PSI - Issue 59

Sviatoslav Motrunich et al. / Procedia Structural Integrity 59 (2024) 58–65 Sviatoslav Motrunich et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 2. Electron-beam cold hearth melting scheme (a): 1 - 5 – electron-beam guns; 6 – expendable workpiece; 7 – intermediate capacity; 8 – crystallizer; 9 – molten ingot and (b) melting process of Ti-Al-Nb alloy ingot (110 mm diameter).

To assess the metal quality of the obtained ingots, a study of the chemical composition of samples taken along the length of the ingot from the upper, middle and lower parts was carried out. The nondestructive ultrasonic method was used to obtained information about titanium ingots quality and provide defectoscopy in order to detect possible internal defects in the form of non-metallic inclusions, as well as pores and leaks. Ultrasonic flaw detector UD4-76 using the echo-impulse method with the contact variant of control was used. The operating frequency of the analysis was 1.25 MHz, which provided the maximum signal-to-noise ratio. To avoid noises in the area of the input of probing signals, which mainly are caused by the loose fitting of the sensor to the surface of the ingot, "sealing" lubricant was used.

The macro and microstructure of the ingots was studied on transverse templates cut from the middle part of the ingot. Microstructure study was carried out using opt і cal microscope on etched templates of metal in a 15% solution of hydrofluoric acid with the addition of 3% nitric acid at room temperature. Uniform test specimens with polished surface for tensile and fatigue testing were produced (Fig. 3). Templates for specimens were machined from the middle part of the ingot. Testing was performed on MTS 318.25 servohydraulic machine with load capacity of 250 kN.

Fig. 3. Uniform specimen for tensile and fatigue testing.

Tensile testing was performed according to ISO 6982-1 at initial speed of 5 mm/min using extensometer with gauge length of 25mm. Fatigue testing was performed according to ISO 1099 at force-controlled loading to fracture with stress ratio R S = 0.1 and loading frequency of 25 Hz. Fracture analysis of specimens after fatigue testing was

provided using scanning microscope JSM-840 (JEOL). 3. Results of experimental study and their discussion

During ultrasonic nondestructive examination of the ingots multiple low-amplitude reflections were observed (Fig. 4.a). This is typical of cast metal and is the result of signal reflection from grain boundaries. The analysis revealed no reflections that could be interpreted as large non-metallic inclusions, shrinking shell pores, etc. The reflection of the bottom pulse is clearly determined on the scan. The macrostructure examination shows that the ingot metal is dense, uniform, with the absence of zones that are etched differently across the cross section (Fig. 4.b). There is no significant difference in the structure of the central zone of the ingot and the peripheral zone. Defects in the form of pores, shells, cracks and non-metallic inclusions were not detected. No segregation of alloying elements characteristic of EBCHM ingots was found. Thus, the conducted studies showed that ingots of Ø110 mm titanium alloy Ti -6Al-7Nb obtained by the EBCHM method, there are no leaks, non-metallic inclusions larger than 1 mm, as well as dense clusters of smaller inclusions.