Investigation of fracture for friction welded joint between pure nickel and pure aluminium with post-weld heat treatment

• Fracture mechanism of pure Ni/pure Al friction welded joint by PWHT was clarified.
• Mainly NiAl layer generated and grew at the weld interface with PWHT.
• The joints autogenously fractured at the weld interface during cooling stage.
• The fracture occurred by decreasing of joining strength as increasing of NiAl width.

The present paper described the investigation of the fracture of friction welded joint between pure nickel (Ni) and pure aluminium (Al) with post-weld heat treatment (PWHT). Most of joints autogenously fractured from the adjacent portion of the intermediate layer (interlayer) consisting of intermetallic compound (IMC) on the weld interface due to growing of that after long heating time during the cooling process after PWHT. The IMC interlayer was composed with mainly NiAl, and that grew at the weld interface with PWHT. The joint fracture temperature increased with increasing width of the IMC interlayer in the axial direction of the joint. That is, the fracture of the joint occurred at the interface between NiAl layer and Al base metal. The fractured surface was covered with a little Ni2Al3 and/or NiAl3, and that was like as disbonding. Furthermore, when the width of the IMC interlayer was smaller than approximately 40 μm, the joint fracture temperature of the joint was under about 470 K. However, when the width of the IMC interlayer exceeded 50 μm, the joint fracture temperature drastically increased up to about 800 K. Hence, it was able to be estimate that the joint fracture temperature increased with increasing width of the IMC interlayer. Therefore, one of the main reasons for the fracture of the joint could be concluded as remarkable decreasing of the bonding strength between NiAl layer and Al base metal, which was produced with PWHT.