Investigation of cracks during electron beam welding of γ-TiAl based alloy

Abstract

Electron beam welding (EBW) of γ-TiAl intermetallic with duplex microstructure has been conducted with various welding parameters. Experiments and numerical simulation were investigated to reveal the cracking mechanism of the joint. The weld undergoes the martensitic transformation via β→α evolution, resulting from the ultra-fast cooling rate, which eventually results in the formation of α2-Ti3Al dominated basket weave microstructure with poor ductility at low temperatures. The brittle microstructure can hardly deform during the cooling process. The simulation results indicate high-level longitudinal residual tensile stress distributed within the weld zone, which accumulates gradually and even exceeding the tensile strength of the joint. The synthetic effect of brittle microstructure and high residual stress promotes the initiation of cracks. Based on this, an original criterion, initial crack generation time of EB welded γ-TiAl joint, is proposed to evaluate the crack sensitivity of the joint and provide guidance for the improvement of welding process. The EB reciprocating scanning process is adopted to extend the residence time of the joint above the initial crack generation temperature. Crack-free joints with the microstructure of bulk γ phase and the α2/γ lamella colonies are obtained, which dramatically ameliorates the ductility of γ-TiAl joints.