Fiber Laser Welding of Fuel Cladding and End Plug Made of La2O3 Dispersion-Strengthened Molybdenum Alloy

The study investigated the laser lap welding of fuel cladding and end plug made of molybdenum (Mo) alloy. The research results showed that the tensile strength of the welded joint when a weld was located at the Mo tube was significantly larger than that at the fit-up gap between the fuel cladding and end plug. Moreover, preheating can also greatly increase the tensile strength of the lap joint. The weld zone was filled with bulky coarse columnar crystal structures while there were numerous coarse recrystallized structures in the heat affected zone (HAZ). The weld zone and HAZ were both subjected to a significant softening. The tensile strength and elongation rate of fuel cladding made of Mo alloy were about 750 MPa and 36.7%, respectively. The welded joint did not undergo any plastic deformation during the tensile process and presented a brittle fracture. Under the optimum processing conditions, the tensile strength of the welded joint reached 617 MPa, taking up 82.3% that of the base metal. The results of composition analysis indicated that there was only Mo inside the columnar crystals in weld zone while significant oxygen segregation was observed at the grain boundary. This was the main reason causing that the strength of welded joint was lower than that of the base metal. Additionally, under the optimum processing conditions, there were numerous slender columnar crystals on the cross section of the joint entering the weld zone in fuel cladding side from that in end plug side where the crystals were nucleated and grew upwards. The analysis results suggested that the presence of these slender columnar crystals crossing the interface between fuel cladding and end plug was favorable for improving the capacity of the joint for bearing the shear loads.