A traditional semisolid processing based on the rheological properties of semisolid alloy is very difficult in manufacturing the semisolid slurry because of the chase for spherical,fine and high solid fraction of the primary solid in the slurry.Through many experiments,it is found that it is not very necessary to excessively emphasize the glomeration and high solid fraction considering of the facts that the rheological properties of the semisolid slurry is controlled by not only the microstructure of the primary solid but also the rheological conditions.The manufacturing slurry can be simply regarded as preparing the semisolid alloy melt,therefore,a modified semisolid processing technology is presented.Its procedure consists of three basic steps,such as preparing slurry,filling mould cavity and solidification with pressure.The basic aim of the preparing slurry is to obtain the slurry containing 5%-20%(volume fraction) primary solid with particle or stick instead of spherical morphology.The experiment results show that,with the modified rheological processing technology,many steel and nonferrous alloy products are industrially produced.
To evaluate the flow stress of the twin-roll casting magnesium sheet in high temperature forming processes,a new analytical model was proposed by analyzing stress-strain curves measured under various temperatures and strain rates.The model can be parameterized to reflect the volume fraction of semi-sold grain and sold grains undergoing twinning.By applying the new model,the result showed that the simulated stress-strain curves fit the experiment data very well.Such curve fitting can be employed for any magnesium sheet when the deformation conditions are known.Microstructure evolution indicated that it affected the stress-strain curves directly.The mechanisms of dynamic recrystallization(DRX) depended on the operating deformation mechanisms,which changed with temperatures.And the volume fraction and grain size of DRX changed with the Zener-Hollomon parameter and temperatures.
Zhimin Liu,Shuming Xing ,Peiwei Bao,Nan Li,Shuqing Yao and Milan Zhang School of Mechanical,Electronic and Control Engineering,Beijing Jiaotong University,Beijing 100044,China
High temperature tensile properties and microstructure evolutions of twin-roll-cast AZ31B magnesium alloy were investigated over a strain rate range from 10-3 to 1 s-1.It is suggested that the dominant deformation mechanism in the lower strain rate regimes is dislocation creep controlled by grain boundary diffusion at lower temperature and by lattice diffusion at higher temperatures,respectively.Furthermore,dislocation glide and twinning are dominant deformation mechanisms at higher strain-rate.The processing map,the effective diffusion coefficient and activation energy map of the alloy were established.The relations of microstructure evolutions to the transition temperature of dominant diffusion process,the activation energy platform and the occurrence of the full dynamic recrystallization with the maximum peak efficiency were analyzed.It is revealed that the optimum conditions for thermo-mechanical processing of the alloy are at a temperature range from 553 to 593 K,and a strain rate range from 7×10-3 to 2×10-3 s-1.
