| Friction stir welding(FSW) technology is widely used in welding aluminum alloy, magnesium alloy and copper alloy materials because of its lower residual stress and deformation, high strength of welded joints. It has a broad prospect in the fields of aerospace, transportation, shipbuilding, automobile and other industrial manufacturing fields. But friction stir welding technology is still limited by the lifetime of the stir tool in welding materials with high melting point. The tool pin undergoes serious shear and frictional wear in friction stir welding process. Preheating the workpiece can soften the material to be welded. It is one of the effective ways to improve the life of the tool. In this paper, numerical simulation and experimental study on friction stir welding of Q235 steel with laser-assisted heating are conducted.Firstly, a heat source model for laser-assisted friction stir welding including such kinds of heat sources as laser heat, friction heat and shear deformation heat is established. In this model, such factors as the preheating of laser beam to the workpiece material, the friction heat between the shoulder of the stir tool and the workpiece, and the shear deformation heat between the tool pin and the workpiece are considered. During laser-assisted friction stir welding process, laser beam is treated as a Gaussian surface heat source in the above model, which plays a supporting role.Secondly, on the basis of the above heat source model, a heat and mass transfer model is set up. Then, CFD software FLUENT is used to solve the above heat and mass transfer model, and the flow field and temperature distribution are obtained. The influence of such processing parameters as laser power, welding speed and rotational speed of the stir tool on the flow field and temperature field is studied by using the software FLUENT. The influence of the presence or absence of laser heat source on welding temperature field and flow field is also analyzed. Simulation analysis can obtain the distributions of such fields and forces as temperature, stress, strain, velocity, flow pressure and shear forces.Finally, based on the above results of simulation analysis, the experiment of laser-assisted heating friction stir welding of Q235 steel is carried out. The stir tool is made of W-Mo alloy, the welding experiment is conducted using a changed milling machine. The influence of download force, welding speed and rotational speed on the welds appearance is investigated. The microstructure of heat affected zone, thermal-mechanical affected zone and nugget zone is analyzed through the metallographic analysis. Such mechanical properties of welding joints as the microhardness and tensile strength are tested. |
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