Analysis Of The Welding Deformation Mechanism For Small-Section Square Tubes

A large thin-walled weld structure is adopted in the manufacturing of rocket nozzle extension. The welding deformation has important effects on the rocket engine. Thus, it is meaningful to study the deformation laws and control methodology of thin-walled structure for improving the quality of products.In this thesis, the temperature field of small-section square tubes was analyzed and measured firstly, disposing the pipe bottom heat border with contact convection method, considering the changes of material physical parameters with the temperature, convection and radiation heat transfer and latent fusion heat, the temperature-deformation direct coupling numerical model was built. It is shown that the simulation results are in good accordance with the actual results and proved the model is correct.According to the complex conditions of multi-pipe multi-pass welding process, the finite element model has a huge mesh number, slow computing speed, and long cycle. By analyzing the actual process and adopting the active-deactivate element method, the mesh element was activated step by step according to the welding procedure. Meanwhile, the LAN-based distributed computing platform was constructed, using three computers for parallel computing, to improve the calculation speed and saving a lot of computing time.By the mathematical analysis and numerical simulation of small-section tube welding process, the results show that the pipe has a deflection by the effect of vertical thermo stress; the pipe has a bend and a torsion by the effect of transverse stress; the excursion of heat source from the weld centre resulted in unfitness of butt joint ;the weld gap in front of heat source changed dynamically with the move of the heat source. In multi-pipe multi-pass welding, the transverse deformation after weld in symmetry welding was decreased compared with that in ordinal welding, and the weld quality is better. By the above results, adopting subsection welding method and the welding sequence with symmetry structure, the welding distortion can be decreased effectively, and the welding quality can be enhanced too.