From Base Alloy to Welded Joint: Understanding Microstructural Evolution and Mechanical Responses in Electron Beam Welded Near-β Titanium Alloys

To meet the aerospace industry's demand for materials with exceptional strength-to-weight ratio, high toughness, and excellent workability, near-beta titanium alloys have been developed as potential alternatives or supplements to conventional titanium alloys (e.g. Ti-6Al-4V). Electron Beam Welding (EBW), characterized by its high energy density, rapid welding speed, low heat input, and high precision, has become the ideal choice for welding Ti-alloy. However, the welding process also induces complex thermal cycles that can cause microstructural evolution and alter local mechanical properties. In this study, both parent and welded samples were characterized, and the effects of heat treatment parameters on the microstructural modifications and resulting mechanical responses in EB-welded near-β Ti-alloy specimens were also investigated.

Microstructural evolution of both parent and EB-welded specimens with different heat treatment temperatures was characterized using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), while Electron Backscatter Diffraction (EBSD) provided insights into grain orientation relationships as well as phase distributions. In addition, Energy Dispersive X-ray Spectroscopy (EDX) was employed to examine the compositional changes from the base area to fusion zone (FZ). Micro-hardness measurements across the FZ, heat-affected zone (HAZ) and base area were conducted to establish correlations between microstructural modifications and mechanical responses.

The results revealed that the parental samples under the specific heat treatment routes exhibit a typical bi-modal microstructure; whereas significant microstructural changes were found in the welded samples with formation of coarse prior β columnar grains in the FZ and refined equiaxed grains in the HAZ, which ultimately shapes the hardness distribution in such specimens. Moreover, EDX analysis confirmed the compositional evolution and the gradual dissolution of α phase.

A comprehensive understanding of the effects of EBW process and heat treatment parameters, as well as microstructure-property relationships, helps establish a framework for optimizing processing conditions, ultimately contributing to the enhanced performance and reliability of EB-welded near-β Ti-alloy components in demanding aerospace applications.