Showing papers on "Directional solidification published in 2021"

Texture evolution in selective laser melted maraging stainless steel CX with martensitic transformation

Abstract: Due to high local cooling rates and non-equilibrium directional solidification conditions, selective laser melting (SLM) processed metals exhibit microstructural and textural features significantly different from the conventionally processed ones. The evolution of crystallographic orientations in a maraging stainless steel (commercially known as stainless steel CX) sample fabricated by the SLM process was studied through experimental and modelling approaches Electron backscattering diffraction analysis showed that the dominant texture components in martensite and austenite phases are || building direction and || building direction, respectively. Texture simulation indicated that the formation of crystallographic orientations in the studied sample is the result of two consecutive phase transformations, from initially solidified delta ferrite phase with dominant cube fiber texture to austenite and austenite to martensite.

Solidification characteristics and as-cast microstructures of a Ru-containing nickel-based single crystal superalloy

Abstract: The solidification characteristics and as-cast microstructures of a Ru-containing nickel-based single crystal superalloy were systematically investigated through thermal analysis, Thermo-Calc simulation, the planar interface solidification experiment, and the directional solidification quenching experiment. The main solidification transition temperature, segregation behavior, and solidification path were analyzed, and the microstructure evolution and phase formation mechanism were also discussed. The solidification began with the formation of primary γ dendrites (L → γ). Then Ni, Al, Ta, and Ru were enriched in the residual liquid, resulting in the precipitation of β-NiAl phase (L → β-NiAl). As the β-NiAl phase grew, the content of Ta gradually increased while the content of Al gradually decreased. Thus, the peritectic γ′ phases were precipitated on the surface of β-NiAl phase and coarsened by the incomplete peritectic reaction (L + β-NiAl → peritectic γ′ + β-NiAlResidual). The Al content further decreased and the Ta content further increased with the precipitation of the peritectic γ′ phase, leading to the formation of γ/γ′ eutectics on the surface of the γ dendrites or directly from liquid (L → γ/γ′ eutectic). Since the precipitation of β-NiAl phase and the subsequent incomplete peritectic reaction, Cr, Co, Mo, W, and Re were rejected into the residual liquid in the vicinity of β-NiAl phase, providing conditions for the nucleation of the TCP(R) phases (L → R). The wide freezing range of the alloy might be the cause of the severe micro-segregation and the precipitation of some secondary phases in the interdendritic regions.

The microstructure, phase composition and tensile properties of austenitic stainless steel in a wire-feed electron beam melting combined with ultrasonic vibration

Abstract: The growth of coarse columnar grains and dendritic ferrite is a common problem peculiar for Cr–Ni steels produced by electron beam additive manufacturing. For these heterophase additively fabricated steels, the oriented dendritic microstructure in such columnar grains provides the anisotropy of the mechanical properties. Herein, we study the effect of ultrasonic (US) vibrations during wire-feed electron beam additive manufacturing (WEBAM) on microstructure, phase composition and tensile properties of AISI 304L steel. US vibrations during WEBAM processing provide the refinement of the austenitic grains, partially suppress the directional solidification of austenite and reduce δ-ferrite volume fraction (in 1.5–2%). The regular elongated austenitic grains with long and straight dendritic colonies of δ-ferrite are typical of WEBAM-specimens. Austenitic grains are visibly more spherical in WEBAM-US specimens, and their mean grain size (256 ± 17 μm) is smaller than that of WEBAM-fabricated steel (433 ± 145 μm). The δ-ferritic arms are broken and partially dissolved in WEBAM-US specimens and the most pronounced changes are observed in the middle part of the billets. US-assisted grain refinement, partial columnar-to-equiaxed transition and reducing δ-ferrite content – all positively affect the resultant properties of WEBAM-fabricated steel. The results of present study clearly show that application of US during WEBAM is an effective way to obtain a positive influence in additive manufacturing of austenitic Cr–Ni steel.

Microstructure formation and columnar to equiaxed transition during cold crucible directional solidification of a high-Nb TiAl alloy

Abstract: In order to study the factors of columnar to equiaxed transition (CET) of high-Nb TiAl alloys, Ti46Al7Nb0.4W0.6Cr0.1B alloy has been fabricated by cold crucible directional solidification (CCDS) technique under different pulling rate from 3.3 μm/s to 16.7 μm/s. The marco/micro-structure and phase composition near solid–liquid interface have been characterized. Results show that the CET of the high-Nb TiAl alloy occurs with the increase of the pulling rate at the constant temperature gradient. The microstructure of the columnar grain is composed of α2/γ lamellar matrix and a coupling structure of striped-like B2+γ phases. The lamellar colonies in a columnar grain possess the same orientation, while the arrangement direction between the striped-like B2 phase and growth direction is 0° or 45°. A solidification map for CCDS is established which predicts columnar or equiaxed morphology according to the growth rate (R) and temperature gradient (G). The dendrite morphology at the solid–liquid interface after quenching and the CET is controlled by the actual temperature gradient at the tip of the dendrite. Meanwhile, the increase of growth rate and the satisfaction of heterogeneous nucleation conditions are the main factors for CET. The decrease of actual temperature gradient caused by quenching or the increase of liquidus gradient caused by increasing growth rate can increase the maximum supercooling degree ΔTC. When it reached the supercooling degree ΔTN required to form a new nucleus, equiaxed grains will be produced. In addition, the boride in this alloy can act as a heterogeneous nucleation core to promote CET.

Directional Solidification of a Nickel-Based Superalloy Product Structure Fabricated on Stainless Steel Substrate by Electron Beam Additive Manufacturing

Abstract: This article is the first to apply wire-feed electron beam additive manufacturing in vacuum to fabricate a part from a complex nickel-based superalloy with directional structure on a stainless steel substrate. It is shown that the determining factor for the formation of parts with directional structure is the local metallurgy conditions implemented in electron beam additive manufacturing. These conditions are the magnitude and direction of the temperature gradient as well as the geometry (shape) of the solidification front in the molten pool. The substrate effect (both chemical and structural) on the composition and structure of the part material is cancelled out at a distance not exceeding 8.0 mm from the substrate. Thus, the proposed 3D printing method is quite acceptable for the manufacturing of parts from heat-resistant nickel alloys with directional structure on substrates made of less expensive materials.

Microstructural Characterization of Ni-Base Superalloy As-Cast Single Crystal (CMSX-4)

Abstract: The as-cast microstructure of a single-crystal CMSX-4 Ni-base superalloy produced by directional solidification was studied. The 〈100〉 misorientation of the angled and spiral grain selectors was evaluated for 3.33 and 5.0 mm/min withdrawal rates. The dendritic microstructure presented primary dendrite arm spacing around 370 µm (for a withdrawal rate of 3.33 mm/min) and 320 µm (for 5.0 mm/min); and interdendritic γ–γ′ islands, whose volume fraction was around 5.4% (for 3.33 mm/min) and 6.1% (for 5.0 mm/min). The microsegregation profiles of one withdrawal condition (for 5.0 mm/min) were used to extrapolate the solidus and γ′ solvus temperatures using the Thermo-Calc software. These microsegregation profiles of the as-cast microstructure also affected the solid-state γ′ precipitation in terms of shape, size and volume fraction. In the center of the dendrite, the γ′ precipitates were regularly cubic-shaped with a face of approximately 0.5 µm and a volume fraction of 55%. In the periphery of the dendrite, the morphology of the γ′ precipitates was more asymmetric, and the volume fraction of the γ′ phase increased to 65%. Two morphologies of the interdendritic γ–γ′ islands, fine and coarse, were observed, and a eutectic solidification sequence was proposed. The results indicated that the chemical composition of the interdendritic liquid progressed from hypoeutectic (fine interdendritic structure) to hypereutectic (coarse interdendritic structure) during the solidification.

Enhancing the elastocaloric effect in Ni–Mn–Ga alloys through the coupling of magnetic transition and two-step structural transformation

Abstract: Elastocaloric effect driven by uniaxial stress in the Ni–Mn–Ga alloys can be greatly enhanced through introducing magnetic transition or inter-martensitic transformation to martensitic transformation. Here, we present large elastocaloric effect in a ⟨0 0 1⟩A textured Ni55Mn19Ga25Ti1 polycrystalline alloy prepared by directional solidification by exploiting the coupled multiple phase transformations, i.e., paramagnetic-ferromagnetic transition, martensitic transformation, and inter-martensitic transformation. Owing to such magneto-multistructural transformation, the transformation entropy change related to the inverse transformation is enhanced to 29.6 J kg−1 K−1. Consequently, on unloading from a compressive stress of 180 MPa, a large adiabatic temperature change of −12.9 K and specific adiabatic temperature change of −72 K GPa−1 are achieved, being much superior over those in the Ni–Mn–Ga based alloys obtained previously.