 600 °C, while at lower temperatures in some cases higher yield stresses are observed. In addition, the yield stress below 600 °C may also depend on the building direction and may change after a heat treatment. No general improvement of ductility is observed, specifically in case of Fe–30Al–10Ti, where the grain size in the LAM processed samples is one magnitude lower than in the as-cast alloy. However, depending on building direction and subsequent heat treatment some samples become ductile at quite low temperatures, which at least in some cases may be explained by the internal stresses measured by XRD.
Melting and casting of FeAl-based cast alloy
The FeAl-based intermetallic alloys are of great interest because of their low density, low raw material cost, and excellent resistance to high-temperature oxidation, sulfidation, carburization, and molten salts. The applications based on these unique properties of FeAl require methods to melt and cast these alloys into complex-shaped castings and centrifugal cast tubes. This paper addresses the melting-related issues and the effect of chemistry on the microstructure and hardness of castings. It is concluded that the use of the Exo-Melt{trademark} process for melting and the proper selection of the aluminum melt stock can result in porosity-free castings. The FeAl alloys can be melted and cast from the virgin and revert stock. A large variation in carbon content of the alloys is possible before the precipitation of graphite flakes occurs. Titanium is a very potent addition to refine the grain size of castings. A range of complex sand castings and two different sizes of centrifugal cast tubes of the alloy have already been cast.