Characterization of an explosively bonded aluminum proton beam window for the Spallation Neutron Source

An effort is underway at the Spallation Neutron Source (SNS) to redesign the 1st Generation high-nickel alloy proton beam window (PBW) to a 2nd Generation design that utilizes aluminum for the window material. One of the key challenges to implementation of an aluminum PBW at the SNS was selection of an appropriate joining method to bond an aluminum window to the stainless steel shielding structure of the PBW assembly. An explosively formed bond was selected as the most promising joining method for the aluminum PBW design and a testing campaign was conducted to evaluate the strength and efficacy of explosively formed bonds that were produced using two different interlayer materials: niobium and titanium. The characterization methods reported here include tensile testing, thermal-shock leak testing, optical microscopy, and advanced scanning electron microscopy. All tensile specimens examined failed in an aluminum interlayer and measured tensile strengths were all slightly greater than the native properties of the aluminum interlayer, while elongation values were all slightly lower. A leak developed in the test vessel with an niobium interlayer after repeated thermal-shock cycles, which was attributed to an extensive crack network that formed in an interfacial layer of a niobium-rich constituent phase located on the bond interfaces of the niobium interlayer; the test vessel with a titanium interlayer did not develop a leak under the conditions tested. Due to the experience gained from these characterizations, an explosively formed bond with a titanium interlayer was selected for the aluminum PBW design at the SNS.