The Effect of Grain Size on the Diffusion Bonding Properties of SP700 Alloy

Superplastic forming and diffusion bonding (SPF/DB) has been recognized as a viable manufacturing technology. However, the basic understanding of grain size and its effects on the quality of diffusion bonds is still limited. In this study, a certain type of SP700 alloy with different grain sizes is bonded at superplastic temperature. The experimental results indicate that the same materials, if coarse‐grained, may not readily bond under identical conditions of pressure, temperature, and time. This type of bonding is possible because of the presence of many grain boundaries in fine‐grained materials that act as short‐circuit paths for diffusion. In addition, grain‐boundary migration is also faster in fine‐grained than in coarse‐grained materials. Fractographic studies show that the dimples on the coarse‐grained specimen have large dimensions compared with that in the fine‐grained material, indicating that heterogeneous deformation develops in the coarse‐grained specimen during tension.