Prediction And Control Of Laser Welding Distortion Of Thin Sheet For Nuclear Reactor Coolant Pump Can

The canned motor was used in reactor coolant pump of the third-generation nuclear power plants (AP1000). The manufacturing quality of canned motor cans is an important basis for safety and reliable operation of nuclear main pump. The cans were made of Hastelloy C-276alloy thin sheet by cutting and welding. Because the stator can has a wall-thickness of0.3to0.5mm, a length of more than3000mm, a diameter of565±0.076mm and a causative life expectancy up to60years, the requirements of its manufacturing process accuracy are extremely high. In this paper, a pulsed laser welding technique was employed based on the design requirements of forming manufacturing with high precision and long life of cans. The distributions of stress-strain and distortion of the pulsed laser welding (PLW) in Hastelloy C-276thin sheets were analyzed by numerical simulation method. The calculation method of the equivalent thermal load and the control tactics of the welding stress and distortion were proposed. The relationship between control parameters and dimension accuracy of can was revealed. These works will provide theoretic foundation and engineering guidance for the independent manufacturing of nuclear reactor coolant pump can. The main research contents and conclusions are as follows:(1) A three-dimensional finite element model (FEM) of the thermal-mechanical process during the PLW in Hastelloy C-276thin sheets was developed. The changes of material thermal physical and mechanical properties at elevated temperatures and the intermittent loading mode of pulsed laser energy were considered in the FEM. The distribution characteristics of temperature field, transient and residual stress-strain field and distortion field of the PLW were analyzed by the FEM. The welding experiments were carried out, and the relevant test equipments were used to measure the temperature histories and residual distortions. The simulation results agree well with the corresponding experimental measurements, which verifies the accuracy of the simulated results.(2) The mechanism of transverse shrinkage distortion was analysed by the evolution of transverse plastic strain. The residual compressive plastic strain was the direct reason of transverse shrinkage distortion produced. The representation method of transverse shrinkage distortion was proposed by considering the transverse deflection. The effect of workpiece dimensions on the transverse shrinkage distortion was obtained. Based on the distribution characteristics of longitudinal residual stress, the calculation method of equivalent thermal load was proposed by the functional relationship considering fixture restraint of longitudinal residual stress distribution along cross section. The structural stability after welding of stator can with an ultra thin wall-thickness and a large length to diameter ratio was achieved by elastic buckling analysis.(3) The effect of the restraint distance on transient and residual stress-strain field and distortion field was analysed by using displacement restriction assumption instead of fixture rigid fixed restraint. With the decrease of restraint distance, the peak values of the longitudinal residual compressive stresses and transverse residual tensile stresses decrease, while the peak value of the longitudinal residual tensile stresses increase. The welding longitudinal deflection, transverse deflection and transverse shrinkage distortion decrease as the restraint distance decreasing, especially the transverse shrinkage distortion decreases significantly. A relatively smaller restraint distance can be used to control the perimeter dimension accuracy of can.(4) According to the lack of the control effect of fixture rigid fixed restraint on the longitudinal residual tensile stresses, a control method of the longitudinal residual stresses and buckling distortion was proposed by using dynamic thermal tension based on a certain restraint condition. The effect of the trailing heat sink parameters on welding temperature, transient and residual stress, and residual distortion was investigated. The longitudinal residual stresses and deflections decrease significantly as the cooling position approaching heat source or the cooling intensity increasing, while the transverse shrinkage distortion increases. The optimized control parameters can be used to provide production guidance for welding manufacture of nuclear reactor coolant pump can with high dimension accuracy and low residual stress.