Microstructure and mechanical properties of Ti6Al4V alloy and sapphire joint brazed with graphene-AgCuTi

The sapphire (alpha-Al2O3) and Ti6Al4V (TC4) alloy was successfully brazed with GNSs (Graphene)-AgCuTi composite filler. The effects of different GNSs content and different brazing temperatures on the interface and mechanical properties of brazed joints were studied. The typical interfacial microstructure of the brazed joint was: Ti6Al4V/Ti2Cu/TiCu/Ti3Cu4/Ag(s.s)/TiC + GNSs /TiCu/Ag(s.s)/Ti3Cu3O/sapphire. The result shows the diffusion layer (I) and Ti-Cu layer (II) got thickened gradually as the brazing temperature rose. At the same time, the thickness of Ti3Cu3O reaction layer increased from 1.9 m to 4.7 m when the brazing temperature increased from 810 degrees C to 870 degrees C. The thermal expansion coefficient of the composite filler and the mechanical properties of the joint were calculated and analyzed. The addition of GNS can reduce the mismatch of thermal expansion coefficient between the brazing joint and Ti3Cu3O reaction layer. When the 0.1 wt% GNSs-AgCuTi composite filler was put into use, the shear strength of the joint reached 29.1 MPa. However, the agglomeration of excess graphene hindered the diffusion of the elements, making the Ti3Cu3O layer discontinuous and causing a mismatch in the coefficient of thermal expansion among the materials, thus reducing the shear strength of the joint.