Identification of Inhomogeneous Flow Stress of Friction Stir Welded Aluminum Alloy Sheets Using Finite Element-Based Virtual Fields Method

Abstract

Inhomogeneous plastic flow stresses are identified for the friction stir welded aluminum alloy 6061-T6 sheets using two different inverse methods; the finite element-based virtual fields method (FE-VFM) and the non-uniform stress method (nUSM). The FE-VFM is an enhanced virtual fields method developed based on the formulations in a common finite element scheme. In the present FE-VFM, the following novel numerical schemes are further adopted: (1) a two-step identification process, (2) partitioning of the whole domain of interest with an interpolation of the constitutive parameter within each subdomain, and (3) the normal distribution type virtual fields. For comparison, an alternative inverse identification procedure based on the nUSM is also considered. Before applying the developed method to a real experiment, the FE-VFM is verified via numerical feasibility tests using ideal deformation data of the finite element simulations conducted with the artificially generated target welding materials. The feasibility tests show that the proposed FE-VFM can calculate the inhomogeneous flow stresses of the welded aluminum sheets with reliable accuracy. In the experimental applications, plastic properties predicted by the FE-VFM and nUSM are compared in terms of the distributions of yield stress, uniform elongation, and ultimate tensile tests. Also, validation of the identified inhomogeneous flow stresses is conducted by predicting the two tensile tests loaded parallel and perpendicular to the weld line direction. The validation reveals that both FE-VFM and nUSM can be used for the evaluation of the welded material properties within reasonable accuracy. However, the predictions based on the proposed FE-VFM approach give better agreement with the experiments owing to its ability to capture the mechanical properties in the narrow thermo-mechanically affected zone (TMAZ).

Keywords: Virtual fields method, friction stir welding, aluminum alloy, inverse method, mechanical properties