Microstructural Analysis of Diffusion-Bonded AA6061 versus AA6061 Interface as Functions of Hot Isostatic Pressing Temperature and Cooling Rate

Understanding microstructural evolution at the hot isostatically pressed (HIP) cladding AA6061/AA6061 interface is essential for advancing the Zr-laminated U10Mo monolithic fuel system for research and test reactors. In this study, AA6061 plates were diffusion bonded via HIP at two temperatures (450 and 560 °C) and two cooling rates (1 and 45 °C/min). Scanning electron microscopy and quantitative image analysis were conducted to characterize the Mg 2 Si precipitation, with an emphasis on precipitate linear density and size at the AA6061/AA6061 HIP-bonded interface. The results revealed a pronounced accumulation of Mg 2 Si precipitates at the AA6061/AA6061 interface, with the highest precipitate density and largest precipitate size occurring in samples HIP-processed at 560 °C followed by slow cooling. Post-HIP heat treatment experiments demonstrated that water quenching after solutionizing effectively suppressed Mg 2 Si precipitation at the interface. To evaluate the feasibility of applying such post-HIP heat treatments within the full monolithic fuel system, simple growth kinetics estimates were conducted for the Zr/AA6061 and U–Mo/Zr interfaces. Overall, these findings provide valuable guidance for optimizing HIP parameters and potential post-HIP treatments to enhance the structural integrity and performance of monolithic fuel cladding.