A Multi-scale Study Of Size Effect On Rheological Properties Of Metals During Bulk Forming

For bulk forming of the metal material, the different feature sizes or grain sizeswill bring a significant effect on its rheological behavior, but most of the existingstudies of plastic deformation process make simple geometric processing, and thuscannot provide accurate guidance for the production practice.In this paper, based on hot compression experiments, combining stress-straincurves, processing map and crystal plasticity theory, a multi-scale of macroscopic andmicroscopic explored the impact of the blank characteristics size and relative grain sizeon the rheological behavior. Firstly, by hot compression test of TC4titanium alloy,stress-strain curves were obtained under different conditions, as well as the effects ofthe deformation temperature, strain rate and other factors on the rheological propertiesand other basic data; next attempting from the macroscopic and microscopic aspects ofstress-strain curves, processing map, microstructure evolution, polycrystalline featuresize, grain size, deformation texture, friction, etc., to explore the size effects in the bulkforming. The main conclusions are as follows:①In the experimental range, the rheological properties of TC4are subject to theeffects of strain rate, deformation temperature and strain. Among them, the flow stresswith strain rate increases, decreases with increasing temperature. The nonlinearrelational model based on BP neural network has a high generalization performance,suggesting that the predictive value can effectively track of the entire deformationtemperature and strain rate range. By the processing map drawn based on experimentaldata of TC4judge, material processing instability mainly concentrated in thelow-temperature region. As the strain increases, instability region increases. When thestrain rate is large, dynamic recrystallization occurs locally. When the strain rate is low,the local grain group appears not nodulizing phase α, and the material is easy to processinstability. When the deformation temperature is high and strain rate is low,spheroidization of phase α ball is significant, and crystals appeared a number of smallequiaxed and lamellar structure with good mechanical properties.②For samples of different sizes, as the size decreases, the peak stress is increased,that is,"the smaller the stronger." Processable regions are also different.③The interpretation of "the smaller the stronger" based on the microstructureevolution is: the larger sample sizes are relatively more phase α transformation to phase α+β, with more slip systems, smaller deformation resistance, corresponding smallerstress.④The explanation of "the smaller the stronger" based on the crystal plasticitytheory is: for polycrystalline crystals of different feature size, smaller size possess fewernumber of grains. During deformation of the crystal, strain distribution is more uniform,resulting in when a small amount of overall deformation, the local areas of crystal havegreater strain and stress concentration.⑤For the same polycrystalline, as the texture intensity increases, both the yieldstress and peak stress are increased in the initial deformation. Polycrystalline havingdeformation texture or friction coefficient, also appears size effect phenomenon of "thesmaller the stronger", and with increasing intensity or texture coefficient of friction,stress curves are slow decline in the late deformation.