Study on relieving residual stress of friction stir welded joint of 2219 aluminum alloy using cold spraying

Abstract

Friction stir welding is an advanced solid state welding technology, which has been widely used in aerospace and other fields. However, the large tensile residual stress developed in such joints will significantly affect mechanical properties. Cold spraying, as a new method to reduce residual stress of welded joints, can also improve the mechanical properties of the joints. In this paper, the correlation between microstructure and mechanical properties and residual stress of 2219 aluminum alloy joints prior and after coated with cold spraying was studied with experiments and numerical simulation for a 4 mm thick joint. The primary mechanism of stress relief of the joint was identified. Results show the refinement of grain size, with grain size decreasing from 2.1 μm in welded joints to 0.7 μm of cold sprayed joints, while high angle grain boundaries increase from 38.0% in the welded joint to 62.7% in cold sprayed ones. The ratio of recrystallized grains in the joints increases from 19.1% to 46.9%, while the joint strengthening phases θ″ and θ' increase, which increases tensile strength of the joints from 343 MPa to 398 MPa. The elongation is increased from 4.5% to 10.2%, and the average hardness of the joint is increased by 35 HV. The “ shot peening effect “ of cold spraying is found to be the primary factor to reduce residual stresses, while the “ heat flow effect “ has negligible effect on stress reduction. The mechanism of reducing residual stresses is provided by the macroscopic stress-strain theory and the microscopic dislocation theory.