Effect of beam current on the microstructure, crystallographic texture and mechanical properties of electron beam welded high purity niobium

Abstract

Fabrication of superconductive radio frequency (SRF) cavity by electron beam welding (EBW) of pure Niobium (Nb) has been a topic of discussion for quite some time. The influence of beam current on the weld properties of Nb plates has been investigated in light of mechanical properties and microstructure. The widths of the fusion zone (FZ) and heat-affected zone (HAZ) of the welded samples increase with increasing the beam current. The hardness profile across the base metal (BM) to FZ followed a typical symmetrical pattern with a plateau across the FZ. The maximum drop of hardness at FZ compared to BM is only about 15%, making the EBW process a suitable one for the joining of Nb plates for the application of SRF cavity. The development of mechanical anisotropy of the welded joints due to differential heat transfer and the associated change in microstructure in FZ and HAZ has been evaluated by conducting tensile tests along different directions to the line of welding. Elongation of the welded joint is the maximum (~94%) for samples with the tensile direction parallel to the welding direction for a beam current of 70 mA. The Lankford constant (r-value) at different angles (0, 45, and 90°) to the tensile axes has been measured and correlated with crystallographic texture measured by Electron Backscattered Diffraction (EBSD). The dominance of the {554} <225> component for samples with larger heat input is found to be responsible for higher r-values, maximum formability, and lower Kernal average misorientation (KAM) profile. A heat transfer simulation has been used to simulate the dimensions of the weld pool and subsequently correlate its impact on various properties. • Heat input controls the metallurgical phenomena like grain size, hardness and stress-strain response. • The hardness profile decreases from the BM through HAZ until it attains a minimum at the FZ. • Smaller average grain size makes the material stronger and lesser ductile. • Tensile specimens at 0° exhibits deeper and uniformly distributed dimples, an evidence of ductile fracture. • Determine the depth and length of weld pool from the heat-transfer simulation.