Numerical Simulation Of Friction Stir Spot Welding And Dynamic Recrystallization Of The Aluminium Alloy

Friction stir spot welding (FSSW) is a new solid state joining technology, which isbased on the linear friction stir welding and it can be used to replace the conventionalresistance spot welding as well as riveting. FSSW has great advantages in the area oflight alloy joint, especially for aluminium. It has won the favor of customers andrecognition from the areas of aerospace, automobile, shipbuilding industries and so on.However, the theoretical research of FSSW is far behind the practical application atpresent. Some key problems such as heat transfer, thermoplastic material flow andmicrostructure evolution have not yet been studied sufficiently and block the applicationof this advanced technology.Several hot topics were studied in the paper. First, the effects of weldingparameters on the temperature filed and down force have been analysed. The temporaland spatial distribution of the temperature filed close to the joint was obtained by theexperimental measurement. The results show that the temperature increases with therotational speed until a certain speed achieved. The heating rate decreases when thatcertain rotational speed was reached. At meanwhile, the welding temperature increaseswith the down speed of stir-head. When reaching the predetermined depth, theprolongation of the stir-head’s dwell time has no contribution to the weldingtemperature. With the increase of rotational speed, the down force of the stir-headreduced gradually before the shoulder contact with the workpiece. And after theshoulder contact with the workpiece, the downforce increases greatly.Secondly, the typical regions in the joint such as stir zone, thermomechanicalaffected zone and heat affected zone were analysed including the distribution of themisorientation angle, the shape and size of the grain. The stir zone is characterized bythe fine and equiaxed grains, which have been undergone the geometrical dynamicrecrystallization. The grain size in stir zone reduced with the decrease of the rotationalspeed. The distribution of the misorientation angle in stir zone is similar for differentjoints. For example, the misorientation angle above15°and below5°have a bigproportion for all joints, which is almost45%and51%, respectively. Thethermomechanical affected zone is consisting of the grains, which have been undergone partial dynamic recrystallization and the grain size distributes in a wide range. The heataffected zone has a similar grain with the base metal. The grain size in this area is a littlelarger than that in base metal due to the welding thermal cycle.Thirdly, a heat source model and a thermomechanical coupling finite elementmodel have been established. The microstructure model, which is based on thedislocation density evolution, has also been coupled in that finite element model topredict the temperature, effective strain, effective strain rate, material flow andmicrostructure evolution in the joint. The basic idea for the microstructure evolution isthat the dislocation density decides the subgrain size, and then the subgrain sizedetermines the grain size once the geometrical dynamic recrystallization (GDRX) takesplace. The condition for the GDRX is that the effective strain in the material exceeds thecritical strain, which is a key parameter for GDRX. The comparisons of temperature andgrain size between the simulation and the experiment show the validity of the model.Finally, the physical simulation of the stir zone was performed on the Gleeble-3800and the imput data such as temperature, effective strain and strain rate were derivedfrom the numerical simulation. Hot torsion test were employed for the physicalsimulation, because the high strain and strain rate in material can not be achieved in thegeneral hot compression and tensile tests. The microstructure of the torsion specimenshows that the grain size is similar in different area and it is mainly determined by thedeformation temperature once the GDRX occured. The strain and strain rate has littleeffects on the grain size just like in FSSW. However, the distribution ratio of themisorientation angle above15°in torsion specimen is only half than that in stir zone ofFSSW.