PSI - Issue 36

Victor Shapovalov et al. / Procedia Structural Integrity 36 (2022) 262–268 V. Shapovalov, I. Protokovilov, V. Porokhonko / Structural Integrity Procedia 00 (2021) 000 – 000

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The Brinell tests showed that the hardness value were highest in the HAZ (HB 285-295), medium in the base metal (HB 277-282) and the lowest in the weld metal (HB 272-280). The microstructure of the welded joint is illustrated in Fig. 5. The microstructures of the base metal (Fig. 5 a) had an equiaxial (α+β) structure with the Vickers hardness of α phase 3360- 3540 MPa, and β -phase 3300-3540 MPa. The grain size was 10-15 μm . The microstructure of HAZ metal (Fig. 5 b, c, d) contained polyhedral grains of (α+β) structure and also acicular structure. The α -phase plates are directed parallel to each other, as well as at angles of 60 and 90 ° relative to each other. Their Vickers hardness was 3500-3800 MPa. The hardness of the acicular structure was slightly higher and amounted to 3850-3900 MPa. The average grain size of HAZ metal was significantly larger than that of the base metal and amounted to 300-640 μ m. The weld metal had (α+β) structure with α -phase in form of plates and lamellas separated by β -phase layers (Fig. 5 e, f) . Separation of α -phases was observed at the grain boundaries. The width of the boundaries varies from 2.5 to 5.0 μm . Vickers hardness of the α -phase was 3100-3700 MPa. It should be noted that near the fusion line there are areas of acicular structure with increased hardness of 3900 MPa compared to the hardness of the nearby plates 3300 3660 MPa. In addition, in the weld metal there are areas of irregular shape, which have a dispersed structure and increased hardness up to 4090 MPa. The grain size of the weld metal was the largest and amounted to 2-9 mm. In general, except for the areas with a acicular structure and beta phase clusters, the Vickers hardness in the base metal, HAZ and weld metal was similar and ranges from 3200 to 3800 MPa.

Fig. 5. Microstructures of the welded joint: a – base metal; b, c, d – HAZ; e, f – weld metal.

The results of mechanical tests of the welded joint are given in Table 3. The base metal had the highest tensile and yield strengths and the weld metal - the lowest. On average, the strength of the weld metal was about 90 % of the strength of the base metal. The strength of HAZ was slightly higher than that of the weld metal but lower than that of the base metal. It can be assumed that the decrease in the strength of the weld metal is associated with a lower content, in relation to the base metal, of interstitial impurities (O, N, H) as well as alloying elements (Al, V), which increase the strength of titanium (Tables 1, 2). However, the elongation and reduction of area of the weld metal were also lower than those of the base metal. This indicates that the decrease in mechanical characteristics in tension tests is associated mainly with structural factors and the coarse grain size of the weld metal.