Thermal fatigue response of W-EUROFER brazed joints by the application of High Heat Flux loads

The thermal fatigue effect on the microstructure and mechanical properties of the joints that form some com-ponents of the future fusion reactor is a concern within the scientific community. In this study, we analyze the metallurgical modifications caused by thermal fatigue and their impact on the mechanical properties of tungsten-EUROFER brazed joints (blocks measuring 6 x 6 x 4 mm). We conduct the analysis using an actively cooled mock-up subjected to steady-state thermal loads, which provides valuable information about the operating conditions of the reactor. Three different surface conditions of tungsten were evaluated: 600 degrees C (2 MW/m2), 700 degrees C (2.5 MW/m2), and 800 degrees C (3 MW/m2), with varying numbers of applied cycles ranging from 100 to 1000. Throughout the tests, infrared cameras and pyrometers were used to analyze the thermal behavior of the W-EUROFER joint. At 600 degrees C and 700 degrees C target temperatures, no anomalies in the heating and cooling capacity of the W-EUROFER joint were observed. This represents an advancement compared to previous studies that employed Cu20Ti filler, as it demonstrates consistent and efficient cooling capabilities even at surface temper-atures of up to 700 degrees C, without any notable anomalies starting from the previous filler's 500 degrees C. However, in the case of 800 degrees C, the test had to be prematurely stopped. Microstructural analysis revealed the formation of cracks in some cases due to the stresses generated by the mismatch in the coefficient of thermal expansion between the materials used. These cracks affected the mechanical integrity of the joint.