PSI - Issue 53

Serhii Lavrys et al. / Procedia Structural Integrity 53 (2024) 246–253

249

4

Serhii Lavrys et al. / Structural Integrity Procedia 00 (2019) 000 – 000

3. Results and discussion According to the metallographic analysis, it can be observed that the wrought titanium alloy has a typical equiaxed structure. Instead, the AM microstructure of the titanium alloy is characterized by acicular martensitic structure, which is obviously associated with the high rates of heating and cooling of the alloy during AM [11 – 13]. As a result of post HT, the metastable martensitic microstructure transforms into an (α+β) two -phase lamellar structure. It should also be noted that an increase of the post HT temperature from 800°C to 850°C leads to an increase in grain size by approximately 2 times, which is associated with recrystallization processes (Fig. 1).

Fig. 1. Microstructure of Ti6Al4V titanium alloy: wrought (a), AM (b), AM after post HT at 800°C (c) and 850°C (d)

The formation of a martensitic structure during AM is also confirmed by the results of XRD analysis. In the diffraction spectrum of the AM titanium alloy, unlike the wrought one, we record only the α/α' phase, without traces of the β phase (Fig. 2). It is obvious ly that during AM, a non- equilibrium martensitic α' phase wit h a HCP structure, similar to the α phase, is formed, which causes difficulties in its identification. However, the presence of an α' martensitic structure is indicated by extended peaks of the α/α' phase. The formation of the non - equilibrium α' martensitic phase is caused by non-diffusion rapid solidification during AM. Such AM conditions prevent the diffusion of β -doping elements (in our case, V), and as a result, a supersaturated solid solution of v anadium in α' phase is formed [12]. This leads to an increase in stresses in the crystal lattice and an increase in microhardness (Fig. 3).

Fig. 2. Microstructure of Ti6Al4V titanium alloy: wrought (a), AM (b), AM after post HT at 800°C (c) and 850°C (d)

The p ost HT leads to the appearance of peaks of the β -ph ase and a decrease in the width of the peaks of the α/α' phase (Fig. 2). That is, during post HT under the influence of temperature, diffusion processes take place, which leads to the redistribution of alloying elements, namely β -stabilizers (vanadium) from the α / α' phase to the β -phase, which can dissolve more vanadium compared to α and α' -phases. Also, the formation of stable α and β -phases and the reduction of the α' phase in the AM Ti6Al4V titanium alloy is evidenced by the decrease in lattice stresses and microhardness. As the post HT temperature increases, the peaks of the β -phase become more noticeable and the stresses become smaller, which indicates a decrease in the amount of non-equilibrium α' phase in the structure of AM Ti6Al4V titanium alloy (Fig. 2 and Fig. 3). Hence, there is a gradual convergence of α' to the α -phase and an increase in the β -phase.