Characterisation and modification of residual stress in brazed dissimilar material joints for a future fusion divertor

In a future nuclear fusion reactor, the critical divertor component will require dissimilar material joining of an armour and a structural material. Tungsten (armour) and copper or steel alloy (structural) have been identified as suitable candidates. A review of existing divertor designs and material combinations highlighted a requirement for further development of tungsten based dissimilar material joints and the associated residual stresses. An improvement in the residual stress state at the interface is seen as a way to improve the performance of the divertor under cyclic loading. A method of beneficially modifying the residual stress state of vacuum brazed, fusion relevant, tungsten dissimilar material joints is presented in this thesis. Tungsten-copper and tungsten-316L stainless steel parts were successfully brazed using a eutectic gold-copper brazing alloy. A total of 30 dissimilar material brazed joints were successfully manufactured and analysed. The brazed parts have been characterised using a range of metallurgical and residual stress measurement techniques. The residual stress state has been predicted using finite element analysis. Temperature dependent thermo-mechanical material properties were experimentally determined for the gold-copper alloy to facilitate accurate finite element modelling. Good agreement between predicted and X-ray diffraction and Contour Method measured residual stresses for brazed tungsten-316L parts was obtained. The need for further research into the complex interfacial region, and translation of this region into a predictive model, was identified to improve correlation of predicted and measured residual stresses in brazed tungsten-copper parts. Damaging tensile residual stresses in the brittle tungsten material were beneficially reduced in a brazed tungsten-316L part using thermal autofrettage. Reduction of high brazing induced tensile stresses was achieved both analytically and experimentally. This improvement in the residual stress distribution has promising applications in beneficially modifying real divertor dissimilar material tiles.In a future nuclear fusion reactor, the critical divertor component will require dissimilar material joining of an armour and a structural material. Tungsten (armour) and copper or steel alloy (structural) have been identified as suitable candidates. A review of existing divertor designs and material combinations highlighted a requirement for further development of tungsten based dissimilar material joints and the associated residual stresses. An improvement in the residual stress state at the interface is seen as a way to improve the performance of the divertor under cyclic loading. A method of beneficially modifying the residual stress state of vacuum brazed, fusion relevant, tungsten dissimilar material joints is presented in this thesis. Tungsten-copper and tungsten-316L stainless steel parts were successfully brazed using a eutectic gold-copper brazing alloy. A total of 30 dissimilar material brazed joints were successfully manufactured and analysed. The brazed parts have been characterised using a range of metallurgical and residual stress measurement techniques. The residual stress state has been predicted using finite element analysis. Temperature dependent thermo-mechanical material properties were experimentally determined for the gold-copper alloy to facilitate accurate finite element modelling. Good agreement between predicted and X-ray diffraction and Contour Method measured residual stresses for brazed tungsten-316L parts was obtained. The need for further research into the complex interfacial region, and translation of this region into a predictive model, was identified to improve correlation of predicted and measured residual stresses in brazed tungsten-copper parts. Damaging tensile residual stresses in the brittle tungsten material were beneficially reduced in a brazed tungsten-316L part using thermal autofrettage. Reduction of high brazing induced tensile stresses was achieved both analytically and experimentally. This improvement in the residual stress distribution has promising applications in beneficially modifying real divertor dissimilar material tiles