SOLID-STATE DIFFUSION WELDING OF WROUGHT AISI-304 STAINLESS-STEEL TO NIMONIC AP-1 SUPERALLOY POWDER BY HOT ISOSTATIC PRESSING

The diffusion welding characteristics of wrought AISI 304 stainless st eel to a superalloy powder Nimonic AP-1 were examined in this study. A fter encapsulation, evacuation and degassing, the welds were hot isost atically pressed (HIPed) in two different cycles, i.e., 1200 degrees C (2192 degrees F)/3 h/147 MPa and 1270 degrees C (2318 degrees F)/3 h/ 121 MPa. A variation in surface condition of slain less steel was also studied for its influence on properties of the joints. These diffusio n welded joints were characterized for diffusion of alloying elements, microstructural features and tensile properties. Most specimens showe d good weld strength for both the hot isostatic pressing (HIP) cycles, except specimens with sand blasted surfaces, which gave negligible du ctility because of the presence of sand particles embedded on the surf ace. In general, the tensile properties of the diffusion welded specim ens were limited by the mechanical properties of stainless steel and n ot the joint interface. The microstructural examination of a HIP welde d joint showed a thin weld zone of 30 - 40 mu m. While the stainless s teel region of the welded specimen showed a relatively clean structure , the Nimonic AP-1 region showed precipitation of dark particles near the interface extending over 20-60 mu m. Electron microprobe analysis of the diffusion welded zone revealed enrichment of titanium-rich part icles along prior particle boundaries (PPBs) near the interface. Elect ron microprobe analysis further revealed extensive diffusion of Ni, Co and Al elements into stainless steel and that of Fe into Nimonic AP-1 superalloy indicating complex composition of the weld interlayer.