Polovetskiy E. Processes of structure and phase formation of joints of titanium and aluminum alloys in vacuum diffusion welding

The thesis is devoted to investigation of effect of processes of structure and phase formation on quality and mechanical properties of aluminum to titanium alloy joints, made by vacuum diffusion welding (VDW). Metallographic investigations of welded joints obtained at different parameters of modes of diffusion welding, without intermediate layer and applying the layer of different thickness, without and using forming matrices, are carried out. Necessity of application of pure aluminum layer is shown for elimination of negative effect of magnesium diffusing from AMg6 during VDW of AMg6 to VT6. It is determined that optimum thickness of the layer from AD1 is 150 m. It is stated that magnesium diffuses from AMg6 in AD1 alloying powder during welding due to thermal-deformation effect of parameters of welding mode. As a result appearance of area with reduced magnesium content is observed and these changes affect the structure and mechanical properties in zone of AMg6 to AD1 joint. Failure and dissolution of oxide films with further formation of diffusion zone of up to 2 m width due to diffusion of titanium and vanadium from VT6 into AD1 take place in zone of AD1+VT6 joint. It is proved that total quality of AMg6+AD1+VT6 diffusion joint depends on quality in AD1+VT6 zone which can be obtained only in welding using optimum modes, namely Pw = 20 MPa, tw = 20 min, Tw = 540 °C. Mechanical pull tests showed that failure of welded joint, obtained using optimum mode, takes place on aluminum, has tough character and layer of aluminum remains on the alloy surface after it. Relevance of application of forming matrices as one of the method for intensifying of plastic deformation in zone of welded joint is studied. Their application provided no quality improvement in our case. However, application of forming matrices can be recommended for cases when intensifying of plastic deformation is necessary for obtaining of quality joint during dissimilar material welding.