[en] Results of investigation into specific features of strucural-phase transformations occurring under welding of the OT4 titanium alloy with the VN2AEh niobium alloy without use of the protective medium are given. Chemical composition of welded materials is presented. Quality of the formed joint under friction welding of chemically active and refractory metals in the air is shown to be determined by heterodiffusion in the welding zone. Grain grinding and recrystallization of the titanium alloy take place under the effect of heat and pressure during friction. Decrease of the weld strength can be explained by intense dissolution of niobium in the αTi lattice and formation of the orthorhombic phase (Ti, Nb) at reduced values of pressure and rate of relative rotation