Enhanced joint integrity of diffusion-welded Alloy 617 by controlling the micro-chemistry near the surface

The diffusion welding process is critical for manufacturing efficient gas-to-gas heat exchangers. Due to issues affecting structural integrity that were extensively reported for Alloy 617 diffusion weldments, the effect of pre-treatment on micro-chemistry near the surface was investigated to enhance the interface properties. During the pre-oxidation step (930 degrees C/200 h/static air), an Al-depleted zone was created through the formation of internal oxides. A fresh joining surface with a minimal Al content was, thereafter, exposed by conventional mechanical polishing. After diffusion welding (1150 degrees C/2 h/14.7 MPa) and post-weld heat treatment (1120 degrees C/20 h), an interface, in which Al-rich oxides were almost absent, and extensive grain boundary migration across the interface were observed. Tensile specimens were fractured based on a ductile behavior, with more than 60% strain at the rupture point, considering a test temperature of 900 degrees C. In the stress-rupture test at 800 degrees C, the times to rupture of the diffusion weldment were comparable to those of the as-received alloy, and the strain to rupture reached over 17%.