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Friction stir welding (FSW) is a promising welding method for welding dissimilar materials without using welding flux. In the present work, 5A06-H112 and 6061-T651 aluminium alloys were successfully welded by friction stir welding with forced air cooling (FAC) and natural cooling (NC). Nanoindentation tests and microstructure characterisations revealed that forced air cooling, which can accelerate the cooling process and suppress the coarsening of grains and the dissolution of precipitate phases, contributes to strengthening and narrowing the weakest area of the joint. The tensile strength of joints with FAC were commonly improved by 10% compared to those with NC. Scanning electron microscopy (SEM) images of the fracture surface elucidated that FSW with FAC tended to increase the number and reduce the size of the dimples. These results demonstrated the advantages of FSW with FAC in welding heat-sensitive materials and provide fresh insight into welding industries.

a b s t r a c tBased on an improved shear-lag model, the effect of an inhomogeneous interphase on the mechanism of stress transfer in fiber-reinforced composites is investigated. The inhomogeneity of the interphase is represented by the graded feature of the Young's modulus varying according to a power law or a linear one in the radius direction, while the Poisson's ratio and thermal expansion coefficient are assumed to be constants. Considering the effects of the inhomogeneous interphase as well as the Poisson's contraction and thermal residual stress, closed-form solutions to the axial fiber stress and interfacial shear stress are obtained analytically. Comparing the case with a power law to that with a linear one, we find that the fiber stress increases significantly in the former case, while it decreases slightly in the latter one with an increasing interphase thickness. With the same external tensile load and interphase thickness, it is found that the fiber in the power law case is subjected to a larger tensile stress than that in the linear variation one. However, the interfacial shear stress is not sensitive to the interphase thickness in both cases, except that near the two ends of fiber. Under the same external load, the maximum shear stress in the interphase is much smaller in the latter than that in the former. All the phenomena can be characterized by one parameter, i.e., the average Young's modulus of interphase, and denote that an interphase with a power variation law is more effective for stress transfer while the linearly graded one is more advantageous to avoid shear failure. The results should be helpful for engineers to properly design the interphase in novel composites, e.g. a carbon-fiber reinforced epoxy one.

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