Hot Deformation Mechanism and Ring Rolling Behavior of Powder Metallurgy Ti_2AlNb Intermetallics

Powder metallurgy（PM） Ti–22Al–24Nb–0.5Mo（at.%） alloys were prepared by hot isostatic pressing. In order to study the feasibility of PM ＋ ring rolling combined process for preparing Ti_2AlNb rings, thermal mechanical simulation tests of PM Ti_2AlNb alloys were conducted and two rectangular PM rings（150 mm in height, 75 mm in thickness,350 mm in external diameter） were rolled as a validation experiment. Experimental results show that the flow stress of Ti_2AlNb alloys exhibited a significant drop at the very beginning of the deformation（true strain／0.1）, and became stable with the increase in strain. Stress instability phenomenon of PM Ti_2AlNb alloys was more obvious than that of wrought alloy. Flow stress fluctuation at the initial stage of deformation is related to phase transition of Ti_2AlNb alloys which strongly depends on heat treatment and thermal mechanical deformation process. Processing windows during initial stage of ring rolling process is very crucial. A sound PM Ti_2AlNb rectangular ring blank（height = 150 mm, thickness = 30 mm, external diameter = 750 mm） was successfully rolled in two passes by using the improved heat preservation method and optimized rolling parameters. Tensile properties of PM Ti_2AlNb alloy were improved, and the porosity was reduced after ring rolling.

Analysis on Shear Deformation for High Manganese Austenite Steel during Hot Asymmetrical Rolling Process Using Finite Element Method

Based on the rigid-plastic finite element method（FEM）, the shear stress field of deformation region for high manganese austenite steel during hot asymmetrical rolling process was analyzed. The influences of rolling parameters, such as the velocity ratio of upper to lower rolls, the initial temperature of workpiece and the reduction rate, on the shear deformation of three nodes in the upper, center and lower layers were discussed. As the rolling parameters change, distinct shear deformation appears in the upper and lower layers, but the shear deformation in the center layer appears only when the velocity ratio is more than 1.00, and the absolute value of the shear stress in this layer is changed with rolling parameters. A mathematical model which reflected the change of the maximal absolute shear stress for the center layer was established, by which the maximal absolute shear stress for the center layer can be easily calculated and the appropriate rolling technology can be designed.

Modeling deformation resistance for hot rolling based on generalized additive model

A model of deformation resistance during hot strip rolling was established based on generalized additive model.Firstly,a data modeling method based on generalized additive model was given.It included the selection of dependent variable and independent variables of the model,the link function of dependent variable and smoothing functional form of each independent variable,estimating process of the link function and smooth functions,and the last model modification.Then,the practical modeling test was carried out based on a large amount of hot rolling process data.An integrated variable was proposed to reflect the effects of different chemical compositions such as carbon,silicon,manganese,nickel,chromium,niobium,etc.The integrated chemical composition,strain,strain rate and rolling temperature were selected as independent variables and the cubic spline as the smooth function for them.The modeling process of deformation resistance was realized by SAS software,and the influence curves of the independent variables on deformation resistance were obtained by local scoring algorithm.Some interesting phenomena were found,for example,there is a critical value of strain rate,and the deformation resistance increases before this value and then decreases.The results confirm that the new model has higher prediction accuracy than traditional ones and is suitable for carbon steel,microalloyed steel,alloyed steel and other steel grades.

Hot Ductility of Severe Plastic Deformed AA6061 Aluminum Alloy

The hot ductility of 6061 aluminum alloy,which was subjected to two different severe plastic deformations（SPD）,was studied at different temperatures and strain rates.The tensile tests were carried out at the temperature range of 300-500 ℃ and at the strain rates of 0.0005-0.01 s^（-1）.The microstructure evolution was characterized using optical microscopy,transmission electron microscopy and X-ray diffraction technique.The influences of the microstructure after SPD,thermomechanical parameters（temperature and strain rate） and specimen size on the hot formability of this alloy were then analyzed.The results show that a decrease in grains/subgrains exhibited significant effect on the hot ductility of SPDed samples.The constitutive equations were then developed to model the hot formability of the studied alloy.The developed model can be represented by Zener-Hollomon parameter in a hyperbolic sinusoidal equation form.Both the changes of elongation to failure and Zener-Hollomon parameter indicate that the hot ductility of the alloy is more sensitive to the temperature rather than to the strain rate.The uniform elongation is independent of the specimen size,but the postnecking elongation increases dramatically as the ratio of l/A^（1/2） decreases.