On Heat Transfer Model For Friction Stir Welding Of Aerospace Aluminum Alloy Component And Its Application

Friction stir welding,also known as ’the groundbreaking joining technology second only to laser welding’,has been widely used in the field of various equipment manufacturing in recent years.During the welding process,the temperature of workpiece remains below the melting point of the material.This effectively avoids these welding defects that may occur in conventional fusion welding,such as weld heat corrosion cracks,large stress deformation cracks and coarse deformation of weld grains.The welding process involves complex physical processes,chemical reactions,and metallurgical effects,accompanied by complex changes in heat,which greatly affect the quality and mechanical properties of the weld.Therefore,the accurate construction of the heat transfer model is of great significance for ensuring welding quality and optimizing welding parameters.Due to the mobility and local concentration of the welding heat source,the welding materials closure and the welding environment complexity in friction stir welding process,the accurate construction of the workpiece heat transfer model during welding is challenging.This paper aims to make a profound study on the heat transfer model for friction stir welding of aerospace aluminum alloy components based on theoretical derivation,numerical simulation and experimental verification,and apply it to the entropy production analysis of the welding process,so as to ensure welding quality and optimize welding parameters.The specific research work is as follows.Firstly,based on fractal geometry theory and rotational viscometry principle,the interaction between the welding tool and workpiece during the friction stir welding process under actual welding conditions is studied,and the frictional heat source model and viscoplastic heat source model are constructed respectively.Considering the fractal characteristics of the contact surface between the welding tool and workpiece,the kinematics and contact mechanics of a pair of microparticles on the contact surface are studied based on fractal geometry theory,and a fractal calculation model for the frictional heat source in the welding process is derived.Based on this,the viscoplastic heat source model of the workpiece material around the welding tool is established by using the rotational viscometry principle,taking into account the closure and complex flow properties of the welding material around the welding tool.The correctness of the two heat source models is verified using the finite element method with thermal coupling.Secondly,regarding the two welding heat sources as internal heat sources,the nonlinear heat conduction equations of the thermoplastic deformation zone and thermal affected zone of the friction stir welding process are established.Combining with the conformal mapping and the Liouville’s formula,the equations are solved.According to the characteristics of heat generation and dissipation in welding,the boundary conditions of the thermoplastic deformation zone and thermal affected zone are derived.By using the conformal mapping,the nonlinear heat conduction equations of the thermoplastic deformation zone and thermal affected zone are simplified.Based on the derived boundary conditions and Liouville’s formula,the heat transfer model of friction stir welding with undetermined parameters is established.Thirdly,for the undetermined parameters in the heat transfer model of the friction stir welding process,the related vector machine regression theory is used for prediction research.The numerical simulation method is used to obtain the heat transfer model parameters under different welding conditions,construct the required data set for model training,select a suitable kernel function,and optimize it.Based on the constructed data set,the related vector machine regression model is trained,and the training results are verified to obtain the regression prediction model of the heat transfer model parameters of the friction stir welding process.Fourthly,based on the constructed heat transfer model of friction stir welding,an entropy production analysis model of the friction stir welding process is conducted combining the local thermodynamic equilibrium hypothesis and phenomenological theory.Based on the local thermodynamic equilibrium hypothesis,the entropy flow and entropy production of each sub-process in the friction stir welding process are systematically studied,and the entropy balance equation of the welding system is obtained.Applying the constructed heat transfer model of friction stir welding,the entropy balance equation is simplified with phenomenological theory,and the entropy production analysis model of the welding process is derived.Finally,experimental research on the heat source model,heat transfer model,and entropy production analysis model of friction stir welding is conducted.A friction stir welding experimental system is set up,and groups of friction stir welding experiments are designed with 2219 aluminum alloy plate as the experimental object.The temperature and quality of the weld seam obtained under different process parameters are compared to verify the effectiveness and accuracy of the proposed heat source model,heat transfer model,and entropy production analysis model of friction stir welding.