Zhang Qingdong

CONSTITUTIVE MODELING FOR ELEVATED TEMPERATURE FLOW BEHAVIOR OF INCOLOY 800H SUPERALLOY

In order to realize accurate numerical simulation of rolling, forging, extrusion of Incoloy 800H superalloy and formulate reasonable process parameters of hot working, the flow stress behaviors of Incoloy 800H superalloy were investigated in the temperature range of 1273—1473 K and strain rate range of 0.01—10 s �1 on Gleeble-1500 thermo-simulation machine. Utilizing hyperbolic sine function and introducing the strain factor with six polynomial fitting, the constitutive equations of flow stress of Incoloy 800H superalloy at high temperature were established, then the accuracy of the constitutive equation was verified. The results showed that during the hot compression deformation of Incoloy 800H superalloy, the characteristics of dynamic softening and dynamic recovery were observed at lower strain rates and high strain rates conditions, respectively, flow stress increased with increasing strain rate, decreased with increasing temperature; the flow stress of Incoloy 800H superalloy predicted by the proposed models with 6 th order polynomial fit agrees with the experimental value well, relative error is not more than 6.5% at the overwhelming majority (95%) cases of the deformation conditions, and average relative error is only 3.15%.

PREDICTED CONSTITUTIVE MODELING OF HOT DEFORMATION FOR AZ31 MAGNESIUM ALLOY

The uniaxial compression tests of AZ31 magnesium alloy at different strain rates of 0.01—10 s~(-1) and different deformation temperatures of 523—723 K were performed by using Gleeble- 1500 simulator with a maximum strain of 0.916.The influences of deformation temperature and strain rate on the flow stress were investigated.The fine microstructure is attributed to dynamic recrystallization during compression process at 523 K.The stretched grains of dynamic recrystallization and growth up along radial direction were founded in microscopic observation at 723 K.Considering plasticity deformation and friction induced temperature rise,the flow stress was corrected at high strain rate by using temperature compensation.The peak flow stress and unified constitutive model were established based on hyperbolic sine model.Strain sensitivity of flow stress was studied to describe the coupling of materials parameters on the strain,and then the relationship between deformation temperature,strain rate and strain during hot deformation was obtained.Comparing with experimental results,the correlation coefficient and average relative error of predicted and measured values are 0.976 and 5.07%respectively,it is proved that the model reflects the real deformation feature of the AZ31 magnesium alloy.

Model for the Generation of Surface Topography in Steel Strip Temper Rolling

The surface topography plays an important role in surface quality of cold-rolled steel strip.How surface topography changed in temper rolling is investigated.Based on Ritz method,a new numerical solution of steel strip plastic deformation during temper rolling is developed.Combining it with elastic deformation of rolls,the integration model of rolls and strip is built.Then the elastic-plastic contact between work roll and steel strip is simplified to asperity contact between one rough surface and one ideal smooth surface.The micro models of the generation of steel strip surface topography with pressing-in,furrowing and squeezing are built.The models considered the uplift effect of edges and neighbor role of asperities.Combining two kinds of models mentioned above,a model for the generation of surface topography is established considering asperity contact,roll force,relative sliding displacement between work roll and steel strip.The generation of surface topography of high strength steel strip and the influence of process parameters during temper rolling are studied.The results show that the pressing-in,furrowing and squeezing of work roll asperities on steel strip all influenced the strip surface morphology.The surface roughness of steel strip in the outlet with the changes in tension stress,reduction,roll roughness,roughness of steel strip at the entrance are obtained when pressing-in,furrowing and squeezing are dominant respectively.The following experiments validated the calculation results.

Robust Control of Strip Thickness for Cold Rolling Mill

In this paper, we derive a mathematical model for the final stand of the tandem cold rolling mill and design control systems to improve the thickness precision for the strip. On the basis of a linearized model which discussed the uncertainty of stand module, strip stiffness and the time delay of gauge measurement of the rolling mill system, we propose a thickness controller using robust control which effectively rejects the uncertainties and disturbances. The comparison of suboptimal H∞ control and μ Control for strip thickness control is given. It is illustrated by simulations that the robust μ thickness controller yields better performance than H∞ methods when the system uncertainties and a periodic thickness disturbance exist.