Quantitative investigation of precipitation and mechanical behaviour for AA2024 friction stir welds

During friction stir welding, the temperature history and the deformation rate introduced through the joint vary. For precipitation hardening aluminium alloys, these parameters strongly influence the precipitation distribution, which controls the weld properties. This paper presents a quantitative investigation of the effect of welding on precipitation in the 2024 alloy in the T351 and the T6 states, using small angle X-ray scattering, transmission electron microscopy, differential scanning calorimetry and hardness measurements. Depending on the peak temperature, the main precipitation phenomena are Guinier Preston Bagaryatskii zone dissolution, precipitation of S′(S), which can be either a hardening agent at small sizes or detrimental to the mechanical properties at large sizes. The quantification of the microstructure has allowed modelling of the yield stress evolution through the weld according to an appropriate mixing law for the contributions of the different obstacles. The effect of a T6 post-welding heat treatment has also been studied on a T351 weld. This treatment actually results in a further softening of the thermo-mechanically affected zone. In parallel, the susceptibility to strain localisation during tensile testing of the weld has been evaluated by the digital image correlation technique.