Research On Thermal-mechanical Coupling During Friction Stir Welding

Friction Stir Welding (FSW) is a new and advanced welding technology, which is the most noticeable and has the most development potential after the presence of Laser Welding. This is a solid state joining process, which is currently being developed for difficult to-weld using conventional fusion welding technique, like high strength Al alloys, Cu , Mg and Ti alloys and so on. Also, its good welding performance has been paid a great amount of attention by researchers all over the world.This complicated process of Friction Stir Welding involves in such four interactive factors as temperature variation, microstructure transformation, stress-strain and metal flow. So far, to proceed basic research in numerical simulation of the temperature field, flow field , stress-strain field and their variation and interrelationship has become one of the important aspect in FSW researching and developing .Through the study in temperature measuring of 10mm LF2 Al alloy weld joint, analyze the joint temperature distribution and its influencing factor, and analyze the macrostructure, microstructure and microhardness. The results show that, different location in joint have different temperature distribution because the metal at different distance from welding line and different depth experiences different thermal and mechanical effect. Also, the temperature distribution and thermal cycle are important factor for macrostructure, microstructure and microhardness at advancing side , retreating side and various zone.Tensile testing is proceeded for by LF2 Al alloy under the condition of different deformation temperature and deformation rate by using universal material testing machine, to determine its true stress and strain, and analyze the effect of deformation temperature and deformation rate on flow stress. The results show that, the true-stress increase with reducing of deformation temperature and increasing of deformation rate; During the whole tensile, it is an inter-competitive process between work-hardening and softening (between recovery and recrystallization).With the finite element analysis software ANSYS, it is verified that the thermal model having been established is correct, and 3-D numerical stimulation and finite element analysis for thermal-mechanical coupled field of FSW are done through indirect coupled analysis method. The results show that, the variation tend of equivalent stress- strain and temperature is identical at different position of welding joint; more approach from heat source, higher thetemperature is, the increasing rate of temperature is quicker, and the equivalent stress-strain is higher, by contrast, more distant from heat source, lower the temperature is, the increasing rate of temperature is more slowly, and the equivalent stress-strain is lower; Meanwhile, the equivalent stress-strain will occur the same increasing and decreasing with variation of temperature, and when the heat source leave, temperature , stress and strain also decrease greatly.