Creep testing of thick-wall copper electron beam and friction stir welds

• Thick section copper canisters are planned to be used as a corrosion protection of nuclear waste disposal containers for long term underground deposal in Sweden. The copper canisters will have the top and possibly the bottom lid welded to the canister walls using electron beam or friction stir welding. Due to the high external hydrostatic pressure and the relatively high temperature of the waste during the first one hundred years the copper will creep. The creep process will close the manufacturing gap between the cast iron container and the copper canister. The creep ductility must be sufficient to avoid cracking of the weld. Specimens cut from the friction stir welds and the electron beam welds have been creep tested at temperatures ranging from 75 to 175 degreesC. Cross-weld specimens were used for both friction stir and electron beam welds. Weld metal, heat affected zone and base metal were also studied for friction stir welds. The results for the electron beam welds show that the main creep deformation is concentrated to the weld metal where the failure takes place. Weld metal and most cross-weld tests of friction stir weld material show similar creep lives and ductility as base metal tests. Ductility at rupture was found to exceed 30% for friction stir weld specimens, and the Norton power law exponent was determined to be between 30 and 50.

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