Abstract
In this paper, phase transitions and microstructure evolution in titanium Ti-6Al-4V alloy parts produced by electron beam melting (EBM) under hydrogenation was investigated. Hydrogenation was carried out at the temperature of 650 °C to the absolute hydrogen concentrations in the samples of 0.29, 0.58, and 0.90 wt. %. Comparative analysis of microstructure changes in Ti-6Al-4V alloy parts was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Furthermore, in-situ XRD was used to investigate the phase transitions in the samples during hydrogenation. The structure of Ti-6Al-4V parts produced by EBM is represented by the α phase plates with the transverse length of 0.2 μm, the β phase both in the form of plates and globular grains, and metastable α″ and ω phases. Hydrogenation to the concentration of 0.29 wt. % leads to the formation of intermetallic Ti3Al phase. The dimensions of intermetallic Ti3Al plates and their volume fraction increase significantly with hydrogen concentration up to 0.58 wt. % along with precipitation of nano-sized crystals of titanium δ hydrides. Individual Ti3Al plates decay into nanocrystals with increasing hydrogen concentration up to 0.9 wt. % accompanied by the increase of proportion and size of hydride plates. Hardness of EBM Ti-6Al-4V alloy decreases with hydrogen content.
Highlights
Additive manufacturing (AM) including the method of electron beam melting (EBM) is one of the promising directions in the production of structural metal materials with a unique set of properties
According to X-ray diffraction (XRD) the structure of the samples before hydrogenation is represented by the α phase of titanium with a hexagonal close-packed lattice and the β phase of titanium with body-centered cubic crystal modification
It is observed that the volume content of the β phase increases with hydrogen concentration to 0.29 wt. %, since hydrogen is a stabilizer of the β phase [34]
Summary
Additive manufacturing (AM) including the method of electron beam melting (EBM) is one of the promising directions in the production of structural metal materials with a unique set of properties. The properties of titanium alloys produced by electron-beam melting are widely discussed in the literature [1,2,3,4,5,6]. Researchers pay particular attention to the microstructure of the manufactured materials as the mechanical properties of titanium alloys directly. The formation of the structural-phase state of Ti-6Al-4V alloy occurs as the result of powder melting at the temperature of 1900 ◦ C and subsequent rapid cooling to a temperature of ~700 ◦ C followed by cooling to the room temperature. This has been found to be formed from partially melted powder in the surrounding bed
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