Local mechanical properties of thin sheet AA6061 aluminum alloy friction stir welds evaluated by small punch test (SPT)

In Friction Stir Welding (FSW) of AA6061-T6 aluminum alloy, abnormal grain growth (AGG) could take place during solution post weld heat treatment (PWHT), mainly at the weld nugget (WN). It has no effect on microhardness and tensile strength, but it could produce a deleterious effect on the toughness of that zone. There is a lack of information about this issue. Local mechanical properties (including strength, ductility and toughness) can be evaluated using Small Punch Tests (SPT), which could be especially useful for the characterization of welded joints. The objective of this work is to apply a novel approach using SPT to study the local mechanical properties of AA6061 thin sheet friction stir welds, analyzing the effect of the AGG on the mechanical properties of WN. There were obtained local values of yield and tensile strength as well as elongation to fracture and toughness, for the different zones of the weld, estimated by SPT. A good correlation between the estimated strength parameters and microhardness was found, presenting comparable profiles across the weld. Joint efficiency obtained from transversal tensile tests were similar to the one estimated by SPT. After PWHT, the hardness and tensile strength of the “as welded” softened area were recovered, but the toughness and ductility of the WN were reduced due to the (AGG) produced. The novel approach applied has shown to be effective to estimate the deleterious effect of AGG in WN toughness of FSW of AA6061-T6 thin sheets.