OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 Ta b l e 3 Physical and mechanical properties of Ti alloys Alloys → / Measurement methods and properties↓ VT1-0* Ti-6Al-4V* 3D printed VT6sv VT6* Macro-indentation E, GPa Longitudinal 110±8 110±13 XZ 103–131 108±4 Vertical 102±3 111±10 XY 90–100 Micro-indentation DuraScan-10 HV0.1 Longitudinal 168±5 370±23 XZ 334±14 339±6 Vertical 170±6 375±25 XY 304±16 DUH-211S EIT = 0.5 GPa 99±3 94±1 90–100 90±2 HIT = 0.5 N/mm2 1930±152 3913±129 3552±259 3660±105 Note. EIT: Indentation elastic modulus, HIT: Indentation hardness *As-rolled alloys β-phase (VT6, Ti-6Al-4V and VT6sv), the elastic modulus may depend on the ratio between these phases, as the elastic modulus for the α-phase is higher than for the β-phase. Lutfullin et al. attribute changes in the elastic modulus not only to the structure and phase composition of the 3D-printed VT6sv alloy, but also to the crystallographic texture. The latter plays an important role for the single-phase VT1-0 alloy. As reported in [43], this alloy predictably manifests a homogeneous structure and is often used as a standard material for nanoindentation measurements of the elastic modulus for Ti alloys. As for the welding titanium wire VT6sv subjected to remelting and thermal treatment during 3D printing, we observe changes in its structure and phase composition (see fig. 7). In addition, phases and texture modified by temperature conditions in different parts of the specimen, also affect the elastic modulus [34]. It should be noted that temperatures below the β-transus temperature, induce the formation of several structural types in the SLM Ti-6Al-4V alloy, namely: allomorphic crude lamellas, small lamellas/aciculae, and α-phase grains [44]. The formation of these structures can be observed in SLM titanium alloys [27, 30]. Structural elements include a finer grain structure and martensite. The grain size and martensitic component depend on the 3D printing mode, which determines the hardness and elastic modulus of the product. Its hardness significantly exceeds that of the product fabricated from the rolled alloy, i.e., 5 or 6 and 3 or 4 GPa, respectively. As for the elastic modulus, it is slightly lower than that of the product fabricated from cast or rolled Ti alloys, i.e., 107 to 119 GPa and 110 to 125 GPa, respectively. In wire-feed EBAM, the layer thickness is much higher than in SLM forgings, and temperature conditions approach to those of casting. In wire-feed EBAM, the well-defined columnar structure appears throughout the forging height and equiaxial grain structure in the scanning plane (see fig. 7, а, b). Such an alloy structure provides its hardness common to cast alloys, which slightly differs from the hardness in planes of forming and scanning. The elastic modulus obtained for all specimens, is much lower than that measured by ultrasonic gauging (see table 2). The highest difference in its values is conditioned by micro-indentation. The same difference is observed in [43], where the elastic modulus is measured by ultrasonic gauging and nanoindentation; besides the attention was drawn to the fact that the accuracy should be expected to be higher if the indentation covers a larger volume. All findings of the elastic modulus and hardness for the VT6sv alloy fabricated by wire-feed EBAM and Ti-6Al-4V and VT6 alloys in various states are presented in fig. 11, а. According to this figure, the elastic modulus obtained by ultrasonic testing for the printed material and rolled Ti-6Al-4V alloy, is slightly higher than that of initial cast and rolled alloys and those fabricated by EB-PBF in other works. Micro-indentation of the elastic modulus shows lower values than macro-indentation and findings of other researchers. Notably, the hardness of specimens printed from the VT6sv wire is lower than that of the Ti-6Al-4V alloy. This is explained by the VT6sv alloy composition, structure (see fig. 7), and microstructure [34]. The data presented in fig. 11, b correspond to cast alloys.
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