Variant selection in laser melting deposited α + β titanium alloy
Introduction
Additive manufacturing (AM) is a revolutionary digital manufacturing process to manufacture the complicated and near net shape metal components layer by layer [1]. The advantages of additive manufacturing such as no tooling, little material waste and post-deposition machining, short production cycle and no restriction to component size make it very suitable for manufacturing those large and critical components which are made of difficult-to-process metallic or inter-metallic materials such as titanium alloys [2], [3], nickel-base alloys [4], [5] and high-strength steels [6]. The AM technology has attracted increasing and world-wide attentions since middle 1990s and a series of technologies using the same principles but with different names have been developed [7]. One of them is called laser melting deposition (LMD) which is very suitable to manufacture large metal components. The biggest component manufactured by LMD ever reported is 4 m × 3 m × 2 m [8].
Titanium alloy is widely used in modern society because of its high strength, low density, excellent corrosion resistance and good biocompatibility. However, titanium alloy is also well-known for its poor materials-processing and components-manufacturing abilities which make it very difficult to processing using traditional material-processing methods [9]. AM is recognized economically competitive to fabricate large and complex titanium components. Almost all kinds of titanium alloys, such as near α titanium alloys [10], [11], [12], [13], α + β titanium alloys [14], [15], [16], [17] and near β titanium alloys [18], [19], [20], [21], [22] have been successfully fabricated by AM. After AM process, near all the titanium alloys were reported to have inherent solidification textures that large prior beta columnar grains aligned along the build-up directions as a result of the strong epitaxial growth at the pool-bottom during rapid solidification under high temperature gradient conditions. The strong β texture is regarded as the main cause of the anisotropic mechanical properties of titanium alloys [23], [24], [25].
As it is known, titanium exhibits a hexagonal close packed (HCP) α phase at low temperature and transforms into a body centered cubic (BCC) β phase at high temperatures. The phase transformations are governed by Burgers orientation relationship (OR): {0001}α//{011}β, <11 0>α//<111>β with 12 possible α orientation variants that can transform from a single β grain during β→α transformation and 6 possible β orientation variants from one single α grain during α→β transformation [26]. Variant selection mechanisms in both BCC and HCP phases during phase transformations have often been linked to transformation strain and elastic anisotropy as the phase transformation are often companied by anisotropic volume changes [27]. For example, in the case of β grains transform into α under unconstrained conditions in TC4 alloy, contraction in the range of 10.9% is expected along a <100>β direction and expansion of 9.2% along one of the two perpendicular <110>β directions, whereas the other shows no volume changes [28]. Internal stress always lead to a relatively strong α texture in titanium alloys and the exact fractions of the variants are closely dependent on the cooling rate [28], [29]. As the deposition process leads to a significant internal stress under the very high cooling rate, a strong α texture can be expected in AM titanium alloy. However, as announced by some researchers, α texture is very weak that all 12 possible α variants respect to the Burgers OR appears at random in the AM titanium alloy [24], [25], [30], [31]. The former researchers made this conclusion mainly depending on EBSD data directly but with confused evidence when considering the crystallographic symmetry of α phase. It is hard to point out the <11 0>α which respect the Burgers OR from others. Another indirect method to confirm α texture is to measure the volume changes during phase transformations. As mentioned above, volume change is anisotropic no matter from α to β phase or from β to α phase. A strong selected α texture will lead to an anisotropic volume change during phase transformations at macro level.
The present work is aimed to confirm α variant selection in LMD α + β titanium alloy by measuring the linear thermal expansion of LMD TC21 alloy at different directions, and the variant selection mechanisms have also been discussed.
Section snippets
Experimental procedure
The studied alloy, TC21 (Ti–6Al–2Zr–2Sn–3Mo–1Cr–2Nb, [Mo]eq = 4.9) was deposited using a laser melting deposition complete equipment containing a YSL-10000 fiber laser, a BSF-2 powder feeder together with a co-axial powder delivery nozzle, and a Fagor-8055 computer numerical control (CNC) four-axis working table. The sketch map of laser melting deposition procedure is shown in Fig. 1. There were three remarkable directions pointed out on the sketch map. They were longitudinal direction (L,
β morphology and microstructure
The morphologies of prior β grains of LMD TC21 alloy are shown in Fig. 2. There are two kinds of prior β grain morphology in LMD TC21 alloy which are alternately arranged along the T direction (Fig. 2 (a)). The large columnar β grains are 0.5∼2 mm wide and 20–50 mm long, and they growth across many layers (Fig. 2 (b)). The small columnar β grains are 0.2–0.5 mm wide and 2∼3 mm long and they only growth within one single layer (Fig. 2 (c)). The formation process of the two kind prior β grains
Volume change during phase transformation
During heating, titanium alloy will transform from HCP α phase to BCC β phase. The phase transformation respects to Burgers OR: {0001}α//{011}β, α//<111>β. We can create a rectangular coordinate system with the axis [011]β//[0001]α, β//α and [β//α to calculate the anisotropic expansion during phase transformation. The titanium atoms in the (011)β//(0001)α plane and β//α plane are shown in Fig. 6. The dotted circles stand for α phase atoms while the
Conclusions
Variant selection in LMD TC21 alloy was studied with linear thermal expansion, and some conclusions can be made as follows.
- (1)
At micro level, d-spacing change during phase transformation is anisotropic in LMD TC21 alloy. In the case of HCP α phase transforms to BCC β phase, the d-spacing at direction would contract while the d-spacing at and [0001]α direction would expand. Variant selection would lead to anisotropic expansion at macro level.
- (2)
In linear thermal expansion test,
Acknowledgment
This work is financially supported by the National Key Basic Research Program under Grant No. 2011CB606305-2. The authors gratefully acknowledge Dr. Dong Liu, Dr. An Li and Dr. Haibo Tang for their great help on laser additive manufacturing of all the titanium specimens.
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