A novel semisolid rheo-rolling process of AZ91 alloy was proposed.The microstructure formation mechanism of AZ91 magnesium alloy during the process was studied.The results reveal that the eruptive nucleation and the heterogeneous nucleation exist.During the grain growth process,the grain breakage took place and transformed into fine spherical or rosette grains on the sloping plate gradually,the other grain growth style is direct globular growth.Due to the secondary crystallization of the remnant liquids in the roll gap,the microstructure of the strip becomes finer with the increment of the casting temperature from 650℃ to 690℃.But when the casting temperature reached 710℃,a part of the liquid alloy transformed into the eutectic phases,and the primary grains ripened to form coarse dendrites.In the casting temperature range from 650℃ to 690℃,AZ91 alloy strip with fine spherical or rosette grains was prepared by the proposed process.
A novel continuous semisolid rolling process for producing AZ31 alloy strip was developed. The process parameters were optimized, and microstructure and properties of AZ31 alloy prepared by the process were studied. The results reveal that primary grains of the strip become coarse, and the grain structure transforms from round shape to dendrite with the increment of casting temperature gradually. Eutectic phase fraction and primary grain size increase with the increment of roll speed. The primary grain size decreases firstly and then increases with the increment of the vibration frequency correspondingly. When the casting temperature is from 650℃ to 690℃, the roll speed is 0.069 m·s-1 , and the vibration frequency is about 80 Hz, AZ31 alloy strip with a cross section size of 4 mm×160 mm was prepared by the proposed process. The ultimate tensile strength and elongation are improved 1% and 57%, respectively.
Renguo GUANZhanyong ZHAOXiang WANGChunguang DAIChunming LIU
The mathematical model of flow shear constitutive relation during rheo-rolling process has been established. The distribution of velocity and shear stress in rolling cavity was investigated, and the effects of process parameters on shear stress of Sn-15 Pb alloy during rheo-rolling process were studied. In rolling cavity, the nearer the roll is, the bigger the velocity and shear stress are. The shear stress increases with the increment of the roll speed and the roll radius during rheo-rolling process, but deceases with the increment of the thickness of the strip. When the solid fraction of Sn-15 Pb alloy increases from 0.3 to 0.5, the shear stress increases slowly, but when the solid fraction increases from 0.5 to 0.6, the shear stress rapidly.
A novel short process for producing A2017 alloy strips with notable features of near net shape,saving energy,low cost,and high product performance was developed by combining semisolid rolling,deep rolling,and heat treatment.The microstructure and properties of the A2017 alloy strips were investigated by metallographic microscopy,scanning electron microscopy,transmission electron microscopy,X-ray diffraction,tensile testing,and hardness measurement.The cross-sectional microstructure of the A2017 alloy strips is mainly composed of near-spherical primary grains.Many eutectic phases CuAl 2 formed along primary grain boundaries during semisolid rolling are crushed and broken into small particles.After solution treatment at 495 C for 2 h the eutectic phases at grain boundaries have almost dissolved into the matrix.When the solution treatment time exceeds 2 h,grain coarsening happens.More and more grain interior phases precipitate with the aging time prolonging to 8 h.The precipitated particles are very small and distribute homogenously,and the tensile strength reaches its peak value.When the aging time is prolonged to 12 h,there is no obvious variation in the amount of precipitated phases,but the size and spacing of precipitated phases increase.The tensile strength of the A2017 alloy strips produced by the present method can reach 362.78 MPa,which is higher than that of the strips in the national standard of China.
Ren-guo GuanXiang WangZhan-yong ZhaoWei-wei WangFu-rong CaoChun-ming Liu
According to the principle of grain refining and slurry preparation by cooling sloping plate process, the distributions of boundary layers during melt treatment by cooling sloping plate were studied, and mathematic model of cooling rate was established. The calculation value approximately agrees with the experimental result. Laminar flow and turbulent flow exist on sloping plate surface commonly. The thickness of velocity boundary layer and the critical transfer distance from laminar flow to turbulent flow increase with the decrease of initial flow velocity. The thickness of temperature boundary layer increases with the increment of flow distance and the decrease of initial flow velocity. The melt cooling rate and melt thickness have an inverse proportion relationship. The melt cooling rate increases along the plate direction gradually when the initial flow velocity is lower than 1 m/s, the melt cooling rate keeps nearly a constant when the initial flow velocity is 1 m/s, when the initial flow velocity is higher than 1 m/s, the melt cooling rate decreases gradually. The melt cooling rate of cooling sloping plate process can reach 102-103 K/s and belongs to meta-rapid solidification scope.
