By employing a quasi in situ method, we investigated the dynamic evolution of the grain structure con-sidering the material flow, strain, and strain rate in the friction stir welding of pure copper. The tool' stop...By employing a quasi in situ method, we investigated the dynamic evolution of the grain structure con-sidering the material flow, strain, and strain rate in the friction stir welding of pure copper. The tool' stop action' and rapid cooling were employed and a brass foil was used as a marker to show the material flow path. The grain structure along the material flow path was characterised using electron backscatter diffraction. Static recrystallization occurs for the work-hardened base material in the preheating stage in front of the tool In the acceleration flow stage, grains are significantly refined by plastic deforma-tion, discontinuous dynamic recrystallization, annealing twinning during the strain-induced boundary migration and slight continuous dynamic recrystallization. In the deceleration flow stage, due to a strain reversal, the grain first coarsens, and is thereafter refined again. Finally, the hot-deformed material in the shoulder-affected zone is ‘frozen’ directly whereas that in the probe-affected zone undergoes signif-icant annealing;thus, the recrystallized microstructure and 45°-rotated cube texture are obtained in the probe-affected zone.展开更多
Friction stir spot welding (FSSW) was applied to 2.0 mm thick steel plate cold-rolled commercial (SPCC) low carbon steel plates at a very low rotation speed that ranged from 5 to 50rpm, which was much lower than that ...Friction stir spot welding (FSSW) was applied to 2.0 mm thick steel plate cold-rolled commercial (SPCC) low carbon steel plates at a very low rotation speed that ranged from 5 to 50rpm, which was much lower than that generally used for the conventional FSSW technique. Due to the very low heat input, the welding processes could therefore be completed at a peak welding temperature below 160℃. As a result, a significantly refined microstructure with an average grain size of about 0.41μm was formed in the stir zone of the joints and the JI {0-11 }<-211> and J2{1-1O}<-1-12> shear textures were the dominant components, which are different from the D1{11-2}<111> and D2{-1-12}<111> shear textures formed in the conventional FSSW joints. In addition, no heat affected zone could be detected along the crosssectional plane of the joints. Although a few void-like non-bonded areas were still observed along the interface between the upper and lower steel plates, the shear tensile loads of the joints increased to about 10.0 kN when welded at a condition of 8t, 20rpm and 30 s, and the joints fractured through the plug failure mode.展开更多
The tensile rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys were examined. The experiments show that the rupture locations of the joints are different for the two aluminum...The tensile rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys were examined. The experiments show that the rupture locations of the joints are different for the two aluminum alloys, which are influenced by the welding parameters. When the joints are free of welding defects, the AA2017-T351 joints are ruptured in the weld nugget adjacent to the thermo-mechanically affected zone on the advancing side and the rupture surfaces appear as oval contours of the weld nugget, while the AA6061-T6 joints are ruptured in the heat affected zone on the retreating side and the rupture surfaces are inclined at a certain degree to the bottom surfaces of the joints. When welding defects are present in the joints, the AA2017-T351 joints are ruptured in the weld center, while the AA6061-T6 joints are ruptured on the retreating side near the weld center. The rupture locations of the joints are dependent on the internal structures of the joints and can be explained through them.展开更多
Microscopic traffic simulations are useful for solving various trafficrelated problems,e.g.traffic jams and accidents,local and global environmental and energy problems,maintaining mobility in aging societies,and evac...Microscopic traffic simulations are useful for solving various trafficrelated problems,e.g.traffic jams and accidents,local and global environmental and energy problems,maintaining mobility in aging societies,and evacuation planning for natural as well as man-made disasters.The origin-destination(OD)matrix is often used as the input to represent traffic demands into traffic simulators.In this study,we propose an indirect method for estimating the OD matrix using a traffic simulator as an internal model.The proposed method is designed to output results that are consistent with the input of the simulator.The method consists of the following steps:(1)calculating link traffic volume from the OD matrix,and(2)updating the matrix.The estimated matrix is updated iteratively until it converges to a predefined tolerance level.Numerical experiments are then conducted using the proposed method on a grid network and on a representation of an actual road network.Finally,we discuss the characteristics of the proposed method and the non-negative constraint for the traffic volume.展开更多
Weld shape variation for different welding parameters is investigated on pure iron plate under gas tungsten arc （GTA） welding with argon-oxygen mixed shielding. Results showed that small addition of oxygen to the ar...Weld shape variation for different welding parameters is investigated on pure iron plate under gas tungsten arc （GTA） welding with argon-oxygen mixed shielding. Results showed that small addition of oxygen to the argon base shielding gas can effectively adjust the oxygen adsorption to the molten pool. An inward Marangoni convection occurs on the pool surface when the oxygen content in the weld pool is over the critical value, 80×10^-6, for pure iron plate under Ar-0.3%O2 mixed shielding. Low oxygen content in the weld pool changes the inward Marangoni to an outward direction under the Ar-0.1%O2 shielding. The GTA weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is determined by the content of surface active element, oxygen, in the weld pool and the welding parameters. The strength of the Marangoni convection on the liquid pool is a product of the temperature coefficient of the surface tension （dσ/dT） and the temperature gradient （dT/dr） on the pool surface. Different welding parameters will change the temperature distribution and gradient on the pool surface, and therefore, affect the strength of Marangoni convection and the weld shape.展开更多
基金partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the “Innovation Structural Materials Project (Future Pioneering Projects)”a Grant-in-Aid for Science Research from the Japan Society for the Promotion of Science
文摘By employing a quasi in situ method, we investigated the dynamic evolution of the grain structure con-sidering the material flow, strain, and strain rate in the friction stir welding of pure copper. The tool' stop action' and rapid cooling were employed and a brass foil was used as a marker to show the material flow path. The grain structure along the material flow path was characterised using electron backscatter diffraction. Static recrystallization occurs for the work-hardened base material in the preheating stage in front of the tool In the acceleration flow stage, grains are significantly refined by plastic deforma-tion, discontinuous dynamic recrystallization, annealing twinning during the strain-induced boundary migration and slight continuous dynamic recrystallization. In the deceleration flow stage, due to a strain reversal, the grain first coarsens, and is thereafter refined again. Finally, the hot-deformed material in the shoulder-affected zone is ‘frozen’ directly whereas that in the probe-affected zone undergoes signif-icant annealing;thus, the recrystallized microstructure and 45°-rotated cube texture are obtained in the probe-affected zone.
基金financially supported by the New Energy and Industrial Technology Development Organization (NEDO) under the “Innovation Structural Materials Project (Future Pioneering Projects)”a Grant-in-Aid for Science Research from the Japan Society for Promotion of Science and Technology of Japan, ISIJ Research Promotion Grant
文摘Friction stir spot welding (FSSW) was applied to 2.0 mm thick steel plate cold-rolled commercial (SPCC) low carbon steel plates at a very low rotation speed that ranged from 5 to 50rpm, which was much lower than that generally used for the conventional FSSW technique. Due to the very low heat input, the welding processes could therefore be completed at a peak welding temperature below 160℃. As a result, a significantly refined microstructure with an average grain size of about 0.41μm was formed in the stir zone of the joints and the JI {0-11 }<-211> and J2{1-1O}<-1-12> shear textures were the dominant components, which are different from the D1{11-2}<111> and D2{-1-12}<111> shear textures formed in the conventional FSSW joints. In addition, no heat affected zone could be detected along the crosssectional plane of the joints. Although a few void-like non-bonded areas were still observed along the interface between the upper and lower steel plates, the shear tensile loads of the joints increased to about 10.0 kN when welded at a condition of 8t, 20rpm and 30 s, and the joints fractured through the plug failure mode.
文摘The tensile rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys were examined. The experiments show that the rupture locations of the joints are different for the two aluminum alloys, which are influenced by the welding parameters. When the joints are free of welding defects, the AA2017-T351 joints are ruptured in the weld nugget adjacent to the thermo-mechanically affected zone on the advancing side and the rupture surfaces appear as oval contours of the weld nugget, while the AA6061-T6 joints are ruptured in the heat affected zone on the retreating side and the rupture surfaces are inclined at a certain degree to the bottom surfaces of the joints. When welding defects are present in the joints, the AA2017-T351 joints are ruptured in the weld center, while the AA6061-T6 joints are ruptured on the retreating side near the weld center. The rupture locations of the joints are dependent on the internal structures of the joints and can be explained through them.
文摘Microscopic traffic simulations are useful for solving various trafficrelated problems,e.g.traffic jams and accidents,local and global environmental and energy problems,maintaining mobility in aging societies,and evacuation planning for natural as well as man-made disasters.The origin-destination(OD)matrix is often used as the input to represent traffic demands into traffic simulators.In this study,we propose an indirect method for estimating the OD matrix using a traffic simulator as an internal model.The proposed method is designed to output results that are consistent with the input of the simulator.The method consists of the following steps:(1)calculating link traffic volume from the OD matrix,and(2)updating the matrix.The estimated matrix is updated iteratively until it converges to a predefined tolerance level.Numerical experiments are then conducted using the proposed method on a grid network and on a representation of an actual road network.Finally,we discuss the characteristics of the proposed method and the non-negative constraint for the traffic volume.
文摘Weld shape variation for different welding parameters is investigated on pure iron plate under gas tungsten arc （GTA） welding with argon-oxygen mixed shielding. Results showed that small addition of oxygen to the argon base shielding gas can effectively adjust the oxygen adsorption to the molten pool. An inward Marangoni convection occurs on the pool surface when the oxygen content in the weld pool is over the critical value, 80×10^-6, for pure iron plate under Ar-0.3%O2 mixed shielding. Low oxygen content in the weld pool changes the inward Marangoni to an outward direction under the Ar-0.1%O2 shielding. The GTA weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is determined by the content of surface active element, oxygen, in the weld pool and the welding parameters. The strength of the Marangoni convection on the liquid pool is a product of the temperature coefficient of the surface tension （dσ/dT） and the temperature gradient （dT/dr） on the pool surface. Different welding parameters will change the temperature distribution and gradient on the pool surface, and therefore, affect the strength of Marangoni convection and the weld shape.
