A novel semisolid rheo-rolling process of A2017 alloy was achieved by combining the shape rolling mill with the vibrating sloping plate device. The microstructure evolution and solidification behaviors during the proc...A novel semisolid rheo-rolling process of A2017 alloy was achieved by combining the shape rolling mill with the vibrating sloping plate device. The microstructure evolution and solidification behaviors during the process were investigated. The high cooling rate caused by the sloping plate and stirring action caused by the vibration and metal flow lead to a high nucleation rate as well as two primary grain growth patterns, direct globular growth as well as dendrite growth and subsequent breakage, which causes the formation of fine spherical or rosette primary grains. During the rolling process, the grains of the strip were elongated. The primary grain size of A2017 alloy strip increases with the increment of casting temperature. When the casting temperature was between 650 °C and 660 °C, A2017 alloy strip with good quality was produced by the proposed process. The microstructures of the strip are mainly composed of spherical or rosette grains.展开更多
Cooling slope casting has been applied to aluminium casting alloys for producing ingots with non-dendritic microstructure.Semi-solid forming of the AA7075 was studied via cooling slope casting,reheating and thixoforgi...Cooling slope casting has been applied to aluminium casting alloys for producing ingots with non-dendritic microstructure.Semi-solid forming of the AA7075 was studied via cooling slope casting,reheating and thixoforging processes in order to determine the effect of semi-solid casting on the microstructure of the alloy.AA7075 ingots with non-dendritic microstructure were produced with cooling slope pouring.Castings were characterized by light microscopy,image analysis,scanning electron microscopy and EDS analysis.The resulting structures are promising in terms of grain size and sphericity.It was realized that grain coarsening may occur very suddenly in the reheating process.Cooling slope castings were obtained with 30°and 60°inclination angles,and in comparison,60°castings showed better results.Moreover,by using short reheating periods,semi-solid forging causes trace formation in the solid grains.展开更多
The indirect boundary element method （IBEM） is used to study three-dimensional scattering of obliquely incident plane SH waves by an alluvial valley embedded in a layered half-space. The free-field response of the l...The indirect boundary element method （IBEM） is used to study three-dimensional scattering of obliquely incident plane SH waves by an alluvial valley embedded in a layered half-space. The free-field response of the layered half-space is calculated by the direct stiffness method, and dynamic Green＇s functions of moving distributed loads acting on inclined lines in a layered half-space are calculated to simulate the scattering wave field. The presented method yields very accurate results since the three-dimensional dynamic stiffness matrix is exact and the moving distributed loads can act directly on the valley boundary without singularity. Numerical results and analyses are performed for amplification of obliquely incident plane SH waves around an alluvial valley in a uniform half-space and in single layer over half-space. The results show that the three-dimensional responses are distinctly different from the two-dimensional responses, and the displacement amplitudes around alluvial valleys in a uniform haft-space are obviously different from those in a layered half-space.展开更多
In this paper, we study the kinematic mechanism and path planning for a two-caster nonholonomic vehicle (the Essboard) which is a recent variant of skateboard. Different from the most studied Snakeboard, the Essboard ...In this paper, we study the kinematic mechanism and path planning for a two-caster nonholonomic vehicle (the Essboard) which is a recent variant of skateboard. Different from the most studied Snakeboard, the Essboard consists of a torsion bar and two platforms, each of which contains a pedal and a caster. We first investigate the relationship between the tilt angles of the pedals and the wheel directions of the casters. This relationship reveals how to control the wheel directions by adjusting the tilt angles. Next, the rotational radius of the Essboard is derived for a given pair of tilt angles of both pedals. The rotational radius of the Essboard is much different than that of the Snakeboard. Then we develop a path-planning algorithm for the Essboard to move from a start position to the goal, using a series of consecutively connected arcs, which are tangent to each other at the connected points. It is shown from a kinematic point of view that the path planning of the Essboard can be solved by a series of pairs of pedals' tilt angles. Three experiments are conducted to confirm the correctness of the main results. The results in this paper are a foundation for further study of the Essboard.展开更多
基金Project (51222405) supported by the National Science Foundation of Outstanding Young Scholars of ChinaProject (50974038) supported by the National Natural Science Foundation of China+1 种基金Project (132002) supported by the Fok Ying Tong Education Foundation, ChinaProject (2011CB610405) supported by the National Basic Research Program of China
文摘A novel semisolid rheo-rolling process of A2017 alloy was achieved by combining the shape rolling mill with the vibrating sloping plate device. The microstructure evolution and solidification behaviors during the process were investigated. The high cooling rate caused by the sloping plate and stirring action caused by the vibration and metal flow lead to a high nucleation rate as well as two primary grain growth patterns, direct globular growth as well as dendrite growth and subsequent breakage, which causes the formation of fine spherical or rosette primary grains. During the rolling process, the grains of the strip were elongated. The primary grain size of A2017 alloy strip increases with the increment of casting temperature. When the casting temperature was between 650 °C and 660 °C, A2017 alloy strip with good quality was produced by the proposed process. The microstructures of the strip are mainly composed of spherical or rosette grains.
文摘Cooling slope casting has been applied to aluminium casting alloys for producing ingots with non-dendritic microstructure.Semi-solid forming of the AA7075 was studied via cooling slope casting,reheating and thixoforging processes in order to determine the effect of semi-solid casting on the microstructure of the alloy.AA7075 ingots with non-dendritic microstructure were produced with cooling slope pouring.Castings were characterized by light microscopy,image analysis,scanning electron microscopy and EDS analysis.The resulting structures are promising in terms of grain size and sphericity.It was realized that grain coarsening may occur very suddenly in the reheating process.Cooling slope castings were obtained with 30°and 60°inclination angles,and in comparison,60°castings showed better results.Moreover,by using short reheating periods,semi-solid forging causes trace formation in the solid grains.
基金sponsored by Project of Natural Science Foundation of China(50908156,50978183)the Application Basis and Front Technology Research Progamme of Tianjin Municipality under Grant 12JCZDJC29000
文摘The indirect boundary element method （IBEM） is used to study three-dimensional scattering of obliquely incident plane SH waves by an alluvial valley embedded in a layered half-space. The free-field response of the layered half-space is calculated by the direct stiffness method, and dynamic Green＇s functions of moving distributed loads acting on inclined lines in a layered half-space are calculated to simulate the scattering wave field. The presented method yields very accurate results since the three-dimensional dynamic stiffness matrix is exact and the moving distributed loads can act directly on the valley boundary without singularity. Numerical results and analyses are performed for amplification of obliquely incident plane SH waves around an alluvial valley in a uniform half-space and in single layer over half-space. The results show that the three-dimensional responses are distinctly different from the two-dimensional responses, and the displacement amplitudes around alluvial valleys in a uniform haft-space are obviously different from those in a layered half-space.
基金supported by the National Natural Science Foundation of China (Grant Nos. 51105012 and 61175079)
文摘In this paper, we study the kinematic mechanism and path planning for a two-caster nonholonomic vehicle (the Essboard) which is a recent variant of skateboard. Different from the most studied Snakeboard, the Essboard consists of a torsion bar and two platforms, each of which contains a pedal and a caster. We first investigate the relationship between the tilt angles of the pedals and the wheel directions of the casters. This relationship reveals how to control the wheel directions by adjusting the tilt angles. Next, the rotational radius of the Essboard is derived for a given pair of tilt angles of both pedals. The rotational radius of the Essboard is much different than that of the Snakeboard. Then we develop a path-planning algorithm for the Essboard to move from a start position to the goal, using a series of consecutively connected arcs, which are tangent to each other at the connected points. It is shown from a kinematic point of view that the path planning of the Essboard can be solved by a series of pairs of pedals' tilt angles. Three experiments are conducted to confirm the correctness of the main results. The results in this paper are a foundation for further study of the Essboard.
