Structural Scale Levels of Plastic Deformation and Fracture of High-Strength Titanium Alloy Welds

Abstract

Structural scale levels of plastic deformation and fracture of welded joints have been studied for two high-strength titanium alloys with a low (VT18U alloy) and a high (VT23 alloy) content of the bcc ß phase. Ultrasonic forging and its combination with high-current pulsed electron beam treatment were used to activate nanoscale structural levels of deformation and fracture in welds in order to increase the fatigue life of welded structures. Ultrasonic forging provides an effective dispersion and nanostructuring of surface layers in the VT18U welded joints with a 4.6-fold increase in their fatigue life. The dispersion and nanostructuring of the VT23 laser welded joints is achieved only by ultrasonic forging combined with high-current electric pulse treatment, in which longitudinal dispersion of ß bands occurs with the formation of orthorhombic a " nanolaths. In so doing, the fatigue life of the VT23 welds increases twice, but the effect depends on the power of the high-current generator and electrical pulse parameters. The fracture micrographs of the treated VT23 welded joints reveal nanofibrous bands responsible for ductile fracture and for the reduction of the fatigue crack growth rate. The structural changes and the increase in the fatigue life of the studied titanium alloy welds are associated with the activation of nanoscale structural levels of deformation and fracture induced by ultrasonic forging or by its combination with high-current pulsed electron beam treatment.