Modelling of plastic flow localisation and damage development in friction stir welded 6005A aluminium alloy using physics based strain hardening law

Abstract

a b s t r a c tPlastic flow localisation and ductile failure during tensile testing of friction stir welded aluminium specimens are investigated with a specific focus on modelling the local, finite strain, hardening response. In the experimental part, friction stir welds in a 6005A-T6 aluminium alloy were prepared and analysed using digital image correlation (DIC) during tensile testing as well as scanning electron microscopy (SEM) on polished samples and on fracture surfaces. The locations of the various regions of the weld were determined based on hardness measurements, while the flow behaviour of these zones was extracted from micro-tensile specimens cut parallel to the welding direction. The measured material properties and weld topology were introduced into a 3D finite element model, fully coupled with the damage model. A Voce law hardening model involving a constant stage IV is used within an enhanced Gurson type micromechanical damage model, accounting for void nucleation, growth and coalescence, as well as void shape evolution. The stage IV hardening, observed in Simar et al. (2010), was found to increase the stiffness during plastic flow localisation as well as to postpone the onset of fracture as determined by the void coalescence criterion. Furthermore, the presence of a second population of voids was concluded to strongly affect the fracture strain of the high strength regions of the welds. This modelling effort links the microstructure and process parameters to macroscopic parameters relevant to the optimisation of the welds.

Void shape effects and voids starting from cracked inclusion

“…see Nielsen et al, 2010) that the shape evolution from the initial penny-shaped crack is described as that of an oblate void in a homogeneous matrix material. In reality the mode of deformation is very different, more like that at fibre breakage in a metal matrix composite (Tvergaard, 1993(Tvergaard, , 2004, where the fibre or the hard particle deforms very little, while the deformations are highly concentrated in the matrix material around the initial crack-tip.…”

A mesomechanical analysis of the stress–strain localisation in friction stir welds of polycrystalline aluminium alloys

“…The experimental evidence and calculations [25] show that a neck is formed for the most part on the retreating side of the weld near its interface with the base material. We examine the extent to which the deformation and fracture are affected by the FSW-produced microstructure on the retreating side.…”

“…The overwhelming majority of the theoretical works is concerned with simulations of the welding process and velocity and temperature field distributions around the rotated FSW tool [17][18][19][20][21], including the microstructural evolution during the process [22,23]. Investigations dealing with modelling the mechanical loading of metal pieces butted together by the FSW technique are few in number and have been performed for the most part with the use of essentially idealized macroscopic models [18,[24][25][26][27], even though they were based on a multiscale methodology [28]. The welded joint zone as a rule is idealized and represents a regularly shaped region with plane interfaces, whereas the material inside the weld region is homogeneous.…”

Mesomechanical response of microstructure formed on the advancing side of friction stir welded aluminum

“…In essence, this is a new composite material characterized by far more pronounced structural inhomogeneity and by the anisotropy of the physical-mechanical properties as opposed to the base aluminum. In numerical models, an idealized welded joint zone, as a rule, is a regularly shaped plane region characterized by rectilinear interfaces, with the material inside the weld region being considered to be homogeneous [2,3]. The aim of this work is to investigate the effect of the microstructural inhomogeneity observed in the thermo-mechanically affected zone (TMAZ) on the advancing side of the FS weld on the mechanical properties of the material.…”