Finite Element Analysis Of Electronic Current Aided Friction Stir Welding Process

This paper mainly simulates the temperature distribution, the stress and strain aswell as the welding residual stress, the flow of the materials, defects and the wear life ofthe friction tool. Then, the effects of the electricity on the welding behaviors areanalyzed. The results are pivotal to understand the mechanics of EFSW and can providea theoretical guidance.To overcome the difficulties during the application of FSW (friction stir welding),a new method of EFSW is proposed in the current paper. EFSW can compensatevarious defects caused by the insufficient sole heat resource in the traditional FSW. Acombined heat-source model is built. That comprises the friction heat in the interface ofthe shoulder and the plate, the friction heat in the pin and the plate, the heat of plasticwork and the contact resistance heat. In material model, main material properties aretemperature dependent, such as thermal conductivity, thermal expansion coefficient,friction factor and electrical conductivity. Finally, a multiphysics coupled FEM modelof EFSW is built, and the following conclusions are derived from the analysis ofsimulation results①With the introduction of a moving heat source model proposed by M. Song andR. Kovacevic, the local grid refinement technology and moving mesh, a EFSWsimulation model is established. The critical axial pressure is determined using thereverse method and batch parametric scan strategy. When the axial pressure is lowerthan the critical axial pressure, due to the insufficient quantity of heat input the frictionstir welded joints are ineffective. Combining with auxiliary current, the sound frictionstir welded joints can be achieved.②Speed ratio changes affects the development and changes of the temperaturefield of the oints through heat production in shoulder and pin as well as convectioncooling rate. When the rotational speed ratio is1400/300r/mm, the welding speed mustbe300mm/min in order to achieve the enough heat input by friction stir welding; Butfor EFSW, when the welding speed is400mm/min (speed ratio1400/400r/mm), theenough heat input can be achieve for a good connection. Therefore, the EFSW canimprove efficiency by a wide margin.③The welding speed is more important than the current in determine the residualstress, but the current indeed reduce and improve the distribution of residual stress. The horizontal distribution of strain shows bimodal and asymmetric, and the strain peakappeared both in the advancing side and retreating side. But the peak value of theadvancing side is larger than the retreating side of peak strain. Equivalent straindecreased when the current is added and the bimodal of the strain is less sharp thanconventional FSW.④A FSW dynamics semi-analytical model about material flow is established onthe base of the rigid-viscoplastic mechanics equation, material constitutive relationshipand finite element variational theory. Simulation results show that: the materials followthe tool moving on the advancing side substantially presenting a semicircular path ofmovement. The materials near the surface move from the advancing side to the rear ofthe pin. Ultimately, it will be deposited in the retreating side. The introduction of thecurrent can within a certain range enhance the flow velocity of the material, but havelittle effect on its flow path, the friction conditions between the tool and workpiece andthe speed ratio play a decisive role in material path and speed.⑤The rigid viscoplastic three-dimensional model is used to predict unweldedwelds, flash, holes, tunnels, grooves and other defects of conventional FSW withoutcurrent, and the result is compared with the experimental welding defects to prove thecorrectness of the model. With the introduction of the current, the joint defects havebeen significantly improved and fine surface is achieved. So, compared with the FSWprocess, the EFSW can largely reduce the welding defects and generate higher qualitywelds.⑥Seven types of friction tool are designed in this article. Froction tool wearmodel is built based on improved Archard theory. According to the the wear results, thestirring tool with concentric circular concave shaft shoulder has the minimum wear. Theflat shoulder tool has largest amount of wear, the decrease of wear up to a maximum of83.4%. Using6#and7#tool with concave shoulder and the three slope tapered threadspin, we can get the no-defect joints;1#and5#mixing tool can not form weld in theprocess of numerical simulation and produce plenty of welding defects such as grooves,voids. Adding the current to the FSW process, amount of wear of the pin has beengreatly reduced. Despite the shoulder wear increasing, the current on a great the extentreduce the tool wear and greatly extend the life of the friction tool.