2.3. Mechanical properties tests
The microhardness arrangement of the entire weld cross section was detected by a LECO-AMH43 Vickers hardness tester with a testing load of 300 gf and a dwell time of 13 s. The spacing between adjacent indentations was 0.5 mm. Small dog-bone shape tensile testing specimens were machined by wire cut electrical discharge machining and tested on AGS-X-50kN testing machine at a strain rate of 1.3×10-3 s-1. The FCG rate tests were performed on INSTRON-8801 testing machine at room temperature by using the compact tension (CT) specimens. All fatigue testing specimens were polished and tested with a load ratio of R=0.1. Sine wave loading with a frequency of 10 Hz was used. The crack length was measured continuously by the compliance method with crack mouth clip gauge (3541-010M-040M-ST). The crack length and cycle number were recorded at every increment of 0.05 mm in crack length. Fig. 1 shows the dimensions of CT specimens and the sampling position. Fig. 2 shows the macro morphology of FSW joint and the fractured CT specimens after FCG test, illustrating that the welded joint shows good quality with no obvious weld defects.