Microstructure Evolution and Phase Transformation of Welded Metastable Beta-titanium Alloy (Ti-5Al-5V-5Mo-3Cr)

Abstract

Ti-5Al-5V-5Mo-3Cr (Ti5553, in wt%) is a recently developed metastable β titanium alloy, specifically designed to replace Ti-10V-2Fe-3Al (VT22) in the manufacture of large airplane components. The studies of Ti5553 have drawn the attention of many researchers. Ti5553 shows reasonable weldability and can be welded autogenously (without filler metal). Most of the previous studies were directed at forged material behaviour. There is still a lack of understanding of the microstructure evolution and the phase transformation in regards both similar and dissimilar welded material.

The main part of this study investigates the microstructure evolution and phase transformation of Ti5553-Ti5553 similar weldment upon various post weld heat treatment (PWHT). The microstructure analysis was carried out by optical, scanning electron microscopy (SEM) and electron transmission microscopy (TEM). Microstructural results show that the weld zone in Ti5553 can retain the β phase in the as-welded condition. SEM micrographs reveal the different morphology and growth rate of the α precipitation at 500℃ and 600℃ ageing temperature. TEM results in the fusion zone (FZ) area of an as-welded (AW) specimen show the diffraction patterns of the ω phase. This ω phase was retained as athermal ω_a which formed during cooling from the welding process. All evidence proved that ω_a in the weld zone improved the precipitation rate. However, there was no evidence for an isothermal ω at a 500℃ ageing temperature.

The volume fraction and size of α precipitates have a major influence on hardness and tensile strength. At around 30mins of ageing time at either temperature, the α platelets reached an equilibrium of precipitation and hence had the highest volume fraction which resulted in the highest tensile strength. Increasing the ageing time resulted in the α laths coarsening, especially for samples aged at 600℃. The coarsened α laths caused a decrease in hardness. However, changes in tensile strength were not significant. Most of the fractures occurred in the FZ. Fractographic analysis showed dimple rupture via microvoid coalescence for all tensile tested pieces.

Investigation on AW dissimilar welding Ti5553-Ti64 and Ti5553-CPTi suggested that Ti5553 is weldable to the most common titanium alloys (Ti64 & CPTi) and with reasonable tensile strength. All fractures occurred at the low-strength area. Hardness profiles indicated higher hardness in the FZ. Electron probe micro-analysis (EPMA) was employed to investigate material flow in the melt pool.