BEHAVIOR OF FLOW STRESS OF ALUMINUM SHEETS USED FOR PRESSURE CAN DURING COMPRESSION AT ELEVATED TEMPERATURE

The behavior of flow stress of Al sheets used for pressure can prepared by different melt-treatment during plastic deformation at elevated temperature was studied by isothermal compression test using Gleeble1500 dynamic hot-simulation testing machine. The results show that the AI sheets possess the remarkable characteristic of steady state flow stress when they are deformed in the temperature range of 350-500℃ at strain rates within the range of 0.01-10.0s^-1. A hyperbolic sine relationship is found to correlate well the flow stress with the strain rate, and an Arrhenius relationship with the temperature, which implies that the process of plastic deformation at elevated temperature for this material is thermally activated. Compared with the AI pieces prepared by no or conventional melt-treatment, hot deformation activation energy of AI sheets prepared by high-efficient melt-treatment is the smallest （ Q= 168.0kJ/mol）, which reveals that the hot working formability of this material is very better, and has directly to do with the effective improvement of its metallurgical quality.

Hot compression deformation behavior of the Mg-Al-Y-Zn magnesium alloy

The hot deformation behavior of a Mg-Al-Y-Zn magnesium alloy was investigated by hot compressive testing on a Gleeble-1500 thermal simulator at the temperature ranging from 523 to 673 K with the strain rate varying from 0.001 to 1 s^-1. The relationships among flow stress, strain rate, and deformation temperature were analyzed, and the deformation activation energy and stress exponent were calculated. Microstructure evolution of the alloy under different conditions was examined. The results indicated that the maximum value of the flow stress increased with the decrease of deformation temperature or the increase of strain rate. Under the present deformation conditions, dynamic recrystallization （DRX） occurred in the alloy, which was the main softening mechanism during deformation at elevated temperature. The deformation temperature and strain had significant effects on the microstructure of the alloy.

THE HOT DEFORMATION BEHAVIOR AND DYNAMIC RECRYSTALLIZATION MODEL OF 35CrMo STEEL

The isothermal single-stage compression of 35CrMo structural steel has been carried out by using Gleeble 1500 simulator at the temperature range of 950℃ to 1150℃ and strain rate range of 0.01s-1 to 10s-1. The effect of hot deformation parameters, such as strain rate, deformed temperature and initial grain size on the flow stress behavior was investigated. The activation energy of tested alloy was calculated, which is 378.16kJ/mol; The relationships between the peak stress (σp), the peak stain (εp), the critical strain (εc) and Z parameter were established. The micro structure evolution shows the pre-existing austenite grain boundaries constitute the principal nucleation sites for dynamic recrystallization (DRX), and the initial austenite grain size affects the grain size of DRX slightly. The kinetic mathematical model of DRX of 35CrMo is: XDRX=1-exp(-3.23-2.28) and Ddyn = 2.252× 10Z-0.22.

Hot Deformation Behavior of Squeeze Casting SiC_p/2 A50 Matrix Composites

The flow stress behaviors of squeeze casting SiCp/2A50 matrix composites were investigated by means of compression tests on a Gleeble 1500 therma1 mechanical simulator at isothermal constant strain rates ranging from of 0.001 to 1.0 with the testing temperature ranging from 350 to 500 ℃. The experiments showed that the relationship between stress and strain was obviously influenced by the strain rate and temperature. Dynamic recrystallization generally occurred at a higher temperature and a 1ower strain rate. A linear equation could be fitted between the Zener-Hollomon parameter Z and stress in the experiments. The mean value reciprocal of temperature at every true strain had a linear relation with natural logarithm of Z parameter, and the correlation coefficient, R=0.99, which was very significant by examination. The hot deformation activation energy of SiCp/2A50 matrix composites was 163.47 KJ/mol by calculation.

Hot deformation behavior of FGH96 superalloys

The hot deformation behavior of FGH96 superalloys at 1070-1170℃ and 5×10^-4-2×10^-1 s^-1 were investigated by means of the isothermal compression tests at a Gleeble-1500 thermal mechanical simulator. The results show that dynamic recovery acts as the main softening mechanism below 2×10^-3 s^-1, whereas dynamic recrystallization acts as the main softening mechanism above 2× 10^-3 s^-1 during deformation; the temperature increase caused by the deformation and the corresponding softening stress is negligible; the thermal-mechanical constitutive model to describe the hot deformation behavior is given, and the value of the apparent deformation activation energy （Qdef） is determined to be 354.93 kJ/mol.

Dynamic Recrystallization Behaviour of Nb-Ti Microalloyed Steels

The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX was retarded effectively at low temperature due to the onset of dynamic precipitation of Nb and Ti carbonitrides, resulting in higher values of the peak strain. An expression was developed for the activation energy of deformation as a function of the contents of Nb and Ti in solution as well as other alloying elements. A new value of corrective factor was determined and applied to quantify the retardation produced by increase in the amount of Nb and Ti dissolved at the reheating temperature. The ratio of critical strain to peak strain decreases with increasing equivalent Nb content. In addition, the effects of Ti content and deformation conditions on DRX kinetics and steady state grain size were determined. Finally, the kinetics of dynamic precipitation was determined and effect of dynamic precipitation on the onset of DRX was clarified based on the comparison between precipitate pinning force and recrystallization driving force.

Deformation behavior of Fe-Mn-C TWIP steel within the temperature range of hot rolling

The hot-deformation behavior of Fe-Mn-C twinning induced plasticity （TWIP） steel was investigated by conducting hot compression tests within a recommended hot rolling temperature range at various strain rates. Flow resistance curves during hot-deformation were obtained, and strain rate sensitivities and activation energies for plastic deformation were calculated using the power law. It is found that the addition of Al and Si clearly increases the peak stresses for the present alloys, especially at 950℃. But Mn has a minor effect on the stress-strain curves and activation energy when its content varies from 15 mass% to 22 mass% for the present alloys.

Constitutive model and deformation microstructure of fine-grain Mg-Zn-Y alloy solidified under high pressure

Fine-grain Mg95.50Zn3.71Y0.79 alloy was prepared by high pressure solidification. By comparison with the conventional cast-ing alloy, the true stress-strain curve characteristic and deformation microstructure of Mg95.50Zn3.71Y0.79 alloy solidified under high pressure were studied via unilateralism compress tests under the strain rate of 0.001–1 s–1 and deformation temperature of 523–623 K. Constitutive equations were constructed. According to the experimental results, compared to the conventional casting alloy, the true stress-strain curve of the fine-grain alloy solidified under high pressure not only had the high strain hardening characteristic but the dynamic recrystallization softening after the peak stress was more than the working hardening and would soon reach a stable flow stress - strain state. The deformation activation energy of the alloy solidified under high pressure was 151 kJ/mol, around 49 kJ/mol lower than that of the conventional casting alloy. The fine-grain Mg-Zn-Y alloy solidified under high pressure could obtain 95 percent of dynamic recrystallization grain at 573 K during hot deformation process.

Deformation behavior and processing maps during isothermal compression of TC21 alloy

In this study,isothermal compression tests were conducted at a Gleeble-1500 simulator at deformation temperatures ranging from 1073 to 1283 K,strain rates ranging from 0.01 to 5.00 s^（-1）,and height reductions ranging from 20%to 60%.The flow stress and apparent activation energy for deformation and constitutive equation were used to characterize the deformation behavior of TC21 alloy during the isothermal compression.The processing maps combined microstructure observations were established based on dynamic material model（DMM） over a range of strain rates and temperatures.The results show that an initial yield drop is observed above 1203 K or at higher strain rates ranging from 1.00 to 5.00 s^-1,and oscillatory flow curves are presented particularly at a strain rate of 5.00 s^-1.Strain has some influence on the apparent activation energy for deformation during the isothermal compression of TC21 alloy.The Q-values and microstructure observation confirm that dynamic recrystallization（DRX） occurs in the β single-phase region.The constitutive equation during the isothermal compression of TC21 alloy is developed using the Zener-Hollomon parameter in the exponent-type equation.The maximum and minimum relative errors between the calculated and the experimental flow stress are 14.1%and 0.3%,respectively.The peak efficiency of power dissipation at a strain of 0.7 is about 0.51 occurring at a deformation temperature of 1073 K and strain rate of 0.01 s^-1,corresponding to an optimal deformation condition of TC21 alloy.

Hot Deformation Behavior of a Novel Ni-Cr-Mo-B Ultra-heavy Plate Steel by Hot Compression Test

Hot deformation behavior of a novel Ni-Cr-Mo-B heavy plate steel was studied by hot compression tests,which were conducted on a Gleeble-3800thermo-mechanical simulator corresponding to the temperature range of850-1 150℃ with the strain rates of 0.01-10s-1 and the true strain of 0.8.The results suggest that the majority of flow curves exhibit a typical dynamic recrystallization（DRX）behavior with an apparent single peak stress followed by agradual fall towards a steady-state stress.Important characteristic parameters of flow behavior as critical stress/strain for initiation of DRX and peak and steady-state stress/strain were derived from curves of strain hardening rate versus stress and stress versus strain,respectively.Material constants of the investigated steel were determined based on Arrhenius-type constitutive equation,and then the peak stress was predicted by the equation with the hot deformation activation energy of 379 139J/mol,and the predicted values agree well with the experimental values.Furthermore,the effect of Zener-Hollomon parameter on the characteristic points of flow curves was studied using the power law relation,and the ratio of critical stress and strain to peak stress and strain were found to be 0.91and0.46,respectively.