Microstructure and mechanical properties of welded joint of titanium/steel and titanium/copper/steel composite plate

The titanium/steel and titanium/copper/steel composite plates were butt joined by arc welding method. SEM, EBSD, TEM, microhardness, nanoindentation and tensile tests were applied to investigate the microstructure and mechanical properties. The results showed that in the titanium/steel butt joints Cu-V weld was mainly consisted of Cu solid solution and Fe solid solution phases. Localized Fe₂Ti intermetallics were surrounded by the soft Cu solid solution. Cu-Ti and Fe-Ti intermetallics were formed at Cu-V/ERTi-1 interface. Cu-V weld near the TA1/Q345 interface was consisted of Fe-Ti, CuTi₂ and β-Ti phases. A series of Cu-Ti compounds were widely distributed at Cu/ERTi-1 interface in titanium/copper/steel butt joints. Although Fe₂Ti brittle intermetallics had high hardness (20.7 GPa), its limited size had less effect on the global microhardness distribution. These two butt joints had similar microhardness distribution, where high hardness values (400 HV0.3) were located at the Cu-base weld/ERTi-1 interface. The Cu-Ti compounds with wide distribution showed the hardness around 8 ~ 11 GPa.The tensile strength of titanium/steel butt joint and titanium/copper/steel butt joint were 440 MPa and 225 MPa, respectively. Both samples were fractured at the weld metal regions and brittle fracture morphology was observed at the Cu-based weld/ERTi-1 interface regions. Fe-Ti brittle intermetallics were inevitable in titanium/steel butt joint. These brittle phases were suppressed in titanium/copper/stee butt joints. However, the widely distributed Cu-Ti compounds was the weak region of such joints.