Temperature analysis has been performed in friction stir welding (FSW) of ADC12 and A5052 dissimilar aluminum alloys. The temperature at the stir zone and four points on each side of the joint line was measured during FSW in various conditions. In addition, an analytical model assumed that the work generated by the rotation of the tool led to the work for stirring materials and heat generation of the material and the tool. The temperature of the stir zone (SZ) where A5052 were FSWed was about 30 K higher than that of ADC12. The temperature of SZ of the advancing side (AS) : A5052/the retreating side (RS) : ADC12 was about 30 K higher than that of AS : ADC12/RS : A5052. The flow stress of the materials in FSW was calculated based on the temperature obtained in the experiment. The analytical model could predict appropriately that the motor power consisted of the heat generated by friction and the work generated by the rotation of the tool.

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High strain-rate compressive stress–strain characteristics of AA7075-T651 and its welds as produced by the friction stir welding (or FSW) process are investigated using the conventional split Hopkinson pressure bar. Friction stir welded AA7075-T651 joints are made under a fixed set of welding conditions. Cylindrical specimens machined along the thickness direction of both the base material and the friction stir weld nugget region are used in the static and impact compression tests. The micro-hardness tests are conducted across the centerline of the friction stir welded AA7075-T651 joint in order to examine the microstructural change. It is shown that the compressive flow stress of the weld nugget is reduced to about 70 to 85% of that of the base material, and that both the base material and the weld nugget exhibit almost no strain rate effect up to nearly ε˙=10³/s.

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Fullerene was successfully dispersed into various metals such as AZ31-O, A5083-O, and C1020-O using Friction Stir Processing (FSP) . The dispersion of the fullerene promoted the grain refinement of the metal matrix by FSP. Especially, the grain size of the AZ31 amazingly decreased by the uniform distribution of the fullerene molecules. The stir zone of the FSPed AZ31 with the fullerene consisted of ultra-fine grains which should be less than 100 nm. The grain size of A5083 and C1020 was also refined to ~200 nm. The maximum microhardness of the fullerene/AZ31 nanocomposite reached ~126 HV, which is almost three times higher than that of the AZ31 substrate. Additionally, the fine grain and high hardness of fullerene/AZ31 nanocomposite are maintained at the elevated temperature. It is considered that fullerene is the ultimate material for the metal matrix as the additive to refine the grains and enhance the mechanical properties due to its extremely small size and excellent physical properties. Our results show that the FSP with the fullerene is very attractive for the metal matrix to decrease the grain size and increase the mechanical properties.

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Lap joining of a 6022 aluminum alloy (Al) sheet and a low carbon steel, SPCC (Fe) sheet was performed using friction stir welding method (FSW) . The rotating tool with a probe was inserted into Al sheet which was overlapped on Fe sheet and traveled along its side edge. Two different probe tip positions were selected for welding: The tip was located at either 0.1 mm above (−0.1 mm) or 0.1 mm beneath (+0.1 mm) in reference to the Al/Fe interface. The probe tip was kept in Al matrix for the former case and it was inserted into Fe matrix for the other. In the present study, effects of probe tip position on interfacial microstructure, shear and peel strength were investigated and the relationship between fracture behavior and the interfacial microstructure was discussed. Lap joining was successively achieved both in “−0.1 mm” and “+0.1 mm” conditions. Both shear strength and peel strength were larger when the probe tip was inserted into Fe matrix. In the tensile test for the lap joint, rapid crack propagation took place along the joint interface in the former case and resulted in the reduced joint strength. On the other hand, for the latter case, in spite of a part of joint interface fractured in a brittle matter, the successive crack growth took place in Al matrix and the final fracture occurred in the parent material (Al) in a ductile manner. Grain refinement was achieved in the region close to the joint interface both in Al and Fe. Characteristic laminate structure was also observed in a part of the refined Fe grain structure.

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Lap joining was carried out using friction stir welding (FSW) for 6022 aluminum alloy and low carbon steel sheets. Precise microstructural observation by a scanning and a transmission electron microscope were made in order to investigate the interfacial microstructure. Hardness measurement using nano-indenter was also performed to characterize the microstructural change at the weld interface. Characteristic laminate structure was formed in steel matrix in the vicinity of aluminum alloy/steel interface. TEM observation and an electron diffraction pattern analysis revealed that the laminate structure consists of periodical overlapping of two layers. One layer was a film formed by fine Fe grains, and the other was a film formed by the mixture of fine Al and Fe grains. Intermetallic compound particles (Al₁₃Fe₄, Al₅Fe₂) were also observed in the latter layer. Formation of an amorphous phase was also observed at the aluminum alloy/steel interface. It is considered that the laminate structure was formed by a repetitive mechanical alloying of Al and Fe grains below the rotating probe tip, which was slightly inserted into the steel sheet from the overlapped aluminum alloy sheet.

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The influence of nonlinear friction stir welding (FSW) tool control on joints' mechanical properties was investigated. Although FSW is widely applied to linear joints, it is impossible for five-axis FSW machines to maintain all FSW parameters in optimum conditions during nonlinear welding. Nonlinear FSW joints should be produced according to an order of priority for FSW parameters. Tensile test results of butt joints with rectangular change in the welding direction on the plate plane (L-shape butt joints) change with various welding parameters. Results show that a turn to the retreating side is encouraged when the welding direction changes. The method of zero inclination tool angles is effective for nonlinear and plane welding.

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Wrong:See PDF for details.
Right:See PDF for details.

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