1 Introduction
Laser powder-bed fusion (L-PBF) of structural alloys is beginning to find widespread use in a variety of applications thanks to the ability to produce net-shaped components with unparalleled design freedom. To sustain the development of this emerging manufacturing technology, a number of studies have investigated how the unique microstructure imposed by L-PBF affects the macroscale mechanical properties of important engineering alloys, in particular those based on titanium such as Ti-6Al-4V.
The microstructure of Ti-6Al-4V after L-PBF exhibits complex features that span across several length scales (from the nm to mm scale), the morphology, and arrangement of which are influenced by the printing process, such as the laser power, scanning speed, the spacing between raster paths (also known as hatch spacing), position within the part, the scan strategy, and the environmental conditions (temperature and oxygen concentration). Typically, a hierarchical structure made up of fine martensitic
α′ phase (including primary, secondary, tertiary, and quartic martensite plates) within columnar prior-
β grains dominates the microstructure of Ti-6Al-4V in the as-built L-PBF condition.[
1] While the metastable
α′ phase can be readily decomposed into a more ductile
α +
β microstructure by standard post-processing heat treatments,[
2,
3] the columnar prior-
β grains maintain, however, a characteristic elongated morphology. This morphology is an undesirable feature of L-PBF, and more generally in additively made materials. Such columnar prior-
β grains are known to negatively affect the mechanical properties of Ti-6Al-4V, giving rise to anisotropy and low fracture toughness and consequently affecting the fracture modes and fatigue resistance of the alloy.[
4‐
8]
The formation of columnar prior-β grains and, more specifically, their suppression, via suitable modification of the alloy chemistry of Ti-6Al-4V, is the focus of this present study.
The formation of columnar prior-
β grains in Ti-6Al-4V is a consequence of epitaxial growth (from a substrate of previously deposited layers) of the high-temperature
β phase during the layer-by-layer deposition.[
9] It is now accepted that, in a repeating sequence, as a new layer of powder is melted, the top of the previously deposited layer is also re-melted; the melt pool does therefore solidify on pre-existing
β grains that are predominantly oriented with a 〈100〉 direction along the build direction. Under the high solidification speed (
R) and temperature gradients (
G) typical of L-PBF, nucleation events in the melt pool are largely prevented. This gives rise to epitaxial growth from the underlying substrate and planar/dendritic grain solidification mode of elongated grains with 〈100〉 along the dominant heat flow direction.[
4]
Several studies have been carried out with the aim of suppressing the formation of β columnar grains in laser-based additive processing of titanium. The approaches taken in the literature can be broadly divided into: (a) manipulation of the processing space to reduce G/R, (b) post-processing heat treatments below and above the β transus temperature, and (c) modification of the alloy constituents to promote the columnar to equiaxed transition (CET) during solidification.
Manipulation of the parameter space and beam shaping are interesting approaches that have been used to promote the CET.[
10,
11] In the case of Ti-6Al-4V, useful solidification maps have been built to predict the effect of laser parameters on the dominant grain structure morphology for a variety of beam deposition and electron beam additive processes.[
12,
13] Nevertheless, these are not always practical or possible approaches as the ultimate aim of L-PBF is the creation of fully dense components of any arbitrary geometry and indeed evidence suggests limited success in forming dense Ti alloys with an equiaxed microstructure.[
14]
Studies have demonstrated that sub-transus heat treatments are ineffective in changing the morphology of the prior-
β grain boundaries, as the retained
α phase effectively pins the
β grain boundaries already present in the structure.[
15] A full
β-solution heat treatment (above the
β transus temperature) has instead shown that
β grain boundaries can migrate and split realizing equiaxed structures but these treatments lead to inevitable coarsening of the structure at the expense of ductility and strength.[
15,
16]
Great effort has also been spent in modifying the constitution of Ti-6Al-4V to control the solidification mode of the high-temperature
β phase. Recognizing that Al and V provide limited constitutional undercooling in Ti-6Al-4V alloys,[
17] and building on the growth restriction theory developed for casting,[
18] a number of solutes with high-growth restriction factor (
Q) in Ti have been investigated (
Q =
ml·
c0·(
k − 1), where
ml is the slope of the liquidus line on the phase diagram (K/wt pct),
c0 is the solute concentration in the corresponding binary alloy (wt pct), and
k is the partition coefficient of the added solute).
A promising solute for refining prior-
β grains in Ti alloys is silicon (Si). Si has proven to be an effective grain refiner in cast commercially pure (cp-) Ti and Ti-6Al-4V although there is contrasting evidence on whether intermetallic silicides might form during primary solidification and how these affect the mechanical properties of the resulting alloy.[
19,
20] Mereddy
et al. have recently investigated the addition of Si to wire arc additively manufactured cp-Ti and have suggested a mechanism where Si solute segregation hinders lateral growth of
β grains, resulting in
β grains that maintain high-aspect ratio but are effectively refined in width.[
21] Beryllium (Be) that has the highest theoretical
Q as a solute addition to Ti has produced significant grain refinement, although its use as a grain refiner in additive manufacturing (AM) might be limited due to associated health hazards.[
22] Molybdenum (Mo) has been shown to expand the freezing range of Ti-6Al-4V, destabilize the planar growth of Ti-6Al-4V, and reduce the size of the prior-
β grains.[
17] Mo particles are, however, generally retained in the microstructure as a result of the thermo-physical property differences with Ti causing microstructural and chemical inhomogeneities that can lead to undesirable scatter in mechanical properties.[
23,
24] Additionally, partial dissolution of Mo is intrinsically linked to relatively low efficiency, as a large amount of Mo seems not to take part in the solidification process. Chromium (Cr) has also been considered as a grain refiner for Ti.[
18,
25] Although research on casting demonstrated that during the primary solidification Cr super-saturates
α +
β phases, the complex thermal history of laser AM processing has been shown to produce grain boundary precipitates in the form of TiCr
2 Laves phases that have a deleterious effect on the properties of the alloy.[
26,
27]
In other approaches, the coupled action of both solutes with high
Q and heterogeneous nuclei in the form of insoluble substrates has also been explored. Boron (B),[
28,
29] tungsten (W)[
30], and a number of rare-earth elements (La, Y,
etc.)[
31‐
33] have been shown to affect the phase transformation and produce a significant constitutional undercooling and an increase in the nuclei population to encourage equiaxed grain formation in Ti and Ti-based alloys. Nevertheless, although B has a large
Q, its use as a grain refiner may be limited when its deleterious effect on ductility caused by the precipitation of titanium borides is considered. Rare-earth oxides and W have been shown to be stable compounds in liquid Ti and thus promising candidates for heterogeneous nucleation of Ti grains. However, after laser AM, the resulting composite microstructure consists of heterogeneous particles distributed within the typical
α +
β microstructure of Ti alloys suggesting poor nucleation efficiency. The distribution of these secondary phases in the microstructure and their effect on the mechanical properties of the alloy is still a matter of ongoing research but appears largely deleterious.
While recognizing the importance, and most likely the necessity, of introducing potent heterogeneous nucleation substrates for promoting CET in laser AM—particularly L-PBF that is characterized by extremely high G and R values—the identification of suitable solute additions that refine prior-β grains without forming deleterious brittle phases remains a scientific challenge in AM and other energy beam-processing methods used to manufacture Ti-based alloys.
The purpose of the present study is to elucidate the influence of iron (Fe) on microstructure formation in a modified Ti-6Al-4V alloy containing Fe as a quaternary solute and to evaluate its suitability for producing a refined grain structure. Fe is characterized by a high
Q value that directly favors the creation of constitutional undercooling (a pre-requisite for the CET),[
25,
34] unfavorable kinetics for the formation of brittle intermetallics,[
35] and thermo-physical properties comparable to Ti that would encourage homogeneous mixing during L-PBF.[
23] Accounting for all these considerations, Fe is potentially a suitable alloy component in Ti-6Al-4V to refine the grain structure while maintaining a well-characterized and predictable balance of
α and
β phases in the final microstructure.
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