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
A mathematic model of rolling pressure during a novel semisolid shearing-rolling process was established. The rolling pressure in this process is higher than that in the conventional rolling. The increment of rolling pressure in the backward slip zone is higher than that in the forward slip zone, and the neutral plane moves toward to the roll gap entrance. The maximum and the average rolling pressures increase with the decrease of strip thickness, and the effects of strip thickness on the rolling pressure is more obvious in the forward slip zone than in the backward slip zone. Meanwhile, the neutral plane moves toward the roll gap exit with the decrease of strip thickness. The maximum and average rolling pressures increase with the decrease of strip width, and the strip width affects the pressure more obviously in the backward slip zone than in the forward slip zone. At the same time, the neutral plane moves toward the roll gap entrance with the decrease of strip width. The maximum and average rolling pressures increase with increasing roll radius, and the neutral plane moves toward the roll gap exit.
Ren-guo GuanZhan-yong ZhaoChao LianQiu-sheng ZhangChun-ming Liu
Heat transfer of flow melt and grain refining mechanism during melt treatment by the cooling sloping plate were investigated.The results show that the cooling sloping plate can refine not only grains of alloys but also can obviously refine pure metal.Cooling ability of the plate is the key factor that induces grain refining,the plate material and the flow amount can affect cooling rate of the melt and thus affect refining effectiveness.The cooling rate of the melt on the cooling sloping plate is much faster than that of the conventional casting process,which can reach 1000 K/s and belongs to meta-rapid solidification scope.The thickness of the temperature boundary layer is much larger than that of the velocity boundary layer on the sloping plate,but the temperature gradient is small in the temperature boundary layer.Under strong cooling action by the cooling plate,most parts of the melt on the plate surface can form undercooling,which causes continuous eruptive nucleation,this is the main grain refining mechanism,and the heterogeneous nucleation on the plate surface is a helpful supplement for the nucleation.
Renguo GUANZhanyong ZHAORunze CHAOHongqian HUANGChunming LIU
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.
Brittle pro-eutectoid cementite that forms along prior-austenite in hypereutectoid steels is deleterious to mechanical properties. The optimum process parameters which suppress the formation of pro-eutectoid cementite in hypereutectoid steels with carbon content in the range of 0.8%-1.3% in mass fraction, were investigated. Pro-eutectoid cementite formation is effectively hindered by increasing the deformation temperature and decreasing the amount of strain. Transformation at lower temperatures close to the nose of the cooling-transformation diagram also reduces the tendency of the formation of pro-eutectoid cementite. Control of prior-austenite grain size and grain boundary conditions is important. Due to larger number of nucleation sites, finer prior-austenite grain size results in the acceleration of transformation to pro-eutectoid cementite. However, large prior-austenite and straight boundaries lead to less nucleation sites of pro-eutectoid cementite. The cooling rate and carbon content should be reduced as much as possible. The transformation temperature below 660 °C and the strain of 0.5 at deformation temperature of 850 °C are suggested.
