On the Formation Mechanism of Banded Microstructures in Electron Beam Melted Ti–48Al–2Cr–2Nb and the Design of Heat Treatments as Remedial Action

Abstract

The formation mechanism of banded microstructures of an electron beam melted engineering intermetallic Ti–48Al–2Cr–2Nb alloy, the solidification behavior, and the heat treatment response are investigated via a process parameter study. Scanning electron microscopy, hardness testing, X‐ray diffraction, electron probe microanalysis, thermomechanical analysis, electron backscatter diffraction, heat treatments, as well as thermodynamic equilibrium calculation, and numerical simulation were performed. All specimens show near‐γ microstructures with low amounts of α2 and traces of βo. Fabrication with an increased energy input leads to an increased Al loss due to evaporation, a lower α‐transus temperature, and to a higher hardness. Banded microstructures form due to abnormal grain growth toward the bottom of original melt pools, whereas α2 in Al‐depleted zones enables a Zener pinning of the γ‐grain boundaries, leading to fine‐grained areas. Via numerical simulation, it is shown that increasing the energy input leads to larger maximum temperatures and melt pool sizes, longer times in the liquid state, and more remelting events. Solidification happens via the α‐phase and increasing the energy input leads to an alignment of (111)γ in building direction. Furthermore, banded microstructures respond heterogeneously to heat treatments. Heat treatment is introduced based on homogenization via phase transformation to obtain isotropic microstructures.

Influence of Two‐Step Heat Treatments on Microstructure and Mechanical Properties of a β‐Solidifying Titanium Aluminide Alloy Fabricated via Electron Beam Powder Bed Fusion

“…In a study on PBF-EB/M of Ti-48Al-2Cr-2Nb, Wartbichler et al found that a higher energy input during processing increased aluminum evaporation, which in turn had a strong influence on phase composition and phase morphology after heat treatment in the (α þ γ)-phase field. [25] Wimler et al observed a similar effect after annealing of PBF-EB/Mmanufactured Ti-44.8Al-4.1Nb-0.7W-1.1Zr-0.4Si-0.5C-0.1B slightly below the γ-solvus temperature. [27] Previous studies investigated the mechanical properties of PBF-EB/M-manufactured TNM-B1 in tensile tests for vertically manufactured specimens [29] and in tensile and fatigue tests for horizontally manufactured specimens [30] for different test temperatures up to 850 °C, respectively.…”

“…In a study on PBF‐EB/M of Ti–48Al–2Cr–2Nb, Wartbichler et al found that a higher energy input during processing increased aluminum evaporation, which in turn had a strong influence on phase composition and phase morphology after heat treatment in the (α + γ)‐phase field. [ 25 ] Wimler et al observed a similar effect after annealing of PBF‐EB/M‐manufactured Ti–44.8Al–4.1Nb–0.7W–1.1Zr–0.4Si–0.5C–0.1B slightly below the γ‐solvus temperature. [ 27 ]…”

“…[19,26] Apart from changes in the overall chemical composition, this effect leads to local fluctuations in the aluminum content since evaporation preferentially occurs at the top of the melt pool. [5,27] Inhomogeneous microstructures and aluminum distribution after PBF-EB/M have been reported for several titanium aluminide alloys, including Ti-48Al-2Cr-2Nb, [20,21,25] Ti-45Al-7Nb-0.3W, [28] Ti-44.8Al-4.1Nb-0.7W-1.1Zr-0.4Si-0.5C-0.1B, [27] and TNM-B1. [5] As the phase transition temperatures in titanium aluminides are highly susceptible to the aluminum content, these local inhomogeneities can significantly affect microstructural composition after heat treatment.…”

Locally Adapted Microstructures in an Additively Manufactured Titanium Aluminide Alloy Through Process Parameter Variation and Heat Treatment

“…[40] It can be noticed that VED-32 exhibits a larger colony size than the other two specimens, presumably because grain growth during heat treatment was not impeded by a second phase. [13] Taking both the DSC results and the cross-sectional images in Figure 6 into account, it seems reasonable to assume that in the case of VED-32, the solution heat treatment temperature of 1290 °C lay above the γ-solvus temperature. For VED-14 and VED-21, however, the same temperature apparently corresponded to a heat treatment in the (α þ γ)-phase field.…”

“…[10][11][12] As a consequence, inhomogeneous microstructures and aluminum distribution can occur after PBF-EB/M processing, which may also affect phase transition temperatures and microstructure evolution during heat treatment. [13,14] Apart from unintentional microstructural variations, these evaporation effects can be used on purpose for microstructural design, e.g., to locally adapt the microstructure and manufacture functionally graded materials (FGMs). [15,16] FGMs are characterized by a spatial variation of chemical composition or microstructure, thereby enabling local control of, e.g., the mechanical properties in different areas of the same component according to the specific requirements.…”