Abstract:
Friction Stir Welding (FSW) is a solid state joining technique patented in 1991 by The Welding Institute (TWI). Currently, the process is successfully applied to join pieces, covering a wide variety of materials -Aluminum Alloys, Steels, etc.- in aeronautic, aerospace, naval, rail, and automotive industries. Due to its distinctive features and novelty -compared to other joining processes-, there is great interest in better understanding the FSW process in order to improve its capabilities and extend its application.
During the process, the material is plastically deformed at high temperatures and high strain rates by a non-consumable tool that rotates and moves along the joint line.
The welding occurs at a temperature below melting point of the material, without filler material. The thermomechanical cycle experienced by the material alters the microstructure, and thus mechanical properties, of the welded joint.
In the present work, the development of a computational model that enables analyze the efect of variables involved in the process on the material is addressed.
A thermomechanical coupled model is solved by the Finite Element Method (FEM).
A flow formulation based on the Eulerian description is used to deal with the large deformations of the material. A rigid-viscoplastic material model is considered for the welding pieces.
The model also includes the welding tool and a backing plate -an inherent feature of the process.
On one hand, the model involves some parameters whose values are not possible to establish in advance. A parameter estimation technique is implemented to find the value of these parameters. Furthermore, a sensitivity analysis is performed to determine the influence of each parameter on the results and assist the estimation procedure.
Moreover, the use of an Eulerian description involves considering an additional method to compute the deformation of the material. According to the kinematics of fluid flow, it is possible to get a measure of the deformation from computing the deformation gradient tensor -an essentially Lagrangian quantity-, by means of velocity field.
A numerical scheme, developed by the FEM, is implemented to solve the evolution equation of the deformation gradient tensor.
Finally, the validity of model results is limited by the availability of reliable experimental data. Therefore, temperature measurements from a test specimen welded by FSW, under known experimental conditions, are compared with temperature results of the model.
The results show that the model (i) includes a great many variables involved in the process #process variables, materials properties, etc.#, enables calculating the temperature, strain rate and deformation of the material, (iii) and is supported by experimental data.
