Natural aging in friction stir welded 7136-T76 aluminum alloy

• The study sheds new light on the natural aging of AA7136 friction stir welds.
• Natural aging notably influences the mechanical properties of friction stir welds.
• The hardness profile can be correlated to the temperature distribution model.
• The GP(I) zones formation is responsible for the increased mechanical properties.
• Natural aging cannot be neglected in practical applications of the FSW process.

The long term natural aging behavior of friction stir welded aluminum 7136-T76 extrusions was investigated. The microstructural characteristics and mechanical properties in the as-welded, three years naturally aged and six years naturally aged conditions were studied and correlated to a coupled thermal/material flow model of the joining process. Hardness profiles taken along the mid-plane thickness of the workpiece displayed the characteristic W-shape typical to friction stir welded aluminum alloys. In the as-welded condition, however, the profile was skewed to the advancing side, such that the advancing side hardness was lower than that on the retreating side. With natural aging, hardness recovery occurred on both sides of the weld, but the position of the hardness minima, particularly on the advancing side, shifted away from the weld centerline. The numerical simulation demonstrated that the temperature profile is also skewed to the advancing side with greater processing temperatures occurring on this side of the weld. When compared to the dissolution temperature of the equilibrium phases, the extent of dissolution was greater on the advancing side and occurred to a greater distance from the centerline than on the retreating side. The hardness behavior upon natural aging, therefore, correlated to the temperature profile developed during welding and the degree to which phase dissolution occurred in the regions adjacent to the stir zone.