Enhancing precipitation phase proportion and mechanical properties in thick plate welded joints of a near β titanium alloy via electron beam scanning welding with waveform design

It is generally difficult to obtain excellent joint properties of near beta titanium alloy thick plates via electron beam scanning welding (EBSW) with traditional circular scanning pattern (Pattern I) due to the precipitation difficulty of the strengthening alpha phase. In this study, EBSW with two novel scanning patterns besides Pattern I, including sine scanning (Pattern II) and sawtooth scanning (Pattern III) was designed to change the moving path of electron beam and regulate the precipitation of alpha phase in 30-mm-thick near beta titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe, which improved the joint properties. Among them, the Pattern III enhanced the joint hardness and strength most, from 299 HV for Pattern I to 340 HV for Pattern III, which was mainly attributed to the largely increased proportion of acicular alpha phase in the fusion zone from 6% for Pattern I to 47% for Pattern III. The moving path of electron beam was in a single direction in Pattern III, avoiding the mutual overlaps between waveforms in Pattern I. It caused the stabilization of the welding pool in advance by continuously stirring in Pattern III, promoting the precipitation of acicular alpha phase. Furthermore, for the joint in Pattern III, the average fracture toughness was 69.3 MPa & sdot;m(1/2) which was 92.0% larger than that of the BM. This study provides a new way to achieve superior EBW joint mechanical properties of near beta titanium alloy thick plates.