Research ArticleTargeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures
Introduction
Metal additive manufacturing (MAM) has in recent years opened up for new opportunities for industrial implementation of titanium, due to its potentially lower buy-to-fly ratio. Titanium is a rather expensive raw material and traditional subtractive manufacturing methods are associated with substantial costs and material waste. Additive manufacturing (AM), as for example laser powder bed fusion (LPBF), only uses material at the desired location by melting of metal powder and subsequent solidification; in principle the remainder of the unused powder can be recycled into subsequent builds [1].
During L-PBF processing a high power laser scans the surface of a powder bed and locally raises the temperature above the melting point of the material [2,3]. One of the most common alloys used in titanium AM is Ti-6Al-4V (ELI), also known as titanium grade 5 (or, for ELI, grade 23), which is a dual-phase α + β-alloy. This alloy has excellent mechanical and chemical properties, hence its widespread application in conventional manufacturing sectors. During AM, the highly localized melting and rapid movement of the laser result in fast solidification and fast cooling through the β-stability region. Such cooling conditions combined with cyclic reheating during deposition of the following layers of the build, result in the formation of a very fine martensitic microstructure [2,4]. The β-grains forming during solidification are elongated along the build direction, because this direction experiences the largest temperature gradient [2,5,6]. The as-built Ti-6Al-4V microstructure is associated with anisotropic mechanical behavior [5,7]. The as-built martensitic microstructure in the prior β-grains is associated with high strength, but the ductility is generally at an unsatisfactorily low level [5,8,9]. Hence, (post-) heat treatment is crucial in order to achieve adequate materials performance with respect to ductility and strength.
In conventionally manufactured Ti-6Al-4V, two types of microstructures are commonly used: bi-modal and fully lamellar. The typical bi-modal microstructure consists of equi-axed grains of α-phase in a lamellar α + β matrix and is known for its combination of good ductility and relatively high strength. Formation of a bimodal microstructure entails thermomechanical processing followed by recrystallization in the α + β region [10]. The fully lamellar microstructure consists of α-lamellae and inter-lamellar β-phase. This microstructure provides excellent crack growth resistance, but it provides lower strength and ductility than the bi-modal microstructure. The fully lamellar microstructure is obtained through isothermal holding above the β-transus temperature, a so-called β-anneal, followed by relatively slow cooling [11].
Obviously, for near-net shape manufacturing processes, such as casting or additive manufacturing, a deformation step as required for obtaining a bi-modal microstructure, is not a viable option. As an alternative, a bi-lamellar microstructure, which combines features of the bi-modal and the fully lamellar microstructures was developed in the 1990s, when the application of titanium castings reached industrial implementation [12]. This process allowed for tailoring the mechanical properties of (coarse) fully lamellar components [13].
Traditionally, the bi-lamellar process consists of three main steps: (i) β-homogenization, (ii) inter-critical annealing in the α + β region followed by sufficiently fast cooling and (iii) aging to precipitate Ti3Al. The critical factors for the β-homogenization are identical to those for fully lamellar processing, i.e. the cooling rate should be fast enough to limit (suppress) excessive formation of GB-α with associated negative impact on the mechanical properties. For inter-critical annealing the temperature must be high enough to ensure that a sufficient fraction of β-phase is available for transformation to produce an effective bi-lamellar microstructure. Furthermore, it must prevent the stabilization of the metastable β-phase by partitioning of α-stabilizers (Al and O) and β-stabilizers (V and Fe), thus ensuring that secondary α-platelets can nucleate within the inter-lamellar β-regions. The temperature must be low enough to guarantee that the composition is inside the α + β-region and to create a certain stabilization of the β-phase, so secondary α-phase can be formed at achievable cooling rates. Then, a fine lamellar α/β-structure develops during sufficiently fast cooling. Moreover, cooling from the inter-critical annealing temperature should be sufficiently fast to guarantee α-precipitation within β, and suppress the growth of primary α at the cost of β. [11] Cooling rates of ∼600 K/min are desirable [13].
The bi-lamellar microstructure is known to improve the fatigue-crack resistance and ductility, while high strength is maintained by precipitation hardening of Ti3Al in the primary α-phase and reduction of the effective slip length by fine α-lamellae in the prior inter-lamellar β-regions [10,12]. The secondary α, present as fine lamellae in the prior inter-lamellar β-regions, is lean in Al due to elemental partitioning during β-homogenization, which limits the Ti3Al precipitation in a final ageing anneal to the primary α-phase.
For additive manufacturing, most research has focused on post-treatment to obtain a fully lamellar microstructure achieved by a full β-homogenization, fast cooling to a martensitic structure followed by decomposition of martensite in the α + β region [5,8,9,14] or insitu martensite decomposition into lamellar α + β during printing [15]. Ter Haar and Becker [6] attempted to replicate the conventional bi-modal microstructure without thermomechanical treatment. Their strategy involved fragmentation of the primary α-lamellae by applying a long holding time just below β-transus, followed by quenching and annealing to transform the martensite created by water quenching from β-transus into lamellar α + β. Generally, fully lamellar microstructures resulted in improved ductility, albeit at a significant loss of strength. On the other hand, the bi-modal microstructure was observed to provide a much better strength/ductility ratio [6].
Additive manufacturing encounters similar challenges as conventional casting due to its near-net-shape character. Accordingly, attaining a bi-lamellar microstructure from the very fine martensitic initial microstructure after L-PBF deserves further investigation. Furthermore, since AM components often are subjected to fatigue loading, a high resistance to micro-crack propagation becomes a strategic optimization property. The present work focusses on identifying a heat treatment route for establishing a bi-lamellar microstructure in L-PBF Ti-6Al-4V. To the best of the authors’ knowledge, the scientific literature does not describe the formation of this microstructure in L-PBF Ti-6Al-4V parts.
Section snippets
Experimental methods
Specimens were manufactured on a SLM280 machine from SLM solutions. The applied powder was titanium grade 23, which is an extra low interstitial (ELI) version of Ti-6Al-4V. The chemical composition, as provided by the supplier, and the standard tolerance range for titanium grade 23 are given in Table 1.
An anti-parallel scan strategy was used for a layer height of 60 μm with 67° rotation between consecutive layers. The specimens were Ø10 mm × 10 mm cylinders with a Ø3.5 mm × 7.5 mm blind center
Microstructure and hardness of as-built condition
The as-built L-PBF Ti-6Al-4V specimens have a very fine martensitic microstructure, see Fig. 1(c). The prior-β grains are elongated approximately parallel to the build direction Z (bottom to top in Fig. 1(a) and (c)), i.e. parallel to the direction with the largest heat flux. The prior β-grains are slightly inclined with respect to the Z-axis, because the finite size of the specimens implies that heat is also conducted through the powder. In the XY-plane, the grains appear almost square due to
As-built condition
The microstructure of the as-built specimens is consistent with observations concerning the formation of a fully martensitic microstructure, as presented in the literature [2,5,6]. The prior β-grains are elongated, albeit to a lesser extent than observed in other studies [[17], [18], [19]]. This is primarily related to the combination of printing parameters and scanning strategy (see Table 2). In particular, rotation of the scan direction between consecutive layers is important for minimizing
Conclusions
In this study, a bi-lamellar heat treatment was successfully applied to additively manufactured Ti-6Al-4V. The most important process parameters for each process step were investigated in terms of their effect on the resulting microstructure using a variety of characterization techniques. Based on these investigations the following conclusions can be drawn:
- (1)
The cooling rate from the β homogenization step is highly important for the resulting microstructure, both in the fully lamellar condition
Acknowledgements
This research was conducted in connection with the AM-LINE 4.0 project (No. 7076-00074B) funded by the Danish Innovation Fund. Specimens were manufactured by the Danish Technological Institute. The authors would like to thank Dr. Xiaoxu Huang for facilitating the collaboration with Chongqing University and Dr. Kristian V. Dahl for providing Thermo-Calc plots.
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